Thermal Hydraulic Behavior in Upper Plenum during Refill-Reflood Phase of BWR LOCA, (II) : Characteristics of Residual Two-Phase Pool in Upper Plenum

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In the first report of this study, dealing with CCFL and CCFL breakdown phenomena associated with the injection of emergency core cooling spray water into upper plenum dur-ing refill-reflood phase of a BWR LOCA, the following tests results were obtained. The injected water maintained two-phase pool across the top of entire core after CCFL breakdown. The pool level oscillated near spray elevation. The objective of this paper is to clarify the mechanism of these phenomena, evaluating steam and spray flow effects on CCFL breakdown. It is found that when spray flow rate was slightly larger than the CCFL drainage deter-mined by core steam flow, pool maintained at some constant level near spray elevation, after CCFL breakdown. On the other hand, when spray flow was appreciably larger than CCFL drainage, pool level slowly oscillated. The oscillation was caused by significant changes in steam condensation rate, and the corresponding subcooling penetration into the fuel bundles, when the pool level passed the spray elevation. The TRAC-BD1 analysis of test results suggested the small sector wall effect of test apparatus on CCFL breakdown phenomena.