In general, considerations should also be to those hazard combinations that include coincident independent hazards without any correlation. Combination of independent hazards should be identified and selected by applying screening methods accompanied with expert judgement. In absence of screening a comprehensive list of hazard combinations, including the extreme weather hazard in question, could be assembled that is not possible to assess in an appropriate manner due to the large number of the identified combinations. In the following Table 1 some criteria, which application should lead to the list of possible combinations of external events (considerably smaller than mentioned above [30] and practically treatable) are presented. The relevant SKI Report for External Events Analysis” [30] is summarised in various ASAMPSA_E reports (e.g. “Guidance [34]). Criteria for irrelevant combinations of external events (hazards) Individual event selection Independence Definition Influence criteria Events are independent in time Events occur AND Events are independent independently of each Events affect the same in time Individual external events other in time NPP safety function AND selection criteria are AND AND Several external events applicable to the Probability of Combined effect on are included in individual combinations as well. simultaneously occurring safety function is not event definition events is low larger than what would be in case of most severe event. The criteria for selection of events (hazards) combinations may be related to the consideration of these conditions: More general definition of events Several external events are included in a more general definition of event. Events interdependence Base for the definition of important external events (their combinations) is whether they are dependent or not. Different safety functions of NPP are affected If condition 2 is satisfied, then other condition is considered: whether different NPP safety functions are affected. If events in combination affect the same function, the 4th condition has to be applied. The degree of impact on NPP safety functions If two dependent external events affect the same safety function, these events still might make an important combination. The effect in combination may be more severe than effect of separate events. Individual external events criteria If a combination is judged to be important even after the application of all the above rules, then individual external events criteria has to be applied for the combination as well.
In general, considerations should also be to those hazard combinations that include coincident independent hazards without any correlation. Combination of independent hazards should be identified and selected by applying screening methods accompanied with expert judgement. In absence of screening a comprehensive list of hazard combinations, including the extreme weather hazard in question, could be assembled that is not possible to assess in an appropriate manner due to the large number of the identified combinations. In the following Table 1 some criteria, which application should lead to the list of possible combinations of external events (considerably smaller than mentioned above [30] and practically treatable) are presented. The relevant SKI Report “Guidance for External Events Analysis” [30] is summarised in various ASAMPSA_E reports (e.g. [34]). Criteria for irrelevant combinations of external events (hazards) Independence Definition Influence Individual event selection criteria Events occur independently of each other in time AND Probability of simultaneously occurring events is low Events are independent in time AND Several external events are included in individual event definition Events are independent in time AND Events affect the same NPP safety function AND Combined effect on safety function is not larger than what would be in case of most severe event. Individual external events selection criteria are applicable to the combinations as well. The criteria for selection of events (hazards) combinations may be related to the consideration of these conditions: More general definition of events Several external events are included in a more general definition of event. Events interdependence Base for the definition of important external events (their combinations) is whether they are dependent or not. Different safety functions of NPP are affected If condition 2 is satisfied, then other condition is considered: whether different NPP safety functions are affected. If events in combination affect the same function, the 4th condition has to be applied. The degree of impact on NPP safety functions If two dependent external events affect the same safety function, these events still might make an important combination. The effect in combination may be more severe than effect of separate events. Individual external events criteria If a combination is judged to be important even after the application of all the above rules, then individual external events criteria has to be applied for the combination as well.
ASAMPSA_E-D50.17-REPORT-3-PSAEXTREME-WEATHER.docx