External hazards EH

ISSUE TITLE Need for systematic assessment of seismic effects (EH 1) 

To identify the vulnerabilities of the design of a nuclear power plant as built and modified over the plant lifetime with respect to seismic effects, a sytematic assessment needs to be carried out using a combination of deterministic and PSA techniques. 

The main objective of the assessment is to compare the seismic demand (input) with the capacity of the plant and to decide on the need for upgrades. 

A seismic PSA was performed in the Individual Plant Examination of External Event (IPEEE) programme in USA and in some other member states. It was shown in these programmes that the probability of core damage caused by an earthquake is of a comparable order as the core damage probability caused by internal events. 

For nuclear power plants built according to earlier standards and for many of die WWER NPPs, die seismic design basis (i.e. seismic input parameters) is generally not in accordance witii current international practice. The as-built status of NPP structures, components and distribution systems have to be checked against the site specific seismic loads in the event of an earthquake of the design basis level. 

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