VOLUME 115 NO. 10 OCTOBER 2015

advanced metals initiative

         

28–30 October 2015

our future through science advanced metals initiative

ADVANCED METALS INITIATIVE TECHNOLOGY NETWORKS - expand the country’s technical The Advanced Metals Initiative (AMI) was The AMI’s technology networks include: capacity; established by the Department of Science • the Light Metals Development Network - develop the use of metals in new and Technology to facilitate research, (LMDN) for titanium and aluminium co- applications and more diverse ordinated by the Council for Scientific development and innovation across the industries; and and Industrial Research (CSIR); advanced metals value chain. - develop industrial localisation. • the Precious Metals Development Network (PMDN) for gold and the GOAL platinum group metals (PGMs), co- LIGHT METALS DEVELOPMENT NETWORK To target significant export income and new ordinated by Mintek; • Global demand for ultralight, industries for South Africa by 2020 through • the Nuclear Materials Development Network (NMDN) for hafnium, ultrastrong, recyclable metals is the country becoming a world leader in zirconium and monazite co-ordinated growing as the world switches to low- sustainable metals production and manu- by the Nuclear Energy Corporation of emission vehicles, energy-saving facturing via technological competence and South Africa (Necsa). devices and sustainable products. optimal, sustainable local manufacturing of • the Ferrous Metals Development • Aluminium demand is forecast to value-added products, while reducing Network (FMDN) for ferrous and base increase by 30% in the near future, environmental impact. metals, co-ordinated by Mintek. while for the emerging industrial light metal, titanium, the sky is the limit. To lead a global revolution in advanced metals generating significant export • For its part, South Africa has a mature STRATEGY income and new industries for South Africa aluminium industry, which is among the while reducing environmental impact. country’s top exporters, and one of the The AMI takes an integrated approach world’s richest titanium resources on across the entire value chain from resource which to build a new industry. development to metal production and the LMDN NMDN PMDN FMDN manufacture of end-products, to achieve its The LMDN sees South Africa becoming a Lighter, Beneficiation for Value-added Beneficiation of goal, through: functional alloy nuclear PGM products resources and world leader in light metals. materials for the materials used (Autocatalysts, materials • Reducing the energy required to automotive and in nuclear PGM coatings, solutions for the The LMDN conducts scientific research aerospace reactors. etc.). transportation, produce metals by 30%; industries. energy and activities along the entire value chain, from petrochemical resource development to primary metal • Increasing asset productivity by 30%; industries. production, fabrication, casting, joining • Developing technologies that can technologies and manufactured products. enable new industries for South Africa; Enabling Platform of Projects and Partnerships and, The AMI promotes collaborative research The aim is to create a globally competitive • Reducing the full life cycle between the science councils, higher integrated light metals industry, to develop environmental impact of metals education institutions, and industry. superior cost-effective technologies and manufacturing systems, and to reduce products by 50%. Human resource development is critical for energy use, greenhouse emissions and the networks to: environmental impact. CONTACT DETAILS Llanley Simpson Tel: +27 12 843 6436 Fax: +27 86 681 0242 Cell: +27 83 408 6910 Email: [email protected] advanced metals initiative

NUCLEAR MATERIALS FERROUS METALS DEVELOPMENT DEVELOPMENT NETWORK NETWORK produce high-end ferrous products, • The global upsurge in energy demand has especially those that are needed by other The FMDN presents a unique opportunity to led to a renewed focus on nuclear energy critical sectors of the economy, such as simultaneously add value to several and related nuclear materials like petrochemical, energy generation, zirconium and hafnium. minerals that South Africa possess in large transportation, etc. quantities such as iron, chromium, • Zirconium is used as cladding in nuclear manganese, vanadium, etc. while • Generation of local know-how (innovation). reactors and zirconium carbide has addressing key material challenges • Human Capital Development which will applications in future nuclear reactors. experienced by strategic sectors of the alleviate the shortage in scientific and economy such as the transportation, energy technological qualifications and skills in • South Africa has a vast resource of zircon and petrochemical industries. The FMDN these sectors and thereby ensuring the and supplies 30% of the world market. R&D programmes are done within a sustainablity and the competitiveness of the local industry. This will also improve The NMDN seeks to beneficiate zirconium tripartite collaborative framework involving SA’s attractiveness as an investment and hafnium across the value chain through industry, academia and science councils. destination. the preparation and purification of The broad objectives of the FMDN can thus • intermediate metal salts, metal be summarised as follows: • Promotion of local and international manufacturing and optimum zirconium- Beneficiation of South Africa’s ferrous collaboration in the field of ferrous alloys. resources to stages 3 and 4 undefined. metallurgy. The NMDN targets alternative, novel, economic and environmentally friendly manufacturing processes for the metal pair zirconium/hafnium via existing plasma and fluorochemical expertise.

PRECIOUS METALS DEVELOPMENT NETWORK Precious metals are characterised by their high density and cost, which make them less attractive for use in bulk components and more viable in coating/deposition technologies (chemical and physical) for various applications in which the unique The key focus is on: properties of these high-value metals are • creating new industries; beneficial. • supporting existing industries; The PMDN assists South Africa in retaining the precious metals value matrix through the • localisation of technology. identification, research and promotion of new technologies and applications to supported by support the long-term development of the mining industry.

• Improvement of the country’sour capabilityfuture through science to  
S.J. Ramokgopa (2002-2003) T.R. Stacey (2003–2004) F.M.G. Egerton (2004–2005) W.H. van Niekerk (2005–2006) R.P.H. Willis (2006–2007) R.G.B. Pickering (2007–2008) A.M. Garbers-Craig (2008–2009) J.C. Ngoma (2009–2010) G.V.R. Landman (2010–2011) J.N. van der Merwe (2011–2012) G.L. Smith (2012–2013) M. Dworzanowski (2013–2014) J.L. Porter (2014–2015) D.G. Maxwell (1961–1962) J.P. Hugo (1972–1973) 1974) R.E. Robinson (1975–1976) G.Y. Nisbet (1981–1982) A.N. Brown (1982–1983) J.D. Austin (1984–1985) H.E. James (1985–1986) H. Wagner (1986–1987) C.E. Fivaz (1988–1989) O.K.H. Steffen (1989–1990) R.D. Beck (1991–1992) J.P. Hoffman (1992–1993) J.A. Cruise (1994–1995) D.A.J. Ross-Watt (1995–1996) N.A. Barcza (1996–1997) R.P. Mohring (1997–1998) J.R. Dixon (1998–1999) M.H. Rogers (1999–2000) L.A. Cramer (2000–2001) * A.A.B. (2001–2002) Douglas * H. Simon (1957–1958) * M. Barcza (1958–1959) * R.J. Adamson (1959–1960) * W.S. Findlay (1960–1961) * J. de V. Lambrechts (1962–1963) * J.F. Reid (1963–1964) * D.M. Jamieson (1964–1965) * H.E. Cross (1965–1966) * D. Gordon Jones (1966–1967) * P. Lambooy (1967–1968) * R.C.J. Goode (1968–1969) * J.K.E. Douglas (1969–1970) * V.C. Robinson (1970–1971) * D.D. Howat (1971–1972) * P.W.J. van Rensburg (1973– * R.P. Plewman (1974–1975) * M.D.G. (1976–1977) Salamon * P.A. (1977–1978) Von Wielligh * M.G. Atmore (1978–1979) * D.A. Viljoen (1979–1980) * P.R. Jochens (1980–1981) * R.P. King (1983–1984) * B.C. Alberts (1987–1988) * H.G. (1990–1991) Mosenthal * H. (1993–1994) Scott-Russell        Honorary Legal Advisers Van Hulsteyns Attorneys Auditors Messrs R.H. Kitching Secretaries The Southern African Institute of Mining and Metallurgy Fifth Floor, Chamber of Mines Building 5 Hollard Street, Johannesburg 2001 • P.O. Box 61127, Marshalltown 2107 Telephone (011) 834-1273/7 • Fax (011) 838-5923 or (011) 833-8156 E-mail: [email protected] 1936) PAST PRESIDENTS *Deceased * W. Bettel (1894–1895) * A.F. Crosse (1895–1896) * W.R. Feldtmann (1896–1897) * C. Butters (1897–1898) * J. Loevy (1898–1899) * J.R. Williams (1899–1903) * S.H. Pearce (1903–1904) * W.A. Caldecott (1904–1905) * W. Cullen (1905–1906) * E.H. Johnson (1906–1907) * J. Yates (1907–1908) * R.G. Bevington (1908–1909) * A. McA. Johnston (1909–1910) * J. Moir (1910–1911) * C.B. Saner (1911–1912) * W.R. Dowling (1912–1913) * A.(1913–1914) Richardson * G.H. Stanley (1914–1915) * J.E. Thomas (1915–1916) * J.A.(1916–1917) Wilkinson * G. (1917–1918) Hildick-Smith * H.S. Meyer (1918–1919) * J. Gray (1919–1920) * J. Chilton (1920–1921) * F. (1921–1922) Wartenweiler * G.A. (1922–1923) Watermeyer * F.W. Watson (1923–1924) * C.J. Gray (1924–1925) * H.A. White (1925–1926) * H.R. Adam (1926–1927) * Sir (1927–1928) Robert Kotze * J.A. (1928–1929) Woodburn * H. Pirow (1929–1930) * J. Henderson (1930–1931) * A. King (1931–1932) * V. (1932–1933) Nimmo-Dewar * P.N. Lategan (1933–1934) * E.C. Ranson (1934–1935) * R.A. Flugge-De-Smidt (1935– * T.K. Prentice (1936–1937) * R.S.G. Stokes (1937–1938) * P.E. Hall (1938–1939) * E.H.A. Joseph (1939–1940) * J.H. Dobson (1940–1941) * Theo Meyer (1941–1942) * John(1942–1943) V. Muller * C. Biccard Jeppe (1943–1944) * P.J. Louis Bok (1944–1945) * J.T. McIntyre (1945–1946) * M. Falcon (1946–1947) * A. Clemens (1947–1948) * F.G. Hill (1948–1949) * O.A.E. Jackson (1949–1950) * W.E. Gooday (1950–1951) * C.J. Irving (1951–1952) * D.D. Stitt (1952–1953) * M.C.G. Meyer (1953–1954) * L.A. Bushell (1954–1955) * H. Britten (1955–1956) * Wm. Bleloch (1956–1957)   
 A. Mainza D. Muma S. Ndiyamba C.W. Mienie C.A. van Wyk P. Bredell I. Ashmole N.M. Namate H. Wagner S.D. Williams L.E. Dimbungu S. Maleba I.J. Corrans, R.J. Dippenaar, A. Croll, C. Workman-Davies J.J.L. Cilliers, N.A. Barcza J-M.M. Rendu, P.C. Pistorius ii Zambia Zimbabwe Zululand Western Cape Northern Cape Pretoria Johannesburg Namibia Corresponding Members of Council Branch Chairmen Ordinary Members on Council Z. Botha V.G. Duke I.J. GeldenhuysM.F. Handley W.C. Joughin M. Motuku D.D. Munro M.H. Solomon G. Njowa A.G. Smith J.D. Steenkamp M.R. Tlala D. Tudor van Niekerk D.J. J.L. Porter Honorary Treasurer Immediate Past President President Elect Vice-Presidents President Honorary Vice-Presidents Mosebenzi Zwane South Africa Minister of Mineral Resources, Rob Davies South Africa Minister of Trade and Industry, Naledi Pandor South Africa Minister of Science and Technology, Honorary President Mike Teke of South Africa President, Chamber of Mines OFFICE BEARERS AND COUNCIL FOR THE AND COUNCIL OFFICE BEARERS SESSION 2015/2016 Australia: Austria: Botswana: United Kingdom: USA: Botswana DRC Past Presidents Serving on Council N.A. BarczaR.D. BeckJ.R. DixonM. DworzanowskiF.M.G. Egerton H.E. James G.V.R. Landman M.H. Rogers J.C. Ngoma G.L. Smith S.J. Ramokgopa W.H. van Niekerk C. Musingwini S. Ndlovu A.S. Macfarlane C. Musingwini R.T. Jones The Southern African Institute of Mining and Metallurgy Institute of Mining African The Southern L Editorial Board R.D. Beck J. Beukes P. den Hoed M. Dworzanowski B. Genc M.F. Handley R.T. Jones VOLUME 115 NO. 10 OCTOBER 2015 W.C. Joughin J.A. Luckmann C. Musingwini J.H. Potgieter R.E. Robinson Contents T.R. Stacey Journal Comment Editorial Consultant by J.T. Nel ...... iv–v D. Tudor President’s Corner Typeset and Published by by R.T. Jones ...... vii The Southern African Institute of Mining and Metallurgy P.O. Box 61127 Where should the national R&D in materials science fit into South Africa’s future nuclear Marshalltown 2107 power programme? Telephone (011) 834-1273/7 by W.E. Stumpf ...... 893 Fax (011) 838-5923 E-mail: [email protected] Friction processing as an alternative joining technology for the nuclear industry Printed by by D.G. Hattingh, L. von Wielligh, W. Thomas and M.N. James ...... 903 Camera Press, Johannesburg Neutron- and X-ray radiography/ tomography: non-destructive analytical tools for the Advertising Representative characterization of nuclear materials Barbara Spence by F.C. de Beer ...... 913 Avenue Advertising Telephone (011) 463-7940 Non-destructive characterization of materials and components with neutron and X-ray E-mail: [email protected] diffraction methods The Secretariat by A.M. Venter...... 925 The Southern African Institute Fluorine: a key enabling element in the nuclear fuel cycle of Mining and Metallurgy by P.L. Crouse ...... 931 ISSN 2225-6253 (print) ISSN 2411-9717 (online) Titanium and zirconium metal powder spheroidization by thermal plasma processes by H. Bissett, I.J. van der Walt, J.L. Havenga and J.T. Nel...... 937 Plasma technology for the manufacturing of nuclear materials at Necsa by I.J. van der Walt, J.T. Nel and J.L. Havenga ...... 943 Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma THE INSTITUTE, AS A BODY, IS NOT RESPONSIBLE FOR THE operating at low to atmospheric pressures STATEMENTS AND OPINIONS ADVANCED IN ANY OF ITS by J.H. van Laar, I.J. van der Walt, H. Bissett, G.J. Puts and P.L. Crouse...... 949 PUBLICATIONS. Copyright© 1978 by The Southern African A redetermination of the structure of tetraethylammonium mer- Institute of Mining and Metallurgy. All rights
2 reserved. Multiple copying of the contents of oxidotrichlorido(thenoyltrifluoroacetylacetonato- -O,O')niobate(V) this publication or parts thereof without by R. Koen, A. Roodt and H. Visser ...... 957 permission is in breach of copyright, but permission is hereby given for the copying of titles and abstracts of papers and names of authors. Permission to copy illustrations and A theoretical approach to the sublimation separation of zirconium and hafnium in the short extracts from the text of individual contributions is usually given upon written tetrafluoride form application to the Institute, provided that the source (and where appropriate, the copyright) by C.J. Postma, H.F. Niemand and P.L. Crouse ...... 961 is acknowledged. Apart from any fair dealing for the purposes of review or criticism under Glow discharge optical emission spectroscopy: a general overview with regard to The Copyright Act no. 98, 1978, Section 12, of the Republic of South Africa, a single copy of nuclear materials an article may be supplied by a library for the purposes of research or private study. No part by S.J. Lötter, W. Purcell and J.T. Nel ...... 967 of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior permission of The influence of niobium content on austenite grain growth in microalloyed steels the publishers. Multiple copying of the contents of the publication without permission by K.A. Annan, C.W. Siyasiya and W.E. Stumpf...... 973 is always illegal. U.S. Copyright Law applicable to users In the The influence of thermomechanical processing on the surface quality of an AISI 436 U.S.A. The appearance of the statement of copyright ferritic stainless steel at the bottom of the first page of an article appearing in this journal indicates that the by H.J. Uananisa, C.W. Siyasiya, W.E. Stumpf and M.J. Papo ...... 981 copyright holder consents to the making of copies of the article for personal or internal use. This consent is given on condition that the copier pays the stated fee for each copy of a paper beyond that permitted by Section 107 or International Advisory Board 108 of the U.S. Copyright Law. The fee is to be paid through the Copyright Clearance Center, R. Dimitrakopoulos, McGill University, Canada VOLUME 115 NO. 10 OCTOBER 2015 Inc., Operations Center, P.O. Box 765, Schenectady, New York 12301, U.S.A. This D. Dreisinger, University of British Columbia, Canada consent does not extend to other kinds of E. Esterhuizen, NIOSH Research Organization, USA advanced metals initiative copying, such as copying for general All papers featured in this edition were presented at the H. Mitri, McGill University, Canada Nuclear Materials Development Network Conference distribution, for advertising or promotional 28–30 October 2015 purposes, for creating new collective works, or M.J. Nicol, Murdoch University, Australia for resale. E. Topal, Curtin University, Australia our future through science

       
  
 iii L J.T. Nel  
that is dedicated to the annual AMI NMDN Coordinator and Conference Chairman Journal are a selection from the papers that will be presented are a selection from the papers that will be The NMDN and Necsa express their sincere gratitude I hope that everyone will find the conference an enjoyable Human capital development forms a fundamental pillar of Human capital development forms a fundamental journal of the The papers that are published in this special Thorium is envisaged to be an important nuclear fuel for Thorium is envisaged the AMI and the NMDN over many years. the future Generation IV high-temperature gas-cooled nuclear the future Generation ingredient in permanent magnets that for example, is a crucial to generate electricity. Monazite is a are used in wind turbines mineral sand industry in South by-product from the heavy in hard-rock orebodies, for example Africa, and is also found Zandkopsdrift in the Western Cape at Steenkampskraal and rare earth phosphate, is extremely Province. Monazite, a procedures chemically inert. Conventional chemical extraction and produce are very harsh, environmentally unfriendly, new plasma radioactive waste. The NMDN is investigating REEs and to fluoride beneficiation methods to recover the values. separate the contained thorium and uranium students the AMI. In 2015, the NMDN has 28 postgraduate to contribute enrolled at various universities in South Africa base in nuclear to the building of a sound local knowledge in the technology. The participation of these universities postgraduate NMDN projects is highly appreciated. The gratitude towards students of the NMDN express their extreme launching platform the DST for creating this programme as a for their careers. Journal of the AMI, of at the 2015 academic peer-reviewed process postgraduate which this conference is the grand finale for NMDN and will students. This year it is being hosted by the University in take place at the Nelson Mandela Metropolitan It is noteworthy that Port Elizabeth from 28–30 October 2015. that the special this is the first time in the history of the AMI edition of the conference. In this conference will be published ahead of the regard the Chairman thanks the organizing committee of the conference, the referees, and the SAIMM publications team for their hard work towards achieving this historic goal. towards the Honourable Minister Naledi Pandor and the Honourable Director General of the Department of Science and Technology, Dr Phil Mjwara, for their continuing support of event and that this special edition of the SAIMM Journal will constitute a major contribution to the nuclear science and engineering fraternity of South Africa. reactors or molten salt nuclear reactors. The mineral monazite reactors or molten salt concentration of thorium along with contains a significant (REEs) such as neodymium valuable rare earth elements Nd, (Pr) and yttrium (Y). (Nd), cerium (Ce), praseodymium        Journal Comment Journal   
 The Integrated Resource Plan for Electricity for South The Integrated Resource Plan for Electricity of the Zirconium alloys are used as cladding material The unfortunate Fukushima incident is still fresh in the iv Africa (generally referred to as the IRP2010) identified Africa (generally referred to as the IRP2010) to the total nuclear power as an important contributor add 9.6 GW of electricity mix of the future. It is planned to national grid by electricity generated by nuclear power to the power plants 2030. This implies that six to eight new nuclear technology (NPPs) need to be built in South Africa. Nuclear important factors in localization and local content will be very the realization of the nuclear new-build programme. that is used in nuclear fuel in NPPs. All the zirconium metal zircon, of which this application is extracted from the mineral in the world. The South Africa is the second-largest producer fluoride NMDN has developed a unique plasma and zirconium beneficiation process to manufacture nuclear-grade is always metal powder from locally mined zircon. Hafnium has to be separated associated with zirconium in nature, but it absorption from zirconium due to its high thermal neutron cross-section. This property of hafnium is, however, being exploited by the nuclear industry in applications where absorbance of neutrons is required. Apart from its nuclear applications, hafnium is also used in electronics, optics, high- temperature-resistant ceramic materials, and in the aerospace industry. The NMDN has consequently also embarked on the development of hafnium products for nuclear and non-nuclear applications. minds of everybody. High temperatures and rapid oxidation of the zirconium cladding material in contact with the coolant water led to the generation of hydrogen gas and the spectacular hydrogen explosions that were witnessed by everyone on television. Since then, there has been a major drive in the nuclear industry to make zirconium alloys more resistant to oxidation at high temperatures. The application of zirconium carbide and silicon carbide layers on zirconium nuclear fuel tubes is but one of the research programmes that the NMDN is pursuing in this regard. This special edition of the Journal of the Southern African This special edition of Metallurgy is dedicated to the Nuclear Institute of Mining and Network (NMDN) of the Advanced Materials Development of South Africa’s Department of Metals Initiative (AMI) (DST). The AMI consists of four Science and Technology Development Network (LMDN) networks: the Light Metals by the CSIR, the Precious Metals which is coordinated (PMDN) and the Ferrous Metals Development Network both coordinated by Mintek, and the Development Network, by the South African Nuclear NMDN which is coordinated Ltd (Necsa). The NMDN focuses on Energy Corporation SOC improvement of nuclear materials in the development and the order to enhance the safety of nuclear reactors, analytical characterization of nuclear materials by various the beneficiation techniques including nuclear techniques, and value chain with of South African minerals across the whole in South Africa in the aim to manufacture nuclear components the future. L Mining in the global village President’s

Corner t has been just over fifty years since the concept of the ‘global village’ was introduced by Marshall McLuhan in 1964, yet it remains relevant to Iour everyday experiences. Modern communication technologies have seemingly shrunk the world even further since then. Ray Tomlinson, in 1971, was the first person to send mail from one computer to another over a network (and also initiated the practice of using the @ sign to direct the networked electronic mail message to a particular user at a particular computer). In 1997, e-mail volume overtook postal mail volume, as more and more people recognized the convenience of this almost immediate, yet still asynchronous, mode of communication. That same year, 1997, saw the registration of the google.com domain name, and searching for information was transformed for ever, as people came to rely on the Google search engine to navigate the World Wide Web (invented by Tim Berners-Lee in 1989). It seems hard to believe that it’s been only about 20 years since the mass popularization of the World Wide Web (arguably one of the world’s greatest inventions since the wheel). Nowadays, we can almost instantly read about (or watch) events happening anywhere in the world. The interconnectedness of today’s world has led to a direct link between the slowing down of the rate of growth in urbanization in China and the state of the economy in Rustenburg, for example. There is also much mobility of people between countries and continents. Many engineers trained in South Africa work in Australia, and many Australian engineers work in the USA, and so on. The SAIMM maintains strong links with similar societies in other countries. In November 2011, an inaugural meeting was held in London between several leading international mining and metallurgical societies – AusIMM (Australasian Institute of Mining and Metallurgy), CIM (Canadian Institute of Mining, Metallurgy and Petroleum), IOM3 (Institute of Materials, Minerals and Mining), SAIMM (Southern African Institute of Mining and Metallurgy), and SME (Society for Mining, Metallurgy and Exploration). The meeting was intended to foster cooperation between the various organizations, to discuss opportunities for improving and sharing benefits to members, and to benchmark the institutions against each other. Further meetings between these societies were held in September 2012 in Las Vegas (SME), in February 2013 in Denver (SME), in February 2014 in Cape Town (SAIMM), in October 2014 in Vancouver (CIM), and in March 2015 in Hong Kong (AusIMM). Agreements have been signed between these societies, resulting in the formation of what is known as the Global Mineral Professionals Alliance (GMPA). Discussions were held about the state of the mining industry in the various countries, as well as the structure and strategies of the societies represented. There was broad agreement that the societies would offer services to each other's members at member rates. This is a significant benefit to SAIMM members, as they can attend international conferences held by AusIMM, CIM, IOM3, and SME at the same cost as members of those societies. Calendars of events are circulated between the organizations to coordinate major events and minimize clashes. The flagship project of the GMPA is OneMine.org, a database of over 100 000 technical papers that is freely available to be used by the members of GMPA societies. Support of this project – both financially and by sharing technical papers – is a necessary precondition for a society to belong to the GMPA. Participating societies also agree to publicize their GMPA affiliation on their websites, and to share meeting calendars and information about each other’s international events. Representatives of each society meet once a year to exchange information, to maintain a common set of standards for technical events, and to look for further ways to increase member benefits with reciprocal arrangements. This also provides an opportunity to share approaches and resources to deal with global problems shared by all. Until asteroid mining becomes accepted practice, we will have to settle for this global approach on Planet Earth.

R.T. Jones President, SAIMM

       
  
 v L MINTEK – a global leader in minerals and metallurgical research and development

TODAY THIS CENTRE OF TECHNOLOGICAL EXCELLENCE, with its teams of highly trained and experienced scientists, engineers, researchers and specialists, continues to develop and provide DGYDQFHG WHFKQRORJ\ IRU WKH PRUH HIIHFWLYH H[WUDFWLRQ XWLOLVDWLRQ DQG EHQHÀFLDWLRQ RI RXU mineral wealth. 0LQWHNSURYLGHVH[SHUWLVHLQDOOFRPPRGLWLHVDQGPHWDOOXUJLFDOÀHOGV ` One of the world’s longest-running and largest ` Very large integrated test and pilot plant minerals technology facilities — based in facilities. Johannesburg, South Africa. ` Comprehensive accredited analytical and ` Metallurgical support throughout all stages of mineralogical services. project development. ` “One-stop shop” service — spanning all ` Extensive knowledge database of Southern metallurgical disciplines. African and worldwide mineral resources. ` Unique, proprietary technologies and products. ` 81-year track record in providing quality work.

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200 Malibongwe Drive, Randburg, South Africa. Private Bag X3015, Randburg 2125, South Africa. Phone: +27 (011) 709 4111 Fax: +27 (011) 709 4326 E-mail: [email protected] A global leader in mineral and http:// www.mintek.co.za metallurgical innovation http://dx.doi.org/10.17159/2411-9717/2015/v115n10a1 Where should the national R&D in materials science fit into South Africa’s future nuclear power programme? by W.E. Stumpf*

significant climate change confronting the Synopsis human race, as the IPCC cautioned in 2013, and will have to adjust its future energy South Africa recently announced a resurgence in its commercial nuclear power programme. The implications for the development of the necessary reliance to a more balanced combination of high-level manpower within South Africa’s tertiary educational system sources. and its national research and development (R&D) capacity in materials After many years of international science and engineering, as well as in other engineering disciplines, are conferences, meetings, and working group placed into perspective. An organized national process of developing this sessions, the world is no nearer to finding an manpower by moving away from the previously high-risk and costly ’large equitable and binding international agreement programmes’ to rather a selection of ‘small and better’ research projects on measures to curb climate change. It is, and a redefinition of what constitutes ‘nuclear materials’ are proposed as therefore, highly unlikely that the more parts of this strategy. acceptable low-emission scenarios such as the Keywords RCP2.6 (Figure 1) are realistic, and current materials science, pressurised water reactor, zirconium based cladding trends appear to indicate that the world is materials, uranium enrichment, steam generator. facing a more pessimistic climate change future, such as the RCP8.5 scenario. Does this mean that South Africa will need to completely phase out coal-fired power in the Introduction medium to long term? No, that would be Since 2008, and more particularly in impossible, and even irresponsible, but it does 2014/2015, South Africa has woken up to the mean that a future energy mix of about 50% fact that significant steps need to be taken to coal-fired, 25% nuclear-based, and 10% ensure sufficient electricity generating capacity imported gas-fired power, with the remaining for the future, even beyond the coal-fired 15% consisting of renewable energy sources, stations at Medupi and Kusile currently under would be a typical future to plan for.Such a construction. It is, therefore, encouraging to scenario would constitute a baseload capacity see some active large-scale wind farms in the of about 80–85% with the remainder Eastern Cape near Jeffrey’s Bay, in the Couga comprising renewable energy sources, mainly area, and others in the Western Cape already wind and solar. in operation. In addition, many solar energy Such a turnaround from a very high to a projects are also progressing from the small more reasonable dependence on coal plus a localized scale to larger programmes in the still limited nuclear dependence will place Northern Cape, which may contribute some heavy demands on South Africa’s technical capacity on a national basis. South Africa expertise to select, evaluate, and later to needs to tap into its renewable resources of supply the materials that are ‘fit for purpose’ wind and solar much more, but will these in the planned nuclear power programme. projects solve the country’s long-term industrial needs? Unfortunately not. One Broad classification of nuclear materials cannot run mines and trains on solar cells. in a pressurized water reactor Industry needs reliable baseload capacity, and Although a modern nuclear power reactor such with very limited easily accessible hydro- as a pressurized water reactor (PWR) consists capacity this leaves really only coal, possibly natural gas, and nuclear power as options. South Africa’s current over-reliance of about 90% on coal–fired power, however, places it in an internationally vulnerable position, and * Professor in Physical Metallurgy, University of diversification into a more equitable energy Pretoria. mix should be a national priority for the © The Southern African Institute of Mining and medium to long term. South Africa cannot Metallurgy, 2015. ISSN 2225-6253. Paper received simply ignore the mounting evidence of Aug. 2015 and revised paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 893 ▲ Where should the national R&D in materials science fit into South Africa’s future nuclear power programme? in essence of the same main components as those for a coal- fuel cladding to boron-based control rod materials, from fired plant, i.e. a heat source, a steam generating system, and electrically conductive copper to cathodically protected tube a steam-driven turbine/generator combination, the operating sheet and, last but not least, oxide fuel pellets. Production and safety requirements make a typical PWR a far more and manufacturing processes for these materials range from complex system that requires specialized materials. Figure 2 cast components to wrought and welded tubes and sheet, shows a broad overview of the typical materials currently in from passivated surfaces to corrosion-resistant weld- use in a modern PWR. Note the wide range, from low-alloy cladding, from sophisticated to more conventional heat steel to more sophisticated ferrous and stainless steel alloys, treatments, from high purity to standard material purities, from nickel-based creep-resistant alloys to corrosion- from solid to porous sintered items, and so on. resistant titanium condenser tubes, from zirconium-based Such a wide range of materials of construction poses a tremendous challenge to South Africa’s materials engineers and scientists if they wish to grow into and actively participate in an expanding nuclear power programme. To simply sit back while all of the know-how is imported, even in the long term, is not an option. On the other hand, to consider actively mastering the know-how for all of the above materials is also unrealistic. Some hard choices, therefore, need to be taken to rather focus on those areas where maximum benefit can be gained within the limited research resources at the country’s disposal.

The need for materials science in PWR technology In assessing the broad research focus areas of South Africa’s science, engineering, and technology (SET) sector in preparation for a future resurgence of nuclear power, one needs to firstly recognize the somewhat onerous process of development, testing, evaluation, and safety assessment before adoption, as described so elegantly by Hoeffelner (2011) (Figure 3). Figure 1 – Estimated IPCC global surface temperature changes for The entire cycle of materials development, from various models of climate control through curbing CO2 emissions conceptual definition until final introduction in practice, can (IPCC, 2013) in essence be separated into two main focus areas:firstly, the

Figure 2 – Typical structural materials in use in a modern PWR (Zinkle and Was, 2013) ▲ 894 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Where should the national R&D in materials science fit into South Africa’s future nuclear power programme?

‘Should South Africa’s materials scientists and engineers attempt to be technology leaders as in the past, or should we rather aim to be technology followers, but in the process improve on an incremental basis what others have already done, i.e. a ‘small and better’ focus? The answer to this basic question can be sought inter alia in the path taken by Japan after the end of the Second World War, when a shattered country with limited own resources had to ’climb out of its ashes’ by emulating what others had done, but doing it incrementally better. Within a decade or two, Japan had become internationally renowned for its high- quality cameras, binoculars, television sets, and many other electronic and engineering goods. There is a lesson to be learnt here. South Africa should avoid ‘large and new’ high-risk technology programmes and focus rather on the ‘small and better’ technical areas that will incrementally draw South African R&D, together with local industry, into growing participation in the future nuclear

Figure 3 – The process of new materials development, testing and power programme. evaluation, design, and safety assessment in nuclear materials before A second strong argument for ‘small and better’ lies in acceptance and introduction (Hoeffelner, 2011) the inherent safety and performance guarantees that have to be provided by the reactor vendor. Local participation in the supply of key components, particularly those associated upper issues of technology, and secondly, the lower issues of directly with the so-called ‘nuclear island’, will most likely be design and safety assessment.The two focus areas go hand- very limited for many years to come, at least until South in-hand, and South Africa’s endeavours in nuclear Africa’s industrial base has reached a level of sophistication technology over the past three or four decades have taught equal to those of the reactor vendor countries. Does this, some hard lessons of the consequences of focusing primarily therefore, mean that no local R&D resources should be only on the development of the technology, without planning focused on these materials? No, not at all! for the resources to bring the technology into safe, reliable, For purposes of design evaluation, operational and cost-effective commercial fruition, which placed the optimization, and safety evaluation as depicted in the lower entire process at risk of termination. This was a classic half of Figure 3, decision-makers need to have a clear technology push instead of a technology pull approach. understanding of the limits of structural materials and a The demands of the entire development cycle as depicted feeling for the behaviour of these materials under severe in Figure 3 can partly be recognized in the unfortunate operational conditions, which often requires much more than terminations of the uranium enrichment programmes (both ‘literature knowledge’.This route will be called the Vortex and the Molecular Laser Isotope Separation (MLIS) ‘understanding better’. systems) and the development of high-temperature gas- In designing a roadmap for South Africa’s materials R&D cooled reactor technology in the pebble bed modular reactor capacity in the future nuclear power programme, one can (PBMR) programme. In all of these programmes the therefore once more return to the model of Hoeffelner in technology developed by South Africa’s scientists and Figure 3 and identify two main areas that need to be engineers was on an equal level with international norms, addressed: leading even to international participation in the MLIS ➤ Research aimed at the incremental development or program. During a visit in the early 1990s to the Vortex improvement of the upstream technology of materials uranium enrichment Z-plant for low-level enrichment, a aimed at future supply into a growing nuclear power group of international engineers from a leading country in generating capacity, i.e. the ‘small and better’ route the area of enrichment just shook their heads and ➤ Developing an understanding of the positive and commented: ‘we would never have been able to design and negative limits of those materials for design, life build such a plant’. assessment, and safety evaluation purposes, i.e. the Why, then, did all three of these programmes falter in the ‘understanding better’ route. end?The exact reasons are, of course, different in each case, Each of these will be explored in some detail, with but overall all three really faltered due to one common factor: specific examples from the nuclear industry. the lack of sustainability of resources needed to take each one through the entire life-cycle of development shown in Figure 3, and then into full commercial viability. It was as Research focus area: technology of nuclear materials simple as that! South Africa must recognize that it is a Zirconium-based cladding materials relatively small country with very limited resources, and furthermore, new technology always carries a high risk of A very visible illustration of the resources required for a failure even if the R&D is on a par with international norms. ‘large and new’ programme is provided by the development The crucial question regarding South Africa’s future of improved zirconium-based cladding materials for PWR nuclear power programme, is therefore a fundamental one: technology.

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Figure 4 – The development of potential advanced zirconium-based cladding materials (CEA-INSTN, 2008)

Figure 6 – In-reactor oxide thickness measurements of Zircaloy-4 (red and yellow data points) and the new Zr-Nb alloy M5 (bottom black and green data points) (CEA-INSTN, 2008)

Figure 5 – Technical challenges to be addressed in the development of any advanced zirconium-based cladding materials (CEA-INSTN, 2008) Considering all of the above, it is therefore quite clear that for South Africa to embark on such a ‘large and new’ programme of cladding alloy development would be illogical, Firstly, where would one have to focus in selecting a new particularly since a country such as France has seemingly development area? The leading nuclear countries of the world more than 100 engineers and scientists working on such a are committing very substantial human and research programme alone. Does this mean that South Africa should resources to the development of new alloys. withdraw completely from any zirconium-based research? The difficulty in choosing a new alloy towards which The answer is, clearly, ‘no!’ South Africa could make a meaningful contribution is South Africa, together with Australia, supplies most of immediately apparent. The development of any new alloy the world’s zirconium-based minerals, and herein lies a involves a wide range of difficult technological challenges, particular opportunity in the ‘small and better’ route. The which often require a compromise in final properties. There is current three major processes followed by companies in the therefore an inherent risk in any choice of R&D on advanced USA, India, and France all start off with zircon (ZrO2.SiO2), cladding materials. which always contains small amounts of hafnium (Hf) Finally, the new alloy needs to be proven by means of substituted for Zr, and use various refining processes to numerous costly and lengthy in-reactor tests to evaluate its arrive at ZrCl4 followed by reduction to hafnium-free safety performance. zirconium metal in the magnesium-based Kroll process. All ▲ 896 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Where should the national R&D in materials science fit into South Africa’s future nuclear power programme? three processes are batch operations, and all of them have 2013), and is superseded by African countries such as environmental and safety implications. Niger (7.6%), Namibia (7.3%), and even Malawi South Africa has the zirconium-based mineral resources, (1.9%). Because of this unfortunate co-existence of and is recognized internationally for its pyrometallurgical gold and uranium, South Africa had only 5.5% of the process technology. world’s ‘Reasonably Assured Reserves’ (RAR, i.e. Here is a prime example of a ‘small and better’ strategy to recoverable at a cost of less than US$130 per kilogram ‘re-invent’ the whole, or only the upstream steps, of the U), in 2009 (TradeTech, 2010), a decline from 6.5% of current zirconium production routes with better technology. historical production up to 2008 The incremental improvement in the three somewhat ➤ Countries to the north of South Africa, however, do similar production routes for nuclear-grade zirconium produce uranium as a primary product and these certainly falls within the capabilities of South African countries contain some noteworthy reserves. Could scientists and engineers, and can benefit both the country South Africa serve as a regional uranium enrichment and the world’s nuclear industry in the long term. Necsa, centre for Africa? A select working group that was with its internationally competitive capability in fluorine and tasked by the Director-General of the IAEA in Vienna high-temperature plasma technology, is uniquely placed to in 2007, and of which the author was a member, accept such a challenge. defined the boundaries of such regional nuclear fuel centres with one of the main criteria being ‘majority Uranium enrichment multinational control’ from outside the region, most In any discussion of South Africa’s future nuclear likely with the participation of one or more of the five programme, the question of uranium enrichment will permanent members of the UN Security Council.To inevitably arise. Should South Africa once more embark on bring such a possibility into fruition, however, is such a ‘large and new’ venture for its future nuclear fraught with a number of difficult questions, both programme? This question is probably somewhat easier to politically and technically: answer today than some decades ago. This is due to the • Politically, nuclear non-proliferation and uranium following reasons. enrichment will always be very sensitive topics. This ➤ South Africa’s uranium resources are found primarily is not made easier by concerns about the real in gold-bearing ores. When South Africa was one of intentions of Iran and North Korea (both signatories the world’s major gold producers, it simply made sense of the Non-Proliferation Treaty or NPT) and those still to also beneficiate the uranium to so-called outside the NPT, notably India, Pakistan, and Israel. ‘yellowcake’ as a by-product. South Africa has since With memories of the Cold War in the previous lost its place in the ranking of the top few gold century still fresh in people’s minds and the current producers. Furthermore, the fall in the price of instabilities in many parts of the world, it is to be yellowcake has made it quite uneconomical to extract expected that the five permanent members of the UN the uranium from the gold recovery processes. South Security Council, the so-called ‘haves of nuclear Africa currently (2013) accounts for only 0.9% of weapons’ within the NPT, would strongly resist any world uranium production (World Nuclear Association, measures to spread uranium enrichment technology.

Figure 7 – Process flow sheets for the production of nuclear-grade zirconium of (left) Wah Chang in the USA, (middle) NFC of India, and (right) Cesuz of France (CEA-INSTN, 2008)

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• On a regional basis there is, of course the recognized. Yes, South Africa’s SET capacity could, in declaration of Africa as a Nuclear Weapon-free principle, once more go down that road with centrifuge Zone, the so-called ‘Pelindaba Treaty’ originally technology, but it will always be a high-risk strategy established in 1996 and finally ratified by the 28th with the very real possibility of another failure on Member State of the African Union on 15 July 2009. commercial grounds. The low-risk strategy of using the This could be offered as a guarantee of the peaceful currently most competitive uranium enrichment intentions of such a regional uranium enrichment centrifuge technology of URENCO under a licence centre situated in South Africa. Internationally, agreement, as France has done for some years, needs however, it is to be expected that serious questions to be considered. will be raised as to whether such a regional treaty is Considering all of the above, it seems that a possible ‘watertight’ against proliferation. South African re-entry into the area of uranium enrichment • The next stumbling block arises from the question: would be faced with almost insurmountable hurdles, and ‘What happens to the depleted UF6 from inter- would need to be very carefully analysed before it is even African ‘imported’ uranium after enrichment?’ considered. Technically, natural uranium imported into South Africa from any other country would be a tradeable The ‘small and better’ approach for South African commodity, but not the depleted UF6, which now is SET in materials research most likely labelled as ‘hazardous waste’. As a signatory to the Bamako Convention to ‘Control the Recognize development trends in commercial power Ban of Imports into Africa and the Control of reactor trends Transboundary Movement of Hazardous Wastes The development of commercial power reactor technology has within Africa’, which was signed on 30 January progressed a long way towards the ‘Generation IV’ (GEN IV) 1991 in Bamako, Mali and came into force on 10 light water reactor (LWR), with enhanced economy and March 1999, South Africa must abide by its safety, minimal waste generation, and, last but not least, undertaking not to allow the movement of any increased proof against proliferation. South Africa is one of hazardous waste materials across international the participating countries in GEN IV and is, therefore, well borders within Africa. South Africa, in its role of placed to use this association in planning its nuclear power such a regional nuclear fuel centre, would therefore reactor programme for the future. have to remain the host of all of the depleted Note the key targets of better economy, enhanced safety, uranium after the feed material had been converted less waste, and proliferation resistance. In each of these to UF6 and then enriched to low enriched uranium areas, South Africa’s SET capacity can certainly make a (LEU). ‘smaller and better’ contribution. ➤ Finally, should all of the above political questions be somehow resolved, the technical question of ‘interna- Should South Africa focus its nuclear materials tional cooperation’ versus ‘go-it-alone’ in selecting the research on in-core or ex-core components? technology for enrichment, needs to be understood. In considering this question for LWR technology, one Here the lessons from the past should once again be inevitably focuses on UO2 and zirconium-based cladding

Figure 8 – -The Generation IV LWR development path (US Department of Energy, 2002) ▲ 898 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Where should the national R&D in materials science fit into South Africa’s future nuclear power programme? material, but any modifications, even those that are only Some typical incremental advances with a high modest, without ultimate proof of performance under impact in the LWR nuclear industry irradiation conditions, would be almost a futile exercise (Figure 6). Is the commercial transfer of in-core technology The development of advanced steels for pressure from a reactor vendor into South Africa an option? Here vessels and steam piping again, lessons from the past need to be recognized, as in the The steels used for high-temperature steam piping for transfer in the 1980s of Koeberg fuel manufacturing pressure vessels in both conventional coal- and nuclear- technology from France in Necsa’s former BEVA programme. powered generating stations still require better understanding Although the transfer was technically successful (the BEVA in terms of properties such as creep behaviour, corrosion, fuel elements in Koeberg performed on par with those in weldability, and end-of-life assessment. For instance, the France,) the transfer of technology incurred high costs over a weldability of P91 (nominally a 9%Cr-1%Mo-V-Nb steel) in period of about four years. At that point, however, the reactor the Medupi power station required detailed attention to meet vendor had already moved on with its next, more advanced the design requirements, and for application in a nuclear fuel element design, which would have required another power station the welding codes will even be stricter. significant investment for South Africa to ‘stay in the game’, The steady improvements in the service performance of a situation that would recur every few years. The message P91 (Figure 10) shown below is evidence of ’small and here is ‘don’t even consider investing heavily in the local better’ improvements over time, achieved only through manufacture of critical items in the nuclear island unless a dedicated research. significant percentage of South Africa’s electricity generating Innovative materials science in solving a stress capacity will be nuclear-based, thus warranting such an corrosion cracking (SCC) problem through grain investment’. This raises the obvious question of whether South Africa boundary engineering (GBE) really needs a reactor such as SAFARI I at all.The answer to Watanabe (1974) once had an ‘eureka’ moment when he this question is an overwhelming YES. South Africa, as one changed the relationship defined by the well-known Hall- of the top three medical isotope producers in the world, Petch equation, that a reduction in grain size leads to the should not relinquish its position. This was achieved under ‘holy grail’ of higher strength with improved ductility, by difficult conditions and the replacement of the ageing SAFARI asking what would happen if we were to change not the size, I reactor needs to take that into account. Should ‘SAFARI II’ but the nature of the grain boundaries. This opened up many then be only an isotope-producing reactor?This would be a studies towards understanding so-called coincident site very unfortunate retrograde step, as the growing use of lattice (CSL) boundaries and how to increase their percentage SAFARI I for non-nuclear industrial tests such as residual in a mixture of low- and high-angle grain boundaries stresses and texture formation in metals, as well as neutron (LAGBs and HAGBs), twin boundaries (TBs), and then CSL research, constitutes a powerful training and research boundaries. CSL boundaries are high-angle boundaries but instrument for South Africa’s national SET institutions. possess the special characteristics of LAGBs. The percentage In the ‘small and better’ strategy, South Africa’s SET of CSL boundaries can be measured with little difficulty by capacity should, therefore, rather focus on ex-core many modern scanning electron microscopes fitted with an components with the general aim of assisting local industry electron backscatter diffraction (EBSD) capability. This to participate in a meaningful manner in the future nuclear innovation is now being applied to the vexing problem of construction programme, but always focusing on the general intergranular stress corrosion cracking (IGSCC) in the overriding aims of the Generation IV LWR. tubing of steam generators of PWR stations.

Figure 9 – The allowable operating stresses for a design life of at least 300 000 hours for three steels typically used in LWR technology. SA 508 Grade 3 is a low-carbon manganese-molybdenum steel ’optimized for Figure 10 – 100 000-hour creep rupture strength and temperature of the nuclear industry. X20 (2.25Cr-Mo steel) and P91 (X10CrMoVNb9-1) 9–12%Cr steels in general over time with the introduction of P91 steel are both used for high-temperature steam piping (Buckthorpe, 2002) by the late 1980s (Klueh and Harries, 2001)

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Figure 11 – A schematic cutaway showing the internals of a PWR steam generator (Staehle, 2007) and general corrosion problems experienced in LWR systems with the IGSCC Alloy 600 used in the U-tubes in the PWR steam generators (top of the figure) since the inception of commercial PWR technology (Palumbo, 1993)

Figure 12 – Alloy 600 (Ni –15.74Cr – 9.1Fe) sensitized for 1 hour at 600°C followed by 120 hours of corrosion testing according to ASTM G28 with (i) its conventional microstructure and (ii) after grain boundary engineering (Lin et al., 1995)

In a number of groundbreaking patents and publications the improvements in creep strength were achieved with a (Palumbo, 1993; Aust, Erb, and Palumbo,1994; Palumbo, relatively modest increase in CSL boundary density, with no Lehockey, and Lin, 1998; Was, Thaveeprungsriporn, and further improvements at higher CSL densities. Crawford, 1998), Palumbo and others have found the means to increase the density of CSL boundaries in Alloy 600 (a Ni- Shifting the focus from ‘technology’ to ’technology 16Cr-9Fe alloy) through iterative strain-annealing or iterative plus design and safety’ strain-recrystallization, typically from less than 10% CSL boundaries in the unprocessed alloy to as high as almost Up till now, the focus was very much on the technology of 50% in the iteratively strain-annealed or strain-recrystallized nuclear materials, but as Hoeffelner (2011) has shown form. Note the very clear micrographic evidence of less grain (Figure 3), this is barely half the story necessary for a boundary penetration from the surface for a grain-boundary- resurgence of a national nuclear power programme. The other engineered Alloy 600 that has been subjected to laboratory- half will require a significant body of high-level SET capacity simulated stress corrosion cracking. with better understanding to technically evaluate offers from Note that in the coarse-grained microstructure in Figure vendors, ask the right questions, retrieve the required design 14, the increase in the CSL boundary density from 20% to and performance data, and critically compare differences in 34% was sufficient to dramatically lower the alloy’s creep rate design approach from the point of view of: at 360°C, while in a fine-grained Alloy 600 the improvement ➤ Expected performance in creep strength appears to be even greater. In both cases, ➤ Life assessment ▲ 900 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Where should the national R&D in materials science fit into South Africa’s future nuclear power programme?

➤ The demonstrated safety of the system being offered postgraduate engineer or scientist who has gained experience under all possible external or internal scenarios in one aspect of the arc welding of zirconium-based cladding before an operating licensing by the NNR (National material for a Masters or a PhD degree will have absorbed the Nuclear Regulator) can even be considered. In the short term intricate nature of this alloy far more than in a few months of this can be achieved by hiring in nuclear consultants reading in a library or attending postgraduate courses, (preferably with no national affiliation with the vendor- without going through the demanding process of an in-depth country), but this cannot be sustained in the long term for literature review, research planning and execution, reporting reasons of cost. This approach would also represent a lost of results, and finally modelling these results in an opportunity for developing that high-level manpower informative discussion; with all of this being critically required for firstly the pre-operational evaluation phase of reviewed at the end by one or more external examiners from each offer, then the construction and operational stage of a the nuclear industry. In addition, international publications number of power stations, and finally waste management that are peer-reviewed by experts in the field add to and the eventual decommissioning of the power stations. demonstrating ‘better understanding’. Building up such a body of high-level manpower may However, a basic change will, have to occur in the appear to be a daunting task, but it is no accident that definition of which materials are nuclear and which are not, Hoeffelner (Figure 3) places the technology focus at the point as Figure 2 has shown. Nuclear materials in a resurgent of entry for design and safety assessment. For instance, a nuclear power programme are not simply limited to zirconium, uranium, and a very few others while the rest are seen as ‘conventional’ and therefore technically outside the current narrow definition of ‘nuclear materials’. A steam generator on a PWR can never be viewed as simply another type of heat exchanger, and thereby not warranting the level of regulatory supervision that a nuclear reactor pressure vessel does (and even the latter material is traditionally not viewed, in South Africa at least, as a ‘nuclear material’). For reasons of public safety and acceptance/assurance, the process of obtaining an ASME Section III N-certificate on equipment used in a nuclear reactor are far more onerous than if the same equipment is used in a conventional power station. This places most of the materials listed in Figure 2 in an entirely new class, that should be dealt with appropriately. Some typical areas in the category of ‘better understanding’ of an advanced PWR to consider in research could include: ➤ Projects that entail the welding of various components, such as fuel cladding, pressure vessel steel, steam piping, steam generator items, etc. Figure 13 – Dependence of total grain boundary cracked fraction on ➤ Projects that entail the strength/ductility/creep/fracture CSL boundary fraction in Alloy 600 (Ni-16Cr-9Fe) (with varying amounts of carbon in solution and for the case of grain boundary carbides) for relationships of the above materials at room strains of 15% and 20% with testing in 360°C PWR primary circuit water temperature as well as at typical steam operating at a strain rate of 3 ×10-7 per second (Alexandreanu, Capell, B., and conditions Was, 2001) ➤ Corrosion properties, including IGSS, pitting corrosion,

Figure 14 – Constant load creep data on Alloy 600 (Ni-16Cr-9Fe) of a (i) coarse-grained and (ii) a fine-grained microstructure in solution annealed and grain boundary engineered conditions. The creep tests were carried out under argon at 360°C with creep stresses of 300 and 450 MPa respectively. In (iii), the dependence of the steady-state creep rate and the percentage of cracked boundaries on the fraction of CSL boundaries on the coarse-grained material is shown (Was, Thaveeprungsriporn, and Crawford, 1998)

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hydriding of zirconium-based cladding, iodine SCC of IPCC. 2013. Climate Change 2013. The Physical Science Basis. IPCC Working zirconium alloys, SCC of steam piping and steam Group 1 Contribution to the Fifth Assessment Report of the generator components, etc. Intergovernmental Panel on Climatic Change. Stocker, T.F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Conclusions Bex, V., and Midgley, P.M. (eds).AR5, WGI 5th Assessment report. Cambridge University Press, Cambridge, UK and New York, NY, USA, The following aspects need to be incorporated into a national 1535 pp. manpower training programme that should be well under way long before the first reactor vendor’s technical offer is received. KLUEH, R.L., and HARRIES, D.R. 2001. High chromium ferritic and martensitic ➤ Set up a Governmental Advisory Board consisting of steels for nuclear applications. Monograph 3. ASTM International, West the main role-players in an expanded nuclear power Conshohocken, PA. programme. These would include Eskom, the NNR, Necsa, senior representatives from industry, public LIN, P., PALUMBO, G,. ERB, U., and AUST, K.T. 1995. Influence of grain boundary representatives, and the like. The constitution of such a character distribution on sensitization and intergranular corrosion of Alloy body would, however, have to be very carefully drafted 600. Scripta Metallurgica et Materialia, , vol. 33. p. 1387. to totally exclude items not associated purely with training of high-level manpower, as any other issue raised in such a forum could compromise the PALUMBO G. 1993. Thermomechanical processing of metallic materials. Int. independence of the NNR in ruling on safety issues Patent Application PCT/CA1993/000556. ➤ Establish initially at least one (and later possibly a second or third) research centre at a South African PALUMBO, G., LEHOCKEY, E.M., and LIN, P. 1998. Journal of Metals, February. university that will undertake the postgraduate training p. 40. and development necessary for scientists, engineers, and technologists required by the programme. This needs to be done in close association with Necsa PWR STEAM GENERATORS ➤ Ensure that the local R&D effort eschews end-use https://www.google.co.za/search?biw=1366&bih=673&noj=1&site=webh nuclear materials, but rather focuses on upstream p&source=hp&q=pwr+steam+generators&oq=PWR+Steam+Generators&gs processes in a ‘small and better’ focus _l=hp.1.0.0i30.8889.22694.0.25409.20.19.0.1.1.0.642.4064.2-13j5- ➤ Expand Necsa’s mandate to include research and 1.14.0.msedr...0...1c.1.62.hp..5.15.4080.OZJadVVd4AE [Accessed 6 development on materials covered by the broader March 2015]. definition of nuclear material.The Advanced Metals

Initiative of DST can play a deciding role in overseeing STAEHLE, R.W. 2007. Anatomy of proactivity. International Symposium on healthy cooperation between the Ferrous Metals Research for Aging Management of LWR and its Future Trends. 15th Development Network managed on behalf of the AMI anniversary of the Institute of Nuclear Safety Systems Inc., Fukui City, by Mintek and the Nuclear Metals Development Japan, 22–23 October 2007. Network managed on behalf of the AMI by Necsa.

Acknowledgments TRADETECH. Not dated. World Uranium Production and Requirements. http://www.uranium.info/world_uranium_production_and_requirements.p The views expressed in this presentation are the author’s hp [Accessed 6 March 2015]. own and do not necessarily reflect official policy of the University of Pretoria or of the South African Government. The author would also like thank many colleagues at Necsa US DEPARTMENTOF ENERGY. 2002. A Technology Roadmap for Generation IV and the University of Pretoria for valuable input and Nuclear Energy Systems. comments. This contribution is published with the permission http://nuclear.inel.gov/gen4/docs/gen_iv_roadmap.pdf [Accessed 6 March of the University of Pretoria. 2015].

References WAS, G.S., Thaveeprungsriporn, V., and Crawford, D.C. 1998. Grain boundary ALEXANDREANU, B., CAPELL B., and WAS, G.S. 2001. Combined effect of special misorientation effects on creep and cracking in Ni-based alloys. Journal of grain boundaries and grain boundary carbides on IGSCC of Ni-16Cr-9Fe- Metals, February, pp. 44–49. xC alloys. Materials Science and Engineering, vol. A300, p94–104.

AUST, K.T., ERB, U. and PALUMBO, G. 1994. Interface control for resistance to WATANABE, T. 1974. An approach to grain boundary design for strong ductile intergranular cracking. Materials Science and Engineering.A, vol. A176, polycrystals. Res Mechanica, vol.11. p. 47. p.329.

BUCKTHORPE, D. 2002. https://odin.jrc.ec.europa.eu/htr-tn/HTR-Eurocourse- 2002/Buckthorpe_582.pdf [Accessed 6 March 2015] WORLD NUCLEAR ASSOCIATION. Not dated. World Uranium Mining.http://www.world-nuclear.org/info/inf23.html [Accessed 8 March CEA-INSTN. 2008. Zr in the Nuclear Industry. CEA-INSTN Seminar, April 2015]. 2008.

HOEFFELNER, W. 2011. Materials databases and knowledge management for advanced nuclear technologies. Journal of Pressure Vessel Technology, vol. ZINKLE S.J. and WAS G.S. 2013. Materials challenges in nuclear energy. Acta 133, no. 1. pp. 014505 1-4. doi:10.1115/1.4002262. Materialia, vol. 61. pp. 735–758. ◆ ▲ 902 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a2 Friction processing as an alternative joining technology for the nuclear industry by D.G. Hattingh*‡, L. von Wielligh*, W. Thomas† and M.N. James‡

with many fossil-fuel fired plants reaching end-of-life, there is renewed interest in Synopsis installing additional nuclear capacity. The process of joining materials by friction is based on generating the heat Proposing friction processing as an alternative necessary to create a solid-state mechanical bond between two faying (and relatively untried) joining technology for surfaces to be joined. In simple terms, the components to be joined are the nuclear industry might be viewed as subjected to frictional heating between rubbing surfaces, causing an potentially perilous because of stringent increase in interface temperature and leading to localized softening of requirements for validation of weld and repair interface material, creating what is described as a ’third body’ plasticized layer. This plasticized zone reduces the energy input rate from frictional procedures. The intention in this paper is to heating and hence prevents macroscopic melting. The plasticized layer can introduce some modern developments in the no longer transmit sufficient stress as it effectively behaves as a lubricant friction processing arena and to outline the (Boldyrev and Voinov, 1980; Godet, 1984; Singer, 1998; Suery, Blandin, potential that these processes hold for and Dendievel, 1994). The potential for this solid-state frictional joining manufacturing of new components and for process to create high-performance joints between, for example, dissimilar maintenance and life extension of ageing materials with limited detrimental metallurgical impact, and reduced nuclear power plant. defect population and residual stress level, has had a very significant The World Nuclear Association (WNA, impact on fabrication and repair in industrial sectors such as transport. 2015) states that as at April 2015, there were This paper presents a brief overview of the advances made within the 437 nuclear reactors in operation, with another family of friction processing technologies that could potentially be exploited in the nuclear industry as alternative joining and repair 65 under construction and a further 481 either techniques to fusion welding. in planning or proposed. Clearly, there is Modern friction processing technologies can be placed into two main significant potential for the application of categories: those that make use of a consumable tool to achieve the friction processing techniques in this power intended repair or joint (friction stud and friction hydro-pillar processing) sector. The age of reactors varies, from first- and those making use of a non-consumable tool (friction stir welding). generation reactors developed in the 1950s to The most mature friction joining technology is friction rotary welding, the current generation III reactors introduced where a joint is formed between original parent materials only. A new in 1996 in Japan, with generation IV reactors addition in this category is linear friction welding, which opens the still under development with a proposed potential for joining complex near-net-shape geometries by friction introduction date of 2020. The main reactor heating. The continuous innovation in friction processing over the last 25 types currently operational are pressurized years has led to the development of a number of unique processes and applications, highlighting the adaptability of friction processes for water reactors (PWRs) – 273 units, boiling- specialized applications for high-value engineering components. water reactors (BWRs) – 81 units, pressurized heavy-water reactors (CANDU-PHWR) – 48 Keywords units, gas-cooled reactors (AGR and Magnox) friction processing, joining, leak sealing, weld repair technology. – 15 units, light-water graphite reactors (RBMK and EGP) –15 units, and fast neutron breeder reactors (FBR) – 2 units (WNA, 2015). The majority of reactors currently in Introduction operation are relatively elderly in terms of their Assessing the potential for using alternative design life and would require decommissioning joining and repair techniques for specific applications in a given industrial sector is informed by an appreciation of the current size and potential growth in that sector. Power generation by nuclear fission has been out of favour over the past few years, due to several * Nelson Mandela Metropolitan University. high-profile nuclear incidents, e.g. the † Co-Tropic Ltd., TWI. Fukushima Daiichi nuclear disaster, as well as ‡ University of Plymouth. concerns over disposal of nuclear waste and © The Southern African Institute of Mining and spent fuel rods. However, due to rapid Metallurgy, 2015. ISSN 2225-6253. Paper received increases in the demand for electricity, coupled Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 903 ▲ Friction processing as an alternative joining technology for the nuclear industry in the short term if life extension methodologies are not generation) point onwards. Steam generated by controlled adopted. The high capital cost and extensive lead time nuclear fission is used to drive turbines, which are intercon- involved in bringing new nuclear plants online make the nected with electrical generators. A number of the main whole idea of accurately determining the remaining life of materials used in the construction of nuclear power plants, existing plants very topical. including Zircaloy, can be joined or repaired by friction-based The graphs in Figure 1 and Figure 2 were populated from processes. The joining of thin-wall sections intended for use data published by the WNA (2015) in 2015 and give an in fuel rod application by rotary friction processes is currently indication of the current operational plants, plants under being studied at the Nelson Mandela Metropolitan University construction, and proposed plants. Considering Figure 1, (NMMU) Work is also being done on Cr-Mo-V alloy steels which shows the percentage contribution from nuclear and various grades of stainless steel. fission, over the decade 2003 to 2013, to the electric power needs of both the ‘BRICS’ nations and of developed countries, Friction processing technologies it is clear that most of these countries made very little capital One of the simplest ways to warm up your hands is to rub investment in additional nuclear generation capacity, with them together. This action generates heat in proportion to the most countries showing a moderate decline in nuclear contri- rapidity of the rubbing motion, the pressure applied on the bution. With the exception of France, and to a lesser extent rubbing surfaces, and the duration. Similar concepts are used Brazil, most countries currently obtain less than 10% of their to provide the primary thermal energy source in friction electricity needs from nuclear. The future for nuclear power processing (FP), allowing lower temperature solid-state joints looks much more promising when one considers the potential and repair processes to be applied to a wide variety of additional capacity from the current build programme, and materials (particularly the magnesium and aluminium light the planned and proposed additions to the nuclear fleet over alloys) used in manufacturing industry (Thomas and the next 20 years (Figure 2). Duncan, 2010). In summary, friction processing comprises a Therefore, taking an overview of the installed, planned, set of solid-state joining techniques that are carried out by and proposed future nuclear capacity, and factoring in generating heat at a contact interface. This softens the developments in respect of the safer, more standardized and material and allows plastic flow to occur which, in cost-effective generation IV reactors, there is considerable combination with an applied pressure, can be used to create opportunity to plan for the implementation of alternative strong, low-defect, and low tensile residual stress bonds joining and repair technologies in the nuclear industry. (Maalekian , 2007; Gibson, 1997; ASM International, 1993; The basic principles of generating electricity from nuclear Serva-Tech Systems, n.d. (a); Thomas and Dolby, 2001). fission are very similar to those used in fossil-fired thermal There are various types of friction processes, as power plants, if viewed from the secondary circuit (steam illustrated in Figure 3. The main friction processing joining technologies considered in this paper are rotary friction welding (RFW), friction stir welding (FSW), friction hydro-pillar processing (FHPP, which can employ either tapered or parallel sided configurations), and linear friction welding (LFW). LFW is a

Figure 1 – Nuclear power share for the period 2003 to 2013 (WNA, 2015)

Figure 2 – Nuclear power generation landscape expressed in MWe Figure 3 – Friction process technologies (TWI) (Thomas and Dolby, (WNA, 2015) 2002) ▲ 904 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Friction processing as an alternative joining technology for the nuclear industry relatively new and versatile process that permits the joining drive welding being done with a constant rotational speed of irregular shapes, allowing for near-net-shape fabrication. that may be varied at different stages of the weld cycle, while In this range of friction technologies, there are autogenous during inertia welding the process begins at a relatively high processes that form a joint by using parent material only rotation speed, which gradually reduces to zero (Thomas and (RFW and LFW), i.e. without the addition of any filler metal, Duncan, 2010). processes that make use of a consumable tool (FHPP), and Friction hydro-pillar processing (FHPP) processes that use non-consumable tools (FSW). There are four basic classifications for relative motion in Like most of the modern friction technologies, FHPP (Figure 6) friction welding (BSI Group, 2000; Serva-Tech Systems, n.d. was invented and patented by TWI (World Centre for Materials (b)), namely: rotary, angular oscillation, linear reciprocation, Joining Technology) in the UK in the early 1990s. A and orbital motion. Figure 4 illustrates these various description of the process, as given by Thomas and Nicholas motions. (1992), states that friction hydro-pillar bonding is achieved by Fabrication by frictional heating is a well-established rotating a consumable tool coaxially in a hole while under load. technology. The first use of friction knowledge to process and The frictional heat generated results in the production of a shape materials dates back to 1891, when Bevington (1891) pillar of continuous plasticized layers until the hole is filled. used heat generated by friction to form and join tubes. The plasticized material consists of a series of shear layers or interfaces which solidifies under the influence of the applied Rotary friction welding (RFW) force. One of the main original observations was that the RFW is a well-established ‘workhorse’ friction joining plasticized material advances more rapidly than the axial feed technology that has been widely used for over a century in rate of the consumable tool, which results in the rising of the numerous applications ranging across the automotive, frictional interface along the consumable tool to form the construction, aerospace, and medical sectors. Two alternative dynamically recrystallized deposit material. During the process RFW techniques, known as continuous drive and inertia a good degree of plasticization must be maintained at the friction welding, are currently in use; the latter is the most interface, allowing hydrostatic forces to be transmitted axially widely adopted process. In both techniques joints are made and radially to the inside of the hole in order to achieve a good by placing a rotating component in contact with a stationary metallurgical bond (Thomas and Nicholas, 1992). component while applying a load perpendicularly to the A large body of reported work from various laboratories contact interface. Once the interface reaches the appropriate has shown that good mechanical integrity can be achieved in welding temperature to plastically displace and fuse the FHPP welds, with metallographic examination indicating a materials by forging, rotation is stopped and the weld is fine-grained microstructure in the welded zone (TWI Bulletin, allowed to solidify while maintaining the forging force 1997). The original geometry proposed for the FHPP process (Thomas and Duncan, 2010; Thomas, Nicholas, and Kallee, was based on a horizontal cylindrical arrangement, with 2001) as illustrated in Figure 5. The differences between the specific clearance between the tool and the sides of the hole. two methods are mainly in process control, with continuous However, in more recent applications tapered holes and consumables are becoming more common, in which the angle of the two tapers is slightly different (Thomas and Temple- Smith, 1996; Bulbring et al., 2012; Meyer, 2001). Wedderburn (2013) reported that the tapered arrangement allows for an additional reactive force to develop horizontally from the sidewall in addition to the vertical hydrodynamic force, which can be utilized for heat generation when making the joint, and that this is beneficial for materials with poor extrusion properties.

Figure 4 – Relative motion classification in friction welding (Wedderburn, 2013)

Figure 5 – Schematic of the basic rotary friction welding process Figure 6 – Schematic representation of the friction hydro-pillar process (Pinheiro, 2008) (Thomas, 1997)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 905 ▲ Friction processing as an alternative joining technology for the nuclear industry

A number of research institutes have claimed that good capability of welding dissimilar metals as well as joining quality FHPP welds can be made in steel and certain non- certain metals that are difficult to weld by fusion processes, ferrous materials using a parallel hole geometry. In this e.g. 2xxx series aluminium alloys. The process can best be context a ‘good quality’ weld is characterized by high impact, described as a continuous ‘hot-shear’ process involving a tensile, and bend properties (TWI Bulletin, 1997). This non-consumable tool that is responsible for mixing and sentiment is supported by Wedderburn (2013) Bullbring, et forging the metals across the joint line (Delany et al., 2005). al., (2012) who reported that good mechanical properties Selection of the tool geometry and material are important were achieved with a taper configuration on most thick-wall considerations in achieving a sound joint. The most steam pipe materials evaluated. Where welds were made important aspects of tool design are the pin and shoulder using parallel tool and holes, periodic changes in the geometry, as these are responsible for forming the joint. microstructure were observed, where regions within the The basic principle of the FSW process involves plunging deposit material were not fully transformed. This effect was a rotating tool between abutting faces of the work piece and more noticeable when a high tool rotation speed and then traversing it along the joint line (Figure 7). Rotary consumable displacement (upset) rate were used. motion of the tool generates frictional heat at the contact Microstructural variations were also exacerbated when the surfaces, softening the material and creating a plasticised hole depth to hole diameter exceeded a ratio of about 3.5:1 zone around the tool pin and below the shoulder. (Meyer, 2003). Apart from generating heat, the shoulder also contains There is limited published work on the influence of the plasticized material in the joint zone, which provides a process parameters on FHPP welds. From the work by Meyer forging action that assists with the formation of a defect-free (2003), Wedderburn (2013), and Bulbring et al., (2012) on solid-state joint. In the nuclear industry, FSW has been the use of FHPP for high-strength low-alloy steel, it is applied to the manufacture of copper canisters for encapsu- apparent that hole geometry is more critical than tool shape. lating nuclear waste (Andersson and Andrews, 1999) as well Meyer (2003) also noted that the influence on heat as for leak sealing or material reprocessing techniques used generation and bond quality is similar to that observed in in the repair of surface-breaking or near-surface defects (Von conventional friction welding, although the rotational speed Wielligh, 2012). required for a good FHPP weld is significantly lower than in Linear friction welding (LFW) conventional friction welding. The forging force, which has a major influence on weld properties in conventional friction (LFW is a joining technology that is ideal for welding non- welding, appears to have little or no influence on most FHPP symmetrical components. This solid-state joining process welds and its effect is limited to the upper area near the generates frictional heat through the linear forced interaction surface region (Wedderburn, 2013; Bullbring, et al., 2012; between a stationary surface and a reciprocating surface Meyer, 2003). (Nentwig, 1995; Nicholas, 2003). The process requires a large force applied perpendicular to the weld interface and, as Friction stir welding (FSW) there is no containment of the hot plasticized material, a FSW must be considered as one of the leading innovations in continuous layer of flash is expelled during the rapid linear solid-state joining in the last 50 years. Invented and patented movement. Since this softened ’third-body’ material is not by TWI (1991), FSW is a joining technique that allows both contained, surface oxides and other impurities are ejected low and high melting point metals, including aluminium, together with the plasticized ‘flash’ (Bhamji et al., 2011). lead, magnesium, steel, titanium, zirconium, and copper, to Unfortunately, LFW requires a major capital investment be continuously welded with a non-consumable tool due to the complexity and size of the welding platforms. The (Nicholas, 1998; Reynolds, Seidel, and Simonsen, 1999; high process force must be contained within the platform Dawes, 2000). The wear rate and cost of refractory tooling is a limiting factor in welding steel alloys. One of the key benefits of this solid-state friction welding technique is the

Figure 7 – Principle of friction stir welding (Thomas, Nicholas, and Figure 8 – Schematic diagram of the linear friction welding process Kallee, 2001) (Bhamji et al., 2011) ▲ 906 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Friction processing as an alternative joining technology for the nuclear industry structure, while achieving the required level of displacement also considered the use of FSW. The main consideration in and process control necessary to ensure joint integrity and selecting FSW as a potential alternative process was the geometrical accuracy means that tight specifications are set solid-state nature of the joining process, together with the on all system components and control algorithms. Hence LFW ability to make high-quality welds with a fully automated is considered feasible for the production of high-value platform and standardized tool technology. engineering components, e.g. joining of aero-engine The investigation of FSW as an alternative joining compressor blades to compressor disks (Wanjara and Jahazi, process started by considering the welding of 10 mm thick 2005). LFW applications are predicated on the suitability of copper plate strips about 0.5 m in length. This study was the process for joining materials with good high-temperature used to find a suitable tool material as well as evaluate properties, low thermal conductivity, and acceptable different tool geometries. The only reference study at the time compressive yield and shear strength properties (Wanjara involved FSW of thick-section aluminium plate with melting and Jahazi, 2005). Low thermal conductivity helps confine temperatures (659°C) well below that of copper (1083°C). heat to the interface region, while appropriate high- However, copper tends to soften at lower a temperature, temperature properties (high yield strength and melting which allows the use of FSW on thick copper sections point) allow a high level of frictional heat generation to occur (Swedish Nuclear Fuel and Waste Management Co., 2001). before plastic collapse. Materials with good high-temperature This study led to the specification of process forces and mechanical properties and low thermal conductivities, e.g. power requirements for welding 50 mm copper plate. This titanium, zirconium, and nickel alloys, are particularly was done by progressively increasing the plate thickness to suitable for LFW applications. 50 mm while checking that the weld microstructure remained acceptable for the identified application. The weld zone Applications of friction technologies in the nuclear showed a fine equiaxed grain structure in the weld nugget industry region. Based on the success of these initial trials, SKB proceeded with the development of a FSW platform to weld A number of studies have shown that friction welding can be lids to short (2 m) sections of 50 mm thick tubes. The tube used to make sound joints and cost-effective repairs in the was placed in a vertical position with the welding head and nuclear industry. These include the work done in a Swedish tool in a horizontal position while the welding head rotated programme that evaluated FSW for encapsulating nuclear fuel around the tube. For each weld, specific weld parameters waste (Andersson and Andrews, 1999), while TWI were recorded, providing a complete record of all the essential demonstrated that thermocouple probes can be fitted to boiler variables and their control during the welding process. These header domes forged from 316 austenitic stainless steel for results, in conjunction with non-destructive testing, form an the Hartlepool and Heysham nuclear power stations (TWI important resource for interpreting weld parameters and data. Global, 2014). Hy-Ten, an independent rebar and accessory Once adequate circumferential friction stir-welded joints supplier, has gained approval for supplying friction welded were achieved, attention was focused on eliminating the exit rebar couplers to be used in concrete construction by the hole left at the end of the weld run. This project has clearly nuclear industry (Maguire, 2012). Locally, the NMMU has demonstrated the feasibility of fabricating capsules by FSW to developed two technologies with potential applications in the enclose nuclear waste for long-term storage. nuclear industry; one relating to leak sealing by FSW that employs a low-force partial-penetration methodology, and the Leak sealing by FSW other the registered Weldcore® process, which employs FHPP Nuclear facilities situated close to the ocean could experience principles to core and repair sites used for creep assessment material degradation, especially of stainless steel vessels and as part of remaining life evaluation. pipework, arising from environmentally-induced stress Nuclear waste storage encapsulation using FSW corrosion cracking (SCC). This prompted an investigation into identifying a suitable refurbishment technique that would This project formed part of a Swedish programme that provide a cost-effective and permanent leak-sealing/SCC evaluated new ways of encapsulating nuclear fuel waste repair technique. Implementation of the repair process was before storing it underground. The capsules were required to have no influence on the operation of the plant, manufactured from copper and consisted of a copper cylinder and also to fulfil the stringent nuclear safety regulations. The with a base and lid (Andersson and Andrews, 1999). The NMMU was therefore requested to investigate the feasibility capsule demand was estimated at 200 per year. Important of using FSW as a potential leak-sealing technique for 304L considerations in fabrication included inspection of the welds stainless steel. This study focused on establishing a and guarantees on achieving defect-free joints that would technology procedure for the identified application, which prevent water entering the canister during long-term storage entailed reviewing the process, tool material, and tool underground (Swedish Nuclear Fuel and Waste Management geometry designs to develop a solution suitable for high- Co. (SKB), 2001). Typical canister dimensions were diameter temperature FSW of unsupported, partial penetration leak 1050 mm, length 4830 mm, and a total weight of 27 t. To sealing of 304L stainless steel with water backing. satisfy the design requirements for environmental and The approach taken in the investigation was initially to chemical resistance, a copper alloy equivalent to EN 133/63 establish a theoretical processing parameter window from with a 50 mm wall thickness was selected for the canisters. published literature. Subsequently, a suitable tool alloy and To improve creep resistance of the copper, 50 ppm of tool profile were identified before preliminary experimental phosphorus was specified in the alloy (Swedish Nuclear Fuel work led to a low-force processing window for 304L stainless and Waste Management Co. (2001). The project developed an steel. Various experiments were carried out to determine the electron-beam welding solution for sealing the capsules and effect of process parameters on weld quality and the

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 907 ▲ Friction processing as an alternative joining technology for the nuclear industry processing forces required to achieve high quality. The stainless steel that minimized plate deflection during welding governing factor was found to be the downwards forging in an unsupported condition. The investigation led to the force required to achieve a fully consolidated, partial development of a new dual-control tool plunge strategy, in penetration weld that would seal a leak in stainless steel which the operator controls the maximum downwards Z-force sheet resulting from, for example, SCC. Finally, a complete during the plunge stage while simultaneously controlling the weld procedure specification was proposed and evaluated by plunge rate and depth (see Figure 9). Plunge rate is, however, welding on a mock-up of the real application with water also a function of the Z-force feedback, which could be seen seeping through the simulated crack during the welding as a significant advantage because the maximum plate process. In principle, this is a similar concept to FSW under deflection at the start of the weld can therefore be limited water, which has been reported by Ambroziak and Gul (Von Wielligh, 2012). (2007). The minimum Z-force required for repeatable weld Such applications require refractory tool materials, and consolidation using tools with a 14 mm, 12 mm, or 10 mm W-25wt%Re was selected for the leak-sealing application as diameter shoulder was found to be 14.25 kN, 10.5 kN, and it provides good resistance under conditions of uneven and 9 kN respectively (Von Wielligh, 2012). This investi- surfaces, high vibratory loads, and plate deflection (Von gation showed that water cooling and water seepage through Wielligh, 2012). Additional benefits include the ability to re- the simulated SCC had only a minor effect on weld quality profile the tool using standard machining processes, hence and that the dominant factor influencing weld quality was increasing tool life. Process development focused on plate deflection. A full preliminary welding procedure specifi- establishing a low-force processing window for the 304L cation was developed for both the 14 mm and 12 mm shoulder diameter tools, but only the procedure for the 14 mm tool was deemed robust enough for industrial application. Un-etched images of typical weld cross-sections obtained during the repair trials of SCC in stainless steel are shown in Figure 10. This figure shows the defect population observed at the start, middle, and end positions in two welds, A and B, made using different weld parameters while water was seeping through the SCC, using two different sets of process parameters. On the etched specimen in the macrograph in Figure 11, the lighter etching material clearly shows a well-defined flow pattern in the weld cross-section. This material was found to be tungsten, a by-product of tool wear that has diffused into the weld. The darker line extending beyond the flaw intended to simulate a stress corrosion crack at the bottom of weld Figure 9 – Difference in the Z-force feedback for different control cross-section is termed a ‘lazy S’ defect in FSW and is not strategies (Von Wielligh, 2012) part of the simulated crack. It is generally ascribed to the

Figure 10 – The defect population found in weld cross-sections along the weld length representing the start, middle, and end of welds for two different welding parameter sets (Von Wielligh, 2012) ▲ 908 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Friction processing as an alternative joining technology for the nuclear industry presence of oxidation and corrosion by-products of the wire Figure 13 shows an example of such a weld repair with cutting process, and serves as a good indication of how five overlapping welds, covering an area of 120 mm × 20 actual corrosion products are likely to be distributed across mm. In this figure, all the loosely attached flash has been the weld. removed. Vickers microhardness tests did not show any significant This investigation has shown that leak sealing of stress- change in the hardness of the weld area or heat-affected zone corrosion cracks in an unsupported tank surface is possible compared with the parent material. This is to be expected, as by using overlapping, partial penetration, friction stir austenitic stainless steels are not hardenable by heat welding. The investigation also highlighted the fact that due treatment (Von Wielligh, 2012). to material thinning during the FSW process, the total area The feasibility of applying this FSW process to crack than can be repaired has to be limited, based on allowable repair in industry was tested using a mock-up of the plate thinning. A reduction in the original plate thickness industrial application of a tank containing a stress-corrosion occurs with each successive overlapping weld. The decrease crack. A simulated crack was successfully welded in an in plate thickness is likely to be a function of plate thickness; unsupported 304L stainless steel plate surface in the in other words, dependent on constraint. Furthermore, FSW annealed condition with water backing up to a depth of tool exit holes can be partially eliminated by using tools with 2 mm. a retractable pin. In summary, FSW is a potential repair The weld repair process required making a number of technique for SCC, although for unsupported welding careful overlapping friction stir welds, with a step distance between consideration must be given to the influence of plate them equal to the pin diameter. The step direction was thickness, surface condition of the area to be processed and towards the retreating side of the weld, because flash its size, and deflection during welding, as well as the formation arising from plate deflection made it impractical to permanent deformation (thinning) induced by the process. step towards the advancing side. FSW is also likely to alter the metallurgical and mechanical The typical weld repair pattern is shown in Figure 13. The properties in the processed area, and this aspect also needs consideration. Associated friction processes such as friction first weld, W1, is carried out from left to right with a surface dressing or friction cladding could also be considered clockwise spindle rotation. The retreating side of the weld is for such repair procedures if it is accepted that the main thus located at the bottom. The second weld, W2, is carried purpose of the process is not to recover structural integrity, out from right to left with an anticlockwise spindle rotation but rather top-seal seepage of the liquid inside the tank such that the retreating side remains at the bottom of the through stress corrosion cracks. Friction cladding is an weld. This process of alternating welds is continued until the additive process, which has the potential to reduce plate cracked area has been covered. deflection and permanent deformation imposed by the friction process. Figure 14 shows the heat generation in the tool while traversing along a repair zone with water leaking through the simulated crack. The installation of the developed platform is shown in Figure 15.

Figure 11 – Macrograph of a typical friction stir processed SCC, etched with Marbles reagent [30]

Figure 13 – The visual appearance of a friction stir processed tank surface area with the loosely connected flash removed on the advancing side of the weld (Von Wielligh, 2012)

Figure 12 – The welding pattern of five consecutive welds used to friction stir process an area of 120 × 20 mm (Von Wielligh, 2012). (Welds W1, W3, and W5 were welded from left to right and welds W2 Figure 14 – FSW in process as water leaks through the simulated crack and W4 from right to left) (Von Wielligh, 2012)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 909 ▲ Friction processing as an alternative joining technology for the nuclear industry

Figure 15 – Set-up for the repair process feasibility investigation, showing the FSW platform attached to a curved stainless steel water-filled test coupon (Von Wielligh, 2012)

Weldcore® sample removal and repair technology metallurgical sample and hole preparation for repair in four Ongoing assessment of the remaining life of high- steps, as illustrated in Figure 16. The hole is then repaired temperature and -pressure (HTP) components in nuclear using friction taper hydro-pillar processing (FTHPP) as power stations is of paramount importance in ensuring their illustrated in Figure 17. safe and cost-effective operation. Although failure can have Current Weldcore® applications are primarily in a coring severe consequences, economic considerations are requiring and repair procedure for local metallurgical sample removal operators to extend the operating lives of plants currently in sites. The advantages of the process are that the metallurgical operation. To ensure safe operation, more direct experimental core can extend much closer to the centre of a thick-walled assessments have to be made of remaining life to pressure pipe, and that the coring and repair can be complement and underpin prediction models. Neutron accomplished during plant operation. The core hole may be irradiation embrittlement is one of the concerns for the either tapered or vertical-sided, depending upon the thickness nuclear industry, as this could limit the service life of reactor of the material to be welded and the depth of the hole. For pressure vessels. Improved understanding of the underlying deeper repair welds, a cylindrical configuration for the hole causes of embrittlement and better calibration of prediction models will provide both regulators and power plant operators with improved estimates of vessel remaining life (Odette and Lucas, 2001). The NMMU, in collaboration with Eskom, has developed the patented Weldcore® process, which is a sampling and repair process that is currently used in fossil fuel power stations to assist in sample removal for creep damage assessment. The process involves removal of a cylindrical

Figure 16 – Illustration of four Weldcore® coring stages (Hattingh et al., Figure 17 – Schematic of the FTHPP process with macro appearance of 2012) a cross-section of a weld (Hattingh et al., 2012) ▲ 910 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Friction processing as an alternative joining technology for the nuclear industry and rotating tool is recommended to ensure that the process acquisition downtime. The process has been tested on steam force and torque are limited. The consumable rotating tool is pipes and turbine components in the power generation typically made of the same alloy as the base metal, although industry and is in the final phases of being tested on it is possible to achieve a matched, over-matched, or under- structures on a petrochemical plant. The main benefit of the matched weld zone. On completion of the weld, the remaining Weldcore® process is a reduction in the risk of failure and portion of the consumable rotating tool can be cut off and hence better management of safe life in high-value ground flush with the metal surface. Since the ’weld’ does not engineering components. produce a liquid weld puddle, the orientation of the weld is not affected by gravity, hence making the process position- Discussion independent. The introduction of a new or alternative joining and repair The main factors making the Weldcore® process processes into the nuclear industry entails a number of attractive to the nuclear fraternity relate to platform size challenges from the engineering and regulatory points of (which is compact and hence suitable for on-site work) and view. Nuclear power, for obvious reasons, is a tightly a high degree of process automation. The Weldcore® regulated industry, but the authors propose that by platforms were initially developed for high-temperature introducing improved life assessment standardization, the steam pipes used in power stations. However, the success of risk and complexity of safely operating these types of the platform has led to further developments for applications facilities could be further reduced. Cost-effective condition that include turbine disc and reducer sections. monitoring is an implicit part of life extension for ageing Figure 18 shows the modular arrangement and plant, and of the through-life cost and performance adaptability of the welding platforms that were developed at optimization of new installations. the NMMU for specific applications of the Weldcore® process Friction processing presents a viable alternative in the power generation industry. methodology for some aspects of condition monitoring and The in-situ application on steam pipes required the repair of thermal power plant, with significant advantages design and development of a portable reconfigurable platform accruing from its solid-state nature (which opens the door to that allowed operation at various locations throughout a welding of dissimilar metals). A solid-state joining process power station. The modular arrangement of the welding limits microstructural phase transformations, while lower platform, involving a frame, spindle, and drive motor, process temperatures result in a significant reduction in facilitates ease of transport, positioning, and mounting. thermal stresses and associated distortion compared with Electrical and hydraulic power and control connections are conventional fusion welding processes. The lower thermal operated via a remote control unit. gradient results in a narrow heat-affected zone with a fine The process and portable platform allow for the in-situ equiaxed weld nugget and limited grain growth in the removal of a representative sample of metal from a structure thermomechanically transformed region. These (pipes, turbine rotors, etc.) that can be used for accurate microstructures are normally associated with good strength metallurgical assessment of the remaining life of the and toughness. As illustrated in the applications presented in structure. It provides superior data to other current methods this paper, friction processing lends itself to a high degree of as the excised core is a far more representative sample of the automation and process control, giving repeatable welds and through-thickness than other techniques, with a shorter allowing sophisticated monitoring of weld quality. Friction

Figure 18 – Developmental progression of Weldcore® equipment (Hattingh et al., 2012)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 911 ▲ Friction processing as an alternative joining technology for the nuclear industry welding offers the power generation industry a clean, NICHOLAS, E.D. 2003. Friction processing technologies. Welding in the World, reconfigurable welding process suitable for complex and vol. 47, November. pp. 11–12. bespoke applications and which can greatly reduce process ODETTE, G.R. and LUCAS, G.E. 2001. Embrittlement of nuclear reactor pressure time and cost of condition monitoring and repair. vessels. Journal of Materials (JOM), vol. 53, no. 7. pp. 18–22.

References PINHEIRO, G.A. 2008. Local reinforcement of magnesium components by friction processing: determination of bonding mechanisms and assessment of joint AMBROZIAK, A. and GUL, B. 2007. Investigations of underwater FHPP for properties. Dissertation, Vom Promotionausschuss der Technischen welding steel overlap joints. Archives of Civil and Mechanical Engineering, Universität Hamburg-Harburals. vol. 7, no. 2. pp. 67–76. REYNOLDS, A.P., SEIDEL, T.U., and SIMONSEN, M. 1999. Visualization of material ANDERSSON, C-G. and ANDREWS, R.E. Fabrication of containment canisters for flow in an autogenous friction stir weld. Proceedings of the 1st nuclear waste by friction stir welding. Proceedings of the 1st International Symposium on Friction Stir Welding, Rockwell Science International. Symposium on Friction Stir Welding, Rockwell Science Center, Thousand Oaks, California, 14–16 June 1999. Center, Thousand Oaks, California, 14–16 June 1999. SERVA-TECH SYSTEMS. Not dated (a). Friction Welding Process Basics – report 1. ASM INTERNATIONAL. 1983. Metals Handbook. Volume 6 – Welding Brazing, and http://www.frictionwelding.com/report1.htm Soldering. 9th edn. Materials Park, Ohio. SERVA-TECH SYSTEMS. Not dated (b). Direct vs Inertia Friction Welding – report 4. BEVINGTON, J. (1891. Spinning tubes mode of welding the ends of wire, rods, etc, Not dated. http://www.frictionwelding.com/report4.htm and mode of making tubes. US patent 463134, 1891.

BHAMJI, I. PREUSS, M., THREADGILL, P.L., and ADDISON, A.C. 2011. Solid state SINGER, I.L. 1998. How third-body process affects friction and wear. MRS joining of metals by linear friction welding: a literature review. Materials Bulletin, June. pp. 37–40. Science and Technology, vol. 27, no.1. pp. 2–12. SUERY, M., BLANDIN, J.J., and DENDIEVEL, R. 1994. Rheological behaviour of two BOLDYREV, R.N. and VOINOV, V.P. 1980. Possible reasons for the formation of phase superplastic materials. Materials Science Forum, vol. 170/172. extremum of torque during heating in friction welding. Welding pp. 167–176. Production, no. 1. pp. 10–12. SWEDISH NUCLEAR FUELAND WASTE MANAGEMENT CO. 2001. Development of BSI GROUP. (2000. Welding – friction welding of metallic materials. British fabrication technology for copper canisters with cast inserts. Technical Standard BS EN ISO 15620-2000. London, UK. Report TR-02-07. Stockholm, Sweden.

BULBRING, D.L.H., HATTINGH, D.G., BOTES, A, and ODENDAAL, D.H. 2012. Friction THOMAS, W. and DUNCAN, A. 2010. Friction based technology for joining and hydro pillar process as an alternative repair steel structures. FERROUS material processing – an introduction. International Conference on 2012. Ferrous and Base Metals Development Network Conference, Welding and Joining of Materials (ICWJM), Pontificia Universidad Católica Magaliesburg, South Africa, 15–17 October 2012. Southern African del Perú, Lima, Peru, 9–11 August 2010. Institute of Mining and Metallurgy, Johannesburg. THOMAS, W.M. 1997. Gas additions boost friction performances. TWI Connect DAWES, C.J. 2000. Faster and faster - welding speed increases with tool Press. The Welding Institute, Cambridge, UK. www.twi.co.uk. development - one of a series of steps. Bulletin, vol. 41, no. 4. pp. 51–55.

DELANY, F., LUCAS, W., THOMAS, W., HOWSE, D., ABSON, D., MULLIGAN, S., and THOMAS, W.M. and DOLBY, E. 2002. Friction stir welding developments. 6th BIRD, C. 2005. International Forum on Welding Technologies in Energy International Conference on Trends in Welding Research, Pine Mountain, Engineering. Shanghai, China, 21–23 September 2005. Georgia. USA, 15–19 April 2002.

GIBSON, S.W. 1997. Advanced Welding. Chapter 12 – Friction Welding. THOMAS, W.M. and NICHOLAS E.D. 1992. Friction hydro pillar processing TWI. McMillan, London. Leading Edge. TWI Connect Press. www.twi.co.uk

GODET, M. 1984. The third-body approach: a mechanical view of wear. Wear, THOMAS, W.M. and TEMPLE-SMITH, P. 1996. Friction plug extrusion. UK Patent vol. 100. pp. 437–452. Application, GB 2306365. The Welding Institute, Cambridge, UK.

HATTINGH, D.G., DOUBELL, P., SCHEEPERS, R., NEWBY, M., VON WIELLIGH, L., THOMAS, W.M., NICHOLAS, E.D., and KALLEE, S.W. 2001. Friction based ODENDAAL, D., and WEDDEBURN, I.N. 2012. Novel core sampling technique technologies for joining and processing. TMS Friction Stir Welding and for HP turbine rotor remaining life study. 10th Electric Power Research Processing Conference, Indianapolis, Indiana, USA, November 2001. Institute Conference, Florida, USA.` TWI BULLETIN. 1997. The need for gas shielding - positive advantages for two MAGUIRE, A. 2012. Hy-Ten celebrates nuclear approvals for friction welded friction processes. The Welding Institute, Cambridge, UK. pp. 84-88. couplers in concrete construction.

http://www.sourcewire.com/news/73407/hy-ten-celebrates-nuclear- TWI GLOBAL. 2014. Friction welding proves perfect in nuclear work. approvals-for-friction-welded-couplers-in-concrete# http://www.twi-global.com/news-events/case-studies/friction-welding-

MEYER, A. 2003. Friction hydro pillar processing - bonding mechanisms and proves-perfect-in-nuclear-work-097/. properties. Dissertation, Von der Gemeinsamen Fakultät für Maschinenbau VON WIELLIGH, L.G. 2012. Un-supported friction stir processing of stress und Elektrotechnik der Technischen Universität Carolo-Wilhelmina zu corrosion cracks in a 304L stainless steel tank surface. Technical Report Braunschweig als. Dissertation angenommene Arbeit. GKSS- TS008-A. eNtsa, Nelson Mandela Metropolitan University. Forschungszentrum Geesthacht GmbH. WANJARA, P. and JAHAZI, M. 2005. Linear friction welding of Ti-6Al-4V: MAALEKIAN, M. 2007. Friction welding - critical assessment of literature. Journal processing, microstructure, and mechanical-property inter-relationships. of Science and Technology of Welding and Joining, vol. 12, no. 8. Metallurgical and Materials Transctions A, vol. 36A, no. 8. pp. 708–729. pp. 2149–2164. NENTWIG, A.W.E. 1995. Untersuchungen zum linear-reibsscheissen von metallen. Schweissen und Schneiden, vol 47, no. 8. pp. 648–653. WEDDERBURN, I.N. 2013. Development of a creep sample retrieval technique and friction weld site repair procedure. PhD thesis, Nelson Mandela NICHOLAS, E.D. 1998. Developments in the friction stir welding of metals. 6th Metropolitan University, Port Elizabeth, South Africa. International Conference on Aluminium Alloys, ICAA-6, Toyohashi, Japan. Sato, T., Kumai, S., Kobayashi, T., and Murakami, Y. (eds). Japan Institute WORLD NUCLEAR ASSOCIATION (WNA). 2015. http://www.world- of Light Metals, Tokyo. nuclear.org/info/Facts-and-Figures [Accessed 2 May 2015]. ◆ ▲ 912 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a3 Neutron- and X-ray radiography/ tomography: non-destructive analytical tools for the characterization of nuclear materials by F.C. de Beer*†

Davies (2000) describes the role and value Synopsis of NDT during maintenance and in-service A number of important areas in nuclear fuel cycle, at both the front end inspection of nuclear power plants during and back end, offer ideal opportunities for the application of non- outages, and particularly the monitoring of destructive evaluation techniques. These techniques do not only provide material degradation to prevent failure. opportunities for non-invasive testing of e.g. irradiated materials, but also Ultrasonic testing (UT), magnetic testing (MT,) play an important role in the development of new materials in the nuclear and electrical testing (ET) play a major role as sector. The advantage of penetrating radiation used as probe in the NDT methods for monitoring materials investigation and testing of nuclear materials makes X-ray and neutron degradation in-situ, while atomic- and nuclear radiography (2D) and tomography (3D) suitable for various applications physics-based methods such as positron in the total nuclear fuel cycle. The unique and different interaction modes annihilation, neutron diffraction, as well as X- of the two radiation probes with materials provide several opportunities. ray and neutron tomography are limited to Their complementary nature and non-destructive character makes them most suitable for nuclear material analyses, analytical method laboratory-scale experimentation. However, development, and the evaluation of the performance of existing nuclear conventional film-based X-ray and gamma-ray material compositions. This article gives an overview of the X-ray and radiography (RT) techniques are being applied neutron radiography/tomography applications in the field of nuclear throughout many areas of material testing in material testing, and highlights a few of the success stories. Several the nuclear fuel cycle. selected areas of application in the nuclear fuel cycle are discussed to The ‘nuclear fuel cycle’ refers to the entire illustrate the complementary nature of these techniques as applied to range of activities associated with the nuclear materials. production of electricity from nuclear fission, Keywords entailing (International Atomic Energy neutron radiography, X-ray radiography, SAFARI-1; non-destructive Agency, n.d.): testing. ➤ Mining and milling: from mined uranium to yellowcake ➤ Conversion: from yellowcake to gas ➤ Enrichment: increases the proportion of Introduction the fissile Isotope During the development of new materials for ➤ Deconversion: depleted uranium. the nuclear industry, materials testing and ➤ Fuel fabrication: UO2 pellets – fuel pins – characterization are of the outmost importance fuel elements to maintain safety standards and reliability. No ➤ Electricity generation: fuel burn-up compromise on safety in the workplace in any ➤ Storage: spent fuel area within the total nuclear fuel cycle can be ➤ Reprocessing: spent fuel tolerated, and therefore most in-situ material ➤ Radioactive waste: safe storage. characterization and testing is conducted by The nuclear fuel cycle includes the ‘front certified and qualified personnel schooled in end’, i.e. preparation of the fuel, the ‘service destructive and non-destructive testing (DT period’ in which fuel is used in the reactor to and NDT) methods. Certification and qualifi- generate electricity, and the ‘back end’, i.e. the cation in NDT can be obtained through many training centres in South Africa in accordance safe management of spent fuel, including with European, American, and other interna- reprocessing and reuse and disposal. tional standards (SGS, n.d.; SAIW, n.d.; African NTD Centre, n.d.). The testing of new methods and materials related to the nuclear fuel cycle is essential for the continuing development and safety of * Radiation Science Department, Necsa, Pelindaba. nuclear-related materials and processes. These † School of Chemical and Minerals Engineering, fundamental research initiatives are mostly North-West University, Potchefstroom. performed at the laboratory scale by material © The Southern African Institute of Mining and and instrument scientists, researchers, and Metallurgy, 2015. ISSN 2225-6253. Paper received most likely postgraduate students. Aug. 2015 and revised paper received Aug. 2015.

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The general nuclear fuel cycle is schematically depicted in Analytical methods based on penetrating radiation Figure 1, showing the various activities in the production of Information about the internal structures of objects, for energy through the nuclear fission process. Every activity example the hydrogen content of Zr cladding, can be obtained requires conventional NDT techniques to be conducted to by destructive analytical methods, e.g. cutting a the fuel rod maintain the safe working and operation of the plants and in a 2D plane for analysis by electron microscopy, or sieve facilities. The standard NDT methods applied to e.g. analysis for particle size distribution of a soil sample. In most inspection of welds in piping, are (Willcox and Downes, n.d): cases, once the sample has been destroyed, no other ➤ Radiography testing (RT) analytical tests are possible and the larger picture (volumetric ➤ Magnetic particle crack detection (MT) hydrogen distribution and particle size distribution in the ➤ Dye penetrant testing (PT) soil) is lost. ➤ Ultrasonic flaw detection (UT) More valuable, unique, and in some cases more accurate ➤ Eddy current and electromagnetic testing (ET). results can be obtained only when three-dimensional This paper does not focus on the so-called conventional information is available. For research purposes the most NDT techniques and their application in the nuclear sector, acceptable way to obtain information while maintaining the but rather on the non-conventional NDT techniques that are sample integrity is to apply a non-destructive test using used as needed, and which constitute important research penetrating radiation (either with X-rays, gamma rays, or tools. In particular, penetrating radiation probes as realized in neutrons). It is worthwhile to mention that the neutron, the radiography/tomography are described with specific fission product of the nuclear fuel cycle, can be used as a applications in material research. Quantitative and/or probe to investigate the integrity of the nuclear fuel itself. qualitative data obtained through applying these novel After irradiation the physical condition of fuel pellets, while techniques in a laboratory environment adds value to many still intact in the fuel pin, can be obtained only by means of areas within the nuclear sector. The following specific radiography. This manner of non-invasive investigation activities, ranging from mining the ore to security of the keeps the sample intact, leaves the sample in its original waste generated, and where radiography and tomography are form, and it is possible for other tests to be conducted applied, are highlighted in this paper: subsequently on the real sample as if it was not touched. The ➤ Mining and geosciences: quantification of ore deposits non-invasive process allows for the generation of valuable ➤ Fuel fabrication: development and testing of new qualitative information. However, when digital data is materials transformed into three-dimensional tomographic data ➤ Electricity generation: fuel rod performance ; post- (Banhard, 2008), it is possible to obtain high-resolution irradiation examination (PIE) quantitative information of the internal structures and ➤ Radioactive waste: safe storage, civil engineering. properties of the object. An example is the volumetric pore size distribution of voids within nuclear-encapsulating concrete matrixes, as well as their physical distribution throughout the sample (McGlinn et al., 2010). X-ray, gamma-ray, and neutron radiation are attenuated (absorbed and scattered by the sample) according to an exponential law (De Beer, Middleton, and Hilson, 2004):

[1]

where I is the intensity of the transmitted radiation beam, I0 the intensity of the incident radiation beam, μ the attenuation coefficient (cm2/g) of the material under investigation for the specific radiation type, ρ the density of the sample (g/cm3), and x the thickness of the sample (cm). The attenuation coefficient μ expresses the total attenuation, due to both the scattering and capture processes for the incident radiation. The term μρ is also called the total absorption coefficient of the sample. We assume that the quantity μρ is linearly related to its constituents:

[2]

where μi is the radiation attenuation coefficient of constituent i, ρi the density of constituent i, Vi the volume fraction of constituent i, and ∑ the summation symbol for the ith components. Composites of materials will thus have a different radiation attenuation property from the individual elements. The parameters I(E), I0(E), and μ(E) reflect radiation Figure 1 – Schematic depiction of the nuclear fuel cycle (Pixshark, n.d.) energy dependency. This dependency, in a radiography ▲ 914 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Neutron- and X-ray radiography/tomography context, means that materials will attenuate different the detector is called a radiograph and contains the integrated radiation types by different magnitudes and thus will yield radiation transmitted information for the total sample in a different radiological images, and also that an element has certain orientation with respect to the source and detector different attenuation properties at different energy levels for configuration. the same radiation type. This implies that an element can be The information captured in the radiograph differs in transparent to fast neutrons (MeV energies) but can be principle for X-ray and neutron radiography/tomography. detected easily using thermal neutrons (eV energies). An The two probes are mostly utilized within the nuclear fuel example is the thermal neutron scattering and absorption of cycle as non-destructive techniques in research and for hydrogen and boron (Domanus, 1992). nuclear material qualification and quantification. These Although the basic interaction of X-rays and neutrons principles are discussed in more detail in the following with the elements differs, the principle of conducting paragraphs. radiography to obtain a two-dimensional radiograph/image X-ray radiography of the sample is the same. Figure 2 schematically illustrates X-ray interaction with materials depends on the density of the basic components and layout of a radiography facility. the sample – i.e. the electron cloud density (Banhard, 2008). A source of radiation emits penetrating radiation towards The area detector registers a two-dimensional image a sample. For example, a sample contains either a defect, an (radiograph) of the object representing the internal structure inclusion of another material, or a void that is abnormal for density. Elements with low electron densities are not easy to the sample, or an area that differs completely in terms of resolve in a radiograph, but they are easily penetrated to composition from the basic matrix of the sample, which reveal denser materials embedded within the sample matrix. results in a lower or higher density at the location within the Figure 3 presents the different X-ray attenuation coefficients sample. The incident radiation will be attenuated (scattered (cm-1) for 125 kV X-rays for the full spectrum of elements in and/or absorbed) differently due to the abnormality. A the periodic table. It clearly shows the increase in absorption sensitive area detector, with a high quantum efficiency for of X-rays (darker shading) at higher atomic numbers. the detection of the specific type of penetrating radiation, Neutron radiography registers the difference in attenuated radiation that has passed through the sample. The 2D data (image) obtained by The interaction of neutrons with materials is totally different to that of X-rays, since neutrons, being neutral particles, interact only with the nucleus of the atom. Neutrons are not affected by even a dense electron cloud, e.g. of a lead atom (μρn = 0.38 cm-1). The thermal neutron attenuation coeffi- cients depicted in Figure 4 shows a totally different, and in some instances an opposite attenuation capability (grey scale) to that for X-rays (Figure 3). Hydrogen, as a highly attenuating material (μρn = 3.44 cm-1), will be easy to detect and clearly visible on a neutron radiograph when embedded in e.g. a ZrTM (μρn = 0.29 cm-1) fuel pin, which is nearly transparent to neutrons. A radiograph with low- to intermediate-energy X-rays is possible as the ZrTM tube (μρX Figure 2 – Principle and layout of a 2D radiography set-up. A similar = 2.47 cm-1) attenuates most of the X-ray radiation and with set-up is used for tomography, with the sample rotating in the radiation no photons remaining, the H (μρX = 0.02 cm-1) cannot be beam (Domanus, 1992) registered/detected on the X-ray radiograph.

Figure 3 – Periodic table with X-ray attenuation coefficients of the elements for 125 kV X-ray energies (Grünauer, 2005)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 915 ▲ Neutron- and X-ray radiography/tomography

Figure 4 – Periodic table with thermal neutron energy attenuation coefficients of the elements (Grünauer, 2005)

Tomography Mining The word ‘tomography’ comes from the Greek words ‘to cut X-ray-, gamma-ray, and neutron tomography have or section’ (tomos) and ‘to write’ (graphein) (Banhard, demonstrated their potential in the earth sciences as 2008). Tomography is also known as computer tomography important diagnostic tools to generate volumetric data on (CT) or computer assisted tomography (CAT) as in diagnostic geological compositions, especially advances in the area investigations in the medical field. For the purpose of this borehole core investigations, as depicted in Figure 5. This article, the following semantics are adopted: CT in general aspect is being explored further with optimum resolution description, XCT for X-ray computer tomography, and NT for obtained through the application of micro-focus X-ray neutron tomography. CT is a radiographic inspection method tomography, as CT complements conventional destructive analytical thin-sectioning of drill core samples (De Beer and that uses a computer to reconstruct an image of a cross- Ameglio, 2011). sectional plane (slice) through an object (ISO 15708-1). The The raw material for nuclear fuel is uranium, which is a resulting cross-sectional image is a quantitative map of the relatively common element that can be found throughout the linear radiation attenuation coefficient, μ, at each point in the world. Uranium is present in most rocks and soils, in many plane. The linear attenuation coefficient characterizes the rivers, and in seawater. Uranium is about 500 times more local instantaneous rate at which the incident radiation is abundant than gold and about as common as tin. attenuated during the scan, by scatter or absorption, as it The largest producers of uranium are currently Australia, propagates through the object. Canada, and Kazakhstan, with Namibia rated 5th and South To obtain this ‘map’, the sample is radiographed and thus Africa 11th globally (World Nuclear Association, 2015). The projection data is gathered from multiple directions through many angles of the sample. For the purpose of this article, no detailed description of the 3D reconstruction process of the sample is presented. To put it simply, multiple 2D-projections are fed into a dedicated computer with a specialized computer algorithm to create cross-sectional planes of the sample. When these cross-sectional planes are stacked together, a full virtual three-dimensional image (tomogram) of the sample can be viewed and analysed.

Application of radiography and tomography within the nuclear fuel cycle In each of the following sectors of the nuclear fuel cycle, radiography and/or tomography have been applied using either X-rays or neutrons. In some areas the application was pioneered in the early second half of the 20th century by Figure 5 – Left: the grouped pyroxene mineral (coloured red) can be means of film techniques. The techniques applied and the clearly seen in norite (top), and different concentration of feldspar are observed in anorthosite (bottom) in a thin slice. Right: transparent description thereof is not within the scope of this article. corresponding neutron tomograms. The minerals shown are only However, the outcomes of these film-based investigations pyroxenes that are present in both norite and anorthosite but at and the results obtained will be described, together with the different concentrations [archives from Necsa’s Nrad and MIXRAD recent digital methods in this field. system] ▲ 916 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Neutron- and X-ray radiography/tomography concentration of uranium in the ore can range from 0.03 to technique cannot be used as an independent tool for mineral 20%. Conventional mining is by open cut or underground characterization, but rather in support of existing methods. Uranium ore can be produced from a mine specif- mineralogical techniques. However, valuable information is ically for uranium, or as a by-product from mines with a added into the nuclear value chain via 3D-CT. No sample different main product such as copper, phosphate, or gold preparation is needed other than cutting a small piece of rock (International Atomic Energy Agency, n.d.). for analysis. The sample integrity is maintained due to the Using micro-focus X-ray CT with 100 kV potential to non-destructive nature of the technique, as it provides 3D distinguish between gold (μρX = 358 cm-1) and uraninite information for the total volume of the sample, including (μρX = 283 cm-1), both the minerals are easy to distinguish internal components. Results can be obtained within about 1 from the matrix minerals such as pyrite (μρX = 18.4 cm-1), hour scan at high resolution with up to 2000 projections, zircon (μρX = 45.9 cm-1), and brannerite (μρX = 89.6 cm-1) providing resolution down to 6 μm (including due to their much higher elemental densities. The use of CT reconstruction). as a sorting method is still a challenge, as it is difficult, at the Enrichment 100 kV X-ray energy CT capability available, to distinguish between gold and uraninite (Chetty et al., 2011). In a follow- Enriched uranium is uranium in which the concentration of up 3D micro-focus X-ray computer tomography (μXCT) U-235 has been increased through the process of isotope study using 120 kV, the contrast and resolution of the separation. U-235 is the only nuclide existing in nature (in minerals were well defined and individual minerals could be any appreciable amount) that is fissile with thermal neutrons separated and distinguished from other minerals (Sebola, (OECD Nuclear Energy Agency, 2003). Natural uranium is 2014). For the detection of uranium, 3D-CT was 99.284% U-238 isotope, with the U-235 isotope constituting benchmarked against 2D mineralogical results from optical only about 0.711%. microscopy and scanning electron microscopy (SEM). The very first application of neutron radiography was in Uraninite, brannerite, and uraniferous leucoxene are the the early 1960s for nuclear fuel characterization using the uranium-bearing minerals present in the samples (from the film technique. The use of neutron radiography in the Vaal Reef) and were quantified by μXCT-3D analysis for their monitoring of isotopic enrichment in fuel pellets loaded into a sizes, shapes, and distribution with respect to other mineral fuel pin has been demonstrated by Frajtag (n.d.) (Figure 8). components in the samples. Uraninite was found to be the Gosh, Panakkal, and Roy (1983) investigated the major mineral, occurring mainly in the quartz matrices and possibility of monitoring plutonium enrichment in mixed- also associated with carbonaceous matter as depicted in oxide fuel pellets inside fuel pins using neutron radiography Figure 6. The uraninite and gold in the matrix occurred as as early as 1983. Recently, Tremsin et al., (2013) rounded grains up to 200 μm in size, as observed by 2D investigated the very large difference in the absorption cross- mineralogical techniques. CT allows for 3D grain size sections of U-235 and U-238 isotopes, as shown in Figure 9, analyses of the uraninite grains in the total volume of the and deduced that very accurate non-destructive spatial sample, and in this study also their association with matrix mapping of the enrichment level of fuel pellets, as well as of minerals, as depicted in Figure 7. The CT observations the distribution of other isotopes in the spent fuel elements supported the results acquired by conventional mineralogical (Nd, Gd, Pu, etc.), can be achieved. techniques, suggesting that 3D μXCT can be used to Additionally, information on the distribution of isotopes complement other mineralogical techniques in obtaining 3D can then be used for the investigation of fuel burn-up rates information. However, 3D μXCT has limitations such as for fuel elements placed at different rod positions in the spatial resolution, partial volume effect, and overlapping of reactor core. mineral grey-scale values. It is therefore suggested that the Large differences in transmission spectra allow very accurate mapping of isotopic distributions in the samples using either transmission radiography or neutron resonance absorption characteristics of the respective isotopes (Tremsin et al., 2013). One of the attractive features of energy-

Figure 6 – X-ray CT-tomogram slice showing the distribution of uraninite grains in the quartz-pyrite matrix and the carbonaceous Figure 7 – Grain size distribution of uraninite in the matrix and the matter (Sebola, 2014) carbonaceous matter (Sebola, 2014)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 917 ▲ Neutron- and X-ray radiography/tomography

Figure 8 – Neutron radiograph (film) of fresh fuel pellets in a fuel pin with varying degrees of isotopic U-235 enrichment. Pellets with higher enrichment appear darker (Frajtag, n.d.)

Figure 9 – Neutron attenuation of 100 μm thick U-235 and U-238 isotopes calculated from the tabulated data on the total cross-sections as a function of neutron energy (Tremsin et al., 2013)

Figure 10 – Thermal neutron transmission radiographs obtained by grouping the energy-resolved images of different neutron spectra. The ranges of neutron energies used to build each radiograph are shown in the respective legends (Tremsin et al., 2013)

resolved neutron radiography is the ability to enhance performance criteria which have to be verified. Non- contrast, and in some cases enable quantification, as shown destructive testing of the fuel ensures that other tests can be in Figure 10. It was observed that the contrast between the performed subsequently and that the material can still be pellets of different density depends strongly on the range of applied in its specific environment. Some of the NDT tests are neutron energies used. The more thermal part of the beam applied to characterize and/or quantify the integrity of the spectrum (neutron energies above 19.7 MeV) reveals the fuel or as quality assurance tests. X-ray radiography cannot pellet with the lowest density as an object with the highest be used for irradiated fuel inspection, whereas neutron transmission. The coldest part of the neutron spectrum radiography becomes possible due to the following reasons (neutron energies even below 6 MeV) shows the least dense (Lehmann, Vontobel, and Hermann, 2003): pellet as a darkest in the assembly. ➤ Uranium has a very high attenuation coefficient for X- -1 Nuclear fuel fabrication and testing (I & PIE) rays (about 50 cm at 150 keV). The diameter of fuel pellets is in the order of 10 mm and penetration by X- Nuclear fuel types range from isotopic sources in a form of rays is impossible. High-energy gamma radiation (>1 salt or disks to pressurized water reactor (PWR) fuel in the MeV) is, however, suitable for quality control of fresh form of UO2 fuel pellets inside ZrTM -cladded fuel pins. fuel pellets Nuclear fuel is subjected to stringent manufacturing and ➤ The neutron attenuation coefficient for the natural ▲ 918 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Neutron- and X-ray radiography/tomography

-1 composition of uranium is low (0.8 cm ) and it is easy Table I to transmit neutrons through thicker assemblies Quantitative information deducted from the XCT ➤ U-235 and U-238 have very different interactions with thermal neutron beams. Due to the 60 times higher Description: Information Value Unit cross-section of U-235, it is very easy to distinguish between the two isotopes and to quantify the amount Needle inscription 100 µg of the fissile isotope U-235 Official source activity 20 mCi ➤ Contact dose rate 1.1 mSv/h Lead is used as shielding material around fuel samples Diameter of tube 1.67 mm for radiation protection purposes, and thus X-ray Diameter of internal void 1.03 mm radiography fails in transmission experiments. Length of tube 9.72 mm Neutrons, on the other hand, penetrate lead shielding Length of internal void 7.57 mm Wall thickness 0.34 mm with a thickness of about 15 cm and allow neutron Length of salt 3.01 mm radiography investigations Inner volume 6.31 mm3 ➤ Additional substitutes in fuel compositions, which are Volume of salt 2.51 mm3 in use as burnable poisons but are strong neutron absorbers (e.g. B, Li-6, Dy, or Gd), are easily identified with neutron methods temperature, pressure, and radiation level. Thermal neutron ➤ After long-term exposure, hydrogen can be found in radiography investigations were conducted with the conven- the cladding outer region of fuel rods under some tional film technique due to the radioactivity of the objects. circumstances. X-ray radiography fails to visualize The following issues are addressed through the use of these material modifications because of the very low neutron radiography: (a) condition of the fuel assembly, contrasts obtained for elements with with low atomic including fuel rod condition, (b) detection of leaks such as numbers. Neutrons, on the other hand, have a high ingress of water, and (c) quality control, including functional sensitivity for hydrogen, thus allowing quantification and dimensional evaluation and inspection of irradiation of the hydrogen content in cladding. devices and components. Figure 13 shows a neutron Isotopes radiograph of a fuel pin with pelletized fuel as fabricated (Domanus, 1992). Characterization of isotopic sources is a demanding and Due to the high radioactivity of nuclear fuel after difficult task due to their physical size and natural radioac- irradiation, X-ray radiography cannot be used as an investi- tivity, which makes visual inspection impossible. Hoffman gation technique. Investigations are done within a hot-cell (2012) investigated a small radioactive radium source using laboratory set-up, which allows for the remote handling of high-resolution micro-focus X-ray tomography to determine the radioactive fuel (Klopper, De Beer, and Van Greunen, whether the sample contained a powdered form of radioactive 1998). A research reactor is normally an extension of a hot- material or whether it was solid. The source contained 20 mg cell laboratory, as neutron radiography is one of the major of radium and was in a form of a needle with a diameter of analytical probes in the post-irradiation examination (PIE) of about 1 mm and about 8 mm in length – all sealed within a nuclear fuel. Typical findings using neutron radiography as glass tube. Figure 11 is an XCT tomogram of the ampoule an analytical probe on irradiated fuel pins are the condition of showing its serial number clearly on the outside, while Figure the fuel pellets and of the ZrTM -cladding material. 12 is a slice from the 3D tomogram revealing the position of Fuel pellet investigations reveal fabricated conditions the radioactive material inside the needle. such as cracks, chips, change of shape or location, voids, Valuable metrology quantitative information could be inclusions, corrosion, nuclear properties, and coolant. Figure deducted from the 3D tomogram of the isotope (Table I). The 14 shows examples of these findings on film neutron most important aspect for further processing of the isotope is radiographs. the quantification of the volume of radioactive salt present in the needle. Nuclear fuel A major field of neutron radiography application is the inspection of nuclear fuel and control rods, reactor materials and components, and of irradiation devices for the testing of nuclear fuels and materials. The fuel rods are used under extreme conditions such as very high power density,

Figure 12 – Slice of μXCT tomogram with enlarged end pugs revealing Figure 11 – μXCT of a Ra isotopic source ampule (Hoffman, 2012) Ra isotope internal information (Hoffman, 2012)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 919 ▲ Neutron- and X-ray radiography/tomography

Figure 13 – Neutron radiograph of nuclear fuel prior to irradiation (Domanus, 1992)

Figure 14 – Neutron radiographs (film prints) of irradiated nuclear fuel and their conditions (Domanus, 1992). Top: random cracks in pellets, middle: typical longitudinal cracks in pellets

Domanus (1992) describes a number of other fuel pellet the study of even small quantities of hydrogen ingress in the properties revealed by neutron radiography, including central cladding, which is an important mechanism for cladding voids and the accumulation of Pu in the central void. Fuel rod embrittlement, as depicted in Figure 15. inspections include deformed cladding, hydrides in cladding, Furthermore, through proper characterization and with plenum and spring, dislocated disks, condition of the bottom the aid of digital radiographs, Nrad allows for the investi- plug, and a picture of a melted thermocouple inside the fuel gation of the absolute hydrogen content and its distribution. rod. Lehmann, Vontobel, and Hermann (2003) report on the in situ investigations provide new information about the extensive utilization of the NEUTRA and ICON neutron kinetics of hydrogen uptake during steam oxidation and of radiography (Nrad) facilities at Paul Sherrer Institute in hydrogen diffusion in zirconium alloys. Nrad-studies are the Switzerland, where a dedicated detection station is available only way to investigate and understand the phenomenon of for the inspection of irradiated fuel assemblies. Aspects such hydrogen ingress in the ZrTM cladding. A linear dependence as fuel enrichment, fuel poisoning, and hydrogen content in of the total macroscopic neutron cross-section on the H/ZrTM the fuel cladding are being addressed and investigated by atomic ratio, as well as on the oxygen concentration, was neutron radiography. Due to the importance of fuel cladding found, while no significant temperature dependence of the investigations, the utilization and function of neutron total macroscopic neutron cross-sections of hydrogen and radiography is addressed in the following paragraph. oxygen was found, depending that zirconium and oxygen do Hydrogen embrittlement not change their structures. Additionally, it was found that It is well known that hydrogen agglomeration is deleterious rapid hydrogen absorption takes place in the absence of the in any material. More than a few hundred ppm hydrogen in oxide layer covering the metallic surface of the ZrTM cladding the cladding surface of fuel rods at compromises the (Grosse et al., 2011). Figure 16 displays the results of in-situ structural stability of the cladding tube significantly, with the Nrad investigations of hydrogen uptake during steam consequence of possible failure, especially when mechanical oxidation with the time dependence of hydrogen concen- loading is also involved. The ability of neutrons to penetrate tration of ZrTM-4 materials at 1273 K and higher, where a uranium is considerably higher than for X-rays and allows very rapid hydrogen uptake was found in the first couple of for the structures of the nuclear fuel rods to be inspected. seconds after the steam flow was switched on. At temper- Furthermore, the probability of neutron interaction with atures of about 1273 K a phase transformation occurs and is hydrogen is very high, while for X-rays it is effectively zero. accompanied by a volume change and the formation of a This allows neutron radiography to be effectively utilized for pronounced crack structure. When the cracks are formed, the ▲ 920 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Neutron- and X-ray radiography/tomography

Figure 15 – Neutron radiograph of nuclear fuel and cladding material showing (black spots) hydrogen accumulation within the ZrTM tubing (Frajtag, n.d.)

(Necsa, 2006). Figure 18 shows the misalignment of the fuel within the carbon matrix of the fuel pebble as well as the location and identification of a TRISO particle within the fuel- free zone. The inhomogeneous distribution of the TRISO particles at the top of the fuel pebble can be clearly seen. Three-dimensional quantitative data of the misalignment of the fuel particles becomes available in the tomograms and is presented in Figure 19, showing the extent of correction in X-, Y- and Z-directions to be introduced in the manufacturing process of the fuel pebble. Lehmann, Vontobel, and Hermann (2003) reported the successful application of neutron tomography to the 3D scanning of PBMR fuel pebbles at the NEUTRA facility of the SINQ spallation source at PSI in Switzerland (see Figure 20). A Figure 16 – Neutron radiography (Nrad) investigation: kinetics of sphere-type fuel element from the high-temperature reactor hydrogen uptake and release during steam oxidation (Frajtag, n.d.) (HTR) programme was studied with neutron tomography. This sample is 6 cm in diameter and contains about 8500 individual fuel pebbles (diameter 0.5 mm). No shielding of the fresh fuel hydrogen uptake increases by nearly an order of magnitude element was necessary for the tomographic inspection. The (Grosse et al., 2008; Grosse, 2010). The decrease in investigation was aimed at the visualization of the 3D distri- hydrogen concentration is due to the consumption of the bution of the fuel particles in the graphite matrix in order to ß-ZrTM phase, which contains most of the absorbed determine its uniformity and the fuel sphere’s content of fissile hydrogen. material. TRISO fuel particles are an integral part of the fuel design for current and future HTRs. A TRISO particle comprises four Pebble bed modular reactor (PBMR) fuel concentric spherical layers encasing a fuel kernel, namely the Figure 17 shows the composition of a 60 mm outer diameter buffer (porous carbon), inner pyrolytic carbon (IPyC), silicon high-temperature reactor (HTR) fuel pebble consisting of carbide (SiC), and outer pyrolytic carbon (OPYC) layers (see thousands of 0.5 mm diameter low-enriched uranium oxide Figure 17). Each layer performs specific functions. The fuel fuel particles with a tri-structural isotropic (TRISO) coating, kernel, consisting of uranium or uranium carbide, provides the embedded in a graphitic matrix. The pebble was analysed fissile material and retains some of the fission products. The using X-ray tomography technique prior to irradiation at the buffer layer, a highly porous carbon structure, provides some SANRAD facility located at the SAFARI-1 nuclear research free volume for gaseous fission products, and protects the SiC reactor in South Africa. The aim of the investigation was to layer from damage by high-energy fission products. The IPyC observe the homogeneity of the TRISO particles within the layer provides structural support for the subsequent SiC layer carbon matrix and to direct the manufacturing process to and prevents the chlorine compounds required for SiC ensure the centralization of the fuel within the carbon matrix deposition interacting with the fuel kernel. The SiC layer forms

Figure 17 – Composition of PBMR fuel (Weil, 2001)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 921 ▲ Neutron- and X-ray radiography/tomography

Figure 18 – X-ray tomography of a PBMR fuel pebble. Left: the non- centralized fuel sphere within the carbon matrix. Right: Location and identification of a TRISO particle inside the fuel-free zone (Necsa, 2006)

Figure 20 – Neutron tomogram generated at PSI, Switzerland showing the exact location and homogeneity of the TRISO (Lehmann, Vontobel, and Hermann, 2003)

Figure 19 – Graphical presentation of the deviation of the fuel zone of a BPMR pebble from the centre of the carbon matrix in three dimensions (Necsa, 2006)

the main diffusion barrier for fission products. It acts as a Figure 21 – Micro-focus X-ray radiograph of a TRISO particle (Necsa) pressure vessel, providing mechanical strength for the particle during manufacture of the nuclear fuel compact or pebble bed. The OPyC layer protects the SiC layer during fuel fabrication as the TRISO particle is pressed into a larger fuel compact or pebble. Lowe et al., (2015) examined the applicability of multi- scale X-ray computer tomography (CT) for the non-destructive quantification of porosity and thickness of the various layers of TRISO particles (see Figure 21) in three dimensions, and compared this to the current destructive method involving high-resolution SEM imaging of prepared cross-sections. An understanding of the thermal performance and mechanical properties of TRISO fuel requires a detailed knowledge of pore sizes, their distribution, and interconnec- tivity. Pore size quantification (false color coding) and distri- bution in an X-ray tomogram of the SiC (D) and OPyC (E) layers within a TRISO particle is shown in Figure 22. Direct comparison with SEM sections indicates that destructive sectioning can introduce significant levels of coarse damage, especially in the pyrolytic carbon layers. Since it is Figure 22 – Xradia VersaXRM orthoslice showing the SiC (D) and OPyC non-destructive, multi-scale time-lapse X-ray CT opens the (E) layers with the pore volumes superimposed. Pore diameter is colour- coded to identify large pore clustering (Lowe et al., 2015) possibility of intermittently tracking the degradation of TRISO structure under thermal cycles or radiation conditions in order to validate models of degradation such as kernel movement. X- ray CT in situ experimentation on TRISO particles under load Research reactor control rod verification and temperature could also be used to understand the internal Nrad is being applied as a verification and analytic technique changes that occur in the particles under accident conditions. at the SAFARI-1 nuclear research reactor on the control rods ▲ 922 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Neutron- and X-ray radiography/tomography prior to their installation in the core of the reactor. The properties of concrete such as porosity, permeability, and quality control assurance test entails the verification of the sorping characteristics are obtained through applying neutron neutron attenuation cross-section of the control rod against a radiography as a non-destructive analytic tool. The aim of standard consisting of Cd. The inspection entails a visual these investigations is to maximize the properties to prevent clarification of the attenuation of the thermal neutrons by water sorption and leaching of concrete structures and optimize inspection of the neutron radiograph of the control rod. one of the physical properties which is sometimes neglected in Additionally, due to the digital radiography capability of the the criteria to develop structures for nuclear waste encapsu- neutron camera detection system, the first-order neutron lation (De Beer, Strydom and Griesel, 2004 ; De Beer, Le Roux transmission calculation can be made using the pixel and Kearsley, 2005). To improve the durability of concrete, the greyscale values on the radiographs of both the standard and capillary and pore size within the concrete matrix must be control rod sample. Pixel greyscale values represent a linear restricted to a minimum. This is why hydration as well as W/C relationship in the neutron attenuation of materials. In this ratio properties is of great importance, and creates thus an instance a dramatic decline in greyscale pixel values is seen ideal opportunity for neutron radiography to play a role in due to the high thermal neutron absorption by the Cd section obtaining the needed information in a non-destructive manner (μρn =115.11cm-1) of the control rod. Neutron radiographs of to optimize these parameters. The visualization of the sorption the Cd standard and a control rod are depicted in Figure 23. of water by means of neutron tomography of a laboratory-size concrete structure is depicted in Figure 25. Radioactive waste Low- and intermediate-level nuclear waste is normally Conclusions encapsulated in in some form of barrier to protect the waste X-ray and neutron radiography in two or three dimensions from the environment and vice versa. Intermediate-level play an important role in many dedicated areas within the nuclear waste is firstly encapsulated in a steel drum, nuclear fuel cycle. The advantage of these methods is their compressed, and finally embedded normally in a concrete drum completely non-destructive nature. Visualization of the and safely stored underground in a remote location such as structure of samples, as well as quantitative description, are Vaalputs in the Karoo region in South Africa (Necsa, n.d. (b)) important aspects in materials research. The important roles of (see Figure 24). A site is normally chosen with low rainfall and X-ray and neutron radiography/tomography as non-invasive suitable surface and groundwater conditions. analytic techniques within specific areas within the nuclear fuel Concrete is a porous medium and the characterization of transport of water through concrete structures is well described by De Beer, Strydom and Griesel (2004) and De Beer, Le Roux and Kearsley (2005). It is especially important to understand the transport of water through concrete because nearly all concrete structures contain steel reinforcing, and in the case of nuclear waste, intermediate-level nuclear waste in compressed steel drums. When cracks in the concrete, caused by the transport of liquid through it, reach the reinforcing, an environment conductive to the corrosion of steel is created. Corrosion affects the strength of the structural members, as the steel is a major contributor to the tensile and compressive strength of the members. Severe leakage of radioactive materials into the surrounding environment is thus possible if the integrity of the concrete barrier is compromised. Neutron radiography studies of concrete and mortars enable the direct physical visualization and quantitative detection of water inside concrete structures. The physical Figure 24 – Vaalputs intermediate-level waste storage site, South Africa (Necsa)

Figure 23 – Neutron radiographs of a Cd standard (left) and control rod Figure 25 – Neutron radiographs showing the effect of 70%, 60%, and (right) showing Cd (black) indicating high thermal neutron absorbing 50% W/C ratio on the sorptivity of water into a concrete slab (De Beer, materials (Necsa, 2010 (a)) Strydom and Griesel, 2004)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 923 ▲ Neutron- and X-ray radiography/tomography cycle should not be underestimated. X-rays and neutrons are GHOSH, J.K., PANAKKAL, J.P., and ROY, P.R., 1983. Monitoring plutonium produced by very different methods, and also interact with enrichment in mixed-oxide fuel pellets inside sealed nuclear fuel pins by neutron radiography. NDT International, vol. 16, no. 5, October 1983. pp. materials in different manners. In the nuclear environment, 275–276. DOI: 10.1016/0308- 9126(83)90127-X each type of radiation has its own field of utilization due to GROSSE, M. 2010. Neutron radiography—a powerful tool for fast, quantitative and their different characteristics, but in some instances their non- destructive determination of the hydrogen concentration and distri- applications complement each other to reveal comprehensive bution in zirconium alloys. Proceedings of the 6th International ASTM information. Symposium on zirconium in the Nuclear Industry, Chengdu, China, 9–13 May 2010. Neutron transmission analysis is a very helpful tool to GROSSE, M., VAN DEN BERG, M., GOULET, C., LEHMANN, E., and SCHILLINGER, B. 2011. obtain information on the properties of, and changes in, in situ neutron radiography investigations of hydrogen diffusion and nuclear fuel material. Scientists and researchers in the absorption in zirconium alloys. Nuclear Instruments and Methods in Physics geosciences in South Africa have, in the availability of the Research, vol. A651. pp. 253–257. doi: 10.1016/j.nima.2010.12.070 tomography facilities at Necsa, the capabilities to conduct GROSSE, M., STEINBRUECK, M., LEHMANN, E., and VONTOBEL, P. 2008. Kinetics of hydrogen absorption and release in zirconium alloys during steam quantitative analytical measurements at state-of-the-art oxidation. Oxidation of Metals, vol. 70, no. 3. pp. 149–162. radiation imaging facilities that compare to similar facilities GRÜNAUER, F. 2005. Design, optimization, and implementation of the new neutron elsewhere in the world. radiography facility at FRM-II. Dr. Rer. Nat. dissertation, Faculty of Physics, Within the mining area, 3D computer tomography shows Technischen Universität München. potential for further development, and can be already used to HOFFMAN, J.W. 2012. Process description for micro-focus X-ray investigation of source at MIXRAD facility. Necsa internal report, DOC NO: RS-MFX-PRO- complement and add value to current conventional 2D 12002. mineralogical techniques. Neutron radiography analysis is able INTERNATIONAL ATOMIC ENERGY AGENCY. Not dated. Getting to the Core of the Nuclear to derive the hydrogen content in fuel cladding both qualita- Fuel Cycle. Department of Nuclear Energy, Vienna, Austria. tively and quantitatively, with high sensitivity and precision. https://www.iaea.org/OurWork/ST/NE/NEFW/_nefw- documents/NuclearFuelCycle.pdf The results presented here illustrate how recent advances ISO 15708-1:2002(E). Non-destructive testing — Radiation methods — in laboratory-based X-ray CT instruments allow the Computed tomography — Part 1: Principles. examination of TRISO particles at the nano- and micro-scales KLOPPER, W., DE BEER, F.C., and VAN GREUNEN, W.S.P. 1998. Overview of hot cell in 3D. In this case study, high-resolution X-ray CT has been facilities in South Africa. Proceedings of the European Working Group ‘Hot shown to be a viable tool for profiling the TRISO particles in Laboratories and Remote Handling’. two important aspects; to characterize the individual TRISO LEHMANN, E.H., VONTOBEL, P., and HERMANN, A. 2003. Non-destructive analysis of nuclear fuel by means of thermal and cold neutrons. Nuclear Instruments layers with variations in thickness and their subsequent and Methods in Physics Research A, vol. 515, no. 3. interactions, thus allowing manufacturing validation as well as pp. 745–759. doi:10.1016/j.nima.2003.07.059 assisting in working towards a mechanistic understanding of LOWE, T., BRADLEY, R.S., YUE, S., BARII, K., GELB, J., ROHBECK, N., TURNER, J., and fabrication and in-service issues. WITHERS, P.J. 2015. Microstructural analysis of TRISO particles using multi- scale X-ray computed tomography. Journal of Nuclear Materials, vol. 461. The availability of these techniques in South Africa opens pp. 29–36. http://dx.doi.org/10.1016/j.jnucmat.2015.02.034 new possibilities for research, quantitative analysis, and non- MCGLINN, P.J., DE BEER, F.C., ALDRIDGE, L.P., RADEBE, M.J., NSHIMIRIMANA, R.B., destructive evaluation. National capacity as well as interna- BREW, D.R.M., PAYNE, T.E., and OLUFSON, K.P. 2010. 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CHETTY, D., CLARK, W., BUSHELL, C., SEBOLA, P.T., HOFFMAN, J.W., NSHIMIRIMANA, http://www.radwaste.co.za/vaalputs%20information%20pamflet.pdf. R.B., and DE BEER, F.C. 2011. The use of 3D X-ray computed tomography for gold location in exploration drill cores. Proceedings of the 10th [Accessed 1 May 2015]. International Congress for Applied Mineralogy (ICAM), Trondheim, Norway, OECD NUCLEAR ENERGY AGENCY. 2003. Nuclear Energy Today. OECD 1-5 August 2011. pp.129–136. http://www.mintek.co.za/wp- Publishing. p. 25. content/uploads/2011/11/ch-17.pdf PIXSHARK. Not dated. http://pixshark.com/images-of-nuclear-fuels.htm DAVIES, L.M. 2000. Role of NDT in condition based maintenance of nuclear power SAIW (Southern African Institute of Welding). Not dated. http://www.saiw.co.za/ plant components. 15th World Conference on Non-Destructive Testing, SEBOLA, P. 2014. Characterisation of uranium-mineral-bearing samples in the Rome, 15-21 October 2000. Vaal Reef of the Klerksdorp Goldfield, Witwatersrand basin. MSc disser- http://www.ndt.net/article/wcndt00/papers/idn078/idn078.htm tation, Faculty of Science, University of the Witwatersrand, Johannesburg. DE BEER, F.C. and AMEGLIO, L. 2011. Neutron, X-ray and dual gamma-ray http://wiredspace.wits.ac.za/handle/10539/16820?show=full radiography and tomography of geomaterial – a South African perspective. http://hdl.handle.net/10539/16820 Leading Edge, June 2011. Special Edition Africa. SGS SOUTH AFRICA (PTY) LTD. Not dated. http://www.sgs.co.za/en.aspx DE BEER, F.C., MIDDLETON, M.F., and HILSON, J. 2004. Neutron radiography of TREMSIN, A.S., VOGEL, S.C., MOCKO, M., BOURKE, M.A.M., YUAN, V., NELSON, R.O., porous rocks and iron ore. Applied Radiation and Isotopes, vol. 61. BROWN, D.W., and FELLER, B. 2013. Non-destructive studies of fuel pellets by pp. 487–495. neutron resonance absorption radiography and thermal neutron DE BEER, F.C., STRYDOM, W.J., and GRIESEL, E.J. 2004. The drying process of radiography. Journal of Nuclear Materials, vol. 440. pp. 633–646. concrete: a neutron radiography study. Applied Radiation and Isotopes, vol. WEIL, J. 2001. Pebble-bed design returns. Nuclear Power gets a Second Look. 61, no. 4. pp. 617–623. IEEE Spectrum Special Report. http://spectrum.ieee.org/energy/nuclear/ DE BEER, F.C., LE ROUX, J.J., and KEARSLEY, E.P. 2005. Testing the durability of pebblebed-design-returns [Accessed 20 June 2015]. concrete with neutron radiography. Nuclear Instruments and Methods in WILLCOX, M. and DOWNES, G. Not dated. A brief description of NDT techniques. Physics Research A, vol. 542. pp. 226– 231. Insight NDT, paper T001. DOMANUS, J.C. 1992. Practical Neutron Radiography. Kluwer Academic Publishers. http://www.turkndt.org/sub/makale/ornek/a%20brief%20description%20of FRAJTAG, P. Not dated. Radiation Protection and Radiation Applications: Gamma %20ndt.pdf and Neutron Radiography. WORLD NUCLEAR ASSOCIATION. 2015. http://www.world- nuclear.org/info/Nuclear- http://moodle.epfl.ch/pluginfile.php/1593971/mod_resource/content/2/RRA Fuel-Cycle/Mining-of-Uranium/World-Uranium-Mining- Production/ -EPFL- FS2014-Week14a.pdf [Accessed 1 May 2015]. [Accessed 13 May 2015]. ◆ ▲ 924 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a4 Non-destructive characterization of materials and components with neutron and X-ray diffraction methods by A.M. Venter*

Background Synopsis Approximately 95% of solid materials can be The availability of advanced characterization techniques is integral to described as crystalline. By applying the wave the development of advanced materials, not only during development properties of X-rays (keV energies as phases, but in the manufactured components as well. At Necsa, two generated in laboratory-based units, or high- modern neutron diffractometers equipped with in-situ sample power synchrotron facilities) and neutrons environments, as well as complementary X-ray diffraction instruments, (meV energies) having wavelengths of the are now available as User Facilities within the National System of same order as atomic spacings, i.e. 10-10 m, Innovation in support of the South African research and industrial unique information can be obtained. Their communities. Neutrons and X-rays, owing to their different interaction interaction with the crystalline ordering leads mechanisms with matter, offer complementary techniques for probing to constructive interference described by crystalline materials. Both techniques enable nondestructive investigation of phenomena such as chemical phase composition, residual stress, and Bragg’s law of diffraction. This leads to texture (preferred crystallite orientation). More specifically, the superior characteristic diffraction patterns for each penetration capabilities of thermal neutrons into most materials allows for phase that essentially are fingerprints of the the analysis of bulk or localized depth-resolved properties in a wide chemical phase content (Fourier transform of variety of materials and components. Materials that can be investigated the atomic arrangement). Both the peak include metals, alloys, composites, ceramics, and coated systems. In positions (corresponding to lattice plane particular, depth-resolved analyses using neutron diffraction complements spacings) and the relative intensities surface investigations using laboratory X-rays in many scientific and (corresponding to the atomic species and engineering topics. The diffraction techniques can add significant arrangement in the unit cell) in the diffraction downstream value to the anticipated nuclear industry development patterns are indicative of specific phases. X- activities. ray photons scatter through an electromagnetic Keywords interaction with the electron charge cloud of residual stress; crystallographic texture; chemical phase identification, the material, while neutrons are scattered by neutron and X-ray diffraction. interaction with the nuclei. In general the interaction strength of X-rays with matter is directly related to the atomic number of the Introduction materials being investigated, whereas neutron scattering lengths are approximately equal in Materials characterization is central in magnitude for most atoms. Apart from the understanding the relationship between the interaction strength differences, their structure, properties, and performance in order penetration depths are also dependent on the to engineer materials that fit the performance atomic species, as summarized in Table I for a criteria for specific applications. This is number of technologically important materials. conveniently represented in the form of a The different interaction mechanisms of tetrahedron, generally known as the material neutrons and X-rays with matter offer comple- science paradigm (Figure 1). mentary techniques for investigating The availability of advanced characteri- crystalline materials at the microstructural zation techniques is integral to the development of advanced metals, not only during development phases, but in the form of manufactured components. At Necsa (South African Nuclear Energy Corporation), modern X-ray and neutron diffraction instruments are now accessible to the South African research * Research and Development Division, The South and industrial communities. These facilities African Nuclear Energy Corporation SOC Ltd., enable nondestructive investigations of (Necsa), Pretoria, South Africa. materials and components that could add © The Southern African Institute of Mining and significant downstream value to the anticipated Metallurgy, 2015. ISSN 2225-6253. Paper received nuclear industry development activities. Aug. 2015 and revised paper received Aug. 2015.

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Neutrons of adequate flux for neutron diffraction applications are produced as a by-product of the fission of 235U in neutron research reactors (such as SAFARI-1 operated by Necsa in South Africa), or in accelerator-based facilities when very high-energy protons strike a target producing ‘spallation’. These high-energy (MeV) neutron products are then thermalized and filtered to the thermal energy range (meV). The neutron diffraction instruments at SAFARI-1 operate in constant wavelength mode where a highly monochromatic thermal neutron beam (<1% wavelength spread with wavelengths selectable in the range 1.0–2.0 Å, typically 25 Figure 1 – Material science paradigm meV energies) with a flux in the order of 106 neutrons per square centimetre per second is extracted from the fission energy spectrum and directed to a sample. Bragg’s law of level. Both techniques enable nondestructive investigation of elastic diffraction phenomena such as chemical phase composition, residual stress, and texture (preferred crystallite orientation). nλ = 2dhklsinθhkl Materials include metals, alloys, composites, ceramics, and coated systems. In particular, depth-resolved analysis using describes the geometrical condition for coherent diffraction that neutron diffraction complements surface investigations using can be measured to high precision on a diffractometer. In this laboratory X-rays in many scientific and engineering equation λ is the monochromatic wavelength in Å (10-10 m), disciplines. dhkl is the interatomic spacing between the parallel crystallite Neutrons complement X-rays due to the following planes of Å dimension (hkl refers to the Miller notation of the θ properties: crystal planes) and hkl is the angle at which the diffracted peak ➤ Neutrons are electrically neutral. This enables orders of is measured. In the angular dispersive operational mode of the magnitude deeper penetration of bulk SAFARI-1 neutron diffraction instruments, the wavelength is material/components: selected with the instrumental setup and thus accurately θ • Allows nondestructive bulk analysis known, with dhkl and hk being the only variables. The latter is •Ease of in situ experiments, e.g. variable measured experimentally to high precision with the diffraction temperature, pressure, magnetic field, chemical instrument, which comprises a goniometer for sample reaction, etc. positioning, and a detector that is precisely rotated around the ➤ Neutrons detect light atoms even in the presence of goniometer axis in the horizontal plane. The diffracted intensity heavy atoms (organic crystallography) – especially is measured with a high-sensitivity neutron detector that uses hydrogen. This property has been decisive in the 3He as ionization medium for the accurate measurement of the investigation of high-temperature superconductors diffraction angles from all coherently scattered Bragg peaks ➤ Neutrons distinguish atoms adjacent in the periodic from which respective values can be calculated. table, and even isotopes of the same element (changing As is the case with X-rays, all crystalline materials (even scattering picture without changing chemistry). This is chemically multi-phased materials) placed in the neutron beam particularly applicable to the transition metal series produces a diffraction pattern. This document provides an ➤ Neutrons have a magnetic moment. This enables overview of a subset of diffraction techniques that may be of nondestructive investigation of magnetic phenomena relevance to the materials beneficiation aims of the Advanced from direct observation of the reciprocal lattice. Materials Initiative.

Table I Neutron and X-ray scattering parameters for a number of elements that comprise the common engineering alloys. Half attenuation lengths correspond to the thickness of material that reduces the intensity by 50%. 1.5 Å X-rays correspond to Cu radiation (8 keV). 0.15 Å X- rays correspond to synchrotron radiation (100 keV)

Element Neutron coherent X-ray scattering Half attenuation lengths scattering length (bc) length 1.8 Å neutrons 1.5 Å X-rays 0.15 Å X-rays (f0 at sin θ/λ = 0.2 [mm] [μm] [mm ] [fm = 10-15 m ] [fm = 10-15 m]

Mg 5.375 0.246 43 100 24 Al 3.449 0.258 66 52 15 Ti -3.37 0.451 12 11 6 Fe 9.54 0.565 20 20 2.4 Co 2.49 0.594 2 14 2 Ni 10.3 0.624 3 16 1.8 Zr 7.16 0.884 24 8 1.1 Hf 7.77 1.715 1 3 0.1 W 4.86 1.762 5 2 0.1 U 8.417 2.172 9 1 0.2 ▲ 926 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Non-destructive characterization of materials and components

Applications calculated from the strain distributions. Residual stresses are Results from selected typical applications pursued at the a double-edged sword in material science applications. Necsa facilities are reported to provide an indication of the Compressive residual stresses are beneficial in applications potential value adding that these techniques could offer. where fatigue performance is required, being able to mitigate crack initiation and propagation. Tensile residual stresses are Chemical phase identification generally considered to be detrimental as they can lead to The most widespread use of powder (polycrystalline) crack initiation and propagation. Specifically, residual stress diffraction is in chemical phase analysis. This encompasses tailoring can render substantial improvements in the phase identification (search/match), investigation of high- optimization of component design. and low-temperature phases, solid solutions, and determi- X-ray and neutron radiation enable nondestructive nations of unit cell parameters of new materials. A multi- probing at different penetration depths (Hutchings et al., phase mixture, e.g. a soil sample, will show more than one 2005; Fitzpatrick and Lodini, 2003; Reimers et al., 2008; pattern superposed, allowing for determination of the relative Webster, 2000; Ohms et al., 2008; Engler and Randle, 2010) concentrations of phases in the mixture. (Table I). The stress tensors are obtained from the measured The powder diffraction method is well suited for charac- strain tensors and application of Hooke’s law of elasticity terization and identification of polycrystalline phases. This is done against the International Center Diffraction Data (ICDD) database, which contains in excess of 50 000 inorganic and 25 000 organic phases. Figure 2 shows a neutron diffraction where S1 and 1/2S2 are the diffraction elastic constants. pattern of a multi-phased sintered iron powder sample that Examples from recent investigations performed using was investigated to quantify the constituent phases. The neutrons are presented to illustrate potential applications. quantification was done with the Rietveld profile refinement Welded mild steel plate technique (Rietveld, 1969; Taylor, 2001), in which a theoretical line profile (a combination of the crystal system As part of an investigation into the influence of differential and instrumental model) is matched in a least-squares hardness between the weld metal and base metal on residual refinement approach to the measured data. Such quantitative stress and susceptibility to stress corrosion cracking, a phase analysis is now routinely used in industries ranging comprehensive two-dimensional (2D) mapping of the from cement manufacture to the oil industry and can provide residual stress field, through the 17 mm plate thickness and detection limits of 1 weight per cent (wt.%). across the weld was completed. Figure 3a indicates the 2D map of the longitudinal stress component, which is Residual stress maximally influenced due the differential longitudinal The total stress that a component experiences in practical use contraction. Values can be as large as the yield stress of the is the vector sum of the applied and residual stresses. The material. Figure 3b indicates the 2D map of this stress applied stresses result from the loading forces in use and can component after the sample had undergone a post-weld heat be calculated to high precision. Residual stresses, consisting treatment. This treatment has been very successful in having of locked-in stress that remains in a material or component completely relaxed the tensile stress values. Such analyses after the external forces that caused the stress have been are only possible owing to the penetrating capabilities of removed, are mostly only approximated qualitatively. These thermal neutrons and the nondestructive nature of the stresses can be introduced by any mechanical, chemical, or measurement technique. thermal process, such as machining, plating, and welding. Stress analysis by diffraction techniques is based on accurate Laser shock-peened aluminium plate measurement of lattice strain distributions (variations in Laser shock-peen (LSP) is an emerging cold work process interatomic spacing). Using Hooke’s law, stresses are used to induce compressive residual stresses in metallic

Figure 2 – Neutron powder diffraction results (measured diffracted intensities as function of diffraction angle) of a mixed-phase sintered iron sample, shown as dots (underneath the red solid line at positive intensities). The multiphase quantification results from a Rietveld analytical approach (red solid line at positive intensities) are indicated in the legend. The curve along the zero intensity level depicts the difference between the measured and analysed data and gives the goodness-of-fit. Vertical lines at negative intensities represent the Bragg peaks (hkl) corresponding to each phase

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 927 ▲ Non-destructive characterization of materials and components

Figure 3 – Two-dimensional maps of the longitudinal residual stress component in SA 516 Gr 70 pressure vessel steel welded plates: (a) as-welded condition, (b) post-weld heat-treated condition. The weld centre-line is at 0 mm

Figure 4 – Through-thickness stress profile in aluminium plate: (a) parent material, (b) laser shock-peened plate, (c) net contribution by laser shock-peen

Figure 5 – 2D map of the longitudinal residual stress component in aluminium laser beam welded plates: (a) as-welded condition, (b) post-weld-treated condition after laser shock-peen treatment. The weld centre-line is at 0 mm components to depths up to 1 mm. The purpose of this it introduces adverse tensile residual stresses as well as treatment is to reduce or remove tensile stresses in the component distortions. Figure 5 indicates 2D maps of the surface region to improve the fatigue performance. Figure 4 residual stress field measured through a 3.3 mm thick summarizes the through-thickness in-plane residual stress AA6056-T4 aluminium plate and across the weld. distribution in an as-rolled aluminium AA6056-T4 plate, Here again, the longitudinal stress component is most taken as the reference, and the stress distribution after laser dominant, as shown in Figure 5a. To improve this adverse shock-peen using a 3 GW laser beam. The net effect, purely stress condition, the weld region has been subjected to laser due to the peen action, was determined by subtracting the shock-peen treatment using a 3 GW laser pulse. Compared to reference values. the post-weld heat treatment results given previously, the laser treatment has completely altered the tensile stress so as Laser-welded aluminium with laser shock peen to be substantially compressive. treatment Laser beam welding is a newly established joining technology Additive-manufactured Ti-6Al-4V using no filler material. Similar to other welding techniques, In the selective laser beam melt (SLM) additive manufac- ▲ 928 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Non-destructive characterization of materials and components

Figure 6 – Measured 2D stress component mapped perpendicular to the build direction in a SLM produces Ti-6Al-4V sample: (a) sample geometry showing the ‘internal plane’ investigated nondestructively, (b) 2D residual stress map

turing process, a high-power laser beam is used to melt and Texture analysis deposit successive layers of powder to form complex three- Most solid-state matter has a polycrystalline structure dimensional metal parts. The highly localized heat input composed of a multitude of individual crystallites or grains. leads to large thermal gradients. This in turn produces The crystallographic orientation can have various complex residual stress states inside the part. Figure 6 shows arrangements, ranging from completely random to the a stress map measured in the centre of the sample perpen- development of preferred alignment. The significance of this dicular to the build direction in a Ti-6Al-4V sample. This texture lies in the corresponding anisotropy of many material reveals the existence of tensile residual stresses in the near- properties. The influence could be as much as 20–50% of the surface regions, while the central region of the sample is in property value. X-ray and neutron diffraction analysis compression. methods are well established, rendering results referred to as

Figure 7 – Pole figure representations of selected reflections from two ferritic stainless steel samples subjected to different heat treatments, measured with neutron diffraction

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 929 ▲ Non-destructive characterization of materials and components macro or bulk texture (Engler and Randle, 2010). Analyses REIMERS, W., PYZALLA, A.R., SCHREYER, A., and CLEMENS, H. (eds). 2008. can also be supplemented by methods whereby individual Neutrons and Synchrotron Radiation in Engineering Materials Science. orientations are measured by transmission or scanning Wiley-Vch Verlag GmbH. electron microscopy and are directly related to the microstructure, which has given rise to the term ‘microtexture’. RIETVELD, H.M. 1969. A profile refinement method for nuclear and magnetic The preferred orientation is usually described in terms of structures. Journal of Applied Crystallography, vol. 2, no. 2. pp. 65–71. pole figures. The inverse pole figure gives the probability of doi:10.1107/S0021889869006558 finding a given specimen direction parallel to crystal (unit cell) directions. By collecting data for several reflections and combining several pole figures, the complete orientation TARAN, Y., BALAGUROV, A., SABIROV, B., DAVYDOV, V., and VENTER, A. 2014. distribution function (ODF) of the crystallites within a single Neutron diffraction investigation of residual stresses induced in niobium- polycrystalline phase that makes up the material can be steel bilayer pipe manufactured by explosive welding. Materials Science Forum, vol. 768-769. pp. 697-704. determined (Engler and Randle, 2010). doi:10.4028/www.scientific.net/MSF.768-769.697 Shown in Figure 7 are bulk pole figures measured with neutron diffraction on ferritic stainless steel specimens that have been subjected to different heat-treatment parameters TAYLOR, J.C. 2001. Rietveld made easy: a practical guide to the understanding of after rolling. Significant texture development is evident, the method and successful phase quantifications. Sietronics Pty. Ltd., which will contribute to anisotropic properties that could lead Canberra. to physical phenomena such as earing, warping, and general dimensional changes and distortions.

TROIANO, E., UNDERWOOD, J.H., VENTER, A.M., IZZO, J.H., and NORRAY, J.M. 2012. Conclusions Finite element model to predict the reverse loading behavior of These examples serve to demonstrate the capabilities of the autofrettaged A723 and Hb7 cylinders, Journal of Pressure Vessels and X-ray and neutron diffraction techniques available at Necsa Piping, PVT-11-1204, 041012-1. for the characterization of new materials and advanced beneficiation techniques to enhance component performance in various applications. The vast field of applications can be VENTER, A.M., LUZIN V., and HATTINGH, D.G. 2014. Residual stresses associated reviewed in the extensive literature references provided, as with the production of coiled automotive springs. Material Science Forum, well as recent publications from Necsa (Taran et al., 2014; vol. 777. pp. 78–83. doi:10.4028/www.scientific.net/MSF.777.78 Troiano et al., 2012; Venter, Luzin, and Hattingh, 2014; Venter et al., 2008, 2013, 2014a, 2014b; Zhang et al., 2008). Due the nondestructive nature of the techniques, samples can VENTER, A.M., OLADIJO, O.P., CORNISH, L.A., and SACKS, N. 2014a. be investigated at various stages of their manufacture or Characterisation of the residual stresses in HVOF WC-Co coatings and utilization lifetimes. Substrates. Material Science Forum, vol. 768-769. pp. 280–285. doi:10.4028/www.scientific.net/MSF.768-769.280 Acknowledgments Specific acknowledgment is attributed to the various co- VENTER, A.M., LUZIN, V., OLADIJO, O.P., CORNISH, L.A., and SACKS, N. 2014b. Study workers from Necsa and academia and their willingness that of interactive stresses in thin WC-Co coating of thick mild steel substrate the results may be used in this document: Professor Johan de using high-precision neutron diffraction. Materials Science Forum, vol. Villiers (University of Pretoria), Deon de Beer (University of 772. pp. 161–165. doi:10.4028/www.scientific.net/MSF.772.161 Pretoria), Victoria Cain (University of Cape Town), Daniel Glaser (University of the Witwatersrand), and Deon Marais from Necsa. VENTER, A.M., OLADIJO, O.P., LUZIN, V., CORNISH, L.A., and SACKS, N. 2013. Performance characterization of metallic substrates coated by HVOF References WC–Co. Thin Solid Films, vol. 549. pp. 330–339.

ENGLER, O. and RANDLE, V. (eds). 2010. Introduction to Texture Analysis, Macrotexture, Microtexture, and Orientation Mapping. CRC Press, Taylor and Francis. VENTER, A.M., VAN DER WATT, M.W., WIMPORY, R.C., SCHNEIDER, R., MCGRATH, P.J., and TOPIC, M. 2008. Neutron strain investigations of laser bent samples. Materials Science Forum, vol. 571–572. pp. 63-68. FITZPATRICK, M.E. and LODINI, A. 2003. Analysis of Residual Stress by Diffraction using Neutron and Synchrotron Radiation. Taylor and Francis.

WEBSTER, G.A. (ed.). 2000. Neutron Diffraction Measurements of Residual HUTCHINGS, M.T., WITHERS, P.J., HOLDEN, T.M., and LORENTZEN, T. (eds). 2005 Stress in a Shrink-fit Ring and Plug. VAMAS Report No. 38. Introduction to the Characterization of Residual Stress by Neutron Diffraction. Taylor and Francis.

ZHANG, S.Y., VENTER, A.M., VORSTER, W.J.J., and KORSUNSKY, A.M. 2008. High OHMS, C., MARTINS, R.V., UCA, O, YOUTSOS, A.G., BOUCHARD, P.J., SMITH, M., energy synchrotron X-ray analysis of residual plastic strains induced in KEAVEY, M., BATE, S.K., GILLES, P., WIMPORY, R.C., and EDWARDS, L. 2008. Proceedings of 2008 ASME Pressure Vessels and Piping Conference (PVP shot peened steel plates. Journal of Strain Analysis, vol. 43, no. 4. pp. ◆ 2008), Chicago, Illinois, 27–31 July 2008. (PVP2008-61913). 229-241. ▲ 930 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a5 Fluorine: a key enabling element in the nuclear fuel cycle by P.L. Crouse

The aim of this paper is to give a brief Synopsis overview of the role of fluorine in the nuclear Fluorine – in the form of hydrofluoric acid, anhydrous hydrogen fluoride, fuel cycle, and to outline the current South elemental gaseous fluorine, fluoropolymers, volatile inorganic fluorides, African fluorochemical capability. and more – has played, and still plays, a major role in the nuclear industry. In order to enrich uranium, the metal has to be in the gaseous state. While Hydrogen fluoride and elemental fluorine more exotic methods are known, the standard and most cost-competitive The major barrier to entry into the fluoro- way of achieving this is by means of uranium hexafluoride (UF ). This 6 chemical industry is the ability to manufacture compound sublimates at low temperatures, and the vapour is enriched using centrifugal processes. The industrial preparation of uranium and, in general, to handle fluorine and its hexafluoride requires both elemental fluorine gas and anhydrous precursor, hydrogen fluoride. Both are extraor- hydrogen fluoride (HF). HF is prepared by the reaction of sulphuric acid dinarily difficult and dangerous substances with fluorspar (CaF2). Fluorine gas in turn is prepared by the electrolysis with which to work. of HF. Yellowcake is first converted to uranium tetrafluoride (UF4), using Hydrogen fluoride (HF) is produced by the HF, after which the compound is treated with fluorine to yield UF6. After reaction of fluorspar (calcium difluoride) with enrichment, the UF6 is reduced to UO2 for use in fuel elements in pellet sulphuric acid: form. South Africa has the largest reserves of fluorspar internationally, and [1] is the third largest producer after Mexico and China. Fluorine technology has many associated difficulties, because of the reactivity of fluorine and the toxicity of HF. The main barriers to entry into the fluorochemical The reaction is endothermic and reactors industry are thus the abilities to produce both HF and F2. Both these substances are produced locally, at the industrial scale, at Pelchem SOC are generally run at temperatures above 200°C. Ltd. Should South Africa contemplate developing its own nuclear fuel HF is a clear liquid, with a boiling point of cycle as part of the awaited new-build nuclear project, it will be imperative 19.6°C. It readily dissolves in water and, in its to leverage the existing skills with respect to fluorine technology, resident aqueous form, is known as hydrofluoric acid. at both Pelchem and Necsa, for this purpose. Unlike the other common mineral acids, it is This paper summarizes the fluorochemical skills developed locally over weak acid, thus does not readily deprotonate. the past several decades, and suggests strategies for maintaining the Hydrofluoric acid is distinguished by its ability technology base and developing it for the next generation of scientists and to dissolve glass, and as a consequence cannot engineers. be used in ordinary laboratory glassware. Keywords Both hydrogen fluoride and hydrofluoric flourine, hydrogen flouride, nuclear fuel cycle. acid are enormously hazardous substances (Bertolini, 1992; Smith, 2004). Upon contact, human skin is not immediately burnt by the Introduction action of the hydronium ion; rather, because of the small size of the HF molecule, it diffuses Fluorine chemistry has always had a very close through the skin and precipitates and relationship with the nuclear industry. inactivates biological calcium and magnesium Although the first mention of fluorspar was subcutaneously, causing tissue necrosis. The recorded in the sixteenth century, and the wounds are extremely painful, and difficult to fundamentals had been well-developed by the treat. In general the dead flesh has to be Second World War, it was only during the Manhattan Project (Banks et al., 1994), when almost unlimited funds were made available for the large-scale industrialization of fluorine and related compounds, that the technology took off. Expertise in both the chemistry and * Fluoro-Materials Group, Department of Chemical Engineering, University of Pretoria. the engineering aspects of fluorine is essential © The Southern African Institute of Mining and for the design and running of several of the Metallurgy, 2015. ISSN 2225-6253. Paper received unit processes in the nuclear fuel cycle. Aug. 2015 and revised paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 931 ▲ Fluorine: a key enabling element in the nuclear fuel cycle surgically removed, and a calcium gluconate solution injected two noble gases helium and neon (Cotton et al., 2007). In into the tissue underneath the wound to prevent deeper general the reactions of fluorine are highly exothermic, and diffusion of the HF. A typical HF wound is shown in Figure 1. because of its reactivity, materials of construction are of HF is also very corrosive, and materials selection is critical importance for safe operation. In general, expensive critically important in the development of an HF-related nickel-containing alloys are required. It should be noted that industrial process. HF sells for US$1–3 per kilogram, while F2 sells for Although HF was first synthesized in the eighteenth US$15–20 per kilogram. The high cost of fluorine can be century, and was known to contain an element, it was only in attributed to electrical requirements and the high the late nineteenth century that fluorine was first isolated. maintenance costs of the electrolysis cells. This was accomplished by Henri Moissan (Argawal, 2007). South Africa is richly endowed with fluorspar (Roskill, Because it is a weak acid, anhydrous HF (AHF) does not 2009). Relative production and reserve figures are given in conduct electrical current. Moissan’s discovery was that the Table I. At present South Africa is the third largest producer molten salt KF.xHF does indeed conduct electrical current and of the ore, and has the largest reserves. China and Mexico, can be electrolysed. As a melt, e.g. with x=2, it dissociates being closer to the larger international markets, are the two and forms the equilibrium: top producers.

[2] A brief history of fluorine chemistry and technology (Ameduri, 2011) Moissan (Figure 2) was awarded the Nobel Prize in A timeline for the major discoveries and developments in Chemistry in 1906. Since the first isolation of fluorine, the fluorine chemistry is listed below. electrolysis process has undergone a few technical changes, ➤ Georg Bauer first describes the use of fluorspar (CaF2) but has remained more or less static throughout the past few as a flux in 1530 – as a flux aiding the smelting of ores decades. Comprehensive descriptions of the technology can by German miners be found in Slesser and Schram (1951), Rudge (1971), and ➤ Heinrich Schwanhard finds, in 1670, that fluorspar Shia (2004). dissolved in acid and the solution could be used to etch Fluorine itself is the most reactive element in the periodic glass table, and reacts with all other elements, excluding only the ➤ From the 1720s, the effect on glass by adding sulphuric acid to fluorspar is studied ➤ Scheele (a Swedish scientist) ‘discovers’ fluoric acid (HF) in 1771 ➤ Several chemists try unsuccessfully to isolate fluorine, and several die of HF poisoning during separation experiments ➤ The French chemist Moissan is the first to isolate elemental fluorine gas. He is awarded the Nobel Prize in 1906 ➤ Swarts discovers the Cl/F exchange chemistry of SbF3 ➤ Midgley discovers Freons in 1928 ➤ In the 1930s General Motors begins using Freon-type fluorocarbons (CFCs) as replacement for hazardous materials e.g. NH3. CFCs also finds use in propellants and fire extinguishers ➤ 1938 Plunkett of DuPont discovers Teflon® ➤ WW II and uranium enrichment ➤ In 1947 Fowler discovers the CoF3 method of perfluori- Figure 1 – A hydrogen fluoride wound being treated nation

Figure 2 – Henri Moissan at his bench (left), and his original fluorine cell (right) ▲ 932 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Fluorine: a key enabling element in the nuclear fuel cycle

Table I International fluorspar reserves and production (Roskill, 2009)

World mine production (1000 t/a) World reserves (Mt) 2002 2003 Reserve Reserve base

China 2450 2450 China 21 110 Mexico 630 650 South Africa 41 80 South Africa 227 240 Mexico 32 40 Mongolia 200 190 Mongolia 12 16 Russia 200 200 Russia - 18 France 105 110 France 10 14 Kenya 98 100 Kenya 2 3 Morocco 95 95 Morocco NA NA Namibia 81 85 Namibia 3 5 Spain 130 135 Spain 6 8 USA 0 0 USA 0 0 Other countries 310 320 Other 110 180 Total 4550 4540 Spain 230 480

➤ 1949 sees Simons’ discovery of electrochemical fluori- Finally, the uranium tetrafluoride is fluorinated to the nation hexafluoride, using elemental fluorine gas: ➤ In the mid-1950s 3M invents ScotchguardTM ➤ Fried’s initial pioneering work in ‘medicinal fluorine [5] chemistry’ commences in 1954 ➤ Neil Bartlett’s discovery of noble gas chemistry in 1962 Enrichment now takes place, with fissionable U235 (XePtF6) separated from U238. This is normally done by centrifuge ➤ Rowland and Molina’s model for ozone depletion is technology. The enriched uranium is used as solid uranium published in 1974 dioxide. There is more than one way of carrying out the ➤ Hargreave’s ‘direct’ perfluorination discoveries in 1979 reduction. A standard method is in a hydrogen-fluorine flame ➤ Fluorocarbon gases start finding application in the reactor. The high temperature is needed to initiate the semiconductor industry in late 1980s reaction, given as ➤ 2003 sees O’Hagan’s isolation of first fluorinating enzyme [6] ➤ Fluorine has become ubiquitous in pharmaceuticals, and is essential in medicinal chemistry. Solid uranium dioxide powder is pressed into pellets At present there are more than 50 industrial producers of which are housed in Zircalloy tubes. These are bundled into HF (Roskill, 2009), the source precursor for all industrial fuel elements (in the case of pressurized-water reactors) fluorochemicals. Because of cost considerations, it is always ready for use. preferable in practice to employ a synthesis route that uses UF6 is itself a powerful oxidant and fluorinating gas. HF rather than F2. Materials of construction for plants handling UF6 are thus similar to those for plants that have fluorine as reactant or The nuclear fuel cycle product. Seals, filters, bearings, etc., are machined from various fluoropolymers, predominantly polytetrafluoroethylene As indicated in Figure 3, fluorine plays a critical role in (PTFE). Being fully fluorinated, PTFE is resistant to attack by several of the unit processes in the nuclear fuel cycle. fluorinating agents (Drobny, 2009; Ebnesajjad, 2013). Generally, the uranium arrives at the conversion plant in the For comprehensive information about the nuclear fuel form of U O . In order for isotope separation to be effected, 3 8 cycle, the reader is referred to Barré and Bauquis (2007), Kok uranium is required in the form of a gaseous compound. This (2009), Konings (2012), Tsoulfanidis (1996), Wilson compound is UF . U O is converted to UF in a three-step 6 3 8 6 (1996), and Yemelyanov (2011). process, each requiring its own plant. The oxide is first converted to UO2 in a hydrogen atmosphere, according to the reaction South Africa’s fluorochemical capability Highlights in the history of South African fluorochemical [3] technology platform (Naidoo, 2015) are listed below. ➤ Nuclear conversion starts at the Atomic Energy Note that U3O8 is a mixed valence oxide, thus reduction Corporation (AEC) (now the South African Nuclear of a single U+6 to U+4 takes place. Subsequent to this, the Energy Corporation, Necsa) in the 1960s uranium dioxide is converted into uranium tetrafluoride in ➤ Anhydrous hydrogen fluoride (AHF) and fluorine (F2) the substitution reaction: are required for uranium hexafluoride (UF6) production. AECI acquires the technology in the 1970s [4] ➤ AECI stops producing AHF in 1984 ➤ Necsa commissions an HF plant in 1985

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 933 ▲ Fluorine: a key enabling element in the nuclear fuel cycle

Figure 3 – The nuclear fuel cycle (World Nuclear Association, 2015)

➤ Industrialization and commercialization – 1992 ➤ The Fluorochemical Expansion Initiative (FEI) is adopted as a national initiative – 2006 ➤ Pelchem mandated to champion FEI by Necsa Board of Directors – 2007 ➤ Two research chairs are founded via the National Research Foundation’s SA Research Chairs Initiative (SARChI), one at the University of KwaZulu-Natal (UKZN) and one at the University of Pretoria (UP). At present Necsa and its wholly-owned subsidiary Pelchem SOC Ltd are the main centres of South African fluorochemical expertise, along with two university chairs, one at the University of KwaZulu-Natal (UKZN) and the other at the University of Pretoria (UP). Pelchem runs a commercial 5000 t/a HF plant. Figure 4 shows a photograph of the rotary kiln HF reactor. The company also operates some 20 fluorine electrolysis cells (Figure 5). Pelchem supplies a range of fluorine products to the local and international markets. These include xenon difluoride, nitrogen trifluoride, various organofluorine compounds, perfluorinated alkanes, and a variety of inorganic fluoride salts. Although a full fuel cycle existed on the Pelindaba site, it was abandoned in 1995. The technology in effect does not exist anymore, and if a new fuel cycle is to be established in South Africa, it will have to be a start-up from scratch rather than resuscitation of the old technology. Should this come to pass, our fluorochemical expertise, both existing and under development, will be invaluable if not essential. This will be Figure 4 – HF rotary kiln at Pelchem SOC Ltd ▲ 934 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Fluorine: a key enabling element in the nuclear fuel cycle

Figure 5 – Pelchem fluorine cells

the case whether the conversion is purchased off the shelf or References developed locally. The operation of a conversion plant AGARWAL, A. 2004. Nobel Prize Winners in Chemistry (1901-2002). APH requires detailed and extensive fluorochemical expertise. Publishing,, New Delhi, India.

AMEDURI, B. 2011. History of fluorine chemistry. Personal communication. The road ahead BANKS, R.E., SAMRT, B.E., and TATLOW, J.C. (eds). 1994. Organofluorine Since the inception of the Fluorochemical Expansion Initiative Chemistry; Principles and Commercial Applications. Springer Science, New (FEI), South Africa has made considerable inroads into the York. development of its fluorochemical capability. The Necsa BARRÉ, B. and BAUQUIS, P.R. 2007. Nuclear Power: Understanding the Future. research effort has been strengthened and expanded, the Ronald Hirlé, Strabourg and Paris. SARChI Chair at UKZN has been extremely productive BERTOLINI, J.C. 1992. Hydrofluoric acid: A review of toxicity. Journal of regarding research into various thermodynamic aspects of Emergency Medicine, vol. 10, no. 2. pp. 163-168. industrial fluorochemical processes, UP is developing a COTTON, F.A., WILKINSON, G., MURILLO, C.A., and BOCHMANN, M. 2007. Advanced fluoropolymer capability, and Pelchem SOC Ltd has commis- Inorganic Chemistry, Wiley, New Delhi, India. sioned a new pilot plant, known as the Multipurpose DROBNY, J.G. 2009. The Technology of Fluoropolymers, CRC Press, Boca Raton, Fluorination Pilot Plant (MFPP). The next few years are FL. critical. A number of things need to happen for the South EBNESAJJAD, S. 2013. Introduction to Fluoropolymers: Materials, Properties, African research and development effort to continue Applications. Elsevier, Amsterdam. progressing, and for the technology to be leveraged for the KOK, K.D. 2009. Nuclear Engineering Handbook. CRC Press, Boca Raton, FL. nuclear build plan. These are: ➤ KONINGS, J.M. 2012. Comprehensive Nuclear Materials Vol 2. Elsevier, The next phase of FEI needs to be commercially Amsterdam. successful, with visible new products ➤ NAIDOO, R. 2015. History of fluorine and HF at Necsa. Personal communication. The new cohort of scientists and engineers, trained via Pelchem SOC Ltd. funding by FEI, SARChI, and the Advanced Metals ROSKILL INFORMATION SERVICES. 2009. The Economics of Fluorspar. London. Initiative (AMI), have to find employment in the fluorine/nuclear industry RUDGE, A.T. 1971. Preparation of elemental fluorine by electrolysis, ➤ Introduction to Electrochemical Processes. Kuhn, T. (ed.). Elsevier, The decision about the nuclear new-build programme Amsterdam. Chapter 5. has to be taken sooner, rather than later. Within the next 5–7 years the majority of the last generation of SHIA, G. 2004. Fluorine. Kirk-Othmer Encyclopedia of Chemical Technology. Hoboken, NJ. Necsa senior scientists and engineers will have retired ➤ The current postgraduate training programme has to be SLESSER, C. and SCHRAM, S.B. (eds). 1951. Preparation, Properties, and Technology of Fluorine and Organic Fluoro Compounds. McGraw-Hill, accelerated, with Necsa senior scientists retained for New York. co-supervision of dissertations and theses. SMITH, R.A. 2004. Hydrogen fluoride. Kirk-Othmer Encyclopedia of Chemical Technology. Hoboken, NJ. Acknowledgements TSOULFANIDIS, N. 2010. The Nuclear Fuel Cycle. American Nuclear Society, The author acknowledges the South African National Scientific Publications, La Grange Park, IL. Research Foundation for financial support via the SARChI WILSON, P.D. 1996. The Nuclear Fuel Cycle: From Ore to Waste. Oxford programme, and Department of Science and Technology for Scientific Publications, Oxford. funding through their Fluorochemical Expansion Initiative. WORLD NUCLEAR ASSOCIATION. 2015. The nuclear fuel cycle.. http://www.world- Rajen Naidoo, current acting CEO of Pelchem, is thanked the nuclear.org/info/Nuclear-Fuel-Cycle/ [Accessed July 2015]. plant photographs, and Dr Johann Nel and Gerard Puts are YEMELYANOV, V.S. and YESVSTYUKHEN, A.I. 2011. The Metallurgy of Nuclear Fuel: acknowledged for comment and assistance with graphics, Properties and Principles of the Technology of Uranium, Thorium and respectively. Plutonium. Pergamon Press, Oxford. ◆

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Copyright © 2015, Weir Slurry Group, Inc.. All rights reserved. WARMAN is a registered trademark of Weir Minerals Australia Ltd. http://dx.doi.org/10.17159/2411-9717/2015/v115n10a6 Titanium and zirconium metal powder spheroidization by thermal plasma processes by H. Bissett, I.J. van der Walt, J.L. Havenga and J.T. Nel

and Revington, 2010). Direct selective laser Synopsis sintering (SLS) or, more specifically when considering the processing of metal, direct New technologies used to manufacture high-quality components, such as metal laser sintering (DMLS), are some of direct laser sintering, require spherical powders of a narrow particle size distribution as this affects the packing density and sintering mechanism. these technologies. Implants used in the The powder also has to be chemically pure as impurities such as H, O, C, N, medical industry to repair or replace bone and S causes brittleness, influence metal properties such as tensile structure must be strong, ductile, and biocom- strength, hardness, and ductility, and also increase surface tension during patible. Materials used for this purpose are processing. stainless steel, cobalt, chromium alloys, or Two new metal powder processes have been developed over the past pure titanium, with titanium (Ti) being the few years. Necsa produces zirconium powders via a plasma process for use material of choice (Taylor, 2004; Bertol et al., in the nuclear industry, and the CSIR produces titanium particles for use in 2010). Titanium is also used for applications the aerospace industry. in the aerospace industry due to its light Spheroidization and densification of these metal powders require re- weight, excellent corrosion resistance, high melting of irregular shaped particles at high temperature and solidifying strength, and attractive fracture behaviour (Li the resulting droplets by rapid quenching. Spherical metal powders can be obtained by various energy-intensive methods such as atomization of et al., 1997). In order to manufacture a high- molten metal at high temperatures or rotating electrode methods. Rapid quality component such as an implant, the heating and cooling, which prevents contamination of the powder by titanium powder used as feed material should impurities, is, however, difficult when using these methods for high- be dense and spherical. Powder particle size melting-point metals. For this reason plasma methods should be can affect the material spreading and the considered. sintering rate due to the fact that the shape, Thermal plasmas, characterized by their extremely high temperatures density, and size of the particles have an effect (3000–10 000 K) and rapid heating and cooling rates (approx. 106 K/s) on their packing density, sintering mechanism, under oxidizing, reducing, or inert conditions, are suitable for and the flowability of the powder during spheroidization of metal powders with relatively high melting points. feeding (Gignard, 1998, p. 34; Despa et al., Thermal plasmas for this purpose can be produced by direct current (DC) plasma arc torches or radio frequency (RF) inductively coupled discharges. 2011). In order to obtain chemically pure spheroidized powder, plasma gases such The chemical purity of the titanium powder as N2, H2, O2, and CH4 cannot be considered, while Ar, Ne, and He are during processing is also very important. suitable. Neon is, however, expensive, while helium ionizes easily and it is Surface oxidation should be prevented as this therefore difficult to obtain a thermal helium plasma at temperatures increases the surface tension, hindering higher than 3000 K. Therefore argon should be used as plasma gas. material from flowing during sintering. Residence times of particles in the plasma region range from 5–20 ms, but Oxidation also results in poor bonding this is usually sufficient as 7–8 ms is required for heating and melting of between sintered lines affecting the titanium or zirconium metal particles in the 30 μm size range at 3500 K. manufactured structures, while nitrides reduce In this study the melting and spheriodization of titanium powders was the material’s corrosion resistance (Gignard, investigated by DC non-transferred arc and RF induction plasma methods. 1998, p.34). The powders were characterized before and after plasma treatment by optical microscopy and scanning electron microscopy (SEM) to observe if South Africa has an opportunity to gain any melting or spheroidization had occurred. benefit from its abundant titanium-bearing mineral reserves through value beneficiation. Keywords The potential applications and markets for plasma, zirconium, titanium, spheroidization. titanium are in aerospace, the armaments

Introduction Contemporary product design processes have been affected by new technologies that assist * The South African Nuclear Energy Corporation SOC Ltd., (Necsa). the manufacturing sector to meet the specifi- © The Southern African Institute of Mining and cations of specialized components used in the Metallurgy, 2015. ISSN 2225-6253. Paper received aerospace and medical industries (Williams Aug. 2015 and revised paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 937 ▲ Titanium and Zirconium metal powder spheroidization by thermal plasma processes industries, naval applications, offshore oil industries, RF induction plasma architecture, and medicine. By establishing a local titanium In an induction torch, the energy coupling between the industry in South Africa, enterprise development across all electric generator and the plasma itself is done by a segments is expected (Van Vuuren, 2009). cylindrical coil (Gignard, 1998, p. 6). A typical induction set- A convenient way to produce spherical particles, and in up is illustrated in Figure 1. particular titanium particles, is to re melt irregularly shaped The ‘flame’ properties for a RF induction plasma are titanium particles at high temperatures and solidify the dependent on the power input, frequency, and the gas droplets by rapid quenching. This can be done by using pressure and composition. Less power is required at higher thermal plasmas, which are characterized by their extremely frequencies and lower gas pressures to maintain a plasma. high temperatures (3 000–10 000 K) and rapid heating and Comparing argon and hydrogen, less power is required to cooling rates (approx. 106 K/s) under oxidizing, reducing, or maintain an argon plasma due to the fact that argon ionizes inert conditions. Plasma is a partially or fully ionized gas in a more easily than hydrogen (Gignard, 1998, p. 9), while condition of quasi-neutrality. Thermal plasmas used for higher plasma gas temperatures can be obtained at higher particle spheroidization are generally produced by devices operating pressures. such as direct current (DC) arc plasma torches and radio Although various RF induction plasma systems are frequency (RF) inductively coupled discharges. Thermal available, several research studies have utilized those plasmas with temperatures as high as 2 × 104 K can be available from Tekna Systems Incorporated. Most of these obtained (Li and Ishigaki, 2001). studies have used the Tekna PL 50 torch, which can be This paper presents a short background on the DC plasma operated at various frequencies (0.3, 2, or 3 MHz) and power and RF induction plasma systems used for spheroidization, inputs (30, 40, 50 and 100 kW). For spheroidization, Ar or as well as some background on the properties of powder Ar/H2 is used as plasma gas (Jiang and Boulos, 2006; Li and before and after plasma treatment. A few experimental results Ishigaki, 2001; Gignard, 1998, pp. 35-42). The use of the relating to the spheroidisation of titanium powder using DC Tekna PL 035LS torch has also been reported (Károly and plasmas and RF induction plasmas at the South African Szépvölgyi, 2005). Nuclear Energy Corporation (Necsa) are discussed. DC plasma Thermal plasmas for spheroidization There are two types of DC torches, these being non- Although various plasma methods can be used, RF induction transferred and transferred arc torches. In non-transferred arc plasmas are the preferred method for the spheroidization and DC torches, the electrodes between which the arc is created densification of particles. This is due to the longer residence are inside the body of the torch itself. The plasma gas moves time in the plasma and also the lower possibility of contami- through the arc and is ionized to form the plasma jet. In nation caused by electrode erosion. The residence times Figure 2 a typical configuration for a non-transferred arc commonly employed for particle melting in RF plasmas range plasma torch is shown. from 5 to 20 ms (Gignard. 1998, p. 24). DC arc plasma Non-transferred arc plasma torches are also used in torches, although not regularly used for spheroidization, are plasma spraying. In this method a powder is introduced into also an option. Necsa has expertise in the design and the plasma jet. In this instance, however, spherical powders operation of these plasma torches. A short description of the with a narrow size distribution are used to spray dense, even RF and DC plasma torch methods is given, but the complete coatings onto substrates. The purpose of this method is not to system, including as particle feeding, quenching, and spheroidize the powder during the process, but the principle collection methods, will not be discussed. Information on the remains similar to spheroidization. In both instances the spheroidization of titanium metal powders by thermal powder will be quickly melted followed by rapid quenching. plasmas has not been found in the public domain, and literature available on spheroidization by DC thermal plasmas Powder properties is limited to in house designed plasma torches. Properties such as particle size, bulk density, morphology, impurities levels, etc. are pivotal in selecting a suitable powder to be used in SLS. Particle size affects the material spreading and the sintering rate. The morphology, density, and size of the particles have an effect on their packing

Figure 1 – RF induction plasma within the confinement tubes with an induction coil Figure 2 – A typical non-transferred arc plasma torch ▲ 938 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Titanium and Zirconium metal powder spheroidization by thermal plasma processes density, sintering mechanism, and the flowability of the Table I powder during feeding (Gignard, 1998, p. 34; Despa and Spheroidization of titanium powder in a Tekna Gheorghe, 2011). Thermal treatment of a powder results in argon plasma torch spherical particles with an increased bulk density and decreased in impurity levels, while improving the flowability Properties Before plasma treatment After plasma treatment of the powder. Tap density 1.0 g/cm3 2.6 g/cm3 Density and flowability of powders Production rate - 10 kg/h at 100 kW Bulk density refers to the density of an uncompacted mass of Particle size -140 + 400 mesh / -140 + 400 mesh / powder taking into account the interparticular voids. The tap 40 100 μm 40 100 μm Hall flow test No flow 37s/50 g density refers to density of a compacted mass of powder [O] ppm 2050 1800 where efforts have been made to eliminate the inter- [C] ppm 160 120 particular voids by repeated tapping of the powder. A [H] ppm 166 114 standardized Hall flow method is used to investigate the [N] ppm 90 90 [Cl] ppm 1200 360 apparent density and flow characteristics of free-flowing [Al] ppm 80 58 metal powders (ASTM. 2006). In Table I the increases in tap density and Hall flow rate are indicated for titanium powder treated by a Tekna torch operated at 50 kW (Boulos et al., occurred during thermal plasma treatment. Figure 3 shows 2011). SEM images of titanium powder before and after treatment by Impurities levels a Tekna torch operated at 50 kW using an argon plasma. The Titanium becomes brittle due to its affinity for oxygen, images clearly indicate that the rapid heating followed by nitrogen, and hydrogen. Contamination of titanium by air cooling resulted in spheroidization of the particles. (more specifically oxygen) and hydrogen is thus a problem during welding or sintering. This contamination causes an Experimental increase in tensile strength and hardness but reduces The titanium powder used in this study was obtained from ductility, resulting in crack formation. For welding, 0.3% the Council for Scientific and Industrial Research (CSIR). Due oxygen, 0.15% nitrogen, and 150 ppm hydrogen are seen as to the limited quantity of powder available, the as received the maximum tolerable limits. Surface discolouration gives a powder was characterized using SEM and an in-house good indication of the degree of atmospheric contamination. manufactured Hall flow meter according to ASTM (ASTM. The colour of the metal changes from silver to a light straw 2014). The amount of powder available for experimentation colour (shades of yellow), then dark straw, dark blue, light was not sufficient for post-plasma measurement of the blue, grey and finally white (TiO2) as contamination flowability using the Hall flow funnel, and for this reason, increases. The light and dark straw colours indicate light SEM and optical microscope images of the powder before and contamination, which is usually acceptable. Dark blue after plasma treatment were used to assess the success of the indicates more contamination, while light blue, grey and spheroidization. Similarly, no assessment of the impurity white indicate high levels of contamination (TWI, 2014). levels of the powder before and after plasma treatment was Impurity levels of elements such as O, C, H, N, and S can possible. be determined by using combustion methods or instrumental As-received powder characterization gas analysis (IGA). In Table I the decrease in impurity levels is clearly The as-received and plasma-treated powders were charac- indicated for titanium powder treated by a Tekna torch terized by SEM using a Quanta FEI 200 D instrument. Optical operated at 50 kW using an argon plasma. images of the powders were also obtained where possible. A Zeiss Discovery V20 stereo microscope was used for this Visual characterization of powders purpose, and the images recorded utilizing Zeiss Axiovision Particle morphology can be investigated by using optical software. microscopy or scanning electron microscopy (SEM) to Owing to the large variation in the sizes of the particles determine whether melting or spheroidization of the powder observed, the as-received powder was separated into four

Figure 3 – SEM images of titanium powder (A) before and (B) after plasma treatment

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 939 ▲ Titanium and Zirconium metal powder spheroidization by thermal plasma processes fractions using sieve mesh sizes of 75, 125, 250, and 425 (resistance) cannot be increased sufficiently. In order to μm. The four fractions were classified as < 75 μm, 75-125 obtain high enough temperatures using argon or helium, the μm, 125-250 μm and 250-425 μm. power supply would need to be operated at currents between Spheroidization by thermal plasmas 400 and 600 A, which fall outside the window of operation of the power supply used in this study. The plasma torch was Two high-temperature plasma systems were available for the mounted in a water-cooled reactor and operated at 150 A and experiments, namely a RF induction plasma and a non- 200 V. The plasma gas was fed through the torch at a rate of transferred arc DC plasma. 1.43 g/s. The plasma gas temperature was estimated by RF thermal plasma system calculating the plasma gas enthalpy (kJ/kg) and relating this The RF plasma system is shown in Figure 4. This system was to temperature. Once the plasma was stable, the as received used for conceptual work and therefore the exact conditions powder was fed near the tail flame (plasma jet) of the plasma (gas flow rates, powder flow rates, and input power) of the at a rate of 5 g/min. experiments are not given at this stage. The RF induction coil was connected to a 3 MHz power supply with a maximum Results and discussion power input of 10 kW. Inside the induction coil, two quartz As-received powder characterization tubes of different diameters were mounted in such a manner as to allow deionized cooling water to flow between them in Figure 5 shows an SEM image of the as-received powder. It is order to cool of the quartz tube. Above the quartz tube, a clear that the powder consisted of various particles sizes and hopper loaded with titanium powder was mounted and morphologies. Some particles appeared to be crystalline while connected to a 1 mm orifice. Once the system was leak-tight, others appeared amorphous. The particles had rough needle- the system was evacuated and a non-thermal plasma- like structures, porous round structures, and porous irregular initiated. Argon gas was slowly introduced. The plasma was structures. gradually changed from a non-thermal to a thermal plasma The SEM images of the four size fractions of the powder by increasing the operating pressure (increasing argon flow) are shown in Figure 6. Even after size classification, large and input power until a stable thermal plasma was variations in particle morphology and structure were maintained. At this stage the titanium powder was fed observed. through the thermal plasma. Once a sufficient amount of a RF thermal plasma treatment given powder fraction had been treated, the plasma was The temperature of the RF thermal argon plasma could not be extinguished and the reactor allowed to cool. The powder was calculated or determined at this stage. It is estimated that the then removed for imaging by SEM and optical microscopy. temperature of the plasma was near 3300 K (enthalpy of 1.6 The temperature of the plasma gas was not estimated due to MJ/kg for argon) due to the fact that particle melting was the complexity of the temperature determination method. All achieved. four powder fractions were treated. Figure 7 shows an SEM micrograph of the as-received DC thermal plasma system powder after RF thermal plasma treatment. It can be seen that A schematic of the non-transferred arc DC plasma system is the smaller particles were affected by the thermal treatment. shown in Figure 2. The system utilized a 30 kW DC power Some spherical particles were observed, as indicated by the supply with a maximum current input of 150 A. The system arrows. was operated at pressures slightly higher than atmospheric. Figure 8 shows a SEM image of the < 75 μm powder In order to obtain temperatures high enough for fraction after plasma treatment. In most instances it appeared spheroidization of titanium (approx. 3300 K) the system was as though spherical particles smaller than 150 μm were operated using a nitrogen plasma. The reason for this is that obtained, although some agglomeration occurred resulting in argon or helium ionize easily and therefore the voltage large irregularly-shaped particles.

Figure 4 – The RF induction plasma system Figure 5 – SEM image of the CSIR Ti metal powder ▲ 940 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Titanium and Zirconium metal powder spheroidization by thermal plasma processes

Figure 6 – SEM images of the < 75 μm, 75-125 μm, 125-250 μm and 250-425 μm powder fractions

Similar results were obtained using the 75125 μm DC thermal plasma treatment fraction, although slightly fewer spherical particles were The DC plasma torch efficiency was calculated to be 61%. observed. The large fractions (125-250 μm and 250-425 μm) Therefore 18.3 kW of the rated 30 kW was available for the were not affected by the plasma treatment. These results plasma gas, which at a nitrogen gas feed rate of 1.43 g/s suggest that particles smaller than 125 μm can be relates to a gas enthalpy of 12.8 MJ/kg for nitrogen. From spheroidized. Agglomeration of melted particles is a concern, thermodynamic data it is estimated that the gas temperature however, and for this reason a sheath gas is required. near the arc was approximately 6200 K. Further from the arc Agglomerate formation is possibly due to the collection of the temperature was approximately 3300 K (enthalpy of 3.7 fine particles near the quartz tube. The particles ‘roll’ down MJ/kg for nitrogen). For the DC plasma treatment, only the the tube surface, forming relatively large agglomerates. The sheath gas is not only necessary to prevent agglomeration as-received powder was used and not the separate fractions. from occurring, but will also prevent the collection of fine The estimated enthalpy cost per unit mass titanium for particles near the coil region of the quartz tube, a regularly spheroidization was 60 kWh/kg (216 MJ/kg). observed occurrence. Once titanium metal particles collect on Optical micrographs of the treated powder are shown in the surface of the quartz tube, induction heating of the Figure 9. A high degree of spheroidization is evident. Due to particles occurs, resulting in a reduction of the plasma gas the fact that nitrogen was used as plasma gas, surface temperature. contamination occurred, which is evident from the colours

Figure 7 – SEM image of the as-received powder after RF thermal Figure 8 – SEM image of the < 75 μm fraction after RF thermal plasma plasma treatment. The arrows indicate spheroidized particles treatment

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 941 ▲ Titanium and Zirconium metal powder spheroidization by thermal plasma processes

Figure 9 – Optical micrographs of the DC thermal plasma treated powder

observed in the images. The light and dark straw colours indicate light contamination, which is usually acceptable. Dark blue indicates more contamination, while light blue indicate high levels of contamination (TWI, 2014). A SEM image of the treated powder is shown in Figure 10. The variation in the contrast of the particles indicates various degrees of contamination by either nitrogen or oxygen. Again, a high degree of spheroidization is evident. A nitrogen plasma was used in this instance, and it is expected that an argon plasma will yield similar results with respect to spheroidization of the particles, but without surface contami- nation occurring.

Conclusions Available literature indicated that titanium metal particles can be spheroidized in a thermal plasma by re melting of Figure 10 – A SEM image of the DC thermal plasma treated powder irregularly shaped particles at high temperatures and solidifying the droplets by a rapid quench. Thermal plasma treatment of powders results in improved powder flow charac- teristics and an increased density of individual particles, pivotal BOULOS, M., HEBERLEIN, J., and FAUCHAIS, P. 2011. Thermal plasma processes, in selecting a suitable powder to be used in SLS or DMLS. fundamentals and applications. Short course presented at the University of Pretoria, 28 29 October. 2011. Pretoria, South Africa. The titanium powder received from the CSIR consisted of DESPA, V. and GHEORGHE, I.G. 2011. Study of selective laser sintering: a various particle sizes and morphologies, where some particles qualitative and objective approach. Scientific Bulletin of Valahia appeared to be crystalline and others amorphous. University Materials and Mechanics, vol. 6. pp. 150 –155.

In this study SEM images indicated that the powder could GIGNARD, N.M. 1998. Experimental optimization of the spheroidization of be spheroidized by an RF thermal plasma using argon gas metallic and ceramic powders with induction plasma. Thesis, National without increasing impurity levels significantly. Various Library of Canada, Sherbrooke, Quebec, Canada. . powder fractions were tested. Only the < 75 μm and 75-125 JIANG, X. and BOULOS, M. 2006. Induction plasma spheroidization of tungsten μm fractions could be spheroidized; the larger fractions (125- and molybdenum powder. Transactions of Nonferrous Metal Society of China, vol. 16. pp. 13 –17. 250 μm and 250-425 μm) were not affected by the plasma LI, Z. GOBBI, S.L. NORRIS, I. ZOLOTVSKY, S., and RICHTER, K.H. 1997. Laser welding treatment. techniques for titanium alloy sheet. Journal of Materials Processing SEM and optical microscope images also showed that Technology, vol. 65. pp. 203 – 208. titanium powder could be effectively spheroidized by a DC LI, Y. and ISHIGAKI, T. 2001. Spheroidization of titanium carbide powders by thermal plasma using nitrogen gas. In this instance the powder induction thermal plasma processing. Journal of the American Ceramic was used as-received and not sieved into fractions. The Society, vol. 84, no. 9. pp. 1929 –1936. particles changed colour, indicating surface contamination by TAYLOR, C.M. 2004. Direct laser sintering of stainless steels: thermal nitrogen. It is, however, expected that spheroidization without experiments and numerical modelling. PhD thesis, School of Mechanical Engineering, University of Leeds, UK. Chapter 2, p. 5. contamination will be possible by DC thermal plasma treatment TWI. http://www.twi-global.com/technical-knowledge/job-knowledge/welding- using argon gas at a gas temperature near 6200 K. of-titanium-and-its-alloys-part-1-109 [Accessed 18 July 2014].

VAN VUUREN, D.S. 2009. Titanium – an opportunity and challenge for South References Africa. Keynote address: 7th International Heavy Minerals Conference ‘What Next’, Champagne Sports Resort, Drakensberg, South Africa, 20–23 ASTM INTERNATIONAL. 2006. Standard test method for apparent density of free- September 2009. Southern African Institute of Mining and Metallurgy, flowing metal powders using the Hall flowmeter funnel. Designation: Johannesburg. pp. 1 –8. B212 99. West Conshohocken, PA. WILLIAMS, J.V. and REVINGTON, P.J. 2010. Novel use of an aerospace selective BERTOL, L.S., JÚNIOR, W.K., DA SILVA, F.P., and AUMUND, K.C. 2010. Technical laser sintering machine for rapid prototyping of an orbital blowout report Medical design: Direct metal laser sintering of Ti 6Al 4V. Materials fracture. International Association of Oral and Maxillofacial Surgeons, vol. and Design, vol. 31. pp. 3982 – 3988. 39. pp.182 – 184. ◆ ▲ 942 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a7 Plasma technology for the manufacturing of nuclear materials at Necsa by I.J. van der Walt, J.T. Nel and J.L. Havenga

Enriched UF6 is converted to UO2, which is pressed into pellets that are used in nuclear Synopsis power plants. Plasma technology could also be The development of plasma technology at Necsa started in the early 1980s, used in a modified and optimized conversion when the applicability of high-temperature plasmas in the nuclear fuel process. cycle was investigated. Since 1995, this plasma expertise has expanded to Zirconium alloys are used as nuclear fuel other industrial applications, for example mineral beneficiation, nanotech- cladding material. In the 1980s, UCOR nology, fluorocarbon production and waste treatment, all of which are also developed plasma process to produce of relevance to the nuclear industry. zirconium metal from ZrCl4, using hydrogen as Necsa has demonstrated the manufacture of plasma-dissociated a reductant (Nel et al., 2011). zircon, zirconium metal powder, carbon nanotubes, silicon carbide (SiC), With the termination of South Africa’s zirconium carbide (ZrC) and boron carbide (B4C) at the laboratory and pilot plant scale. These materials are commonly used in the nuclear nuclear programme in the 1990s, the South industry. Zirconium alloys are used as fuel cladding material for nuclear African government asked the then Atomic fuel assemblies. Energy Corporation (AEC) to investigate the Necsa manufactured the monomer tetrafluoroethylene (TFE), using possible re-aligning of already established 150 and 450 kW DC plasma systems, from which the polymer polytetraflu- plasma technology, expertise and equipment to oroethylene (PTFE) was synthesized for use in filters and as seals in investigate other applications for plasma nuclear plants. With the nuclear renaissance at hand, it was demonstrated technology in the non-nuclear industry. From that plasma technology can be used to produce hydrofluoric acid (HF), this the so-called Metox (Metal Oxide) which is used in the manufacture of fluorine gas (F ) for the production of 2 programme was born. This programme uranium hexafluoride (UF ) directly from the mineral calcium fluoride 6 investigated alternative means of zircon (CaF2) without the use of sulphuric acid as in the conventional process. The recovery of valuable uranium from nuclear waste such as filters, oils, beneficiation, as well as the production of and solids with plasma processes will also be discussed. The destruction of high-temperature-resistant ceramic low-level nuclear waste by a plasma gasification system can reduce the compounds that are used in the nuclear volume of this waste by several orders in magnitude, resulting in huge industry, such as alumina, zirconia, silica, savings in the storage costs. Another product of plasma technology is the silicon carbide, zirconium carbide, and boron encapsulation process for nuclear waste and the production of vitrified carbide. By changing the process parameters, product, which could be used as filler material for medium-level nuclear nanoparticles of the abovementioned products waste. were also synthesized in a plasma system. Keywords Polytetrafluoroethylene (PTFE) was plasma, zirconium, fluorocarbons, nuclear waste, nanomaterials. extensively used in nuclear plants as seals, in valves and pipes, as containers, filters, etc. PTFE filters used in the enrichment plant that are contaminated with uranium products can Introduction be destroyed with a plasma process and simultaneously, in the same process, the During the 1980s, a plasma research and uranium values can be recovered. development programme was launched at the There is no PTFE or fluoropolymer erstwhile Uranium Enrichment Corporation of production facility in South Africa and all these South Africa (UCOR) at Valindaba outside products are imported. In 1994, Necsa started Pretoria. The main purpose of this programme was to develop alternative and more economical processes for the processing and manufacturing of uranium compounds used in the nuclear fuel cycle. The conversion of UO2 (as received from uranium mines) to UF6, which is needed for enrichment, conven- * The South African Nuclear Energy Corporation SOC Ltd. (Necsa), Pretoria. tionally involves several laborious, expensive, © The Southern African Institute of Mining and chemical processing steps. Plasma processes Metallurgy, 2015. ISSN 2225-6253. Paper received can eliminate several of the intermediate steps. Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 943 ▲ Plasma technology for the manufacturing of nuclear materials at Necsa a project on the production of TFE (C2F4), the precursor for hydrofluorinated with hydrofluoric acid (HF(g)) to form UF4, PTFE, using a plasma process. a solid green compound, which is then fluorinated with South Africa is the world’s fourth-largest producer of fluorine gas to form UF6(g). This process is schematically calcium fluoride (fluorspar or CaF2), but very little or no local presented in Figure 2. Hydrofluoric acid is produced by the beneficiation takes place. In the nuclear fuel cycle, reaction of CaF2 with H2SO4 according to Equation [1]. F2 gas hydrofluoric acid is produced from CaF2 to manufacture is produced by electrolysis of KF.HF. fluorine gas, which is used to make UF for the enrichment 6 CaF + H SO → CaSO + 2HF(g) [1] purposes. It was proved that HF can be manufactured directly 2 2 4 4 in a plasma system instead of using the conventional The AEC proved in concept that a uranium oxide could be sulphuric acid route. The destruction, encapsulation, and directly fluorinated to UF6 with capacitive and inductive vitrification of nuclear waste can also be accomplished with a coupled non-thermal plasma according to Equation [2] plasma process. (Jones, Barcza, and Curr, 1993). This will eliminate the multi- Under the Advanced Metals Initiative (AMI) programme step process presented in Figure 2 with a major economic of the Department of Science and Technology (DST), a new advantage. process was developed to make nuclear-grade zirconium 2UO + 3CF → 2UF + 3CO [2] metal in a continuous plasma process. 3 4 6 2 The purpose of this paper is to discuss the abovemen- Uranium metal can also be fluorinated by F2 or CF4 or tioned processes in more detail. mixtures thereof in an inductive coupled non-thermal plasma (Equation [3]). Plasma technology in the nuclear fuel cycle U(m) + F2/CF4 → UF6 [3] The nuclear fuel cycle is schematically presented in Figure 1. The AEC (now Necsa), demonstrated the use of a direct The red arrows indicate where plasma technology can be current (DC) plasma process to convert UF to UF using used to replace conventional processes. 6 4 hydrogen or cracked ammonia as reductant. This process was UF is a gas that is used for the enrichment of uranium. 6 scaled to a production rate of 1 kg UF per hour in a Processed uranium ore as received from the mines may 4 continuous operation. It was estimated that the process could consist of various chemical compounds, depending on the be 30% more economical than the conventional process. The specific process that the mine uses. The conventional manufacture of depleted uranium metal from UF was also conversion route for UF production is a complex process 4 6 demonstrated, and a total of 13 kg of depleted uranium was involving multiple steps, each of which requires a separate, manufactured using this process by the AEC using a scaled- complete chemical plant. There are many minor variations on up laboratory system. Processes have been proposed by this process, but in general it can be described in as follows. Toumanov (2003) and Fridman (2008, p. 449) for the direct Uranium oxide, which can be UO or U O , (also referred 3 3 8 conversion of UF to uranium metal (Equation [4]). to as yellowcake) or ammonium diuranate (ADU or 6 → (NH4)2U2O7) is calcined to UO2 with hydrogen. The UO2 is UF6(gas) Usolid + 3F2(g) [4]

Figure 1 – A schematic of the nuclear fuel cycle. (World Nuclear University, n.d.) Red arrows indicate where plasma technology can be used ▲ 944 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Plasma technology for the manufacturing of nuclear materials at Necsa

Figure 2 – The conversion of uranium oxide to UF6

This process is highly endothermic and takes place at temperatures above 5000 K. High quenching rates in the order of 10-7 seconds are required to prevent the reverse reaction from taking place. Beta-UF5(s) is an unwanted reaction product that forms under certain conditions during the enrichment process. A high-frequency non-thermal plasma process was utilized for the in situ back-fluorination of beta-UF5 to UF6 using a mixture of CF4/argon or fluorine/argon. Unfortunately, further developments regarding the abovementioned nuclear plasma processes were halted when South Africa’s nuclear programme was terminated in 1993.

Nuclear nano-materials Plasma technology is an extremely useful and effective technique for the manufacture of nanoparticles, especially with regard to advanced ceramics (Fridman, 2008, pp 417–498, 566; Van der Walt, 2012; Boulos, Fauchais and Figure 3 – Schematic of a typical plasma system for the manufacturing Pfender, 1994). A typical plasma system for the manufac- of nano-powders turing of nanopowders is presented in Figure 3. The process involves the evaporation of reactants in the high-temperature environment of a plasma arc, followed by rapid quenching of the vapour in order to nucleate the particles. Ceramic powders Nano-sized ZrC can also be manufactured by a plasma such as carbides, nitrides and oxides have been synthesized process. in this way. Ceramic materials like SiC, ZrC, B4C and ZrO2, which are stable at high temperatures, have numerous Fluorocarbons in the nuclear industry applications in the nuclear industry, and will become even PTFE is one of the few fluorocarbons used in the nuclear more prominent in Generation IV high-temperature gas- industry. A sintered filter was produced from PTFE and used cooled nuclear reactors (HTGRs) (Konings et al., 2012; Kok, in the enrichment stages. 2009). A thermal chemical process was developed to produce CF4 Necsa has produced several of these nanomaterials. gas by reacting solid carbon with pure fluorine. A 99 % Nano-zirconia can be produced by the plasma dissociation of conversion was achieved, and the gas was used in a plasma zircon, followed by selective removal of the formed process to produce PTFE monomer. The chemical formulae amorphous silica by fluorinating agents. Necsa commissioned governing these reactions are presented in Equations [6] and and operated a pilot plant with a capacity of 100 kg/h to [7]: produce plasma-dissociated zircon and a 10 kg/h plant for C(s) + 2F2(g) → CF4 [6] the production of nano-silica. Boron carbide is often applied in nuclear technology as a CF4 + C(s) + Plasma → C2F4 [7] neutron shielding material and as control rods inside nuclear reactors. It has a high neutron absorbance cross-section for Various CF4 plasma plants for the production of TFE thermal neutrons of 755 barns and a high melting point of (C2F4) were constructed and commissioned at Necsa. A small 2723 K (Kirk-Othmer Encyclopedia of Chemical Technology; 30 kW laboratory system and 100, 150, and 450 kW pilot Lipp, 1965). Necsa produced nuclear-grade B4C powder in a systems were successfully developed and operated. A typical 30 kW non-transferred arc DC plasma system according to plasma TFE production system is presented in Figure 4. Equation [5]. BCl3 was evaporated at 60°C and fed into the As part of a DST-funded Fluorochemical Expansion plasma reactor at a rate of 120 litre per hour. The reaction Initiative, the C2F4 (TFE) was used in a suspension polymer- took place at about 2200°C, and nitrogen was used as a ization process to produce PTFE. After PTFE had been quench gas. A particle size of between 80 nm and 100 nm produced successfully, the second stage of the project to was obtained, depending on the plasma parameters. Nano- produce FEP was developed and proven. A schematic diagram sized B4C powder has enhanced compaction properties for the of the system is presented in Figure 5. Since neither of these production of B4C pellets. polymers are produced in South Africa, this an opportunity 4BCl3 + CH4 + 4H2 → B4C + 12HCl [5] for a new business.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 945 ▲ Plasma technology for the manufacturing of nuclear materials at Necsa

matrix. This aspect is crucial in the treatment and disposal of nuclear waste due to the long periods of storage required. With the addition of a glass/ceramic mixture that encapsulates the waste, the resistance to leaching and chemical attack can be optimized (Petijean et al., 2002). The matrix developed for the containment of radionuclides should take into account the following issues (Luckscheiter and Nesovic, 1996; Bisset and van der Walt, 2009): ➤ The waste must be accommodated within the matrix composition ➤ Waste loading ➤ Characteristics such as resistance to solubility, phase separation, and devitrification ➤ Long-term behaviour such as thermal stability, irradiation resistance and chemical and mechanical stability ➤ Process considerations such as melting temperature, viscosity and electrical conductivity. A thermal plasma is able to decompose various types of waste into a gas and a residue by exposing it to a very high Figure 4 – Schematic of a plasma TFE production process temperature. Because of the very high temperature in the plasma, no sorting is necessary, thus human exposure is minimized. This system also has the capability of containing the uranium contamination in the molten pool of metal (if the Plasma treatment of nuclear waste waste is in a drum) as well as in ceramics and non- Vitrification combustible materials. The cost implications will include a capital installation Vitrification is a recognized standard method of reducing the cost and an operational cost. If designed properly, a plasma environmental impact of harmful waste. In vitrification, the volume reduction system for nuclear waste can generate solid component of the treated waste is encapsulated by high- electricity to feed back into the process, reducing the temperature treatment. In recent years a glass formulation operating cost to a minimum. There is a case to be made has been added to the vitrification process that allows the whether the long-term cost of disposal at a waste repository tailoring of the waste matrix. Nuclear waste containment in like Vaalputs will be comparable to the costs of volume glass has several advantages: it guarantees the stability of reduction of nuclear waste by a plasma process. The the waste package over very long time periods (50–300 additional benefits of the plasma system are the production of years) and could reduce the waste volume. Disposal safety a vitrified product and the reduction of the waste volume criteria can be more easily met by adaptation of the waste (van der Walt and Rampersadh, 2011).

Figure 5 – Schematic presentation of the polymerization reactor ▲ 946 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Plasma technology for the manufacturing of nuclear materials at Necsa

Uranium recovery system using O2 and water gas. The HF is scrubbed by The recovery of valuable constituents such as uranium from means of a KOH scrubber. A schematic diagram of this historical and current waste streams is important and may system is presented in Figure 7. also be economically viable. Plasma technology could replace other competing processes for this purpose, and may have The AMI zirconium process important advantages over the conventional processes. In 2005, the DST initiated the Advanced Metals Initiative Conceptual research was done on one such waste stream, (AMI) programme and asked Necsa to coordinate the New namely the coated uranium kernels used in the pebble bed Metals Development Network, with the specific focus on the modular reactor (PBMR) project. manufacturing of nuclear-grade zirconium metal from the An alternative recovery method to the mechanical mineral zircon (Zr(Hf)SiO4). The request from DST was crushing of off-specification tri-structural-isotropic (TRISO) specifically to develop a new and more economical way to coated fuel microspheres was demonstrated. It was shown manufacture Zr metal using Necsa’s plasma and fluoro- that the inert SiC layer can be completely removed by etching chemical expertise and existing facilities. The conventional with the active fluorine species by an inductively coupled methods are described in detail in the scientific and patent radio-frequency CF4 glow-discharge impinging a static bed literature dating back to the early 1920s (Blumenthal, 1958). from the top, at a working pressure of 1 kPa (van der Walt et Significant industrial manufacturing of nuclear-grade al., 2011). The apparatus is shown in Figure 6. zirconium started in the late 1940s after World War II, when The recovery of uranium from contaminated PTFE filters the advantages of using zirconium in nuclear power plants was also investigated by Necsa. A process was developed that were realized. In general, most industrial manufacturing depolymerizes PTFE and thereby separates the matrix from processes for nuclear-grade zirconium use high-temperature the extremely valuable uranium. This system makes use of carbochlorination of zircon, selective separation of ZrCl4 and radio frequency (RF) induction heating to heat a reactor up to SiCl4, the separation of Zr and Hf by means of solvent the depolymerization temperature of PTFE. Inside the reactor extraction, distillation, selective crystallization, etc., and the PTFE is depolymerized and the product gas is cooled, eventually the reduction of purified ZrCl4 with magnesium separated from residual uranium particles, scrubbed and via the Kroll process (Equation [2]). These conventional evacuated to a destruction facility where fluorocarbons are processes can consist of between 16 and 18 individual steps, converted into CO2 and HF by means of a DC thermal plasma making the manufacturing of nuclear-grade zirconium one of the most expensive operations in the nuclear fuel cycle.

ZrCl4 + 2Mg → Zr + 2MgCl2 [8] Zircon is an extremely chemical inert mineral. However, activating zircon with a plasma process to produce plasma- dissociated zircon (PDZ) makes it very reactive, especially towards fluoride-containing compounds such as HF and ammonium bifluoride (ABF, NH4F.HF) (Nel et al., 2011a, 2011b). Necsa operated a semi-commercial PDZ plant from 1995 to 2003, which had a capacity of 100 kg/h using a 3 × 150 kW DC plasma torch configuration (Havenga and Nel, 2011). Necsa developed a plasma process for the manufac- turing of Zr metal powder from PDZ, making use of the reactivity of PDZ with ABF to produce ZrF4 (Makhofane et al., 2011). ZrF4 or ZrCl4 is reduced with magnesium in a in a 30 kW DC non-transferred arc plasma plasma process

Figure 6 – Non-thermal CF4 plasma system Figure 7 – Uranium recovery system with plasma waste destruction

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 947 ▲ Plasma technology for the manufacturing of nuclear materials at Necsa

Figure 8 –Block flow diagram of the AMI zirconium metal process

(similar to the Kroll process) (Nel et al., 2012). The whole developed plasma applications in the nuclear fuel cycle, process is generally referred to as the AMI zirconium metal including the manufacture of nuclear ceramics and nanopar- process (Nel et al., 2013). The plasma process has, however, ticles that are being (or can be) used in nuclear reactors, the additional advantage that it can be modified to make it a especially in high-temperature gas-cooled reactors. continuous process, unlike the conventional Kroll process. Fluoromonomers, which are used as precursors for many The AMI zirconium metal process making use of fluoropolymers, can also been made via plasma processes. plasma/fluoride technology consists of only six steps, and Fluoropolymers are extensively used as filters, seals, and offers the potential of huge cost savings in comparison with containers in the nuclear industry. Nuclear waste destruction the conventional processes. can also be accomplished by plasma processes. Many of these This process is schematically presented in Figure 8. processes have been developed to pilot scale, while others were developed only to the laboratory scale and proof-of- Conclusions concept. Necsa has patented plasma and fluoride processes Plasma processing has numerous potential applications in for the manufacturing of nuclear-grade zirconium metal nuclear science and technology and for the manufacturing of powder using the mineral zircon as starting material. ◆ nuclear materials. Over a period of three decades, Necsa has

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Maelgwyn Mineral Services Africa (Pty) Ltd Tel +27 (0)11 474 0705 Fax +27 (0)11 474 5580 Email [email protected] www.maelgwynafrica.com ▲ 948 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a8 Synthesis and deposition of silicon carbide nanopowders in a microwave- induced plasma operating at low to atmospheric pressures by J.H. van Laar*†,I.J. van der Walt*, H. Bissett*, G.J. Puts† and P.L. Crouse†

nanopowders in the nuclear industry, for Synopsis example, allows for fast recovery of irradiation-induced defects (Vaßen and Stöver, Silicon carbide nanopowders were produced using a microwave-induced 2001). plasma process operating at 15 kPa absolute and at atmospheric pressure. Various synthesis methods for SiC Methyltrichlorosilane (MTS) served as precursor, due to its advantageous stoichiometric silicon-to-carbon ratio of unity, allowing it to act as both nanoparticles have been reported in the carbon and silicon source. Argon served as carrier gas, and an additional literature. These include carbothermic hydrogen feed helped ensure a fully reducing reaction environment. The reduction (Dhage et al., 2009), pulsed laser parameters under investigation were the H2:MTS molar ratio and the total deposition (Kamlag et al., 2001), sol-gel enthalpy. The particle size distribution ranged from 20 nm upwards, as processes (Ahmed and El-Sheikh, 2009), determined by SEM and TEM micrographs. It was found that an increase in microwave heating (Satapathy et al., 2005; enthalpy and a higher H2:MTS ratio resulted in smaller SiC particle sizes. Moshtaghioun et al., 2012) as well as The adhesion of particles was a common occurrence during the process, numerous plasma techniques ranging from resulting in larger agglomerate sizes. SiC layers were deposited at 15 kPa inductive radio-frequency (RF) (Károly et al., with thicknesses ranging from 5.8 to 15 μm. 2011; Sachdev and Scheid, 2001) to Keywords microwave plasma-assisted chemical vapour silicon carbide, microwave plasma, methyltrichlorosilane, nanoparticles. deposition (Tang et al., 2008; Honda et al., 2003; Vennekamp et al., 2011). Previous work by the authors (Van Laar et al., 2015) reported the empirical study of the synthesis of SiC Introduction powders using a microwave-induced plasma at It is well known that methyltrichlorosilane atmospheric pressure only. (CH3SiCl3 or MTS) decomposes to form silicon In this work a microwave plasma system carbide (SiC) and hydrogen chloride (HCl) as similar to that reported in the earlier paper shown in Equation [1]. The reaction kinetics (Van Laar et al., 2015) was used. The study is and mechanisms for this reaction are expanded here to include initial results of SiC thoroughly reported in the literature (Sone et deposition at low pressure (15 kPa) onto a al., 2000; Papasouliotis and Sotirchos, 1999; quartz substrate. MTS was chosen as precursor Kaneko et al., 2002; Wang et al., 2011). due to its advantageous stoichiometric silicon- to-carbon ratio of unity, allowing it to act as CH SiCl (l) →SiC(s) + 3HCl(g) [1] 3 3 both a carbon and a silicon source. A Owing to its physical and mechanical combination of its liquid state, high vapour properties, SiC has found application in several pressure at standard conditions, and high areas of high-power, high-frequency, and volatility also allows for easier feeding of the high-temperature technology (Harris, 1995; MTS into the system at both low and high Saddow and Agarwal, 2004). Amongst its pressures. Hydrogen was fed into the reactor seemingly limitless applications, SiC is gaining to serve as a reductant for driving the increased attention as a nuclear ceramic due to its excellent mechanical properties and dimensional stability under irradiation (Katoh et al., 2012). SiC composites have also been proposed to act as fuel cladding in light water reactors as a direct replacement for zirconium alloy cladding (Katoh et al., 2012). * Applied Chemistry Division, South African Nuclear Conventional ceramics exhibit certain Energy Corporation (NECSA) SOC Ltd., Pelindaba, South Africa. drawbacks such as low ductility and high † Fluoro-Materials Group, University of Pretoria, brittleness; however, the fact that South Africa. nanopowders can overcome these © The Southern African Institute of Mining and shortcomings has encouraged the use of SiC in Metallurgy, 2015. ISSN 2225-6253. Paper received various fields. The application of SiC Aug. 2015 and revised paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 949 ▲ Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma conversion reaction. Argon served as the carrier gas. Changes waveguide and served as the reaction zone. Support flanges in total enthalpy and the H2:MTS molar ratio were studied at at the top and bottom of the tube also served as gas inlets. atmospheric pressure. Argon and hydrogen flow rates were controlled using calibrated Aalborg rotameters. This set-up is similar to the Experimental one used previously (Van Laar et al., 2015). An illustration of the physical layout of the reactor assembly is shown in Apparatus Figure 1, and a schematic representation of the flow path is The experimental set-up consisted of a 1.5 kW power supply shown in Figure 2. with a MOS-FET amplifier, a microwave generator operating At atmospheric pressures, argon was bubbled through the at 2.45 GHz, a water-cooled magnetron head, and rectangular MTS in order to help vapourize and carry the MTS through waveguide with sliding short and stub tuners. The quartz the system. The MTS-rich argon stream was then mixed with tube, with internal diameter of 2 cm and a length of 30 cm, hydrogen and argon streams in predetermined ratios before was positioned through the middle and perpendicular to the entering the reactor. At low pressures, the MTS was vapourized under vacuum, and the flow rate controlled using a valve placed before the entrance to the reactor. Calibration curves were reported in previous work (Van Laar et al., 2015). The exiting gas was passed through a CaCO3 scrubber to remove HCl and any unreacted MTS before entering the extraction system. A T-connection and valve assembly immediately following the scrubber allowed for shifting between vacuum and atmospheric operating pressures. Characterization of the particles was performed using the available equipment at the University of Pretoria. Particle size distribution was determined with a ZEN 3600 Malvern Zetasizer Nano System. Scanning electron microscopy (SEM) was performed on the particles using a high-resolution (6 Å) JEOL 6000 system, and transmission electron microscopy (TEM) was performed using the JEOL JEM2100F TEM (JEOL Japan). Powder X-ray diffraction was conducted with a PANalyticalX`pert Pro diffractometer using Co Kα radiation. The peaks were assigned using the databases supplied by the Figure 1 – Physical layout of the reactor assembly instrument manufacturer.

Figure 2 – Schematic diagram of the experimental set-up ▲ 950 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma

Figure 3 – Operation of the reactor at (a) atmospheric pressure and (b) low pressure

Method The average particle sizes, as determined from SEM and At the start of each experimental run the argon plasma was Zetasizer results, are also listed elsewhere (Van Laar et al., initiated under vacuum at approximately 15 kPa (Figure 3b), 2015). Particle agglomerates were a common occurrence. using an Alcatel 2010I duel-stage rotary vane pump. The Based on the results, the best-fitted model included deposition experiments were performed at this pressure. quadratic and 2-factor interaction terms. Analysis of Atmospheric pressure was reached by gradually increasing variance (ANOVA) results for the agglomerate sizes the operating pressure, at which point filamentation of the indicated that enthalpy had the greatest effect on the plasma structure occurred (Cardoso et al., 2009) as shown in agglomerate sizes, whereas the H2:MTS ratio was found to Figure 3a. be least significant (Van Laar et al., 2015). Hydrogen and MTS were then fed into the system. Zetasizer and SEM results were analysed using response Depending on the H2:MTS ratio, stable plasmas were possible surface analysis (RSA). The resulting surface contour plots at applied powers between 200 and 1500 W. High hydrogen are shown in Figure 4 and Figure 5 respectively. Figure 4 concentrations tended to extinguish the plasma, presumably suggests that particle size decreases with increasing due to increased energy demand for dissociation of the H2 enthalpy and H2:MTS ratio. Considering the effect of the bonds. During experiments at atmospheric pressure, a black two studied parameters separately, higher enthalpy values powder deposited on the inner walls of the quartz tubes. presumably allow de-agglomeration to occur more readily These tubes were removed after each experimental run and due to more energetic particle collisions. This results in flushed using distilled water. The water was collected and smaller particle sizes. Higher H2:MTS ratios result in a more evaporated in a drying oven at 80°C, after which the black reducing environment, leading to smaller particle sizes. powders were collected. Quartz tubes with smaller diameters When considering the effect of both parameters in (15 mm) were used as substrates for the deposition conjunction, two contrasting trends are seen. At high experiments. These smaller tubes were placed inside the enthalpy values (195–220 MJ/kg), particle sizes seem to larger tubes, and held in place using zirconium wool. increase with increasing H2:MTS ratios. This trend could possibly be attributed to the increasing energy demand for Results and discussion hydrogen dissociation with increasing H2:MTS ratio. At low Synthesis of SiC at atmospheric pressure enthalpy values (70 –120 MJ/kg), particle size decreases with increasing H :MTS ratio. This is in contrast to trends The results of the synthesis experiments are reported 2 seen at high enthalpy values. It is speculated that this trend elsewhere (Van Laar et al., 2015). The enthalpy values are at low enthalpy values occurs because the energy supply is those of the system enthalpy, H , which combines all the T not adequate to allow for hydrogen dissociation, increasing chemical species (argon, hydrogen, and MTS). These the reducing environment and allowing more available enthalpy values were determined by Equation [2]: energy for de-agglomeration. The particle size distributions determined by the [2] Zetasizer show that lower enthalpies produce larger agglomerates, but that the agglomeration process is much where Pf is the forwarded power, Pr is the reflected power, • more sensitive to H2:MTS ratios, with higher ratios and mT is the total mass flow rate. The enthalpy of the MTS, negatively influencing the agglomerate size. HMTS, was calculated from Equation [3]: Figure 6 and Figure 7 present SEM micrographs [3] showing agglomerate particle sizes down to approximately 50 nm.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 951 ▲ Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma

Figure 4 – Effect of total enthalpy and H2:MTS molar ratio on individual particle size Figure 7 – SEM image of SiC (2)

al., 2008), making it extremely difficult to obtain a reliable particle size distribution. From the images in Figure 6 and Figure 7, it can be seen that agglomerates exist in a wide variety of sizes. The smallest particle sizes to be confidently identified from SEM images were approximately 50 nm. The TEM micrographs are presented in Figure 8 and Figure 9. Figure 8 shows particle sizes down to approxi- mately 20 nm, and Figure 9 shows larger structures, presumably those of the agglomerates. X-ray diffraction results show diffraction peaks at positions indicative of beta (β) SiC (also referred to as cubic) shown in Figure 10. Also present are peaks indicative of silicon, although in much lower amounts than SiC. This could Figure 5 – Effect of total enthalpy and H2:MTS molar ratio on imply the presence of silicon in the SEM and TEM images. agglomerate size Other elements present within the plasma were verified using an optical emission spectrometer, reported previously (Van Laar et al., 2015). The majority of peaks were in good agreement with experimental values of elemental silicon, carbon, and argon (Kramida et al., 2014), suggestive of MTS decomposition in the plasma. The presence of elemental silicon in the gas phase as well as silicon in the product material suggests that the addition of hydrogen to the plasma drives the conversion reactions too far into the reductive regime. The equilibrium thermodynamics and formation mechanisms of Equation [1] have been reported in the literature (Deng et al., 2009). The thermodynamics software package TERRA (Trusov, 2006) was used to confirm the optimum conditions for the formation of β-SiC. The results predict that optimum yield for β-SiC formation is achieved at temperatures of around 1400 K. Microwave plasmas are known to achieve temperatures in the region of 1000–10 000 K (Tendero et al., 2006). The temperatures of the Figure 6 – SEM image of SiC (1) experiments reported in this paper were measured using a pyrometer, giving an indication of the SiC temperature inside the reactor. SiC is chemically inert, with excellent microwave Nanoparticles tend to form strongly bound agglomerates. absorption and heat-conducting properties (Isfort et al., Ultrasound sonification is often used to de-agglomerate 2011). This enabled a rough estimation of the temperatures nanoparticles; however, there is insufficient knowledge on inside the reactor, which were measured to range from 1100 the de-agglomeration of nanoparticulate systems (Sauter et to 1400 K. ▲ 952 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma

Figure 8 – TEM image of silicone carbide (1) Figure 9 – TEM image of silicone carbide (2)

Figure 10 – XRD spectrum of a product sample synthesized at an H2:MTS ratio of 4. SiC is found in the β phase

Deposition of SiC at 15 kPa Figure 13 shows the XRD analysis of the SiC layers. No The experimental results of the initial deposition experiments SiC peaks could be detected, indicating the presence of are shown in Table I. All five runs were performed at the amorphous SiC. The absence of peaks could also be attributed • to the small layer thickness and curvature of the quartz same conditions, namely P = 500 W, mMTS = 0.11 g/min, and substrates. The small peak broadening at 25° is believed to QAr = 100 sccm. The run times varied randomly, as the plasma extinguished at unpredictable times. The efficiency is be that of the amorphous quartz substrate. an indication of the percentage of MTS mass converted and Typical EDAX results are shown in Table II, indicating the deposited onto the quartz tubes. The layer thickness was relative amounts of carbon, oxygen, silicon, and chlorine calculated using deposition rate, run time, and assumed SiC found in the SiC layers. The presence of oxygen and chlorine density of 3.21 g/cm3 (Harris, 1995). is indicative of SiO2 and HCl. SEM images of deposited SiC layers are shown in Figure 11 and Figure 12. These images were taken from experiment Conclusions and recommendations number B, and show what appear to be layered structures. A microwave-induced plasma operating at atmospheric Figure 12 shows agglomerates and smaller structures pressure was used to decompose methyltrichlorosilane to down to approximately 50 nm, supporting the presence of form SiC nanoparticle agglomerates with sizes down to 20 nm nanoparticles. as determined from SEM and TEM micrographs.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 953 ▲ Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma

Table I Experimental results of SiC deposition

Exp no. Run time (s) Deposition rate (g/min) Efficiency (%) Layer thickness (μm)

A 320 0.013 12.16 8.352 B 900 0.009 18.48 15.12 C 360 0.008 9.692 5.927 D 160 0.018 9.898 5.854 E 180 0.016 12.08 5.776

Figure 11 – SEM image of SiC layers Figure 12 – SEM image of SiC layer showing smaller nanostructures

Figure 13 – XRD analysis of SiC layers, indicating an amorphous structure

Table II Agglomerates were a common occurrence, resulting in larger EDAX results indicating the relative elemental particle size distributions when measured by the Zetasizer. β abundances The presence of -SiC and silicon was confirmed using X-ray diffraction studies as well as optical spectroscopy. The first Element Weight % Atomic % Net int. Net int. error part of the investigation focused on the effect on particle size by varying the H2:MTS molar ratio and the total enthalpy. C 12.16 21.37 6.55 0.1 SEM, TEM, and Zetasizer results showed that higher enthalpy O 25.22 33.27 74.05 0.02 values and higher H :MTS ratios produced smaller particle Si 51.65 38.82 638.18 0.01 2 Cl 10.97 6.53 77.49 0.03 sizes. Furthermore, RSA results indicated that at high ▲ 954 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Synthesis and deposition of silicon carbide nanopowders in a microwave-induced plasma enthalpy values (195–220 MJ/kg), particle size increases with KRAMIDA, A., RALCHENKO, Y., READER, J., and NIST-ASD-TEAM. 2014. NIST Atomic Spectra Database (version 5.2). National Institute of Standards and increasing H2:MTS ratio. This trend could possibly be attributed to the increasing energy demand for hydrogen Technology, Gaithersburg, MD. http://physics.nist.gov/asd [Accessed 23 dissociation with increasing hydrogen amounts. October 2014]. The second part of the investigation included initial MOSHTAGHIOUN, B.M., POYATO, R., CUMBRERA, F.L., DE BERNARDI-MARTIN, S., experiments on SiC layer deposition. Operating and synthesis MONSHI, A., ABBASI, M.H., KARIMZADEH, F., and DOMINGUEZ-RODRIGUEZ, A. conditions remained similar to those in the first part of the 2012. Rapid carbothermic synthesis of silicon carbide nano powders by study, except the process was run at low pressure (15 kPa). using microwave heating. Journal of the European Ceramic Society, vol. Initial results indicate the successful deposition of SiC layers 32. pp. 1787–1794. with thicknesses ranging between 5.8 and 15 µm. XRD results indicate amorphous crystalline structures, although PAPASOULIOTIS, G.D. and SOTIRCHOS, S.V. 1999. Experimental study of further analysis is needed. atmospheric pressure chemical vapor deposition of silicon carbide from The use of microwave-induced plasma shows promise for methyltrichlorosilane. Journal of Materials Research, vol. 14. the deposition of SiC layers. Further experimental work is pp. 3397–3409. needed, however, in order to determine the quality and structure of these deposited layers. Pending these results, it SACHDEV, H. and SCHEID, P. 2001. Formation of silicon carbide and silicon is recommended that the use of microwave plasma systems carbonitride by RF-plasma CVD. Diamond and Related Materials, vol. 10. be considered for the encapsulation of nuclear waste. pp. 1160–1164.

Acknowledgements SADDOW, S. and AGARWAL, A. 2004. Advances in Silicon Carbide Processing and Applications. Artech House, London. The authors acknowledge the South African National

Research Foundation for financial support, and the South SATAPATHY, L.N., RAMESH, P.D., AGRAWAL, D., and ROY, R. 2005. Microwave African Nuclear Energy Corporation for use of their synthesis of phase-pure, fine silicon carbide powder. Materials Research equipment. Bulletin, vol. 40. pp. 1871–1882.

References SAUTER, C., EMIN, M.A., SCHUCHMANN, H.P., and TAVMAN, S. 2008. Influence of hydrostatic pressure and sound amplitude on the ultrasound induced AHMED, Y.M.Z. and EL-SHEIKH, S.M. 2009. Influence of the pH on the morphology of sol–gel-derived nanostructured SiC. Journal of the dispersion and de-agglomeration of nanoparticles. Ultrasonics American Ceramic Society, vol. 92. pp. 2724–2730. Sonochemistry, vol. 15. pp. 517–523.

CARDOSO, R.P., BELMONTE, T., NOËL, C., KOSIOR, F., and HENRION, G. 2009. SONE, H., KANEKO, T., and MIYAKAWA, N. 2000. In situ measurements and Filamentation in argon microwave plasma at atmospheric pressure. growth kinetics of silicon carbide chemical vapor deposition from methyl- Journal of Applied Physics, vol. 105. pp. 093306-093306-8. trichlorosilane. Journal of Crystal Growth, vol. 219. pp. 245–252. DENG, J., SU, K., WANG, X., ZENG, Q., CHENG, L., XU, Y., and ZHANG, L. 2009. Thermodynamics of the gas-phase reactions in chemical vapor deposition TANG, C.-J., FU, L.-S., FERNANDES, A.J.S., SOARES, M.J., CABRAL, G., NEVES, A.J., of silicon carbide with methyltrichlorosilane precursor. Theoretical and GRÁCIO, J. 2008. Simultaneous formation of silicon carbide and Chemistry Accounts, vol. 122. pp. 1–22. diamond on Si substrates by microwave plasma assisted chemical vapor DHAGE, S., LEE, H.-C., HASSAN, M.S., AKHTAR, M.S., KIM, C.-Y., SOHN, J.M., KIM, deposition. New Carbon Materials, vol. 23. pp. 250–258. K.-J., SHIN, H.-S., and YANG, O.B. 2009. Formation of SiC nanowhiskers by carbothermic reduction of silica with activated carbon. Materials Letters, TENDERO, C., TIXIER, C., TRISTANT, P., DESMAISON, J., and LEPRINCE, P. 2006. vol. 63. pp. 174–176. Atmospheric pressure plasmas: A review. Spectrochimica Acta Part B: HARRIS, G. 1995. Properties of Silicon Carbide, INSPEC, London. Atomic Spectroscopy, vol. 61. pp. 2–30.

HONDA, S.-I., BAEK, Y.-G., IKUNO, T., KOHARA, H., KATAYAMA, M., OURA, K., and RUSOV HIRAO, T. 2003. SiC nanofibers grown by high power microwave plasma T , B. 2006. Terra - Phase and Chemical Equilibrium of Multicomponent chemical vapor deposition. Applied Surface Science, vol. 212–213. Systems. Bauman Moscow State Technical University, Moscow. pp. 378–382. VAN LAAR, J.H., SLABBER, J.F.M., MEYER, J.P., VAN DER WALT, I.J., PUTS, G.J., and ISFORT, P., PENZKOFER, T., PFAFF, E., BRUNERS, P., GUNTHER, R.W., SCHMITZ-RODE, CROUSE, P.L. 2015. Microwave-plasma synthesis of nano-sized silicon T., and MAHNKEN, A.H. 2011. Silicon carbide as a heat-enhancing agent in carbide at atmospheric pressure. Ceramics International, vol. 41. microwave ablation: in vitro experiments. Cardiovasculat Intervent pp. 4326–4333. Radiology, vol. 34. pp. 833–8.

KAMLAG, Y., GOOSSENS, A., COLBECK, I., and SCHOONMAN, J. 2001. Laser CVD of VAßEN, R. and STÖVER, D. 2001. Processing and properties of nanophase non- cubic SiC nanocrystals. Applied Surface Science, vol. 184. pp. 118–122. oxide ceramics. Materials Science and Engineering: A, vol. 301. pp. 59–68. KANEKO, T., MIYAKAWA, N., SONE, H., and YAMAZAKI, H. 2002. Growth kinetics of

hydrogenated amorphous silicon carbide films by RF plasma-enhanced VENNEKAMP, M., BAUER, I., GROH, M., SPERLING, E., UEBERLEIN, S., MYNDYK, M., CVD using two kinds of source materials. Thin Solid Films, vol. 409. MÄDER, G., and KASKEL, S. 2011. Formation of SiC nanoparticles in an pp. 74–77. atmospheric microwave plasma. Beilstein Journal of Nanotechnology, vol. KÁROLY, Z., MOHAI, I., KLÉBERT, S., KESZLER, A., SAJÓ, I.E., and SZÉPVÖLGYI, J. 2. pp. 665–673. 2011. Synthesis of SiC powder by RF plasma technique. Powder

Technology, vol. 214. pp. 300-305. WANG, X., SU, K., DENG, J., LIU, Y., WANG, Y., ZENG, Q., CHENG, L., and ZHANG, L.

KATOH, Y., SNEAD, L., SZLUFARSKA, I., and WEBER, W. 2012. Radiation effects in 2011. Initial decomposition of methyltrichlorosilane in the chemical vapor deposition of silicon-carbide. Computational and Theoretical Chemistry, 9 SiC for nuclear structural applications. Current Opinion in Solid State and ◆ Materials Science, vol. 16. pp. 143–152. vol. 67. pp. 265–272.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 955 ▲ Subscription to the SAIMM Journal for the year January to December 2015 The SAIMM Journal all you need to know!

SUBSCRIBE • R1 800.00 NOW!! local • US$486.50 overseas per annum per subscription. & Less 15% discount to agents only & PRE-PAYMENT is required & The Journal is printed monthly & Surface mail postage included & ISSN 2225-6253 In the new world of work, we all have  to achieve more  at a faster pace  with less resources  against greater competition  in a global economy tougher than ever before The SAIMM Journal gives you the edge!  with cutting-edge research  new knowledge on old subjects  in-depth analysis For more information please contact: The Southern African Institute of Mining and Metallurgy

Kelly Matthee ¥ The Journal Subscription Department 27-11-834-1273/7 P O Box 61127 u MARSHALLTOWN, 2107 South Africa [email protected] or  [email protected] Website: http://www.saimm.co.za http://dx.doi.org/10.17159/2411-9717/2015/v115n10a9 A redetermination of the structure of tetraethylammonium mer- oxidotrichlorido(thenoyltrifluoroacetyl acetonato-κ2-O,O')niobate(V) by R. Koen, A. Roodt and H. Visser

of the electron-donating and -withdrawing groups of the β-diketone on the activity Synopsis induced and reaction mechanisms at these The tetraethylammonium salt of the mono-anionic coordination compound metal centres. mer-oxidotrichlorido(thenoyltrifluoroacetylacetonato-κ2O,O’)niobate(V) (NEt4)[NbOCl3](ttfa)], has been prepared under aerobic conditions and Experimental characterized by single-crystal X-ray diffraction. (NEt4)[NbOCl3](ttfa)] crystallized in the monoclinic P21/c space group, with a = 11.483 (5), b = Materials and instruments 12.563 (5), c = 17.110(5) Å, and β = 100.838 (5)º. The complex structure All chemicals used for the synthesis and exists in a 50.0% (NbA) : 50.0% (NbB) positional disorder ratio. preparation of the complexes were of analytical Keywords grade and were purchased from Sigma- Bidentate, niobium(V), disorder. Aldrich, South Africa. The 1H-, 13C-, and 19F FT-NMR solution- state spectra were recorded on a Bruker AVANCE II 600 MHz (1H: 600.28 MHz; 13C: 150.96 MHz; 19F: 564.83 MHz) or Bruker DPX Introduction 300 MHz (1H: 300.13 MHz; 13C: 75.47 MHz; Complexes containing organometallic type β- 19F: 282.40 MHz) nuclear magnetic resonance diketone ligands with O,O and O,N donor spectrometer using the appropriate deuterated atoms are used widely in coordination solvent. Chemical shifts, δ, are reported in chemistry and have applications in catalysis, ppm. 1H NMR spectra were referenced radiopharmaceuticals, etc. (Schutte et al., internally using residual protons in the 2011; Roodt, Visser and Brink, 2011; Brink et deuterated solvents, Acetonitrile-d3 [CD3CN = al., 2010; Otto et al., 1998). These ligand 1.94(5) ppm]. 13C NMR spectra were similarly systems are very useful because of their highly referenced internally to the solvent resonance coordinative nature, high solubility, and also [CD3CN = 1.39(4) ppm and 118.69(8) ppm] due to their ability to be functionalized with with values reported relative to tetramethyl- various substituents on the carbonyl carbon silane (δ 0.0 ppm). atoms (Schutte et al., 2011). The X-ray intensity data was collected on a Only a small number of β-diketonate Bruker X8 ApexII 4K Kappa CCD area detector ligands have successfully been coordinated to diffractometer, equipped with a graphite a Nb(V) metal centre, with only a select few monochromator and MoKα fine-focus sealed being characterized by X-ray crystallography tube (λ = 0.71069 Å, T = 100(2) K) operated (Viljoen, 2009; Bullen, Mason and Pauling, at 2.0 kW (50 kV, 40 mA). The initial unit cell 1965; Preuss, Lamding and Mueller-Becker, determinations and data collection were done 1994; Funk, 1934; Davies, Leedlam and Jones, by the SMART (Bruker, 1998a) software 1999; Allen, 2002. The synthesis and crystal package. The collected frames were integrated structure determination at room temperature of using a narrow-frame integration algorithm mer-oxidotrichlorido (thenoyltrifluoroacety- and reduced with the Bruker SAINT-Plus and 2 lacetonato-κ O,O’)niobate(V) XPREP software package (Bruker, 1999) {(NEt4)[NbOCl3(ttfa)]} was first reported by Daran et al. in 1979. Accordingly, for this current investigation, (NEt4)[NbOCl3(ttfa)] was re-evaluated at 100 K to determine if the structural features might change with temperature. * Department of Chemistry, University of the Free This study of this structure forms part of State, South Africa. an AMI-funded programme to better © The Southern African Institute of Mining and understand the solid-state characteristics of Metallurgy, 2015. ISSN 2225-6253. Paper received Ta(V) and Nb(V) complexes and the influences Aug. 2015 and revised paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 957 ▲ Tetraethylammonium mer-oxidotrichlorido(thenoyltrifluoroacetylacetonato-κ2-O,O')niobate(V) respectively. Analysis of the data showed no significant Results and discussion decay during the data collection. The data was corrected for The title compound was previously prepared by Daran et al. absorption effects using the multi-scan technique SADABS (1979), with X-ray diffraction data collected at room Bruker, 1998b) and the structure was solved by the direct temperature. For this study, the reaction was modified as methods package SIR97 (Altomare et al., 1999) and refined described above and the data collected at 100 K. using the WinGX (Farrugia, 1999) software incorporating The compound crystallizes in the monoclinic space group, SHELXL (Sheldrick, 1997). The final anisotropic full-matrix P21/c, with four molecules in the unit cell (Z = 4). The least-squares refinement was done on F2. The aromatic asymmetric unit consists of a Nb(V) metal centre surrounded protons were placed in geometrically idealized positions (C–H by three crystallographically independent chlorido groups = 0.93 – 0.98 Å) and constrained to ride on their parent (Cl1A – Cl3A), an oxido (O3A) and one O,O’-bonded thenoyl- atoms with Uiso(H) = 1.2Ueq(C). Non-hydrogen atoms were trifluoroacetonato ligand and a tetraethylammonium cation. A refined with anisotropic displacement parameters. The graphic illustration is shown in Figure 1. The complex molecule graphics were obtained with the DIAMOND program and the counter-ion are disordered over two positions in a 50 A B (Brandenburg, 2006) with 50% probability ellipsoids for all Nb : 50 Nb ratio as shown in Figure 2. General crystallo- non-hydrogen atoms. graphic details are presented in Table I, while selected bond lengths and bond angles are listed in Tables II and III respec- Synthesis of (NEt4)[NbOCl3(ttfa)] (1) tively. A distorted octahedral geometry is displayed for NbA and (Et4N)[NbCl6] (0.500 g, 1.147 mmol) was added to 4,4,4- trifluoro-1(2-thienyl)-1,3-butanedione (ttfaH) (0.327 g, NbB. The Nb-Claxial distances for NbA vary between 2.428(1) 1.147 mmol) in acetonitrile (20 cm3). The resulting solution and 2.507(1) Å and the Nb1A-Cl1A and Nb1A-O3A have was heated to 50ºC and stirred for 30 minutes. The excess distances of 2.390(1) and 1.733(1) Å, respectively. When comparing Nb1A-O1A and Nb1A-O2A bond lengths, distances solvent was evaporated and dark yellow plate-like crystals of of 2.357(1) vs. 2.037(1) Å are observed. This difference is due the title compound (1), suitable for X-ray diffraction, were to the effects of the electron withdrawing, CF substituent on obtained (0.565 g, yield 89 %). IR (ATR, cm-1): ν = 3 (Nb=O) the bidentate ligand backbone causing a longer NbA-O1A bond 952. 1H NMR (300.13 MHz, Acetonitrile-d , ppm): δ = 5.88 3 length. The trans Cl2-Nb1-Cl3 angle is 168.11(1)º, while the 13 (s, 1H), 6.83 (m, 1H), 6.93 (dd, 1H), 7.40 (dd, 1H). C NMR O1-Nb1-O2 bite angle is 79.65(1)º. A similar distortion is δ (75.47 MHz, Acetonitrile-d3, ppm): = 30.1, 118.8, 123.9, observed for NbB, with bond- lengths and angles in accordance 130.0, 137.4, 142.0, 182.3. 19F NMR (564.83 MHz, with NbA. Acetonitrile-d3, ppm): -73.37. The coordination plane constructed through Cl1A, Cl2A, Cl3A, and O2A, as indicated in Figure 3, shows the niobium metal centre is slightly shifted out of this plane by 0.2751(3) Å. The molecular packing within the unit cell is illustrated in Figure 4. The packing illustrates a ‘head-to-tail’ arrangement

Table I Crystallographic and refinement details of the title compound

Crystallographic data (NEt4)[NbOCl3(ttfa)]

Empirical formula C16H24C13F3N1Nb1O3S Formula weight 566.68 Crystal system, space group Monoclinic, P2 /c Figure 1 – Graphic illustration of the mer-[NbOCl3(ttfa)] anion showing 1 general numbering of atoms. Numbering of the disordered complex a, b, c (Å) 11.483(5), 12.563(5), 17.110(5) α, β, γ (°) 90.000, 100.838(5), 90.000 denoted by A = 50.0%. The displacement ellipsoids are drawn at 50% Volume (Å3), Z 2424.3(16), 4 probability displacement level. Hydrogen atoms and counter-ion Density (calculated) (mg/m3) 1.553 omitted for clarity Crystal colour, crystal size (mm3) Yellow, 0.99 × 0.79 × 0.31 Absorption coefficient μ (mm-1) 0.951 Theta range, F(000) 2.664 – 27.99°, 1144 Index ranges -16<=h<=15, -15<=k<=15, -22<=l<=22 Reflections collected, 5834, 5209, 0.0574 independent reflections, Rint Completeness (%) 99.6 Max. and min. transmission 0.750 and 0.412 Data, restraints, parameters 5834, 894, 501 Goodness-of-fit on F2 1.0980 Final R indices [I>2sigma(I)] R1 = 0.0260 wR2 = 0.0740 Figure 2 – Graphic illustration of the mer-[NbOCl3(ttfa)] anion illustrating R indices (all data) R1 = 0.0303 the disorder in an overlay. (red) NbA = 50.0%; (blue) NbB = 50.0%. wR2 = 0.0796 -3 Hydrogen atoms and counter-ion omitted for clarity Largest diff. peak and hole (e.Å ) 0.58, -0.58 ▲ 958 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Tetraethylammonium mer-oxidotrichlorido(thenoyltrifluoroacetylacetonato-κ2-O,O')niobate(V)

Table II

Selected bond lengths of the two disordered parts in the title compound mer-[NbOCl3(ttfa)] anion, denoted by NbA and NbB

NbA NbB Atoms Bond length (Å) Atoms Bond length (Å)

Nb1A-Cl1A 2.390(1) Nb1B-Cl1B 2.389(1) Nb1A-Cl2A 2.507(1) Nb1B-Cl2B 2.373(1) Nb1A-Cl3A 2.428(1) Nb1B-Cl3B 2.339(1) Nb1A-O1A 2.357(1) Nb1B-O1B 2.254(1) Nb1A-O2A 2.037(1) Nb1B-O2B 2.095(1) Nb1A-O3A 1.733(1) Nb1B-O3B 1.745(1)

Table III

Selected bond angles of the two disordered parts in the title compound mer-[NbOCl3(ttfa)] anion, denoted by NbA and NbB

NbA NbB Atoms Bond angle (º) Atoms Bond angle (º)

O1A-Nb1A-O2A 79.65(1) O1B-Nb1B-O2B 76.82(1) Cl1A-Nb1A-O3A 96.28(1) Cl1B-Nb1B-O3B 103.50(2) O1A-Nb1A-O3A 171.69(2) O1B-Nb1B-O3B 166.36(2) Cl1A-Nb1A-O2A 163.58(1) Cl1B-Nb1B-O2B 166.38(1) Cl2A-Nb1A-Cl3A 168.11(1) Cl2B-Nb1B-Cl3B 160.12(1) C2A-C3A-C4A 120.60(2) C2B-C3B-C4B 120.99(2)

when viewed along the c-axis. There are no classical hydrogen bonds or interactions observed in this structure. In Figure 5, two coordination planes are illustrated; the first one constructed through O1A, O2A, and Nb1A, and the second through O1A, C2A, C3A, C4A, and O2. The angle between planes revealed the slight, 1.677º out-of-plane bend of the coordinated O,O’-bonded thenoyltrifluoroacetonato ligand. This also contributes to the distorted octahedral geometry. The crystal structure determination of the published complex was performed at room temperature (298 K), while the synthesized analogue (1) was determined at 100(2) K. Data obtained for the title compound (1) correlates well with Figure 3 – Side view of the axial plane illustrating the out-of-plane the previously published structure (Daran et al. 1979). The B distortion. Displacement ellipsoids are drawn at the 50% probability disordered part denoted by Nb differs less from the displacement level published structure and is probably a better representation of the anionic complex. Table IV illustrates a comparison between bond angles and distances of the published structure vs. the structure collected at 100K. The greatest difference

Figure 5 – Illustration of the out-of-plane bend of the coordinated O,O'- Figure 4 – Packing of (NEt4)[NbOCl3(ttfa)] (NbA) along the c-axis. bonded thenoyltrifluoroacetonato ligand. Displacement ellipsoids are Displacement ellipsoids are drawn at the 50% probability level drawn at the 50% probability level

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 959 ▲ Tetraethylammonium mer-oxidotrichlorido(thenoyltrifluoroacetylacetonato-κ2-O,O')niobate(V)

Table IV

Comparison of bond lengths and bond angles of (NEt4)[NbOCl3(ttfa)] (Bullen, Mason, and Pauling, 1965) collected at room temperature vs. (NEt4)[NbOCl3(ttfa)] at 100 K

(NEt4)[NbOCl3(ttfa)]-NbA (100 K) (NEt4)[NbOCl3(ttfa)] (298 K) Atoms Bond length (Å) Atoms Bond length (Å) NbA NbB

Nb1-Cl1 2.390(1) 2.389(1) Nb1-Cl1 2.367(1) Nb1-Cl2 2.507(1) 2.373(1) Nb1-Cl2 2.365(2) Nb1-Cl3 2.428(1) 2.339(1) Nb1-Cl3 2.422(2) Nb1-O1 2.357(1) 2.254(1) Nb1-O1 2.285(3) Nb1-O2 2.037(1) 2.095(1) Nb1-O2 2.044(3) Nb1-O3 1.733(1) 1.745(1) Nb1-O3 1.704(3) Atoms Bond angle (º) Atoms Bond angle (º) NbA NbB O1-Nb1-O2 79.65(1) 76.82(1) O1-Nb1-O2 78.7(1) Cl2-Nb1-Cl3 168.11(1) 160.12(1) Cl2-Nb1-Cl3 165.0(1) C2-C3-C4 120.60(2) 120.99(2) C2-C3-C4 122.9(1)

between the two complexes is the positional disorder BRUKER AXS Inc. 1998a. Bruker SMART-NT Version 5.050, Area-Detector observed in the newly synthesized product. Software Package. Madison, WI, USA.

BRUKER AXS Inc. 1998b. Bruker SADABS Version 2004/1, Area Detector Conclusions Absorption Correction Software. Madison, WI, USA.

A simplified method to obtain (NEt4)[NbOCl3(ttfa)] in aerobic BRUKER AXS Inc. 1999. Bruker SAINT-Plus Version 6.02 (including XPREP),. conditions is reported. This highlights the misrepresentation Area-Detector Integration Software. Madison, WI, USA. of the ‘extreme sensitivity’ of niobium(V) complexes to air BULLEN, G.J., MASON, R., and PAULING, P. 1965. The crystal and molecular and water. Clearly, the exclusion of oxygen is not that structure of bis(acetylacetonato)nickel (II). Inorganic Chemistry, vol. 4. important, while the exclusion of water is, since it will pp. 456–462. increase hydrolysis and thus the loss of chloride in favour of DARAN, J., JEANIN, Y., GUERCHAIS, J.E., and KERGOAT, R. 1979. The crystal aqua, hydroxide, or oxo coordination. The crystallographic structure of tetraethylammonium trichlorooxo(1,1,1-trifluoro-4-thenoyl- investigation revealed that this structure exhibited a 50:50 2,4-butanedionato)niobate(V). Inorganica Chimica Acta, vol. 33. positional disorder. The electron withdrawing effects of the pp. 81–86. CF substituent on the bidentate ligand backbone is 3 DAVIES, H.O., LEEDLAM, T.J., and JONES, A.C. 1999. Some tantalum(V) β- illustrated by the longer Nb-O bonds of Nb1A-O1A and diketonate and tantalum(V) aminoalcoholate derivatives potentially Nb1B-O1B vs. Nb1A-O2A and Nb1B-O2B. All bond lengths important in the deposition of tantalum-containing materials. Polyhedron, and angles of the complex were found to be in accordance vol. 18. pp. 3165–3174. with similar structures in the literature. FARRUGIA, L.J. 1999. WinGX suite for small-molecule single-crystal crystal- lography. Journal of Applied Crystallography, vol. 32. pp. 837–838. Acknowledgements FUNK, H. 1934. Über die einwirkung von niob- und tantalpentachlorid auf Financial assistance from the Advanced Metals Initiative organische verbindungen (IV. Mitteil.). Berichte der Deutschen (AMI) of the Department of Science and Technology (DST) of Chemischen Gesellschaft, vol. 62. pp. 1801–1808. South Africa, through the New Metals Development Network OTTO, S., ROODT, A., SWARTS, J.C., and ERASMUS, J.C. 1998. Electron density (NMDN) managed by the South African Nuclear Energy manipulation in rhodium(I) phosphine complexes: structure of acetylacet- Corporation Limited (Necsa) is gratefully acknowledged. onatocarbonylferrocenyl diphenylphosphinerhodium(I). Polyhedron, vol. Gratitude is also expressed towards SASOL, PETLabs 17. pp. 2447–2453. Pharmaceuticals, and the University of the Free State for PREUSS, F., LAMDING, G., and MUELLER-BECKER, S. 1994. Oxo- und thiotan- financial support of this research initiative outputs. tantal(V)-verbindungen: Syntese von TaOX, und TaSX (X = OR, SR), Z. Furthermore, this work is based on research supported in Zeitschrift für Anorganische und allgemeine Chemie, vol. 620. part by the National Research Foundation of South Africa pp. 1812–1820. (UIDs 71836 and 84913). ROODT, A., VISSER, H.G., and BRINK, A. 2011. Structure/reactivity relationships and mechanisms from X-ray data and spectroscopic kinetic analysis. References Crystallography Reviews, vol. 17. pp. 241–280. ALLEN, F.H. 2002. Cambridge Structural Database (CSD) Version 5.35, SCHUTTE, M., KEMP, G., VISSER, H.G., and ROODT, A. 2011. Tuning the reactivity November 2013 update. Acta Crystallographica, vol. B58. pp. 380–388. in classic low-spin d(6) rhenium(I) tricarbonyl radiopharmaceutical ALTOMARE, A., BURLA, M.C., CAMALLI, M., CASCARANO, G.L., GIACOVAZZO, C., synthon by selective bidentate ligand variation (L,L'-Bid, L,L ' = N,N', N,O GUAGLIARDI, A., MOLITERNI, A.G.G., POLIDORI, G., and SPAGNA, R. 1999. & O,O' donor atom sets) in fac-[Re(CO) (L,L'-Bid)(MeOH)]n complexes. SIR97: a new tool for crystal structure determination and refinement 3 Journal of Applied Crystallography, vol. 32. pp. 115-119. Inorganic Chemistry, vol. 50. pp. 12486–12498.

BRANDENBURG, K. 2006. DIAMOND, Release 3.0e. Crystal Impact GbR, Germany. SHELDRICK, G.M. 1997. SHELXL97. Program for crystal structure refinement. BRINK, A., ROODT, A., STEYL, G., and VISSER, H.G. 2010. Steric vs. electronic University of Göttingen, Germany. anomaly observed from iodomethane oxidative addition to tertiary phosphine modified rhodium(I) acetylacetonato complexes following VILJOEN, J.A. 2009. Speciation and interconversion mechanism of mixed halo O,O’- and N,O-bidentate ligand complexes of hafnium. MSc dissertation, progressive phenyl replacement by cyclohexyl [PR3 = PPh3, PPh2Cy, ◆ PPhCy2 and PCy3]. Dalton Transactions, vol. 39. pp. 5572–5578. Universtry of the Free State. 132 pp. ▲ 960 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a10 A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form by C.J. Postma*, H.F. Niemand* and P.L. Crouse†

In this paper, a sublimation model is Synopsis developed to predict the sublimation rates of The separation of zirconium and hafnium is essential in the nuclear both ZrF4 and HfF4 in an inert gas. These rates industry, since zirconium alloys for this application require hafnium are used to calculate the partial pressures of concentrations of less than 100 ppm. The separation is, however, very the two fluorides exiting a sublimer and difficult due to the numerous similarities in the chemical and physical entering a desublimer where the one properties of these two elements. tetrafluoride is selectively removed from the Traditional methods for separation of zirconium and hafnium rely gas stream, thereby separating the two predominantly on wet chemical techniques, e.g. solvent extraction. In tetrafluorides. contrast to the traditional aqueous chloride systems, the AMI zirconium metal process developed by Necsa focuses on dry fluoride-based processes. Literature survey and theoretical Dry processes have the advantage of producing much less hazardous chemical waste. discussion In the proposed AMI process, separation is effected by selective Sublimation methods used for the separation sublimation of the two tetrafluorides in an inert atmosphere under of Zr and Hf are reported in literature, but controlled conditions, and subsequent selective desublimation. Estimates these methods are all under vacuum are made for the sublimation rates of the two tetrafluorides based on the conditions (Monnahela et al., 2013; Solov’ev equilibrium vapour pressures. A sublimation model, based on the and Malyutina, 2002a). sublimation rates, was developed to determine if the concept of separation Sublimation is, however, a general method by sublimation and subsequent desublimation is theoretically possible. used for the purification of ZrF4 by removing Keywords most trace elements, e.g. Fe, Co, Ni, and Cu sublimation separation, zirconium tetrafluoride, hafnium tetrafluoride. (Abate and Wilhelm, 1951; Dai et al., 1992; Kotsar’ et al., 2001; MacFarlane et al., 2002; Pastor and Robinson, 1986; Solov’ev and Introduction Malyutina, 2002b; Yeatts and Rainey, 1965). The addition of baffles (Abate and Zirconium requires several purification steps to Wilhelm, 1951; Kotsar’ et al., 2001; Yeatts and conform to nuclear-grade specifications. Little Rainey, 1965) is used quite frequently to help information is available on the sublimation reduce the mechanical carry-over of impurities. separation of Zr and Hf compounds, especially These baffles are merely plates positioned in the fluoride form, the majority of which between the charge and the cold finger. These deals with sublimation under vacuum impurities impart a greyish colour to ZrF4, conditions. On the industrial scale, only whereas pure ZrF4 is much whiter. vacuum sublimation of ZrF4 has been The literature also describes the use of a reported. No records were found for the gettering agent (Monnahela et al., 2013; sublimation of ZrF4 in an inert atmosphere. Solov’ev and Malyutina, 2002a), which seems Information on the sublimation rate of ZrF4 or to reduce the number of steps required to HfF4 in an inert atmosphere is also limited. produce nuclear-grade ZrF4. Getters used The rate is assumed to be dependent on include NiF2, zirconium oxides, and/or several factors, of which temperature, area, zirconium oxyfluorides. and composition are considered the most important. In the process currently under investi- gation, the separation technique envisaged is by sublimation/desublimation in the * The South African Nuclear Energy Corporation SOC tetrafluoride form. The separation involves the Ltd. (Necsa). sublimation of the metal tetrafluorides in an † Department of Chemical Engineering, University of inert atmosphere under controlled conditions, Pretoria. followed by desublimation (i.e. condensation) © The Southern African Institute of Mining and of the one metal fluoride in preference to the Metallurgy, 2015. ISSN 2225-6253. Paper received other. Mar. 2015 and revised paper received July 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 961 ▲ A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form

Area-dependent sublimation rate Table I

MacFarlane et al. (2002) calculated the area-dependent rate Combined vapour pressure correlations for ZrF4 of sublimation of ZrF4 and obtained a value of approximately and HfF4 1.87 g/m2/s at 850–875°C. Product composition Component Vapour pressure Temperature range [kPa] [°C] Ti, Esyutin, and Scherbinin (1990a, 1990b) found that pure ZrF4 has a higher sublimation rate than industrial-grade ZrF4 log(P) = 12.096−11879/T 600 – 920 HfF4 log(P) = 12.391−12649/T 600 – 950 ZrF4, which contains a degree of impurities. They concluded that this might be due to the accumulation of low-volatile components in the near-surface layer of the sample, making diffusion and evaporation increasingly difficult and resulting Figure 2. The concept under consideration is to pass a stream in decreased sublimation flux. of pre-heated dry nitrogen as a carrier gas over a bed of Layer height subliming ZrF4 and HfF4 (Figure 3); the gas exiting the sublimer then enters a desublimer operating at a slightly In a study on the influence of layer height on the vacuum lower temperature. The difference between the vapour sublimation rate of ZrF , Ti, Esyutin, and Scherbinin (1990c) 4 pressures as function of temperature is used to determine an concluded that the sublimation rate does not necessarily optimum temperature for the desublimer. In the desublimer, depend on the height of the sample in the sublimator. one of the tetrafluorides is desublimed in preference to the Vapour pressure of ZrF4 other, thus effecting separation. The remainder of the gas Figure 1 gives a range of vapour pressures from the literature mixture exits the desublimer and enters a water-cooled cold for both ZrF4 and HfF4 at temperatures above 600°C finger for collection of the remaining ZrF4 and HfF4, which (Benedict et al., 1981; Cantor et al., 1958; Koreneo et al., can be subjected to a further separation step. 1972; Sense et al., 1954, 1953). The sublimer (Figure 3) consists of two rectangular The data in Figure 1 was combined and can be expressed sections; a bottom section containing the bulk mass to be as Antoine constants for both ZrF4 and HfF4. These two sublimed, and a top section that facilitates the movement of expressions are given in Table I. the nitrogen gas and carries the sublimed tetrafluorides in the gas phase to the desublimer. Experimental concept to be modelled The desublimer is a long cylindrical pipe that is heated to a predetermined temperature, depending on the partial The flow diagram for the proposed process is presented in pressures of the ZrF4 and HfF4 entering the desublimer.

Figure 1 – Literature vapour pressures for ZrF4 (Benedict et al., 1981; Figure 3 – Sketch of the sublimer Cantor et al., 1958; Koreneo et al., 1972; Sense et al., 1954, 1953)

Figure 2 – Block flow diagram for the sublimation separation of ZrF4 and HfF4 ▲ 962 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form

Modelling [5] Flux model

The rate model for the sublimation of ZrF4 and HfF4 is based on the work of Smith (2001), who predicted evaporation σAB is the collision diameter, a Lennard-Jones parameter rates for liquid spills of chemical mixtures by employing in Å, where A refers to nitrogen and B to either ZrF4 or HfF4. vapour-liquid equilibrium. As sublimation progresses, the Since σ is denoted as the Lennard-Jones diameter of the bed height decreases with time, which causes changes in the respective spherical molecule (Welty, 2001), an estimation mass transfer coefficient resulting in a change in the was made for the diameter of a ZrF4 and a HfF4 molecule sublimation rate of the two tetrafluorides. The rate model for assuming sphericity. The sizes of the respective molecules both ZrF4 and HfF4 is given in Equation [1]: were calculated at room temperature with the use of SpartanTM software. The equilibrium geometry was calculated [1] using the Hartree-Fock method with the 6 31* basis set. Estimated values for the collision diameters of ZrF4 and HfF4 where ri is the sublimation rate of ZrF4 (or HfF4) in mol per with N2 were calculated as 4.146 and 4.127 Å respectively. unit sublimation area per unit time, ki,t is the mass transfer The collision integral (ΩD) is a dimensionless parameter * coefficient in m/s at time t, Pi is the vapour pressure in kPa, and a function of the Boltzmann constant Κ (1.38 × 10-16 ’ pi is the partial pressure in the bulk gas, xi,t is the mol ergs/K), the temperature, and the energy of molecular fraction of the respective tetrafluoride in the unsublimed bulk interaction ∈AB. The boiling points (Tb) for ZrF4 (912°C) and mass, R is the ideal gas constant (8.314 kPa.m3/kmol/K), HfF4 (970°C) (Lide, 2007) were used to calculate the values and T is the temperature in K. for ∈i with the use of an empirical correlation, given by In order to calculate the total flux along the length of the Equation [6]: sublimation pan (nj,t), the pan is divided into segments each of length ΔL. The flux in each successive segment is calculated by [6] adding the flux in the previous segment to the sublimed masses of ZrF4 and HfF4 in segment j (Equation [2]). Estimated values for the collision integrals for ZrF4 and HfF4 in N2 were calculated as 0.980 and 0.987, respectively. [2] The diffusion coefficients were calculated at several temperatures and are listed in Table II. Model results where Zt is the height of the headspace above the solid bed at any given time t. Limitations Model parameters One practical problem encountered in the design of the experimental set-up is the working temperature of the valves Mass transfer coefficient (650°C). At this temperature, the vapour pressures are The mass transfer coefficient (ki) is required for calculating relatively low, which results in a very long sublimation time. the rate of sublimation and is a function of the Sherwood Since all of the components of the valves are metallic, and it number (Shi), the diffusion coefficient (DAB), and the is general knowledge that these valves have a built-in safety equivalent flow diameter (De) (Equation [3]): factor, the decision was made to operate slightly above the maximum specification temperature of the valves, i.e. at [3] 700°C. Sublimation rates Sherwood numbers differ for each experimental set-up. In the case of convective mass transfer for forced convection The initial load to be sublimed was taken as 80 g, which over a flat plate (in this case a sublimation pan), and for includes the HfF4 and other impurities. The full sublimation laminar flow conditions with Reynolds number < 5 × 105, area is 0.0075 m2. The average area-dependant rates of sublimation at Prandtl number > 0.6, and Shmidt number (Sci) > 0.5, the Sherwood number can be calculated using Equation [4] several temperatures calculated from the model are given in (Çengel, 2006).

[4] Table II

Diffusion coefficients Diffusion coefficients for ZrF4 and HfF4 in nitrogen The diffusion coefficient can be estimated using the Lennard- at a pressure of 1 atmosphere Jones potential to evaluate the influence of the molecular forces between the molecules. This correlation (Equation Temperature (°C) (cm2/s) (cm2/s)

[5]), also known as the Chapman-Enskog equation, holds for 700 0.693 0.677 binary gas mixtures of non-polar, non-reacting species 750 0.757 0.740 (Green, 2008; Welty, 2001), which is the case for ZrF4 and 800 0.824 0.806 850 0.893 0.874 HfF4 in nitrogen.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 963 ▲ A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form

Figure 4, which is a sum of the average rates of ZrF4 and increasing, since the ZrF4 has a higher rate of sublimation. HfF4 at several time intervals. It can be seen that the rate is a The sublimed ZrF4 and HfF4 diffuse into the nitrogen power function of the temperature, illustrating the effect of a stream, exit the sublimer, and enter a desublimer operating at higher temperature on the rate. a slightly lower temperature than that of sublimation. The The total sublimation time is dependent on the area of desublimation temperatures for the two tetrafluorides can be sublimation and the temperature of sublimation, the latter calculated from the vapour pressure correlations based on the controls the two vapour pressures. The dependence of the partial pressure of the respective fluorides entering the total sublimation time on temperature is shown in Figure 5. desublimer. Figure 8 indicates the desublimation temper- At 850°C, the total sublimation time equals approximately 33 atures obtained from the model at a sublimation temperature minutes, whereas at 700°C the total sublimation time was of 700°C for ZrF4 and HfF4. It is evident that a desublimer calculated to be approximately 24.5 hours. The vapour operating temperature of between 540°C (maximum pressure ratio between 850 and 700°C is 42.7, which temperature for HfF4) and 610°C (minimum temperature for indicates that the rate at 700°C should be lower by at least ZrF4 desublimation) is required to ensure that, according to this factor, which amounts to a total sublimation time of 23.5 the model calculations, no HfF4 will desublime in the hours. The other factor influencing the rate is the partial desublimer. pressure in the gas stream, which is also higher at the higher temperature, which may contribute to the difference in the total sublimation time at 700°C. The mass sublimed with respect to time at 700°C is presented in Figure 6. Here it is evident that, according to the model calculations, some HfF4 will remain in the sublimation pan once all the ZrF4 has sublimed. Theoretically this implies that the sublimation can be stopped after a certain time, thereby separating most of the HfF4 from the ZrF4 in the cold finger. The rates of both HfF4 and ZrF4 sublimation are given in Figure 7. From this figure it is evident that the sublimation rate of HfF4 becomes increasingly significant as the sublimation progresses, i.e. as the bed height lowers with Figure 6 – Mass of ZrF4 and HfF4 sublimed with time at 700°C time. This is probably due to the mass fraction HfF4

Figure 7 – Rate of sublimation for ZrF4 and HfF4 at 700°C as a function Figure 4 – Rate of sublimation of impure ZrF4 at different temperatures of the sublimation time

Figure 5 – Total sublimation time calculated at different sublimation Figure 8 – Desublimation temperatures of ZrF4 and HfF4 at a temperatures sublimation temperature of 700°C ▲ 964 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy A theoretical approach to the sublimation separation of zirconium and hafnium in the tetrafluoride form

ZrF4 lost to cold finger DAI, G., HUANG, J., CHENG, J., ZHANG, C., DONG, G., and WANG, K. 1992. A new preparation route for high-purity ZrF . Journal of Non-Crystalline Solids, In the case of ZrF , the desublimer operating temperature is 4 4 vol. 140, no. 1-3. pp. 229–232. still relatively high for all the ZrF4 to desublime while passing through the desublimer. CANTOR, S., NEWTON, R., GRIMES, W., and BLANKENSHIP, F. 1958. Vapor pressures A desublimer operating temperature of 30°C higher than and derived thermodynamic information for the system RbF-ZrF4. Journal of Physical Chemistry vol. 62, no. 1. pp. 96–99. that of the maximum temperature for HfF4 (i.e. 570°C) results in a vapour pressure of ZrF which is still relatively 4 GREEN, J.D. 2000. II / distillation / sublimation. Encyclopedia of Separation large, and some of the ZrF4 will therefore not desublime and Science. 2nd edn). Wilson, I.D. (ed.). Academic Press, Oxford. will be collected on the cold -finger. KORENEO, Y., SOROKIN, I., CHIRINA, N., and NOVOSELO, A.V. 1972. Vapor-pressure Comparison to literature data of hafnium tetrafluoride. Journal of Inorganic Chemistry, vol. 17, no. 5. pp. 1195. The area-dependent rates of sublimation for both ZrF4 and HfF at 850°C were calculated. The average rate over the 4 KOTSAR’, M.L,. BATEEV, V.B., BASKOV, P.B., SAKHAROV, V.V., FEDOROV, V.D., and entire duration of sublimation of the total sublimated mass SHATALOV, V.V. 2001. Preparation of high-purity ZrF4 and HfF4 for optical amounts to approximately 5.36 g/m2/s, which is 2.87 times fibers and radiation-resistant glasses. Inorganic Materials, vol. 37, no. 10. higher than the value estimated from literature data (1.87 pp. 1085–1091. g/m2/s). The difference between the literature and model rates may be attributable to the impurities present in the LIDE, D.R. 2008. CRC Handbook of Chemistry and Physics. CRC, London. sample used by Macfarlane et al., (2002), since the presence MACFARLANE, D.R., NEWMAN, P.J., and VOELKEL, A. 2002. Methods of purification of impurities can have a direct influence on the rate of of zirconium tetrafluoride for fluorozirconate glass. Journal of the sublimation. American Ceramic Society, vol. 85, no. 6. pp. 1610–1612.

Conclusions MONNAHELA, O.S., AUGUSTYN, W.G., NEL, J.T., PRETORIUS, C.J., and WAGENER, J.B. 2013. The vacuum sublimation separation of zirconium and hafnium A sublimation model has been developed to predict the tetrafluoride. AMI Precious Metals 2013 Conference, Protea Hotel, Cape sublimation rates and the partial pressures of ZrF4 and HfF4 Town, 14-16 October 2013. in the tetrafluoride form and in an inert gas. The gas exits a sublimer and enters a desublimer where the one tetrafluoride NEL, J.T., DU PLESSIS, W., CROUS, P.L., and RETIEF, W.L. 2011. Treatment of is desublimed in preference to the other, separating the two zirconia-based material with ammonium bi-fluoride. Patent WO2011013085 A1. tetrafluorides. The model revealed an area-dependent sublimation rate PASTOR, R.C. and ROBINSON, M. 1986. Method for preparing ultra-pure zirconium that is 2.87 times higher than the value estimated from and hafnium tetrafluorides. US patent 4,578,252 A. literature data (1.87 g/m2/s) at 850°C. This indicates that the SENSE, K.A., SNYDER, M.J.and CLEGG, J.W. 1953. Vapor pressures of beryllium rates obtained from the model are within an acceptable range. fluoride and zirconium fluoride. US Atomic Energy Commission Technical The difference between the literature and model rates may be Information Services, Oak Ridge, Tennessee. attributable to the impurities present in the sample used by Macfarlane et al., (2002), since impurities can have a direct SENSE, K.A., SNYDER, M.J., and FILBERT, R.B.J. 1954. The vapor pressure of influence on the rate of sublimation. zirconium fluoride. Journal of Physical Chemistry, vol. 58, no. 11. Due to experimental/equipment constraints, the operating pp. 995–996. temperature of the sublimer should be in the range of 700°C. SMITH, R.L. 2001. Predicting evaporation rates and times for spills of chemical Optimal temperature selection is imperative, since low mixtures. Annals of Occupational Hygiene, vol. 45, no. 6. pp. 437–445. temperatures result in a low sublimation rate and high temperatures increase the level of impurities in the sublimed SOLOV’EV, A.I. and MALYUTINA, V.M. 2002a. Production of metallic zirconium product. tetrafluoride purified from hafnium to reactor purity. Russian Journal of Non-Ferrous Metals, vol. 43, no. 9. pp. 14–18. At a sublimer operating temperature of 700°C, the model indicates that an operating temperature for the desublimer of SOLOV’EV, A.I. and MALYUTINA, V.M. 2002b. Metallurgy of less-common and between 540°C (maximum temperature for HfF4) and 610°C precious metals. Production of metallurgical semiproduct from zircon concentrate for use in production of plastic metallic zirconium. Russian (minimum temperature for ZrF4 desublimation) is required to Journal of Non-Ferrous Metals, vol. 43, no. 9. pp. 9–13. ensure that, according to the model calculations, only ZrF4 and no HfF4 will desublime. Selection of an optimal TI, V.A., ESYUTIN, V.S., and SCHERBININ, V.P. 1990a. The dependence of the temperature of the desublimer is also critical, since too high a zirconium tetrafluoride sublimation rate in vacuum upon the process temperature will result in more ZrF4 lost to the cold finger. temperature and product composition. Kompleksnoe Ispol'zovanie It is recommended that the model results be compared Mineral'nogo Syr'ya, vol. 9. pp. 63–64. with experimental values, including if necessary the TI, V.A., ESYUTIN, V.S., and SCHERBININ, V.P. 1990b. The influence of the sample sublimation kinetics to account for any possible time height on the zirconium tetrafluoride sublimation process. Kompleksn. dependencies of the vapour pressures. Ispol’z. Miner. Syr’ya, vol. pp. 60–61.

References TI, V.A., ESYUTIN, V.S., and SCHERBININ, V.P. 1990c. The dependence of the zirconium tetrafluoride sublimation rate upon the process pressure. ABATE, L.J. and WILHELM, H.A. 1951. Sublimation of zirconium tetrafluoride. US Kompleksnoe Ispol'zovanie Mineral'nogo Syr'ya, vol. 10. pp. 61–63. Atomic Energy Commission, Ames Laboratory. WELTY, J.R., WICKS, C.E., WILSON, R.E.and RORRER, G. 2001. Fundamentals of BENEDICT, M., PIGFORD, T.H., and LEVI, H.W. 1981. Nuclear Chemical Momentum, Heat and Mass Transfer. Wiley, New York. Engineering, McGraw-Hill, New York. YEATTS, L.B. and RAINEY, W.T. 1965. Purification of zirconium tetrafluoride. ÇENGEL, Y.A. 2006. Heat and Mass Transfer. McGraw-Hill, New York. Technical report ORNL-TM-1292, US Atomic Energy Commission. ◆

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 965 ▲ advanced metals initiative Ferrous 2016 FERROUS AND BASE METALS DEVELOPMENT NETWORK CONFERENCE 2016 15–17 October 2016 C o BACKGROUND Through its Advanced Metals Initiative (AMI) the South African Department of Science and Technology (DST) promotes research, development n and innovation across the entire value chain of the advanced metals field. The goal of this initiative is to achieve sustainable local mineral f beneficiation and to increase the downstream value addition of advanced metals in a sustainable manner. The achievement of this is envisioned to be through human capital development on post-graduate and post-doctoral level, technology transfer, localization and ultimately, e commercialisation. The AMI comprises four networks, each focussing on a different group of metals. These are Light Metals, Precious Metals, Nuclear Materials r and Ferrous and Base Metals (i.e. iron, steel, stainless steels, superalloys, copper, brass, etc.). The AMI FMDN 2015 Conference aims to bring together stakeholders from the mineral sector, academia, steel industry, international research e institutions and government in order to share and debate the latest trends, research and innovations, specifically in the areas of energy, n petrochemical, corrosion, materials for extreme environments and transport, local mineral beneficiation and advanced manufacturing related to these materials. c Keynote speakers to be invited include international specialists in the fields of ferrous metals, computational materials science, high temperature corrosion and mineral beneficiation. e The Ferrous and Base Metals Development Network (FMDN) of the DST’s Advanced Metals Initiative (AMI) programme will host the AMI’s annual conference in 2016. The conference seeks to share insight into the state of R&D under the AMI-FMDN programmes and explore and debate the following broad themes: A  Development of high performance ferrous and base metal alloys for application in the energy and petrochemical industries n  Development of corrosion resistant ferrous and base metal alloys  Development of lightweight and/or durable steels for cost-effective transportation and infrastructure, and n  Panel discussions on possible future value-adding R&D programmes under FMDN within the South African National Imperatives. o u OBJECTIVES WHO SHOULD ATTEND n Insight into ferrous and base metal materials R&D for application Stakeholders from the energy, petrochemical, corrosion and in the areas of energy, petrochemical, corrosion, extreme transportation industries where ferrous (i.e. iron, steel, stainless steels, c environments, improved processing technologies and advanced superalloys, etc.) and base (i.e. copper, brass, etc.) metals are used in e alloys for the transport industry in South Africa and globally. their infrastructure. Also included in this invitation are local and international Higher Education Institutions (HEIs), Government m EXHIBITION/SPONSORSHIP Departments and Science Councils that are involved and/or have interest in R&D in these areas. e Sponsorship opportunities are available. Companies wishing to n sponsor or exhibit should contact the Conference Co-ordinator. For further information contact: t Head of Conferencing, Raymond van der Berg SAIMM, P O Box 61127, Marshalltown 2107 Tel: +27 11 834-1273/7 ·Fax: +27 11 833-8156 or +27 11 838-5923

our future through E-mail: [email protected] · Website: http://www.saimm.co.za science http://dx.doi.org/10.17159/2411-9717/2015/v115n10a11 Glow discharge optical emission spectroscopy: a general overview with regard to nuclear materials by S.J. Lötter*†, W. Purcell* and J.T. Nel†

profiles of materials that contain layers of Synopsis varying composition and thickness allows not only for the quantification of a sample, but Glow discharge optical emission spectroscopy (GD-OES) is an analytical technique used in the analysis of solid, conducting materials. Though also the evaluation of its composition in terms primarily of interest as a depth profiling technique on samples with of homogeneity. Unfortunately the method is varying layers of both conducting and non-conducting materials, it is also not yet as well established as other techniques. capable of rapid bulk analysis of homogenous, solid samples. GD-OES In order to be at its most effective it requires combines the advantages of ICP-OES (wide detection range, speed, and conductive samples. GD-OES was developed by lack of interferences) with the solid sampling of XRF techniques. This Grimm in 1967 (Payling, Jones, and Bengtson, allows the analyst to not only quantify the elemental composition of a 1997). It was initially used to analyse solid sample, but to evaluate it in terms of homogeneity with depth, a field in metallic samples (bulk analysis) where all which auger electron spectroscopy (AES) and secondary ion mass surface effects were eliminated by a pre-burn. spectrometry (SIMS) have traditionally been the primary techniques. In the 1970s, Roger Berneron, among others, Although GD-OES does not replace these useful techniques, it does offer began investigating the phenomena that various advantages over them, making it an excellent complementary analytical tool. occurred during this pre-burn period and In GD-OES analysis, a low-pressure glow discharge plasma is developed GD-OES as a surface analytical generated with the sample material acting as a cathode, accelerating the technique. In 1988 M. Chevrier and Robert cations in the plasma towards the sample surface. This bombardment Passetemps first successfully applied an RF causes the sample material to ‘sputter’, essentially knocking free atoms or potential to a conventional Grimm-type source molecules of analyte material, which are then drawn into the plasma (GD) to produce an RF device that was able to where they are excited. The light emitted by this excitation is then also analyse non-conductive samples. diffracted to separate the wavelengths emitted by the specific elements Currently, GD-OES is still used mainly in and detected by a spectrophotometer. The intensity of the signal is directly the analysis of solid metallic samples proportional to the quantity of analyte element present in the sample, (Azom.com, 2004). Its most useful application allowing simple calibration and quantitative determination of the elements. is as a depth profiling technique on samples Technological improvements made in the past twenty years or so have with varying layers of both conducting and significantly increased the usefulness of GD-OES for surface analysis. non-conducting materials (Shimizu et al., Faster plasma stabilization and start-up allow for the quantification of 2003), but it is also capable of rapid bulk surface layers as thin as 1 nm. Sputter rates have been accurately analysis of homogenous samples (both measured for many common materials, allowing them to be built into conducting and non-conducting). A typical software libraries in the instrument’s control software. This dramatically analysis takes only a few minutes, which is expands the usefulness of this software and the ease of performing one of the main advantages of GD-OES, analyses. especially where quick results are required, for GD-OES analysis of nuclear materials allows for the rapid determi- example at a smelter. In GD-OES analysis, nation of the elemental composition without the requirement of initial when using a Grimm-type direct current (DC) dissolution. The thickness of any corrosion layers on nuclear materials can also be determined. lamp source, a potential difference of the order of 1 kV is applied between two electrodes in a Keywords cell containing a gas, usually argon, at low glow discharge optical emission spectrometry, GD-OES, nuclear materials. pressure (around 1 Torr) (Bogaerts and

Introduction Glow discharge optical emission spectroscopy (GD-OES) combines the detection range, * Department of Chemistry, University of the Free speed, and lack of interferences of an State, South Africa. inductively coupled plasma optical emission † The South African Nuclear Energy Corporation SOC Ltd. spectroscopy (ICP-OES) instrument with solid © The Southern African Institute of Mining and sampling reminiscent of X-ray fluorescence Metallurgy, 2015. ISSN 2225-6253. Paper received (XRF) techniques. Its ability to perform depth July 2015 and revised paper received Aug. 2015.

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Gijbels, 1998). This causes the gas to ionize into positive and individual wavelength intensities can then be detected using negative ions, forming the plasma. The electrons are either a charge-coupled device (CCD) or a photomultiplier accelerated towards the anode, sustaining the plasma and tube (PMT). The intensity of the signal is directly propor- generating more positive ions. The positive ions are similarly tional to the quantity of analyte element present in the accelerated towards the cathode, the sample material. The plasma, allowing simple calibration and quantitative determi- resulting bombardment of the sample material causes it to nation of elements. In practice this is often achieved using a ’sputter’, essentially knocking free atoms or molecules of Rowland circle (Figure 3) with PMTs positioned behind exit analyte material. These atoms are then drawn into the plasma slits that are positioned at the wavelength corresponding to where they too are excited. A diagram of this process can be characteristic emission lines of various elements. This allows seen in Figure 1. A radio frequency (RF) lamp may also be the instrument to read all of these lines simultaneously, used for analysing non-conducting samples (Winchester and greatly increasing sample throughput rate at the cost of the Payling, 2004), see Figure 2. The sputtered analyte is then ability to generate a complete spectrum. As there is only a drawn into the high-energy plasma where it is excited into a single PMT installed for most elements in this type of higher electronic energy state. In order to return to their spectrometer, detection of interferences is more difficult than ground state the analyte ions emit a photoelectron of a with scanning sequential instruments. A sequential wavelength characteristic of the element emitting it. instrument would, however, not be capable of the most useful In GD-OES the characteristic light emitted by this aspect of the GD-OES, which is its ability to perform depth excitation/de-excitation process is passed into a spectropho- profiling. The signals that are captured by the PMTs are tometer through an entrance slit, and is then diffracted by a recorded by a computer at a typical rate of 10 000 to 30 000 concave holographic mirror grating that contains 3600 lines measurements per second. per millimetre. This is done in order to separate the emitted Technological improvements made in the past twenty light into wavelengths that are specific for each element. The years or so have significantly increased the usefulness of GD- OES for surface analysis (Azom.com, 2004). Faster plasma stabilization and start-up times allow for the quantification of surface layers as thin as 1 nm. Sputter rates have been accurately determined for many common materials, allowing them to be built into the software libraries of the instrument’s control software. This significantly expands the usefulness of the software. Improvements in calibration systems allow for the changing of instrumental conditions without the need to completely recalibrate, saving time and effort. In glow discharge mass spectrometry (GD-MS), a mass spectrometer is attached to the GD source rather than an optical detector. This allows for a much lower limit of detection at the cost of linear dynamic range. The instrument is otherwise essentially the same as a GD-OES instrument. A collision cell can be used at the interface between the glow discharge plasma and mass spectrometer in order to minimize interference from polyatomic species formed in the plasma. Figure 1 – Schematic of the various processes occurring during glow discharge (GD) (Betti and de las Heras, 2004)

Figure 2 – Diagram of a Rowland circle used in the LECO GDS850A Figure 3 – Conceptual diagram of a radio frequency glow discharge (Leco Corporation, 2007) device (RF GD) (Winchester and Payling, 2004) ▲ 968 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Glow discharge optical emission spectroscopy

As GD-MS still makes use of a glow discharge plasma When positioning a sample it is preferable to ensure that sampling system it shares the GD-specific advantages and no previous GD craters overlap with the surface to be limitations of the GD-OES. Due to the cost of a mass analysed. Care must be taken that samples are non-porous spectrometer a GD-MS instrument is generally significantly and smooth. If they are not, the vacuum will not be strong more expensive than an OES instrument. enough to form the GD plasma, the instrument will detect Modern instruments are equipped with sophisticated this, and the sample will be ejected before analysis can begin. software packages that use libraries containing the sputter If the seal is tight the sample will then be held in place by the rates of known materials and advanced algorithms. These are reamer, a pneumatically controlled ‘air jack’ with an used to calculate both the depth and mass composition of the integrated drill bit. This reamer serves multiple functions, layers, as seen in Figure 4 and Figure 5. Figure 4 shows the including holding the sample in place, cleaning the anode depth profile of a hard-disk platter with seven distinct layers between analyses, and serving as a conductor to allow the clearly visible, while Figure 5 shows the composition of the sample to act as cathode (when using the DC lamp). When surface of a photovoltaic cell. moving into position the reamer impacts the sample with some force, which can break brittle samples. Instrumentation The LECO GDS850A (Figure 7) supports both a DC and A GD-OES instrument is a fairly simple piece of equipment to an RF lamp. The RF lamp is intended for use with non- operate, in comparison to other instruments capable of conducting materials and has a lower effective applied power. similar analyses. Once a sample is properly prepared, sample It was found that using this lamp with the copper anode introduction consists of simply placing a sample over the O- allowed only for the analysis of very thin, regular, non- ring at the lamp opening and allowing the vacuum to hold it conducting layers on a conducting substrate. The DC lamp is in place. A simple cross-section of a standard Grimm-type GD able to generate a greater applied power, which can increase lamp can be seen in Figure 6. sensitivity but is also more prone to short-circuits.

Figure 4 – Three replicate quantitative depth profiles (QDPs) of the surface of a hard disk exhibiting seven resolved layers at a depth of 100 Figure 6 – Cross-section of a glow discharge lamp showing vacuum, nm (Leco Corporation, 2007) cooling, and gas flow lines (Payling, Jones, and Bengtson, 1997)

Figure 5 – Depth profile of the absorber layer of a copper indium gallium selenide (CIGS) thin film photovoltaic (PV) cell by pulsed RF GD-OES (Horiba Scientific, 2014) Figure 7 –The LECO GDS850A (Leco Corporation, 2007)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 969 ▲ Glow discharge optical emission spectroscopy

In order to ensure that the Rowland circle is in alignment, plants. Examples of these are high nickel-containing alloys, the instrument is equipped with two PMTs mounted at normal and special stainless steels (e.g. 316, 304, and 17- diffraction angles for detecting the characteristic wavelengths PH), Monel, Inconel, aluminium alloys, copper, and brass, of iron. These two iron ‘lines’ are used when profiling the etc. The impurity specifications for nuclear materials are instrument. This is done by analysing a sample containing usually extremely strict and the low limit of detection of GD- iron. Detecting these two lines allows the instrument to OES makes this technique extremely powerful in the nuclear confirm the position of the spectrometer’s entry slits. As the industry. instrument stores calibration data it is necessary to drift- In the current programme of the Advanced Metals correct a method’s calibration before quantitative analysis. Initiative (AMI), and especially the Nuclear Materials This is performed by analysing standard samples, pre- Development Network (NMDN), one of the main focuses is selected in the method, and adjusting the calibration the development of nuclear-grade zirconium metal and the according to the intensities detected. This drift must be done study of corrosion of zirconium alloys. whenever a different method is used but the warm-up and GD-MS has been used in the analysis of spent uranium profiling need to be performed only once per working period. oxide fuel (Robinson and Hall, 1987) prior to recycling. It is necessary to confirm that the reprocessing plant is working The use of GD-OES in the nuclear industry efficiently and that fission products, activation products, and As previously mentioned, the most striking capability of GD- corrosion products are successfully removed. GD-MS offers a OES is its ability to accurately and rapidly determine a depth faster and more quantitative approach than traditional profile of multiple layers of varying composition on a methods, such as spark source mass spectrometry (SSMS), substrate. The raw data from a GD-OES analysis is given in without the need for time-consuming interpretation of results time versus intensity of emission, as can be seen in Figure 8, by highly skilled operators. where layers of UO2F2·1.5H2O, and UF4 are seen bonded to a nickel substrate. This data, along with known sputter rates Experimental for each material, allows for the characterization of the Samples of Zircaloy 2 and Zircaloy 4, obtained from ATI Wah various uranium species present on the surface of the Chang, were exposed to air at 600°C for periods of either 16 material. or 32 hours. The samples were allowed to cool rapidly in air In the 1980s the erstwhile Atomic Energy Corporation of and then analysed by GD-OES to determine the thickness of South Africa (AEC) used GD-OES intensively to study surface the oxide layer formed. A sample of zirconium metal, and interface phenomena like corrosion, surface cleanness, obtained from Alfa Aesar, was analysed as well in order to passivation of surfaces, decontamination efficiency, plating, provide a baseline from which to compare the degree of the etc. (Nel, 1991). Valuable information could be obtained by growth of the oxide layer. The elemental composition of the analysing surfaces of materials that were exposed to UF6, as Zircaloy materials can be seen in Table I. indicated in Figure 8. The types of uranium compounds that The GD-OES instrument was calibrated using NIST were found on a surface were used by nuclear engineers and standards 1212a, 1234, 1235, 1236, 1237, 1238, and 1239, scientists to choose the best materials for construction or to obtained from Pelindaba Analytical Laboratories. The diagnose chemical and material problems that occurred photomultiplier tube power settings were determined using during plant operation. the automated software tool built into the control software, GD-OES was also routinely used for the bulk analysis of with applied voltages ranging between 400 and 900 V. materials that are used in nuclear and uranium enrichment Instrument settings can be seen in Table II. As the RF lamp was used the Teflon isolation puck was utilized in all determinations and was kept at a constant temperature of 12°C by an external chiller. This puck is similar to the cooling puck used with the DC lamp but, in addition to cooling, also electrically isolates the sample from the reamer.

Results and discussion Oxygen readings in all cases were higher than expected, but this was likely a calibration issue. As the oxygen calibration values were for dissolved oxygen in the metal standards rather than the stoichiometric levels found in the oxide

Table I Elemental composition of Zircaloy 2 and Zircaloy 4 (Roskill Information Services, 2011, p. 322)

Sample Zr % Sn % Fe % Ni % Cr %

Zircaloy 2 98.23 1.5 0.12 0.05 0.1 Figure 8 – Depth profile of a thin layer containing UO2F2·1.5H2O and UF4 Zircaloy 4 98.28 1.5 0.12 - 0.1 on a nickel substrate (Nel, 1991) ▲ 970 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy Glow discharge optical emission spectroscopy

Table II GDS850A settings

Setting Value

Voltage (V) 1200 True plasma power (W) 20 Vacuum (Torr) 6 - 12 Analysis time (min) 10

Figure 12 – Quantitative depth profiles of Zircaloy 2 after 16 and 32 hours’ exposure in air at 600°C

Figure 9 – Quantitative depth profile of a clean plate of zirconium metal before corrosion

Figure 13 – Quantitative depth profiles of Zircaloy 4 after 16 and 32 hours’ exposure in air at 600°C

Table III Measured thicknesses of Zircaloy corrosion layers Figure 10 – Quantitative depth profiles of Zircaloy 2 and 4 after 16 Sample Time at 600°C (h) Layer thickness (μm) hours’ exposure in air at 600°C Zircaloy 2 16 35 Zircaloy 2 32 100 Zircaloy 4 16 31 Zircaloy 4 32 60

layers, the intensities detected were several orders of magnitude higher than the calibration range. Oxygen values must thus be described as semi-quantitative. As the objective of the study was to determine the thickness of the layers rather than their elemental composition, this was not considered too great of a difficulty. The oxide layer formed naturally at room temperature by zirconium metal was found to be approximately 100 nm thick with approximately another 200 nm required for the transition to mostly pure zirconium, as seen in Figure 9. The thickness of the corrosion layers varied between 31 and 100 Figure 11 – Quantitative depth profiles of Zircaloy 2 and 4 after 32 µm, depending on the duration of and temperature at which hours’ exposure in air at 600°C the corrosion test was performed (Table III). The least

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 971 ▲ Glow discharge optical emission spectroscopy corrosion was exhibited by Zircaloy 4, with layer thickness BOGAERTS, A. and GIJBELS, R. 1998. Fundamental aspects and applications of marginally thinner than those seen with Zircaloy 2 at 16 glow discharge spectrometric techniques. Spectrochimica Acta Part B, vol. 53. pp. 1–42. hours but significantly thinner after both had been corroded for 32 hours. These results are clearly visible in Figure 10 HORIBA SCIENTIFIC.2014. Ultra fast elemental depth profiling. and Figure 11. The increase in oxide layer thickness of 285.7 http://www.horiba.com/fileadmin/uploads/Scientific/Documents/Emission /GDPROFILER_Series.pdf [Accessed 18 April 2015]. % for Zircaloy 2 can clearly be seen in Figure 12, while the Zircaloy 4 showed an increase of only 193.5 % (Figure 13). LECO CORPORATION. 2007. GDS850A Glow Discharge Spectrometer. As expected, Zircaloy 4 exhibited a significantly better http://www.leco.com/component/edocman/?task=document.viewdoc&id=4 9&Itemid=0 [Accessed 4 September 2013]. resistance to corrosion than Zircaloy 2. These results clearly show the obvious utility of GD-OES NEL, J.T. 1991. Oppervlakanalises van uranielfluoriedlagies met 'n gloeiontlad- for the rapid determination of oxide layers on nuclear fuel ingslamp. DPhil thesis, University of Pretoria. cladding. It can be highly useful as a method to quantify the PAYLING, R., JONES, D.and BENGTSON, A. 1997. Glow Discharge Optical Emission corrosion of known and experimental materials for nuclear Spectrometry. Wiley, Hoboken, NJ. application. Its usefulness extends to the determination of ROBINSON, K. and HALL, E.F.H. 1987. Glow discharge mass spectrometry for layer thickness and composition for known and potential nuclear materials. Journal of Metals, vol. 39. pp. 14–16. coatings on nuclear and non-nuclear materials. ROSKILL INFORMATION SERVICES. 2011. Zirconium: Global Industry Markets and Outlook. 13th edn. London. References SHIMIZU, K., HABAZAKI, H., SKELDON, P. and THOMPSON, G.E. 2003. Impact of RF- AZOM.COM. The A to Z of Materials. 2004. GD-OES in practical surface analysis. Spectrochimica Acta Part B, vol. 58. http://www.azom.com/article.aspx?ArticleID=2449 pp. 1573–1583. [Accessed 9 July 2013].

BETTI, M. and DE LAS HERAS, L.A. 2004. Glow discharge spectrometry for the WINCHESTER, R.M. and PAYLING, R. 2004. Radio-frequency glow discharge characterization of nuclear and radioactively contaminated environmental spectrometry: a critical review. Spectrochimica Acta Part B, vol. 59. samples. Spectrochimica Acta Part B, vol. 59. pp. 1359–1376. pp. 607–666. ◆

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BMG-MIN25092015 BEE3 ▲ 972 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a12 The influence of niobium content on austenite grain growth in microalloyed steels by K.A. Annan, C.W. Siyasiya and W.E. Stumpf

in-situ monitoring of austenite grain growth at high temperatures is impossible. Modelling the Synopsis grain growth behaviour based on available The relationship between niobium content and austenite grain growth has data is the ideal option. To quantitatively been investigated through hot rolling simulation on a Bähr dilatometer. describe austenite grain growth therefore The effect of delay time between passes during rough rolling in Nb- requires development of a sound mathematical microalloyed steels with nitrogen contents typical for electric arc furnace austenite grain growth equation that accounts (EAF) melting was studied. The results indicate that the grain growth for the effects of the varying microalloying constants n, Q, and A increase with an increase in Nb content. The elements in inhibiting austenite grain growth. activation energy for austenite grain growth Q was found to be in the Numerous attempts have been made to range of 239 to 572 kJ/mol, the exponential constant n ranged from 2.8 to develop an empirical model based on the 6.2, and the material and processing condition constant A from 4.24 × 1012 to 4.96 × 1028, for steels with niobium contents ranging from 0.002% Nb to general equation developed by Sellars and 0.1% Nb. A general constitutive equation for the prediction of austenite Whiteman (1979). Many of these models do grain growth in these Nb-microalloyed steels under rough rolling not account for the direct effects of the conditions has been developed. Good agreement between the experimental microalloying elements such as Nb in austenite and the predicted values was achieved with this constitutive equation. grain growth control [Fu et al., 2011; Wang Keywords and Wang, 2008; Wang et al., 2006; constitutive equation, austenite grain growth, microalloying, deformation. Shanmugama et al., 2005; Banerjee et al., 2010; Pous-Romeroa et al., 2013). The current work has considered this limitation, taking into account the direct effect of niobium in grain growth control during thermal Introduction processing. This is done by incorporating the Grain refinement has been found to increase initial austenite grain size Do and the microal- both the strength and toughness of steels (Gao loying element niobium in the development of and Baker, 1998; Sharma, Lakshmanan, and a constitutive equation for grain growth Kirkaldy, 1984; Seok et al., 2014; Maalekian prediction in Nb-containing microalloyed et al., 2012). It is also known that the steels. austenite grain size directly influences the microstructure, and thus the mechanical Materials and techniques properties, of the steel (Maalekian et al., 2012; Table I shows the chemical compositions of the Yue et al., 2010). Effective grain growth five microalloyed steels used in the study, control is reported to be achieved through which were cast by vacuum induction melting addition of precipitate-forming elements, such into ingots of 16 kg. The compositions were as Nb that, slow down the grain boundary chosen to test the effect of the Nb content on migration through pinning and solute drag the austenite grain growth while the other mechanisms (Yu et al., 2010; Olasolo et al., elements were kept approximately constant. 2011; Alogab et al., 2007). Much work has Note, however, that the melting and casting been carried out on austenite grain lsize procedure used for these laboratory steels control by the addition of precipitate-forming elements that have a strong affinity for interstitial elements, such as carbon and nitrogen, which form the dispersed pinning particles to inhibit the austenite grain growth (Alogab et al. 2007; Hodgson and Gibbs, 1992; Nanba et al., 2003; Flores and Martinez, * Department of Materials Science and Metallurgical 1997; Rollett, Srolovitz and Anderson, 1989). Engineering, University of Pretoria, South Africa. Austenite grain growth can be described using © The Southern African Institute of Mining and a conventional or a modelling approach. With Metallurgy, 2015. ISSN 2225-6253. Paper received conventional approaches (e.g. metallography), Aug. 2015 and revised paper received Aug. 2015.

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Table I Chemical composition of the steels used in the study

Steel Chemical composition (wt %) CMnSiNbTiV AlN Ni

0.002% Nb 0.17 1.13 0.11 0.002 0.001 0.007 0.05 0.018 0.15 0.03% Nb 0.16 1.15 0.15 0.03 0.02 0.008 0.02 0.018 0.15 0.05% Nb 0.17 1.15 0.15 0.05 0.02 0.009 0.03 0.019 0.15 0.07% Nb 0.14 1.10 0.15 0.07 0.02 0.006 0.04 0.020 0.17 0.10% Nb 0.18 1.27 0.15 0.10 0.02 0.009 0.04 0.020 0.17

resulted in somewhat higher nitrogen contents, which are tridecylbenze sulphonate, and 2-3 drops of triton. Etching nearer to those expected from electric arc furnace (EAF) than was carried out at a temperature of 60–70°C for times ranging from basic oxygen furnace (BOF) steelmaking. from 3 to 14 minutes. The samples were observed under an TM Compression test on a Bähr dilatometer Olympus BX51M optical microscope to reveal the prior austenite grain boundaries. The austenite grain size was Cylindrical samples (5 mm diameter × 10 mm length) measured using the average linear intercept method machined from the laboratory cast slabs were heated by according to ASTM E112 (Van der Voort, 1984). To obtain a induction in the Bähr dilatometer to 1150°C at a rate of 5°C s-1 statistically acceptable grain size distribution, more than 300 and held at 1150°C for 5 minutes to achieve homogenization. intercepts were measured on each sample. The samples were then cooled at 5°C s-1 to the testing temperature (1000, 1050, 1100, and 1150°C) where a single- Results hit compression was applied after holding at the test temperature for 20 seconds. A strain of 0.4 was used at a The solubility behavior of precipitates in microalloyed strain rate of 0.1 s-1. After compression, the samples were steels predicted by Thermo-CalcTM then held at the deformation temperature in the Bähr The volume fractions of precipitates as a function of the dilatometer for times of 0, 10, 30, 60, 90, or 120 minutes to temperature predicted by Thermo-CalcTM for the studied simulate the delay time after rough rolling, before rapid steels are shown in Figure 2. These show that the reheating -1 cooling at a rate of 600°C s to room temperature. Oxidation temperature range of 800–1250°C will lead to dissolution of a of the specimens during compression was prevented by substantial amount of Ti,Nb(C,N) carbonitrides but not to a passing a continuous flow of high-purity helium through the complete dissolution of all of these precipitates. NbN precip- system. The cooled samples were then tempered at 490°C for itates within the temperature region of 1000–1200°C, while 72 hours to improve the response of prior austenite grain TiN does not go into solution at temperatures considered in boundaries to etching. In this study, the grain size of the this study. The higher dissolution temperatures shown for samples held for zero (0) minutes after deformation was used NbN in these microalloyed steels are due to their higher N as the initial grain size D0 for the steel at that temperature. content of about 0.019 wt%, which is more typical for steel The scheduled profile followed in the single hit compression produced in an EAF (N in solute-rich regions preferentially tests is shown in Figure 1. combines with Nb). The presence of precipitates means that a The samples were mounted, ground, polished, and then unimodal grain structure can be predicted if these precipitates etched with picric acid solution containing 100 ml saturated are effective in pinning the grain boundaries (Fernández, aqueous picric acid, 100 ml distilled water, 4 g sodium Illescas and Guilemany, 2007; Van der Voort, 1984; Zener, 1948 (as cited by Ringer, Li and Easterling, 1989). The volume fraction of Nb precipitates with respect to temperature as predicted by Thermo-CalcTM is shown in Figure 3. This shows that the volume fraction at a given temperature is highly dependent on the Nb content in the steel, thus the higher the Nb content, the higher the volume fraction of the precipitate at any given temperature. Austenite grain growth behaviour in the steels after deformation Figure 4 shows the grain size distribution in the Nb-bearing steels after high-temperature deformation followed by tempering at 490°C for 72 hours. The steels showed a unimodal grain structure during deformation. These distrib- utions and structures attest to the fact that no abnormal grain growth, i.e. no bimodal distribution, was visible in these steels at the thermomechanical controlled processing (TMCP) Figure 1 – Schematic representation of the deformation process on the conditions employed in this work, confirming the presence of Bähr dilatometer pinning particles in the steels as predicted by Thermo-CalcTM. ▲ 974 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy The influence of niobium content on austenite grain growth in microalloyed steels

Figure 2 – Thermo-CalcTM predictions of the volume fraction of precipitates in the high-N laboratory-produced steels with (a) 0.03 wt%, (b) 0.05 wt%, (c) 0.07 wt%, and (d) 0.1 wt% Nb

basic relationship for the pinning force of particles on grain boundaries

(Zener, 1948, as cited by Ringer, Li, and Easterling, 1989), which predicts a stronger effect of pinning with increased volume fractions of small precipitates at constant particle sizes. The higher Nb content accounts for a greater volume fraction of NbN precipitates as shown in Figure 3, providing a greater area fraction of solute-rich regions compared to the steels containing less Nb (Xu and Thomas, 2011; Brewer, Erven and Krauss, 1991; Deus et al., 2002; Akamatsu, Senuma and Hasebe, 1992). The quantitative analysis of the

Figure 3 – Effect of Nb content on volume fraction of niobium optical measurements of austenite grain growth, is shown in precipitate as predicted by Thermo-calcTM Figure 6, which confirms the substantial influence of an increased Nb content on austenite grain growth inhibition in these steels. Increased Nb content and austenite grain growth in Quantitative evaluations of the average grain size as a the microalloyed steels function of austenitizing time and temperature in the 0.03 The observed effect of an increased Nb content in the wt% Nb microalloyed steel are shown in Figures 7 and 8 microalloyed steels is shown in Figure 5. It is evident that respectively.A comparative analysis of Figures 7 and 8 there was limited grain growth in the steel containing 0.1 indicates the expected greater effect of temperature compared wt% Nb, as it produced the smallest final grain sizes, while with time on grain growth, as recorded by numerous studies the highest grain growth was seen in the 0.002 wt% Nb steel, (Seok et al., 2014; Yue et al., 2010; Nanba et al., 1992; thereby following the volume fraction of Nb precipitates in Akamatsu, Senuma and Hasebe, 1992; Sha and Sun, 2009; the steel. The observed results are in agreement with Zener’s Zhao et al., 2011)

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Figure 4 – Optical micrographs and grain size distribution for (a) an 0.07 wt% Nb deformed at 1100°C at a strain rate of 0.1 s-1 to a strain of 0.4, (b) 0.05 wt% Nb deformed at 1150°C at a strain rate of 0.1 s-1 to a strain of 0.4

Figure 5 – Optical micrographs of (a) 0.002 wt%, (b) 0.03 wt%, (c) 0.05 wt%, (d) 0.07 wt%, and (e) 0.1 wt% Nb steels deformed at a temperature of 1100°C and held for 120 minutes ▲ 976 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy The influence of niobium content on austenite grain growth in microalloyed steels

Development of a constitutive equation It is generally accepted that the grain boundary migration velocity (v) is proportional to the driving force (Seok et al., 2014; Maalekian et al., 2012; Yue et al., 2010; Olasolo et al., 2011; Alogab et al., 2007; Hodgson and Gibbs, 1992; Nanba et al., 1992), an approximation justified at the relatively small driving forces involved in grain growth. This velocity is given by the expression (Stumpf, 2010)

[1]

where M is the grain boundary mobility, Fd is the driving force for grain growth, and Fp is the pinning force induced by precipitates. From the rate of movement:

Figure 6 – Influence of Nb content on the average austenite grain size (AGS), showing a strong inhibition in Nb-containing steels [2]

The grain boundary mobility M is given by the expression

and the grain growth rate equation therefore becomes:

[3]

Under isothermal conditions

[4]

The differential solution to Equation [4] is given by (Sellars and Whiteman, 1979)

[5]

Figure 7 – Isothermal grain growth behaviour of austenite in steels, where T is the absolute temperature, t is the time, Q is the showing an increase in the AGS with time in the 0.03 wt% Nb steel after activation energy for grain boundary migration, A is a hot deformation constant parameter dependent on the material and processing conditions, and R is the universal gas constant. A general solution to Equation [4] in a linear form is expressed in Equation [6]:

[6]

Equation [5] has been used by many authors (Seok et al., 2014; Maalekian et al., 2012; Nanba et al., 1992; Florez and Martinez, 1997; Rollett, Srolovitz and Anderson, 1989; Sellars and Whiteman, 1979; Fu et al., 2011; Wang and Wang, 2008; Wang et al., 2006) by assuming the value of Do to be zero or constant. This simplifies the plot of lnD as a function of 1/T. In the current study Do, which was experi- mentally measured, was not discarded or kept constant but was included and used in Matlab programming and an Excel solver to plot

as a function of 1/T in generating the constants used in the

Figure 8 – Isothermal grain growth behaviour of austenite in steels, development of the constitutive equations. This was done showing an increase in the AGS with temperature in the 0.03 wt% Nb through regression analyses of the first and second derivative steel after initial hot deformation at the same temperature orders of the experimentally derived regression value R2.

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The grain growth constants n, Q, and A The constant n was obtained by regression analysis of experimentally measured grain sizes through iterative tests of the straight-line regression coefficients R2, from a plot of Equation [6] where a second differential of the R2, (dR2)2/dn2 with respect to n gave a maximum correlation value of n at (dR2)2/dn2 = 0. Figure 9 shows a plot of ln(−) as a function of the inverse temperature 1/T, which was used in determining the constants. Thus, a plot of Equation [6] provided a linear relationship between and 1/T. Comparison of Equation [6] with the equation of a straight line made it possible for the constants Q and A to be Figure 9 – Plot of the natural log of the average grain size as a function determined from the plots shown in Figure 9. Thus, of the inverse of the absolute temperature for the 0.07 wt% Nb steel

(while the slope is negative) and A = ec/t where, c is the growth equation for austenite grain growth prediction in intercept from the graph and t is the time. 0.002–0.1% Nb microalloyed steels during deformation The dependence of the grain growth constants n, Q, and within the temperature range of 1000–1150°C and delay A on the Nb content was quantitatively analysed by multiple times ranging from 0–120 minutes is constituted for DDef in regression to obtain the following reliance equations, where μm: [Nb] is the wt% in the steel:

The statistical analysis resulting from the multiple where Do is the initial austenite grain size in μm and [Nb] is regression analysis presented in Table II, showing that the in wt %. activation energy Q has a strong linear relationship with the Nb content, while there is no linear relationship between A and the Nb content. There is, however, a weak linear Table II relationship between the grain growth constant n and the Nb content. Statistical results of the multiple regression The constants derived from experimentally measured analysis of the dependence of the austenite grain grain sizes for the microalloyed steels under deformation growth constants on varied Nb content conditions with and without taking Do into consideration are Regression statistics Grain growth constants presented in Table III. Qn A General constitutive equation for austenite grain Adjusted R square 0.91 0.64 0.40 growth prediction in Nb-bearing microalloyed steels Standard error 32.31 0.83 1.7 × 1028 with high N contents Number of observations 5 5 5 ANOVA 0.0072*** 0.066* 0.152 Based on the analysis of the isothermal grain growth kinetics *** p<0.01 (this means there is a very strong relationship with probability <1%) in Nb-microalloyed steels, the following isothermal grain * p<0.1 (this indicates a weak relationship with probability <10%)

Table III Grain growth constants generated from deformation data of Nb-microalloyed steels

Constants determined from deformed Nb-microalloyed steels

Steel (wt % Nb) Constants using measured values of Do Constants with Do= 0 Difference in constants from neglecting D0

nAQnAQΔn ΔA ΔQ

0.002 2.8 4.24E+12 276 2.8 3.00E+11 239 0 3.94E+12 37.27 0.03 5.5 1.33E+22 417 5.5 6.60E+21 384 0 6.70E+21 33.10 0.05 5.6 7.94E+23 474 5.5 5.00E+23 428 0.1 2.94E+23 46.31 0.07 6.0 9.63E+24 489 6.1 4.62E+24 459 -0.1 5.01E+24 30.41 0.1 6.2 4.96E+28 572 6.2 1.22E+28 559 0 3.74E+28 12.67 ▲ 978 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy The influence of niobium content on austenite grain growth in microalloyed steels

Discussion this study, as shown in Figure 10. The correlation R2 values obtained from the comparison of the experimentally Austenite grain growth behaviour of Nb-bearing measured grain sizes with the predicted grain sizes are 0.94, microalloyed steels 0.91, and 0.90 for 0.03 wt% Nb, 0.05 wt% Nb, and 0.07 wt% The grain growth rate was significantly slower in Nb steels respectively. The 0.1 wt% Nb steel, however, microalloyed steels with higher contents of Nb. Grain growth recorded a poor correlation value of R2 = 0.63, showing the control in microalloyed steels has been shown to be inability of the equation to predict grain sizes for steels dependent on the amount of microalloying additions (Alogan containing 0.1 wt% Nb and also most likely for those with et al., 2007; Hodgson and Gibbs, 1992). The greatest higher Nb contents. This is due to the inability of the inhibition here was recorded in the steel containing 0.1 wt% developed equation to account for the excess precipitates that Nb. It was also found that the average austenite grain size form owing to the high addition of the microalloying element. decreased with increasing Nb content in the steels. Generally, it has been reported that increasing the N content in the steel Conclusions leads to a beneficial increase in grain refinement (Alogan et ➤ A general constitutive equation for predicting austenite al., 2007; Hodgson and Gibbs, 1992; Nanba et al., 1992), grain growth in Nb-bearing steels that incorporates the which without doubt results from the larger volume fraction Nb content and then the starting grain size Do after of the precipitates that act as pinning particles. It should be simulated rough rolling has been developed noted, however, that while increased N content is beneficial ➤ There is a linear relationship between Nb content and for grain refinement, excessive levels of N dissolved in the activation energy for grain growth austenite may be detrimental to other properties of the steel, ➤ The austenite grain growth exponent n, the activation such as hardenability, through the decrease of the Nb fraction energy Q, and the material and processing conditions dissolved in the austenite, which is a key in improving most constant A all increase with an increase in Nb content, of the desired mechanical properties of the steel (Alogan et most likely from increased effectiveness of grain- al., 2007; Hodgson and Gibbs, 1992; Nanba et al., 1992). boundary pinning in the microalloyed steel with Predictive potential of the equation for grain growth increased Nb content in microalloyed steels in the current study ➤ The constitutive equation shows a poor correlation for A logical degree of precision in predicting austenite grain the steel containing 0.10 % Nb, with an R2 value of growth in Nb bearing steels has been achieved by comparison 0.63 recorded in the plot of measured against predicted of experimentally measured grain sizes with predicted grain grain sizes for the 0.1 wt% Nb steel. The equation is sizes using the general constitutive equations developed in most likely not applicable at such high values of Nb.

Figure 10 – Comparison of predicted and measured austenite grain sizes in Nb-bearing steels

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 979 ▲ The influence of niobium content on austenite grain growth in microalloyed steels

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in the through-thickness direction during and after hot rolling in carbon RINGER, S.P., LI, W.B.and EASTERLING, K. E. 1989. On the interaction and steels. ISIJ International, vol. 32. pp. 377–386. pinning of grain boundaries by cubic shaped precipitate particles. Acta Metallurgica, vol. 37, no. 3. pp. 831–841.

OLASOLO, M., URANGA, P., RODRIGUEZ-IBABE, J.J.M. and LÓPEZ B. 2011. Effect of austenite microstructure and cooling rate on transformation characteristics ZHAO, Y., SHI, J., CAO, W., WANG, M.and XIE, G., 2011. Kinetics of austenite in a low carbon Nb–V microalloyed steel. Materials Science and grain growth in medium carbon niobium-bearing steel. Journal of Engineering A, vol. 528. pp. 2559–2569. Zhejiang University SCIENCE A, vol. 12. pp. 171–176. ◆ ▲ 980 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy http://dx.doi.org/10.17159/2411-9717/2015/v115n10a13 The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel by H.J. Uananisa*, C.W. Siyasiya*, W.E. Stumpf* and M.J. Papo†

al., 2008), while at the same time presenting Synopsis good mechanical properties similar to those The need to reduce weight while maintaining good mechanical properties offered by austenitic steel grades. Moreover, in materials used in the automotive industry has over the years seen an some standard ferritic grades such as 409, increased exploitation of various steels to meet this new demand. In line 410, and 430 are readily available all over the with this development, the ferritic stainless steel family has seen a wide world, and are already very successfully used application in this industry, with the AISI 436 type increasingly being used for various applications, such as washing- for automotive trims and mufflers for exhaust systems, as well as a machine drums and exhaust systems in the significant part of this steel’s application being for the manufacture of automotive industry, and actually have a much wheel nuts and wheel nut caps in trucks, mainly through the deep drawing process. However, there have been reports of some poor surface broader application potential in many fields roughening of this material during deep drawing, with tearing and/or (Charles et al., 2008). Since new designs in cracking also reported in some instances. This has been suspected to the automotive industry are driven by safety possibly be associated with some local differences in localized mechanical and environmental concerns, the need to properties between grains and grain clusters of the rolled and annealed reduce the weight of the exhaust systems material. while maintaining good resistance to thermal In order to investigate the poor surface roughness exhibited by AISI 436 fatigue is continuing to see the exploitation of ferritic stainless steel (FSS) during deep-drawing, Lankford values (R- the ferritic stainless steel family for alternative Δ mean and r), grain size, and microtextures of various sheet samples from solutions. Ferritic stainless steels have a this steel were studied. The chemical composition range for the samples relatively low coefficient of thermal expansion was 0.013–0.017% C, 17–17.4% Cr, 0.9–1% Mo, and 0.4–0.5% Nb. The and, therefore, some efforts have been made to steels were subjected to various hot and cold rolling processing routes i.e. involving industrial direct rolling (DR) or intermediate annealing rolling create new ferritic stainless steels with high (IR), and the drawability and final surface qualities of the steels were yield strengths at elevated temperatures, compared. It was found that the DR route gave an average R-mean and Δr particularly by the addition of niobium (Nb), value of 1.9 and -1.4 respectively, while the IR route yielded an average R- which increases the initial high-temperature mean and Δr value of 1.6 and 0.52 respectively. The high Δr value for the strength through solid solution hardening DR processing route had a substantial adverse effect on the drawability. IR (Sello and Stumpf, 2010). The ferritic type samples exhibited a smoother surface finish on visual inspection, while AISI 436 is increasingly used for automotive clear flow lines were visible on the DR samples, despite the fact that DR is trims, with a major application being for lug the preferred industrial processing route due to the reduced production nuts or wheel nuts on trucks. It has also seen costs it offers. This observation was also confirmed through SEM extensive use in the production of both the examinations. The difference in the surface quality was attributed to microtexture. However, the mechanism responsible for this difference still central and rear mufflers of exhaust systems, needs to be identified. due to its good hot and wet corrosion resistance properties (Charles et al., 2008). Keywords However, during the deep drawing process ferritic stainless steel, formability, microtexture, EBSD. associated with the manufacture of most of these products, tearing or cracking can sometimes occur. Deep drawing is one of the Introduction most common processes for forming metal Owing to the price volatility of nickel over parts from sheet metal plates, and it is widely recent years, the need to cut down on the use used for the mass production of parts in the of nickel in the steelmaking industry has become a matter of keen interest, hence the rising interest in ferritic stainless steels to replace their austenitic counterparts for various industrial applications. Ferritic grades, * University of Pretoria, Department of Material containing chromium and possibly other Science & Metallurgical Engineering, South Africa. stabilizing elements (Ti, Mo, Nb, etc.) are well † Advanced Materials Division, Mintek. known as cost-saving materials as they do not © The Southern African Institute of Mining and have the expensive nickel additions (Charles et Metallurgy, 2015. ISSN 2225-6253. Paper received Aug. 2015.

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 981 ▲ The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel

emphasis on the processing route, influence this poor surface quality behaviour of AISI 436. This was achieved by studying some crystallographic parameters of the steel, and correlating those parameters to surface topography, in order to help understand the mechanism behind the surface roughening of the steel in subsequent studies. The ultimate objective is to develop possible solutions to the problems experienced in the deep drawing of AISI 436.

Figure 1 – A multi-stage deep-drawing process for wheel-nut covers Experimental procedure Various samples of commercial AISI 436 were received and characterized. These were sampled from a production trial packaging industry, automotive industry, and many others. A operation, as shown in Figure 2, in which two processing typical example of this process is illustrated in Figure 1, routes, direct rolling (DR) and intermediate rolling (IR), were which shows some wheel-nut covers manufactured through a investigated. The emphasis in this study was limited to the six-stage deep-drawing process. final cold-rolled and annealed samples marked F1, F2, F3, In the reported cases of poor surface roughness, tearing, and F4 in Figure 2, with thicknesses ranging from 0.46 mm and cracking, the surface of the drawn products appears to to 0.50 mm. have roughened, possibly due to local differences in All the samples were processed from the same ‘production mechanical properties between grains or grain clusters of the heat’ and hence had the same chemical composition, shown raw material. This behaviour has been suspected to be a in Table I. crystallographic texture effect similar to ridging and roping, Each sample was first mechanically polished, etched in an which is the very common susceptibility of ferritic stainless aqua regia solution, and examined under an optical steel to develop narrow ridges on the sheet surface during microscope to determine the grain structure morphology. forming (Knutsen and Wittridge, 2002; Raabe et al., 2003; Grain-size measurements were determined by the ‘Image J’ Shin et al., 2003). The ridges or undulations result in a very line-intercept method. The samples were then further dull surface appearance, which in turn reduces the surface mechanically prepared to a final fine polish in a colloidal shine and quality of the formed product (Knutsen and silica medium (OP-S) for electron backscatter diffraction Wittridge, 2002). The purpose of this study is to investigate (EBSD) analysis, with the scans being performed using a Jeol how various thermomechanical processes, with particular JSM-IT300LV scanning electron microscope (SEM).

Figure 2 – Block diagram showing the thermomechanical trial processing routes for the AISI 436 samples used in this work ▲ 982 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel

Table I Chemical composition of the AISI 436, wt.%

CMnSiP S CrMoNiAlCuNbTiSnN

0.013 0.42 0.43 0.022 0.002 17.14 0.94 0.25 0.003 0.09 0.41 0.001 0.012 0.017

Table II Chemical composition of the deep-drawn AISI 436 samples, wt.%

Sample C Mn Si P S Cr Mo Ni V Cu Nb Ti Co N

D1 0.015 0.46 0.55 0.02 0.0005 17.04 0.94 0.15 0.12 0.08 0.50 0.002 0.02 0.02 D2 0.017 0.56 0.38 0.02 0.0005 17.23 0.90 0.24 0.10 0.10 0.37 0.003 0.02 0.02

Orientation distribution functions (ODFs) were further of the deep-drawn items (Maruma et al., 2013). calculated by the series expansion method (lmax=22) using Samples of the steel from different production heats, with the SALSA post-processing software of the CHANNEL5 the compositions shown in Table II, were then deep-drawn package. The mean R-values (Rm) and planar anisotropy into lug-nut covers through the multi-stage forming process values (Δr) for the various samples were measured after 10% shown in Figure 1, and analysed for surface roughness. strain along the longitudinal (0°), transverse (90°), and Sample D1 was intermediate rolled (IR) prior to the deep- diagonal (45°) directions. The Rm and Δr values were then drawing process, while D2 was directly rolled (DR). This calculated using the following standard equations: analysis was used to investigate the effect of the production processing route on the surface roughness of the steel.

[1] Results and discussion Microstructural analysis [2] Figure 3 shows the microstructures of the cold-rolled and annealed samples by optical microscopy. Grain size The subscripts 0°, 45°, and 90° refer to the longitudinal, measurements showed an average grain size value of about diagonal, and transverse directions with respect to the rolling 25 μm for the four final cold-rolled and annealed samples (F1 direction. The planar anisotropy (Δr) gives an indication of to F4), with the respective measured values shown in Table the amount of necking or earing that will occur on the edges II. It is evident from these results that the processing route

Figure 3 – Microstructures of the samples after a 30-second deep etching in aqua regia solution. (a) F1 sample, (b) F2 sample, (c) F3 sample, and (d) F4 sample

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 983 ▲ The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel

Table III R-mean, Δr, and measured grain sizes

Sample RHF Dis. temp (°C) Rolling route Annealing after Steckel Rm delta-r Grain size (μm)

F1 1079 IR No 1.55 0.53 24.0 ± 3.7 F2 1083 IR Yes 1.55 0.51 27.6 ± 3.8 F3 1079 DR No 1.65 -1.38 24.2 ± 2.1 F4 1160 DR No 2.07 -1.39 22.0 ± 2.5

had no significant effect on grain size as the average grain as may be seen from the distribution of the ‘hills and valleys’ sizes measured were within an average ±3 μm standard on the micrographs, some areas on the IR sample seem not to deviation from each other. have undergone any deformation (circled regions on ‘D1’ in The optical microstructures also clearly reveal various Figure 4). This observation calls attention to a phenomenon grains that are strongly etched (darker) for all samples, while mentioned by Knutsen and Wittridge (2002), who emphasize others are lightly etched. This disparity in grain morphology the influence of grain size banding in the light of the Hall- is caused by differences in grain orientations (Raabe and Petch effect, hence possible resultant differential yielding Lucke, 1992). In order to evaluate the drawability of the under tension. That is to say, clusters consisting of different sheet samples, measurements of the R-mean (Lankford grain sizes would obviously respond to deformation parameters – Rm) values for each sheet were determined differently, thereby resulting in distortion of the surface. through tensile tests. The results, also shown in Table III, Texture evolution indicated a lower average Rm value for the IR route at about Many studies have shown that ferritic stainless steels, which 1.6 compared to an average Rm value of 1.9 for the DR route. It is of particular interest that the planar anisotropy parameter have a body-centred cubic (bcc) structure, have a tendency to (Δr) for both DR samples is significantly higher in magnitude form a preferred orientation of their grains (fibre texture) during rolling and annealing. Siqueira et al. (2011), Huh et and negative (average value of -1.4) compared to that of the al.,(2005), and Maruma Siyasiya, and Stumpf (2013) have IR samples (0.52). This behaviour is indicative of a rotation elaborated on the fact that rolling deformation in most cases of earing from an angle of 0° and 90° with respect to the leads to a texture characterized by two orientation fibres – an rolling direction (RD) to a 45° or 135° direction with respect α-fibre texture typically comprising orientations with a to RD, which could be attributed to the poor resulting common <110> direction parallel to RD (RD//<110>, and a γ- formability of the DR samples despite their higher R value. m fibre texture comprising orientations of the {111} plane The severity of the surface roughness of the DR samples parallel to the RD. Normally, subsequent annealing of cold- was further illustrated by examining one sample from each rolled sheets increases the γ-fibre component at the expense route by SEM. The micrographs are shown in Figure 4. Both of the α-fibre, with possible improvements in the formability samples were obtained from the ‘wall area’ of a stage two of the sheet metal (Yazawa et al., 2003). Figure 5 shows the sample (as shown in Figure 1) from each route, with D1 orientation distribution function (ODF) maps of the four being from the IR process and D2 from the DR process. It is samples in this work alongside a Bunge notation texture evident, even by visual inspection of this early forming stage diagram showing the main texture fibres in bcc taken at of the two samples, with scans at the same magnification, Φ=45°. The F1 and F2 IR sample textures are predominantly that the DR sample shows severe surface roughness characterized by a strong γ-fibre, notably {111}<011>, compared to the flatter surface for the IR sample. Moreover, {111}<123>, and {111}<112>, as well as {554}<225>. The

Figure 4 – SEM micrographs of deep-drawn AISI 436 sheets after the second stage of the wheel-nut cover forming process. D1: smoother surface of IR sample, and D2: severe surface roughness of DR sample ▲ 984 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel striking difference between the two samples is observed in annealing texture (Maruma et al., 2013). This is possibly the intensities of these γ-fibres, with the F1 sample showing compounded by the absence of subsequent ‘intermediate a stronger intensity of a maximum of about 11 compared to a annealing’ in the DR process, which hindered an increase of maximum of about 5 for the F2 sample. This observation the γ-fibre evolution. Similarly, the F4 DR sample shows a clearly illustrates the insignificance of the AP1 annealing weak α-fibre near {322}<258> as well as a weak γ-fibre in process, which is clearly an additional industry cost. the vicinity of {554}<225>, despite it having the highest The F3 DR sample, however, is characterized by a strong Lankford parameter value (Rm = 2.1). The high negative intensity near or between {322}<258> and {322}<236>. earing parameter (Δr), which has already been alluded to, is Thus, it is evident that a strong texture component in the suspected of adversely affecting the formability of these {100} plane would always have an adverse influence on the sheets (F3 and F4), despite their good ductility parameters.

Figure 5 – Orientation distribution function (ODF) maps for the four samples studied. The adjacent diagram shows the texture fibre positions in Euler space (Raabe and Lucke, 1992)

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 115 OCTOBER 2015 985 ▲ The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel

Figure 6 – Inverse pole figure maps //ND for the first stages of the deep-drawn samples, showing clear indications of grain size clustering

Effect of thermomechanical processing on grain size References distribution CHARLES, J., MITHIEUX, J.D., SANTACREU, P.O. and PEGUET, L. 2008. The ferritic The grain size clustering effect mentioned above is clearly stainless steel family: the appropriate answer to nickel volatility. 6th illustrated by the inverse pole figure (IPF) maps in Figure 6, European Stainless Steel Conference, Helsinki, 10–13 June 2008. particularly on the map for sample D2 (the DR sample), in which there is evident clustering of smaller grains separately G. SEARCH, “Euler space - recrystallization textures of metals,” [Online]. in a particular region, almost forming a band of smaller Available: www.google.com. [Accessed 12 May 2015]. grains, and clustering of larger grains in other regions within HUH, M.-Y., LEE, J.-H., PARK, S.H., ENGLER, O. and RAABE, D. 2005. Effect of the microstructures. However, despite this grain size through-thickness macro and micro-texture gradients on ridging of 17% clustering, which may be responsible for differences in Cr ferritic stainless steel sheet. Materials Technology - Stainless Steels, yielding across the sheet metal plates and hence the poor vol. 11, no. 76. pp. 797–806. surface roughness, no clear texture banding along a particular direction was observed in the microstructures or KNUTSEN, R.D. and WITTRIDGE, N.J. 2002. Modelling surface ridging in ferritic the SEM images, as is normally the case in ridging and stainless steel. Materials Science and Technology, vol. 18. pp. 1279–1285. roping. This leads to the conclusion that another localized grain-related mechanism, rather than ridging, is at play in MARUMA, M.G., SIYASIYA, C.W.and STUMPF, W.E. 2013. Effect of cold reduction this case. and annealing temperature on texture evolution of AISI 441 ferritic stainless steel. Journal of the Southern African Institute of Mining and Conclusions Metallurgy, vol. 113, no. 2. pp. 115–120. ➤ Grain size measurements suggested that the processing route had no significant influence on average grain RAABE, D. and LUCKE, K. 1992. Influence of particles on recrystallization size, but had an effect on grain size distribution textures of ferritic stainless steels. Steel Research, vol. 63, no. 10. ➤ The intermediate rolling (IR) route resulted in the pp. 457–462. desired γ-fibre texture, and hence superior surface qualities (surface roughness) in comparison to the RAABE, D., SACHTLEBER, M., WEILAND, H., SCHEELE, G. and ZHAO, Z., 2003. Grain- direct rolling (DR) route scale micromechanics of polycrystal surfaces during plastic straining. Acta ➤ In addition, the IR route also resulted in lower planar Materialia, vol. 51. pp. 1539–1560. anisotropy (average Δr = 0.52) compared to the DR route (Δr = -1.4), suggesting superior deep-drawability SELLO, M.P. and STUMPF, W.E. 2010. Laves phase embrittlement of the ferritic as observed stainless steel type AISI 441. Materials Science and Engineering Section A, ➤ Grain-size clustering as opposed to texture banding is vol. 527. pp. 5194–5202. suspected as a possible factor responsible for surface roughening. SHIN, H.-J., AN, J.-K., PARK, S.H. and LEE, D.N. 2003. The effect of texture on ridging of ferritic stainless steel. Acta Materialia, vol. 51. pp. 4693–4706. Acknowledgements SIQUEIRA, R.P., SANDIM, H.R.Z., OLIVEIRA, T.R. and RAABE, D. 2011. Composition The authors gratefully acknowledge the financial contribution and orientation effects on the final recrystallization texture of coarse- provided by the Advanced Metal Initiative (AMI) of the grained Nb-containing AISI 430 ferritic stainless steels. Materials Science Department of Science and Technology (DST) through the and Engineering Section A, vol. 528, no. 9. pp. 3513–3519. Ferrous Metals Development Network (FMDN), and Columbus Stainless (Middelburg, South Africa). Special thanks are also YAZAWA, Y., OZAKI, Y., KATO, Y. and OSAMU, F. 2003. Development of ferritic due to Mr Dave Smith and Mr Jaco Kruger (Columbus stainless steel sheets with excellent deep drawability by {111} recrystal- Stainless) for their immense contribution and input into this lization texture control. Society of Automotive Engineers (SAE) of Japan, study. vol. 24. pp. 483–488. ◆ ▲ 986 OCTOBER 2015 VOLUME 115 The Journal of The Southern African Institute of Mining and Metallurgy 
    

14–17 March 2016, Gaborone International Convention Centre Workshop: 14 March 2016 Conference: 15 –16 March 2016 Technical Visit: 17 March 1026

BACKGROUND eing the sixth conference in the series, the Diamonds—still Sparkling Conference targets the full spectrum of the diamond pipeline from exploration through to sales Band marketing. The last conference was held in 2013 at Misty Hills, Muldersdrift: the 2016 conference is returning to Botswana which previously hosted in 2010. Photographs of diamonds courtesy Petra Diamonds

   > Cutting and polishing Sponsors: > he objective of the conference will be to provide a Marketing and sales > forum for the dissemination of information relating Diamontiers T to the latest mining methods and technologies > Mine managers applicable to the diamond mining industry. This will > Mining companies consider all stages of the value chain, from exploration > Students mining industry through mine design, drilling and blasting production, Conference Supporter Media Partner and processing, to cutting, marketing and sales.   > Geology and exploration
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2015 12–14 October 2015 — Slope Stability 2015: 13–14 April 2016 — Mine to Market Conference 2016 International Symposium on slope stability in open pit South Africa mining and civil engineering Contact: Yolanda Ramokgadi In association with the Surface Blasting School Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 15–16 October 2015 E-mail: [email protected] Cape Town Convention Centre, Cape Town Website: http://www.saimm.co.za Contact: Raymond van der Berg Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 17–18 May 2016 — The SAMREC/SAMVAL Companion E-mail: [email protected] Volume Conference Website: http://www.saimm.co.za Johannesburg Contact: Raymond van der Berg 20 October 2015 — 13th Annual Southern African Student Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 Colloquium E-mail: [email protected] Mintek, Randburg, Johannesburg Website: http://www.saimm.co.za Contact: Yolanda Ramokgadi Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 21–28 May 2016 — ALTA 2016 E-mail: [email protected] Perth, Western Australia Website: http://www.saimm.co.za Contact: Allison Taylor Tel: +61 (0) 411 692 442 21–22 October 2015 — Young Professionals 2015 E-mail: [email protected] Conference Website: http://www.altamet.com.au Making your own way in the minerals industry Mintek, Randburg, Johannesburg May 2016 — PASTE 2016 International Seminar on Paste Contact: Camielah Jardine and Thickened Tailings Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 Kwa-Zulu Natal, South Africa E-mail:[email protected] Contact: Raymond van der Berg Website: http://www.saimm.co.za Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 E-mail: [email protected] 28–30 October 2015 — AMI: Nuclear Materials Website: http://www.saimm.co.za Development Network Conference Nelson Mandela Metropolitan University, North Campus 9 –10 June 2016 — 1st International Conference on Solids Conference Centre, Port Elizabeth Handling and Processing A Mineral Processing Perspective Contact: Raymond van der Berg South Africa Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 Contact: Raymond van der Berg E-mail: [email protected] Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 Website: http://www.saimm.co.za E-mail: [email protected] 8–12 November 2015 — MPES 2015: Twenty Third Website: http://www.saimm.co.za International Symposium on Mine Planning & Equipment 1–3 August 2016 — Hydrometallurgy Conference 2016 Selection ‘Sustainability and the Environment’ Sandton Convention Centre, Johannesburg, South Africa in collaboration with MinProc and the Western Cape Branch Contact: Raj Singhal Cape Town E-mail: [email protected] or E-mail: [email protected] Contact: Yolanda Ramokgadi Website: http://www.saimm.co.za Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 E-mail: [email protected] 2016 Website: http://www.saimm.co.za 14–17 March 2016 — Diamonds still Sparkle 2016 16–19 August 2016 — The Tenth International Conference Heavy Minerals Conference ‘Expanding the horizon’ Gaborone International Convention Centre Sun City, South Africa Contact: Yolanda Ramokgadi Contact: Camielah Jardine Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 Tel: +27 11 834-1273/7, Fax: +27 11 838-5923/833-8156 E-mail: [email protected] E-mail: [email protected] Website: http://www.saimm.co.za Website: http://www.saimm.co.za

       
  
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Namakwa Sands (Pty) Ltd Namakwa Sands (Pty) Limited New Concept Mining (Pty) - Zondereinde Northam Platinum Ltd SA (Pty) Ltd Osborn Engineered Products Limited Outotec (RSA) (Proprietary) PANalytical (Pty) Ltd Paterson and Cooke Consulting Engineers (Pty) Ltd Thyssenkrupp Polysius A Division Of Ltd Industrial Solutions (Pty) Precious Metals Refiners Rand Refinery Limited Redpath Mining (South Africa) (Pty) Ltd Rosond (Pty) Ltd Royal Bafokeng Platinum Roymec Tecvhnologies (Pty) Ltd Runge Pincock Minarco Limited Rustenburg Platinum Mines Limited SAIEG Salene Mining (Pty) Ltd Sandvik Mining and Construction Delmas (Pty) Ltd Sandvik Mining and Construction RSA(Pty) Ltd SANIRE Sasol Mining(Pty) Ltd Scanmin Africa (Pty) Ltd Sebilo Resources (Pty) Ltd SENET Senmin International (Pty) Ltd Shaft Sinkers (Pty) Limited Sibanye Gold (Pty) Ltd Smec SA SMS Siemag South Africa (Pty) Ltd SNC Lavalin (Pty) Ltd Sound Mining Solutions (Pty) Ltd South 32 SRK Consulting SA (Pty) Ltd Technology Innovation Agency Time Mining and Processing (Pty) Ltd Tomra Sorting Solutions Mining (Pty) Ltd Ukwazi Mining Solutions (Pty) Ltd Umgeni Water VBKOM Consulting Engineers Webber Wentzel Weir Minerals Africa WorleyParsons (Pty) Ltd        Engineering and Project Company Ltd Engineering and Project eThekwini Municipality Exxaro Coal (Pty) Ltd Exxaro Resources Limited Fasken Martineau Ltd FLSmidth Minerals (Pty) Elbroc Mining Products (Pty) Ltd Elbroc Mining Products Ltd Fluor Daniel SA (Pty) Johannesburg Franki Africa (Pty) Ltd Fraser Alexander Group Glencore Goba (Pty) Ltd Hall Core Drilling (Pty) Ltd Hatch (Pty) Ltd Herrenknecht AG HPE Hydro Power Equipment (Pty) Ltd Impala Platinum Limited IMS Engineering (Pty) Ltd JENNMAR South Africa Joy Global Inc. (Africa) Leco Africa (Pty) Limited Longyear South Africa (Pty) Ltd Lonmin Plc Ludowici Africa Lull Storm Trading (PTY)Ltd T/A Wekaba Engineering Magnetech (Pty) Ltd Magotteaux(PTY) LTD MBE Minerals SA Pty Ltd MCC Contracts (Pty) Ltd MDM Technical Africa (Pty) Ltd Metalock Industrial Services Africa (Pty)Ltd Metorex Limited Metso Minerals (South Africa) (Pty) Ltd Minerals Operations Executive (Pty) Ltd MineRP Holding (Pty) Ltd Mintek MIP Process Technologies Modular Mining Systems Africa (Pty) Ltd MSA Group (Pty) Ltd Multotec (Pty) Ltd Murray and Roberts Cementation Nalco Africa (Pty) Ltd Company Affiliates Company   
 The following organizations have been admitted to the Institute as Company Affiliates to the Institute as have been admitted organizations The following viii AECOM SA (Pty) Ltd AEL Mining Services Limited Air Liquide (PTY) Ltd AMEC Mining and Metals (Pty) Ltd AMIRA International Africa Ltd ANDRITZ Delkor(Pty) Anglo Operations Ltd Anglo Platinum Management Services (Pty) Ltd Anglogold Ashanti Ltd Atlas Copco Holdings South Africa (Pty) Limited Aurecon South Africa (Pty) Ltd Aveng Moolmans (Pty) Ltd Axis House (Pty) Ltd Bafokeng Rasimone Platinum Mine Barloworld Equipment -Mining BASF Holdings SA (Pty) Ltd Bateman Minerals and Metals (Pty) Ltd BCL Limited Becker Mining (Pty) Ltd BedRock Mining Support (Pty) Ltd Bell Equipment Company (Pty) Ltd Blue Cube Systems (Pty) Ltd Bluhm Burton Engineering (Pty) Ltd Blyvooruitzicht Gold Mining Company Ltd BSC Resources CAE Mining (Pty) Limited Caledonia Mining Corporation CDM Group CGG Services SA Chamber of Mines Concor Mining Concor Technicrete Council for Geoscience Library CSIR-Natural Resources and the Environment Department of Water Affairs and Forestry Deutsche Securities (Pty) Ltd Digby Wells and Associates Downer EDI Mining DRA Mineral Projects (Pty) Ltd DTP Mining Duraset L  EXHIBITS/SPONSORSHIP Companies wishing to sponsor and/or exhibit at any of these events should contact the 
 conference co-ordinator as soon as possible

SAIMM DIARY 2015 or the past 120 years, the N SYMPOSIUM Southern African Institute of International Symposium on slope stability in open pit mining and civil engineering FMining and Metallurgy, has 12–14– October 2015 promoted technical excellence in In association with the Surface Blasting School the minerals industry. We strive 15–16 October 2015, Cape Town Convention Centre, Cape Town N COLLOQUIUM to continuously stay at the cutting 13th Annual Southern African Student Colloquim 2015 edge of new developments in the 20 October 2015, Mintek, Randburg, Johannesburg mining and metallurgy industry. N CONFERENCE The SAIMM acts as the Young Professionals 2015 Conference 21–22 October 2015, Mintek, Randburg, Johannesburg corporate voice for the mining N CONFERENCE and metallurgy industry in the AMI: Nuclear Materials Development Network Conference South African economy. We 28–30 October 2015, Nelson Mandela Metropolitan University, actively encourage contact and North Campus Conference Centre, Port Elizabeth N SYMPOSIUM networking between members MPES 2015: Twenty Third International Symposium on Mine and the strengthening of ties. Planning & Equipment Selection The SAIMM offers a variety of 8–12 November 2015, Sandton Convention Centre, Johannesburg, South Africa conferences that are designed to bring you technical knowledge 2016 and information of interest for the N CONFERENCE good of the industry. Here is a Diamonds still Sparkle 2016 Conference 14–17 March 2016, Gaborone International Convention Centre glimpse of the events we have N CONFERENCE lined up for 2015. Visit our Mine to Market Conference 2016 website for more information. 13–14 April 2016, South Africa N CONFERENCE The SAMREC/SAMVAL Companion Volume Conference 17–18 May 2016, Johannesburg N SEMINAR PASTE 2016 International Seminar on Paste and Thickened Tailings May 2016, Kwa-Zulu Natal, South Africa N CONFERENCE 1st International Conference on Solids Handling and Processing A Mineral Processing Perspective For further information contact: 9 –10 June 2016, South Africa Conferencing, SAIMM N CONFERENCE P O Box 61127, Marshalltown 2107 Hydrometallurgy Conference 2016 Tel: (011) 834-1273/7 1–3 August 2016, Cape Town Fax: (011) 833-8156 or (011) 838-5923 N CONFERENCE E-mail: [email protected] The Tenth International Heavy Minerals Conference ‘Expanding the horizon’ 16–19 August 2016, Sun City, South Africa

Website: http://www.saimm.co.za