Strength of Silicon Carbide Layers of Fuel Particles for High-Temperature Gas-Cooled Reactors

Abstract

A fuel design being developed for the high-temperature gas-cooled reactor consists of microspheres (particles) of a very small kernel of dense, sintered, enriched [sup 235]UCO encapsulated by several layers of pyrolytic carbon and a layer of silicon carbide (SiC). The coated fuel particles are often called TRISO[reg sign] particles. The SiC is derived via thermal decomposition of methyltrichlorosilane. This strong, dense layer is very important to the integrity of the particle and the retention of fission products. A fundamental understanding of failure mechanisms of unirradiated fuel particles is elucidated by measuring their failure particles is elucidated by measuring their failure rates when exposed to mechanical stresses. This was accomplished by compression testing of whole particles in two modes: (a) point loading and (b) dimple loading. Finite element stress modeling showed that point loading primarily exposed a small portion of the inner surface of the SiC layer to a maximum tensile stress. Stress analysis for the dimple loading showed that a significant area (inner and outer surface) of the SiC layer and a large volume of the SiC layer were stressed to near-maximum tensile levels. Various batches of archived particles were tested. Weibull methodology was used for analyses of failure statisticsmore » for groups of 500 particles. A scanning electron microscope was used for fractography, which identified critical flaws that were the likely fracture origins.« less