Simulant melt experiments on in-vessel retention through external reactor vessel cooling

Abstract

External reactor vessel cooling (ERVC) is one of the major severe accident management strategies for operating nuclear power plants. Flow circulation inside the reactor pressure vessel (RPV) insulator should be effective enough to ensure sufficient heat removal via ERVC. Confirmation experiments for different configurations of the RPV insulator were performed using alumina-iron thermite melt as a corium simulant. For precise evaluations on the flow path inside the insulator, flow analyses using the RELAP5/MOD3 code were performed. Because of the limited steam venting through the insulator, steam binding occurred inside the annulus in the tests that were performed to simulate the operating conventional insulator design. This steam binding brought about incident heatup of the vessel outer surface. On the contrary, in the test that was performed to simulate the advanced design of insulator considering ERVC, sufficient water ingression and steam venting through the insulator resulted in effective cooldown of the vessel lower head characterized by nucleate boiling. The results of flow analyses using the RELAP5/MOD3 code confirmed the steam binding in case of the limited steam venting. From the current experimental results, it could be found that the proposed modification of the insulator design allowing sufficient water ingression and steam ventilation could increase the possibility of in-vessel corium retention through ERVC.