Kernel migration for HTGR fuels from the Th--U--C--O--N system

Abstract

Nuclear fuels for the high-temperature gas-cooled reactor (HTGR) consist of spherical kernels of actinide compounds contained within gastight pyrolytic carbon and SiC. This fuel is subjected to a significant in-reactor temperature gradient that leads to migration of the fuel kernel up the temperature gradient and into the coating layers. This phenomenon has been studied both in the laboratory and in-reactor for fuel kernels from the Th-U-C-O-N system. Application of theory to the analysis of in-reactor migration data indicates that a solid-state diffusion process operating across the kernel controls the migration rate in fissioned Th/sub 0/./sub 84/U/sub 0/./sub 16/C/sub 2/, UO/sub 2/, ThO/sub 2/, and Th/sub 0/./sub 8/U/sub 0/./sub 2/O/sub 2/ particles. The theoretically-based kernel migration coefficient (KMC), (cm/sec) /sup 0/K/sup 2/ (/sup 0/K/cm)/sup -1/, is thus used to correlate the data. The in-reactor KMC values were apparently not dependent on the extent of fission, the fission of either /sup 233/U or /sup 235/U, or the presence of a SiC coating layer. Laboratory KMC values were obtained from unirradiated dicarbide particles; these values were in excellent agreement with previously published laboratory KMC data and did not appear to be inconsistent with in-reactor dicarbide KMC values. Laboratory KMC values for ThO/sub 2/ agreed with in-reactor values. The irradiation performance of nitrogen-containing oxide or carbide kernels was no better than that of analogous nitrogen-free compounds.