Assessment on the applicable model in RELAP code to simulate downcomer boiling phenomena

The downcomer boiling phenomena at the late reflood period have been issued recently. In late reflood period, the injection water flow-rate is small compared to the that of refill and early reflood period due to the termination of large cooling water source, that is, the Safety Injection Tanks(SITs). At this situation, the reactor vessel wall is still in a high temperature and injected water is partly vaporized near the reactor vessel wall surface. Some of system codes (e.g. RELAP, TRACE) predict this generated steam prevents the penetration of safety injection water into core and eventually degrades the core cooling capability. In this concern, KAERI performed a separate effect test on the downcomer boiling phenomena in the reactor downcomer during the LBLOCA reflood period. In this paper, we assessed the several models (EPRI, Bestion, and Blasius) in RELAP3.3 code of downcomer boiling phenomena using the experiment by KAERI