DESIGN LIMITATIONS ON METALLIC URANIUM FUEL ELEMENTS FOR A GAS-COOLED REACTOR

The use of natural U a graphite moderated gas-cooled power reactor for operation at a gas-outlet temperature of 750 to 800 deg F imposes severe restrictions upon the fuel element design. Experience with the irradiation behavior of uranium metal at useful surface temperatures has shown that at temperatures above 400 deg C, a phenomenon called swelling'' must be considered. This phenomenon is associated with the expansion of the fission gases in the weakened metal lattice and results in serious increases in volume of the U. The design of a fuel element must provide for this swelling.'' Since the low multiplication factor possible in the natural uranium-graphite system dictates that the fuel be restricted to high density U, strengthening of the base metal by introducing alloying agents is limited to very low weight percentages of additives. The low multiplication factor also requires the lumping'' of the U in bodies of approximately 1 in. diameter. The practical results of these considerations are that a cylindrical metal rod of analloyed or low alloy U metal must be considered as the basic fuel element type. The low reactivity of the natural U system also restricts the choice of structural and canning materials which canmore » be used in the reactor core. The only metals that can be present in any appreciable amounts in such a system are Al, Mg, Be, and Zr. The bases for the design of fuel elements using natural uranium metal under gas-cooled power reactor conditions are discussed and several possible fuel assembly designs are described. (auth)« less