Safety margin evaluation of RSG-GAS during LOFA considering ageing-induced degradation of coast-down flow and recent decay heat measurement

This study evaluates the safety performance of the RSG-GAS research reactor during a Loss of Flow Accident (LOFA), with a particular focus on the effects of aging on primary pump coast-down behavior and post-shutdown decay heat. As the reactor has operated for over 38 years, mechanical degradation, especially in the pump flywheel, has led to a more rapid decline in coolant flow following a pump trip. To capture this effect, updated coast-down flow and decay heat profiles were experimentally measured and incorporated into LOFA simulations using the PARET/ANL code. The analysis compares reactor behavior under commissioning and current conditions at initial power levels of 15 MW and 30 MW. Results show that while the updated profiles slightly reduce safety margins, evident in lower burnout ratios and higher coolant temperatures, all key safety parameters remain within the reactor’s Operational Limits and Conditions (OLCs). The passive safety system, based on natural convection, effectively removes decay heat even with degraded pump inertia. These findings underscore the importance of incorporating real-time experimental data into safety evaluations and confirm that the RSG-GAS reactor continues to operate safely under ageing conditions.