| 摘要 |
Material for use as a nuclear fuel, a moderator or a control-rod is made by mixing the corresponding nuclear fuel, moderator or absorber material, in finely-divided state, with a metallic constituent comprising one or more metals, also finely divided, the mixture being sintered in an atmosphere containing hydrogen, a halogen and a chromium halide, the surface layer formed being substantially purely metallic. The metallic constituent comprises one or more of the following: Fe, Ni, Co, Cr, Mo and alloys containing at least one of these metals. The nuclear fuel, moderator or absorber material must have a heat of formation, if a compound, high enough to prevent it being reduced by the metallic constituent during the sintering operation and form halides which are both stable and volatile under sintering conditions. The nuclear fuel may be UO2, U3O8, ThO2 or mixtures of these and may be mixed with a finely-divided moderator substance such as BeO. The absorber material may be boron. The sintering atmosphere may either be an atmosphere of hydrogen- and halogen-containing gas, e.g. a hydrogen halide which is dissociated at the temperatures employed, together with a chromium halide; or an atmosphere of hydrogen and halogen (as above) which forms chromium halide in situ, e.g. when the body to be sintered is packed in chromium fragments. The halogen in the sintering atmosphere is preferably fluorine. The metallic surface layer is plastic as well as being firmly bonded to the underlying matrix so that recompressions e.g. between two sintering stages, can be applied as on a press or in extruding and wire-drawing equipment without the risk of cracking. When U3O8 is used, a reduction to UO2 may occur under some operating conditions with the formation of Cr2O3, if chromium is present in the matrix, and this may result in a hardening of the matrix. Various oxides, e.g. Fe2O3, CeO, NgO and Al2O3 may be added to a UO2-containing mixture and during the sintering release oxygen which forms Cr2O3 in situ to harden the matrix. The mixture, prior to sintering, may be shaped and coated with a layer of Cr powder, an Fe-Cr alloy powder or a Cr-Ni alloy powder. A sheath of refractory material may be bonded to the metallic surface layer of the sintered material. Many examples of processes according to the invention are given. The Mo and Fe constituents may be present as fibres and Cd and W powders may also be added to the matrix. Uranium carbide may also be used.
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