This study aims to assess the applicability of the in-vessel melt retention (IVMR) strategy with external vessel water cooling to the reactors of VVER-1000/v320 type. IVMR strategy is one of the feasible solutions to mitigate reactor vessel failure and further fission products release to the containment and to the environment outside. The reference power plant for this investigation is VVER-1000/v320 reactor sited at Unit 5 and 6 of Kozloduy NPP. The ASTECv2.0r3 severe accident computer code was used to simulate Large Break LOCA(2 × 850 mm) with full Station blackout (SBO) in VVER-1000/v320 reactor model. The external water cooling of the vessel bottom head during the accident was also simulated by the code. The results from four calculations have been assessed: two integral calculations made withASTECv2.0r3with and without simulation of external vessel water cooling and two stand-alone calculations made with the ICARE module of ASTECv2.0r3. The heat fluxes from the corium to the vessel and the heat fluxes from the vessel to the outside water have been predicted by the code. There were also accounted periods of maximum heat input from the corium to the vessel steel wall.