# Safety of Nuclear Power Plants with Respect to the Fault Displacement Hazard
## 6 Discussion of the Results  
The adequate level of conservatism, use of proven methodologies and the graded approach to the design that is achieved for seismic design should be extended for the load case PGD. On the basis of the above requirement, in Section 2, the concept of design for the safety of a nuclear power plant is briefly presented that should be based on the derivation of the design basis PGD from the deaggregation of seismic design basis hazard. The PGD for design basis can be correlated with the closest and largest magnitude earthquake contributing to the hazard, as per Equation (7). If the site selection is made reasonably, multiple sources could be excluded from the definition of PGD. The annual rate of this PGD will be less than the annual rate associated with seismic design basis that ensures a reasonable level of conservatism that may compensate the uncertainty in the correlation between the magnitude and ground displacement. A conservative screening criterion is proposed that is based on the maximum potential magnitude of the source and on the probabilistic criterion for the early and large releases (Equation (8)). The displacement that is defined for the maximum potential magnitude of the source affecting the site can also be used as a beyond design basis PGD if the annual frequency assigned to maximum potential magnitude is about an order of magnitude less than the seismic design basis hazard level. This assumption is in line with the generic concept of seismic margin assessment. A key element of the derivation of the design basis PGD is the conditional probability \(\mathrm{P}[\mathrm{D}>\mathrm{d}|\mathrm{m}]\) (or the \(\mathrm{P}[\mathrm{D}>\mathrm{d}|\mathrm{m},\mathrm{r}]\)) similar to the probabilistic ground displacement analysis. The aleatoric and epistemic uncertainties related to the seismotectonic bases of the evaluation of the hazard are accounted for in the probabilistic seismic hazard analysis. Uncertainties in the empirical correlations between surface displacement and fault movement characteristics need a coordinated effort of a researcher to correct and evaluate the field observations; this is promoted, e.g., in [47]. The ground shaking and permanent surface displacement evaluation methods should be better supported by physics-based rupture modelling, e.g., [48]. A better understanding of the effects of plastic, soft rock, thick soil layers on the surface manifestation of the fault movement should also be achieved, see, e.g., [49]. A qualitative assessment of the sensitivity of structures, systems and components needed for the fundamental safety functions is performed in the paper that is supported by the JANSI-FDE-03 case studies and on the analysis of Paks Nuclear Power Plant reactor building for uneven settlement. Most important conclusion of this qualitative assessment is that the nuclear island structure, piping and components of safety systems are rather rugged with respect to permanent ground displacement. Although permanent ground displacement effects were not accounted for in the design of studied in [22; 41] existing reactors, the safety-related structures, systems and components can withstand moderate ground displacements (0.2 to 0.3 m in the referenced studies). According to [PERSON] and Coppersmith [28], a PGD of 0.2 to 0.3 m can be expected at Mw 6 to 6.5 for practically all styles of faulting. These findings should encourage and motivate the designers of nuclear power plants to consider the design for permanent ground displacement as \"business as usual\". Although the fragility analyses [23; 24] show higher than 0.2-0.3 m High Confidence for Low Probability of Failure PGD values for both the foundation and containment of some existing reactor designs, the analytical pieces of evidence are not sufficient to make a judgement about plant vulnerability if just below the site earthquake magnitude exceeds 6.5.