An evaluation of the effects of SiC layer thinning on failure of TRISO-coated fuel particles

The fundamental design for a gas-cooled reactor relies on the behavior of the coated particle fuel. The coating layers surrounding the fuel kernels in these spherical particles, consisting of pyrolytic carbon and silicon carbide layers, act as a pressure vessel that retains fission products. Many more fuel particles have failed in US irradiations than would be expected when only one-dimensional pressure vessel failures are considered. Several multi-dimensional failure mechanisms that may have contributed to these failures have been previously studied, such as (1) irradiation-induced shrinkage cracks in the inner pyrocarbon (IPyC) layer, (2) partial debonding between the IPyC and SiC layers, and (3) deviations from a perfectly spherical shape. A further phenomenon that could lead to particle failures is thinning of the SiC layer caused by either thermal decomposition or interaction with fission products. Results of a study of the effects of SiC thinning and criteria for evaluating this behavior in a fuel performance code are presented.