Investigation of IVMR strategy with external vessel water cooling in VVER1000 reactor type with ASTECv2.1.1.0 computer code

It is investigated the applicability of In-Vessel Melt Retention (IVMR) strategy with external vessel water cooling for the VVER-1000 reactor type. The investigation concerns a calculation made with ICARE and CESAR module of ASTECv2.1.1.0 computer code. ASTEC computer code was developed by IRSN (France) and GRS (Germany) to be a European computational tool for simulation of severe accidents in the different reactor designs. The selected reference nuclear power plant for this analysis is Units 5 and 6 of the Kozloduy NPP equipped with VVER-1000 reactor. This type of reactor is a pressurized water reactor with 3000 MW thermal power and 1000 MW electric power. A LBLOCA (double ended guillotine break of the cold leg: 2 × 850 mm) simultaneously with SBO at VVER-1000 reactor design was the postulated SA transient chosen as the most challenging for this type of reactor. The purpose of this calculation is to compute a realistic heat flux profile on the inner side of the vessel and from the vessel to the external water. The periods of maximal heat input from the corium to the vessel steel wall were also accounted. Keywords: In-Vessel Melt Retention, Severe accident, VVER-1000, ASTEC v2.1.1.0 computer code.