Event sequence analyses of a forest fire heat effect on a sodium-cooled fast reactor for an external hazard PRA methodology development

As a part of a probabilistic assessment methodology development on a forest fire, this paper deals with an event sequence by forest fire heat effect on a decay heat removal function of a sodium-cooled fast reactor. Related to the potential vulnerability, an event scenario was developed using conservative assumptions such as loss of off-site power due to a damage of high-voltage lines in the course of the forest fire approaching to a nuclear power plant. An event tree was developed with an initiating event of the loss of off-site power, and the headings are related to 'external diesel fuel tanks', 'emergency diesel generator and its auxiliary system', 'alternative cooling system and its power source', and 'decay heat air cooler'. A failure probability on each heading was given from a fragility curve as a function of reaction intensity or by assumptions based on conservative models. A core damage frequency, under the conditional of the loss of off-site power, was conservatively evaluated around 10^-7/year. A key heading in the event tree with large effect on the frequency is the intactness of the external diesel fuel tanks. (author)