A Study on Natural Circulation Flow under Reactor Cavity Flooding Condition in Advanced PWR

Cavity flooding is an important severe accident management measure for in-vessel retention of a degraded core by external reactor vessel cooling in advanced PWRs. A code simulation study on the natural circulation flow in the gap between the reactor vessel wall and insulation material under cavity flooding condition is performed by using a detailed mechanistic thermal-hydraulic code package RELAP 5. By simulating of an experiment carried out for studying the natural circulation flow for APR 1400 shows that the code is applicable for analyzing the circulation flow under this condition. Molten corium pool in vessel lower plenum imposes highest heat load on the vessel lower head wall in large-break loss of coolant accident. The analysis results show that heat removal capacity of the natural circulation flow in AP1000 is sufficient to prevent thermal failure of the reactor vessel under this bounding heat load. By sensitivity analysis, some insights are provided for increasing the thermal margin between the heat source in the corium pool of the reactor lower plenum and the heat transfer from the lower vessel wall to the coolant in the gap by improvement of the vessel/insulation configurations.