Abstract This paper presents the results of thermal–hydraulic calculation of the “Blackout” analysis for VVER-1000/V320 units at Kozloduy nuclear power plant (KNPP), done during the development of symptom based emergency operating procedures (SB EOPs) for this plant. These kinds of analyses are designed to provide the response of monitored plant parameters to identify symptoms available to the operators, timing of the loss of critical safety functions and timing of operator actions to avoid the loss of critical safety functions of core damage. The results of the thermal–hydraulic analyses have been used to assist KNPP specialists in analytical validation of EOPs. The principal acceptance criteria for EOPs are averting the onset of core damage. The response of the VVER-1000/V320 nuclear steam supply system to the loss of all ac power initiating event is analyzed to identify the behavior of important VVER process variables. A limited set of potential operator actions have been evaluated. An investigation of plant response to different combinations of component availabilities in which the operator can be effective in averting core melt is presented in the paper. The RELAP5/MOD3.2 computer code has been used in performing the analyses in a VVER-1000 nuclear power plant (NPP) model. A model of VVER-1000 based on Unit 6 of Kozloduy NPP has been developed for the systems thermal–hydraulics code RELAP5/MOD3.2 at the Institute for Nuclear Research and Nuclear Energy – Bulgarian Academy of Sciences (INRNE – BAS), Sofia and was used for simulation of “Blackout” transient. To analyze blackout transient for VVER-1000/V320, the acceptance criterion which was used was fuel cladding temperature not more than 1473 K (1200 °C). This report was possible through the participation of leading specialists from Kozloduy NPP and with the assistance of Pacific Northwest National Laboratory (PNNL), under the International Nuclear Safety Program (INSP) of the United States Department of Energy.