About definition and practical application of probabilistic safety indicators related to large accidental release

The probability of a large early release is one of the safety indicators that reflects the risk of fatalities during an accident at a nuclear power plant. Despite the fact that this indicator is very important, it is not defined in the federal rules and regulations of the Russian Federation. The article analyzes the worldwide experience related to determining criteria for large early release taking into account the statistical data of the specific country under consideration. Based on the analysis, the definition of a large early accidental release and the criteria for its achievement are formulated taking into account measures to protect the population living within the protective action planning zone (PAPZ), and an approach for their definition is proposed. A conclusion has been made about the sustainable nature of the goals identified using the NRC approach. A critical analysis of the safety indicators established worldwide allowed for the conclusion that it is necessary to improve the methodology for assessing the safety goals taking into account the specifics of NPP design, regional characteristics, phases of accident progression and other factors. The paper outlines the proposed approaches to determining high-level safety goals based on probabilistic targets – quantitative values for probabilistic health objectives (PHO) designated as PHO1 and PHO2 for the Russian Federation. A definition of a "large early release" is formulated and criteria for its achievement are proposed taking into account measures to protect the population living within the PAPZ. Thus, when assessing the degree, to which these goals are achieved in practice, it is proposed to take into account not only the likelihood of a large release during an accident at a nuclear power plant, but also measures aimed at protecting the population, in particular, evacuation. In this case, early deaths occur only when the population is not evacuated before the critical health dose loads from the release are reached.