The paper is devoted to one of the problems of system analysis for nuclear power plants (NPP) in touch with the modelling and simulation of the potentially hazardous objects in nuclear power safety including severe accidents at the nuclear power plants. Development and implementation of the mathematical models for analysis and computer simulations of the passive protection systems against severe accidents and mathematical modelling and simulation of the potentially hazardous objects (PHO) with account of different deviating arguments concerning the real features are considered. It has concern to a need of the planning and decision making in the environmental and other tasks.