Estimating Allowable Operator Action Time for Mitigating Multiple-Failure Accidents in Korean Advanced Power Reactor

In the case of multiple-failure accidents in light water reactors, it is imperative to demonstrate that a significant fuel damage can be prevented via prearranged mitigation measures performed by operators. However, the time elapsed from the initiation of the event to the implementation of mitigation actions significantly influences the progress and consequences of the accident. Therefore, assuming a specific time for operator mitigation actions does not provide sufficient insights for safety assessments, thereby calling for further investigations into the effect of the time delay from the prescribed value. In this study, the allowable operator action time for multiple-failure accidents of the Advanced Power Reactor 1400 was estimated by means of the sensitivity analysis using multi-dimensional analysis for reactor safety-KINS standard (MARS-KS), a best-estimate thermal–hydraulic analysis code used by the Korean nuclear regulatory authority. Three multiple-failure accidents were investigated, including small-break loss-of-coolant accident with the loss of safety injection (SI), intermediate-break loss-of-coolant accident with the loss of SI, and total loss of feedwater accident. The upper limit of the mitigation action time, which prevents core damage, was estimated based on the requirement for peak cladding temperature. This was in turn used to derive the tolerable delay from the assumed action time in the Korean accident management program for each multiple-failure accident.