**1. Introduction**

## **1.1. Background**

Experience of the March 11 2011, Great Tohoku earthquake clearly demonstrated that the earthquakes might be the dominating contributors to the overall risk of nuclear power plants (Institute of Nuclear Power Operations [INPO], 2011); International Atomic Energy Agency [IAEA], 2007). The seismic probabilistic safety assessments of several nuclear power plants also provided similar results. On the other hand, experiences show that plants survive much larger earthquakes than those considered in the design base, as it was the case of Kashiwazaki-Kariwa plant, where the safety classified structures, systems and components survived the Niigata-Chuetsu-Oki earthquake in 2007 without damage and loss of function (IAEA, 2007). In spite of the nuclear catastrophe of the Fukushima Daiichi plant caused by the tsunami after Great Tohoku earthquake 11th of March 2011, the behaviour of thirteen nuclear unit in the impacted area on the East-shore of the Honshu Island demonstrated high earthquake resistance. Consequently, proper understanding and assessment of the safety for the case earthquake (and generally for the external hazards) is very important for the operating nuclear power plants.

For the operating plants basic questions to be answered are, whether the nuclear power plant (NPP) is safe enough within the design basis and whether the operation can be continued safely if an earthquake hits the plant.

The designer and operators were mainly focusing on the first question, i.e. whether the reactor can be shut down, cooled-down, the residual heat can be removed from the core and spent-fuel stored at the plant, and the radioactive releases can be limited below the acceptable level in case of an earthquake. The second question became important especially after series of events when large nuclear capacities were shutdown for assessment of plant

© 2012 Katona et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Katona et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

post-earthquake condition and justification of safety before their restart (Onagawa NPP in 2005, Shika NPP in 2007, Kashiwazaki-Kariwa NPP in 2007, Hamaoka NPP in 2009). Obviously, there is a need for reliable justification of plant safe status after felt earthquake for avoiding long shutdown time and consequent economic losses. Recently, the importance of the rapid assessment of the post-event plant status became very important from the point of view of the emergency management. This is one of the lessons learnt from the stress tests of nuclear operators following the Fukushima Dai-ichi accident.

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 79

Paks NPP was one of the most complex one. The implementation of measures was completed in 2003. Therefore the peculiarities of the programme, its scope and the applied methodologies could not be properly addressed and interpreted in the referenced above review papers. Originally the Paks NPP has not been designed and qualified for the earthquake loads. The seismic safety programme at Paks NPP has therefore aimed at design basis reconstitution. The re-evaluation of site seismic hazard included all required geological, geophysical, seismological and geotechnical investigations. The seismic design basis had been newly defined. Formally the compliance with design basis requirements has to be ensured by design methods and standards. It was already recognised that a consequent and full scope re-design in line with design codes and standards and subsequent upgrading might be impossible at Paks NPP. It should be recognised that use of methodologies developed for the justification of the seismic safety of operating plants does not ensure the compliance with design basis requirements and cannot be directly applied for VVER plants. The qualification of the nuclear power plant have been executed for the newly defined design basis earthquake by applying procedures and criteria for the design, combined with the methods and techniques developed for seismic re-evaluation of operating nuclear power plants. The selection and use of methodologies has been graded in accordance with safety and seismic classification of the SSCs. After implementing the measures for design basis reconstitution, the achieved level of safety has been quantified via seismic PSA, which

The question of the safe continuation of operation became very important as the World largest Kashiwazaki-Kariwa plant was shutdown for long-term after Niigata-Chuetsu-Oki earthquake in 2007 that caused a 0.67g ground motion acceleration at the site (value measured at the Unit 1 base mat). The safety classified SSCs designed for PGA 0.27g survived the earthquake without damage and loss of function while the non-safety

The decision on the continuation of the operation is rather simple if the earthquake does not exceed the operational base (OBE) level. The case becomes more difficult if the OBElevel is exceeded and there are obvious damages in place. The justification of operability is even more complex if the earthquake loads exceed the design base level and there are no obvious failures/damages as it happened at the Kashiwazaki-Kariwa plant. Obviously, the judgement on the continuation of operation should be based on the set of information regarding capability of SSCs to survive an earthquake and on the post-event inspections, tests and analyses. It would be very reasonable to have in advance an assessment method for the plant status to ensure the effectiveness of the post-earthquake walk-downs and other actions, and to limit the time of shutdown. The methods for judgement on the safe continuation of operation can be developed on the basis of the design information. The results of the seismic probabilistic safety analysis (seismic PSA) or margin assessment provide useful additional information regarding weak-links. The design provides deterministic type information that no failure or damage should be expected if the earthquake loads do not exceed the design base level. However, the

structures were heavily damaged. The justification of the safety took two years.

provides the core damage frequency.

Operators of nuclear power plants worldwide performed seismic re-evaluation and upgrading programmes of nuclear power plants during last three decades. A summary of international effort is given e.g. in (Campbell, et al, 1988; Gürpinar & Godoy, 1998) and in the special report issued by Nuclear Energy Agency, thereafter NEA, (NEA, 1998). The reevaluation and upgrading of the seismic safety of the operating nuclear power plants were motivated mainly by the changing understanding of the seismic hazard at plant sites and/or recognition of inadequacy of design and/or qualification of certain safety related systems, structures and components relative to the seismic hazard or state-of-the-art of the technique and the requirements. In some countries, the existence of necessary margins with respect to the beyond design base earthquakes and avoidance of the cliff edge effects have to be demonstrated. The scope of the seismic re-qualification and upgrading programmes includes the definition of the pre-earthquake preparedness and post-earthquake actions at the plants.

All operating nuclear power plants in the United States are conducting an Individual Plant Examination of External Events, including earthquakes beyond the design basis, and about two-thirds of the operating plants are conducting parallel programs for verifying the seismic adequacy of equipment for the design basis earthquake; see (Campbell, et al, 1988). Western European countries also performed some re-evaluation of their older nuclear power plants for seismic events (NEA, 1998). Most extensive programmes have been performed in Eastern-European countries, where the operators implemented comprehensive programmes for evaluating and upgrading the seismic safety of their operating nuclear power plants (Gürpinar and Godoy, 1998; IAEA, 1995, 2000). Seismic re-qualification has been performed at following VVER plants:


The scope of seismic safety programmes at VVER-440/213 plants was the most extensive. It includes the re-evaluation of the hazard, reinforcement of structures and components, qualification of the active equipment, installation of seismic instrumentation and development of appropriate procedures. The seismic safety programme implemented at Paks NPP was one of the most complex one. The implementation of measures was completed in 2003. Therefore the peculiarities of the programme, its scope and the applied methodologies could not be properly addressed and interpreted in the referenced above review papers. Originally the Paks NPP has not been designed and qualified for the earthquake loads. The seismic safety programme at Paks NPP has therefore aimed at design basis reconstitution. The re-evaluation of site seismic hazard included all required geological, geophysical, seismological and geotechnical investigations. The seismic design basis had been newly defined. Formally the compliance with design basis requirements has to be ensured by design methods and standards. It was already recognised that a consequent and full scope re-design in line with design codes and standards and subsequent upgrading might be impossible at Paks NPP. It should be recognised that use of methodologies developed for the justification of the seismic safety of operating plants does not ensure the compliance with design basis requirements and cannot be directly applied for VVER plants. The qualification of the nuclear power plant have been executed for the newly defined design basis earthquake by applying procedures and criteria for the design, combined with the methods and techniques developed for seismic re-evaluation of operating nuclear power plants. The selection and use of methodologies has been graded in accordance with safety and seismic classification of the SSCs. After implementing the measures for design basis reconstitution, the achieved level of safety has been quantified via seismic PSA, which provides the core damage frequency.

78 Nuclear Power – Practical Aspects

the plants.

at following VVER plants:




post-earthquake condition and justification of safety before their restart (Onagawa NPP in 2005, Shika NPP in 2007, Kashiwazaki-Kariwa NPP in 2007, Hamaoka NPP in 2009). Obviously, there is a need for reliable justification of plant safe status after felt earthquake for avoiding long shutdown time and consequent economic losses. Recently, the importance of the rapid assessment of the post-event plant status became very important from the point of view of the emergency management. This is one of the lessons learnt from the stress tests

Operators of nuclear power plants worldwide performed seismic re-evaluation and upgrading programmes of nuclear power plants during last three decades. A summary of international effort is given e.g. in (Campbell, et al, 1988; Gürpinar & Godoy, 1998) and in the special report issued by Nuclear Energy Agency, thereafter NEA, (NEA, 1998). The reevaluation and upgrading of the seismic safety of the operating nuclear power plants were motivated mainly by the changing understanding of the seismic hazard at plant sites and/or recognition of inadequacy of design and/or qualification of certain safety related systems, structures and components relative to the seismic hazard or state-of-the-art of the technique and the requirements. In some countries, the existence of necessary margins with respect to the beyond design base earthquakes and avoidance of the cliff edge effects have to be demonstrated. The scope of the seismic re-qualification and upgrading programmes includes the definition of the pre-earthquake preparedness and post-earthquake actions at

All operating nuclear power plants in the United States are conducting an Individual Plant Examination of External Events, including earthquakes beyond the design basis, and about two-thirds of the operating plants are conducting parallel programs for verifying the seismic adequacy of equipment for the design basis earthquake; see (Campbell, et al, 1988). Western European countries also performed some re-evaluation of their older nuclear power plants for seismic events (NEA, 1998). Most extensive programmes have been performed in Eastern-European countries, where the operators implemented comprehensive programmes for evaluating and upgrading the seismic safety of their operating nuclear power plants (Gürpinar and Godoy, 1998; IAEA, 1995, 2000). Seismic re-qualification has been performed

The scope of seismic safety programmes at VVER-440/213 plants was the most extensive. It includes the re-evaluation of the hazard, reinforcement of structures and components, qualification of the active equipment, installation of seismic instrumentation and development of appropriate procedures. The seismic safety programme implemented at

of nuclear operators following the Fukushima Dai-ichi accident.

The question of the safe continuation of operation became very important as the World largest Kashiwazaki-Kariwa plant was shutdown for long-term after Niigata-Chuetsu-Oki earthquake in 2007 that caused a 0.67g ground motion acceleration at the site (value measured at the Unit 1 base mat). The safety classified SSCs designed for PGA 0.27g survived the earthquake without damage and loss of function while the non-safety structures were heavily damaged. The justification of the safety took two years.

The decision on the continuation of the operation is rather simple if the earthquake does not exceed the operational base (OBE) level. The case becomes more difficult if the OBElevel is exceeded and there are obvious damages in place. The justification of operability is even more complex if the earthquake loads exceed the design base level and there are no obvious failures/damages as it happened at the Kashiwazaki-Kariwa plant. Obviously, the judgement on the continuation of operation should be based on the set of information regarding capability of SSCs to survive an earthquake and on the post-event inspections, tests and analyses. It would be very reasonable to have in advance an assessment method for the plant status to ensure the effectiveness of the post-earthquake walk-downs and other actions, and to limit the time of shutdown. The methods for judgement on the safe continuation of operation can be developed on the basis of the design information. The results of the seismic probabilistic safety analysis (seismic PSA) or margin assessment provide useful additional information regarding weak-links. The design provides deterministic type information that no failure or damage should be expected if the earthquake loads do not exceed the design base level. However, the probability of damage is not zero even if the loads are less than the design base one. The seismic PSA provides the core damage frequency as the output of the analysis, which is a measure of the seismic safety. The PSA is generally failure oriented. The seismic PSA shows the weak links. This knowledge can be very useful for the planning of the postevent inspections. Similar information is provided by the seismic margin analysis, which quantifies the capability of the plant to survive an event greater than the design basis one.

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 81

of the application of seismic design methods combined with those developed for the re-

New areas of the seismic safety evaluation of operating plants are also addressed in the Chapter that were triggered by recent events, the Kashiwazaki-Kariwa plant and the Fukushima Dai-ichi plant, that are focusing on the assessment and assurance of the beyond




Section 2 of this Chapter defines the basic principles of seismic safety. Section 3 provides an overview of the methodologies applicable: Section 3.1 outlines the objective and scope of the seismic safety programmes. Section 3.2 provides an overview of applicable methodologies. Sections 3.3 address the issues of restart after earthquake. Section 3.4 outlines the questions of accident management. Sections 3.5 to 3.7 address the walk-down, design of upgrading and role of the peer-review. Section 4 is devoted to the pre-earthquake preparedness and post-earthquake actions. The practical and full scope example of seismic re-evaluation and upgrading is shown in Section 5. Section 6 and 7 are related to the maintenance of the seismic qualification during operation and periodic safety reviews. Extensive list of

The fundamental safety objective of design and operation of nuclear power plant is to protect human life and environment in case of any malfunctions, failures of the plant systems, structures and components which may occur during the plant lifetime including those caused by rarely occurring earthquakes. The generic approach for ensuring this safety objective is the application of the concept of the defence in depth. In accordance with this

design base capability of the nuclear power plants, periodic review of safety, etc.


evaluation of the beyond design base capabilities of the plants,

evaluation of existing plants is demonstrated.


references is provided to the Chapter in Section 8.

concept, the following requirements are applicable:

**2. Basic principles of seismic safety** 


**1.3. Scope of the Chapter** 

Scope of the Chapter covers

actions,

safety review.

**1.4. Structure of the Chapter** 

After the severe accident at Fukushima NPP the operators in European Union, U.S. and some other countries including Japan performed comprehensive safety and risk evaluation of operating nuclear power plants, see e.g. (European Commission, 2011). These tests/reviews will launch different re-evaluation and upgrading programmes with regard to seismic safety and for improvement of the capability to cope with the beyond design base earthquake and associated events (fire, flood) at the existing plants. This process includes at some sites the re-assessment of the site hazard motivated by recent events and/or new scientific evidences, for example in the U.S. (NRC, 2011). These lessons learned will also affect the projects under preparation and/or implementation. For the new plants, it has to be demonstrated that the plant has sufficient margins with respect to the design basis extension earthquake loads of and avoiding the cliff-edge effect. Consequently, the lessons learned from the former projects for evaluation of the seismic safety and upgrading of operating plants are still of great practical importance.
