Nuclear power plants are the greenest and cheapest non-renewable energy sources in normal operation, but they are very dangerous for people and environment at emergency. Nuclear disasters with leakage of radioactive substances make huge areas uninhabitable for hundreds of years. Therefore, safety of the nuclear power plants is the most important task of developing and exploiting them. In this work, one of possible emergencies is analysed: extremal frost and full de-energizing of all systems in a reactor building. In this case, backup power should provide enough energy for safety systems, but not for microclimate control. Simulation of temperature conditions has been performed using k- model of turbulent flows (convection) and Fourier-Kirchhoff equation for thermal conductance. The results show no danger of frosting because of internal heat gains and thermal storage in concrete. More dangerous is overheating of heat stressed rooms. There are some recommendations for avoiding the overheating. The book is intended for scientists, post-graduate or doctoral students and professionals of design and exploiting of nuclear power plants.