Nuclear Safety
2019

CSNI Technical Opinion Paper
No. 17

Fire Probabilistic Safety Assessments
for Nuclear Power Plants: 2019 Update

NEA

Nuclear Safety

CSNI Technical Opinion Paper
No. 17
Fire Probabilistic Safety Assessments
for Nuclear Power Plants: 2019 Update© OECD 2019
NEA No. 7417
NUCLEAR ENERGY AGENCY
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NUCLEAR ENERGY AGENCYThe OECD Nuclear Energy Agency (NEA) was established on 1 February 1958. Current NEA membershipconsists of 33 countries: Argentina, Australia, Austria, Belgium, Canada, the Czech Republic, Denmark,Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Korea, Luxembourg, Mexico,
the Netherlands, Norway, Poland, Portugal, Romania, Russia, the Slovak Republic, Slovenia, 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,
economic and technical analyses of the nuclear fuel cycle, nuclear law and liability, and public
information. The NEA Data Bank provides nuclear data and computer program services forparticipating countries.
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Cover photos: Cooling tower, Mülheim-Kärlich NPP (Wikimedia Commons, Wolkenkratzer); Reactor hall, Ignalina NPP (Jefffrey RobertDonovan/IAEA).

FOREWORD

Foreword

Probabilistic safety assessment (PSA) has been considered a necessary complement
to traditional deterministic safety analysis in nuclear power plants for more than
30 years because of its disciplined, integrated and systematic approach. The
Nuclear Energy Agency (NEA) has been working to advance understanding of PSA
and to enhance its use in improving the safety of nuclear installations – primarily
through the NEA Working Group on Risk Assessment (WGRISK) of the Committee
on the Safety of Nuclear Installations (CSNI) and via publications such as the CSNITechnical Opinion Paper No. 1: Fire Probabilistic Safety Assessment for Nuclear Power
Plants (NEA, 2002).
Significant changes have nonetheless occurred in the area of fire probabilistic
safety assessment (fire PSA) over the years. Many of the methods, models andcomputational tools for performing fire PSA have improved, additional fire-related
operational experience has been gained and experimental results have been
generated. In addition, analyses have been used to support major changes to designand operations for many plants, and research programmes have been formulated
and initiated to address key, remaining issues in relation to PSA.
The purpose of the present paper, CSNI Technical Opinion Paper No. 17: Fire
Probabilistic Safety Assessment for Nuclear Power Plants, is to provide the current
international view on the state of fire PSA as performed in support of nuclear
power plant design and operation. The viewpoints and perspectives contained inthis technical opinion paper (TOP) are the result of the work of the WGRISK task
group, which includes experts on the subject of fire PSA. 1 The report is also basedon the results of an international workshop on fire PRA, organised by WGRISK
in 2014 and documented in the “Proceedings of International Workshop on Fire
PRA” (NEA, 2015b).
This paper takes into consideration operating experience in nuclear power
plants (NPPs), particularly with regard to fire events that were collected andanalysed within the NEA Fire Incidents Records Exchange Database Project (NEA
FIRE). In addition, consideration has been given to recent results from fire-related
experimental NEA projects, more specifically the Fire Propagation in Elementary,
Multi-room Scenarios PRISME 2 project, and PRISME 2, which takes into account fire1.

The abbreviations PRA and PSA are used synonymously in this report, consistent withusage across NEA member countries.

2

From the French “Propagation d’un incendie pour des scénarios multi-locauxélémentaires”.

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FOREWORD

behaviour and spreading in nuclear specific complex geometries under different
boundary conditions. The NEA High Energy Arcing Fault (HEAF) project also provides
insights on high energy arcing faults with the potential of ensuing fires.
The fire PSA community generally agrees on the overall approach for performing
fire PSA, with the level of realism in terms of fire PSA for an NPP often representing
a trade-off between the level of modelling efforts and the needs of the plant’s fire
PSA application. Key sources of uncertainty in fire PSA, including potential nonconservatisms as well as conservatisms, have been identified, and many are being
addressed through ongoing research and development (R&D). Continuing efforts todevelop guidance and provide training are expected to improve practice andbroaden the base and expertise of analysts, reviewers and potential decision makers.
This opinion paper is not intended to be a guidance document for performing fire
PSA, although Annex B does provide some sample guidance documents.
Fire PSA is a valuable tool that provides useful results and insights in support ofrisk-informed decision making. As with all risk-informed decisions, it is importantthat the decision makers are informed of potential biases (both conservative andnon-conservative, with magnitudes that are typically uncertain) and otheruncertainties associated with these results and insights.
This last point is worthy of particular emphasis. Ongoing discussions regarding
the maturity and realism of fire PSA, although a positive driver for improvement
activities, including R&D, database maintenance and guidance development, should
not overshadow two fundamental points: fire can be an important contributor toplant risk, and fire PSA is a useful tool for risk-informed decision making. Knowledgeof the uncertainties and potential biases in fire PSA can and should be included
when weighing available evidence.
It should also be recognised that fire PSA judged suitable for a fire risk
management issue may need to be improved when used to address an issue
involving the full set of hazards relevant to plant risk. It is expected that ongoingand future improvement activities will strengthen the broad acceptance of fire PSA
for a potentially wider set of applications, including enterprise risk management
applications not yet widely envisioned.
Some of the key messages of this report are as follows:

4•

At the industry level, fire continues to be an important risk contributor for
many NEA member countries.

•

The risk due to fires at a particular plant site, as with many other hazards, isstrongly dependent on plant-specific factors in design and operation.

•

Fire PSA insights in relation to the major contributors to the total fire risk are
generally aligned with operating experience and appear to be largely
representative of the expected plant responses. However, some aspects of
the quantitative results of modern fire PSA can be conservative. Overall,there is a wide range of views within the PSA technical and user communities
regarding the realism of fire PSA and the consequent implications for theappropriate use of fire PSA results. Fire PSA methods, models, tools and data
continue to improve, and the practice of fire PSA continues to mature.

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FOREWORD/ACKNOWLEDGEMENTS

The NEA recommends that organisations continue to support R&D activities on
key topics identified by the fire PSA technical and user communities, as well asactivities to collect, manage and provide access to quality data. Organisations
should also continue to support activities facilitating the sharing of challenges,
solutions, uncertainties, uses and good practices, along with activities to develop
practical guidance based on these factors. Organisational support is in fact essential
to ensure appropriate co-operation between the multiple disciplines andcommunities involved in the performance, review and use of fire PSA.

Acknowledgements

The NEA wishes to thank the experts listed below, who provided valuable time andconsiderable knowledge towards the development of this report:•

Kevin Coyne, Nuclear Regulatory Commission (NRC), United States;•

Per Hellström, Swedish Radiation Safety Authority (SSM), Sweden;•

Marina Röwekamp, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS)gGmbH, Germany;•

Nathan Siu, NRC, United States.

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TABLE OF CONTENTS

Table of contents

List of abbreviations and acronyms ................................................................................ 9
Chapter 1. Introduction .................................................................................................... 11
Background .................................................................................................................... 11
Fire complexity and challenges to fire PSA ............................................................... 12
Chapter 2. Key messages and report structure ............................................................ 13
Chapter 3. Fire as a risk contributor .............................................................................. 15
Chapter 4. Plant-specific fire risk ................................................................................... 17
Chapter 5. Realism of fire PSA results and insights.................................................... 19
Fire risk perspective ...................................................................................................... 19
Total risk perspective ................................................................................................... 20
Chapter 6. Fire PSA methodology and practice............................................................ 23
Consensus approach to fire PSA ................................................................................. 23
Trade-offs in practical analyses .................................................................................. 24
Areas for fire PSA research and development........................................................... 25
Fire PSA guidance and training ................................................................................... 27
Chapter 7. Use of fire PSA in risk-informed decision making .................................. 29
Chapter 8. Concluding remarks and recommendations ............................................ 31
References .......................................................................................................................... 33
Annex A. Viewpoints on conservatisms in fire PSA results...................................... 35
Annex B. Additional resources ....................................................................................... 37
Fire PSA methodology and standards ........................................................................ 37
Fire PSA operational experience and data ................................................................. 38
General fire safety ......................................................................................................... 38
Special topics ................................................................................................................. 38

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LIST OF ABBREVIATIONS AND ACRONYMS

List of abbreviations and acronyms

CCDP

Conditional core damage probability

CSNI

Committee on the Safety of Nuclear Installations (NEA)

FIRE

Fire Incidents Records Exchange (NEA Database Project)

HEAFHigh Energy Arcing Fault (NEA Joint Research Project)

IAEA

International Atomic Energy Agency

LOOP

Loss of off-site power

MCR

Main control room

NEA

Nuclear Energy Agency

OECD

Organisation for Economic Co-operation and Development

PRA

Probabilistic risk assessment

PRISME

Propagation d’un incendie pour des scénarios multi-locauxélémentaires (Fire propagation in elementary, multi-room
scenarios, NEA Joint Research Project)

PSA

Probabilistic safety assessment

R&D

Research and development

TOP

Technical opinion paper

WGRISK

Working Group on Risk Assessment (NEA CSNI)

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INTRODUCTION

Chapter 1. Introduction

Background
Since the publication of the CSNI Technical Opinion Paper (TOP) No. 1: Fire Probabilistic
Safety Assessment for Nuclear Power Plants (NEA 3948) in 2002, significant changes haveoccurred in the area of fire probabilistic safety assessment (fire PSA). Many of themethods, models and computational tools for performing fire PSA have improved,
and additional fire-related operational experience has been gained and experimental
results generated. In addition, analyses have been used to support major changes todesign and operations for many plants, and research programmes to address key
remaining issues have been formulated and initiated. Examples of these changes
have been discussed at numerous international conferences and workshops,
including the NEA Workshop on Fire PRA in 2014 (see “Proceedings of International
Workshop on Fire PRA” – NEA, 2015b).
Recognising the inherent complexity of the phenomenological behaviour of fireand of potential plant and operator responses to fire-initiated scenarios, it is not
surprising that the results of fire PSA demonstrate considerable uncertainty. With
the increasing use of fire PSA in major fire-protection related applications, and withthe recognition that fire PSA can play a significant role even for non-fire-related riskinformed applications, there has been (and continues to be) healthy discussion on
the maturity and realism 3 of fire PSA methods, models, tools and data. Numerous
stakeholders with varying points of view, including fire protection engineers, PSA
analysts and senior decision makers, have been involved in the discussion. The
purpose of this TOP revision is to provide a current, international view on the stateof fire PSA as performed in support of nuclear power plant design and operations.
The authors of this TOP are PSA experts from NEA member countries affiliatedwith the NEA Working Group on Risk Assessment (WGRISK). The opinions expressed,
and the arguments employed in this report are those of WGRISK members, and in3.

For the purposes of this TOP, the term “maturity” refers to the developmental state of atechnical discipline while the term “realism” refers to the extent that the PSA model
faithfully represents the response of the as-built, as-operated nuclear power plant. Thoughrelated, maturity and realism are separate concepts. Maturity refers to the global state of
the technical discipline and generally depends on factors such as the characteristics of theuser community, active research topics and application areas. Realism is generally
discussed in the context of a specific modelling application. The degree of realism
achieved often represents a trade-off between the cost and added value of additionalmodelling effort.

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INTRODUCTION

particular, of the authors. These opinions do not necessarily represent those ofOECD or other international experts outside the WGRISK community. Members of
nuclear safety regulatory organisations, technical support organisations and private
industry concerned with the practice and use of fire PSA will find this report ofparticular interest. Government authorities, nuclear power plant operators andother international organisations (including CSNI working groups and members of
NEA projects), as well as the general public, may also find the report of interest.

Fire complexity and challenges to fire PSA
Although the basic principles of fire behaviour are easily understood – given
sufficient fuel, oxygen and heat, a fire will grow – fire is an extremely complex
phenomenon to model in practice. Consider, for example, a fire in a cable tray. The
availability of fuel depends on the pyrolysis of the cable jacket and insulationmaterials, a chemical process that depends on the physical properties ofjacket/insulation (which can vary considerably), as well as the temperature of thecable surface. The transport of oxygen to the combustion zone is affected by suchfactors as the size and location of gaps between cables and the strength of buoyancy
forces generated by the fire. Heating of the combustion zone is provided both fromthe flame and radiative feedback from the fire’s surroundings. Under supportive
conditions, oxygen flowing to the flame supports combustion, leading to heatgeneration which further heats the fuel (increasing pyrolysis) and increases
buoyancy forces (increasing oxygen flow rates). The outcome of this positive
feedback loop is fire growth. Under negative conditions, oxygen starvation (e.g. due
to insufficient flow area), dissipation of heat through the cable or blockage ofradiative heat feedback (due to obstructions), or simple fuel burnout can lead to fireself-extinguishment.
Furthermore, local fire physics and chemistry are not the only determinants offire evolution and impact. The transport of heat and smoke away from the fire isaffected by obstructions in the room (“clutter”), ventilation conditions, and theconditions of room boundary elements (e.g. penetration seals, ventilation dampers,
doors). The effectiveness and speed of efforts to control and extinguish the firedepend on the fire detection time, the time to apply suppressants, and theeffectiveness of suppressants and their application. In many cases, detection andsuppression can involve plant staff actions (e.g. local detection by nearby personnel,
manual actuation of installed fire protection systems, manual suppression via
portable extinguishers or fire hoses). The analysis also needs to account for potential
detection and suppression system unavailability (e.g. failure to restore equipment
after testing) or failure during the event (including functional failures, e.g. failure of
suppression systems as a result of an insufficient amount of suppressants). For
severe fires, fire suppression can require the aid of off-site fire departments.

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KEY MESSAGES AND REPORT STRUCTURE

Chapter 2. Key messages and report structure

The key messages of this report are as follows.
•

At the industry level, fire continues to be an important risk contributor for
many NEA member countries.

•

The risk due to fires at a particular plant site, as with many other hazards, isstrongly dependent on plant-specific factors in design and operation.

•

Fire PSA insights in relation to the major contributors to total fire risk are
generally aligned with operating experience and appear to be largely
representative of expected plant responses. However, some aspects of thequantitative results of modern fire PSA can be conservative. Overall, there isa wide range of views within the PSA technical and user communities
regarding the realism of fire PSA and the consequent implications for theappropriate use of fire PSA results.

•

Fire PSA methods, models, tools and data continue to improve, and thepractice of fire PSA continues to mature.

The fire PSA community generally agrees on the overall approach for performing
fire PSA, although the level of realism in the fire PSA for a plant often represents a
trade-off between the level of modelling effort and the needs of the plant’s fire
PSA application.
Key sources of uncertainty in fire PSA, including potential non-conservatisms aswell as conservatisms, have been identified and many are being addressed throughongoing R&D.
Ongoing efforts to develop guidance and provide training are expected to
improve practice and broaden the base and expertise of analysts, reviewers andpotential decision makers. Generally, it has been confirmed that fire PSA is avaluable tool that provides useful results and insights in support of risk-informed
decision making. As with all risk-informed decisions, it is important that thedecision makers are informed of potential biases (both conservative and nonconservative, with magnitudes that are typically uncertain) and other uncertainties
associated with these results and insights.
The remainder of this report discusses each of these messages and concludes
with some final remarks.

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FIRE AS A RISK CONTRIBUTOR

Chapter 3. Fire as a risk contributor

At the industry level, fire continues to be an important risk contributor for many
NEA member countries.
International operating experience has shown that some of the most challenging
precursors to a core damage accident have involved internal fires. 4

4.

•Browns Ferry cable fire (United States, 1975): a candle-ignited penetration
seal fire spread to multiple cable trays in units 1 and 2. Multiple safetysystems were affected in unit 1. Fire suppression was delayed for seven hours
because of the reluctance to use water on an electrical fire. Nonproceduralised operator actions were required to achieve safe shutdown.

•

Narora turbine building fire (India, 1993): a turbine blade failure led to ahydrogen explosion and severe oil fire. The ensuing scenario involved main
control room (MCR) abandonment (as a result of smoke), station blackout(which lasted 17 hours), operator inability to use the unit 1 emergency
control panel (due to loss of power), and a complete loss of indications forseveral hours. Plant operators entered unit 1 containment to obtain direct
instrumentation readings and energised essential equipment to prevent
core damage.

•

Maanshan high energy arc fault (Chinese Taipei, 2001): salt spray and an
essential bus ground fault caused a loss of off-site power. Both of the unit 1
emergency diesel generators were unable to supply power and station
blackout ensued. Smoke from a high-energy arc fire (induced by the ground
fault) led to heavy smoke that prevented operators from restoring one of theunit emergency diesel generators. Operators were able to connect andactuate a swing diesel, terminating the station blackout after two hours.

•

Krümmel transformer fire (Germany, 2007): a long-duration main stationtransformer fire caused a manual reactor trip. Heavy smoke from the fire
adversely affected MCR personnel in carrying out manual actions.
(Ventilation was inadequate for this type of event.) Operators were
nevertheless able to shut down the reactor and prevent core damage.

Notable examples are included in: Nowlen, S. et al. (2011), Risk Methods Insights Gained from
Fire Incidents; NEA (2015b), “Proceedings of International Workshop on Fire PRA”; andNEA (2016), “Combinations of Fires and Other Events – The Fire Incidents Records
Exchange Project Topical Report No. 3”.

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FIRE AS A RISK CONTRIBUTOR•

Robinson high energy arc fault (United States, 2010): a non-vital bus arc
flash and fire, followed by a subsequent breaker failure, led to reactor trip,
an uncontrolled reactor coolant system cooldown, and automatic safety
injection actuation. Plant response was complicated by equipment
malfunctions leading to loss of reactor coolant pump seal cooling andoperating crew errors (including reinitiation of the electrical fault, leading
to a second fire).

Numerous improvements in plant design and operations, many of which havebeen identified by fire PSA, have been made over time. (Some of these improvements,
e.g. the installation of improved reactor coolant pump seals in pressurised water
reactors, have been prompted by broad risk considerations, but have considerable
impact on fire PSA results.) However, the results of many current fire PSA show thatfire remains an important contributor to core or fuel damage frequencies and tolarge and/or large early release frequency, and therefore, public health risk (NEA,
2015b). This holds true for operations in low power and shutdown operational states,
as well as during full power operation.
As discussed subsequently in this report, there are often conservatisms in fire
PSA results, even after plant improvements are credited. However, there are no
indications that more realistic analyses will change the essential message that fire isa hazard worthy of serious consideration in a risk-informed decision-making
environment. This message has been consistently reported over the years, assummarised in multiple NEA reports on the use and development of PSA in NEA
member countries and discussed at a 2014 WGRISK workshop on fire PSA (see the“Proceedings of International Workshop on Fire PRA” – NEA, 2015b). It is also reflectedin the strong participation of NEA member countries in international fire-related
projects (including the following NEA projects: Fire Incidents Record Exchange – FIRE;
High Energy Arcing Fault – HEAF; and Fire Propagation in Elementary, Multi-Room
Scenarios – PRISME), and the continuing discussion of fire PSA at major international
PSA conferences.

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PLANT-SPECIFIC FIRE RISK

Chapter 4. Plant-specific fire risk

The risk of fire at a particular plant, as with many other hazards, is strongly
dependent on plant-specific factors in design and operation.
A risk-significant fire scenario generally involves weaknesses in somecombination of fire prevention, fire detection and alarm, fire suppression andequipment fire separation. In such a scenario, ignition of a self-sustaining fire leads
to damage of key components, and subsequent failure of plant systems and failureof operators to prevent core or fuel damage. The likelihood of each of these scenario
elements is strongly affected by plant-specific features. These features include:

5.

•

fire prevention, which includes housekeeping (particularly control of
combustibles), routine preventative maintenance and inerting theenvironments of selected critical areas;•

the location of key equipment (components and supporting electrical cables);•separation between redundant trains of equipment important to safety bypassive fire protection features (fire barriers, encapsulations), distance or a
combination of the two;•

active ventilation controls (e.g. fire dampers, compartment pressurisation,smoke control and ventilation fan control);•the location and type of fixed fire detection and suppression systems (bothautomatic and manually actuated);•susceptibility to fire-induced damage of key equipment, considering both
thermal and non-thermal (e.g. smoke-related) effects, and the possibility offire-induced spurious actuations; 5•fire response design provisions (e.g. the location, protection and capabilities ofalternate control stations);•

fire response procedures, alongside co-ordination with emergency operatingprocedures and associated training;•

non-fire specific plant design features (e.g. improved reactor coolant pumpseals).

Note that the increased use of fibre optic cables reduces the likelihood of such actuations.
Note also that a particular class of fire-induced spurious actuations – those affecting
isolation valves on emergency core cooling system lines penetrating containment – canbe important contributors to fire-induced containment bypass scenarios (and hence fire
contributions to large release frequency).

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PLANT-SPECIFIC FIRE RISKAll these features can differ significantly from plant to plant. For example,
consider the location and separation of key equipment. Unlike nuclear steam supply
systems (NSSS), which are largely standardised for a given reactor type, equipmentlayouts for safety and support systems can be highly plant-specific, especially in thecase of older plants. The location of important rooms (switchgear rooms, cablespreading rooms) and passageways between these rooms (notably cable tunnels andvaults) can vary even for nominal sister plants with the same NSSS, of the same
vintage and with the same architect/engineer. Some plants can have key equipment
and cables in the turbine building (which poses specific fire hazards, which may
include the presence of large quantities of lubricating oil and hydrogen) and othersnot. These particularities are typically identified by the deterministic fire hazard
analysis, representing in many countries one of the basic elements for fire PSA.
Note that modern plant designs incorporate lessons from past fire studies. For
example, evolutionary plant designs typically provide strong spatial separation for
their redundant safety divisions. This separation is maintained up to the MCR.
The above discussion applies to the layout at the plant level. Within rooms,
specific equipment locations, particularly cable routings, can vary significantly,
even for newer plants. Moreover, these routings often change over time with plant
equipment modifications. Such detailed differences in equipment location can be
an important factor in terms of risk. For example, whether a critical set of cables isin a tray directly above a switchgear cabinet or in cable trays a few feet away will
significantly impact the likelihood that a fire in that cabinet will damage thosecables. For plants that lack a detailed database for cable routings, one of the major
expenses of performing fire PSA involves the development of such a database.
It is important to recognise that, in addition to the effect of variability in plantspecific features, fire risk estimates can be affected by analyst choices. As discussed
in Chapter 6, fire PSA is generally performed as an iterative process in order to focus
analysis resources on the most important contributors to risk. For each fire scenario
addressed, practical analyses typically start with conservative simplifying
assumptions regarding fire extent (e.g. any fire in a room causes loss of the entire setof equipment in the room) and consequences (e.g. a main station transformer firecauses an unrecoverable loss of off-site power – LOOP). More refined analyses are
performed only for those scenarios where such refinements are judged worthwhile.
(This judgement is typically based on the expected impact to risk estimates.)
Variations in such judgements, as well as in the resources available to perform thefire PSA, can lead to study-dependent variations in fire risk estimates and associateduncertainties.
Past NEA reports, and more recent discussions of fire risk results at international
meetings, make reference to such plant-to-plant fire PSA results variations. It cantherefore be emphasised that this variability is related to both plant- and studyspecific features.

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REALISM OF FIRE PSA RESULTS AND INSIGHTS

Chapter 5. Realism of fire PSA results and insights

Fire PSA insights in relation to the major contributors to total fire risk are generallyaligned with operating experience and appear to be largely representative of
expected plant responses. However, some aspects of the quantitative results ofmodern fire PSA can be conservative. Overall, there is a wide range of views within
the PSA technical and user communities regarding the realism of fire PSA and theconsequent implications for the appropriate use of fire PSA results.
When discussing the realism of PSA results and insights, 6 it is important torecognise that the notion of “risk” includes qualitative aspects as well as
quantitative ones. In particular, consideration can be given to the widely-used triplet
definition of risk proposed by Kaplan and Garrick (1981), in response to threequestions:•

What can go wrong?

•

How likely is it?

•

What are the consequences?

The accident scenarios identified by PSAs appear to provide an answer to the firstquestion. Thus, for example, some of the important insights resulting from a fire PSA
include identification of the types and locations of fires, as well as subsequent
equipment failures and failed actions on the part of operators that may contributesignificantly to plant fire risk.
It is also important to recognise that, as discussed in Chapter 7, the results andinsights of fire PSA can, in principle, be used in both fire-protection oriented riskinformed applications (e.g. the development of measures to reduce fire risk) andbroader applications (e.g. the modification of allowed outage times for key
equipment). Discussions of realism thus need to consider fire PSA results andinsights from both a fire risk perspective and a total (all-hazard) risk perspective.

Fire risk perspective
Past fire PSA studies, taken as a whole, have consistently found that fires involving
electrical cables and/or cabinets in key plant areas (including MCRs, emergency
switchgear rooms, cable spreading rooms, cable vaults and tunnels) are the6.

See Footnote 3 for the definition of “realism” as used in this report.

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REALISM OF FIRE PSA RESULTS AND INSIGHTS

dominant contributors to fire risk. In a number of these areas, the risk-significant
scenarios can involve localised fires that damage important, co-located cables.
However, there are some plants for which larger scale fires, e.g. turbine building
fires and fires inducing MCR abandonment, are important contributors to overall
fire risk. Such risk information has facilitated decisions on the prioritisation ofvarious fire risk compensatory measures.
Past studies have also consistently shown that risk-significant accident
sequences triggered by fires are generally dominated by some form of transient(e.g. loss of feedwater, LOOP and loss of various support systems). However, lossof-coolant accidents (LOCAs), including reactor coolant pump seal LOCAs andtransient induced LOCAs involving stuck open relief valves, are important
contributors to risk for some plants.
From a qualitative perspective, fire PSA insights in relation to important plant
locations, fire scenarios and plant response modelling appear to reasonably alignwith operating experience. In particular, operating experience (such as that
discussed in Chapter 3) have highlighted the significant risk posed by high energy
arcing fault events, fire-induced losses of off-site power, spurious equipment
operation and large turbine building fires.
In recent years, many fire PSAs have moved from approaches postulating fires
that immediately cause the loss of all equipment in a room to approaches
postulating local fire sources that might grow and damage additional equipment(e.g. using Fire PRA Methodology for Nuclear Facilities, “Volume 1: Summary andOverview”, “Volume 2: Detailed Methodology” and “Supplement 1: Fire Probabilistic
Risk Assessment Methods Enhancements” [EPRI/NRC 2005 and 2010], andsupporting documents). Although the NEA has not undertaken a formal analysis
through its Working Group on Risk Assessment (WGRISK), the understanding of
WGRISK members is that the overall qualitative results of such studies are
generally consistent with the above points.

Total risk perspective
There is a wide range of views within the PSA technical and user communities
regarding the realism of fire PSA and the consequent implications for theappropriate use of fire PSA results. On the one hand, some users argue that theresults are overly conservative and cannot be effectively used in risk-informed
applications that require consideration of the risk from all hazards (including but
not limited to fire). On the other hand, others argue that the objective evidence forassessing conservatisms is not definitive. Furthermore, in a truly risk-informed
decision-making process, analysis limitations and uncertainties can (and should)
be recognised and accommodated in a risk-informed, holistic decision-making
approach (see discussion in Chapter 7).
A fire PSA that provides an overly conservative representation of fire risk, given
the current state of knowledge, could bias decisions by plant and regulatory decision
makers and could also inappropriately deprioritise (“mask”) hazards of greater
importance. Less intuitively, an overly conservative analysis could undervalue the20

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REALISM OF FIRE PSA RESULTS AND INSIGHTS

effectiveness of proposed fire-related improvements and lead to non-conservativedecisions. More broadly, perceptions of significant departures from realism, either
conservative or non-conservative, can damage stakeholder trust in fire PSA andperhaps even in non-fire-related PSA applications.
Based on discussions within the fire PSA community, and on the authors’experiences, it appears that fire PSAs, as performed using currently available tools andguidance, are providing conservative quantitative results in some analysis areas. 7
However, it has proved to be challenging to provide estimates of the degree ofconservatism. Moreover, whether the degree of conservatism is excessive depends on
subjective judgements, as well as the needs of the intended application. It should be
noted that, similar to PSA treatments of internal events and of other hazards, there
are topic areas that are difficult to model and are not yet routinely addressed incurrent fire PSAs. Some of the topic areas (e.g. secondary fires initiated in the courseof the event, such as that seen during the Robinson fire discussed in Chapter 3) are
unique to fire; others (e.g. multiple potential damage mechanisms, 8 operator errors ofcommission) are not. These topic areas constitute sources of uncertainty in thecompleteness of the fire PSA and potential sources of non-conservatism. Current use
of fire PSA in support of decision making generally accounts for these factors by using
a risk-informed process such as that discussed in Chapter 7.

7.

See Annex A for further details.

8.

As discussed in NUREG/CR-6738, in some extreme instances, a fire event can involve
explosions resulting in blast overpressures and missiles, as well as heat.

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FIRE PSA METHODOLOGY AND PRACTICE

Chapter 6. Fire PSA methodology and practiceFire PSA methods, models, tools and data continue to improve, and the practice offire PSA continues to mature.
The complexities with fire as a phenomenon (see Chapter 1) compound the“normal” challenges associated with performing PSAs in nuclear power plants.
(Examples of these normal challenges include dealing with sparse operational data,
addressing the likelihood and consequences of different equipment failure modes,
such as spurious operations, treating operator actions, identifying and prioritisinga multitude of potential, contributing scenarios, and characterising uncertainties.)
This chapter discusses the maturity and realism of current fire PSA technology(i.e. methods, models, tools and data) and practice, addressing the following key
points:•

The fire PSA community generally agrees on the overall approach for
performing fire PSA.

•

The level of realism in the fire PSA for a plant often represents a trade-off
between the level of modelling effort and the needs of the plant’s fire PSA
application.

•

Key sources of uncertainty in the fire PSA, including potential nonconservatisms as well as conservatisms, have been identified, and many arebeing addressed through ongoing R&D.

•

Ongoing efforts to update practical guidance and provide training areexpected to improve practice and broaden the base and expertise of analysts,
reviewers and potential decision makers.

It is important to emphasise that, as discussed in other chapters of this report,
fire PSA is a useful tool for risk-informed decision making. The technicalcommunity’s acknowledgement and discussion of uncertainties and potential
biases, which ultimately leads to the identification of research needs driven bypractice, is in fact an indicator of a reasonably mature discipline (see, for example,
Cornell, 1981).

Consensus approach to fire PSA
The fire PSA community generally agrees on the overall approach for performing
fire PSA.

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FIRE PSA METHODOLOGY AND PRACTICE

In a typical fire PSA, potentially important scenarios are identified throughconsideration of the fire hazards (both permanent and transient combustibles andignition sources) within the plant and of the plant equipment that may be damaged
by a fire. Of particular interest are those event scenarios with fires involving thetriggering of a plant transient and the response of plant systems and operators,
potentially leading to core and/or fuel damage. The frequencies of such scenarios
are quantified by estimating:•the frequency of fire occurrence; 9•

the conditional probability of fire-induced damage to structures, systems andcomponents (SSC) given the fire;•the conditional probability of core and/or fuel damage given fire-induced
damage to SSC.

If the fire PSA is performed as part of a Level 2 PSA, 10 large (and early) releasefrequencies can be quantified for fire event scenarios. Many analyses consider therisk associated with low power and shutdown conditions, as well as operations atfull power. General guidance documents for performing a fire PSA are provided in
Annex B of this report.
Fire occurrence frequencies are typically estimated based on plant-specific andgeneric data from the operating experience, by applying simple statistical models
for fire occurrences. The likelihood of fire damage is estimated using combinationsof deterministic and probabilistic models for the physical processes of fire growth,
detection, suppression and equipment damage. The likelihood of core or fueldamage is estimated using event tree and fault tree models that address the
combined effect of fire-induced failures and random failures of equipment notdamaged by the fire.

Trade-offs in practical analyses
The level of realism in the fire PSA for a plant often represents a trade-off between
the level of modelling effort and the needs of the plant’s fire PSA application.
As a result of the fundamental complexities discussed in Chapter 1, as well as
the demands of characterising and treating a variety of plant-specific features
identified in Chapter 4, a detailed fire PSA can require considerable resources. In
practice, the fire PSA is usually performed iteratively, starting with highly simplified

9.

Consistent with the current state of practice for fire PSA event screening, a fire event thatis screened into the PSA analysis is typically assumed to result in a plant transient. This
assumption may result in the overestimation of fire induced transients (compared tooperating experience) if the screening process includes fire events that do not directly
result in a transient event.

10. Within Level 2 PSA models, the modelling of a plant's response to accident sequences that
resulted in reactor core or fuel element damage is typically referred to as severe accident
analysis.

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FIRE PSA METHODOLOGY AND PRACTICE

and conservative modelling assumptions (e.g. assuming that any fire in a room will
induce the failure of all the equipment in that room) and refining these assumptions(e.g. through the modelling of specific equipment and the use of detailed fire models)
only for those scenarios where refinement is judged important to the final results
and insights. This iterative approach enables an efficient analysis.
It is important to emphasise that fire PSA is a practical tool used to support
decision making (see Chapter 7). Different fire PSA applications can have different
technical support needs and therefore may require different levels of technicalrefinement. For example, demonstrations that a plant’s fire risk meets specified
criteria, demonstrations that a fire PSA has an appropriately balanced treatment of
key elements in fire protection (e.g. prevention, detection and mitigation) that is
capable of supporting fire programme improvements, and analyses that identify andprioritise the important contributors to overall plant risk (including non-fire as wellas fire scenarios) can place different demands on the analysis. Project decisions
regarding the purpose and degree of iteration will depend on the cost and added
value of more detailed analysis.

Areas for fire PSA research and development
Key sources of uncertainty in fire PSA, including potential non-conservatisms aswell as conservatisms, have been identified, and many are being addressed throughongoing R&D.
As discussed at the beginning of this chapter, there are many drivers to theuncertainties and biases in the results of fire PSAs. Considerable thought, reflecting
lessons from a variety of sources (including past R&D, operational experience, andthe performance and review of fire PSAs) has gone into the identification andprioritisation of national and international activities to improve current methods,
models, tools, data and guidance. Some of the activities involve R&D, while others
are aimed at maintaining or improving important infrastructure. R&D activities
include, inter alia:•

experimental and analytical investigations of the likelihood and potential
severity of high energy arcing fault (HEAF) events, particularly those
involving aluminium in medium-to-high voltage switchgear and bus bars;•

experimental and analytical investigations of the growth, classification andseverity of transient combustible fuel packages to refine the heat release rate
methodology and fire growth profiles;•

experimental and analytical investigations to improve fire modelling
techniques associated with electrical cabinets, including improvements to
enclosure fire spread, enclosure effects on horizontal radiation heat transfer
and alternative methods for main control board fire modelling;•

analytical investigations into the effectiveness of plant personnel detection
and suppression efforts, and the relationship to fire frequency fromoperating experience;

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25

FIRE PSA METHODOLOGY AND PRACTICE•

experimental investigations on the severity of electrical fires, in particular
cable and electrical cabinet fires and corresponding model improvements;•

experimental investigations
compartments;•

analytical and experimental investigations of the effectiveness of fire
detection and suppression systems (including very early warning fire
detection systems – VEWFDS – which are designed to detect incipientconditions prior to actual combustion);•experimental investigations concerning the impact of smoke on electricaland electronic equipment;•

expert panel elicitations to interpret experimental results and develop
practical inputs for fire PSA;•

expert panel elicitations to improve the heat release rate distributions for inplant transformers (excluding large outdoor oil filled transformers);•

the development of practical screening methods to deal with fire events
involving additional, secondary events (e.g. turbine failures leading tomissiles, explosions and even flooding);•

the development of improved human reliability analysis methods to address
the likelihood of personnel failures (including ex-control room actions, aswell as actions within the MCR) during a fire scenario.

of

fire

growth

withinandbetweenConsistent with a maturing technology, one observation is that these research
activities have shifted from the general exploratory activities conducted decades ago
to more focused research projects intended to address specific high risk and/or highuncertainty areas.
The infrastructure activities include support of operational experience
databases (such as that administered by the NEA FIRE Incidents Records Exchange
Database Project), the development of guidance, and the execution of training.
Guidance and training are further discussed in the following section.
Outside of the fire PSA area, activities are underway to address more general
PSA topics that are relevant to the performance of fire PSA (e.g. site level PSA dealing
with multiple units and other on-site radioactive sources) and to the use of fire PSA
results (e.g. the aggregation of PSA results across hazards, plant operational statesand on-site radiological sources).
As can be expected, for a field where both the risks and the potential benefits of
PSA improvements are believed to be significant, the fire PSA R&D environment isdynamic. Ongoing activities are being pushed to completion with high urgency inorder to speed up the use of research results in practical problem solving.
Unexpected research results (notably the discovery of the potential severity of HEAF
events involving aluminium) and lessons from fire PSA applications (e.g. regarding
modelling assumptions that appear to affect fire PSA realism) are leading to newresearch activities and modified priorities in terms of existing activities. It is beyond
the scope of this report to comment on the nature or priority of specific elements of

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FIRE PSA METHODOLOGY AND PRACTICE

research plans. It has been observed that the fire PSA R&D community is paying
particular attention to all the major elements of a fire scenario (i.e. initiation, growth,
detection and suppression, equipment damage, plant response) and is aware ofmajor analysis challenges identified from operational experience reviews. These
challenges, some of which involve potential non-conservatisms in current fire PSAs,
include the treatment of multiple hazards and events (e.g. fires induced byearthquakes or flooding), low power and shutdown operations, as well as at-powerstates, and multi-unit and/or multi-source impacts. Activities to address some ofthese challenges are at an early stage of development.

Fire PSA guidance and training
Ongoing efforts to update practical guidance and provide training are expected to
improve practice and broaden the base and expertise of analysts, reviewers andpotential decision makers.
In addition to analytical methods, models, tools and data, the practical
performance of fire PSA requires the availability of appropriate guidance andtraining. With regard to guidance, a variety of current technical standards andreports are available. In addition, a number of activities are underway to develop
improved guidance for both detailed issues arising during the application of a fire
PSA (e.g. the modelling of cabinet to cabinet fire propagation; HEAF; and heat release
rates for transformers, motors and transient combustibles) and more general fire
PSA issues (e.g. analyses of fires during low power and shutdown operations). Key
documents are provided in Annex B of this report.
In terms of training, several workshops have been held, covering fire PSA andrelated topics and aspects (e.g. electrical circuit analysis, fire modelling and humanreliability analysis), and the training materials are available in a variety of forms(including slides, reports, videos and simulation model files). The International
Atomic Energy Agency (IAEA) has also conducted several fire PSA training courses
in various countries.

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USE OF FIRE PSA IN RISK-INFORMED DECISION MAKING

Chapter 7. Use of fire PSA in risk-informed decision making

Fire PSA is a valuable tool that provides useful results and insights in support of riskinformed decision making. As with all risk-informed decisions, it is important thatthe decision makers are informed of potential biases (both conservative and nonconservative, with magnitudes that are typically uncertain) and other uncertainties
associated with these results and insights.
Fire PSA has a long history of supporting risk-informed decision making in the
nuclear power industry. Early applications include a late 1970s analysis that
supported design development for a high-temperature, gas-cooled reactor, and an
early 1980s analysis that supported licensing hearings for the Indian Point nuclear
power plant. Currently, fire PSA (as one part of a comprehensive PSA) is being
increasingly used, even in those countries that employ a deterministic regulatory
approach. Applications include the identification and prioritisation of plant changes(including specific, fire protection improvements, as well as major changes in fire
protection programmes), the development of safety cases as part of the periodic
safety review requirements for operating reactors in many countries, and the
support of the development and approval of new reactor designs. Fire PSA results
and insights are also being used to identify and prioritise R&D activities.
With the continuing use of the fire PSA, the fire PSA technical and user
communities have improved their understanding of the different strengths andweaknesses of the capabilities and practice of the fire PSA. Currently, as evidenced
by practical applications, it appears that there is reasonable confidence in the use offire PSA to support applications focused on fire considerations alone (e.g. firevulnerability assessments, ranking of fire contributors). For applications requiring
relative comparisons of fire risk with other sources of risk (e.g. when aiming for abalanced design) or reasonable estimates of the absolute levels of fire risk (e.g. whenusing decision processes, such as those involving the modification of allowed outage
times for key equipment, which rely on demonstrations that a plant has achieved aprescribed safety level), the community views are considerably more varied. Thissituation is providing a strong driver for efforts to better understand uncertainties
and improve the usefulness of fire PSA (as discussed in Chapter 6), and it is expectedthat these efforts will yield useful improvements.
While there may be some limitations associated with the current state ofpractice for fire PSA (as discussed in Chapter 6), these limitations can be mitigatedthrough the use of risk-informed, integrated decision-making processes that also
consider traditional engineering insights, safety margins, defence in depth andanalysis uncertainties. (For example, see IAEA, 2011 and NRC, 2018.) In particular,
fire PSA can provide substantial decision-making support through the identificationof key sources of uncertainty and their impact on the plant response.

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USE OF FIRE PSA IN RISK-INFORMED DECISION MAKING

Looking to the future, it is expected that ongoing R&D will enable new, yet
untested applications for fire PSA. For example, a fire PSA could be used to support
a risk-informed examination of proposed cable qualification standards. A fire PSA
that addresses the frequency and consequences of scenarios leading to non-coredamage, but still undesirable plant states, could also support enterprise risk
management activities that might address both economic and safety risks.

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CONCLUDING REMARKS AND RECOMMENDATIONS

Chapter 8. Concluding remarks and recommendations

In the last several years, considerable discussion has taken place on the maturity
and realism of fire PSA methods, models, tools and data, with a variety of views
continuing to co-exist on the subject. Although such discussions are a positive driverfor improvement activities – including R&D, database maintenance and guidancedevelopment – it should not overshadow two fundamental points: fire can be an
important contributor to plant risk, and fire PSA is a useful tool for risk-informed
decision making. Developing a thorough understanding of the key drivers of
uncertainties in the fire PSA is therefore critical for the appropriate use of risk
insights that the fire PSA can provide to decision makers.
Many PSA practitioners, when comparing the fire PSA with an internal events
analysis, consider fire PSA to be less mature and its results subject to greater
uncertainties. As discussed in this report, the results for some areas of a given fire
PSA can be conservative. Nevertheless, it is clear that the fire PSA provides asystematic analytical tool for dealing with the complex issues that need to beaddressed when ensuring fire safety at a nuclear power plant. The fire PSA has
proven useful for supplementing deterministic analyses on which reactor design
and fire protection are based, as it highlights the strong and weak points of a plant’s
design and operation with respect to fire hazards, while supporting a variety of risk
management applications – cable routing management being one example of animportant application.
As is the case with all PSA applications, the results and insights from fire PSA
should be used as part of an overall risk-informed decision-making process, rather
than providing the sole technical basis for decisions. Knowledge of the uncertainties
and potential biases (both conservative and non-conservative, with magnitudes thatare typically uncertain) in the fire PSA can (and should) be included when weighing
available evidence. One, important observation is that in such a decision support
framework, a fire PSA judged suitable for a fire risk management issue may need to
be improved when used to address an issue involving the full set of hazards relevant
to plant risk. Ongoing and future improvement activities are expected to improve thebroad acceptance of fire PSA for a wider set of potential applications, including
enterprise risk management applications not yet widely envisioned.
Of course, continued improvement requires sustained organisational support. It
is recommended, therefore, that organisations continue to prioritise fire PSA
improvements as an essential activity, and that organisations continue to support:•

R&D on key topics identified by the fire PSA technical and user communities;•

activities to collect, manage and provide access to quality data;

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CONCLUDING REMARKS AND RECOMMENDATIONS•

activities facilitating the sharing of challenges, solutions, uncertainties, usesand good practices;•

activities to develop practical guidance based on the above points.

In terms of sharing information, as a multidisciplinary activity, fire PSA has been
demonstrated to involve the interaction of a number of technical communities.
Historically, interactions between some communities (e.g. the fire PSA andoperational experience communities) have been limited, and organisational support
is likely needed to facilitate improved co-operation.

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REFERENCES

References

Cornell, C.A. (1981), Structural safety: some historical evidence that it is a healthyadolescent, Proceedings of Third International Conference on Structural Safety
and Reliability (ICOSSAR’81), Trondheim, Norway, 23-25 June 1981.
EPRI/NRC (2010), “Supplement 1: Fire Probabilistic Risk Assessment Methods
Enhancements”, Fire PRA Methodology for Nuclear Facilities, EPRI TR 1011989 –NUREG/CR 6850, Palo Alto, CA and Rockville, www.nrc.gov/reading-rm/doccollections/nuregs/contract/cr6850/s1.
EPRI/NRC (2005), “Volume 1: Summary and Overview” and “Volume 2: Detailed
Methodology”, Fire PRA Methodology for Nuclear Facilities, EPRI TR 1011989 –NUREG/CR 6850, Palo Alto, CA and Rockville, www.nrc.gov/reading-rm/doccollections/nuregs/contract/cr6850.
IAEA (2011), A Framework for an Integrated Risk Informed Decision Making Process,
INSAG-25, IAEA, Vienna.
Kaplan, S. and B. J. Garrick: On the Quantitative Definition of Risk, Risk Analysis, Vol 1,
Issue 1, pp. 11-27, March 1981.
Nowlen, S. et al. (2011), Risk Methods Insights Gained from Fire Incidents, NUREG/CR6738, US NRC, Washington, DC, SAND2001-1676P, SNL, Albuquerque, NM,
www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6738.
NEA (2016), “Combinations of Fires and Other Events – The Fire Incidents Records
Exchange Project Topical Report No. 3”, NEA/CSNI/R(2016)7, www.oecdnea.org/documents/2016/sin/csni-r2016-7.pdf.
NEA (2015a), “FIRE Project Report: Collection and Analysis of Fire Events (2010-2013)– Extensions in the Database and Applications”, NEA/CSNI/R(2015)14,
www.oecd-nea.org/nsd/docs/2015/csni-r2015-14.pdf.
NEA (2015b), “Proceedings of International Workshop on Fire PRA”, NEA/CSNI/
R(2015)12, www.oecd-nea.org/nsd/docs/2015/csni-r2015-12.pdf.
NEA (2002), CSNI Technical Opinion Papers – No. 1: Fire Probabilistic Safety Assessment for
Nuclear Power Plants, No. 2: Seismic Probabilistic Safety Assessment for Nuclear
Facilities, NEA 3948, OECD, Paris.
NEI (2017), Sustainable Use of Risk-Informed Regulation to Improve Plant Safety, US NRC
Briefing, www.nrc.gov/reading-rm/doc-collections/commission/slides/2017/201
70511/industry-20170511.pdf.
NRC (2018), “An Approach for Using Probabilistic Risk Assessment in Risk-Informed
Decisions on Plant-Specific Changes to the Licensing Basis”, Revision 3, Regulatory
Guide 1.174, Revision 3, www.nrc.gov/docs/ML1731/ML17317A256.pdf.

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VIEWPOINTS ON CONSERVATISM IN FIRE PSA RESULTS

Annex A. Viewpoints on conservatisms in fire PSA results

A common assertion supporting the view that fire PSA is overly conservative is that“fire PSA results do not comport with operational experience” (e.g. see NEI, 2017). In
particular, it is argued that fire PSA over-predicts the frequency of fire-induced
scenarios that pose significant challenges to plant safety as measured by theconditional probability of core damage – CCDP – a measure of “how close” a scenario
comes to core damage. 11
Empirical information from operational experience, including data on fireoccurrences, characteristics and qualitative plant impacts is extremely valuable for
PSA and risk-informed decision making. Additionally, quantitative measures ofscenario-level operational experience, such as the CCDPs for actual events, arepotentially valuable, provided that they are compared to operating experience in atechnically appropriate manner using defensible statistical testing approaches. As
suggested by critics of current fire PSAs, such information can be used to calibrate
PSA model estimates for many scenarios. Calibration, in turn, can increase
stakeholder confidence in the PSA. Even further, a calibrated model would likely be
useful for additional applications, e.g. non-regulatory enterprise risk management
activities to reduce the economic risk associated with non-core and non-fuel damage
scenarios. Such applications could represent a “win-win” approach to safety andproduction concerns. It has been noted that scenario-level operational experiencehas been somewhat overlooked in the broader PSA community, and greater use
would assist many risk-informed applications, not just those related to fire.
Of course, all sources of information used to infer the likelihood of rare events
have their limitations, and scenario-level measures such as CCDP are no exception.
As with conventional PSA results, CCDP generation and use require fundamental
modelling assumptions regarding the applicability of the modelled events to theassessment at hand, as well as technical assumptions associated with the PSAoriented modelling of these events. They should therefore be considered asproviding an additional, rather than an alternate, perspective on risk. With respect
to the comparison of fire PSA estimates with expectations based on precursor
analysis results, a few cautions should be provided.
•

By their nature, precursor analyses are well-suited for identifying andprioritising potential accident scenarios that are more extreme versions ofobserved events. These analyses can thus address important scenarios that

11. The CCDP measure is widely used in assessments of the severity of operational events
such as those performed as part of the accident sequence precursor programmes of several
NEA member countries.

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VIEWPOINTS ON CONSERVATISM IN FIRE PSA RESULTS

involve unlikely combinations of somewhat likely failures. They are notaimed at low likelihood/high consequence scenarios for which there has
been little or no warning. However, such scenarios can be important
contributors to a nuclear power plant’s risk profile. This caution applies
especially to extreme natural phenomena (e.g. large earthquakes or external
floods) but also seems applicable to fires; the lack of observations ofchallenging fires in sensitive locations does not necessarily mean that suchfires are unimportant contributors.
•

Conditional probability of core damage (or CCDPs) computed using fire PSA
models and those computed using standard accident precursor models are notnecessarily equivalent, because of the differing purposes of these models (andtherefore their differing underlying assumptions and boundary conditions).

•

As discussed in the main body of this report, although fire PSA is performed
iteratively to refine initially conservative assumptions, the analysis
iterations are aimed at refining total core damage frequency estimates, notat refining estimates of the frequencies of various pre-core damage plantdamage states. Clearly, as currently practised, there are no guarantees ofaccuracy for such intermediate frequencies.

Finally, it is important to recognise that the benchmarking of fire PSA resultswith operational experience has a qualitative as well as a quantitative dimension,and that qualitative comparisons appear to be reasonably favourable. For example:i) scenarios identified as being important in fire PSA (e.g. fire-induced loss of off-site
power) have been observed in significant fire events, and ii) significant fire events
have involved mechanisms (e.g. high energy arcing fault) typically addressed by fire
PSAs (which is expected since many scenarios in PSAs are developed withconsideration of operating experience). However, in terms of their level ofimportance/applicability within a fire PSA, such conclusions can also be limited
and/or overstated since a strict interpretation of such events does not address plant
improvements that may have occurred since event occurrence, a fact which isparticularly important for older events (e.g. the aforementioned Browns Ferry fire in1975). In addition, operational experience can also underestimate nonconservatisms since it cannot fully capture insights from fires that did not take place(although some precursors, e.g. those associated with degraded plant conditions,may provide additional information).

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ADDITIONAL RESOURCES

Annex B. Additional resources

Fire PSA methodology and standards
BfS (2005a), Daten zur Quantifizierung von Ereignisablaufdiagrammen und Fehlerbäumen
(in German only), BfS-SCHR-38/05, BfS, Salzgitter, https://doris.bfs.de/jspui/handle/urn:nbn:de:0221-201011243838.
BfS (2005b), Methoden und Daten zur probabilistischen Sicherheitsanalyse für Kernkraftwerke(in German only), BfS-SCHR-61/16, BfS, Salzgitter, https://doris.bfs.de/jspui/handle/urn:nbn:de:0221-2016091314090.
BfS (2005c), Methoden zur probabilistischen Sicherheitsanalyse für Kernkraftwerke (inGerman only), BfS-SCHR-37/05, BfS, Salzgitter, http://doris.bfs.de/jspui/handle/
urn:nbn:de:0221-201011243824.
IAEA (2010), Development and Application of Level 1 Probabilistic Safety Assessment for
Nuclear Power Plants, IAEA Safety Standards Series No. SSG-3, STI/PUB/1430, ISBN
978-92-0-114509-3, IAEA, Vienna, www-pub.iaea.org/MTCD/publications/PDF/
Pub1430_web.pdf.
IAEA (1998), Treatment of Internal Fires in Probabilistic Safety Assessment for Nuclear
Power Plants, Safety Reports Series, No. 10, IAEA, Vienna.
NFPA (2001), Performance-Based Standard for Fire Protection for Light Water Reactor Electric
Generating Plants, NFPA 805, Quincy, MA.
NRC (2016), Verification and Validation of Selected Fire Models for Nuclear Power Plant
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Fire PSA operational experience and data
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General fire safety
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Special topics
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38

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Türschmann, M., M. Röwekamp and J. von Linden (2005), Systematisches
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Türschmann, M. and M. Röwekamp (2016), “Probabilistic safety assessment of fire
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FIRE PROBABILISTIC SAFETY ASSESSMENTS FOR NUCLEAR POWER PLANTS, NEA No. 7417, © OECD 201939

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CSNI Technical Opinion Paper

No. 17CSNI Technical Opinion Paper No. 17: Fire Probabilistic Safety Assessments for Nuclear
Power Plants: 2019 Update provides an authoritative review of the current status anduse of the fire PSA in nuclear power plants. The report demonstrates that while fires
at a particular plant site are highly dependent on plant and site specific factors, they
are nonetheless an important contributor to overall risk. Insights from fire PSAs are
generally found to be aligned with operating experience and to be representative of
the expected plant response, making them valuable in addressing risk. This report
should be useful for regulators overseeing the use of fire PSAs in nuclear installations,practitioners in understanding the considerations for performing or reviewing fire PSAs,
and researchers in identifying areas requiring further study.

Nuclear Energy Agency (NEA)
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NEA No. 7417