The total loss of power supply (station black out) is an accident which may lead to severe consequences in nuclear power plants operating a water cooled reactor. The biggest problem during the station black out accident is a dramatic deterioration of a nuclear fuel cooling capability. It is necessary to note that the nuclear fuel cooling is crucial not only during the operation of a reactor, but also after its shut down, due to so-called after-heat power. Insufficient cooling may end in the core uncovery, nuclear fuel overheating and generation of gaseous hydrogen. Beside of the possible core meltdown, which itself is a very severe effect of such accident, a release of hydrogen to an oxygen rich atmosphere leads to possible explosions. The threat of a nuclear reactor containment structure damage and emission of radioactive material to the environment appears. Fukushima Dai-ichi power plant accident in 2011 has shown how such situation may evolve, and what may be consequences. The lesson learnt from Fukushima accident resulted in stress tests and significant improvements on emergency and safety systems implemented to all existing, and also constructed nuclear power plants. The war in Ukraine has brought two situations – in Zaporozhiye Nuclear Power Plant and in Chernobyl Nuclear Power plant - which could develop and evolve like a station black out accident. Multivariant simulations of selected station black out scenarios for the power unit with a VVER 1000 nuclear reactor were realized within the framework of this work. The VVER 1000 reactor simulator was used to accomplish the simulations, and available data for Zaporozhiye Nuclear Power Plant unit 1 were used. The main goal of this analysis was to check the risk of hydrogen generation and detonation. Results indicate that a risk of such severe consequences as observed in Fukushima accident is very low.