**6. Justification of design tools and solutions**

The design tools/solutions can be proven:

a.Directly by NPP experiences (events and damages).

b.Indirectly by:


From our point of view, the most important are the real NPP experiences regarding natural hazard events and consequences. There are several sources archiving the experiences of extreme natural events. The International Atomic Energy Agency International Seismic Safety Centre collected the information on the earthquake experiences reported by the operators. There are several hundreds of significant earthquakes registered within 300 km epicentral distance from NPPs.

The World Nuclear Association has also collected the data of nuclear accidents that could be compared by other industrial activities [39].

The European Commission Joint Research Centre also published a study on the external hazard-related events at NPPs [40]. According to this study, apart from earthquakes and tsunamis (Fukushima case), the fouling events (biological fouling of water intakes affecting also the ultimate heat sink and chemical fouling causing corrosion) and extreme weather conditions, including lightning strikes and floods, are dominating. A few events reported have safety significantly according to the International Nuclear Event Scale.

In the USA the Nuclear Energy Institute published a fact sheet on the response of US NPPs to natural events starting with June 2011 Missouri River (Nebraska) flooding up to September 2018 as the Hurricane Florence threatened the NPPs in the Southeast region of the USA and including also the 23rd of August 2011 beyonddesign-basis seismic event at North Anna NPP in Virginia [41].

The examples show that the nuclear plants can withstand and properly respond to extreme natural events, if the design basis defined is adequate that was not the case at the Fukushima site with respect to the tsunami. The industry has the tools, the analytical and testing capabilities, and the consolidated standards to design and build safe plants.

**41**

and upgrading if necessary.

*Natural Hazards and Nuclear Power Plant Safety DOI: http://dx.doi.org/10.5772/intechopen.83492*

*6.1.1 Earthquakes: vibratory ground motion*

ground vibratory effects of earthquakes.

really negligible [43].

respectively.

*6.1.2 Flooding*

**6.1 Justification for possibility protection by the experiences of NPPs**

There are plenty of examples demonstrating that the codes and standards accepted in the nuclear praxis ensure sufficient capacity of SSCs to withstand the

Although the recorded ground motions exceeded those values for what the plants were designed, the safety consequences of the earthquakes were negligible. That was the case of Miyagi earthquake (August 2005) at the Onagawa NPP and the Chūetsu offshore earthquake (July 2007) at the site of the Kashiwazaki-Kariwa NPP [42]. In case of the Great Tohoku earthquake, the behavior of 13 nuclear units in the impacted area on the East shore of the Honshu Island demonstrated high resistance against ground vibrations due to earthquake. Even the Fukushima Dai-ichi plant survived the strong motion period of the earthquake. In August 2011 the North Anna plant in Virginia, USA, also survived a beyond-design-basis earthquake thanks to the designed and built margins. The North Anna case demonstrated also the adequacy of definition of damage criteria formulated in terms of cumulative absolute velocity and justified the correctness of predefined measure of margin. Although the ground motion experienced at the site exceeded the design-basis level, the damaging effect of the earthquake was found below the margin evaluated, and the damages were

Sufficient capability of plants to withstand beyond-design-basis vibratory motion of earthquakes has been demonstrated by the stress tests performed in the European Union and by focused reviews implemented in other countries. The stress tests have been aimed to the review of seismic hazard assessments for sites of nuclear power plants and to the verification of the design bases, as well as to the evaluation of margins against external hazard (mainly earthquakes and floods) effects, whether the beyond-design-basis hazard effects can cause cliff-edge effect, that is, sudden loss of safety functions due to effects exceeding the design-basis one. Information on these programs in the European Union is provided at http://www. ensreg.eu/EU-Stress-Tests. Information regarding post-Fukushima measures in the USA are available at http://www.nrc.gov/reactors/operating/ops-experience/japandashboard.html and for Japan at https://www.nsr.go.jp/english/library/index.html,

Food safety can be ensured by combination of technical and procedural measures, reducing the power generation or shutting down the reactors. The protection of plants against floods is feasible even at rather unfortunate sites like the Tricastin one [21]. In spite of this, floods at some sites caused safety issues. For example, at Fort Calhoun site in 2011 [41], the plant should be protected by extraordinary temporary measures. The flood and fire resulted in a 3-year shutdown of the plant. At Blayais Nuclear Power Plant in 1999 [44], the high tide and storm flooded the plant and caused an event Level 2 according to the International Nuclear Event Scale. Safety upgrading measures and improved procedures have been developed and implemented to achieve the required safety level. The case turned the attention to event combinations that are capable to cause extreme flood event. Both cases reveal the importance of design-basis definition, regular review of the hazard characterization, and checking the protection capabilities

*Natural Hazards - Risk, Exposure, Response, and Resilience*

**6. Justification of design tools and solutions**

a.Directly by NPP experiences (events and damages).

• Experiments (shaking table tests, wind tunnels, etc.).

earthquakes registered within 300 km epicentral distance from NPPs.

that could be compared by other industrial activities [39].

design-basis seismic event at North Anna NPP in Virginia [41].

International Nuclear Event Scale.

The design tools/solutions can be proven:

• Numerical analysis and experiments.

example, in [37].

b.Indirectly by:

best-estimate approach can be adopted for the evaluation of this margin [34]. The high-confidence of low-probability of failure (HCLPF) could be the measure of the seismic margin [35, 36]. For new plants, depending on the regulatory framework and design practice, a HCLPF capacity of at least 1.67 [37] or 1.4 [38] times the design-basis peak ground acceleration is required to be demonstrated. These values are based on the conservatism of the nuclear design standards and justified by extensive studies. In the standard ASCE/SEI 43–05 [33], it is proposed to accept the probability of unacceptable performance less than about 10% for a ground motion equal to 150% of the design-basis ground motion, while for the design basis,

The above concept can be adopted for other hazards as it is proposed, for

• Reconnaissance and analysis of event consequences and damages (nonnuclear).

From our point of view, the most important are the real NPP experiences regarding natural hazard events and consequences. There are several sources archiving the experiences of extreme natural events. The International Atomic Energy Agency International Seismic Safety Centre collected the information on the earthquake experiences reported by the operators. There are several hundreds of significant

The World Nuclear Association has also collected the data of nuclear accidents

The European Commission Joint Research Centre also published a study on the external hazard-related events at NPPs [40]. According to this study, apart from earthquakes and tsunamis (Fukushima case), the fouling events (biological fouling of water intakes affecting also the ultimate heat sink and chemical fouling causing corrosion) and extreme weather conditions, including lightning strikes and floods, are dominating. A few events reported have safety significantly according to the

In the USA the Nuclear Energy Institute published a fact sheet on the response of US NPPs to natural events starting with June 2011 Missouri River (Nebraska) flooding up to September 2018 as the Hurricane Florence threatened the NPPs in the Southeast region of the USA and including also the 23rd of August 2011 beyond-

The examples show that the nuclear plants can withstand and properly respond to extreme natural events, if the design basis defined is adequate that was not the case at the Fukushima site with respect to the tsunami. The industry has the tools, the analytical and testing capabilities, and the consolidated standards to design and

the probability of unacceptable performance less than about a 1%.

**40**

build safe plants.
