Nowadays, for the risk assessment of nuclear power plants, we consider - based primarily on the PSA method - a computer assisted analysis using a rather complicated model of the safety related technological systems. In many cases, the purpose of the analysis is to identify quantitative indicators, suitable for verifying the fulfillment of the criteria determined in the offiicial regulations. The target of the analysis, of course, might focus on other purposes and needs, but in each case, a well-built PSA model expected to provide suitable results.