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Published: 1962

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An experimental program has been undertaken to explore the feasibility and limits of UO/sub 2/ fuel rod operation with a molten central core. Three UO/sub 2/ pellet fuel assemblies were irradiated at high thermal performance levels, under pressurized water conditions, in the GETR. The fuel rods operated with 0.565 OD peak surface heat fluxes ranging from approximately 600,000 to 1,100,000 Btu/hr-ft/sup 2/. UO/sub 2/ central melting occurred in the fuel rods operating above 600,000 sion products was evident in the higher thermal performance fuel rods. Ten of the twelve fuel rods irradiated exhibited severe clad swelling in the peak heat flux zone, with a maximum diameter increase in one iuel rod of 0.073 inch. The mechanism for clad swelling has been identified as the UO/sub 2/ volume expansion on melting. A special test assembly was fabricated incorporating internal free volume in the fuel to accommodate the UO/sub 2/ expansion. Short time irradiation of this assembly at high thermal performance conditions produced negligible clad swelling. The assembly has been returned to the loop for a 25-day irradiation cycle. Continued freedom from swelling in this assembly following extended irradiation will permit resumption of the original program irradiation schedule. A final maximum thermal performance pellet fuel assembly remains to be irradiated. This assembly contain; fuel rods to operate at peak surface heat fluxes in the range of 1.2 to 1.3 x 10/sup 6/ Btu/hr-ft/sup 2/. Following fuel rod modifications similar to that in the special test assembly above, this assembly irradiation will be undertaken and continued to high burnup. The target burnup for the assembly is 20,000 Mwd/t. An identical sequence of assembly irradiations is scheduled using compacted powder UO/sup 2/ fuel rods. (auth).