Analysis of critical pipe break sizes leading to reactor pressure vessel liquid level collapse and core uncovery with APROS

Collapse of liquid level in reactor pressure vessel (RPV) and uncovery of core in typical pipeline rupture accidents, such as steam generator tube rupture (SGTR), main steam line break (MSLB), and loss of coolant accident (LOCA) for pressurized water reactor (PWR), may occur. This study used APROS to establish a full-scale simulation model of VVER440 reactor and evaluated different break sizes that can lead to RPV liquid level collapse and core uncovery during these accident processes. It is demonstrated that in SGTR accident, the liquid level in RPV collapsed under the condition of four or more heat transfer tube ruptures without the risk of core uncovery in all analyzed cases. In the MSLB accident, RPV liquid level collapse and core uncovery were absent due to the integrity of the primary circuit. In the LOCA accident, the RPV liquid level collapsed under a break size of 0.5% of pipe area (PA) of cold and hot leg. Notably, core uncovery occurred when the break area was 1.8% of PA in the cold leg and 65% of PA in the hot leg. The time required until core uncovery was earlier in the cold leg than that in the hot leg.