Kernel Migration for HTGR Fuels from the System Th‐U‐Pu‐C‐O‐N

Abstract

Nuclear fuels for the high‐temperature gas‐cooled reactor (HTGR) consist of spherical kernels of actinide compounds contained in gaslight pyrolytic carbon and SiC. The fuel kernels migrate up the temperature gradient and into the coating layers. A theoretical analysis of in‐reactor migration data indicates that a solid‐state diffusion process controls the migration rate in fissioned Th0.84U0.16C2, UO2, ThO2, PuO2‐x, and Th0.8U0.2O2 particles. The theoretically based kernel migration coefficient (KMC) measured in units (cm/s) °K2 (oK/cm)‐1, is thus used to correlate the laboratory and in‐reactor data. The KMC values for Pu‐containing particles may be dependent on the initial and in‐reactor O/Pu values. The other in‐reactor KMC values were apparently not dependent on the extent of fission, the fission of either 233U or 235U, or the presence of an SiC coating layer. Laboratory KMC values were obtained for unirradiated ThO2, UO1.65 N0.25,UC2 ThC2, and Th0.84U0.16C2 particles and generally appeared to be consistent with in‐reactor data.