Method of negative pressure analysis for secondary containment of BWR-6 plant

Abstract

In the analysis of design basis loss of coolant accident (DB LOCA) for BWR, the time it takes the Secondary Containment to reach the negative pressure condition, typically of 0.25 in. wg, is termed positive pressure period (PPP). In accordance with the USNRC regulatory guide, the filtration system can only be credited to reduce the radioactivity released to the environment at the time after PPP. Hence the PPP time is an important input parameter for the dose calculation during the DB LOCA. In view of this, an analysis method used to determine PPP time, based on the transient calculation of the GOTHIC computer code, is developed in the study. The analysis is performed for the Secondary Containment of the Kuosheng BWR-6 plant. A lumped parameter model of GOTHIC is used to model the Secondary Containment. A standby gas treatment system (SGTS) used for drawing the air from the Secondary Containment and discharging it to the ambient is modeled in conjunction with an in-leakage modeling. The results show that the GOTHIC predicts well the pressure and temperature response of the Secondary Containment. The corresponding PPP time thus can be determined appropriately based on the response of the pressure difference between the Secondary Containment and the ambient. Sensitivity study is also performed for the key parameters affecting the PPP time.