Evaluation of in-vessel corium retention through external reactor vessel cooling for small integral reactor

A criterion to prevent reactor vessel failure using in-vessel corium retention through the external reactor vessel cooling (IVR-ERVC) of a small integral reactor has been evaluated during a severe accident. A thermal load analysis from the corium pool to the outer reactor vessel wall in the lower plenum of the reactor vessel was performed to determine the heat flux distribution. The critical heat flux (CHF) on the outer reactor vessel wall was determined to fix the maximum heat removal rate through the external coolant between the outer reactor vessel and the insulation of the reactor vessel. Finally, the thermal margin was evaluated through a comparison of the thermal load with the maximum heat removal rate of the CHF on the outer reactor vessel wall. The maximum heat flux from the corium pool to the outer reactor vessel is estimated as approximately 0.25 MW/m2 in the metallic layer, owing to the focusing effect. The CHF of the outer reactor vessel is approximately 1.0–1.1 MW/m2 because of a two-phase natural circulation mass flow rate. Since the thermal margin for the IVR-ERVC is sufficient, the reactor vessel integrity is maintained during a severe accident in the small integral reactor.