Modification in the stress calculation of PyC material properties in TRISO fuel particles under irradiation

Tristructural-isotropic (TRISO) coated fuel particles are a key component in the high-temperature gas-cooled reactor fuel elements. The stresses in a TRISO-coated particle depend upon the coating layer properties which are affected by neutron irradiation. The modifications of pyrolytic carbon (PyC) material properties under irradiation require the use of time-dependent material parameters in the stress calculations, which was performed with an analytic solution in this study. The experimental results indicated that PyC density increased at the early stages of irradiation while PyC anisotropy increased obviously at the later stages of irradiation, both of which caused particle stresses to increase. The PyC creep coefficient was modeled as increasing with neutron fluence. This significantly decreased the silicon carbide and inner pyrolytic carbon tangential and interface radial stresses. Increasing the Young's modulus of the PyC had little effect on the particle stresses. As a result, compared with constant material properties, the dynamic variation of PyC material parameters under irradiation further improved particle stress calculations and made the stress model closer to actual reactor conditions.