Preliminary investigation on improvement of FP management during BWR severe accident with MELCOR-2.2

According to the current Boiling Water Reactor (BWR) accident management procedure, the Reactor Pressure Vessel (RPV) is not depressurized until the core water level has decreased to a predetermined threshold level (e.g., 10% of the active fuel length from the bottom of the active fuel (BAF)) when the backup low pressure water injection is unavailable. In the meantime, significant Fission Products (FPs) may be released due to core degradation. In such case, excessive FP deposition on the RPV dome may lead to heat up and failure of the top flange sealing of the Primary Containment Vessel (PCV) due to radiation heat. For BWR4/Mark-I TQUV event, the relationship between the different depressurization timings and FP release and depositions (represented by CsI and CsOH) on RPV (especially, steam dryer and RPV dome), D/W, W/W have been highlighted with MELCOR-2.2 analyses. Overall, it may be understood that the later the depressurization timing, the later the RPV failure timing, but the larger the amount of FP released to RPV. The remaining FP not trapped by water pool were mostly deposited on the RPV steam dryer. Clear correlation between the FP deposition amount on the steam dryer and its temperature could be confirmed. However, more investigations may be necessary to reveal what determines the RPV dome temperature, which seemed to have limited influence on the D/W head temperature profile.