# Safety of Nuclear Power Plants with Respect to the Fault Displacement Hazard
## 1 Introduction
The safety regulations require one to account for severe external loads, including \(10^{-4}\) to \(10^{-5}\) per annum earthquakes, severe tornados, postulated crash of military and large commercial airplanes, etc. Contrary to this, no nuclear power plant has been ever designed for PGD. Fortunately, empirical information regarding response of nuclear power plants to the permanent ground displacement is completely missing. Nevertheless, there are opinions (see, e.g., [20]) that the structures, systems and components of nuclear power plants might have good chances to withstand effects due to PGD thanks to designing for mentioned above extreme loads. Unfortunately, the number of rigorous analyses on the subject is limited. The ANSI/ANS-2.30-2015 standard [21] defines the rules for hazard evaluation and sitting. The JANSI-FDE-03 document [22] provides insight for safety evaluation for permanent ground displacement. In spite of these achievements, detailed rules for the PGD design should be developed that would fit into the system of nuclear requirements. It is reasonable to promote these activities, since it could not be excluded that the safety of some existing plants should be justified for PGD hazard, or a new plant should be designed accounting for the PGD hazard. The latter can happen because of difficulties in finding a new nuclear site that would be accepted by the society. Therefore, the paper intends to contribute to the development that makes the design of nuclear power plants for PGD as \"business as usual\". The paper is composed of two parts. The first part aimed to develop a reasonable method for the definition of design basis PGD. The probabilistic safety criteria and their use in the definition of the design basis natural hazards is the starting point. It is also assumed that seismic design basis is defined by the probabilistic method as per International Atomic Energy Specific Safety Guide SSG-9 [2]. Two hypotheses are formulated and discussed in the first part of the paper:  
* The PGD that should be considered in the design basis and safety evaluation can be defined on the basis of deaggregation seismic design basis hazard taking into account the localized sources with magnitude contributing to the seismic hazard. The annual frequency that could be associated with this ground deformation should be less than the hazard level for design basis earthquake. This ensures the conservatism of the design basis for PGD. * The PGD associated to the source identified, as above, can be conservatively approximated by the displacement due to the maximum potential magnitude of the source. This conservatively defined PGD can be considered as beyond design basis phenomenon. Based on the probabilistic nuclear safety criteria and on the deaggregation of the design basis seismic hazard, a method for the definition of the design basis permanent hazard has been proposed. It should be emphasized that the review and discussion of the state-of-the-art of PGD hazard assessment is beyond the scope of this paper. Basic references for hazard characterization are referred in the ANSI/ANS-2.30-2015 [21] and JANSI-FDE-03 rev.1 [22]. Criteria for the capability of the fault to cause significant surface displacement are understood as per International Atomic Energy Agency (IAEA) Specific Safety Guide SSG-9 [2]. In the second part of the paper, a qualitative analysis of the sensitivity/vulnerability of the structures, systems and components of nuclear power plants that are needed for fundamental safety function is presented. The baseline information on the resistance of important for safety systems, structures and components with respect to PGD is given in the JANSI-FDE-03 document [22]. The most recent fragility analyses of the structural response to ground displacement, [23; 24], are also used as supporting information. The analyses performed for Nuclear Power Plant Paks in Hungary for the differential settlement will also be referred as practical example.