Creep of fissile ceramic materials under neutron irradiation

Theoretical estimation of the irradiation-induced creep rate of U02 by a modification of the Nabarro-Herring model for diffusional creep resulted in a creep rate range between about 6 x 10-6 to 8 x 10-5 h-1 for a fission rate of 1 x 1014 f/cm3 s and a stress of 2 kgf/mm2. Accordingly, the creep rate is enhanced by irradiation at temperatures below 10000 to 12000C. It is essentially due to the 'thermal rods' along the fission fragment tracks. Therefore, irradiation-induced creep rates should depend only slightly on temperature and must be markedly lower for carbide and nitride fuel. In-reactor creep experiments on UO2 were performed at fuel temperatures between 2500 to 8500C. At burnups between 0.3 to 3% the steady-state compressive creep rates are proportional to stress (0 to 4 kgf/mm2) and to fission rate (1 x 1013 to 2 x 1014 f/cm3 s), and are in the range estimated before. The increase in the creep rate with increasing temperature is low and corresponds to an apparent activation energy of only 5200 cal/mol. At burnups above 3 to 4% the stress exponent of the irradiation-induced creep rate increased from n = 1 to n = 1.5. Creep measurements on UO2 to 15 wt-%Pu02 (mechanically mixed, sintered density 86% TD) showed the same temperature dependence as UO2 below 7000C. However, the creep rates were higher by a factor of about 20 compared to fully dense UO2. This difference may be explained by assuming a high 'effective' porosity. In-pile creep tests on some UN samples resulted in creep rates that were lower by an order of magnitude than for UO2 under comparable conditions. (author)