Multiphysics simulation of TRISO fuel compacts and the effects of homogenization on silver release predictions

This work studies the impact of explicit and homogenized modeling approaches on the multiphysics simulation of TRistructural ISOtropic (TRISO) fuel compacts in prismatic High Temperature Gas Reactors (HTGRs). TRISO fuel compacts exhibit complex double heterogeneity that significantly affects heat conduction, neutron transport, and silver fission products release. In this work, we use Cardinal, a multiphysics tool based on the Multiphysics Object-Oriented Simulation Environment (MOOSE) framework, to couple neutron transport, heat conduction, and fission product transport. OpenMC is used for neutron transport and the MOOSE heat transfer module is used for the heat conduction. BISON is used for the silver release predictions. Two different modeling approaches – explicit modeling of individual TRISO particles and homogenized representation using effective thermal properties – are compared at high TRISO packing fractions (20% and 40%) across varying power densities. Results demonstrate that homogenization significantly underpredicts peak temperatures; for the case of high power per TRISO particle, there is a difference of 83.73 K in the maximum temperature. As a result, homogenization underestimates the silver release fraction predictions compared to explicit modeling, especially in the irradiation phase. This work demonstrates the importance of accurately modeling heterogeneity of the TRISO particles to reliably predict fission product release and assess reactor safety margins.