Summary report on main steam header and main feedwater header rupture analysis for WWER-440/213 type NPPs

Several analyses of combined main steam header and main feedwater header rupture accidents were performed for the WWER-440/V-213 type reactor where the above facilities are congested in a room on the +14.7 m floor together with a considerable part of the secondary system equipment. The accidents are similar to the main steam header rupture accident. The main differences are in a poorer feedwater availability for the steam generators and larger secondary coolant leaks in the room on the +14.7 m floor. The hazard of accident extension by a primary coolant leak (LOCA) through the pressurizer safety valve exists in both cases. With the upgraded I+C system, involving modified Reactor Trip System and Engineering Safety Features Actuation System signals for the ''Main Steam Header Rupture'' event, the secondary system leak would be considerably smaller and the transients smoother. Nevertheless, an early operator intervention stopping the high-pressure emergency pump injection and stabilizing the pressurizer level is required to avoid the risk of accident extension to a primary LOCA through the pressurizer safety valves