Effects of Anisotropic Transformation on the Buffer Layer of TRISO Fuel Particles

Abstract

The buffer layer of the tristructural-isotropic (TRISO) nuclear fuel particles contributes to storing gaseous fission products. Any change in the layer’s materials properties may affect the mechanical integrity of the fuel particles, especially at high burn-up conditions. Continued irradiation on the buffer layer is expected to cause anisotropic transformations of the initially isotropic buffer layer, and the following changes in mechanical properties may affect the particle durability. Yet, the effect of anisotropic transformation on the buffer layer’s and eventually the particle’s mechanical properties remain to be elucidated. Despite its porous structure, in the previous studies, the layer was generally considered as a low density yet non-porous pyrocarbon. We attempt to study the mechanical properties of the buffer layer by taking the porous structure into account, instead of defining the buffer layer as a nonporous layer with low density. In this research, we employed a fully coupled multiphysics model to analyze this problem. To model and analyze the coupled system of anisotropy-dependent mechanical properties such as thermal expansion and modulus of elasticity, we implemented the fuel performance code based on MOOSE. The effect of the transformation to the buffer layer were discussed considering several cases in depth.