Code Benchmark of a Depressurized Conduction Cooldown Transient in the High Temperature Test Facility

We present results from modeling of a depressurized conduction cooldown (DCC) at the High Temperature Test Facility (HTTF) in the OECD-NEA Thermal Hydraulics Code Validation Benchmark for High-Temperature Gas-Cooled Reactors using HTTF Data. We briefly describe the benchmark and the models being used. We then present a comparison of steady state and transient results based on the Problem 2 Exercise 1A and 1B. We compare distributions of block and helium temperatures and coolant flow. We also compare energy balance in steady state. All models show comparable mass flow distributions and energy balances. The temperatures within the core and outer regions are comparable in all models too, but inner reflector temperatures can vary significantly. Despite that, we find that the models are in good agreement for the full-power steady state. In the DCC, we look at block temperature at the core midplane and RCCS water exit temperature. The INL and ANL models are found to be in excellent agreement with one another on block temperature over time, and when the KAERI and NRG models are considered agreement is good. Differences in the transient heat removal from the RCCS cause the differences in block temperature over time. The CNL models show similar trends to the INL, ANL, KAERI, and NRG models, but the temperatures are high because the volumes used in calculating the average temperature include the heater rods in the CNL models. The HUN-REN model shows slightly different RCCS behavior than other models, leading to differences in results, but these differences do not suggest fundamental differences.