Failure Probability Analysis of a Uniformly Thinned Silicon Carbide Layer in TRISO Particle Fuel

This article describes a preliminary assessment to quantify the effect of a Pd chemical attack on the stress and failure probability of silicon carbide (SiC) in TRistructural ISOtropic (TRISO) fuel under steady-state and transient reactor conditions. One-dimensional TRISO simulations have been performed using BISON, a finite element fuel performance code developed and maintained by Idaho National Laboratory. The Pd chemical attack is modeled in this work as a uniform thinning of the SiC. Failure probability calculations are performed in BISON based on Weibull statistics. A layer of αPd2Si, the expected phase of palladium silicide at relevant temperatures, has been inserted in this model between the inner pyrolytic carbon (IPyC) and SiC layers. Simulations of a baseline TRISO particle without an αPd2Si layer were performed for comparison. The available elastic and thermomechanical properties for αPd2Si were included in the presented models. Modeled steady-state conditions based on the Advanced Gas Reactor-1 (AGR-1) irradiation test specifications indicate that as the αPd2Si thickness increased, the tensile hoop stress within the SiC layer increased. The baseline SiC (e.g. without a Pd2Si layer) under steady-state conditions showed a maximum fractional failure probability of 6.4 × 10−5. A 1-μm-thick αPd2Si layer, under steady-state conditions, raises this failure probability to 2.7 × 10−4. Postulated accidents were simulated based on a controlled rod withdrawal (CRW) or an overcooling transient in the generic fluoride salt-cooled high-temperature reactor (gFHR) starting from steady-state irradiation at a moderator temperature of 700°C and nominal power (P/P0 = 1). During the CRW reactor conditions, baseline failure probabilities are 8.1 × 10−9. When implanting the αPd2Si layer, the failure probability increases by nearly three orders of magnitude, with a failure probability of 1.7 × 10−6 for the 1-μm-thick αPd2Si layer. Additionally, two accidents for a representative micro high-temperature gas reactor (µHTGR) were simulated: a pressurized loss of forced coolant and a CRW with a loss of flow. The BISON simulations for both µHTGR transients indicate that the SiC layer failure probabilities are on the order of 10−3 in the most extreme case. Finally, two-dimensional simulations of AGR-1 reactor conditions were performed to evaluate the effect of a through-thickness IPyC crack formed during steady-state reactor conditions. Significant increases in both tensile stress and failure probability are apparent when the SiC thickness is reduced by the presence of the αPd2Si layer. The absolute worst case simulated in this work, which combined an extreme thinning of the SiC layer with Pd2Si and IPyC cracking, has a failure probability above 0.1.