Investigations on in-vessel melt retention for VVER-1000 reactors

In recent years, many NPPs have developed and implemented severe accident management guidelines (SAMG). A primary objective of SAMG is prevention or mitigation of the consequences of severe accidents by keeping the reactor pressure vessel (RPV) integrity and reducing the load to the containment.

In-vessel melt retention (IVR) by ex-vessel reactor cooling (EVC) is one of the possible strategies for mitigation of a severe accident. This concept has already been approved by the Finnish Regulatory Agency (STUK) to be a part of the severe accident management procedures for the Loviisa NPP, Finland, with a VVER-440 reactor [1]. Besides its feasibility for such smaller power reactors, the concept is also investigated for some GEN III+ advanced light water reactors with higher core powers, such as the Westinghouse AP-1000. By applying the strategy of in-vessel retention of corium, the possible fuel-coolant interaction in the reactor cavity and thus the pressure loads in the cavity could be avoided.

The late in-vessel phase of a VVER-1000 reactor, applying external cooling of the RPV wall as mitigative severe accident management measure (SAMM), is investigated. The study is based on the FEM computer code ANSYS. Without flooding, the vessel wall would fail, as the necessary temperature for a balanced heat release from the external surface via radiation is near to or above the melting point of the steel. A crucial question regarding the feasibility of the IVR is whether all the decay heat could be transferred through the vessel wall into the water in the reactor cavity. The heat flux distribution from the molten corium to the RPV wall is therefore of high importance.