An Analysis on the TRISO Failure due to Pd-SiC Interactions in a 350-MW_th Prismatic HTR Fuel

A tri-structural isotropic particle (TRISO) is a coated fuel particle of a high temperature reactor (HTR) fuel which consists of a kernel, a low-density pyrocarbon layer called a buffer, an inner high-density pyrocarbon (IPyC) layer, a silicon carbide (SiC) layer, and an outer high-density pyrocarbon (OPyC) layer. he mechanism and modeling on the reaction of palladium with the SiC layer of an HTR TRISO were reviewed. An analysis of TRISO failure due to Pd-SiC interactions has been done for a 350-MW_th prismatic HTR, whose fuel burnup and fast fluence are about 152 GWd/tHM and 8*10²1 n/cm², E_n > 0.1 MeV at 1500 EFPD and packing fraction of 25 %, respectively. - Kernel composition influences the birth of palladium during irradiation because the fission yield of palladium depends on fissile materials. The diffusion from a kernel limits the Pd-SiC interaction rate in a UO₂ kernel. - The currently developed correlation for the Pd-SiC reaction rate describes only the temperature dependence of the rate. It must be modified to include the effects of other factors such as kernel composition, palladium concentration, other fission products, and SiC properties. - The release of palladium from a kernel increases with temperature. The number of palladium atoms released is negligible below 1100℃, about 10 % at 1200℃, and about 40 % at 1400℃. - The TRISO failure due to the chemical attack of palladium is negligible at temperatures below 1200℃. The failure fractions calculated using COPA and TRIPPER are in good agreement.