Experimental investigation of early initiation of primary cooldown by secondary-side depressurization in a PWR inadequate core-cooling accident

The thermal-hydraulic phenomena of inadequate core cooling caused by a cold -leg small-break loss-of-coolant accident (SBLOCA) were investigated experi mentally at the Institute of Nuclear Energy Research Integral System Test f acility. The experiments were performed under the conditions of different b reak sizes (0.5 and 2%) in the cold leg followed by failure of the high-pre ssure injection system. The primary system cooldown is implemented by the s econdary-side depressurization. The effectiveness of early initiation of th e recovery action on reactor safety and related thermal-hydraulic phenomena are examined. The initiation criterion for recovery action considered here is determined by core water levels instead of core exit temperature based on the current emergency operating procedures. The impact of emergency core -cooling flow bypass phenomenon may significantly deteriorate the effective ness of the recovery operation for a cold-leg SBLOCA. The results showed th at the early initiation of secondary-side depressurization can effectively minimize the risk of core damage by preventing fuel rods from heating up th roughout the transient. In addition, the core suffers a rather moderate the rmal stress during the cooldown process.