Evaluation of Alternative Safety Cooling Systems for Open Pool Research Reactor with Downward Flow

Modern open pool research reactors utilize flywheels and flap valves as safety systems to manage core cooling during loss-of-flow accident (LOFA) initiating events. The feasibility of using large flywheels at high power levels is currently under scrutiny. Specifically, for high powers and high heat flux values, the required size of the flywheels to meet safety requirements, especially for reactors with downward flow, renders them impractical. This impracticality arises from concerns regarding the reliability of a large wheel rotating for extended periods (t > 100 s) and the significant power consumption during the startup of the primary pumps. This study evaluates the departure from nucleate boiling ratio as the acceptance criterion and sets an upper limit of 2 min for the maximum total coastdown time induced by the flywheel. By adhering to these constraints, the threshold values for power and heat fluxes in research reactors with downward flow during a LOFA initiating event were determined. To enhance inherent safety and delay core flow reversal, the study proposes different alternatives to replace pump flywheels, thereby increasing the reactor power that can be removed by coolant flowing downward. These proposed alternative cooling systems include a passive auxiliary tank open to the atmosphere and an auxiliary pump integrated with the primary cooling system. To assess these proposals, a generic research reactor was modeled, and a typical LOFA event was simulated using the RELAP5 MOD3.2 code. The implementation of these alternative cooling mechanisms allows for the design of research reactors with downward flow that can operate at higher power levels and heat fluxes during LOFA transients, thereby significantly improving reactor safety and performance.