The main purpose of performed work is the probabilistic analysis of extreme external events, which have a potential influence on safety of the present nuclear power plant (NPP) in Lithuania. This analysis can also be related to the future NPP in Lithuania at the same site. At first, the methodology was established for screening out hazardous events, which impact on the present Ignalina NPP safety is not significant. For risk estimation, the following external events were considered in detail: forest fire, external flood, airplane crash, extreme wind. In order to estimate probabilities of hazards occurrence the statistical data related to various external events were collected, mathematical models were constructed and probabilities of these events occurrence were determined. Statistical, meteorological and other updated data from the Republic of Lithuania has been used to estimate probability of the most important hazardous events. Due to many factors affecting the inaccuracy of any result it is not enough to calculate only the estimate of the probability of the event. There is a need also to evaluate errors or variations of result made by such estimation. For such purpose uncertainty and sensitivity analysis was performed for a probability estimate, i.e. frequency of the event. This analysis determines the parameters that have the greatest influence on the probabilistic model results. The modelling of external events occurrence and its impact on the NPP is significant in order to evaluate the safety of NPP and to prevent failure of the systems important to the safety.

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