# Safety of Nuclear Power Plants with Respect to the Fault Displacement Hazard
## 1 Introduction  
The lessons learned from the accident of Fukushima Dai-ichi Nuclear Power Plant changed the safety paradigm radically. Instead of designing for external events with rather low probability, e.g., earthquakes with a return period of ten thousand years, preparedness for practically impossible events is required. The experience of the Fukushima accident and other cases when earthquakes affected the nuclear power plants demonstrated that the effects due to vibratory ground motion were well sustained by the nuclear power plants, even if these exceed the design basis levels. Therefore, the focus of the post-Fukushima safety reviews worldwide was to update the site hazard evaluations with respect to adequacy and completeness of the design basis and assess the beyond design basis hazards and plant response to these hazards. One of the objectives of the site hazard re-evaluation was to prove whether the potentially active faults at or in the vicinity of the nuclear site are capable of causing permanent ground displacements/deformations (indicated further as PGD) that could have a significant impact on nuclear safety. In case of a new site, the nuclear safety regulations require abandoning the potential sites that could be exposed to a fault displacement hazard [1, 2], if the potential effects of the PGD on the nuclear safety cannot be compensated for by means of a combination of measures for site protection and design features of the nuclear installation. Compared to the earlier generic avoidance, this approach reflects a tendency to consider the PGD as a load case that could be accounted for in the design of nuclearpower plants. This conceptual change in the regulation is motivated by several factors: (1) Experience shows that some structures could survive the effects of direct surface rupture, [3; 4]. (2) The design experience of non-nuclear facilities is convincing regarding the possibility of protective measures, [3; 4]. (3) There is essential progress in PGD hazard characterization [5; 6; 7; 8]. (4) There are certain experiences in hazard evaluation for nuclear sites, for example, [9; 10]. (5) Highly sophisticated analytical tools are available capable to simulate the response of the foundation medium and the structure to PGD, for example, [11; 12]. There are some other, more interest-based motivations: the selection of a new site is very difficult from both technical and socio-political aspects. Therefore, quite often, the already existing sites are envisaged for the new projects. If it is a case, even for the accepted sites, a state-of-the-art site characterization might be needed. For example, it is the new-build project at the site of Paks, Hungary, that includes updates of fault mapping and the re-evaluation of fault capability. The hazard re-evaluations, as it has been made after the Fukushima accident, can reveal new scientific information suspecting the capability of faults at or around the site, as it happens at Higashidori, Tsuruga, Ohi, Mihama, Shika and Monju nuclear power plants [13; 14; 15]. There are sites where the assessment of the activity of the mapped faults in the site vicinity or even within the site location cannot be assessed due to specific geologic context. There, the simple presence of the fault suggests suspicion of the capability, see examples in [16; 17], and the doubts are inevitably considered as an issue in nuclear safety. The evaluation of a PGD hazard became important for the design of new plants as well as for the justification of safety of the old plants due to some new scientific evidences related to secondary faulting. The recognition that the fault movement can cause surface displacements at a rather extended area increased the need of investigation of distributed faulting that can potentially affect the nuclear sites, see, e.g., [18; 19]. Nuclear safety is a very complex matter that can be properly understood in a holistic way. The design is code conservative, and the design rules and standards are proven by experiments and practice.