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 or gases, etc. as 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 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 emitted per fission, it is considered as an respectively to fissile 239Pu and 233U. The nuclear fuels important fissile fuel species in fast breeder reactors. The are generally fabricated by judicious combination of these fissile 241Pu, with a half life of 14.4 years decays primarily three fissile and two naturally occurring fertile nuclides in by  – emission to alpha –active non-fissile 241Am which the forms of metals, alloys, inter-metallic compounds, further decays to 237Np. The nuclides, 238Pu and 241Am, ceramics (oxide, carbide, nitride, silicide), or dispersion are highly alpha - active and generate heat which increases types, depending on the reactor designed. progressively with the aging of plutonium. The daughter Uranium – 235: The naturally occurring fissionable products of isotope 236Pu (Tl and Bi) are hard gamma - 235U although can burn with neutrons of all energies in emitters and may be present in high burn up plutonium. nuclear reactors, but the natural 235U-content (~0.72%) is In the reactor, besides the production of plutonium not sufficient enough to sustain a fission chain reaction as isotopes, host of other fission products and ‘minor’ actinides such and an increase in specific concentration of 235U is like Np, Am, Cm, etc., are also produced in the fuels. essentially needed to initiate the desired nuclear chain Plutonium produced in a reactor during irradiation is reaction in reactors. separated from spent fuels mainly by PUREX process. In In nuclear technology, uranium recovered from natural the process, the spent fuel is first dissolved in nitric acid ores is primarily concentrated as ‘Yellow Cake’ for and then uranium along with most of the fission products fabrication of nuclear fuels. In production of reactor grade are separated from plutonium nitrate by solvent extraction/ nuclear fuel isotopically enriched in 235U, the pure uranium ion exchange processes. Plutonium left in nitrate solution product has to be treated further by some sophisticated is precipitated as an oxalate and then calcined to get enrichment procedures in which uranium is fed as UF6 plutonium as PuO2 powder. Conversion of PuO2 to metal formed by direct fluorination of UF4 as UF4 + F2  plutonium is performed by its fluorination to PuF4 followed UF6. Most of the reactors operating in the world use by metallo-thermic reduction. enriched uranium. In reactors using ordinary water as Since late eighties, 239Pu, because of its specific moderator, the fuel has to be enriched to about 3 to 4% in nuclear characteristics, is better utilized as mixed oxide 235U. If natural uranium is to be used as fuel, the moderator (MOX) fuels in light water fast breeder reactors (FBRs) in has to be heavy water. Enhancement in isotopic a significant way. Plutonium is a highly radiotoxic alpha - concentration of fissile 235U in uranium more than that active material and is utilized with outmost care. Attempt occurs in nature is termed as uranium enrichment. Fuel for disposition of plutonium in fast reactors has been made enrichment varies from natural to 90% of 235U. Uranium by replacing the depleted UO in MOX fuels by some inert for strategic application needs to be enriched beyond 90% 2 materials like SiC and MgO stabilized in ZrO and the of 235U. 2 axial blanket material by some specific inert matrix. Plutonium - 239: In harnessing nuclear energy, Uranium-233: The fissile uranium 233U, artificially uranium is the most common fuel material, the other being produced in FBRs, is an important source of nuclear energy artificial plutonium. Plutonium does not exist in nature. It for the future. The nuclide, 233U, produced by transmutation

6 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 of 232Th through the thorium-uranium breeding cycle significant fission with thermal neutrons, but can undergo provides a large source of fissile materials in advanced fission with fast neutrons. power breeder reactors10. The primary aim of development Uranium – 238: The naturally occurring fertile of the FBR concept is to breed the fissile nuclides, 239Pu uranium nuclide 238U through a series of nuclear reactions and 233U, through the two respective breeding cycles, 238U gets converted into the most important fissionable nuclide, — 239Pu and 232Th — 233U. In producing 233U, the   239Pu. In FBRs, the nuclear chain reactions are sustained natural thorium species, 232Th, like 238U, needs to absorb by fast neutrons and the reactor cores are surrounded by a neutron to get converted into an unstable nuclide, 233Th. blankets of natural uranium or thorium to capture the which on two successive - decays leads to the fissionable  neutrons leaking from the cores producing the respective 233U with a half life of 1.6 x 105 years. The involved fissile nuclide, 239Pu or 233U. The element, in general, nuclear reactions may be presented as should be very low in impurities like boron, cadmium,  gadolinium, samarium, etc. as these species are high neutron 232 233 233 233 Th90 (n, )–– Th90 –– Pa91 –– U92 absorbers. 23.5 m 27.4 d In nature, uranium occurs in rocks of almost all Thorium based nuclear fuels have some added geological ages and are generally categorized as the advantages since the fissile nuclide, 233U, has favorable Unconformity-related (33%), Sandstone type (18%), nuclear characteristics both in thermal and fast neutron Breccia complex (17%), Quartz-pebble conglomerates spectrum and the 232Th–233U fuel cycle can be made self- (13%) and Vein type (10%) mostly located in Australia, sustaining using external fissile material, 235U, 239Pu or an USA, Kazakhstan, Uzbekistan, Canada and South Africa. accelerator driven neutron source. On irradiation of thorium Exploration and evaluation of uranium deposits are in FBRs, along with fissile 233U, small quantities of 232U generally performed following the estimation of ore reserves (68.9 yr), a hard gamma emitter, is also formed through and radiometric reconnaissance survey of uranium (n, 2n) nuclear reaction in thorium fuel cycle which may mineralization. The total world’s reserves of uranium is ~ pose some technological problems. 2.49 million tones and the annual world production of uranium concentrates as U O is estimated to be around The ability to convert significant quantities of fertile 3 8 40,000 tons. material into useful fissile material is depended on the magnitude of its reproduction factor,  (eta) i. e. the number Uranium, in most of its natural deposits, generally of neutrons produced per neutron absorbed in the fuel. The exists in its tetravalent state. The tetravalent uranium, value of  in fast neutron spectrum of 239Pu is however, gets converted to its water soluble hexavalent comparatively higher than that of 235U or233U implying that form and becomes mobile in oxidizing environment. The more number of neutrons become available for breeding process of concentration of uranium from ores involves 239Pu from 238U in 238U-239Pu fuel cycle than that in 232Th- several stages like (i) dressing of the ores by physical 233U fuel cycle and hence the breeding ratio. Actually, 239Pu methods of crushing, grinding, gravity separation, froth is the best fissile material in fast neutron spectrum. floatation, etc., (ii) treatment of the dressed materials with dilute acid or alkali carbonate to dissolve uranium Fertile Materials depending on the chemical composition of the ores and

The potentiality of the virtually inexhaustible nuclear (iii) recovery of uranium as U3O8 concentrate (70 – 80%) energy locked in natural uranium and thorium was realized from the leach solutions. Uranium is further purified either since mid 1940s2,9,10. The naturally occurring heavier through solvent extraction utilizing organo-phosphoric acid uranium, 238U, and thorium, 232Th, as precursors of fissile esters or long chain alkyl amines as suitable extractants or 239Pu and 233U, are designated as the fertile materials and by effective ion exchange procedures as anionic complexes. are efficiently utilized in breeding and recycling of the Practically, a better grade of uranium is obtained through fissile species in FBRs. Presently, in most of the advanced solvent extraction, especially when amines are employed power reactors, the fission chain reactions are maintained as extractants. at a controlled and steady rate by transformation or Thorium – 232: The naturally occurring thorium is breeding of fresh fissile isotopes, 239Pu and 233U, mono-isotopic, 232Th (100%) and as a promising fertile respectively from 238U and232Th. The fissile nuclides have element like uranium238U, plays a prominent role in world’s high cross sections for thermal neutron induced fissions nuclear energy programs10. Thorium-232, because of its but the fertile nuclides, 238U and 232Th, do not experience ability to get converted into fissile 233U on exposure to

VOL. 81, NOS. 1–2 7 neutron, is utilized as an efficient fertile fuel material with oxide ceramic fuels are the undisputed choice for majority fertile uranium-238 in breeder reactors. Thorium based of the nuclear power reactors presently in operation all over nuclear fuels have excellent nuclear characteristics in both the world. The ceramic mixed carbide and nitride fuels, thermal and fast neutron spectrum. The thermal neutron having excellent compatibility with coolant like liquid absorption cross section of thorium is 7.4 barns as against sodium, are sometimes being used as nuclear fuels. The 235 2.7 barns for natural uranium and is very effective in UO2, either natural or enriched with U, and PuO2 are neutron capture. Countries which have large deposits of iso-structural and completely solid soluble in the whole thorium but with limited uranium reserves, the option for range of UO2 - PuO2 composition as mixed oxide. using 233U fuel based FBRs for generation of nuclear power Compared to metal or alloy fuels, oxides of uranium, is of primary importance. plutonium and thorium have good chemical stability and compatibility with different dual-use nuclear structural In FBRs, thorium, in the form of metal or oxide, is materials. used as the reactor core blanket to produce 233U by neutron capture followed by  - decays. The application of thorium In addition to these fissile and fertile materials, the as a nuclear fuel comprises several major steps like (i) radionuclides, 241Am, 238Pu, 210Po, 137Cs, 90Sr, 3H, etc., conversion of natural 238U fed as fuel to a reactor to fissile under specific conditions, sometimes act as radioisotopic 239Pu, (ii) transfer of the fissile 239Pu to another reactor nuclear fuels. Similarly, the byproducts that are produced using thorium as fuel where thorium is converted to fissile or made radioactive in nuclear reactors as well as the 233U resulting in production of fissile material more than tailings and wastes produced during ore processing of that actually consumed and (iii) to continue the process, uranium and thorium as source materials, are also all that is required to feed the breeder reactor is addition considered as nuclear materials. of thorium. In FBR fuel-assembly, thorium, the precursor of fissionable 233U, acts as an excellent host for 239Pu and Dual-use Structural Nuclear Materials enables deeper burning of plutonium. In fuel performance The nuclear fuel primarily comprising of the fissile point of view, thorium (ThO2) is suggested to be superior and fertile species is the heart of a reactor. In successful to uranium (UO2), as it has better thermal conductivity, operation of a reactor, in addition to nuclear fuels, it always lower thermal expansion, higher radiation resistance and needs some ‘dual-use’ special elements or compounds as 233 the long lived minor actinides produced by fission of U structural materials with favorable nuclear characteristics are also much less in quantities. highly compatible with the fuels as in-core and out-of-core structural components 2,5,11-14. In an on-going fission In reprocessing of thorium fuel, the highly stable ThO2 is dissolved in nitric acid in presence of ions. The process, a large number f highly radioactive neutron-rich chemical procedures involved in solvent extraction fission fragments of masses of around 100 and 135 with separation of thorium-uranium in the irradiated fuels are half lives ranging from a fraction of a second to several almost similar to that of separation of uranium-plutonium hundred of years, are produced causing changes/ system. The extractant, Tributyl phosphate (TBP), in degradations in thermo-chemical properties of the exposed suitable diluent is used to extract 233U, 228Th and 229Th or structural materials. 233 for selective extraction of U alone. The dual-use structural materials, in general, are In nature, thorium primarily occurs in orthophosphate effectively utilized in different forms5 such as (i) fuel dispersing medium like aluminum, aluminum oxide, etc., rare earth minerals, monazite, as ThO2 (~9%). It is about three times as abundant as uranium. Thorium is recovered (ii) neutron moderators like light water, heavy water, from phosphate minerals by dissolving in nitrate solution graphite, beryllium oxide, etc., (iii) cladding materials like from which it is precipitated as oxalate and finally zircaloys, stainless steels, etc. (iv) coolants like light water, converted to oxide by calcination. The oxide is reduced to heavy water, liquid sodium, carbon dioxide, etc., (v) thorium metal powder and processed to the desired form reflector of neutrons like heavy water and (vi) radioactive by powdered metallurgical techniques. shields like lead, steels, water, concrete etc. In designing an advanced reactor, adequate knowledge of physico- Ceramic Nuclear Fuel Materials: To achieve higher chemical properties and nuclear characteristics of the burn up levels than that obtained by using metallic fuels structural materials, in addition to the fissile and fertile in a reactor, development of ceramic mixed oxide, (U, elements, are essentially needed in predicting the fuel Pu)O and non-oxide carbide (U, Pu)C and nitride (U, 2, performance as well as the aspect of nuclear safety. Pu)N, nuclear fuels have been initiated8. The U-Pu mixed Characteristic properties of some of the important ‘dual-

8 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 use’ structural materials effectively employed in fuel sub- Boron carbide in the form of powder and as assembly as essential components like control rods, composites like Bocarsil (composites of flexible rubber moderators, claddings, coolants, etc. in reactors are briefly sheets and boron carbide), Boral, (composites of boron discussed. carbide and aluminum), Poly-boron, (composites of boron in polyethylene matrix), etc. have varieties of applications Control Rods in plugging neutron leaks surrounding beam holes in The control rods control the nuclear power level. The research reactors, neutron shielding in nuclear instruments substances capable of absorbing neutrons are called and in storage of radioactive materials. The compound, ZrO , as sintering additive to born carbide, is found to be controlled materials2,5,11. To achieve controlled fission in 2 highly effective in its densification in reduced temperatures. a reactor, the control materials in general, must have very high cross sections for neutron capture so as to absorb In early designed Pressurized Heavy Water Reactors, excess neutrons to keep the multiplication factor at a state some specific components containing boron or other suitable for maintaining desired nuclear reactivity in the neutron absorbers like Cd, Dy, Eu, Gd, Hf, 135Xe, etc. were reactor core. In this context, boron with a neutron capture used along with the fuel rods as nuclear Burnable Poisons2 cross section of 764 barns is one of the best choices as to achieve optimized high burn up of the nuclear fuels. control rod material. Presently, the neutron poison is added directly to fuel pellets as boron coating and also as gadolinium oxide Boron: The element boron and its compounds, boron doping to allow more flexibility in their use. In fuel carbide and boride, having high neutron absorption processing plants, especially in storage tanks and vessels, cross sections, are extensively used as important neutron poison tubes containing materials like gadolinium are absorbing materials in control rods in nuclear reactors. In incorporated so as to cause substantial reduction in neutron operating condition, the reactivity of a reactor gradually population. The stainless steel vessels clad with cadmium decreases due to decrease in its fissile content and increase or borated coatings are also sometimes used for the in formation of fission products. The steady reactivity, as purpose. Addition of dysprosium (ZrO2-Dy2O3) as burnable desired, is however maintained by insertion of control rods neutron absorber to the central rod used in connecting the containing appropriate control materials to absorb neutrons. 232 233 fuel pins loaded with ( Th- U)O2 has helped in The control rods, as part of the fuel-assembly, generally achieving the desired reactivity in Advanced Heavy Water contain the element like boron in the form of boron steel Reactors. Cadmium and with high capture cross or boron carbide as neutron absorber. Boron, besides its sections but with low melting points are sometimes used essential use as a potential neutron absorber in control rods, in the forms of alloys as neutron absorbers. finds important applications also in human shielding against In nature, boron generally occurs in , neutrons. Na4B4O2.10H2O (~10% of boron) and Kernite, Boron has two principal isotopes, 10B and 11B. Of Na2B4O7.4H2O minerals. The element is mainly produced these, 10B with high neutron absorption cross section through Moissan process by reduction of boric oxide with

(thermal neutron cross section, 3835 b) is highly effective magnesium, B2O3 + 3Mg  2B +3MgO. In the as neutron absorber. The (n, ) reaction products of boron, laboratory, it can be produced by thermal decomposition 10 helium and , as shown by the reaction, B + n  of diborane, B2H2, or through thermal/electrolytic reduction 4He + 7Li, are stable and non-radioactive and hence the techniques. The element is not affected by either HCL or problems of decay heating during reactor shut down or HF acid but gets oxidized by hot oxidizing acids like transfer of depleted control rods become minimal. Boron HNO3, H2SO4, HClO4, etc. It is extremely difficult to get carbide enriched in 10B isotope has now become the most pure boron as it reacts with a majority of metals at high preferred control rod material for controlling neutrons in temperatures to form metallic carbides or borides. It is an FBRs. Similarly, titanium di-boride with high melting point extremely hard refractory solid material and classified as a and good thermal conductivity is also considered as a metalloid or semiconductor. suitable control rod material, especially for high Besides boron, pure graphite has also got important temperature reactors. Boron of natural isotopic composition use as a moderator in control rods. Graphite is a form of 10 as well as enriched with B is used in neutron counters as carbon with significant resistance to corrosion, low neutron neutron absorber/sensor in nuclear instrumentations. absorption, high temperature resistance, good resistance to Colloidal boron solution is sometimes applied in fabricating ablation and good machinability. To reduce fuel pellet- clad coated sensors for neutron counters. interaction, the inner surfaces of the fuel tubes are

VOL. 81, NOS. 1–2 9 sometimes thinly (5-9 microns) coated with graphite. by using natural uranium metal as fuel in heavy water Similarly, the metal hafnium with high thermal neutron moderated reactors first installed in 1944 at Argonne cross-section (~ 600 times more than Zr) make it ideal for National Laboratory12. use as control rods in fission reactors. The Hf-control rods In case of water as moderator, the fast neutrons have long life as many of the hafnium isotopes produced produced by fission events in reactors are reduced to by absorption of neutrons are also strong neutron absorbers. thermal energies or moderated by collisions with low mass Hf-oxide is often used in the production of shields for nuclei of 1H or 2H, usually as light water or heavy water. nuclear materials. Comparatively heavy water has been found to be a better moderator as D has a neutron capture cross section of only Moderators 0.5 mb and the moderating capacity is only slightly inferior L. Szilard and E. Fermi were the first to introduce to that of 1H, whereas light water has a significant neutron the concept of moderator in slowing down neutrons. capture cross section (0.3 b for 1H). Presence of one Substances capable of slowing down or thermalizing additional neutron in the nucleus of D in D2O considerably neutrons are generally termed as moderators2, 10 -12 . The reduces its neutron absorption capacity compared to that first nuclear pile constructed in the University of Chicago of H2O making D2O as a good moderator for neutrons to study the controlled nuclear chain reactions used graphite and thus plays a significant role in sustaining a nuclear blocks as moderator. Since the moderator which usually chain reaction in reactors with natural uranium. In heavy surrounds the fuels and the coolants in reactors should have water reactors, the concentration of T in D2O gradually a light nucleus and a low neutron capture cross section, increases as it is formed by D through neutron capture the essential requirements have limited the choice of and also in the fission of uranium. Heavy water is a good moderator materials to a few, namely, two hydrogen reflector of neutrons and is not consumed in reactor isotopes, 1H and 2H (D), respectively in the forms of light operation. water (H O) and heavy water (D O), carbon in the form 2 2 The oxides of H, D and T, light water, heavy water of graphite and beryllium. In case of graphite, because of and tritiated water, considerably differ in their physical its higher mass, the moderating characteristics are not as properties as the mass differences of the hydrogen isotopes good as those of water, although a wide variety of coolants differ greatly. It is about 10% more dense than regular water can be used. High toxicity of beryllium has restricted its but visibly indistinguishable and non-radioactive. There are use as a moderator. Amongst these moderators, heavy water some differences in their chemical behaviors also such as is most prominent and perhaps the best (moderating ratio the neutral heavy water has a pD of 7.4 as D2O ionizes to of D2O, H2O and graphite is 21000 : 72 : 188). a lesser extent than H2O and the solubility of some salts is Heavy Water: The lightest element, hydrogen, with comparatively lower in D2O than that in H2O. There is three naturally occurring isotopes, hydrogen 1H, deuterium, also preferential combination of O2 with H2 compared to 2H (D) and tritium, 3H (T), is the most abundant matter in D2. On radiolysis, the net decomposition for D2O is lower the universe. Of these three isotopes, 1Hand 2H are stable than that for H2O. and 3H is radioactive (12.26 yr) which undergoes  -decay Heavy water is generally produced by distillation or to form 3He. Separation of the hydrogen isotopes is of great electrolysis of water and through different chemical importance in nuclear science. For example, deuterons (a exchange processes. Amongst the important chemical few MeV) are used as projectiles in studies of nuclear exchange procedures involving H S-H O, NH -H O and reactions and deuterium and tritium are used in nuclear 2 2 3 2 H O-H systems generally employed in production of heavy fusion. 2 2 water, the H2S-H2O exchange process meets over 90% of The natural occurrences of H2O, D2O and T2O are in its global requirement. Cryogenic distillation of hydrogen the ratio of about 1 : 1.5 x 10-4 : 2x-16 and the abundance is an important process for separation of both deuterium of heavy water in normal water is ~150 ppm. Deuterium and tritium. discovered by H.C. Urey in 1931, was concentrated in the In reactor technology, it is an advantage that heavy form of its oxide, D O, as heavy water, by electrolysis of 2 water, D O (~100%), as a moderator and coolant, permits ordinary water in 1932. The Light Water Reactors (LWRs) 2 the use of natural uranium as fuel in thermal reactor systems which are in maximum use in the world with enriched eliminating the need of uranium enrichment. Presently, the (~3%) uranium oxide as fuel utilize light water both as heavy water reactors fuelled by natural uranium are moderator and coolant. The requirement of enriched fruitfully utilized to generate fissile 239Pu to feed the fast uranium as fuel in LWRs has to some extent been overcome

10 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 breeder reactors to breed fissile 233U from fertile natural in maintaining a predetermined geometric integrity of the thorium, 232Th as a nuclear fuel for generation of much reactor cores. needed electricity. Since World War II, due to strategic as Zirconium: Zirconium as a congeneric pair of well as commercial importance of heavy water, major elements always occurs together in nature with hafnium research on development of heavy water processes have (~2 – 3%). Since hafnium has high thermal neutron been carried out in the countries like Canada, France, absorption cross section, it is essential to have hafnium- Germany, U. K., Sweden and India. free zirconium for its use as cladding materials in reactor In Molten Breeder Reactors with a molten salt technology. It encapsulates fuels in water-cooled thermal mixture of UF4, LiF, BeF2 and ThF4 fuels, the graphite nuclear power reactors as fuel cladding and other internal rods suspended in the molten salt are used as moderators. reactor components. Zirconium having the property of In 1954, in Aircraft Nuclear Propulsion Program (ANP), a corrosion resistance in water and stream, U-Zr alloy molten salt, NaF-ZrF4-UF4, was used as fuel and blocks cladding have been used in early nuclear submarines, first of BeO served as moderator. The Russian ‘RBMK’ reactors in the nuclear submarine, ‘the Nautilus’, USA, in 1951. In using enriched uranium as fuel are graphite moderated Enrico Fermi Atomic Power Plant, the U-Mo fuel was also reactors. bonded with zirconium cladding. Pure zirconium metal has the unique characteristic Claddings properties of (i) low neutron absorption cross section (~0.8 The primary function of the cladding materials 2, 3, 13 b), (ii) dimensional stability at high temperatures, (iii) high is to encase the fuel pellets isolating the fuels from the reactivity and alloying behaviors with various elements and coolants in reactor cores. Claddings act as the barriers or (iv) better corrosion resistance, etc. Addition of traces of primary containments to fission products entering the tin to pure zirconium further enhances its corrosion coolant stream and minimize the transfer of radioactivity resistance. The exceptional favorable properties of the into the reactor shell and beyond. It is the first line of element make it an ideal material for use in reactor cores defense for radioactivity. The claddings and the wrapper and more than 90% of zirconium production is consumed tube materials in a fast reactor fuel sub-assembly13 are by commercial nuclear power generation. usually subjected to high operational temperatures, high Research and development on cladding materials over energy neutron flux, chemical interactions with various the years have resulted in the production of different fission products and coolants. The cladding materials, in zirconium alloys like zircaloy-2 and zircaloy-4 having general, should have the preferable properties of (i) lower excellent compatibility with varieties of nuclear fuels and affinity for neutron absorption, (ii) high corrosion resistance structural materials, especially in thermal power reactors (iii) good compatibility with fuels, coolants and other for generation of electricity. Currently, almost all structural materials and (iv) resistance to irradiation induced commercial power reactors, in general, use zircaloy-2 (Sn- void swelling, creep and embrittlement. 1.5%, Fe-0.12%, Cr-0.1%), or zircaloy-4 (slightly different In operating condition, chemical compatibility of the in composition from zircalloy-2) as the cladding material. fuel-cladding systems varies from reactor to reactor and Addition of some alloying elements like Fe, Cr and Ni to even from point to point in a given reactor and thus the zircalloy-2 provides better mechanical properties for its use phenomenon of the fuel pellet-clad interactions (PCI) in reactors. Zircaloy-4 has higher resistance to hydrogen demand special attention in reactor technology. The uptake than zircaloy-2. The problem of hydriding the diffusion and deposition of corrosion and fission products, surface of the clad by hydrogenous impurities resulting in particularly the gases like iodine, which can easily reach blistering or cracking is eliminated by inclusion of a the claddings along the fuel cracks and pellet-pellet hydrogen getter inside the fuel rods. Development of a thin interfaces, are responsible for fuel failure. For example, in liner of pure zirconium ‘barrier’ inside cladding reduces case of stainless steel claddings, deposition of the corrosion the incidence of PCI failure in water reactors. Similarly, a products, Fe, Ni, Cr, etc. on fuel surface as oxide or uptake thin layer of copper or titanium coating on the inside wall of hydrogen by zirconium alloy claddings may lead to of zircaloy clad tubing can provide an effective diffusion cladding embrittlement resulting damage to fuel assembly. barrier against diffusion of iodine. Zirconium alloys meet In nuclear power reactors, the special dual-use almost all required criteria for ideal claddings and provide structural materials like zirconium alloys, stainless steel, major incentive for use in reactors especially in LWRs using aluminum, etc. as nuclear claddings play significant roles UO2 or (U, Pu) O2 as fuels. The fission product, cesium forms compounds like Cs2ZrO3 and Cs2UO4 near pure

VOL. 81, NOS. 1–2 11 zirconium layer resulting considerable reduction in PCI. addition of suitable stabilizing elements or by modification For stainless steel or zirconium alloy claddings, sometimes, of the structural compositions. incorporation of a fuel-clad barrier layer of graphite helps In course of study, varieties of austenitic stainless steel in reducing the PCI in different designed reactors. alloys like 304, 304L (N), 306, 316L (N), 316-Ti, 15-15Ti, Development of varieties of zirconium alloying systems like D9, etc.; high nickel alloys like PE-16, INC-706, etc. and Zr-Nb, Zr-Ti, Zr-Hf, Zr-Ta, Zr-Fe, Zr-Ni, Zr-Cu, Zr-Cr, Zr- ferritic/martensitic stainless steel alloys like HT9, T91, H, Zr-Al, Zr-Sn, Zr-O, Zr-N, Zr-Nb-Cu, Zr-Sn-Nb, etc. are EM10, EM2, etc., as effective nuclear cladding/wrapper under investigations. materials have been developed. These alloy claddings with Zirconium as a non-fissile dual-use material, in some favorable high temperature mechanical and irradiation specific conditions, can act as a thermal bond between the resistances exhibit excellent compatibility with different binary U-Pu fuel system in the form of a ternary U-Pu-Zr coolants especially with sodium in fast reactors. The CO2 alloy fuel like ‘Na’. The Zr-bonded fuel, in some cases, cooled British AGR reactor has utilized stainless steel alloy has some added advantages over the Na-bonded fuel. In (Fe – 20Cr-25Ni-1Nb) with good ductility, corrosion Experimental Breeder Reactors, the U-Zr (~2%) alloy fuel resistance and reasonable strength at high temperatures as rods encased in Type 347 stainless Steel tubes were cladding. To have better irradiation resistance, in sodium individually loaded in the core inside zircaloy tube. The cooled Prototype Fast Breeder Reactor (PFBR), D9 has important role played by zirconium includes increasing of been used as cladding/wrapper in-core component and the solidus temperature, overcoming of the problems of PCI, low carbon-nitrogen alloys 304 L(N) and 316 L(N) have stabilization of the isotopic phases, etc. been selected as the main out-of-core structural components. In case of the stronger ferritic/martensitic type The TRIGA reactors and the CABRI reactor in France alloy claddings, HT9 and T91, swelling is comparatively used U O dispersed in graphite moderator as fuel and 3 8 less than that in case of other austenitic stainless steel zircaloy as claddings. In TRISO fuel particles, the SiC alloys. The modified ferritic-Cr-Mo steels of the type coating layers are sometimes replaced by stable ZrC layers 2.25Cr-1Mo and 9Cr-1Mo, with microstructure stability and in forming more advantageous ZrC-TRISO fuel particles. good compatibility with the coolants, sodium and water, The thoria and urania fuels, clad in zircaloys using the are used as stream generator components. In PFBR, nickel- blanket concept are being investigated in USA, Canada, based colmonoy alloys are sometimes selected as suitable Italy and other countries. replacement for the cobalt-based stellite alloys in steam The element, zirconium, discovered by Berzelius in supply system components. Similarly, austenitic stainless 1824, occurs in nature mainly as zircon, ZrSiO4, and steel of AISI type 304 is the favored structural material baddeleyite. In separation of the refractory element for the construction of FBR reprocessing and waste zirconium, it is first recovered as oxide from zircon and management plants. then converted to its metallic form through two major steps Generally, the oxide fuels, UO and (U, Pu) O , are of (i) chlorination of the oxide to zirconium tetrachloride 2 2 very inert in contact with stainless steels and zircaloys. and (ii) its reduction with magnesium to metal zirconium. However, in fast reactors with stainless steel alloy cladding Applications of Van Arkel iodine refinement process and of U-Pu fuels, there is a possibility of forming a low the Kroll’s process led to separation of highly pure melting eutectic liquid phase of uranium and plutonium with zirconium as sponge. The extractant, TBP, has been found the alloying elements, Fe, Cr and Ni, if there is a loss of to be very effective in solvent extraction separation of coolant-flow. Addition of zirconium to U-Pu fuels helps in highly pure zirconium from hafnium and other associated forming an U-15Pu-10Zr alloy with good compatibility with impurities. Countries like France, USA, Russia and India austenitic stainless steel type 304 cladding and also with are capable of producing reactor grade zirconium sponge, sodium coolant which acts as high conductivity bond zirconium alloys and a wide variety of related dual-use between the fuel and the clad during reactor operation. The structural components effectively used in power reactors. metallic fuel-clad compatibility is further enhanced by the Stainless Steel: In 1940, plutonium alloy clad in mild addition of Nb, V and Mo to cladding materials under steel was first used as fuel in mercury cooled fast reactor, specific conditions. In case of carbide fuels, there is a Clementine, in USA. Since then various types of both in- possibility of forming inter-metallic compounds like (U, core and out-of-core dual-use stainless steel components Pu)(Fe, Ni)2 and (U, Pu)(Ni, Fe)5 with stainless steel as nuclear claddings and other major structural materials claddings and also carburization of stainless steels due to for advanced reactor systems are being evolved either by dissolution of carbon in sodium coolant.

12 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 The uranium compound, U-Fs (Fs is an alloy of Mo, liquid coolants effectively used in different designed Ru, Rh, Pd, Zr and Nb), used as driver fuel in the reactors include light water, heavy water, liquid metals like Experimental Breeder Reactor-II, has been found to be of sodium, lead, NaK, Pb-Bi eutectic (LBE) and molten salts poor compatibility with stainless steel claddings and it has of FLiNaK and FLiBe. The most commonly used gaseous been replaced by the metal fuel, U-Pu-Zr, with better coolants are CO2 and helium. The Enrico Fermi Atomic compatibility with the claddings in ANL. The Russian BR- Power Plant at Michigan was a U-Mo fueled and sodium

5, a sodium cooled reactor, was fuelled with PuO2 pellets cooled nuclear plant. clad in Type 321 stainless steel and with helium bonding. Liquid Sodium: The un-moderated high neutron flux Development on British DFR (Dounreay Fast Reactor), led FBRs with high temperature small reactor core volumes to the use of two metal claddings, namely, vanadium for demand very efficient cooling systems. The high neutron inner cladding and niobium for the outer depending on flux causes radiation damage and degradation of the reactor temperatures. claddings and duct materials and may restrict the life of Aluminum: The metal aluminum and its alloys have the fuel elements. The alkali metals especially liquid sodium got important applications both as fuel matrix as well as a having favorable thermodynamic and nuclear properties, fuel cladding materials particularly in research reactors. The suitable compatibility with different cladding materials and first light water-cooled and moderated Material Testing higher heat transfer capabilities has been extensively 14 Reactor (MTR) installed in 1952 used an alloy fuel of UAl3 employed as an effective coolant in FBRs . The nuclear clad in aluminum. In British Magnox type reactor utilizing heat from the reactor core is removed by circulating liquid ‘adjusted uranium’ as fuel, the magnesium-base alloy, sodium from a cold pool and after transporting its heat to Magnox Al80 (Al 0.8%, Be 0.002 –0.05%, Ca 0.008% the heat exchangers mixes again with the cold pool. and Fe 0.006%) applied as cladding material provided good Utilization of liquid sodium metal as coolant makes a fast compatibility with CO2 coolant. Application of aluminum breeder reactor possible to operate at higher temperatures as clad in the plate type uranium inter-metallic compound, enhancing its nuclear fuel cycle efficiency as a whole. UAl or U Si dispersed in aluminum matrix as fuel, 3 3 4, Sodium does not moderate neutrons (fast neutron ensured effective fuel-clad contact resulting improvement absorption cross section, ~ 0.87 mb) and its possible (n,) in corrosion resistance of the reactor fuel. activation product, 24Na, is short lived (15 h) and the The advanced High Flux Beam Reactor (HFBR) at (n, 2n) product, 22Na, also does not build up much limiting ORNL developed mainly to produce transuranic elements the emission of radiation doses to working environment. for research purposes used cermet fuel of U3O8 dispersed The nuclear grade sodium metal has got the properties of in aluminum matrix and an aluminum alloy 6061 (Mg low melting point (~186o C), high boiling point (~1400o 1%,Si 0.6%, Ca 0.25% and Cr 0.25%) with good corrosion C), high thermal conductivity, good corrosion resistance resistance as cladding material. In water-cooled TRIGA and high compatibility with the major dual-use structural reactor developed by General Atomics Corporation in 1950s materials. It is also compatible with mixed carbide fuels used U-Zr hydride fuel clad in aluminum/zirconium. as coolant. Liquid sodium having density and viscosity very close to water permits its use in different hydraulic design Coolants parameters. The liquid metal cooled fast reactors are under In nuclear reactors, the dual-use coolant materials operation in United Kingdom, France, Russia, Japan and which are circulated through the reactor cores, in general, India. ensure the fuel assembly systems to maintain structural Since in case of liquid sodium, there is a possibility boundary integrity, minimize the fuel-clad interactions and of formation of Na3(U, Pu)O4, with mixed U-Pu oxide fuel reduce the out-of-core radiation fields but these themselves resulting in swelling and cladding rapture, alternatives like should not be easily radioactive2, 11, 14. The suitability of a Na-Pb and Pb–Bi alloys have sometimes been considered coolant is dictated by its favorable characteristic properties as coolants. But the characteristic properties of higher of high thermal conductivity, low neutron absorption cross melting point, corrosion and especially generation of section, chemical stability, corrosion resistance and higher radioactive 210Po by neutron capture in 209Bi and 208Pb compatibility with the fuel and other associated structural have to some extent restricted their applications. materials. Both liquids and gases are being used as coolants Sodium metal is generally produced by Down’s primarily to transfer the heat released during nuclear fission process. Sodium being an alkali is highly reactive and forms to a suitable medium to generate electricity or simply compounds with all non-metals excepting nitrogen and dissipated as occurs in a research reactor. The important

VOL. 81, NOS. 1–2 13 noble gases. It burns in air and reacts extensively with for example, in gas filled radiation detectors like water. proportional counters, Geiger Mullar(GM) counters, etc., gases like helium, argon, hydrogen, nitrogen, methane, etc. In Molten Salt Reactors (MSRs), the fluid fuels and are a few fill-gases that have got important applications as the blanket circulation systems are separated by an dual-use nuclear materials. Similarly, BF , 3He and 4He- intermediate coolant like liquid sodium, helium or 7LiF- 3 gas filled detectors are exclusively used for neutron BeF . In MSRs, 7LiF-BeF as coolant, displays excellent 2 2 detection. compatibility with Hastelloy-N (an alloy of Ni with Mo, Cr, Fe, Si and C), a preferred construction material for In gamma-ray spectroscopy, the detectors are also reactor vessels, pumps and heat exchangers, etc. The based on some specific dual-use semiconductor materials electricity generating Gas Cooled Reactors (GCRs), built like germanium, silicon, etc. The inorganic scintillators, Bismuth germinate, Thallium activated NaI and CsI, High in Britain and France generally use CO2 as coolant and graphite as moderator. In High Temperature Gas Cooled Purity Germanium (HPGe) and Lithium drifted Germanium reactors, the fissile and the fertile materials dispersed in detectors Ge(Li) are effectively employed in gamma- graphite moderator are used as fuels and helium is used as spectroscopic measurements. Silicon surface barrier coolant. detectors and Si(Li) detectors are respectively used in Alpha-spectroscopic studies and X-ray measurements. The presence of trace and ultra trace elemental Beryllium metal has an useful application as a metal constituents in nuclear fuels as well as in fuel sub-assembly window for X-ray machine. Organic materials like components has significant affects on the overall efficiency anthracene, stilbane and some specific plastics are also of a nuclear reactor as these may cause significant changes utilized as scintillation detector materials. In early in neutron economy, fuel integrity, thermal, mechanical and developments, paraffin was used in fabrication of moderator corrosion properties of dual-use structural materials and assemblies in neutron counters. In Thermo-Luminescence failure of fuel-clad-coolant interactions. The quality control Dosimeters (TLD), generally used as TLD badges, small of the trace constituents to certain specifications in the capsules of LiF and CaSO4 (50–100 mg) doped with relevant media needs careful consideration in achieving the manganese impurity constitute the thermo-luminescent desired fuel burn-up in a reactor. materials to measure the cumulative radiation doses The management of radioactive wastes15, in general, received by the workers in radiation environment. Thus, is of great environmental concern. In disposal of radioactive varieties of organic and inorganic materials found their wastes, after separation of the most radio-toxic and heat effective applications in nuclear instrumentations (detection generating fission products such as 137Cs and 90Sr, systems/nuclear devices) as essential dual-use nuclear lanthanides and ‘minor’ actinides, Pu, Np, Am, Cm, etc., materials. present in spent fuels, the residual wastes are immobilized Conclusions in an appropriate solid matrix like borosilicate glass, phosphate glass or ceramics like SYNROC and zirconolite Extensive research on the development of advanced

(CaZrTi2O7), the choice of which is governed by its nuclear nuclear fuels and efficient fuel sub-assembly structural compatibility with the quality of the specific radioactive materials having favorable elemental properties especially wastes. The immobilized solid wastes encased in stainless nuclear characteristics highly compatible with the fuels has steel canisters are placed in deep repository of suitable been continued since the installation of Fermi’s first atomic geological formations for long term safety surveillance. pile. Over the decades, development of varieties of improved fuel-assembly designs has resulted in significant In the last decade of the 19th century, Becquerel in increase in higher thermal margins in the reactor cores for his discovery of radioactivity used photographic films as a a longer life-span (40–60 yr). Since the life and efficacy suitable medium to detect and measure the intensity of the of nuclear fuels and the loss of integrity of the dual-use emitting radiations. During this long period, various structural material used in a reactor are primarily dictated sophisticated nuclear techniques and devices have been by the effective changes/degradations caused by the fuel- developed for detection, characterization and precise clad-coolant interactions in its hostile environment, it is measurement of intensities of these emitting radiations in essential to know the out-of-pile properties of the fuels different media. The efficiency of radiation detectors16 and the major and minor structural components particularly depends on the physic-chemical and nuclear characteristics their nuclear compatibilities towards the fuels so that of the specific dual-use materials to be used for nuclear corrective measures can be taken at the design stage of a detection and/or the type of measurement to be made. As reactor.

14 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2015 Knowledge on the evolution of novel nuclear fuels 5. G. Flanagan, Export Control of Nuclear Reaction Components coupled with more efficient new alloys, ceramics, and Dual-use Materials and Equipments, National Nuclear Security Association (NISA), Dept. of Energy, USA. composites, etc. as dual-use structural materials may 6. T. B. Lindermer and T. M. Besmann, J. Nucl. Materials, 100 mitigate the challenges in achieving optimum reactor system (1–3), 178 (1981). performance under demanding conditions. Understanding 7. Nuclear Materials, European Nuclear Society, 11-14 (2014) of the characteristic properties and specifications of the 8. N. R. Das, Science and Culture, 79 (9-10), (2013) fuel as well as fuel sub-assembly components can help to 9. G. A. Rama Rao (Ed), IANCAS Bull., IV(3), (2005) keep pace with the increasing world’s demand of energy 10. R. V. Kamath (Ed)) IANCAS Bull., VIII(2), (2009) with minimum environmental impact but with maximum 11. K. L. Murty and I. Charit, J. Nucl. Materials, 383, 189 (2008) safety as par regulations stipulated by the authorized Bodies 12. A. V. R. Reddy (Ed), Heavy Water, IANCAS Bull., 17(1), like IAEA, ICRP and others.  (2001) 13. Bailly, Menessier and Prunier (Ed), “The Nuclear Fuel of References Pressurized Water reactors & Fast Neutron reactors, Design &Behavors”, (1999) (Lavoisier Publishing, France). 1. Wikipedia, the free encyclopedia, Nuclear Materials, modified on 26. 06. (2013). 14. A. V. R Reddy (Ed), Fast Breeder Reactor, IANCAS Bull., I 2. W. D. Ehmann and D. E. Vance, Radiochemistry and Nuclear (4), (2002) Methods of Analysis, (1991), (John Wiley & Sons, N. Y.). 15. M. R. A Pillai (Ed), Nuclear Waste Management, practice and 3. B. R. T. Frost, Nuclear Fuel Elements, (1981), (Argonne National trends, IANCAS Bull., 11(1), (1997) Laboratory, Pergamon Press, N. Y.). 16. B. S. Tomar(Ed), Chemical Characterization of Nuclear Fuels, 4. N. R. Das, Wonders of Radioactivity, (2011), (Sailee, India). IANCAS Bull. VII(3), (2008)

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