RELAP5 analyses of ROSA/LSTF experiments on AM measures during PWR vessel bottom small-break LOCAs with gas inflow

Takeda, Takeshi 

RELAP5 code analyses were performed on ROSA/LSTF experiments that simulated PWR 0.2% vessel bottom small-break LOCAs with different AM measures under an assumption of non-condensable gas inflow. Depressurization of and auxiliary feedwater (AFW) injection into both steam generators (SGs) as the AM measures were taken 10 min after a safety injection signal. The primary depressurization rate of 55 K/h caused rather slow primary depressurization being obstructed by the gas accumulation in the SG U-tubes. Core temperature excursion thus took place by core boil-off before the actuation of low-pressure injection (LPI) system. The fast primary depressurization by fully opening the relief valves in both SGs with continuous AFW injection led to long-term core cooling by the LPI actuation even under the gas accumulation in the SG U-tubes. The code indicated remaining problems in the predictions of break flow rate during two-phase flow discharge period and primary pressure after the gas inflow.