Procedures guide for performance of a probabilistic safety analysis relating to ''earthquake'' as an external, accident initiating event

Under the sponsorship of the Federal Office for Radiation Protection a procedures guide for a probabilistic earthquake analysis has been prepared as a basis for a discussion. The method follows corresponding analyses already performed in the USA, i.e. evaluating calculations done within the deterministic earthquake analysis of the buildings and the components. The procedure is based on valid documents and is suitable especially for the assessment of operating plants. For all buildings and components of class I corresponding to the German KTA rule 2201.1 a fragility analysis is done, based on the safe-shutdown earthquake. Fragility curves which plot the peak ground acceleration at which the component/building is expected to fail must be developed. They will be derived from the median acceleration value and the logarithmic standard deviations (assuming random variability and uncertainty). The corresponding peak ground acceleration (ASSE) of the safe-shutdown earthquake is multiplied by the overall factor of safety to obtain the median ground acceleration capacity. The overall safety factor is derived from factors such as a ductility factor, related to the equipments inelastic energy absorption capability and others based on the variability of the static strength, the response spectrum, etc. In addition to and independently from this effort a seismic hazard analysis will be performed by geophysics specialists: The historical earthquake occurrences within a radius of 200 km to the plant location are evaluated together with the tectonic underground conditions. The anual exceedance frequency for earthquakes as a function of the intensity level is calculated. Both results are used as input for the level 1+ PSA, differentiating between earthquake-induced accidents and initiating events. The event trees are quantified and the core melt frequencies are calculated. (orig.)