Simulation of LOFA and RIA for the IEA-R1 research reactor using the code MERSAT

Abstract

The thermal hydraulic system code MERSAT has been applied to model the Brazilian IEA-R1 research reactor. The developed MERSAT model consists of detailed description for the primary loop including reactor core, bypass, reactor pool, main pump and coupling valve. The developed model is verified through the simulation of the steady state operation at the nominal power of 5 MW. According to the safety significance of postulated initiating events two selected design basis accidents corresponding to loss of flow accident (LOFA) with reactor scram and reactivity insertion accident (RIA) without scram have been analyzed. The results of LOFA simulation demonstrate the phenomena of flow reversal characterizing the flow turnaround from forced downward to natural upward circulation. The prevailing natural circulation after the flow reversal guarantees the removal of residual decay heat. The maximum clad temperature of the hot channel reaches its maximum of about 73 °C after 35 s after the initiation of the transient. The step reactivity insertion of 2 mk leads to increasing the reactor power up to 1.7 times the nominal value. Consequently, the maximum clad temperature of hot channel reaches about 99 °C which is still far below the onset of subcooled boiling. Hence, the established thermal hydraulic conditions ensure a considerable safety margin even if the reactor protection system should fail to shutdown the reactor 400 s after the begin of the reactivity insertion event. The results of LOFA and RIA demonstrate the safety features of IEA-R1 under the investigated conditions. In both cases the thermal hydraulic design limits have not been exceeded. However, for the RIA case the maximum clad temperature exceeds the limit of 95 °C being chosen as operation criteria for limiting corrosion on the surface of the fuel elements.