Patent Number: 039432106
Section: summary

The present invention relates to an improved process for preparing metal hydride bodies by powder metallurgy, and more particularly to a powder metallurgy process for producing dense metal hydride bodies having a high hydrogen concentration. Metal hydride compositions such as zirconium and titanium hydrides (group IVB of the periodic table of the elements, "Handbook of Chemistry and Physics", pages 448-449, 44th edition) are employed as moderating materials in nuclear reactors. Hydrogen has the greatest neutron slowing down power of any element, and combined with the group IVB metals, which have reasonable structural properties, it is in a relatively stable, high density form. Homogeneous metal hydride nuclear fuel compositions are of particular interest where small reactor cores of high power density are required, for example, for remote terrestrial bases, mobile applications, and as auxiliary power sources in space vehicles. Reactor systems for such applications have been developed, and for information concerning their characteristics reference is made to Nucleonics, vol. 18, No. 1, January 1960. Hydrogen does not combine with group IVB metals in a fixed stoichiometry; the hydrogen is interstitially absorbed in the metal matrix in variable amounts. The moderating properties of the composition are dependent upon the hydrogen concentration, and it follows that the reactor core size is directly proportional to the hydrogen concentration. Since size and weight are at a premium, particularly in space power plants, it is necessary to increase power density to reduce the size of a reactor core. This has stimulated the development of methods of increasing the hydrogen concentration of metal hydride fuel compositions. Two general methods have been available for the preparation of homogeneous metal hydride fuel elements: massive hydriding and powder metallurgy. In massive hydriding, uranium-zirconium alloy bodies fabricated to approximately the final shape are heated in a hydrogen atmosphere to absorb the desired concentration of hydrogen. Since zirconium expands greatly upon hydriding and the hydrogen distribution must be substantially uniform throughout a fuel rod, massive hydriding requires very long time periods under carefully controlled temperature and conditions to avoid cracking and to assure uniform hydrogen distribution. Such processes are necessarily costly, and there are additional costs resulting from machining losses of the finished product. In principal, powder metallurgy processes offer economies in the preparation of zirconium hydride bodies. Zirconium powders can be hydrided to the desired hydrogen concentration, cold formed, and hot pressed in a short period of time. Process yields can be considerably higher because machining losses are not as great and, further, any losses can be readily recycled. Moreover, it is relatively easy to add nuclear fuels and/or burnable poisons to the product. In spite of such potential economies of powder metallurgical processing, there are certain drawbacks which have resulted in the adoption of massive hydriding methods. It has been considered that the brittle nature and reversible decomposition of zirconium hydride does not make it amenable to normal powder densification processes of sintering or hot pressing and, further, the approximately 90-95 percent densities obtainable had poor mechanical and heat transfer properties. Massively hydrided material, in contrast, has had greater density, higher thermal conductivity, and generally better mechanical properties. An object of the present invention, therefore, is to provide an improved powder metallurgy method of preparing group IVB metal hydride bodies. Another object is to provide an improved powder metallurgy method of preparing uranium-zirconium hydride fuel bodies of high densities. Another object is to provide a powder metallurgy method of preparing dense uranium-zirconium hydride fuel bodies having high hydrogen concentrations. Still another object is to provide such a method which achieves densities approaching 100 percent of theoretical, and a hydrogen-to-zirconium atom ratio of at least 1.2. Still another object is to provide a relatively simple and economical method of obtaining such powder metallurgical bodies which requires neither excessive temperature nor excessive pressure. The above and other objects and advantages of the present invention will become apparent from the following detailed description.