Safety Assessment of Severe Accident Management Strategies for Prevention of High-Pressure Melt Ejection Scenarios in VVER-1000 Reactors

VVER is a Russian designed “Water Cooled, Water Moderated energy reactor” which uses light water as coolant & moderator and uses enriched Uranium as fuel which is housed inside Reactor Pressure Vessel (RPV). During core melt scenarios, molten core i.e., corium is relocated to RPV bottom. After RPV failure, corium is accommodated in core catcher located below the RPV in the concrete vault. If the RPV failure occurs when vessel pressure is much higher than containment pressure then High-Pressure Melt Ejection (HPME) occurs, wherein melt is forcefully ejected into the reactor cavity, finely fragmented, and transported out of reactor vessel cavity. These processes could induce an early containment failure. High pressure core melt scenarios are usually observed during secondary side-initiated scenarios wherein secondary inventory is lost. This leads to primary circuit heat up and pressurization. Cyclic opening of PRZ PSD causes formation of vapor in the primary circuit. With continuous formation of steam, heat transfer from fuel rod to primary coolant is deteriorated which leads to reactor core heat up and subsequent core degradation. To avoid the HPME, provisions are provided in VVER-1000 reactors to depressurize the primary circuit which leads to actuation of Emergency Core Cooling System (ECCS).