RADIATION DAMAGE IN SOLIDS AND REACTOR MATERIALS

Hardly considered a separate area of science General Theory: Radiation Damage in some 20 to 25 years ago, solid state physics has now become a vigorous and rapidly growing discipline. Metals and Alloys The reason for its great stimulation is that modern At the first session of the symposium, theo­ technology has placed increasingly drastic demands retical physicists discussed the basic processes of on materials. Nowhere perhaps is this trend more radiation damage and its effects in solids and other evident than in nuclear technology, where the novel materials. The discussions showed that good pro­ demands for certain optimum combinations of physical gress had been made in recent years in understanding properties have necessitated an almost complete re­ the effects of interatomic collisions in solids, though examination of materials and their associated proper­ more theoretical work was needed to understand fully ties from both fundamental and technological points the nature and mechanism of these effects. of view. One of the most important requirements imposed by this new application is an entirely new one, viz. radiation stability. In order for a material Presenting a paper on analogies between radi­ ation effects on solids caused by nuclear radiation and to be useful within a nuclear reactor it must be able bya flux of slow charged particles, S.V.Starodubtsev to withstand not only high temperatures but also high (USSR) pointed out that at present problems connected fluxes of energetic nuclear radiation for prolonged with changes in the physical properties of various periods. materials under irradiation acquired more and more Nuclear particles passing through reactor ma­ importance. He described some experiments to show terials transfer part of their energy to the atoms of that many phenomena in radiation physics and cathode these materials and eject some of them from their electronics are related. On the basis of his experi­ normal positions in the materials. The cumulative ments, Professor Starodubtsev suggested a pro­ result is manifested in significant modifications of gramme for model studies of the effects of reactor the physical properties of irradiated materials. These radiations on materials. properties include physical dimensions, strength and hardness, conductivity of heat and electricity, mag­ J.R. Beeler and D.G. Besco (USA) presented a netism, resistance to corrosion, and many others. paper in which they described the use of anIBM com­ Because of these modifications it is necessary to know puter in radiation damage studies, while a paper by the service-life of any material considered for use J. Lindhard and P. V. Thomsen (Denmark) discussed in nuclear technology, and this can be done only through radiation effects caused by electrons and other nuclear research on radiation damage. particles. In view of the important role that knowledge of Three more papers presented by scientists from the mechanisms of radiation damage plays in nuclear Belgium, Romania and the USA dealt with certain other technology, the International Atomic Energy Agency aspects of the general theory of radiation damage. held a Symposium on Radiation Damage in Solids and Two sessions of the symposium were devoted to Reactor Materials from 7 to 11 May 1962 in Venice, radiation damage in pure metals and one session to Italy. that in alloys. Over 200 scientists from Austria, Belgium, Canada, Czechoslovakia, Denmark, France, Germany, Discussing the nature of radiation damage in Greece, Hungary, Italy, Japan, the Netherlands, metals, A. Seeger (Federal Republic of Germany) said Norway, Pakistan, Poland, Portugal, Romania, South that work done during the past three years supported Africa, Spain, Sweden, Switzerland, the Soviet Union, the picture of the radiation damage by fast neutrons the United Kingdom, the United States and Yugoslavia given by him at the second International Conference and from EURATOM attended the symposium. Of some on the Peaceful Uses of Atomic Energy in 1958. More 90 papers submitted, 38 were presented individually information on radiation damage had been obtained by and the others considered in panel discussions. The electron transmission microscopy and by ferromag­ papers covered the following fields of radiation damage netic measurements. Considerable work had recently research: General Theory; Pure Metals; Metallic been done on radiation hardening and on the plastic and Ceramic Nuclear Fuel; Graphite and Beryllium deformation of radiation hardened single crystals. Oxide; Moderator Materials; Radiation Damage in Professor Seeger then discussed some aspects of ra­ Semiconductors, Ionic and other Non-Metallic Crys­ diation damage in the noble metals and nickel, caused tals and Special Techniques. by electron and fast neutron irradiation.

15 R.O. Simmons, J.S. Koehler and R. W. Balluffi (USA) stated in a paper that bombardment of pure metals by energetic particles produced various defects and, because most of these defects were of atomic dimensions, their study by direct observation had so far been very limited. The presence of such atomic defects in a metal altered many physical properties of a solid, such as electrical resistivity, density, stored energy and elastic and inelastic properties. In general, the authors said, two types of experiments had been used to learn something about the defects produced by irradiation and therefore about the atomic mechanisms responsible for phenomena observed in irradiated metals. Firstly, the rate and type of de­ fects estimated by theory for a given kind of irradiation could be compared to measured changes in physical properties. This method had been most useful in the case of electron irradiation. Secondly, thermal an­ nealing (very slow regulated cooling) of a metal sub­ sequent to irradiation could be carried out to measure the physical property changes occurring in different At the opening session of the Venice symposium, temperature ranges. This method had been most from left to right: Dr. Henry Seligman, IAEA Deputy useful at the lowest temperatures. The authors Director General in charge of Research and Isotopes; Dr. Vittorio Branca, from Fondazione Cini; pointed out that numerous investigations had been made Mr. Manfredo De Bernart, Prefect of Venice; Dr. Sigvard Eklund, Director General of IAEA; of radiation damage to the crystal structure of metals Mr. Favaretto Fisco, Mayor of Venice; and and many attempts had been made to identify the atomic Prof. Arkadij Rylov, IAEA Deputy Director General defects and mechanisms responsible. While theory in charge of Training and Technical Information had been able to furnish a rough guide to interpre­ tation, many explanations had unfortunately been of an ad hoc character. Many of the difficulties would United Kingdom and the United States, were devoted be resolved only when more precise knowledge con­ to different aspects of radiation damage in pur e metals. cerning the simplest defects in metals became avail­ Discussing radiation damage in alloys, A.C. able. Damask (USA) pointed out that control over the type A. A. Johnson (United Kingdom), Mrs. N. Milasin and distribution of the defects was particularly im­ (Yugoslavia) and F.N. Zein(UAR), in a joint paper on portant in alloys, because any given thermodynamic the embrittlement of metals by neutron irradiation, state of an alloy was characterized by a positional pointed out that experiments in this field had so far relationship of the atoms. Mr. Damask discussed been of three main types. First, attempts had been the ways in which this relationship could be altered made to observe the radiation damage directly by by heavy particle bombardment. electron microscopy. This method had been applied Presenting a paper on radiation damage in to iron and molybdenum but it had been found that in zirconium, zircaloy-2 and stainless steel, L..M. Howe these metals the damage was on too fine a scale to be (Canada) said that the most suitable materials for fuel resolved in the microscope. Secondly, attempts had sheathing and pressure tubes were zirconium alloys been made to deduce the nature of the damage from because they combined the best available properties the changes in mechanical properties which it pro­ of strength, corrosion resistance, neutron economy duced. This approach had been extensively used, but and ease of fabrication. He said that the Canadian was limited by incomplete understanding of the me­ reactors NPD (Nuclear Power Demonstration) and chanical properties of irradiated metals. Finally, CANDU (Canadian Deuterium Uranium) would use changes in physical properties, such as electrical zircaloy-2, a zirconium alloy containing tin and iron, resistivity, had been measured and their recovery nickel and chromium combined. In these reactors followed when the irradiated metal was warmed above the pressure tubes would be operating near the tem­ the irradiation temperature. This technique, the perature of the pressurized coolant flowing within authors stated, was particularly useful in studying them and therefore must have sufficient strength to defects left in the crystal structure by irradiation. contain the coolant and must maintain this strength They reviewed and correlated the published results under the conditions of high temperature and neutron of these three types of experiment and described a irradiation over long periods of time. To increase set of measurements in which all three methods had the mechanical properties of zircaloy-2 a series of been applied to the study of the behaviour of irradi­ irradiation experiments had been carried out. The ated molybdenum when it was warmed in a temperature results showed that in some cases such properties of range of approximately 100 to 300 C. the alloy as tensile and yield strengths were improved. An additional purpose of the experiments was to obtain information on the mechanism of radiation hardening Other papers, presented by scientists from in zircaloy-2 and zirconium. Belgium, France, Germany, Romania, the USSR, the

16 M.S. Wechsler and R.H. Kernohan (USA) re­ mechanisms of the observed damages and to establish viewed the work at the Oak Ridge National Laboratory optimum parameters for fabrication and successful on the effects of radiation on copper-base alloys. operation of beryllium oxide-moderated reactors. These studies indicated that some reactions in the Although appreciable effects in metals are pro­ alloys were enhanced as a result of exposure to duced by radiation of relatively short time in reactors, neutron, electron and gamma-ray bombardment. They the effects are still more apparent in other materials, also indicated the conditions under which irradiated particularly in semiconductors, such as germanium. alloys might be unstable, thus influencing the use of structural materials in radiation environments. At a special session of the symposium, P.Baruch Some of the papers presented on radiation (France) read a survey paper on radiation damage in damage in alloys by scientists from Austria, France, semiconductors, which was followed by a panel dis­ Italy, the USSR and the USA reported on experimental cussion based on eight papers from France, Greece, work done on different kinds of steel, where radiation Poland, Romania and the USA. effects on the mechanical properties are of techno­ J.W. MacKay and E.E. Klontz (USA) said that logical importance, because of the wide use of steel the efficiency for the production of defects in semi­ as structural or pressure vessel material. conductors was strongly dependent on the temperature during irradiation. This was shown by experiments Fissionable Materials, carried out by the authors to study the radiation damage Moderator Materials, Semiconductors mechanism in germanium. P. Baruch and J.C. Pfister (France) presented The symposium next reviewed the state of a detailed report on experiments to study the effects current knowledge of radiation effects in materials of radiations on the diffusion of impurities (such as most important for the construction of reactors: the gallium, boron and phosphorus) in semiconductors. nuclear fuels themselves in the form of metallic uranium or ceramic compounds of uranium with carbon At another panel discussion, which took place and oxygen. This was done in the form of a panel duringthe samesession, E. L. Andronikashvili (USSR) discussion, atwhichA. Herpin (France) surveyed nine presented a survey of seven papers on radiation papers submitted by participants from India, France, damage in ionic and other non-metallic crystals, which the USSR, the United Kingdom and the USA. The were submitted by participants from the Federal Re­ most important phenomena reviewed were uranium public of Germany, Italy, Japan, Poland, Romania cracking and uranium swelling under irradiation and and the USSR. the study of diffusion and precipitation of rare gases in non-irradiated and irradiated fissionable metals. One of these papers, submitted by K. Heine, G.B. Schmidt and W. Herr (Germany), showed that At another panel discussion, W.E. Roake (USA) chemical aspects were also important in explaining presented a survey paper on radiation damage in radiation damage in ionic crystals. ceramic nuclear fuel. In a paper on the effects of neutron irradiation Irradiation behaviour of uranium-carbide fuels on magnetic materials for nuclear techniques, at low and high burn-up levels and the low temperature E. Labusca, N. Andreescu and C. Motoc (Romania) irradiation defects in cast uranium carbide were dis­ described a number of experiments which helped them cussed on the basis of recent research work reported to establish the conditions in which some magnetic in seven papers submitted by scientists from France, materials could be used in devices working in radi­ the Netherlands, the United Kingdom and the USA. ation fields. Another important problem discussed at the symposium was radiation damage in moderators, like Special techniques graphite and beryllium oxide. The panel discussion on this subject, with a survey of graphite presented At the last session of the symposium, scientists by J.H.W. Simmons (United Kingdom), showed that from France, Japan, Romania, the USSR, the United in the last few years great progress had been made Kingdom and the USA presented seven papers on in understanding the phenomena of radiation damage special techniques and methods in studying radiation in graphite. damage. G.W. Keilboltz, J.E.Lee Jr., R.P. Shields R.R.Coltman(USA) described the use for radi­ and W.E. Browning Jr. (USA), in a paper on radiation ation damage studies at the very low temperatures of damage in beryllium oxide, said that the attractive liquid helium, in a special device called a "helium nuclear properties of beryllium oxide and its excel­ cryostat" installed near the centre of the Oak Ridge lence as a high-temperature ceramic recommended Graphite Reactor. With the help of this device, studies its consideration as a moderator or reflector for a were made of electrical resistivity, stored energy variety of power reactors in the USA and in other release, length change, yield stress and other proper­ countries. Satisfactory and economical use of be­ ties. Mr. Coltman said that the use of various re­ ryllium oxide required that the material withstand very actor fuel configurations around the cryostat and of high integrated neutron doses. The authors described a movable shield around the specimens made it possi­ experiments now in progress to determine specific ble to examine the damage caused by thermal neutrons.

17 reactor fast neutrons and fission neutrons quite inde­ of polymeric and other materials were bound together. pendently. He discussed particularly the damage Later this method had been used to increase the caused by gamma rays emitted by the specimens them­ strength of binding rubber surfaces with certain selves as a result of capture of thermal neutrons. He fabrics. The results showed that the binding strength also discussed the importance of these types of was up to four times greater than that reached by other measurement in understanding the basic mechanisms methods. of radiation damage. The participants also had informal meetings to V. E. Goldansky (USSR) described what is known assess the most important current problems in the as the nuclear spot welding method and some of its field and to discuss the experimental and theoretical applications. He said that the emission of heavy methods by which they might be tackled. This helped charged particles in some nuclear reactions causes in defining a number of directions in which research intense heating within the microvolumes at the particle programmes could be planned and conducted. tracks. The introduction of lithium or boron, which The following scientists acted as chairmen of undergo such reactions, into the surface layer dividing the scientific sessions or as leaders of panel dis­ any two materials and the subsequent irradiation of cussions: Professor A. Seeger (Germany), Dr. H.G. these materials by thermal neutrons causes heating VanBueren(Netherlands), Professor S.V. Starodubtsev of a large number of such microvolumes at the sur­ (USSR), Professor E.W.J. Mitchell (United Kingdom), face of contact and results in the so-called "nuclear Dr. E. Nagy (Hungary), Dr. A. Herpin (France), spot welding" - a very strong binding of these ma­ Dr. W.E. Roake (USA), Dr. J.H.W. Simmons (United terials with a relatively weak radiation-chemical Kingdom), Dr. W. Kosiba (EURATOM), Professor P. effect. Mr. Goldansky said that the possibilities of Baruch (France), Professor E. Andronikashvili this method were first demonstrated when a number (USSR) and Dr. L. Giulotto (Italy).

REPORTS FROM IAEA FELLOWS

Training in various branches of nuclear science Alfonso E. Leon Guim, of Ecuador, received and technology is one of the most important ways in nine months' training in the Radiotherapy Department which IAEA has been endeavouring to promote the of the Queen Elizabeth Hospital, Birmingham, UK. peaceful uses of atomic energy throughout the world, He says that during the period of his training he was and by the end of last year more than 500 trainees able to gain wide experience in radiotherapy, includ­ had completed their studies under the Agency's fel­ ing the treatment of patients by radium, X-rays and lowship programme. In an attempt to assess the radioisotopes. He also attended a course at the value of this programme, the Agency requests the Harwell Isotope School and a medical course at the fellows themselves, after they have completed their Christie Hospital and Holt Radium Institute in training and returned to their home countries, to Manchester. After returning home and at the time send brief reports on their training and the use being of writing his report, he was working as honorary made of it in their work. assistant in the General Hospital in Guayaquil City Summaries of a few of these reports were car­ and as a private practitioner. He says: "The ex­ ried in the last issue of this Bulletin; a few more are perience I have had during my training in England summarized here. has given me a new opportunity to work on the diag­ nosis and treatment of cancer with patients in the Raul Brenner, a Brazilian scientist, who has General Hospital and in my private office. " been in charge of the Electronics Department of the Atomic Energy Institute in Sao Paulo since 1958, was Laodamas Sklavenitis, a Greek scientist, re­ awarded an IAEA fellowship for studies and practical ceived ayear's training at the Technische Hochschule training at the Brookhaven National Laboratory, USA. in Munich, Germany, and at Harwell, England. He Apart from designing electronic circuits and servic­ had practical training in Munich, followed by a three- ing electronic instruments at the Institute in Sao month general course at Harwell on radiation protec­ Paulo, he has also been teaching electronics as part tion. After his return to Greece, he has been head of a course in nuclear engineering given at the Insti­ of the Health and Safety Division of the Greek Atomic tute. On the value of his training, he says it gave Energy Commission, his duties including the organi­ him "an invaluable introduction to the new and extra­ zation of the Health and Safety Division of the Com­ ordinary field of semiconductor devices which are mission and of the Health Physics Section of the becoming so important in electronic instrumentation". Democritus Nuclear Centre. He has also been en-

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