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Zirconium Alloys in Nuclear Technology

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Zirconium Alloys in Nuclear Technology Proc. Indian Aead. Sci. (Engg. Sei.) Vol. 4, Pt. 1, April 1981: pp. 41-56. ~) Printed in India. Zirconium alloys in nuclear technology R KRISHNAN and M K ASUNDI Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085 MS received 17 October 1980 Abstract. This paper describes the historical development of zirconium and its alloys as structural materials for nuclear reactors. The various problems encountered in the early stages of the development of zircaloys and their performance in reactors operating presently are described in detail. The development of Zr-2.5~o Nb alloys for pressure tube applications is discussed. The paper concludes with a detailed dis- cussion on the development potential of zirconium alloys for high temperature appli- cations and a brief account of the work carried out at Trombay in this field. Keywords. Zirconium; zirconium alloys; structural materials; nuclear reactors; corrosion; high strength alloys. 1. Introduction The development of zirconium metallurgy is essentially due to the nuclear industry, where zirconium alloys have come to be regarded as the proven structural material. This is primarily because of their unique combination of good corrosion resistance in water near 300~ and low capture cross-section for thermal neutrons (Douglass 1971). It is quite likely that the application of zirconium in the nuclear industry will remain its dominant use. This paper begins with an account of the present day use of zirconium alloys in the nuclear industry mainly to acquaint non-nuclear techno- logists with the various sizes, shapes and functions of such structural materials. Then the history of the development of presently accepted zirconium alloys--zirca- loy-2 and zircaloy-4---is given some consideration. A diseussionon the development of high-strength zirconium alloys follows and the paper concludes with a brief account of the work being done at Trombay. 2. Current applications There are two types of water-cooled power reactors. In the first, the whole reactor core is enclosed in a large steel pressure vessel in which one fluid, usually ordinary water, is both the heat transfer medium and neutron moderator. Such a reactor is based on the pressurised water reactor (PWR) and boiling water reactor (BWR) concepts, where zirconium is used for the cladding tubes which encase the uranium dioxide fuel pellets. The reactor operating at Tarapur near Bombay is an example of the BWR reactor. In the second, the pressure vessel ~ replaced by a large number of tubes each con- veying either light or heavy water coolant over an individual fuel element. These 41 Prec.--3 42 R Krishnan and M K Asundi pressure tubes pass through a calandria tank containing cool, heavy water mode- rator. The pressure tube is separated from the surrounding calandria tube by a gas gap which insulates the moderator from the hot coolant. Operating reactors of this type are the pressurised heavy water-cooled CANDU-PHW and the boiling light water-cooled CANDU-BLW and the British steam-generating heavy water reactor SGHWR. The nuclear power station at Rana Pratap Sagar at Kota in Rajasthan is an example of the CANDU-PHW system. In these reactor systems, the fuel-clad- ding tubes, the pressure tubes and sometimes the calandria tubes are all made of zirconium alloy. Fuel-cladding tubes are thin-walled (0.4 to 0.8 mm) and experience a complex sequence of heterogeneous tensile, creep and recovery strains during their operating life of 15,000 to 30,000 hr. The pressure tubes in CANDU reactors are 103 mm ID and the minimum wall thickness is in the range of 2.4 to 5.1 mm depending on the strength of the alloy and the operating conditions (Ross-Ross 1968). Tubes for the SGHWR are of 130 mm ID and a minimum wall thickness of 5 mm (Watkins & Cockady 1965). These pressure tubes are subjected to the full coolant pressure (55-60 bar in SGHWR, CANDU-BLW and 85-90 bar in CANDU-PHW) and cool- ant temperature of 200-300~ Unlike the fuel-cladding tubes, these tubes experience a simple biaxial stress system with a hoop-to-axial stress ratio of approximately 2:1. The tubes must have very high integrity throughout a plant life perhaps exceeding 200,000 hr. Although failure can be tolerated and faulty tubes replaced, it is very expensive. The zircaloy calandria tubes typically have a wall thickness of 1.24 mm and 107-7 mm ID, in a CANDU-PHW system. These tubes are generally fabricated from rolled sheets by seam welding. The demands made on zircaloy calandria tubes are less exacting than those made on fuel or pressure tubes (Cheadle 1977). 3. Historical development The decision to use zirconium in a nuclear reactor was essentially due to Admiral Rickover of the US Navy (Rickover 1975). He was in charge of the development of nuclear-propelled ships and submarines. The reactor for navigation had necessarily to be compact and had to operate when the ship was rolling or pitching or at an angle, when the submarine was diving or surfacing. Thus a pressurised water reactor (PWR) was envisaged, and hence a metal which would withstand corrosion at high temperatures for long periods of time, which would maintain its integrity in an envi- ronment of intense radiation and which would not absorb neutrons required for the nuclear reaction, was needed. The availability of the metal in sufficient quantities and at reasonable cost would be the additional consideration. Stainless steel, beryl- lium and aluminiurn all had disadvantages which weighed against their use. Zirco- nium also did not appear promising as it was expensive and not produced in quantity. Initial tests with zirconium showed that it absorbed neutrons needed for the fission process. It was discovered later that naturally-occurring zirconium contained about 2 ~ by weight of hafnium, which gave zirconium the high level of absorption cross- section for neutrons. Kaufman at MIT and Pomerance at Oak-Ridge came out with laboratory successes in separating hafnium from zirconium and showed that zirconium in its pure form absorbed only a few neutrons. These early promises Zirconium alloys in nuclear technology 43 led Admiral Rickover to decide that zirconium would be the reference core structural material for the Mark I PWR in the Idaho prototype and for the Mark II reactor being built for the nuclear submarine, Nautilus. Though the decision was taken by the U. S. Navy to use zirconium as the reactor structural material, no technical specifications had been elaborated. The only thing they knew was that the metal should be ' as pure as possible' and the minimum strength limits were to be' as strong as possible without increasing the hardness '. Naturally, the choice fell on the crystal-bar process (also called the Van-Arkel- DeBoer Process) for the production of high-purity zirconium. This process is essen- tially a refining rather than a metal reduction technique. In this process zirconium tetra-iodide is formed at the surface of a bed of impure metallic zirconium main- mined at low temperature (approximately 400~ by reaction with free iodine and is decomposed at the surface of a hot (approximately 1400~ filament, depositing pure zirconium on the filament and liberating iodine vapour to react again with the impure zirconium. The end product was designated as the 'crystal bar' because of the bright surface and the external shape of the grains that grew on the filament. When crystal bar zirconium was corrosion-tested in high temperature water auto- claves, it was found that corrosion-resistant material became black after testing, while non-corrosion resistant samples became white, the different appearances being related to the thickness and physical nature of the zirconium oxide corrosion product film. Initial experiments with crystal bar zirconium showed a bewildering variety of corrosion test behaviour. This led to the problem of detecting and measuring the culprit impurity elements in zirconium. It was found that if the refining process could be carried out under sufficiently ' antiseptic' conditions, a reproducible high level of corrosion resistance could be obtained. At this stage, the decision was taken to use a purer grade of sponge zirconium as the raw material (for crystal bar), which had then become possible by the Kroll process. In the Kroll process, zirconium tetrachloride vapour, prepurified by subli- mation, is brought into contact with the reductant, molten magnesium, using pre- cautions to exclude air during all critical operations. The use of this high-purity raw material coupled with improved operating practices and procedures resulted in an improved yield of corrosion-resistant product. Every one connected with zirconium technology knew that continued purification of the product was not necessarily the correct path. In fact, it was then observed that the crystal bar stock received from two different sources did not behave in a similar manner. Contrary to expectations, it was found that the corrosion resistance of the purer material was lower than that of the impure crystal bar. It was argued that the pick-up of impurity elements like iron, chromium and nickel might be helpful in imparting corrosion resistance to zirconium. Even with intentional additives, zirconium showed poor corrosion resistance in the presence of nitrogen. It was ultimately decided that refining the Kroll sponge into crystal bar zirconium might not be the right step and thus the programme of development of KroU-process sponge- base alloys was started. 44 R Krishnan and M K Asundi 4. Development of zircaloys The first stage in the development of zirconium alloys was to identify those elements which improved the corrosion behaviour of Kroll sponge zirconium to a level appa- rently equivalent to that of the good quality crystal bar. These elements were tin, tantalum and niobium, which, in decreasing order of effectiveness, circumvented the damaging effect of the impurities present in the sponge then available.
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