Effectiveness of AM measures for long-term core cooling during PWR vessel bottom small-break LOCA based on RELAP5 analyses of ROSA/LSTF experiment

Takeda, Takeshi ; Watanabe, Tadashi; Nakamura, Hideo  

Effectiveness of SG depressurization as accident management measure for long-term core cooling in case of PWR 0.2% PV bottom small-break LOCA was investigated by a ROSA/LSTF experiment and parameter analyses with RELAP5/MOD3.2.1.2 code under total failure of HPI system. In the LSTF experiment, the primary depressurization rate of 55 K/h at both SGs and auxiliary feedwater (AFW) injection for 30 min resulted in rather slow primary depressurization due to non-condensable gas inflow. Core temperature excursion took place due to boil-off before actuation of LPI system. The LSTF experiment was analyzed to minimize the discrepancy in the calculated results from the test observations. A fast primary depressurization by fully opening SG relief valves would result in later core uncovery than in the 55 K/h rate case due to smaller loss rate of the primary coolant. Continuous AFW injection would be necessary for long-term core cooling by the LPI actuation without core heatup.