Evaluation of Long Term Core Cooling Capability Considering LOCA-generated Debris

Abstract

An in-containment refueling water storage tank (IRWST) sump contains strainers in series that protect the emergency core cooling system (ECCS) components from debris washed into the sump. During ECCS circulation operation following a postulated loss-of-coolant accident (LOCA), the strainers collect fiber and particulates keeping them from being ingested into the ECCS flow paths. Nonetheless, a portion of the particulates and fibrous material may still be ingested into the ECCS and, subsequently, into the reactor coolant system. This debris could collect on fuel assembly and thereby affect long-term core cooling (LTCC) when circulating coolant from the IRWST sump. In this study, in-vessel effect tests for the advanced power reactor (APR) 1400 were conducted, and problems in the previous studies were identified through sensitivity tests. In addition, an evaluation of LTCC capability for the APR1400 was performed using a RELAP5/MOD3.3 code. A test facility was designed and constructed to perform a prototypical test to confirm that the head losses caused by debris meet the available driving head following a LOCA. The mock-up fuel assembly tested has prototypical grids above the bottom nozzle. The tests demonstrated that for a given debris load, sufficient driving force is available to maintain an adequate flow rate to remove decay heat. Fibrous debris is the most crucial material in terms of causing pressure drops, and was prepared in this study to satisfy the length distribution obtained through a strainer bypass test. Sensitivity studies on pressure drops through LOCA- generated debris deposited on a fuel assembly were performed to evaluate the effects of water chemistry and fiber length distribution. The maximum pressure drop in a mock-up fuel assembly with debris laden pure water was about 55% of that of the tap water test. The chemical precipitates settling rates in the pure water were higher than those in ordinary tap water, and this reflects that the particle size in the pure water is larger than that in the tap water. The tests on in-vessel effects with tap water give conservative results. The maximum pressure drop in a mock-up fuel assembly with WCAP-16793 fiber length distribution was about 33% of the APR1400 specific fiber length distribution test. The tests on in-vessel effects with the APR1400 specific fiber length distribution give conservative results. To simulate the core blockage in a RELAP5/MOD3.3 model, it was assumed that the pressure drops based on in-vessel effect tests of the APR1400 occur at the core inlet during a cold-leg break of double ended guillotine. The effect of crud and chemical deposition were considered to evaluate the decay heat removal capability when LOCA-generated debris is deposited on a fuel cladding. In addition, full core blockage except one fuel assembly during a cold-leg break of double ended guillotine was simulated to investigate the limiting decay heat removal capability. The analyses results showed that sufficient liquid can enter the core when bounding core blockage occurs by LOCA-generated debris, and the peak cladding temperature was maintained below the acceptance basis of 700 K. It is concluded that sufficient driving force is available to maintain an adequate flow rate to remove decay heat once the plant starts safety injection to the reactor vessel, and thus the LTCC capability is adequately maintained in the APR1400.