# Safety of Nuclear Power Plants with Respect to the Fault Displacement Hazard
## 2 Baseline Information for the Definition of Design Basis PGD
### Safety Objectives  
Nuclear power plants should be designed by applying the 'defense-in-depth' principle, that is a systematic implementation of consecutive and independent levels of protection combining box-in-box physical barriers redundant and diverse safety functions and accident management measures. If one level of protection or barrier were to fail, the subsequent level or barrier should be available.  
In case of accidents accounted for in the design basis, the plant should be brought into the safety condition. The reactor should be shut down and the reactivity in the reactor core and spent fuel pool should be controlled, the heat from the reactor core and spent fuel should be removed, and the integrity and leak-tightness of the containment should be ensured. The seismic design basis is defined at \(10^{-4}\) to \(10^{-5}\) annual probability level. The reactor core damage should be the avoided in case of design basis event. The annual probability of core damage (CDF) cumulated for all possible initiating events and external hazards should not exceed \(10^{-5}\)/a level in the case of new designs.  
The conservative design ensures sufficient margins to cope with effects of events exceeding the design basis; thus, the sudden loss of safety functions called cliff-edge effect could be avoided. The design should cope with design extension conditions (DEC) that are postulated conditions not considered in the design basis but that are subject of the design. The safety goal is to avoid the fuel damage and loss of containment function. The assessment of the effects of beyond design basis hazards and the DEC conditions on the facility can be based on realistic assumptions and the single-failure criterion is not applicable. Generally, the total frequency of event sequences resulting in large or early releases (indicated further as ELRF) summarized for all initiating events and phenomena (excluding sabotage) should not exceed ELRF \(\lesssim 10^{-6}\)/a.  
Safe design for PGD should comply with the above safety concept and criteria. Thus, the PGD hazard exceedance probability \(H_{PGD}\) should meet the condition \(H_{PGD}\times P_{F}\leq CDF\), where the \(CDF\) is the acceptable core damage frequency CDF \(\lesssim 10^{-5}/\text{a}\). The \(P_{F}\) expresses the mean probability of unacceptable performance that is the convolution plant fragility (conditional probability of failure given the PGD level) and PGD hazard curve integrated up to the PGD value at \(H_{PGD}\) level.  
Additionally, sufficient margin should be demonstrated by the design for avoiding the early or large releases and cliff-edge effects. This can be expressed as \(H_{PGDm}\times P_{Fm}\leq ELRF\), where \(H_{PGDm}\) is the hazard level of the PGD for what the integrity and functionality is proved. For the case of severe external events, beside of safety systems, additional design features should be in place to avoid the severe accident sequences. Therefore, in margin assessment, the set of systems, structures and components and their annual probability of loss of the function is not necessarily equal with those for core damage. Here, the concept of seismic design could be accounted for.  
It is reasonable to apply the concept of seismic design and safety evaluation. For earthquakes, the plant safety with regard to both core damage frequency and early or large releases' criterion is demonstrated either by probabilistic safety analysis or via demonstrating sufficient margin above design basis earthquake effects. The latter can be done by code deterministic assessment [25]. A minimum 40% margin above design basis maximum horizontal acceleration is required.  
The maximum potential PGD depends on the maximum potential magnitude, \(Mw_{max}\), of the fault/source that can affect the site area, directly or due to distributed faulting. Therefore, as for an ultimate case, it should be demonstrated the possibility of avoiding the severe accident sequence if an earthquake would hit the site with \(Mw_{max}\).