Interaction of LaC2 and NdC2 with SiC in HTGR particles

The reaction rate of either NdC2 or LaC2 kernels with SiC in coated particles similar to those used in a High-Temperature Gas-Cooled Reactor (HTGR) is essentially identical with that observed in irradiated, TRISO-coated HTGR fuel particles containing UC2. The reaction apparently results from destructive distillation of the rare-earth dicarbide in the kernel, transport of rare-earth metal gas down the temperature gradient, formation of the dicarbide at the SiC-pyrocarbon interface, and solution of the SiC in the dicarbide along the SiC grain boundaries. The reaction rate necessary to cause SiC coating failure in four years would apparently be achieved at temperatures above approximately 1565K. It appears that much less than 0.003 of the fissile fuel could fail from this reaction; this compares with a proposed large HTGR design failure fraction of approximately equal to 0.0039 for the fissile fuel