Patent Number: 043671845
Section: summary

Fuels in the form of sintered microspheres containing a high percentage of U.sup.235 have come into general use for nuclear power generation. However, such microspheres can readily be processed to provide fissile material for use in atomic weapons. To promote nuclear weapon control, the nuclear reactor fuel supplied to countries in need of nuclear power generating plants should be in a form that can only be diverted into use in atomic weapons by means of uranium-enriching facilities not generally available. A preferred form of nuclear fuel that can safely be supplied to energy-poor countries consists of high density microspheres produced from 20% enriched uranium and containing up to 30 mole percent UC.sub.2 with the remainder in the form of UO.sub.2. PRIOR ART Extensive investigations have been conducted to determine the nature of the uranium-carbon-oxygen nuclear fuel system with which the present invention is involved. The phase diagram for the system at temperatures in the range of 1300.degree.-1750.degree. C. has been developed in detail, and the preferred process embodiment of this invention will be described by reference to this diagram. The same U-C-O diagram has been previously considered in an article titled "The Application of Thermochemical Principles to the Production of Nuclear Fuel Materials" and published by G. R. Chilton in 1976 as Special Publication No. 30 of The Chemical Society, Burlington House, London, England. Carbothermic reduction of uranium dioxide, which is utilized in the preferred process of this invention, is also described in the aforesaid publication and in an article titled "Carbon Monoxide Equilibrium Pressures and Phase Relations During the Carbothermic Reduction of Uranium Dioxide" and published by J. F. A. Hennecke and H. L. Scherff in 1971 in the Journal of Nuclear Materials 38, North-Holland Publishing Company, Amsterdam, Holland. SUMMARY OF THE INVENTION It is an object of this invention to form microspheres containing uranium compounds in chemical forms and proportions suitable for use as nuclear reactor fuel but not conveniently adaptable for use in atomic weapons. Another object of the invention is to provide a method for making nuclear fuel microspheres consisting essentially of about 1-30 mole percent uranium dicarbide and 70-99 mole percent uranium dioxide and the fuel having a density of about 10.2 to 11.0 g/cm.sup.3. These objects are attained by a preferred process embodiment of the invention wherein microspheres containing uranium dioxide and uncombined carbon are first sintered at a temperature of 1550.degree. C. in an atmosphere containing about 0.5 to 1 mole percent of carbon monoxide and then sintered at the same temperature in an atmosphere containing about 3 mole percent of carbon monoxide.