Investigation of IVMR Strategy with External Vessel Bottom Head Water Cooling in VVER1000/v320 Reactor Design with ASTECv2.1.0.3 Computer Code – R. Gencheva, A. Stefanova, P. Groudev

During a severe accident a large quantity of molten core material may relocate to the lower plenum of the reactor pressure vessel, where it starts to interact with the stainless steel of the vessel. This causes heat up of the lower head vessel and its eventual failure.
In-vessel melt retention strategy through external cooling of the reactor vessel is one of the essential Severe Accident Management (SAM) measures at nuclear power plants. The aim is to terminate the progress of a core melt accident and to ensure the final coolability of the reactor pressure vessel.
The reference power plant for this investigation is VVER-1000/v320 reactor sited at Units 5 and 6 of Kozloduy NPP. In the calculation external water cooling of the vessel lower head was simulated during a severe accident: Large Break LOCA (2850 mm) with full Station blackout (SBO).
Calculation has been performed with ICARE and CESAR module of ASTECv2.1.0.3 computer code and input model for VVER-1000 reactor design. ASTECv2.1.0.3 computer code was used to predict the heat fluxes from the corium to the vessel and the heat fluxes from the vessel to the outside coolant. It highlighted key periods of maximum heat input to the vessel steel wall
that the steel wall has to be capable of sustaining.