Nuclear fuel metallic fuels

Other metals such as beryllium, hafnium, niobium, vanadium, and zirconium are known to have nuclear and other properties which make them desirable materials of construction in various designs of nuclear reactor, but also they have, or may have in the future, important uses outside that field. All these metals except hafnium have been used or proposed for canning materials to clad and protect the nuclear fuel metals from corrosion by the reactor coolants or moderators, air, carbon dioxide, water, heavy water, graphite or molten sodium, etc. In some cases the specifications for neutron-absorbing impurities are of the same order as for the fuel metals uranium and thorium. Hafnium, however, with a high neutron-capture cross-section, is a useful material for reactor control rods and exhibits favourable metallurgical properties under irradiation. 

The metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth s crust than from both uranium and fossil fuels. Any sizable demand from thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties. 

Several components are required in the practical appHcation of nuclear reactors (1 5). The first and most vital component of a nuclear reactor is the fuel, which is usually uranium slightly enriched in uranium-235 [15117-96-1] to approximately 3%, in contrast to natural uranium which has 0.72% Less commonly, reactors are fueled with plutonium produced by neutron absorption in uranium-238 [24678-82-8]. Even more rare are reactors fueled with uranium-233 [13968-55-3] produced by neutron absorption in thorium-232 (see Nuclear reactors, nuclear fuel reserves). The chemical form of the reactor fuel typically is uranium dioxide, UO2, but uranium metal and other compounds have been used, including sulfates, siUcides, nitrates, carbides, and molten salts. 

The determination of critical si2e or mass of nuclear fuel is important for safety reasons. In the design of the atom bombs at Los Alamos, it was cmcial to know the critical mass, ie, that amount of highly enriched uranium or plutonium that would permit a chain reaction. A variety of assembhes were constmcted. Eor example, a bare metal sphere was found to have a critical mass of approximately 50 kg, whereas a natural uranium reflected 235u sphere had a critical mass of only 16 kg. 

Concern about the potential diversion of separated reactor-grade plutonium has led to a reduction ia U.S. governmental support of development of both plutonium recycle and the Hquid metal reactor. This latter ultimately depends on chemical reprocessing to achieve its long-range purpose of generating more nuclear fuel than it bums ia generating electricity. 

The NRC has developed special procedures for the handling, transportation, and storage of nuclear fuel because radioactivity can be a health hazard if not properly shielded. Spent fuel is typically transported by rail or tmck in heavily shielded (Type B), sealed, thick metal shipping containers designed to withstand possible accidents, such as derailments or coHisions, which may occur during transport. The NRC certifies that each shipping container meets federal requirements. The U.S. Department of Transportation sets the rules for transportation. 

The main technological uses for UO2 are found in the nuclear fuel cycle as the principal component for light and heavy water reactor fuels. Uranium dioxide is also a starting material for the synthesis of UF [10049-14-6] 6 (both critical for the production of pure uranium metal and... 

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Fluorides. Uranium fluorides play an important role in the nuclear fuel cycle as well as in the production of uranium metal. The dark purple UF [13775-06-9] has been prepared by two different methods neither of which neither have been improved. The first involves a direct reaction of UF [10049-14-6] and uranium metal under elevated temperatures, while the second consists of the reduction of UF [10049-14-6] by UH [13598-56-6]. The local coordination environment of uranium in the trifluoride is pentacapped trigonal prismatic with an 11-coordinate uranium atom. The trifluoride is... 

Hafnium-free zirconium is particularly weU-suited for these appHcations because of its ductiHty, excellent oxidation resistance in pure water at 300°C, low thermal neutron absorption, and low susceptibiHty to radiation. Nuclear fuel cladding and reactor core stmctural components are the principal uses for zirconium metal. 

Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

Hydrides of the types AnHi (An = Th, Np, Pu, Am, Cm) and AnHs (Pa —> Am), as well as ThaHis (i.e. ThHs.yj) have been so obtained but are not very stable thermally and are decidedly unstable with respect to air and moisture. Borides, carbides, silicides and nitrides (q.v.) are mostly less sensitive chemically and, being refractory materials, those of Th, U and Pu in particular have been studied extensively as possible nuclear fuels.Their stoichiometries are very varied but the more important ones are the semi-metallic monocarbides, AnC, and mononitrides, AnN, all of which have the rock-salt structure they are predominantly ionic... 

Pyrophoric penetrators equal in effectiveness to those containing U are reportedly provided by those containing spark sintered bodies of 49—95% W, 4—50% Zr and a binder of ductile metal such as Ni, Fe, Co or Cu (Ref 115). Even though U is the most pyrophoric of all elements, it is not expected to see use as an incendiary because of its toxicity and its great value as a nuclear fuel... 

Disiloxane, tetramesityl-, 3,206 Disproportionation iridium catalysts, 4,1159 Dissolution nuclear fuels, 6, 927 Distannene, 3,217 Distannoxane, 1,3-dichloro-, 3,207 Distibine, tetraphenyl-, 2,1008 Distibines, 2,1008 Disulfido ligands metal complexes, 2,531-540, 553 bonding, 2, 539 electron transfer, 2, 537 intramolecular redox reactions, 2,537 reactions, 2, 537... 

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