Sensitivity Analysis of an Overcooling Transient in the Generic Fluoride-Salt-Cooled High-Temperature Reactor

The focus of this study is to perform an in-depth analysis of a potential power-cooling mismatch within a Pebble Bed Fluoride-salt-cooled High-temperature Reactor (PB-FHR). The postulated transient that was chosen was an overcooling accident, which can be caused by an increase in heat removal from the primary system. The PB-FHR system that was selected for this study was the Kairos Power generic Fluoride-salt-cooled High-temperature Reactor (gFHR), which is free of proprietary information and encourages modeling and collaboration across multiple institutions. Overcooling is of interest in the gFHR because a decrease in coolant temperature can have significant impacts on the system because of reactivity feedback. Changes in power and fuel temperature need to be investigated to determine how the fuel will respond with respect to fuel performance. Therefore, this study will generate thermal-hydraulic boundary conditions and perform a Sobol sensitivity study to observe the power and temperature behavior during the transient, as well as what parameters had the most impact on the key figures of merit. It was found that the change in the inlet coolant temperature during the transient had significant impact on the total power, while the nominal coolant inlet temperature and the effective thermal conductivity of the fuel region heavily impacted fuel temperatures. The peak power reached about 465 MW(thermal) (66% increase from nominal), and the temperature increase did not exceed 955 degrees C for the fuel, which is lower than a given 1650 degrees C safety limit for TRistructural ISOtropic (TRISO) particle fuel. To investigate the fuel behavior during the transient, the thermal-hydraulic boundary conditions were exported to a one-dimensional BISON model that tested the failure probability of the SiC layer. Under these conditions, the maximum principal stress of the SiC layer reached 1.27 $$MPa,$$MPa, and the overall failure probability of the SiC layer was 8.5 x 10-9. It was concluded that the fuel was not challenged during this transient and helped to demonstrate the robustness of the TRISO fuel form and the inherent safety features of the gFHR.