Some studies of the failure of irradiated triso-coated fuel particles during their anneal at high temperatures

Abstract

The pressure buildup in the buffer layer of TRISO fuel particles is one of the most important factors influencing the mechanical behavior of the fuel. The pressure inside the particles is a driving force for many other phenomena (such as kernel migration, chemical attack of SiC layer, mechanical degradation, etc), therefore even small changes in pressure value could later have a critical impact on the fuel performance. Current TRISO fuel performance models are considering only Xe, Kr and CO (CO2) gases, however at high burnup values low concentration gases also could have essential impact. Therefore, in this paper we analyzed pressure buildup inside TRISO particles including also binary and ternary fission gases. The concentrations of fission gases inside the fuel kernel was calculated by Serpent code. For the calculations of molecular gases Proksch approximation was used. Pressure values inside TRISO particles were calculated using both Redlich-Kwong equation of state and ideal gas law. Finally, the maximum stress on the TRISO particles SiC layer was estimated using thin-shell approximation and the failure fraction of the SiC layer was evaluated. Calculations are done both for UCO and UO2 type fuel kernels for max 150MWd/kgU burnup level.