ARTICLE NUCLEAR MATERIALS – FISSILE, FERTILE AND DUAL-USE STRUCTURAL MATERIALS INVOLVED IN NUCLEAR REACTORS N. R. DAS* The article presents a brief account of nuclear materials, with special emphasis on fissile, fertile and some important dual-use structural materials generally involved in nuclear reactors. The dual- use structural materials utilized in nuclear reactors have got important applications in both nuclear and non-nuclear fields. In the hostile environment, the important phenomena such as interactions between fission products and the surrounding elemental species, radiation-induced effects, corrosion, generation of gases, swelling and so forth, become increasingly complex and the performance of a nuclear reactor system thus becomes very much dependent on the physicochemical stability and nuclear compatibility of the dual-use structural materials used in fuel sub-assembly towards the fuel elements. In this context, the dual-use structural materials like stainless steel, zirconium alloys, etc. as claddings; water, liquid sodium or gases, etc. as coolants and water, boron, etc. as moderators, having good reliability and appropriate nuclear compatibility with the fuels, are of prime importance in reactor technology. In advanced designed reactors, development of novel fuels coupled with efficient dual-use structural materials may mitigate the challenges involved in optimizing the efficiency of the power reactors under specific experimental conditions. Introduction presented as 14N(, p)17O. In 1932, J. Chadwick discovered the fundamental particle, neutron, by bombardment of boron ince the discovery of X-rays by Wilhelm C. with alpha particle through the nuclear reaction,10B( , Roentgen and ‘Becquerel rays’ or ‘Uranic rays’ by n)13N. Subsequently, Irene and Frederic Joliot Curie in Henri Becquerel in the last decade of the nineteenth S 1934 discovered artificial radioactivity and Otto Hahn and century, conscientious activities in nuclear science have F. Strassman along with Lise Meitner in 1939 discovered been pursued in a big way1-4. Pierre and Marie Curie the phenomenon of nuclear fission suggesting the division indicated that the emission of these energetic rays is an of a heavy atomic nucleus into two fragments roughly of atomic property characteristic of the element concerned and equal masses accompanied by the release of a huge amount they introduced the term ‘Radioactivity’ for the of nuclear energy. It was shown that when an atom like phenomenon. Ernest Rutherford in 1919 suggested that in 235U absorbs a neutron of suitable energy undergoes fission a nuclear reaction, interactions of two nuclear particles give splitting the nucleus into two parts as illustrated by the rise to emission of new particles from the interacting system fission reaction, 235U + n 144Ba + 90Kr + 2n + and also result in redistribution of nuclear energy and energy. momentum. He indicated that the nucleus, 14N, on reaction with alpha particles, undergoes transmutation to form 18F The nuclear fission can occur with slow or fast which disintegrates further to give 17O and a proton, neutrons and the release of nuclear energy can be controlled or uncontrolled under specific conditions. The nuclear * Former Professor, Saha Institute of Nuclear Physics, power has thus gradually been acknowledged as an essential Kolkata-700 064. Present Address: P-60, Green View, alternative source of sustainable energy. In harnessing this Kolkata-700 084, India. 4 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 nuclear energy, the elemental species, 233U, 235U, 239Pu and The nuclear materials generally used in reactor to a lesser extent, some heavier transuranic elements are operation may be classified as the nuclear fuels8 and the effectively being utilized as the fissionable nuclear source dual-use structural materials as fuel sub-assembly materials in reactor technology. components mainly in the forms of control rods, fuel- The term “nuclear materials”1-3, according to the claddings, moderators, coolants and shielding (Figure – 1). 233 International Atomic Energy Agency (IAEA), in general, In fabrication of nuclear fuels, the fissionable species, U, refers to uranium, plutonium, and thorium, in any form. 235U and 239Pu, and the fertile species, 238U and 232Th, are Nuclear materials, has been differentiated further into of primary importance and to control the fission chain “source materials”, consisting of naturally occurring reactions in the fuels, the dual-use materials are essentially uranium, thorium and the depleted uranium which is not utilized both as in-core and out-of core structural materials. as such suitable for use as nuclear fuels. Uranium ore The dual-use sub-assembly materials are generally used as concentrates are also sometimes considered as “source (i) control rods containing neutron absorbers like cadmium material”. Presently, the fissile 233U, 235U and uranium or boron, (ii) fuel-claddings like zirconium alloys, stainless enriched in isotopes of 233U, 235U or 239Pu, are defined as steel, etc. to ensure retention of the fission products in the “Special Nuclear Materials (SNMs)” or sometimes as fuel matrix and also to prevent coolant from chemically “Special Fissile Materials”. Of these SNMs, only 235U interacting with the fuel, (iii) moderators like light water, occurs naturally. Advancement in nuclear technology has heavy water and graphite which usually surround the fuels 233 239 helped to have the species, U, and Pu, the respective and the coolants helping in controlling the chain reaction 232 238 nuclear reaction products of Th and U, as two more by slowing down or thermalizing neutrons and (iv) coolants fissionable materials and has made it possible to sustain like light water, heavy water, liquid sodium or gases like fission chain reactions in nuclear reactors. The naturally CO and helium to take away the heat generated in the occurring heavier 238U and 232Th, the respective precursors 2 fuels and transfer over directly or indirectly to steam of the artificial or man-made fissile species 239Pu and233U generator as par requirement. are designated as fertile nuclear materials. It is interesting that even in 1980 Convention on the Physical Protection of Nuclear Materials, thorium has not been includes in the definition of nuclear materials. In addition to these nuclear fissile and fertile source materials, there are some specific materials like boron, graphite, aluminum, stainless steel, zirconium alloys, etc. termed as ‘dual-use-materials’5-7 which have got important applications in both nuclear and non-nuclear fields. Over the years, with the evolution of different types of advanced designed reactors, varieties of ‘dual-use materials” having specific nuclear characteristics are “especially designed or prepared for the processing, use or production of special fissionable material” exclusively suitable as fuel and fuel- sub-assembly components in nuclear reactors. The history of development of nuclear power reactors dates back to the early days of Manhatton Project and the Chicago Pile-1 was the world’s first controlled nuclear fission reactor constructed in 1942 in the University of Chicago. Demonstration of controlled nuclear chain reaction Figure 1. A Generic Nuclear Reactor5 by Enrico Fermi paved the way for the construction of In a reactor, the dual-use structural materials having advanced designed reactors. The nuclear reactors are enhanced physicochemical reliability and favorable nuclear broadly designated as the research reactors primarily used characteristics become exposed to the hostile condition and for material research, training and production of medically hence have to withstand high temperatures, high neutron and industrially important radioisotopes and the power flux and gamma doses as well as the active and changing reactors essentially used in harnessing nuclear energy for chemical environment in the core. generation of electricity. VOL. 81, NOS. 1–2 5 Fissile Materials is an artificial element having a number of isotopes. The most important fissionable plutonium isotope, 239Pu, is The basic requirement for an atomic energy program produced in reactors from uranium 238U by (n,) reaction is the element uranium as it is the primary fissile fuel in followed by two successive - decays which can be nuclear reactors8,9. Uranium, as found in nature, consists presented as of three varieties of isotopes, namely, 234U (~0.0054%), 235U (~0.72%) and 238U (~99. 27%), with specific activity 235 238 239 239 239 of ~ 0.67 Ci/g. Of these uranium isotopes, only U is U92 (n,) –– U92 –– Np93 –– Pu94 fissionable. The predominant naturally occurring heavier 23.45 m 2.36 d isotope, 238U, is not fissile. There are three fissile nuclides, On neutron irradiation of 238U with neutrons, along 233U, 235U and 239Pu, and of these three, nature has thus with fissile 239Pu, some other plutonium isotopes like fissile provided with only one nuclide, 235U, and the other two 241Pu and fertile 236Pu, 238Pu, 240Pu, 242Pu are also produced nuclides, 233U and239Pu, are artificially produced or man- through different nuclear reactions depending on the type made. In producing these two fissile nuclides, 239Pu and of reactor and burn-up of uranium fuel. The fissionable 233U, in reactors, the fertile nuclides, 238U and 232Th, 239Pu has a half life of 24,110 years and undergoes fission capture neutrons first to form 239U and 233Th, and then with neutrons of all energies. Since a higher number of after two successive -decay convert themselves neutrons are