TRISO burnup-dependent failure analysis in FHRs using BISON

Abstract

In this study, we perform a study on TRIstructural ISOtropic (TRISO) Silicon Carbide (SiC) layer failure using the BISON fuel performance code. SiC thermomechanical behavior is analyzed during both steady-state and accident scenarios pertaining to a Fluoride-Salt-Cooled High-Temperature Reactor (FHR). A monodimensional single particle model is adapted from a previous analysis and run at 140 mW/particle for 500 days. The maximum burnup reached is 18.49% FIMA. A total of three accident scenarios are simulated at both fresh fuel conditions and following multiple irradiations at 2% FIMA increments up to 18.49% FIMA. The instances include an overcooling transient, a Control Rod Withdrawal (CRW) and a Loss of Forced Cooling Accident (LOFA). SiC hoop stress and failure probability predictions are evaluated for all simulated cases. A consistent tangential compressive stress state is found as a function of burnup for SiC. The maximum compressive hoop stress reaches approximately -400 MPa. The maximum SiC failure probability as a function of burnup is 2⋅10−13% at steady-state and no increases are predicted during transient simulations. It is concluded that the selected accident scenarios, under our modeling assumptions, are not predicted to pose a challenge to SiC integrity due to their relatively slow progression and low temperature increments, for both fresh fuel and at burnup values up to 18.49% FIMA. A 2D model is used, along with XFEM, to simulate a crack in the Inner Pyrolytic Carbon (IPyC). Conservative IPyC failure predictions are obtained with Weibull statistics approximately 38 days after beginning of irradiation. After crack formation, hoop stress concentrations are predicted in SiC using conservative assumptions, with failure probability increasing to 3.3⋅10−4%. 2D BISON simulations of the base irradiation are then carried out to the maximum burnup and transient conditions are subsequently applied. SiC maximum tensile hoop stress is predicted at the time of crack initiation, after which crack expansion reduces the predicted stress magnitude in the material. The results presented in the work are interpreted under the selected modeling assumptions and conclusions are based on a comparison with a parallel analysis on a High-Temperature Gas-Cooled Reactor design to evaluate the effects of power density and burnup on SiC performance.