Assessment of Modeling Capabilities and Transient Performance for High-Temperature Reactors

Modeling and simulation tools play an important role in the design and licensing of reactors, but models alone are insufficient for licensing reactors. As advanced reactors approach deployment, modeling tools must be validated to ensure their applicability. Code-to-code verification studies can be conducted to assess the performance of simulation tools relative to one another. These studies are particularly important when validation data do not exist. This dissertation uses verification studies to assess modeling capabilities for a fluoride salt-cooled high-temperature reactor (FHR) design called the Generic FHR (gFHR) and code-to-data validation studies to assess modeling capabilities for a high-temperature gas-cooled reactor (HTGR) thermal hydraulics test facility called the High Temperature Test Facility (HTTF). Neutronics and thermal hydraulics studies of the gFHR conducted using SCALE and MELCOR demonstrated a capability to produce results that are comparable to those in the literature. The analysis with SCALE also demonstrated and verified a novel approach for determining the equilibrium state of a pebble-bed reactor. Following verification of the SCALE and MELCOR models, a loss of flow accident (LOFA) was modeled. Sensitivity studies on the LOFA showed that inlet temperature, graphite thermal conductivity, SCRAM time, and friction multiplier had significant control over peak temperatures during the transient. Fuel temperatures remained well within safety limits, but coolant outlet temperatures may slightly exceed safety limits based on the primary loop configuration used in this analysis. The HTTF validation analysis assessed RELAP5-3D. Sensitivity studies revealed that the thermal conductivity of HTTF blocks was the most important parameter controlling their temperature in steady state and transients. Calibration studies aimed at validation demonstrated that the PG-26 depressurized conduction cooldown experiment was challenging to model due to confounding events. The PG-27 pressurized conduction cooldown experiment demonstrated that while the RELAP5-3D model could reproduce steady-state temperatures, it significantly underpredicted the transient temperature rise. This is likely due to the nodalization of the model rather than code deficiencies. Finally, models of HTTF experiment PG-29, a depressurized conduction cooldown demonstrated that the PG-27 calibration was reasonable, but the models could not be validated against PG-29 either.