Technical Advisory Committee of the Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku, Tokyo, 100-8126 Japan
October 14, 2021
Dr. George Apostolakis
Head, Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku
Tokyo, 100-8126 JapanSUBJECT:

FIRE PRA GUIDE

Dear Dr. Apostolakis:
In June 2020, the Nuclear Risk Research Center (NRRC) published the "Fire PRA
Guide – An Update of NUREG/CR-6850 for Use by the Japanese Nuclear Industry".
This is a very important milestone for the NRRC fire risk research activities. We
planned to conduct a review of the Fire PRA Guide during our November 2020 or
May 2021 meetings.
Unfortunately, due to the continuing challenges anduncertainties with the COVID-19 pandemic, we could not hold in-person meetings
with your research teams. Therefore, we used an alternative approach to conduct
our review of this report.
The complete report, in English and Japanese, is available on the NRRC website. In
June, the NRRC research teams also provided us with presentations that summarize
the main elements of the report and selected supporting research activities. We
reviewed the report and those summaries, and we prepared comments andquestions from each of our members, as we would normally do before our meetings.
In lieu of the active meeting discussions that clarify our understanding of each topic,
we sent those comments and questions to you for consideration by the fire researchteam. The researchers provided detailed and thoughtful written responses to ourquestions. We also had a 2-hour video conference with your team to further discussspecific issues and follow-up questions. We then deliberated on those responses
and developed the Committee's consensus observations, conclusions, andrecommendations that are provided in this letter report.
Our experience from this effort has reinforced the vital importance of the dynamic
interactions during our face-to-face meetings. While the approach we used for thisreview achieved our basic objectives, the in-person technical exchanges provide
clarification and understanding that benefit each of us in ways that cannot be
accomplished through written questions and answers. We sincerely hope that we
can return to our normal meetings in 2022.

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技術諮問委員会報告書「火災PRAガイド」（英文）CONCLUSION AND RECOMMENDATIONS
1. We commend the NRRC for completing this major milestone to develop a fire
PRA methodology and guidance that is suitable for use by the Japanese industry.
The Fire PRA Guide has advanced the international state-of-the-practiceguidance for performing a fire PRA.
2. During our review of the Fire PRA Guide and the supporting research activities,
we identified the following topics that merit additional attention. Our specificrecommendations for each topic are summarized in the Discussion section of thisreport. The fire PRA researchers should address these topics while the current
guidance is being used for the trial application. We would like to work closelywith the NRRC fire PRA research team as they address these topics.
• Integration of Japanese fire event data
• Guidance for low power and shutdown modes• Main Control Room fires• Fire effects on digital systems
BACKGROUNDInternational experience has shown that fires are often an important contribution to
nuclear power plant risk. That contribution depends very strongly on the plantspecific design, physical layout, cable routing, operating and maintenance practices,and fire protection program. Therefore, it is very important that each plant should
include a comprehensive analysis of fires in a good-quality full-scope probabilistic
risk assessment (PRA).
The development of fire analysis methods and guidance specific to the Japanese
nuclear industry has been and continues to be a focus of the NRRC research
activities. We have been engaged in discussions on this topic since the TechnicalAdvisory Committee (TAC) was formed in 2014. On June 6, 2015, we wrote a letter
report on the fire research activities, recommending certain priorities and test
programs (e.g., realistic estimates of ignition frequencies). On May 27, 2017, weissued another letter report on the fire PRA research, highlighting, among otherthings, the need for an integrated pilot application of the proposed guidelines.
The NRRC completed the first draft of the Fire PRA Guide in September 2018. The
Japanese utilities and vendors reviewed the draft guide and provided comments andrecommendations. One of our members also provided his personal comments.
During our May 2019 and November 2019 meetings, the fire PRA research teamdiscussed the status of the draft guide, the nature of comments received, plans torevise the guide, and plans for a trial application. In our November 19, 2019 letter
report, we supported issuance of the Fire PRA Guide for general use, without anintermediate trial application and iterative revision. We noted that we wouldcomment separately on technical details of the guidance after the September 2018draft version of the guide was updated. This letter report provides our observations,conclusions, and recommendations on the updated guidance.

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技術諮問委員会報告書「火災PRAガイド」（英文）DISCUSSION
We commend the NRRC for completing this major milestone to develop a fire PRA
methodology and guidance that is suitable for use by the Japanese industry.
The NRRC Fire PRA Guide is founded on the extensive U.S. research and fire risk
assessment experience that formed the basis for the guidance in NUREG/CR-6850.
The original version of NUREG/CR-6850 was issued in 2005. Since that time,
substantial additional experience has been gained from using that guidance tosupport risk-informed changes to the licensing basis and fire protection programs at
almost half of the U.S. nuclear power plants. The U.S. NRC and industry havedeveloped consensus clarifications of the guidance for several complex topics, andseveral major test programs have been undertaken to address specific issues. The
NRRC Fire PRA Guide fully incorporates the results from this subsequent research,and it is organized to take advantage of the U.S. industry's practical experience. As
such, it has advanced the international state-of-the-practice guidance for performing
a fire PRA.
We understand that a trial application of the fire PRA guidance will be conductedusing a "model plant". We have commented extensively on the selection and use of"model plant" PRAs to demonstrate proposed analytical methods and guidance. We
understand the practical constraints that influence those decisions. If the trial
application is performed according to the plan that was summarized by the fire PRA
team, it will demonstrate all tasks and key elements of the Fire PRA Guide.
However, from our technical perspective, we continue to recommend that these trial
applications should use the good-quality PRAs for internal initiating events that are
being developed for Ikata Unit 3 and Kashiwazaki-Kariwa Unit 7. That practice will
provide the best demonstration of how each contribution to risk is consistently
integrated into a full-scope PRA.
During our review of the Fire PRA Guide and the supporting research activities, we
identified the following topics that merit additional attention.
The fire PRA
researchers should address these topics while the current guidance is being used forthe trial application. The next update of the guide can then incorporate the combined
benefits from the trial application practical experience and further enhancements tospecific analysis methods and guidance. We would like to work closely with the
NRRC fire PRA research team as they address these topics.
Integration of Japanese Fire Event Data
We understand that each Japanese utility has compiled data for fire events that haveoccurred at their nuclear power plants. We also understand that NRRC has
evaluated those data and has developed initial estimates for industry-wide genericfire ignition frequencies.
Of course, realistic estimates for the fire ignition
frequencies depend completely on the scope and technical quality of the compiled
fire event data. Therefore, it is essential to have confidence that the data arecollected consistently at each plant and that fire events are not inadvertently
excluded.

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技術諮問委員会報告書「火災PRAガイド」（英文）We have not performed an in-depth review of the guidance for collection andevaluation of the fire event data, screening and categorization of the fire events, orthe process that has been used to derive the generic fire ignition frequencies.
During our review of the Fire PRA Guide summary presentations, we provided very
preliminary comments on some elements of the fire ignition frequency analyses.
However, that effort is not related directly to the current version of the Fire PRA
Guide. We will discuss that guidance separately with the fire PRA team, and it is notaddressed in this report.
Experience has also shown that it is important to develop consistent integrated
guidance for (1) compilation, screening, and categorization of fire events to support
the fire ignition frequencies and (2) evaluation of fire growth, detection, andsuppression in the PRA models and supporting analyses. In particular, if fire events
are removed from the database because the fires were detected and suppressed
before they grew to a "potentially challenging" or "challenging" size, those screening
criteria affect the application of parameters for fire growth, detection, andsuppression in the PRA models for the fire events that are retained in the database.
If the guidance for these analyses is not carefully coordinated, there is a danger of
inappropriately "double accounting" for rapid detection and suppression. For
example, this issue can be especially important if plants have incipient fire detection
systems, and the fire analyses include credit for those systems to quickly detect verysmall fires. In that case, the fire events database should include all recorded fires,
regardless of their size.
In the Fire PRA Guide, some of the detailed fire modeling guidance in Task 14 andsome of the specific numerical values that are recommended in the appendices forthat task depend on consistency between the process that was used to derive thefire ignition frequencies in Table 8.1 and the corresponding assumptions about fireseverity, detection, growth, and possible suppression. Those Task 14 numericalvalues in the current version of the Fire PRA Guide have been carefully coordinatedwith the process and assumptions that were used to derive the U.S. fire ignition
frequencies. They should not be used independently with the Japanese fire ignition
frequencies, without a very careful evaluation of the consistency of the applied
analytical methods, assumptions, and supporting fire event data.
Furthermore, some of the recommended values in the Fire PRA Guide are derived
entirely from U.S. operating experience, and they may not apply for typical Japaneseplant designs and practices. One example is the probability distributions for manualfire suppression times that are tabulated in Appendix 14D. Another example is theguidance for evaluating the effectiveness of incipient detection and enhanced
suppression to control specific fires before they damage any PRA items. The
composite fire severity and non-suppression factors for Main Control Board fires thatare recommended in Appendix 10A similarly depend on the supporting U.S. data andassumptions. Japanese plants may also use different guidance and criteria for
personnel decisions to abandon the Main Control Room.
We request an opportunity to review the following topics:•

Guidance for the fire event data collection, screening, and categorization-4-

技術諮問委員会報告書「火災PRAガイド」（英文）•Development of the generic fire ignition frequencies•

How recommended numerical values in the Fire PRA Guide for fire growth,
detection, and suppression will be revised to consistently account for theJapanese fire event data and Japanese fire protection practices

Guidance for Low Power and Shutdown Modes
Section 1.1 of Chapter 1 of Volume 1 of the Fire PRA Guide ("Introduction andBackground") notes that:"Also, the focus of this guide is on core damage and containment failure
possibility from a fire in an NPP when at power. Similar to NUREG/CR-6850[Reference 1.1], the scope of the methodology and data presented in this guide
does not include:• Non-power operating modes (i.e., low power and shutdown),• Accidents not related to the reactor core (e.g., spent fuel pool accidents), …"With a few exceptions, as noted below, experience from full-scope PRAs has shownthat essentially the same fire analysis methods and guidance can be used for all
plant operating modes.
The fundamental guidance for Task 1 ("Plant Response Model for Fire PRA") does
not depend on the scope of the baseline model. In other words, analysts must
always use the same techniques to examine and modify the internal event PRA
models to account for fire-induced failure modes, to revise the event tree and fault
tree logic to evaluate the functional effects from specific fire scenarios, to define newhuman failure events, etc. Of course, details of the baseline plant response model
for internal initiating events will differ, according to the corresponding scope of the
PRA. For example, some equipment may be de-energized during specific shutdown
plant operating states. Other equipment may be re-aligned from its normal power
operation configuration. The internal event models for each plant operating stateaccount for these conditions. Thus, the functional effects from fire-induced open
circuits and spurious signals will also vary, depending on the status of specificcomponents during each plant operating state. The current guidance for Task 1emphasizes careful analyst evaluation of fire-induced failures when the internal
event PRA models are modified. It appropriately does not provide prescriptive
methods for how to revise specific parts of the models. Thus, in the context of thisFire PRA Guide, the same basic guidance and techniques that are currently
summarized in Task 1 can be used for PRA models that include full-power, lowpower, and shutdown modes; spent fuel cooling; and mitigation of offsite releases.
The guidance should be expanded to address three issues that pertain to the fire
analyses during plant shutdown, compared to power operation: (1) differences insome fire ignition frequencies, (2) differences in the likelihoods that human-caused
fires occur in specific plant compartments, and (3) differences in the status ofcompartment fire barriers.
The ignition frequencies for some fire categories apply only during specific plant
operating modes. Operating experience has shown that:-5-

技術諮問委員会報告書「火災PRAガイド」（英文）•Most equipment-related fires may occur at any time, regardless of the plant
operating mode.

•

Some equipment-related fires may occur only during full-power and low-power
operating modes. The frequencies of human-caused fires that occur during plant
power operation depend on the amount and types of activities that are typically
performed during those operating modes.

•

Some equipment-related fires may occur only during plant shutdown modes.
Different frequencies of human-caused fires apply during plant shutdown,
accounting for the increased amount of work and types of activities that are
performed when the reactor is not operating.

The scope of the Fire PRA Guide can be extended very easily to address thesedifferences. Ignition frequencies for fire categories that apply only during plant
shutdown modes can be derived from the Japanese industry operating experience.
Chapter 8 of Volume 2 of the Fire PRA Guide ("Task 8 – Fire Ignition Frequencies")
would then contain three tables of ignition frequencies: one for fires that may occur
at any time, one for fires that may occur only during power operation, and one forfires that may occur only during shutdown.
Industry operating experience has shown that the amount and types of activities (e.g.,maintenance, testing, system modifications, welding, cutting, etc.) and the quantitiesof transient combustibles in several plant locations are often significantly different
during shutdown, compared to power operation. In practice, some activities are also
performed only during specific plant operating states throughout an outage.
Therefore, the guidance in Chapter 8 (e.g., Section 8.5.6.3) should alert analysts to
re-examine the compartment-level allocations of these human-caused fire ignition
sources during each shutdown plant operating state, compared to the allocations
that apply during power operation modes.
Operating experience has also shown that the status of fire compartment barriers
may be different during specific shutdown plant operating states, compared to power
operation. For example, doors may be blocked open, equipment and personnel
access hatches may be removed, and penetrations may be opened to facilitate
outage-related work. The guidance for Step 12.1 in Chapter 12 of Volume 2 of theFire PRA Guide ("Task 12 – Multi-Compartment Analysis") should alert analysts to
examine the status of fire compartment barriers during each plant operating stateand evaluate differences in the fire propagation scenarios.
The basic methods and guidance for all other tasks apply universally, regardless ofthe plant operating mode.
The fire PRA team should update the Fire PRA Guide as follows:•

Extend the scope of Chapter 8 to include ignition frequencies for equipmentrelated and human-caused fires that may occur only during plant shutdown
modes.

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技術諮問委員会報告書「火災PRAガイド」（英文）•Expand the guidance in Chapter 8 to alert analysts to re-examine the types ofactivities and amounts of transient combustibles in each compartment during
each shutdown plant operating state, compared to power operation.

•

Expand the guidance in Chapter 12 to alert analysts to re-examine the status offire compartment barriers during each shutdown plant operating state, comparedto power operation.

•

Make other editorial changes to remove the limitation that the analysis methods
and guidance apply only for fires that occur during plant power operation.

Main Control Room Fires
We understand that the fire PRA team intends to enhance the methods andguidance for evaluating Main Control Room (MCR) fires after the trial application iscompleted. This section discusses our recommendations for specific elements of
those analyses. To be most responsive to the Japanese industry needs, the fire
PRA team should conduct supporting research and develop extensions of thecurrent methods for these analyses in parallel with the trial application.
Two important issues for the evaluation of MCR fires are guidance for:(1) identification of PRA target sets for fires that affect the Main Control Board (MCB)or operator control consoles, and (2) analyses of fires that force the operators toabandon the MCR.
Several elements of the MCR fire analysis guidance in the Fire PRA Guide arebased on assumptions and models that were developed for older-design U.S.
nuclear power plant control rooms. For example, those designs typically contained
very large central MCB panels with distributed displays, instrumentation, and large
analog control switches. Several of the recommended qualitative assumptions andquantitative screening criteria for the size of an MCB fire that may cause potentiallyrisk-significant damage are based on that general configuration.
The MCRs for newer plants typically contain more compact control panels. The
MCBs for some older plants have also been upgraded with more modern analog ordigital displays and controls. Furthermore, many modern MCRs contain stand-alone
operator control consoles that are separated from the MCB. The guidance for asystematic process to identify PRA target sets for fires that affect the MCB or theoperator control consoles should be re-examined and enhanced to remove implicit
and explicit assumptions that apply only for older plant designs. Consistentguidance for a systematic evaluation and a progressive refinement process should
apply for any MCR design.
Some fires that occur inside or outside of the MCR may force the operators toabandon the MCR, due to loss of reliable instrumentation and controls, or severely
degraded habitability. In the Fire PRA Guide, one of the recommended MCR
abandonment criteria relies on a specific size of MCB fire damage. As noted above,that recommendation seems to be based on assumptions about the functional
damage that may occur from a fire in an older U.S. MCB design. It should be reexamined for consistency with newer plant designs and MCR configurations.

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技術諮問委員会報告書「火災PRAガイド」（英文）Furthermore, criteria for MCR abandonment due to habitability concerns should bebased on Japanese guidance and practices, rather than U.S. criteria.
The Fire PRA Guide describes an initial simplified treatment of fires that force theoperators to leave the MCR. That guidance focuses exclusively on estimating thetotal frequency of conditions that require the operators to abandon the MCR. It does
not address how analysts should account for the functional impacts from the firedamage in the MCR and the progression of the consequential event scenarios thatmust be mitigated after the operators leave the MCR.
We are not familiar with details of the Alternate Shutdown System designs at
Japanese nuclear power plants. In many U.S. plants, operation of only a limited setof equipment is possible from the Alternate Shutdown System panels. Furthermore,it may not be feasible to mitigate the effects from some fire-caused failure modes(e.g., spurious signals) by other local actions that can be accomplished within theavailable time windows (e.g., at switchgear, motor control centers, local manual
operation of valves, etc.). Thus, in practice, depending on the plant-specific design
and the specific fire damage, the MCR fire may functionally disable some equipment
or cause plant responses that cannot be fully reversed by operations at the Alternate
Shutdown System panels or other locations. The Fire PRA Guide should provide
guidance for how the simplified analyses should account for those non-reversible
failure modes.
The fire PRA team should improve the guidance for the following analyses:•

Systematic identification of PRA target sets for fires that affect the MCB oroperator control consoles•

Recommended MCR abandonment criteria•

Treatment of the functional effects from MCR fire damage after the operators
leave the MCR

Fire Effects on Digital Systems
We understand that the fire PRA team also intends to enhance the methods andguidance for evaluating fires that affect digital instrumentation, control, andprotection system cabinets and their interconnecting cables after the trial applicationis completed. This section discusses our recommendations for specific elements of
those analyses. The fire PRA team should conduct supporting research and develop
preliminary methods for these analyses in parallel with the trial application.
The Fire PRA Guide does not contain any guidance for the analyses of fires that
affect digital instrumentation and control systems. Two issues are particularly
important for those analyses: (1) evaluating fire damage to fiber optic cables, and(2) determining the functional effects from fire damage to digital signal processing
cabinets.
Some Japanese nuclear power plants contain fully integrated digital instrumentation,
protection, and control systems. Based on international experience, it seems very-8-

技術諮問委員会報告書「火災PRAガイド」（英文）likely that many plants also contain distributed digital instrumentation and controls for
specific systems and functions (e.g., nuclear instrumentation, feedwater control,
turbine-generator control, some electrical protection functions, newly-installed
accident management systems, etc.). These digital applications will continue toincrease as older plants are progressively upgraded with more modern controls. Of
course, new plants will rely extensively on digital systems.
Fiber-optic cables are used extensively in digital systems and in some hybrid
applications. A limited amount of research and testing has examined thresholds forthermal damage to fiber-optic cables. Other research has generally concluded thatfire damage to fiber-optic cables will not produce spurious signals (or the conditional
probability of a spurious signal is extremely low). To support comprehensive
assessments of the risk from fires in Japanese plants, the Fire PRA Guide should
provide analysts with a clear understanding of the available knowledge and guidancefor how to treat fire damage to fiber-optic cables.
Fire damage and high temperatures in cabinets that contain digital signal processing
modules may cause spurious signals. Therefore, the guidance should clearly
distinguish between the failure modes that may be caused by fire damage to fiberoptic cables, and the failure modes that may be caused by fire damage inside a
digital signal processing cabinet.
The analysis of digital systems is an evolving topic in the international PRA
community. Methods to estimate the reliability of software, evaluate possible
common cause failures, and identify unexpected software responses are not yet fully
mature. However, substantial progress has been made, and analysts are developing
models for these systems. Of course, as we gain more experience with thesesystems and their models, our collective knowledge will increase, and the guidance
for these analyses will certainly be improved. The same perspective applies formany other topics in the science of risk assessment, which are currently included
and quantified in contemporary PRA models. Conservative assumptions, bounding
models, and careful assessments of uncertainties are used to compensate forincomplete knowledge.
Despite the evolving nature of some elements of digital systems analysis methods, it
seems possible to provide analysts with practical guidance that can be used toevaluate the potential effects from fire damage and determine how those effects areincluded in the PRA models.
For example, in practice, analysts should have information about the specific input
and output signals from each digital system cabinet, and they should haveinformation about the functional logic of the signal processing modules. Therefore, it
seems feasible to identify the possible effects from fire damage to the cabinet (e.g.,loss of specific output signals or spurious actuation of those signals). For a very
complex cabinet, it may not be practical to perform detailed analyses that determine
the conditional probabilities for all possible combinations of the signal states.
However, depending on the system design, the functions which are provided by eachcabinet, and the cabinets' spatial configurations, those detailed analyses may not benecessary. It may be sufficient to simply assume that the "worst possible"
combination of output signals occurs, based on the functional effects in the Level 1-9-技術諮問委員会報告書「火災PRAガイド」（英文）and Level 2 PRA models. If the initial analyses indicate that fire damage to aparticular digital signal processing cabinet may be important to risk, then the analysts
will need to examine those circuits in more detail. Similar progressive methods haveoften been used for the analyses of fire damage to cabinets that contain analog
signal processing circuits (i.e., relays and copper wires).
The fire PRA team should develop guidance for the following analyses:•

Thermal damage criteria and functional effects from fire damage to fiber-optic
cables•

Evaluation of functional failure modes from fire damage to digital signal
processing cabinets

We look forward to working with the NRRC fire research team as they continue toimprove the guidance for this very important element of a good-quality full-scope
PRA for each Japanese nuclear power plant.
Sincerely,

John W. Stetkar
Chairman
REFERENCES
1.

Nuclear Risk Research Center, "Fire PRA Guide – An Update of
NUREG/CR-6850 for Use by the Japanese Nuclear Industry," NRRC Report
O20001, Central Research Institute of Electric Power Industry, March 2020
(English), June 2020 (Japanese).

2.

"Fire PRA (FPRA) Guide – Status of NRRC Fire PRA Guide and Relevant
Activities," Presentation to NRRC Technical Advisory Committee, July 2021,
Proprietary.

3.

"Fire PRA (FPRA) Guide – Details of Revised Guide with External Comments
in FY2018," Presentation to NRRC Technical Advisory Committee, July 2021,
Proprietary.

4.

Stetkar, J. W., "Comments and Questions on Fire PRA Guide, March 2020,"
June 27, 2021, Confidential.

5.

Technical Advisory Committee individual members' comments and questions
on "Group A – Comments and Questions on Fire PRA Guide, March 2020,"
July 8, 2021, Confidential.

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技術諮問委員会報告書「火災PRAガイド」（英文）6.

Technical Advisory Committee individual members' comments and questions
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of Revised Guide with External Comments in FY2018," July 8, 2021,
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7.

Technical Advisory Committee individual members' comments and questions
on "Group C – Comments and Questions on Fire PRA (FPRA) Guide – Statusof NRRC Fire PRA Guide and Relevant Activities," July 8, 2021, Confidential.

8.

NRRC Fire PRA Team responses to comments and questions on "Group A –Comments and Questions on Fire PRA Guide, March 2020," July 19, 2021,
Confidential.

9.

NRRC Fire PRA Team responses to comments and questions on "Group B –Comments and Questions on Fire PRA (FPRA) Guide – Details of Revised
Guide with External Comments in FY2018," July 17, 2021, Confidential.

10.

NRRC Fire PRA Team responses to comments and questions on "Group C –Comments and Questions on Fire PRA (FPRA) Guide – Status of NRRC FirePRA Guide and Relevant Activities," July 21 2021, Confidential.

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Nagata, Y., T. Uchida, T. Yoshida, and K. Shirai, "Estimation of Generic Fire
Ignition Frequency Distributions for Japanese Nuclear Power Plants," Draft,
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12.

Technical Advisory Committee of the Nuclear Risk Research Center, "FireResearch Activities," June 6, 2015.

13.

Technical Advisory Committee of the Nuclear Risk Research Center, "Current
Status of Fire PRA Research Activities," May 27, 2017.

14.

Stetkar, J. W., "Comments and Questions on Fire PRA Guide, Draft,
September 2018," November 20, 2018, Confidential.

15.

Technical Advisory Committee of the Nuclear Risk Research Center,"Proposed NRRC Research Plan for Fiscal Year 2020," November 19, 2019.

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U.S. Nuclear Regulatory Commission and Electric Power Research Institute,"EPRI / NRC-RES Fire PRA Methodology for Nuclear Power Facilities,"
NUREG/CR-6850, EPRI 1011989, September 2005.

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U.S. Nuclear Regulatory Commission and Electric Power Research Institute,"Fire
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