Assessment of APR1400 ECCS Capability Against Large-Break LOCA Scenario by RELAP5/MOD3 Code

APR1400 is an evolutionary pressurized water reactor developed in Korea. The emergency core cooling system (ECCS) of APR1400 has been improved by adopting an independent four-train safety injection system. Each train is composed of a safety injection pump and an accumulator with a fluidic device, the passive flow rate controlling equipment. Also, ECCS water is injected directly into the reactor vessel upper downcomer, ~2 m above the cold-leg centerline. With these design characteristics, more complex thermal-hydraulic phenomena can be observed in a large-break loss-of-coolant accident (LBLOCA) scenario. In this paper, the effects of these design characteristics on the LBLOCA scenario are examined using the RELAP5/MOD3.3 code. The code modeling capability in predicting the phenomena important to APR1400 ECCS design is examined using available experiments. It shows that RELAP5/MOD3.3 conservatively predicts the bypass rate and downcomer boiling phenomena. RELAP5/MOD3.3 code analysis of APR1400 LBLOCA with conservative assumptions show that ECCS design is adequate and there is no degradation of core cooling capability and reheat phenomena during the late reflood phase. All fuel rods are quenched in the early reflood phase when the fluidic devices are inactive, showing the effectiveness of the direct vessel injection and fluidic devices against an APR1400 LBLOCA scenario.