Probabilistic Safety Analysis of the Liquefaction Hazard for a Nuclear Power Plant

Liquefaction hazard safety is essential for operating nuclear power plants where the elimination of hazards via engineering measures is not practicable. For this, the core damage frequency should be evaluated via integration of the liquefaction hazard into the seismic probabilistic safety analysis. In the seismic probabilistic safety analysis, the maximum horizontal acceleration is used as the intensity measure and as the engineering demand parameter for a simple calculation of failure rates. According to the studies performed for the Paks Nuclear Power Plant, loss of emergency service water supply due to relative settlement of adjacent structures and structural and functional failures due to tilting are the dominating failure modes. To integrate these failure modes into a seismic probabilistic safety analysis, hazard and fragility should be evaluated as functions of properly identified intensity measures and engineering demand parameters, preferable the maximum horizontal acceleration. Since a generic procedure does not exist in nuclear practice, based on the analyses for the Paks Nuclear Power Plant, two practical options are proposed for integration of the liquefaction hazard into a seismic probabilistic safety analysis, and for the calculation of annual probability of failure of critical structures.