Nuclear Safety
2021

CSNI Technical Opinion Paper
No. 18

Seismic Probabilistic Safety Assessment
for Nuclear Facilities

NE A

Nuclear Safety

CSNI Technical Opinion Paper
No. 18
Seismic Probabilistic Safety Assessment
for Nuclear Facilities© OECD 2021
NEA No. 7486
NUCLEAR ENERGY AGENCY
ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENTORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENTThe OECD is a unique forum where the governments of 38 democracies work together to address the
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views of the member countries of the OECD or its Nuclear Energy Agency.

NUCLEAR ENERGY AGENCYThe OECD Nuclear Energy Agency (NEA) was established on 1 February 1958. Current NEA membershipconsists of 34 countries: Argentina, Australia, Austria, Belgium, Bulgaria, Canada, the Czech Republic,
Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Korea, Luxembourg,
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Spain, Sweden, Switzerland, Turkey, the United Kingdom and the United States. The European
Commission and the International Atomic Energy Agency also take part in the work of the Agency.
The mission of the NEA is:– to assist its member countries in maintaining and further developing, through international
co-operation, the scientific, technological and legal bases required for a safe, environmentallyfriendly and economical use of nuclear energy for peaceful purposes;– to provide authoritative assessments and to forge common understandings on key issues, as
input to government decisions on nuclear energy policy and to broader OECD policy analyses
in areas such as energy and sustainable development.
Specific areas of competence of the NEA include the safety and regulation of nuclear activities,
radioactive waste management and decommissioning, radiological protection, nuclear science,
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information. The NEA Data Bank provides nuclear data and computer program services forparticipating countries.
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Corrigenda to OECD publications may be found online at: www.oecd.org/about/publishing/corrigenda.htm.
© OECD 2021
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Cover photos: The four Candu 6 units at Wolsong, Korea (KHNP); Fukushima Daiichi Nuclear Power Plant, Japan (Nuclear Information
Center, CRIEPI).

FOREWORD

Foreword

The main objective of the Working Group on Risk Assessment (WGRISK) of the
Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI)
is to advance probabilistic safety assessment (PSA) understanding and to enhance
its utilisation for improving the safety of nuclear installations.
In pursuing this goal, the working group performs a number of activities toexchange PSA-related information among member countries. One of these activities
is to develop technical opinion papers (TOPs) on specific subjects of PSA.
The first TOP on seismic probabilistic safety assessment (SPSA) dates back to
2002. Since then, significant progress has been made in the development of SPSA, in
particular in the areas of probabilistic seismic hazard analysis and fragility analysis.
Therefore, this TOP has been revised, with special attention given to the experiencein the member countries applying SPSA.
The NEA Secretariat wishes to thank the following experts who provided
valuable time and considerable knowledge towards the revision of this TOP: KwangII Ahn, Korea; Jon Ake, United States; Attila Bareith, Hungary; Stefan Brosi,
Switzerland; Kevin Coyne, United States; Felix Gonzalez, United States;
Vinod Gopika, India; Dries Gryffroy, Belgium; Yoshikane Hamaguchi, Japan;
Jose Pires, United States; Jorma Sandberg, Finland; Gerhard Schoen, Switzerland;
Robert Sewell, United States; Nathan Siu, United States; Yann Stempfel, Switzerland;
Jim Xu, United States; Smain Yalaoui, Canada.
Thanks are also extended to the members of the Working Group on External
Events (WGEV), Working Group on Integrity and Ageing of Components and Structures(WGIAGE) and WGRISK who have reviewed and commented drafts of this paper.

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TABLE OF CONTENTS

Table of contents

List of abbreviations and acronyms .................................................................................. 7
Executive summary ............................................................................................................. 9
1. Introduction.................................................................................................................... 13
2. Probabilistic seismic hazard analysis ......................................................................... 15
3. Fragility analysis ............................................................................................................ 17
4. Seismic probabilistic safety assessment model ........................................................ 21
5. On the applicability of seismic probabilistic safety assessment ............................. 25
6. Seismic probabilistic safety assessment applications .............................................. 27
7. Areas for seismic probabilistic safety assessment research and development ....... 29
8. Conclusions .................................................................................................................... 31
References .......................................................................................................................... 33

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LIST OF ABBREVIATIONS AND ACRONYMS

List of abbreviations and acronyms

CDF

Core damage frequency

CSNI

Committee on the Safety of Nuclear Installations (NEA)FA

Fragility analysis

GMPE

Ground motion prediction equations

HCLPF

High confidence of low probability of failure

HEPs

Human error probabilities

HRA

Human reliability analysis

LHS

Latin hypercube simulation

LOCA

Loss-of-coolant accident

M&S

Modelling and simulation

NEA

Nuclear Energy Agency

OECD

Organisation for Economic Co-operation and Development

PSHA

Probabilistic seismic hazard analysis

SEL

Seismic equipment list

SPSA

Seismic probabilistic safety assessment

SRA

Seismic response analysis

SSC

Structures, systems, components

SSHAC

Senior Seismic Hazard Analysis Committee

SSI

Soil-structure interaction

TOP

Technical opinion paper

WGIAGE

Working Group on Integrity and Ageing of Components andStructures (NEA)WGRISK

Working Group on Risk Assessment (NEA)

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EXECUTIVE SUMMARY

Executive summary

Seismic probabilistic safety assessment (SPSA) is a systematic method for
examining and evaluating the risk from earthquake-initiated accidents. The
significant advances in the area of SPSA since the initial publication in 2002 of the
NEA/CSNI Technical Opinion Paper (TOP) No. 2: Seismic Probabilistic Safety Assessment
for Nuclear Facilities have prompted a revision of the document. The purpose of thisrevision is to provide the up-to-date international view on the state of the SPSA as
it is currently being applied, including a description of the main elements of SPSA.
The revision was written by seismic PSA experts of the NEA Working Group on Risk
Assessment (WGRISK) member countries. The initial draft and subsequent updates
were presented to WGRISK for review and comments. In addition, comments wereobtained from seismic experts of the Working Group on External Events (WGEV) andWorking Group on Integrity and Ageing of Components and Structures (WGIAGE).
SPSA consists of three main elements, probabilistic seismic hazard analysis(PSHA), fragility analysis (FA) and development of an SPSA model. They can besummarised as follows:•

PSHA provides a comprehensive quantification of the annual exceedance
probabilities for the ground-motion parameters of interest for the SPSA.
There are large uncertainties associated with the location, size and level ofground shaking of earthquakes. PSHA quantifies these uncertainties in orderto produce a realistic description of the ground motion at the site.

•

The objective of the FA is to estimate the seismic failure probabilities of thestructures, systems and components (SSCs) whose failure may contribute to
the risk quantified by the SPSA. There are various established methods for
obtaining the parameters of the fragility function, each potentially requiring
significantly different levels of effort. In practice, for a given SSC, the method
is selected according to the importance of the SSC and to the expected benefit
provided by a more realistic estimation compared to a conservative approach.
Since the mid-1990s, significant developments in the application and use ofprobabilistic seismic response analysis (SRA) have increased the realism of thecomputed structural responses. The increase in computing power has largely
facilitated these developments.

•

By means of an SPSA model, sequences of events caused by the effects ofground motion can be comprehensively modelled and the associated risk,
including uncertainty, can be quantified. In general, an SPSA model isdeveloped using an existing internal-events PSA model. An SPSA model
incorporates the results of the PSHA and of the FA.

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EXECUTIVE SUMMARY

The main technical opinions of this paper on these elements of SPSA and theapplication of SPSA are the following:

10•

Based on the fact that SPSA has served in several countries as a valuable tool
for risk-informed decision-making, it can be concluded that SPSA methods are
mature enough to analyse the risk of core damage accidents and radiologicalreleases that might result from earthquake-initiated events at nuclear power
plants, and to understand the risk significance of SSCs and human actions.

•Due to randomness in nature and the limited available data, especiallyhazard data, large uncertainties are inherent in (i.e. a factual aspect of) any
seismic evaluation. SPSA provides a systematic framework for explicitly
tracking and quantifying uncertainties and their impacts (rather than, forexample, ignoring them or leaving their treatment obscure or implicit). Thisfeature of SPSA is one of its key strengths.

•

By means of the FA, the seismic capacity of SSCs is systematically analysed.
Making also use of the SPSA walk-down findings, the FA has a high potential
to identify cost-effective seismic capacity improvements, some of which canbe easily corrected prior to finalising the SPSA. This has been proven in many
applications and is one of the FA’s strongest attributes.

•

SPSA results themselves, such as on the importance of components, arecrucial to point out effective measures to reduce the plant’s risk. For example,
in case of a plant safety re-evaluation resulting in a significantly different
seismic hazard, SPSA has been used to evaluate the impact on the plantsafety and to identify the most effective safety enhancements.

•

In dealing with the large uncertainties associated with the location, size andlevel of ground shaking of earthquakes, the so-called Senior Seismic Hazard
Analysis Committee (SSHAC) approach is widely accepted in the seismic
hazard community as the state-of-the-art PSHA approach. Over the last
decade, the original SSHAC methodology has been supplemented byexperience-based implementation guidelines.

•

Continuing research aims to better understand and, where possible, reduce theuncertainty in seismic evaluation approaches. However, considering therandomness in nature and the limitations in collecting data (especially hazarddata), it is to be expected that significant uncertainties will remain a real aspect.

•

Challenges faced by SPSA include the following:–

Estimating the maximum magnitude for a PSHA is particularly
challenging in intra-plate regions such as Europe, where large
earthquakes are infrequent compared to the length of earthquake
catalogues, and earthquakes often occur on previously unrecognised
active faults.

–

Although it is possible to conduct scenario-specific seismic Human
Reliability Analysis (HRA) quantifications, the number of unknown
elements such as the plant status, the distraction caused by the multiple
failures of non-safety relevant components, the indications still available

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EXECUTIVE SUMMARY

in the main control room, etc. makes this quantification process onerous
and challenging. A common practice to resolve this issue consists inmodifying the human error probabilities (HEPs) derived for the internalevents PSA.
–•

There are few companies and experts on the market that have been
trained and have experience with PSHA or with fragility analysis.

Limitations associated with the state of the art of SPSA include, for example:–

Ground motion prediction equations (GMPEs) are a very sensitive issue
in seismic hazard assessment. The limited number of instrumental
strong motion records in large parts of Europe makes it difficult to derive
empirical attenuation relations.

–

Traditional PSHA heavily relies on the extrapolation of short records ofearthquake data to the very low occurrence probabilities required as
input parameters for PSA.

In relation to the challenges and limitations listed above, it is important to notethat the majority of them are not SPSA specific. Rather, they are inherent aspects ofany seismic safety evaluation.
Recognising that uncertainties and limitations are an important factual aspectthat needs to be adequately dealt with, many countries have complemented theregulatory framework with risk-informed approaches where PSA is one element of
the decision-making process.
Overall it should be noted that the use of PSA in risk-informed decision-making
is particularly meaningful and comprehensive if SPSA is considered. Ongoing
discussions in some countries regarding the maturity and realism of SPSA should
not overshadow the powerful abilities of SPSA to identify appropriate plant quick
fixes and back-fits and to support the strengthening of the safety of nuclear power
plants worldwide.

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INTRODUCTION

1. Introduction

This technical opinion paper conveys the insights and understanding of risk
analysts and experts in the NEA member countries concerning the state of the art
in seismic probabilistic safety assessment (SPSA) for nuclear facilities.
SPSA is a systematic method for examining and evaluating the risk fromearthquake-initiated accidents. It is composed of the hazard assessment and theconsequence analysis. In both parts, uncertainties are systematically consideredand explicitly quantified.
SPSA has served in many member countries as a valuable tool for risk-informed
decision-making. SPSA methods are mature enough to support the analysis of therisk of core damage accidents and radiological releases that might result fromearthquake-initiated events at nuclear power plants, and to understand the risk
significance of structures, systems, components (SSCs) and human actions. In caseof a plant safety re-evaluation resulting in a significantly different seismic hazard,
SPSA has been used to identify the potential impact on plant safety and mosteffective safety enhancements.
SPSA can also be adapted for use at other nuclear facilities, such as high-level
waste processing and storage centres including final repositories.
This paper describes the main elements of SPSA: Probabilistic Seismic Hazard
Analysis (PSHA), fragility analysis (FA), and development and quantification of the
SPSA model. It also discusses the applicability of SPSA, SPSA applications andongoing research topics.

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PROBABILISTIC SEISMIC HAZARD ANALYSIS

2. Probabilistic seismic hazard analysis

The objective of the probabilistic seismic hazard analysis (PSHA) is to quantify
estimates of annual exceedance probabilities for the ground-motion parameters ofinterest in PSA modelling and quantification. There are large uncertainties
associated with the location, size and level of ground shaking of earthquakes. The
PSHA quantifies these uncertainties in order to produce a realistic description of theground motion at the site.
The PSHA framework published by the US Nuclear Regulatory Commission (NRC)
in 1997 (Budnitz et al., 1997) focuses on systematic treatment of uncertainties, andis now widely accepted within the seismic hazard community as state of the art.
The framework provides a systematic way to produce estimates of the hazard(i.e. annual exceedance probabilities), while simultaneously accounting for all
significant sources of uncertainty and variations in expert judgement, in order toevaluate confidence levels for the annual exceedance probabilities. The original
recommendations (Budnitz et al., 1997) have been supplemented by experiencebased implementation guidelines, e.g. in the United States (Kammerer and Ake, 2012)
and in Switzerland (Swiss Federal Nuclear Safety Inspectorate, 2019).
A PSHA typically includes the following steps:•

Seismic source characterisation: The possible seismic sources are identified andcharacterised as fault sources representing the three-dimensional characterof possible rupture locations on known faults or area sources representing
the two-dimensional zone boundaries for earthquakes within a volume ofthe earth’s crust where fault geometries are unknown. For each source, amagnitude-frequency relationship is determined, which is derived from
catalogues of seismic events (instrumental, historical) and from available
paleo-seismic studies. All credible seismic sources that may contributesignificantly to the seismic hazard at the site are addressed in acomprehensive PSHA.

•

Ground-motion modelling: Relationships quantifying ground-motion parametersof interest (e.g. spectral accelerations and peak ground acceleration [PGA]) interms of earthquake magnitude, source-to-site distance, the site condition at
a selected reference rock level, and various geophysical characteristics aredeveloped. This development is usually based on the review, selection,
adaption, and/or possible generation of available ground motion prediction
equations (GMPE).

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PROBABILISTIC SEISMIC HAZARD ANALYSIS•

Site response analysis: Dynamic response models are developed of the local
soil (and/or rock) deposit; these models convey layer geometries and various
soil/rock properties (e.g. density, low-strain shear-wave velocity, straindependent damping and shear modulus) of each layer between reference
rock and the ground surface. The dynamic models are exercised using
representative ground motions in order to assess modifications to theincoming seismic waves.

•Calculation of the hazard: Seismic hazard curves that describe relationships
between the ground-motion parameters of interest and annual probability ofexceedance are assessed together with uncertainties using suitable PSHA
software. The hazard curve is used as a basis to develop a set of seismic
initiating events and their likelihoods for use in quantifying the PSA model.

•

Additional hazard results: Another common seismic hazard result are theuniform hazard spectra – i.e. response spectra having specified values of
uniform exceedance frequency – which are used to describe the spectral
shape of the ground-motion demand in seismic fragility analyses. The
fractional contribution – of each seismic source, earthquake magnitude leveland source-to-site distance – to the total seismic hazard can be evaluated bymeans of hazard de-aggregation analyses. As input for the fragility analysis,time histories may be required that are consistent with the results of the
PSHA.

To perform a high-level SSHAC-conformant PSHA can take many years andrequire significant investments. In addition, there are few companies and expertson the market that have been trained and have experience with PSHA or withfragility analysis. Another challenge is to estimate the maximum magnitude of aseismic source. This is particularly challenging in intra-plate regions such as Europe,
where large earthquakes are infrequent compared to the length of earthquake
catalogues, and earthquakes often occur on previously unrecognised active faults.
Traditional PSHA relies heavily on the extrapolation of short records ofearthquake data to the very low occurrence probabilities required as input
parameters for PSA. A further limitation associated with PSHA are the GMPEs. The
limited number of instrumental strong motion records in large parts of Europe
makes it difficult to derive empirical attenuation relations.
In relation to the challenges and limitations mentioned above, it is important tonote that these are not PSHA specific. Rather, they are inherent aspects of any
seismic safety evaluation.

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FRAGILITY ANALYSIS

3. Fragility analysis

A seismic fragility analysis for structures, systems, components (SSC) conveys its
failure probability as a function of the seismic ground-motion parameter of interest.
The determination of a fragility involves estimates of the characteristics of theseismic demand on the SSC as well as its capacity. The goal of the fragility analysis(FA) is to estimate the seismic fragilities of the SSC, whose failure may contribute to
the risk quantified by the seismic probabilistic safety assessment (SPSA). The
analysis consists of the following major elements:•

Scope of SSCs and data collection: to initially define the scope of SSCs, a seismic
equipment list (SEL) is developed based on the internal-events PSA model,
supplemented by structures and components that may not be included in theinternal-events model but require consideration in the SPSA. To refine andfinalise the SEL, a seismic capability walk-down is performed. It serves to:(a) identify additional SSCs and seismic failure modes not adequately
captured in the initial SEL; (b) screen out SSCs from further evaluation basedon high-capacity configuration; and (c) ensure that, for the unscreened SSCs
and their seismic failure modes, all field data and reference informationneeded to perform a proper fragility analysis are collected. Walk-downs areused to additionally identify any important seismic interactions – i.e. cases
where an SSC may induce failure of an SPSA-relevant component. The actual
field installation and condition, which may change with age and differ fromdrawings and specifications, are adequately documented for the purpose of
performing realistic capacity analyses. Examples of typical reference
information gathered before, during, or after the walk-down include:
equipment locations, layout drawings, drawings of pipe runs, seismic design
documents and calculations that clarify component layout, dimensions,material properties, anchorage details, failure modes, design methods, andseismic qualification test and/or analysis reports and results.

•

Seismic Response Analysis (SRA): the goal of the SRA is to determine structural
demands and in-structure response spectra (ISRS) (i.e. acceleration, velocity
and/or displacement response at support locations of equipment) for
specified seismic input. Since the mid-1990s, the application and use ofprobabilistic SRA have been developed significantly to increase the realismof structural responses. A significant increase in computing power haslargely facilitated these developments. Modern SRAs include the soilstructure interaction (SSI) as well as additional site response characteristics.
For the SSI, the creation of a soil model (based on the soil properties, such as
density, damping, etc.) and a building model (based on the geometry and theconstruction material) are necessary. For probabilistic SRA based on a

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FRAGILITY ANALYSIS

sampling approach (e.g. Latin Hypercube Simulation [LHS]), the variabilitiesof structural stiffness and damping, as well as the variabilities of soil
properties, are considered together. Input motions for the response analysis
can be defined as acceleration response spectra or more commonly sets ofacceleration time histories compatible with the probabilistic seismic hazard
analysis results. One of the most important advantages of the sampling
approach is that in this way all crucial variables for the structural response,
particularly the seismic hazard, soil and structure properties and SRA issues,
are considered all in a single analysis, and different sources of variability arepropagated through the model. On the other hand, the approach is very time
consuming.

18•

Evaluation of seismic capacity of SSCs: for the quantification of the seismic
capacities of SSCs, different approaches are applied depending on the type ofcomponent and the failure mode. For structures as well as structural andmechanical failures of equipment, capacities are determined according tomodifications of design calculations and/or use of more realistic strength
equations considering geometry and statistical data on material properties.
For functional failures of electrical and mechanical equipment, fragility test
data and other test and/or earthquake experience data are applied in
combination with engineering judgement.

•

Assessment of seismic failure probability: there are various methods for
obtaining the parameters of the fragility function, each potentially requiring
significantly different levels of effort. In practice, for a given SSC, the method
is selected according to the importance of the SSC and to the expected benefit
provided by a more realistic estimation compared to a conservative approach.
A more refined approach is to perform a multiple-stage evaluation whereby
a lower effort method is used first and then, based on the results of an initialrisk quantification, more detailed methods may be used for safety-relevant
components. Common approaches for quantifying seismic fragilities arelisted below (ranked by increasing effort):–

Using generic fragilities data given by references such as EPRI (2013).
Analysts should however provide justification that the generic fragilities
data are applicable to the plant-specific SSCs, including an understandingof the purpose and scope of the source of the generic fragilities data
(e.g. fragility test data, generic seismic qualification test data, andearthquake experience data) (ASME, 2017).

–

Estimating the High Confidence of Low Probability of Failure (HCLPF)
capacity using the Conservative Deterministic Failure Margin (CDFM)
approach (EPRI, 1991).

–

Employing the well-established “separation of variables” (SoV)
methodology documented in EPRI (1994) and EPRI (2009). In thismethodology, median factors of conservatism relative to a reference
analysis (e.g. design analysis or other evaluation), as well as variation
and uncertainty in these safety factors are identified and quantified in acomprehensive manner.

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FRAGILITY ANALYSIS–

In connection with modern methods for the fragility analysis, it issometimes warranted to perform nonlinear dynamic time-history
analyses, possibly considering SSI. It may also be warranted or preferred
to more explicitly address correlation in responses and/or modes inrealistically modelling failure. In such cases, a simulation approach suchas LHS is preferred.

By means of the FA the seismic capacity of SSCs is systematically analysed.
Making also use of the SPSA walk-down findings, the FA has a high potential toidentify cost-effective seismic capacity improvements, some of which can be easily
corrected prior to finalising the SPSA. For example, SPSA often identify anchorage
problems with electrical equipment, pumps, large tanks and relay chatter problems,
which can usually be fixed without large expense. The ability to identify costeffective seismic capacity improvements has been proven in many applications andis one of the FA’s strongest attributes.

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SEISMIC PROBABILISTIC SAFETY ASSESSMENT MODEL

4. Seismic probabilistic safety assessment model

In general, a seismic probabilistic safety assessment (SPSA) model is developed using
an existing internal-events PSA model. New basic events are added in order toconsider seismic failures of the structures, systems, components (SSCs) alreadymodelled in the internal-events PSA as well as seismic failures due to damage of bothpassive SSCs and interacting SSCs. In addition, standards such as the Swiss Federal
Nuclear Safety Inspectorate (2019) and ASME (2017) require the consideration of
secondary hazards induced by seismic events (for example, seismically induced fires
and floods) and methods were developed (EPRI, 2013).
The two most common fundamental approaches to SPSA modelling are thediscrete approach and the convolution approach (EPRI, 2013).
•

Discrete approach: typically, the seismic hazard is represented by about 7 to
20 initiating events each representing a range of horizontal groundaccelerations. The frequency of these initiating events is derived bydiscretising the site-specific seismic hazard curve according to the suitablehorizontal acceleration ranges or bins. In general, the seismic event trees aredeveloped based on a general transient event tree and the event trees forloss-of-coolant accidents (LOCAs). The impact of the seismic initiating event
is considered by incorporating seismic failures of the SSCs, with failure
probabilities determined according to the fragility results. The main
advantage of this method is that the PSA model, and not the analyst,
identifies the seismic sequences/cut-sets. It also allows the use of common
existing PSA software to implement SPSA and thus the implementation of
SPSA as part of the overall PSA model.

•

Convolution approach: the term “convolution approach” does not refer to asingle procedure but rather to an ensemble of methods using convolution
between hazard curves and fragility curves. Convolution is used to computethe seismic core damage frequency (CDF) or to quantify the seismic initiating
events considering also seismic phenomena like seismically induced LOCAs.
This approach allows the hazard and the fragility curves to be considered ina high level of detail.

Regardless of the overall modelling and quantification approach chosen, the
SPSA involves the following modelling aspects:•

Seismically induced LOCA: a seismic event may cause minor cracks or leaks insmall piping (such as instrument lines) connected to the reactor coolant
system pressure boundary. The effect of individual or cumulative leakage ofsmall piping may result in a non-negligible equivalent small seismic LOCA,CSNI TECHNICAL OPINION PAPER: SEISMIC PROBABILISTIC SAFETY ASSESSMENT FOR NUCLEAR FACILITIES, NEA No. 7486, © OECD 2021

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SEISMIC PROBABILISTIC SAFETY ASSESSMENT MODEL

which may be referred to as a small seismic LOCA (EPRI, 2013). This small
seismic LOCA can be conservatively postulated or a more detailed approach,such as the evaluation of pipe specific fragilities, could be used. In order tomodel any additional seismically induced (small or large) LOCA, arepresentative piping section is identified by means of the walk-down andassociated analyses, and based on suitable characterisation, a fragility
function is determined for this representative section.
Then, the effects of seismically induced LOCAs are investigated. Changes inpressure, loss of inventory and associated reduced time windows, the need for along-term injection system, as well as loss of accessibility due to flooding andradioactivity exposure, are typically considered.
•

Seismically induced fire: a review of the operational experience shows that fires
can be induced as a consequence of seismic events (EPRI, 1990). In addition
to seismically induced fires, seismic degradation of the fire protection system
and adverse effects of the inadvertent action of fire suppression systems is
considered in the SPSA.

Based on the existing fire PSA, significant sources of ignition (fuel tanks,hydrogen bottles, etc.) are identified and the potential for seismically induced failureof the component themselves or of components (not necessarily safety relevant)near the ignition sources are investigated and their potential for seismically induced
fires assessed (EPRI, 2013).

22•

Seismically induced flood: the methodology for identifying and assessing theimpact of a seismically induced internal flood is fundamentally similar tothat for seismically induced fires (EPRI, 2013). If the integrity of a water sourceis compromised by the seismic event, then not only is the failure of thatsource considered but also the failure of all components in the flooding zone.

•

Seismically induced external floods should also be addressed, taking into
account the potential clogging of the plant’s heat sink due to debris. The
reactor catastrophe of Fukushima Daiichi is the most potent example ofseismically induced external flood (tsunami). In order to consider properly aseismically induced flood, the risk of tsunami for coastal nuclear power
plants should be investigated. For nuclear power plants located close to ariver, the potential failure of dams upstream due to the seismic event should
also be considered.

•

Other seismic hazards: a screening analysis may be performed, based on sitecharacteristics, to assess whether, in addition to the vibratory ground motion,other seismic hazards, such as fault displacement, landslide, soil liquefaction
or soil settlement are to be included in the SPSA.

•

Seismic Human Reliability Analysis (HRA): although it is possible to conduct
scenario-specific seismic HRA quantifications, the number of unknown
elements, such as the high uncertainty regarding the plant status, thedistraction caused by the multiple failures of non-safety relevant
components, the indications still available in the main control room, thepsychological shock and physical fitness of the operator crew after theseismic event, and the accessibility and operability of the controls makes thisCSNI TECHNICAL OPINION PAPER: SEISMIC PROBABILISTIC SAFETY ASSESSMENT FOR NUCLEAR FACILITIES, NEA No. 7486, © OECD 2021

SEISMIC PROBABILISTIC SAFETY ASSESSMENT MODEL

quantification process onerous and challenging. A common practice toresolve this issue is to base the human error probabilities (HEPs) for the SPSA
on the HEPs derived for the internal-events PSA. This modification (increase)of the HEPs for the seismic case reflects the greater difficulty of the task
under the uncertain conditions listed above. In any case, it is acknowledgedthat the residual uncertainty of the HEPs is high.
For human actions that require an operator to access a specific location withina specific period, the quantification of failure probabilities should consider seismic
failures that may adversely affect accessibility and/or preclude, hinder or prolong
execution of the needed action.
•

Grouping of SSC failures/failure correlation: although it seems intuitive toassume that seismic component failures are somehow correlated, there areno comprehensive empirical data on the performance of multiple proximate
components subjected to similar ground motion (EPRI, 2013; Budnitz et al.,2017). Therefore, analytical models (e.g. scenario simulation approaches) andexpert judgement are usually relied upon to address this issue.

The assumption of full failure correlation (i.e. a failure of one component implies
the failure of all components in the group) is recommended, for instance, if identicalcomponents in proximity are mounted with similar anchorage. The assumption offull independence may be appropriate if the components are more than nominallydifferent, or are located on different floors in different buildings.
Various guidance exists concerning how to model correlations (including partial
correlations) between seismic failures of components under various circumstances(elevation, orientation, Eigen frequency, etc.). A review of such guidance is presentedin (EPRI, 2013).
It is worth noting that assuming full correlation between all components in a
fragility group (i.e. having a single basic event in the PSA model representing theseismic failure of the component type) is only conservative in cases where thecomponents are in parallel on the success path. When the components are in series
on the success path, (a failure of any of the component induces a failure of thewhole), full correlation is not conservative. In this regard, where possible, the SPSA
analyst should seek to determine the most realistic (as opposed to optimistic orconservative) model.
•

Consideration of success probabilities: internal-events PSA models typicallyassume success probability values significantly larger than failure probability
values (i.e. under the rare event assumption). In SPSA, especially for large
accelerations, the failure probability of a number of SSCs will approach, or
exceed, the probability of success. Therefore, outside of rare events,
approximation will lead to an overestimation of the risk if this is notaddressed in quantification. Most codes offer options to treat adequately
successes in fault trees and event trees (although increased computational
cost may need to be managed).

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SEISMIC PROBABILISTIC SAFETY ASSESSMENT MODELA detailed SPSA considering a large number of different earthquake initiators
together with a detailed modelling of the various seismic failures of SSCs may resultin a very large PSA model that cannot be quantified in a reasonable period or at a
sufficiently low truncation limit. Although manageable, it may be challenging to
reduce the model to an adequate level of detail.
By now, a rich toolset has been developed for a broad scope of SPSA modellingand quantification issues. SPSA methods for modelling and quantifying these
aspects of real scenarios are available and described in PSA/PRA standards.

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ON THE APPLICABILITY OF SEISMIC PROBABILISTIC SAFETY ASSESSMENT

5. On the applicability of seismic probabilistic
safety assessment

This chapter discusses the applicability of the seismic probabilistic safetyassessment (SPSA) based on the experience of countries where the methodology has
been applied. Specific aspects that highlight the bases and justifications for applying
SPSA include the following:•

The members of the NEA Working Group on Risk Assessment (WGRISK) earlier
stated (in the former version of this TOP): “Based on the large number ofsuccessful applications of SPSA around the world in recent years, it is clear thatthe methodology is now mature enough for routine use, to analyse the risk ofcore damage accidents and radiological releases that might arise fromearthquake-initiated events at nuclear power plants (NPPs)”.

Subsequent to this statement, the various forms of seismic evaluation, including
SPSA, have been further refined towards enhancing their realism and increasingly
applied in the member countries. As such, the original WGRISK statement has beenfurther validated.
•Significant uncertainty is inherent in (i.e. is a factual aspect of) any seismic
evaluation, and SPSA provides the systematic framework for explicitly
tracking and quantifying uncertainties and their impacts (rather than, forexample, ignoring them or leaving their treatment obscure or implicit). Thisfeature of SPSA is one of its key strengths, lending it broad utility.

•

Probabilistic seismic hazard analysis (PSHA) addresses variabilities associatedwith the size and location of earthquakes, as well as (for each given
earthquake) the variability in ground motion experienced at a site, in order
to assess the site ground-shaking hazard. The approach is systematic as wellas comprehensive concerning the scope of scenarios. As different credible
experts generally make multiple alternative assessments for the PSHA input
variabilities, collectively their suite of assessments leads to a distribution in
PSHA results, from which best-estimate hazard and other hazard statistics
are developed. Approaches for synthesising the knowledge base of multiple
credible experts in PSHA have been developed and extensively tested –e.g. the original Senior Seismic Hazard Analysis Committee (SSHAC)
approach, as mentioned in the previous section, as well as variants on the
SSHAC approach. On this basis, a large number of quality PSHAs have beensuccessful performed in various countries, for diverse application areas.

•

Fragility analysis (FA) employs best practices from both analysis and use ofexperience data (empirical performance data and test data). A comprehensive

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ON THE APPLICABILITY OF SEISMIC PROBABILISTIC SAFETY ASSESSMENT

FA synthesises this information. Thereby, it delivers results in the form ofmultiple seismic capacity distributions (for each component of interest), fromwhich the most realistic result (best estimate) and other statistics are derived.
Making also use of the SPSA walk-down findings, FA facilitates theidentification of cost-effective seismic capacity improvement, aside from its
role in enabling the determination of CDF and large early release frequency(LERF) (and their respective uncertainties). This is one of its strongest attributesand has been proven in many applications.
•

By now, a rich toolset has been developed for a broad scope of SPSA
modelling and quantification issues. Some potentially critical aspects of realseismic accident sequences include: human errors and collateral effects
caused by earthquakes, such as seismically induced fires and floods. SPSA
methods for modelling and quantifying these aspects of real scenarios areavailable and described in PSA/PRA standards. Real seismic events
demonstrate the importance of considering these elements in SPSA.

•SPSA is sufficiently versatile to make use of various types and levels of
information (e.g. from generic data to high-precision site-specific data). As
such, SPSA is capable of graduated use that can progress in accuracy andrealism based on initial findings and study requirements. For achieving
maximum insights, SPSA can be highly plant specific and make use of themost advanced analyses. SPSA is often conducted at Level 1 considering fullpower; however, in an increasing number of countries it is being applied also
for other plant states and PSA levels.

As is the case with other PSA applications, the results and insights from SPSA
are used as part of risk-informed decision-making. The consideration of the PSA
results complements the basis of the decision-making. The use of PSA in the riskinformed decision-making will be particularly meaningful and comprehensive if
SPSA is considered. Ongoing claims regarding a supposed lack in the maturity andrealism of SPSA should not overshadow the fact that earthquakes can be an
important contributor to plant risk, and that SPSA is a useful tool for risk-informed
decision-making. In some countries, SPSA is used or encouraged to complement theexisting deterministic regulatory approaches. Both for determining site-specificseismic design bases, and for establishing the seismic design factors and rules forstructures and equipment, SPSA methods and insights have become embedded inregulations. Additionally, various applications of SPSA have been successfully
undertaken (see next section for additional information on application).
Continuing research aims to better understand and, where possible, to reduce theuncertainty in seismic evaluation approaches. Considering the randomness
associated with earthquakes and the frequent lack of strong motion data, it can be
expected that uncertainties will remain a significant aspect. Performing SPSA allows
the systematic identification of seismic-related weaknesses. The current state of theart in SPSA has already permitted the identification of many significant cost-effective
back-fits in countries applying SPSA.
In summary, SPSA continues to be a most valuable tool providing useful results
and insight and is applicable to support risk-informed decision-making as shown inmany member countries. Specific SPSA applications are described in the following
chapter.
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SEISMIC PROBABILISTIC SAFETY ASSESSMENT APPLICATIONS

6. Seismic probabilistic safety assessment applications

Possible seismic probabilistic safety assessment (SPSA) applications that havealready been implemented in some member countries include the following:•

Design specifications: for future plants, an appropriate estimate of seismic
hazard enables the determination of a suitable seismic design-basis
earthquake for structures, systems, components (SSCs). Using the sitespecific seismic hazard curves (from a probabilistic seismic hazard analysis,PSHA) spectral design requirements (such as the safe shutdown earthquake)can be determined.

For existing plants, modifications may be subject to new design and safety
requirements. For instance, the installation of a new safety system or a new
emergency diesel generator may need to consider the latest seismic hazard asderived from the most-recent accepted PSHA.
•

Seismic safety re-evaluation: the state of the art in PSHA is under constantdevelopment. In various countries, their national seismic hazard institute
may regularly update seismic hazard assessments based on refined and new
methodologies and insights. When new hazard estimates meaningfully differ
from previous results, a re-evaluation of the seismic safety may be generally
necessary.

A probabilistic reassessment of safety involves an update of the SPSA. The
results can be used to identify adequate risk-efficient back-fits.
Likewise, a deterministic reassessment of a seismic safety proof for an existing
nuclear power plant may be supported by probabilistic considerations. For example,
the ground motion input for a deterministic seismic safety proof can be defined bya given annual exceedance probability (WENRA, 2014). In addition, it is admissible
in some countries to use fragility parameters as a basis for deterministic
demonstration of the stability, integrity and function of the corresponding systems.
For example, if the HCLPF capacity is higher than the acceleration of the earthquake
considered in the seismic safety proof, then the seismic safety of the component iscorrespondingly demonstrated for the deterministic assessment.
•

Improving the seismic safety of the plant: the seismic safety is usually improvedin two ways.

First, during the process of performing the SPSA: the detailed analysis ofcomponents (including their anchorages) may reveal deficiencies which can beeasily corrected prior to finalising the SPSA. During the seismic walk-down, it is anadditional objective to identify issues that are easily fixable but nonetheless have a

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SEISMIC PROBABILISTIC SAFETY ASSESSMENT APPLICATIONS

significant impact on the component capacity. Such findings are typically related toequipment anchorage issues and seismic interactions. Interactions may be caused
by a non-safety-relevant item physically impacting, or interfering with, a safetyrelevant component (for instance, a heavy overhead chain colliding with the shaft
or motor of a pump). It is also possible that two safety-relevant components maythemselves adversely interact (e.g. two adjacent electrical cabinets colliding) during
an earthquake if they are not fixed together.
Second, once the results of the SPSA are available: potential plant weaknesses
are identified, thus pointing to effective means for risk reduction. Considering thedominant cut-sets, the risk-importance measures and dedicated sensitivity
analyses, the most effective plant improvements can be identified. In addition, SPSA
allows the quantification of the risk benefit of potential plant improvements.
•

Seismic housekeeping: seismic housekeeping involves the development ofprocedures and the training of plant personnel to avoid exacerbating, or
creating new, seismic safety issues when performing maintenance, replacing
components or adjusting plant configuration. For instance, the seismic
housekeeping may include guidelines for blocking the wheels of the mobile
equipment, for undertaking regular controls to verify that the crane is parked
in a safe position, or for removing all standing ladders from safety-relevantrooms.

•

Emergency preparedness: SPSA can play a role in the development ofemergency operating procedures (EOPs) and severe accident management
guidance covering the failure of multiple components (e.g. emergency diesel
generators) simultaneously. SPSA can highlight accident sequences which
may not be covered by the existing procedures (e.g. multiple instrumentation
failures, inaccessibility etc.). New procedures and guidance can be developed
to address these scenarios. SPSA can also provide a basis for the developmentof accident scenarios for training purposes. Given a predefined value ofseismic acceleration, the SPSA can help to identify the components that arelikely to fail if exposed to this acceleration. These components can then be
assumed unavailable in the accident scenario.

•

Further applications of PSA: other applications of PSA (e.g. changes to technicalspecifications and limiting conditions of operation changes, risk-informed
classification of SSCs, ageing surveillance and risk-informed in-service
inspection) are strengthened by considering the SPSA, in particular if theseismic risk is a significant contributor to the overall plant risk.

Including SPSA in the scope of the PSA accounts for a potentially important risk
contributor, and, thus, stands to further strengthen the overall usefulness of a PSA.

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AREAS FOR SEISMIC PROBABILISTIC SAFETY ASSESSMENT RESEARCH AND DEVELOPMENT

7. Areas for seismic probabilistic safety assessment
research and development

Some important areas of research and development (R&D) for enhancing seismic
probabilistic safety assessment (SPSA) include the following:•

Research on seismic source modelling and empirical ground-motion modellingin the context of probabilistic seismic hazard analysis (PSHA).

•

Investigation and development of advanced modelling and simulation (M&S)
for fully nonlinear soil-structure interaction (NLSSI) analysis, as well as
experimental investigations, in concert with empirical studies, aimed atverification and validation of NLSSI models and simulation results.

•

Modelling and quantification approaches for improved treatment ofcorrelations in seismic input and response motions, as well as in seismic
failures of structures, systems, components (SSCs).

•

Investigation of M&S development for coupled in-structure response spectra,
for improved seismic response and fragility treatment for components having
multiple support locations.

•Continued experimental seismic test investigations (including seismic fragility
testing), in concert with continued collection of earthquake experience data
(i.e. seismic performance of SSCs in real earthquakes), for developing improved
seismic fragilities.

•

Numerical M&S, in concert with experimental and empirical studies, to assess
the impacts of ageing on seismic performance and seismic fragility of SSCs.

•

Development of improved methods to include secondary hazards induced byseismic events (like seismically induced fires and floods) in SPSA.

•

Improvement of the technical basis to support seismic human reliability
analysis (HRA) quantifications as well as development of refined HRA methods
which can be applied with reasonable effort and which are sufficiently
scenario-specific for seismically induced events.

Additionally, work continues on improvement in databases, database andtechnology management, project management and fragility training in SPSA, as wellas updating of the documentation of associated guidance and standards.

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29

CONCLUSIONS

8. Conclusions

Seismic probabilistic safety assessment (SPSA) allows the consideration of seismic
risk in a PSA and in its applications. As part of the SPSA, the probabilistic seismic
hazard analysis (PSHA) includes a comprehensive quantification of the seismic
hazard by systematically considering the various sources of the typically very large
uncertainty. Several methods with different levels of effort and accuracy areavailable to perform the seismic fragility analysis (i.e. to quantify the seismic failure
probabilities of structures, systems, components). Existing modelling techniques
consider earthquake specific phenomena such as the dependence between seismic
failures of similar components, seismically induced loss-of-coolant accidents(LOCAs), other seismically induced transients, or secondary hazards (e.g. seismicallyinduced fires or floods). SPSA thus provides a broad scope of insights relating toplant safety. The inclusion of SPSA in a risk assessment results in a more complete
risk picture, and thus enables more meaningful PSA applications.
Research is ongoing to refine existing methods in various fields of SPSA. These
efforts can help to further improve the quality of SPSA and strengthen its
applications. In many member countries, however, SPSA (or key elements thereof)
are already systematically applied and accepted as a valuable tool for risk-informed
decision-making.

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REFERENCES

References

American Society of Mechanical Engineers (2017), “Case for ASME/ANS RA-Sb-2013,
Standard for Level 1/Large Early Release Frequency Probabilistic Risk
Assessment for Nuclear Power Plant Applications”, ASME/ANS RA-S CASE 1.
Budnitz R.J., G.S. Hardy, D.L. Moore and M.K. Ravindra (2017), “Correlation of Seismic
Performance in Similar SSCs (Structures, Systems, and Components)”,
NUREG/CR-7237.
Budnitz, R.J., et al. (1997), “Recommendations for Probabilistic Seismic Hazard
Analysis: Guidance on Uncertainty and Use of Experts”, Senior Seismic Hazard
Analysis Committee (SSHAC), NUREG/CR-6372, Vol. 1-2.
Kammerer, A.M. and J.P. Ake (2012), “Practical Implementation Guidelines for SSHAC
Level 3 and 4 Hazard Studies”, NUREG-2117 Revision 1.
Electric Power Research Institute (2013), “Seismic Probabilistic Risk Assessment
Implementation Guide”, EPRI 3002000709.
Electric Power Research Institute (2009), “Seismic Fragility Applications Guide
Update”, EPRI 1019200.1
Electric Power Research Institute (1994), “Methodology for Developing Seismic
Fragilities”, EPRI TR-103959. 1
Electric Power Research Institute (1991), “A Methodology for Assessment of Nuclear
Power Plant Seismic Margin”, NP-EPRI-6041, Revision 1.
Electric Power Research Institute (1990), “Survey of Earthquake-Induced Fires inElectric Power and Industrial Facilities”, EPRI NP-6989. 2
NEA (2002), Seismic Probabilistic Safety Assessment for Nuclear Facilities, CSNI Opinion
Papers No. 2, OECD Publishing, Paris.

1.

2.

This reference has been superseded recently by the following report which is so far not
publicly available: Electric Power Research Institute (forthcoming), “Seismic Fragility andSeismic Margin Guidance for Seismic Probabilistic Risk Assessments”, EPRI 3002012994, 27
Sep. 2018.
There is a more recent report, which is so far not publicly available: Electric Power
Research Institute (forthcoming), “Methodology for Seismically Induced Internal Fire andFlood Probabilistic Risk Assessment”, 3002012980, 27 June 2018.

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33

REFERENCES

Swiss Federal Nuclear Safety Inspectorate (2019), “Probabilistic Safety Analysis (PSA):
Quality and Scope”, ENSI-A05/e.
Western European Nuclear Regulators Association (2014), “WENRA Safety Reference
Levels for Existing Reactors”, Issue T4.2.

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CSNI Technical Opinion Paper

No. 18

Seismic probabilistic safety assessment (SPSA) is a systematic method for examining
and evaluating the risk from earthquake-initiated accidents. The significant advancesin the area of SPSA since the initial publication in 2002 of Technical Opinion Paper
No. 2: Seismic Probabilistic Safety Assessment for Nuclear Facilities by the OECD
Nuclear Energy Agency have prompted its revision. The objective of this report is toprovide the up-to-date international view on the state of the SPSA as it is currently
being applied, including a description of the main elements of SPSA. While theuncertainties associated with seismic hazard and the determination of seismic failure
probabilities are typically large, the inclusion of SPSA in a risk assessment results ina more complete risk picture, and thus enables more meaningful PSA applications.

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