EPRI 1019259 NUREG/CR-6850 Supplement 1 

Fire Probabilistic Risk Assessment 

Methods Enhancements 

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Fire Probabilistic Risk Assessment 

Methods Enhancements 

Supplement 1 to NUREG/CR-6850 and EPRI 1011989

1019259 NUREG/CR-6850 Supplement 1

Technical Report, September 2010 

Electric Power Research Institute (EPRI) 

3420 Hillview Avenue 

Palo Alto, CA 94303 

EPRI Project Manager 

K. Canavan 

Division of Risk Analysis 

Office of Nuclear Regulatory Research (RES) 

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U.S. NRC-RES Project Manager 

J.S. Hyslop

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iii 

ABSTRACT 

The U.S. Nuclear Regulatory Commission (NRC) and Electric Power Research Institute (EPRI) 

significantly improved the state-of-art of fire PRA methodology with the development and 

documentation of the joint publication NUREG/CR-6850, EPRI 1011989 “EPRI/NRC-RES Fire 

PRA Methodology for Nuclear Power Facilities.” Although the individual portions of the 

methodology were tested in this joint project, the methodology was not tested in an integrated 

fashion. As a part of the fire PRAs developed for transition to the voluntary risk-informed, 

performance-based fire protection rule, 10CFR 50.48(c), the methodology was tested in a fully 

integrated fashion to yield risk results. 

As a part of their transition to 10CFR50.48(c), the National Fire Protection Association (NFPA) 

Standard 805 pilot plants raised a broad set of fire PRA questions while performing their fire 

PRAs. These questions became part of the overall NFPA 805 frequently asked questions (FAQ) 

program established by the NRC. NRC, EPRI, and the broader industry participated in this FAQ 

program to develop interim responses to these questions. 

The interim responses for these fire PRA FAQs, i.e. NUREG/CR-6850 FAQs, through April, 

2010 are provided in this report. These FAQ responses significantly improve our knowledge 

about this set of fire PRA issues, and can be used to enable more realistic estimates of fireinduced core damage frequency. 

Importantly, although these FAQ responses are considered interim, all 15 are currently publically 

available on ADAMS, and will also be available in this document once it is published. 

Therefore, they are currently in use and will remain so until results are known for research 

programs currently being planned or conducted that can be used to improve our knowledge of 

these issues. Ultimately, these FAQ responses or their enhancements will be integrated into a 

future revision of NUREG/CR-6850 (EPRI 1011989). 

v 

CONTENTS 

ABTRACT.................................................................................................................................... III 

CONTENTS...................................................................................................................................V 

LIST OF FIGURES...................................................................................................................... IX 

LIST OF TABLES........................................................................................................................ XI 

REPORT SUMMARY............................................................................................................... XIII 

CITATION.................................................................................................................................. XV 

ACKNOWLEDGMENTS.......................................................................................................... XVII 

1 INTRODUCTION .................................................................................................................... 1-1

1.1 Background .................................................................................................................. 1-1

1.2 Objectives .................................................................................................................... 1-2

1.3 Overview of the FAQ Process...................................................................................... 1-3

1.4 Report Structure .......................................................................................................... 1-3

2 APPLICATION OF FIRE PRA METHODS TO NFPA 805 CHANGE EVALUATION ............ 2-1

3 IGNITION SOURCE COUNTING GUIDANCE FOR ELECTRICAL CABINETS (FAQ 

06-0016) ..................................................................................................................................... 3-1

3.1 Background .................................................................................................................. 3-1

3.2 Resolution .................................................................................................................... 3-2

4 IGNITION SOURCE COUNTING GUIDANCE FOR HIGH-ENERGY ARCING FAULTS 

(HEAFS) (FAQ 06-0017) ........................................................................................................... 4-1

4.1 Background .................................................................................................................. 4-1

4.2 Resolution .................................................................................................................... 4-2
vi 

5 IGNITION SOURCE COUNTING GUIDANCE FOR MAIN CONTROL BOARD (MCB) 

(FAQ 06-0018) ........................................................................................................................... 5-1

5.1 Background .................................................................................................................. 5-1

5.2 Resolution .................................................................................................................... 5-1

6 MISCELLANEOUS FIRE IGNITION FREQUENCY BINNING ISSUES (FAQ 07-0031) ........ 6-1

6.1 Background .................................................................................................................. 6-1

6.2 Resolution .................................................................................................................... 6-1

6.2.1 Bin 14: Electric Motors ........................................................................................ 6-1

6.2.2 Bin 21: Pumps ..................................................................................................... 6-2

6.2.3 Bin 23: Transformers ........................................................................................... 6-3

6.2.4 Bin 26: Ventilation Subsystems ........................................................................... 6-4

7 BUS DUCT (COUNTING) GUIDANCE FOR HIGH-ENERGY ARCING FAULTS (FAQ 

07-0035) ..................................................................................................................................... 7-1

7.1 Background .................................................................................................................. 7-1

7.2 Resolution .................................................................................................................... 7-1

7.2.1 Summary ............................................................................................................. 7-1

7.2.1.1 Selection of Bus Duct Fire Scenarios ........................................................... 7-3

7.2.1.2 Ignition Source Counting Guidance .............................................................. 7-3

7.2.1.3 Anticipated Analysis Approach for Segmented Bus Ducts ........................... 7-5

7.2.1.4 Fire Frequency and Frequency Partitioning .................................................. 7-6

7.2.1.5 Estimating the Initial Fault Zone of Influence ................................................ 7-9

8 FIRE PROPAGATION FROM ELECTRICAL CABINETS (FAQ 08-0042) ............................ 8-1

8.1 Background .................................................................................................................. 8-1

8.2 Resolution .................................................................................................................... 8-2

9 MAIN FEEDWATER PUMP OIL SPILL FIRES (FAQ-08-0044)............................................. 9-1

9.1 Background .................................................................................................................. 9-1

9.2 Resolution .................................................................................................................... 9-1

10 FIRE IGNITION FREQUENCY (FAQ 08-0048) .................................................................. 10-1

10.1 Background ........................................................................................................... 10-1

10.2 Resolution ............................................................................................................. 10-1

10.2.1 Impact on Current Fire PRA and FAQ Implementation Plan ......................... 10-3
vii 

11 CABLE TRAY FIRE PROPAGATION (FAQ 08-0049) ....................................................... 11-1

11.1 Background ........................................................................................................... 11-1

11.2 Resolution ............................................................................................................. 11-1

12 LOCATION OF FIRES WITHIN ELECTRICAL CABINETS (FAQ 08-0043) ...................... 12-1

12.1 Background ........................................................................................................... 12-1

12.2 Resolution ............................................................................................................. 12-2

12.3 References ............................................................................................................ 12-3

13 INCIPIENT FIRE DETECTION SYSTEMS (08-0046) ........................................................ 13-1

13.1 Background ........................................................................................................... 13-1

13.2 Resolution ............................................................................................................. 13-2

13.3 References .......................................................................................................... 13-10

14 MANUAL NONSUPPRESSION PROBABILITY (FAQ 08-0050) ....................................... 14-1

14.1 Background ........................................................................................................... 14-1

14.2 Resolution ............................................................................................................. 14-1

14.2.1 Introduction .................................................................................................... 14-1

14.2.2 Solution .......................................................................................................... 14-3

14.3 References ............................................................................................................ 14-9

15 SPURIOUS OPERATION PROBABILITY (FAQ 08-0047) ................................................ 15-1

15.1 Background ........................................................................................................... 15-1

15.1.1 Purpose of FAQ 08-0047 ............................................................................... 15-1

15.2 Resolution ............................................................................................................. 15-2

15.2.1 Background .................................................................................................... 15-2

15.2.2 Initial Circuit Failure Probability Determination .............................................. 15-4

15.2.3 Detailed FPRA Circuit Failure Probability Determination ............................... 15-4

15.2.4 Summary ....................................................................................................... 15-5

16 HOT SHORT DURATION (FAQ 08-0051) .......................................................................... 16-1

16.1 Background ................................................................................................................ 16-1

16.2 Resolution .................................................................................................................. 16-2

16.3 References: ................................................................................................................. 16-9

17 TRANSIENT FIRE GROWTH RATE (FAQ 08-0052) ......................................................... 17-1

17.1 Background ........................................................................................................... 17-1
viii 

17.2 Resolution ............................................................................................................. 17-1

17.2.1 Nonsuppression Curves for Transient Fires in the MCR ............................... 17-1

17.2.2 Transient Fire Growth Rates/Times ............................................................... 17-1

17.3 References ............................................................................................................ 17-3

18 REFERENCES ................................................................................................................... 18-1

A SUMMARY OF EXPERIMENTAL INSIGHTS FOR TRASH FIRES ..................................... A-1

B SUMMARY OF EXPERIMENTAL INSIGHTS FOR TRASH BAG FIRES (FAQ 08-

0052) ......................................................................................................................................... B-1 
ix 

LIST OF FIGURES 

Figure 3-1 Counting of electrical cabinets .................................................................................. 3-3

Figure 3-2 Counting of electrical cabinets .................................................................................. 3-4

Figure 7-1 Photograph of the point on the bus duct. The photograph shows where the 

arcing fault at Diablo Canyon was manifested (the fault point). Note that the tops of 

the cabinets to the left and right (the cabinets to the left are shown in Figure 7-2) 

are visible in the lower corners of this photograph. .......................................................... 7-11

Figure 7-2 Photograph of the surface damage and burning. The photograph shows 

damage observed for the more heavily impacted cabinets on one side of the aisleway below the fault point (the cabinet to the left as seen in Figure 7-1). ......................... 7-12

Figure 9-1 Oil fire severity factor guidance ................................................................................ 9-2

Figure 9-2 Main feedwater pump oil fire severity factors ........................................................... 9-3

Figure 13-1 Proposed event tree for assessing fire risk, assuming VEWFDS is installed ....... 13-4

Figure 13-2 Simplified event tree ............................................................................................. 13-8

Figure 14-1 Conceptual relationship between fire detection time, suppression time, and 

fire brigade response time ................................................................................................ 14-2

Figure 14-2 Revised nonsuppression curves Part A .............................................................. 14-19

Figure 14-3 Revised nonsuppression curves Part B .............................................................. 14-20

Figure 14-4 Revised nonsuppression curves for individual groupings ................................... 14-20

Figure 14-5 Revised nonsuppression curves for individual groupings ................................... 14-21

Figure 14-6 Revised nonsuppression curves for individual groupings ................................... 14-21

Figure 14-7 Revised nonsuppression curves for individual groupings ................................... 14-22

Figure 14-8 Revised nonsuppression curves for individual groupings ................................... 14-22

Figure 14-9 Revised nonsuppression curves for individual groupings ................................... 14-23

Figure 14-10 Revised nonsuppression curves for individual groupings ................................. 14-23

Figure 14-11 Revised nonsuppression curves for individual groupings ................................. 14-24

Figure 14-12 Revised nonsuppression curves for individual groupings ................................. 14-24

Figure 14-13 Revised nonsuppression curves for individual groupings ................................. 14-25

Figure 14-14 Revised nonsuppression curves for individual groupings ................................. 14-25

Figure 14-15 Revised nonsuppression curves for individual groupings ................................. 14-26

Figure 16-1 Durations of hot shorts .......................................................................................... 16-3

Figure A-1 Heat release rate for NUREG/CR-4680 test 7, small trash can fire ........................ A-2 

Figure A-2 Heat release rate for NUREG/CR-4680 test 8, small trash can fire ........................ A-2

Figure A-3 Heat release rate for NUREG/CR-4680 test 9, large trash can fire......................... A-3
x 

Figure A-4 Photograph of the trash receptacle tested in NISTFR-4018 ................................... A-4

Figure A-5 Heat release rate test results for the two tests documented in NISTFR-4018 ........ A-4

Figure B-1 Results of the LBL tests involving a trash bag filled with eucalyptus duff ............... B-1

Figure B-2 Results of the LBL tests for trash bags filled with cups and paper towels .............. B-2

Figure B-3 SNL fuel package 3—box with crumpled paper ...................................................... B-3

Figure B-4 The heat release rate profile from SNL test 5 involving fuel package 3 .................. B-3
xi 

LIST OF TABLES 

Table 1-1 Summary of the Status of the FAQs Related to EPRI 1011989, NUREG/CR6850 ................................................................................................................................... 1-4

Table 7-1 Segmented Bus Duct Fire Events Used in Frequency Calculations .......................... 7-6

Table 7-2 Iso-Phase Bus Duct Fire Events Used in Frequency Calculations ............................ 7-8

Table 7-3 Iso-Phase and Segmented Bus Duct Fire Frequency Distributions ........................... 7-8

Table 9-1 FEDB Events Involving Main Feedwater Pump Oil ................................................... 9-5

Table 14-1 Updated Numerical Results for Suppression Curves........................................... 14-10

Table 14-2 Original and Updated Mean Suppression Rates (λ) ............................................ 14-11

Table 14-3 List of Fire Events for Original and Revised Suppression Curves ....................... 14-12

Table 16-1 Probability of duration ≥ time (min) for hot shorts .................................................. 16-4

Table 16-2 Duration data (min) from NEI/EPRI and CAROLFIRE tests .................................. 16-8

xiii 

REPORT SUMMARY 

This report documents the interim guidance of the industry and the U.S. Nuclear Regulatory 

Commission (NRC) on several issues from the National Fire Protection Association (NFPA) 

Standard 805 Frequently Asked Questions (FAQs) Program arising from use of EPRI 1011989, 

NUREG/CR-6850 (a joint report of EPRI and the NRC Office of Nuclear Regulatory Research 

[NRC-RES]) fire PRA methodology for nuclear facilities. The FAQ program was established by 

the NRC Office of Nuclear Reactor Regulation (NRC/NRR) to support the NFPA 805 pilots’ 

transition to the new voluntary rule, 10 CFR 50.48 (c), and is documented in NRC Regulatory 

Issue Summary RIS 2007-19, “Process for Communicating Clarifications of Staff Positions 

Provided in Regulatory Guide 1.205 Concerning Issues Identified During the Pilot Application 

of National Fire Protection Association Standard 805.” The present report is expected to be the 

first in a series of reports that will cover all of the FAQs generated through the use of EPRI 

1011989, NUREG/CR-6850 as well as future enhancements, clarifications, and additions to the 

fire PRA methods. 

Results and Findings 

Early, limited use of EPRI 1011989, NUREG/CR-6850 showed that some of the screening/ 

scoping approaches, methods, and assumptions introduced conservative estimates of fire PRA 

results. In an effort to improve the fire PRA methods, an NFPA 805 Frequently Asked Questions 

process collected and documented questions on the methods and technical analysis and their 

associated responses. This report documents the FAQs related to fire PRA methods and their 

responses. In general, the FAQ solutions improve our knowledge about these respective issues, 

and result in more realistic estimates of fire-induced core damage frequency. 

Challenges and Objectives 

The objective in resolving the FAQs is to provide clarifications and modifications to the 

EPRI/NRC-RES guidance report so that it leads to improvement in core damage frequency 

results. The challenge to accomplishing the objective is to achieve the most realistic approach 

possible while attempting to integrate deterministic and probabilistic factors into the analysis. 

Applications, Value, and Use 

The results of this report are eventually expected to be substantially incorporated into a revision 

to EPRI 1011989, NUREG/CR-6850. Importantly, these FAQ resolutions are considered interim 

and may undergo further enhancement prior to their inclusion in the revision to EPRI 1011989, 

NUREG/CR-6850. The revised report can be used by nuclear plant fire protection and risk 

engineers to perform fire PRAs. Use of the improved guidance will result in improvements in the 

results of fire PRAs. 
xiv 

EPRI Perspective 

EPRI 1011989, NUREG/CR-6850 was written jointly by a team of EPRI and NRC contributors. 

The experience and best knowledge at the time were used to form an initial consensus by the 

team, which was then incorporated into the report. As is often the case in the development of 

guidance in new technical areas, subsequent limited use of the screening/scoping guidance in the 

report resulted in high fire-induced core damage frequencies that may not reflect the practical 

experience in the industry over the last fifty years. Accordingly, the original team undertook an 

effort to identify those areas in the guidance that contributed most to these high results. 

The effort to address these methodology improvements was to develop FAQs under the auspices 

of NFPA 805 and have these issues addressed by a broader set of contributors than just the 

original authors of EPRI 1011989, NUREG/CR-6850. In fact, many of the FAQs were identified 

by the users of the guidance, and often the FAQ originator assisted in the development of the 

initial responses to the FAQs, if not the final resolution of the issue. The FAQs and their 

responses are expected to amend the current guidance for the interim and eventually be 

incorporated, possibly with updated resolutions, into a revision to EPRI 1011989, 

NUREG/CR-6850. 

Although these FAQs represent interim solutions, it should be noted that not all solutions require 

longer-term research to develop a lasting and more complete solution. In addition, EPRI 

1011989, NUREG/CR-6850 represents one way to develop a fire PRA. Alternative methods may 

be acceptable provided they can be supported by the state of the art in fire PRA model 

development. 

Approach 

The approach taken by the resolution team followed the procedure given in RIS 2007-19. For the 

later FAQs identified under the RES/EPRI memorandum of understanding (MOU), a modified 

closure process was developed for resolution and is documented under ADAMS Accession No. 

ML090920045. In this later process NRC/NRR and RES develop a preliminary position. RES 

then interacts with EPRI under an established MOU to obtain feedback. NRR then makes 

changes, if appropriate, to the position and issues the FAQ resolution for public comment. 

Industry and public comments are received and appropriately incorporated. The FAQ resolution 

is then published by NRR; however, it remains an interim solution until formally endorsed in 

Regulatory Guide 1.205. 

Keywords 

Fire protection 

Probabilistic risk assessment (PRA) 

Fire probabilistic risk assessment (FPRA) 

Fire risk 

Risk-informed regulation 

Circuit failure
xv 

CITATIONS 

This report was prepared by 

Science Applications International Corporation (SAIC) 

1671 Dell Avenue #100 

Campbell, CA 95008 

Principal Investigators 

C. Afshar 

B. Najafi 

F. Joglar 

GE Hitachi Nuclear Energy Americas LLC 

3901 Castle Hayne Road 

Wilmington, NC 28402 

Principal Investigators 

Y. Li 

D. Henneke 

M. Warner 

U.S. Nuclear Regulatory Commission 

Office of Nuclear Regulatory Research 

Washington, DC 20555 

Principal Investigators 

J.S. Hyslop 

J. Kratchman 

N. Siu 

This report describes research sponsored by the Nuclear Regulatory Commission (NRC) and the 

Electric Power Research Institute (EPRI). 

The report is a corporate document that should be cited in the literature in the following manner: 

Fire Probabilistic Risk Assessment Methods Enhancements: Supplement 1 to NUREG/CR-6850 

and EPRI 1011989. EPRI, Palo Alto, CA, and NRC, Washington, D.C.: December 2009. 

1019259. 

xvii 

ACKNOWLEDGMENTS 

EPRI and NRC-RES would like to acknowledge the following contributing developers and 

authors who provided significant input into the Frequently Asked Questions (FAQs) and/or 

writing of this report: 

Victoria Anderson, Nuclear Energy Institute 

Patrick Baranowsky, ERIN Engineering and Research 

Harold Barrett, Nuclear Regulatory Commission 

Robert Bertucio, Scientech, Curtiss-Wright Flow Control, Commercial Power & Services 

Biff Bradley, Nuclear Energy Institute 

James Chapman, Scientech, Curtiss-Wright Flow Control, Commercial Power & Services 

Erin Collins, Science Applications International Corporation 

Jason Dreisbach, Nuclear Regulatory Commission 

Mardy Kazarians, Kazarians & Associates, Inc. 

Steven Mays, ERIN Engineering and Research 

David Miskiewicz, Progress Energy 

Steve Nowlen, Sandia National Laboratories 

Mathew Warner, GE Hitachi 

Brandy Weaver, Duke Energy Corporation 

Clarence Worrell, Westinghouse 

Frank Wyant, Sandia National Laboratories 

Kiang Zee, ERIN Engineering and Research 

1-1 

1 

INTRODUCTION 

1.1 Background 

In 2001, the National Fire Protection Association (NFPA) completed the development of NFPA 

805, “Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition” [1]. Consequently, effective July 16, 2004, the U.S. Nuclear 

Regulatory Commission (NRC) amended its fire protection requirements to permit existing 

reactor licensees to voluntarily adopt fire protection requirements contained in NFPA 805 as 

an alternative to the existing deterministic fire protection requirements [2]. NFPA 805 and the 

voluntary rule 10 CFR 50.48(c) that adopts it require that a “change evaluation” be performed in 

the event of a change to a previously approved fire protection program element. Such changes 

may apply to a number of fire protection program elements including (but not limited to) fire 

protection system design, installation, maintenance, and operation; fire protection procedures; 

administrative controls; fire brigade; fire protection impairments; or plant post-fire safe 

shutdown strategy. 

In 2002, the Electric Power Research Institute (EPRI) and the NRC Office of Nuclear Regulatory 

Research (RES) started a program to develop the state of the art for fire probabilistic risk 

assessment (PRA). In 2005, this program produced a joint EPRI/NRC report, EPRI 1011989, 

NUREG/CR-6850, “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities” [3]. 

That report is a detailed compendium of methods and technical bases to estimate risk associated 

with internal fires in a nuclear power plant, covering a wide range of disciplines, including fire 

initiation and effects, impact of fire on plant cables and circuits, and plant response to firegenerated conditions. The EPRI/NRC-RES Fire PRA Methodology provides a powerful tool for 

use in risk-informed applications that include estimating change in risk associated with changes 

in plant design and or operational configuration, such as an NFPA 805 “Change Evaluation.” 

In 2005, the Nuclear Energy Institute (NEI) developed NEI 04-02, “Guidance for Implementing 

a Risk-Informed, Performance-Based Fire Protection Program under 10 CFR 50.48(c)” [4]. 

Section 5.3 and Appendices I and J of that document provide significant guidance for conducting 

change evaluations. The guidance covers a) defining the plant change where a “change 

evaluation” is warranted; b) preliminary risk screening where changes with “No” or “Minimal” 

impact of the risk are eliminated from consideration; c) initial risk evaluation, using fire 

modeling or bounding risk assessment (Sections 5.3.4.1, 5.3.4.2, J.4); d) detailed risk evaluation, 

using detailed fire modeling or detailed fire risk analysis (Sections 5.3.4.3, J.5); and e) criteria 

for evaluation of the change. The guidance for bounding risk assessment (Sections 5.3.4.2, J.4.2)

and detailed fire risk analysis (5.3.4.3, J.5.2) relies on existing fire risk assessments and methods 
Introduction 

1-2 

such as the fire protection Significance Determination Process (SDP).1

 In May 2006, NRC 

published Regulatory Guide 1.205, Revision 0, “Risk-Informed, Performance-Based Fire 

Protection for Existing Light Water Nuclear Power Plants” [5], endorsing, with exceptions, 

NEI 04-02, Revision 1. 

Following a proposal by NEI in 2006, an NFPA-805 Frequently Asked Questions (FAQ) process 

was established between the NRC and the industry, emulating a similar process that had been 

previously implemented for the Reactor Oversight Process (ROP) Performance Indicators 

program. The process is introduced in a letter from Sunil Weerakkody of the NRC to Alexander 

Marion of NEI [NRC ADAMS Accession No. ML061660105] and described in detail in RIS 

2007-19 [NRC ADAMS Accession No. ML071590227]. As stated in RIS 2007-19, the purpose 

of the FAQ process is to provide a mechanism for resolving interpretation issues with NEI 04-02. 

Once closed out as dictated in RIS 2007-19, the resolved FAQs represent the NRC staff 

interpretations of the guidance for licensee transition to risk-informed, performance-based fire 

protection under NFPA 805. In addition, RIS 2007-19 states that the resolved FAQs (via NRC 

closure memos) are preliminary extensions of the implementation guidance in NEI 04-02, as 

approved by the NRC. RIS 2007-19 describes the FAQ process and provides a template for 

documenting the FAQs and their resolution. 

Between 2006 and 2009, a number of early applications of EPRI 1011989, NUREG/CR-6850 

resulted in seemingly conservative fire-induced risk estimates. These early results necessitated 

timely development of clarification and enhancements to the document, to support the plants 

transitioning to the risk-informed and performance-based fire protection rule. This led to 

increased focus on the NFPA-805 FAQs related to fire PRA methods and applications. Meetings 

in 2008 and early 2009 regarding these FAQs indicated that there were still important technical 

considerations that needed to be resolved in order to generate timely resolution to support plants 

transitioning to NFPA 805. In June 2009, the NRC instituted a revised closure process for the 

FAQ that had been identified under the NRC/EPRI MOU. This process is documented in a letter 

from Jack Grobe (NRC) to Alexander Marion (NEI) [NRC ADAMS Accession No. 

ML090920045]. 

1.2 Objectives 

In order to document the responses and final resolution of NFPA-805 FAQs related to the Fire 

PRA Methodology, specifically those in EPRI 1011989, NUREG/CR-6850, a revision of this 

joint document will be undertaken by EPRI and RES once sufficient clarification and 

enhancements to the methods documented in the EPRI 1011989, NUREG/CR-6850 are 

developed. 

 

1

 U.S. Nuclear Regulatory Commission, Inspection Manual Chapter 0609, Appendix F, February 28, 2005. 
Introduction 

1-3 

1.3 Overview of the FAQ Process 

RIS 2007-19 describes the FAQ process for fire PRA FAQs not identified under the NRC/EPRI 

MOU. ADAMS Accession No. ML090920045 describes the approach to resolve the existing 

FAQs related to the EPRI 1011989, NUREG/CR-6850 NRC/EPRI MOU, as follows. 

1. NRR staff, in consultation with RES staff, will develop an Interim Position for each specific 

FAQ related to EPRI 1011999, NUREG/CR-6850 in accordance with the schedule provided 

in the enclosures [enclosures may be found in ML09090045]. This is the start of the process. 

2. NRR will transmit the draft Interim Position to RES within two weeks after the start of the 

process. 

3. RES will engage EPRI under the MOU to obtain comments on the specific FAQ Interim 

Position within two weeks from receipt of the NRR draft. The MOU Team (RES and EPRI) 

may agree, disagree, or concur on additional confirmatory research. 

4. RES will return the specific FAQ Interim Position to NRR with recommendations, as 

appropriate, within five weeks after the start of the process. 

5. NRR will appropriately incorporate recommendations generated through review under the 

RES/EPRI MOU and provide a proposed resolution of the FAQ for industry and other public 

stakeholder consideration within seven weeks after the start of the process. 

6. Industry and other public stakeholder comments will be received and appropriately 

considered in finalizing the FAQ resolution and issuing the final FAQ closure documentation 

within sixteen weeks of the start of the process. 

1.4 Report Structure 

Table 1-1 contains a summary of the status of the issues related to the fire PRA and EPRI 

1011989, NUREG/CR-6850. FAQs 16, 17, 18, 31, 35, 42, 43, 44, 46, 47, 48, 49, 50, 51, and 52 

were solved under the closure process described in Section 1.3. 
Introduction 

1-4 

Table 1-1 

Summary of the Status of the FAQs Related to EPRI 1011989, NUREG/CR-6850 

FAQ No. Title Status 

(As of December 2, 2009) 

In This 

Report 

FAQ 06-0016 Ignition Source Counting 

Guidance for Electrical 

Cabinets 

NRC Closure Memo, 10/05/07 (Ref. NRC 

ADAMS ML072700475 and 

ML070580334) 

Chapter 3 

FAQ 06-0017 Ignition Source Counting 

Guidance for High-Energy 

Arcing Faults (HEAF) 

NRC Closure Memo, 09/26/07 (Ref. NRC 

ADAMS ML072500300 and 

ML071570255) 

Chapter 4 

FAQ 06-0018 Ignition Source Counting 

Guidance for Main Control 

Board (MCB) 

NRC Closure Memo, 09/26/07 (Ref. NRC 

ADAMS ML072500273) 

Chapter 5 

FAQ 07-0031 Miscellaneous Binning 

Issues 

NRC Closure Memo, 12/17/07 (Ref. NRC 

ADAMS ML072840658) 

Chapter 6 

FAQ 06-0035 Bus Duct Counting 

Guidance for High-Energy 

Arcing Faults 

NRC Closure Memo, 07/24/09 (Ref. NRC 

ADAMS ML091620572) 

Chapter 7 

FAQ 08-0042 Fire Propagation from 

Electrical Cabinets 

NRC Closure Memo, 08/04/09 (Ref. NRC 

ADAMS ML092110537) 

Chapter 8 

FAQ 08-0044 Main Feedwater Pump Oil 

Spill Fires 

NRC Closure Memo, 08/04/09 (Ref. NRC 

ADAMS ML092110516) 

Chapter 9 

FAQ 08-0048 Fire Ignition Frequency NRC Closure Memo, 09/01/09, (Ref. 

NRC ADAMS ML092190457) 

Chapter 10 

FAQ 08-0049 Cable Tray Fire 

Propagation 

NRC Closure Memo, 07/30/09 (Ref. NRC 

ADAMS ML092100274) 

Chapter 11 

FAQ 08-0043 Location of the Fire Within 

an Electrical Cabinet 

NRC Closure Memo, 08/04/09 (Ref. NRC 

ADAMS ML092120448) 

Chapter 12 

FAQ 08-0046 Incipient Fire Detection 

Systems 

NRC Closure Memo, 11/23/09 (Ref. NRC 

ADAMS ML093220426) 

Chapter 13 

FAQ 08-0050 Manual Nonsuppression 

Probability 

NRC Closure Memo, 09/14/09 (Ref. NRC 

ADAMS ML092190555) 

Chapter 14 

FAQ 08-0047 Spurious Operation 

Probability 

NRC Closure Memo, 12/4/08 (Ref. NRC 

ADAMS ML082950750 and 

ML082770662) 

Chapter 15 

FAQ 08-0051 Hot Short Duration NRC Closure Memo, 4/1/10 (Ref. NRC 

ADAMS ML100900052) 

Chapter 16 

FAQ 08-0052 Transient Fires – Growth 

Rates and Control Room 

Nonsuppression 

NRC Closure Memo, 08-04/09 (Ref. NRC 

ADAMS ML092120501) 

Chapter 17 
Introduction 

1-5 

The remainder of this report is structured as follows: 

• Chapter 2: Application of the Fire PRA Methods to NFPA 805 Change Evaluation 

• Chapters 3 through 17: FAQ 06-0016 through FAQ 08-0052 

2-1 

2 

APPLICATION OF FIRE PRA METHODS TO NFPA 805 

CHANGE EVALUATION 

NFPA 805 allows for engineering analysis to be performed in support of plant changes. 

This engineering analysis includes the performance of Fire Modeling and Fire PRA (FPRA) 

calculations. The Fire Modeling portion of the NFPA 8052

 is applicable to risk-informed 

decision-making only as it supports the FPRA and therefore it is not covered in this chapter. 

The risk analysis performed using FPRA can be addressed by the requirements in the ASME 

PRA Standard, Part 4, “Requirements for Fires At-Power PRA,” henceforth referred to as the 

“FPRA Standard” [6]. The requirements that should be addressed are highly dependent on the 

analysis being performed for the change evaluation, and as such, may not involve the full FPRA. 

The degree to which a requirement must be met is dependent on its significance in determining 

the risk change; therefore, the needed capability category (i.e., the extent to which the level of 

detail, plant specificity, and realism of plant response are modeled) is dependent on the 

significance of the resulting risk for the change. 

An FPRA prepared in support of transition to the NFPA 805 rule has as its primary objective the 

demonstration that the change in plant risk is acceptable to the Authority Having Jurisdiction 

(AHJ). Consequently, screening and conservative treatments that may not necessarily satisfy the 

requirements of the FPRA Standard may be fully adequate in the context of an NFPA 805 

application. In addition, it should be noted that lower Capability Categories (CCs) in the 

Supporting Requirements (SRs) of the FPRA Standard [6] do not always result in conservative 

risk estimates (e.g., higher CDFs), while in some cases the higher CCs may yield the higher 

CDFs.3

 This issue needs to be considered when performing an analysis to ensure the risk is 

acceptable to the AHJ. 

 

2

 Fire Modeling is described in NFPA 805 and the supporting NEI Implementation Guide NEI 04-02. NFPA 805 and 

NEI 04-02 require that all change analysis shall also include an assessment of risk. Fire Modeling is performed to 

identify and define fire scenarios that would require consideration in the risk assessment. This Fire Modeling option 

includes the comparison of the Limiting Fire Scenario (LFS) to the Maximum Expected Fire Scenario (MEFS), as 

described in NEI 04-02 Appendices D and J. 

3

 Note that there may be offsetting factors. Increased rigor is likely to add more mitigation features as well, so credit 

is being given for more equipment to respond to the fire (rather than assuming the equipment is failed for every 

compartment). As the equipment selection process (overall) heads for Capability Category III, a more realistic 

assessment of the risk will occur, which may not necessarily lead to higher CDF and LERF. However, in some cases 

a higher CDF or LERF may result, and this may be the most realistic. 

3-1 

3 

IGNITION SOURCE COUNTING GUIDANCE FOR 

ELECTRICAL CABINETS (FAQ 06-0016) 

3.1 Background 

FAQ 06-0016, Revision 0, was proposed to clarify the guidance from EPRI 1011989, 

NUREG/CR-6850, “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities,” 

on counting electrical cabinets and panels for NFPA (National Fire Protection Association) 805 

transition applicants. 

EPRI 1011989, NUREG/CR-6850 recommends cabinets to be counted by “visible” vertical 

sections assuming a “typical” electrical cabinet configuration. The available guidance, however, 

did not clarify how to count cabinets departing from this typical configuration that can be found 

in commercial nuclear plants. Consequently, further guidance was necessary to provide such 

clarifications. It should be noted that panel counting impacts only the fire frequency assigned to 

an individual panel or group of panels. Hence, the ultimate criterion for partitioning of the fire 

frequency is the relative number of ignition sources present. The counting guidance does not, in 

particular, have anything to do with the potential for fire spread between vertical sections or out 

of a panel. 

The existing guidance in EPRI 1011989, NUREG/CR-6850 is based on industry data that has 

only been provided with fidelity adequate to support plant-level ignition frequencies for 

electrical cabinets. Although the guidance does address the broad applicability of the data, it 

leaves room for variability that can create issues with PRA quality. It is important that the 

ignition frequency results be of sufficient quality to support not only NFPA-805 transition but 

also the broader scope of regulatory inspection and enforcement issues related to FPRA. The 

guidance proposed will provide more consistency when determining plant-specific electrical 

cabinet ignition frequencies while working within the bounds of the existing data provided by 

EPRI 1011989, NUREG/CR-6850. This should facilitate the review and acceptability of the 

results. 

The guidance provided in EPRI 1011989, NUREG/CR-6850 for Task 6, Fire Ignition Frequency 

(Section 6.5.6, Bin 15), states: 

Bin 15 – Electrical Cabinets (Plant-Wide Components): Electrical cabinets represent such 

items as switchgears, motor control centers, DC distribution panels, relay cabinets, 

control and switch panels (excluding panels that are part of machinery), fire protection 

panels, etc. Electrical cabinets in a nuclear power plant vary significantly in size, 

configuration, and voltage. Size variation range from small-wall mounted units to large 
Ignition Source Counting Guidance for Electrical Cabinets (FAQ 06-0016) 

3-2 

walk-through vertical control cabinets, which can be 20’ to 30’ long. The configuration 

can vary based on number of components that contribute to ignition, such as relays and 

circuit cards, and combustible loading, which also affects the fire frequency. Voltages in 

electrical cabinets vary from low voltage (120 V) panels to 6.9 kV switchgears. Even 

though it is expected that these features affect the likelihood of fire ignition, from a 

simple analysis of the event data involving the electrical cabinets, it was determined that 

the variation by cabinet type did not warrant separate frequency evaluation. Therefore, 

one fire frequency was estimated for the electrical cabinets. 

This issue affects only general electrical cabinets and panels. In the case of switchgears, load 

centers, unit substations, and motor control centers the term “segment” was uniformly interpreted 

to be equal to the individual vertical sections that define these types of components. As applied 

to general electrical cabinets and panels, the term “segments” could be interpreted to mean 

different metrics: 

• A segment could be defined as an enclosed element that is generally independent of size or 

volume (also referred to as a vertical section). 

• A segment could be defined as an individual section of an enclosure regardless of whether it 

was fully enclosed. 

• A segment could be defined based on a “standard” or reference sample panel size. 

Depending on the metric being used, the counting of electrical cabinets would result in varying 

results and consequently, different fire ignition frequency values. While EPRI 1011989, 

NUREG/CR-6850 allows the establishment of plant-specific criteria for counting of electrical 

cabinets, additional guidance is required to achieve a consistent basis for determining the ignition 

frequencies. 

3.2 Resolution 

The recommendations in EPRI 1011989, NUREG/CR-6850 consider it to be impractical to count 

ignition sources inside cabinets (e.g., circuit cards, relays, cable terminations and junctions, etc.) 

directly because (1) the number of ignition sources present in an entire plant is quite large and 

(2) in most cases the analyst will not be free to open multiple panels for routine inspection of 

contents. Hence, the guidance was written in such a way that the analyst is not expected to 

examine the contents of every panel in the plant as a part of the counting process. Examining a 

sample of representative panels will likely prove useful to the analyst, but the guidance presumes 

that this will be possible for, at most, a small sample of panels. 

As an alternative, the guidance in EPRI 1011989, NUREG/CR-6850 is based on the counting of 

“vertical sections” as a surrogate for the counting of ignition sources. The overall objective is to 

strike a balance such that two banks of panels with a similar number of ignition sources would 

result in a similar panel count. 
Ignition Source Counting Guidance for Electrical Cabinets (FAQ 06-0016) 

3-3 

A generalized counting criterion for general electrical cabinets is discussed in the FAQ response 

[NRC ADAMS Accession No. ML072700475]. This proposed criterion would involve two 

elements. For switchgears, load centers, unit substations, and motor control centers the counting 

for the purposes of EPRI 1011989, NUREG/CR-6850, Task 6, Bin 15 would be based on vertical 

section. This counting is illustrated in the following examples. 

Figure 3-1 

Counting of electrical cabinets 

For general electrical cabinets and panels, counting is based on externally apparent vertical 

sections, regardless of the number of horizontal divisions within a vertical section (e.g., ignoring 

the three horizontal divisions in each of the four vertical sections of the second example above). 

No examination of the internal construction is required. 

This proposed counting for electrical cabinets and panels is to be applied for a wide range of 

panel sizes. However, recognizing that the ignition frequency is more a function of the cabinet 

contents than the cabinet size, a basis is needed to address outlier conditions. It is proposed that 

each user be required to establish criteria for identifying the outliers and the basis for counting 

them. As an example, they can be counted by establishing a nominal “standard” or reference 

cabinet size. The count could also be based on evaluating the cabinet internals relative to a 

defined “standard” or reference configuration. For example, a particular user may define a 

cabinet with any horizontal dimension more than 8 feet as an outlier, and a “standard” cabinet as 

being nominally 4 feet in length x 3 feet deep. (Cabinet height is not generally an issue based on 

the use of vertical sections.) Using this example, the following cabinet and panel examples 

would be counted as follows: 
Ignition Source Counting Guidance for Electrical Cabinets (FAQ 06-0016) 

3-4 

Figure 3-2 

Counting of electrical cabinets 
Ignition Source Counting Guidance for Electrical Cabinets (FAQ 06-0016) 

3-5 

The intent is that a basis for the counting of outliers is required. A volumetric comparison is not 

required. Also, to prevent any appearance that this treatment is intended to be based on physical 

measurements, the proposed approach allows only integer counting. The assignment of fractional 

values would not be allowed. In addition, the proposed methodology retains the option for 

screening small cabinets resulting in a count of zero for them (as discussed in EPRI 1011989, 

NUREG/CR-6850). As applied in this case, the user would be allowed to screen cabinets or 

panels based on defined criteria and exclude them from the overall population count. When 

performing detailed fire modeling, the fire should be applied to the actual cabinet footprint by 

vertical section, including outliers. 

4-1 

4 

IGNITION SOURCE COUNTING GUIDANCE FOR HIGHENERGY ARCING FAULTS (HEAFS) (FAQ 06-0017) 

4.1 Background 

The original Frequently Asked Question (FAQ) 06-0017, Revision 0, was proposed to clarify the 

guidance from EPRI 1011989, NUREG/CR-6850, “EPRI/NRC-RES Fire Probabilistic Risk 

Assessment Methodology for Nuclear Power Facilities,” on counting high-energy arcing faults 

(HEAFs) for NFPA-805 transition applicants. Subsequently, this FAQ was split into two parts. 

Part one, labeled as Revision 1 to FAQ 06-0017 and the subject of this chapter, addresses the 

guidance for counting HEAFs associated with switchgear and load centers. Part two, the subject 

of FAQ 07-0035, addresses the guidance for counting HEAFs associated with bus ducts and 

junctions. This is treated separately in Chapter 7. 

It is important that the ignition frequency results be of sufficient quality to support not only 

NFPA-805 transition but also the broader scope of regulatory inspection and enforcement issues 

related to FPRA. The guidance proposed will provide more consistency when determining plantspecific electrical cabinet ignition frequencies while working within the bounds of the existing 

data provided by EPRI 1011989, NUREG/CR-6850. This should facilitate the review and 

acceptability of the results. 

The guidance provided in EPRI 1011989, NUREG/CR-6850 for Task 6, Fire Ignition Frequency 

(Section 6.5.6, Bin 16), states: 

Bin 16 – High-Energy Arcing Faults (Plant-Wide Components): High-energy arcing 

faults are associated with switchgear and load centers. Switchyard transformers and 

isolation phase buses are not part of this bin. For this bin, similar to electrical cabinets, 

the vertical segments of the switchgear and load centers should be counted. Additionally, 

to cover potential explosive failure of oil filled transformers (those transformers that are 

associated with 4.16 or 6.9kV switchgear and lower voltage load centers) may be 

included in vertical segment counts of the switchgear. 

Pilot discussions and benchmarking of EPRI 1011989, NUREG/CR-6850 for Task 6, Fire 

Ignition Frequency, has shown inconsistency in the treatment of high-energy arcing faults 

(Bin 16). Strict interpretation of the guidance is that the HEAF count should mimic the electrical 

cabinet counts for switchgear and load centers. The industry experience and consequently the 

HEAF frequency are based on 3 events occurring on medium-voltage switchgears and 1⁄2 event 
Ignition Source Counting Guidance for High-Energy Arcing Faults (HEAFs) (FAQ 06-0017) 

4-2 

occurring on a 480VAC load center. Because of the relative numbers of switchgears and load 

centers at an individual plant, it is expected that the resultant frequency may be inappropriately 

skewed. 

4.2 Resolution 

The state of knowledge regarding HEAF fires continues to evolve. New insights developed since 

publication of the EPRI 1011989, NUREG/CR-6850 do indicate that an adjustment of fire 

frequencies between low- and medium-voltage equipment is warranted. 

The electrical power community has gained significant knowledge about HEAFs since 

publication of EPRI 1011989, NUREG/CR-6850. The increased awareness and knowledge base 

was driven by adoption of new arc flash protection requirements in NFPA 70E, Standard for 

Electrical Safety Requirements for Employee Workplaces. Discussions with experts close to the 

subject, including a member of the IEEE 1584 standards committee (Guide for Performing ArcFlash Hazards Calculations), revealed that recorded events of HEAFs are actually dominated by 

incidents involving 480V gear. The experts confirm that the higher incidence of 480V events is 

partially attributable to the greater population of installed 480V equipment. However, other 

overlapping factors are also important: 

• A majority of arc flash events are initiated by human error. 

• Low-voltage equipment is worked on/operated more frequently than medium-voltage 

equipment. 

• Workers have a more casual attitude when working on 480V gear, i.e., everyone knows that 

you will probably not get a second chance if you make a mistake working on mediumvoltage equipment but they tend to perceive 480V gear as less threatening. Additionally, it is 

more probable that 480V equipment will be worked “hot”; that is, worked on while the 

equipment is energized. 

• Basic design attributes of medium-voltage gear decrease the likelihood of initiating a 

sustained arcing fault. Key elements include insulated bus bars in lieu of open bus bar work, 

barrier protection, compartmentalization between phases, and increased creepage distances. 

• Arcing faults do occur on 208V systems; however, sustained arcing faults at 208V are rare 

and difficult to reproduce. 

With these observations in mind, the intent of the HEAF analysis (per Appendix M of EPRI 

1011989, NUREG/CR-6850) is to capture “higher-consequence” events that may have a 

substantive impact outside the cabinet of origin. Other arc fault events (e.g., events that did not 

lead to an impact outside the originating panel) are already treated via the general electrical panel 

fire frequency, and this treatment need not be adjusted. Only the “higher-consequence” events 

are under question here. 
Ignition Source Counting Guidance for High-Energy Arcing Faults (HEAFs) (FAQ 06-0017) 

4-3 

Another observation that is evident from the event records amassed by the IEEE standard groups 

is that, even though the general incidence of arc faults in low-voltage equipment may actually be 

higher, the fraction of such events leading to substantive impacts outside the initiating cabinet 

(i.e., higher-consequence events) is actually lower than for similar incidents in medium-voltage 

equipment. In essence, if a sustained arc fault occurs in a 4.16 kV switchgear, the fault will very 

likely have an impact beyond the limits of the panel. In contrast, an arc fault in a low-voltage 

panel is more likely to remain confined to the panel and less likely to have impact beyond the 

panel. This rationale is supported by standardized arc flash calculations; equivalent stand off 

distances are typically greater for medium-voltage equipment, given normal and customary 

overcurrent protection. 

This contention is consistent with both the broader industry experience and with the specific 

nuclear industry experience as cataloged in Appendix M of EPRI 1011989, NUREG/CR-6850. 

That is, the frequency analysis included three events in medium-voltage equipment, and only 1⁄2 

of an event (i.e., one uncertain event) for low-voltage equipment. This assessment included 

consideration of whether each reported event actually had impact outside the panel of origin. 

There are many other low-voltage panel fire events that appear to have involved some degree of 

arc-flash, but that also remained confined to the panel of origin. 

The proposed resolution to the underlying issue raised in the FAQ [NRC ADAMS Accession 

No. ML072500300] is to split fire ignition frequency Bin 16, HEAF, into two bins; namely, “16a 

– HEAF for low-voltage panels (480-1000V)” and “16b – HEAF for medium-voltage panels 

(greater than 1000V).” For each bin, the method of panel counting would then stand unchanged 

(i.e., count vertical sections). Given the split into two bins, the counting method—and hence the 

fire frequency apportioning process—need to be self-consistent within each of the two new bins, 

but there is no longer any cross-over between the low- and medium-voltage equipment. This also 

maintains consistency with the counting method for general thermal fires (i.e., the non-HEAF 

panel fires that must also be treated), which is also a highly desirable feature so that the analyst 

need not maintain two separate population counts for the same set of fire ignition sources. The 

net result is a repartitioning of the “higher-consequence” HEAF events between low-voltage and 

medium-to-high-voltage equipment in accordance with the event data. The revised annual fire 

frequencies for these two new bins are as follows: 

16a: HEAF for Low-Voltage Panels 

(480–1000 V) 

16b: HEAF for Medium-Voltage Panels 

(above 1000 V) 

Mean 4.8E-04 Mean 1.4E-03 

Variance 1.4E-03 Variance 1.2E-02 

5% 1.6E-05 5% 3.8E-05 

50% 2.0E-04 50% 6.2E-04 

95% 1.5E-03 95% 4.1E-03 
Ignition Source Counting Guidance for High-Energy Arcing Faults (HEAFs) (FAQ 06-0017) 

4-4 

In the course of providing the above response, the related issue of whether or not motor control 

centers (MCCs) should be included as potential sources of HEAFs along with the switchgear and 

load centers when counting sources was considered. While not explicitly mentioned in EPRI 

1011989, NUREG/CR-6850, inclusion of MCCs when counting HEAF sources is explicit in the 

Fire Protection Significance Determination Process (Inspection Manual Chapter 609F). 

Consensus was reached regarding this, with the following guidance: 

Only MCCs with switchgear that is used to directly operate equipment such as load 

centers should be counted as HEAF sources. 
5-1 

5 

IGNITION SOURCE COUNTING GUIDANCE FOR MAIN 

CONTROL BOARD (MCB) (FAQ 06-0018) 

5.1 Background 

Frequently Asked Question (FAQ) 06-0018 was proposed to clarify the guidance from EPRI 

1011989, NUREG/CR-6850, “EPRI/NRC-RES Fire Probabilistic Risk Assessment Methodology 

for Nuclear Power Facilities,” on counting “main control boards.” 

The scope of FAQ 06-0018 is the main control board, which is located in the main control room. 

The main control room contains electrical cabinets, identified as (Bin 15) plant-wide components 

in EPRI 1011989, NUREG/CR-6850 for the purposes of determining fire ignition frequency, in 

addition to the main control board. The recommendation in this FAQ is that the definition of the 

main control board provided in EPRI 1011989, NUREG/CR-6850, Appendix L, be accepted as 

also being applicable for Task 6, Bin 4 counting (i.e., main control board), given clarification 

regarding the relationship between Appendix L of EPRI 1011989, NUREG/CR-6850 and Task 6, 

Bin 4, as follows: There is a one-to-one correspondence between the main control board as 

discussed in Appendix L and Task 6, Bin 4, in EPRI 1011989, NUREG/CR-6850. 

The guidance provided in EPRI 1011989, NUREG/CR-6850 for Task 6, Fire Ignition Frequency, 

is subject to application inconsistency in the treatment of the main control board (Bin 4). The 

guidance for Task 6 does not provide any specific definition or characterization of what 

constitutes a main control board (MCB) other than a reference to it being the central element of 

the room. A discussion among the NFPA-805 pilot plants that included consideration of other 

plants in their respective fleets found wide variability in the configuration of the main control 

room. There was a concern that inconsistent treatment of this bin would unnecessarily challenge 

the completion and review of the FPRA. This challenge would be manifested by a notable 

change in the fire frequency assigned to an individual panel, depending on whether it was 

counted as Bin 4 or Bin 15. 

5.2 Resolution 

As discussed in the resolution to this FAQ [NRC ADAMS Accession No. ML072500273], the 

intent of the guidance in EPRI 1011989, NUREG/CR-6850 was to sharply limit the scope of the 

panels to be included in the main control board ignition frequency bin. The main intent was to 

capture the main “horseshoe” and little else. For many plants, the main control board will be the 

main horseshoe and nothing else. This is important given that fires in the main control room that 

occur outside the main horseshoe were binned with the general electrical panel fires and not with 
Ignition Source Counting Guidance for Main Control Board (MCB) (FAQ 06-0018) 

5-2 

the main control board. Changing the definition of a fire frequency bin (i.e., what goes into a 

particular bin) creates an inconsistency with the binning of events (in Chapter 6 of EPRI 

1011989, NUREG/CR-6850) and the resulting fire frequency estimates. 

The additional wording provided in Appendix L of EPRI 1011989, NUREG/CR-6850 (the bullet 

list on page L-2) was intended to allow for some flexibility given the wide variability among control 

rooms around the country. The guidance was not intended to open the door to inclusion of more than 

a small handful of other control room panels. Any panel that is detached from the main horseshoe 

would generally be excluded from this definition of the main control board with few exceptions. 

An exception of the above described guidance may include “bench-board” panels that were detached 

from, but directly in front of, the main horseshoe (at some plants such panels are referred to as 

“consoles”). These bench-board-type cabinets, usually one or two per control room, may be counted 

as part of the main control board. These panels were (1) serving as an integral part of the main plant 

monitoring and control functions; (2) located in the center of the operators’ main work area; and 

(3) manned on a nearly continuous basis.4

Plants may have numerous smaller detached panels housing such equipment as computers and the 

event recording equipment and printers. These panels typically are in full view of the operators 

(generally behind or to the side of their main work area). Nonetheless, they should not be treated as a 

part of the main control board, because they are clearly and distinctly detached from the main control 

board and serve unique functions. 

There may also be numerous “back panels” and other detached panels housing items such as balanceof-plant and off-site power controls and indicators. All of these panels should be excluded from the 

main control board and treated as general electrical panels. In general, the definition of the main 

control board is intended to sharply limit the scope of that bin to the main horseshoe and, under 

certain circumstances, a very small number of other detached panels. The intent is to treat the vast 

majority of the detached panels, and any “back” panels, as general electrical panels, not as a part of 

the main control board.

 

4

 Conditions specified in the example in the FAQ resolution 
6-1 

6 

MISCELLANEOUS FIRE IGNITION FREQUENCY 

BINNING ISSUES (FAQ 07-0031) 

6.1 Background 

Frequently Asked Question (FAQ) 07-0031 was proposed to clarify the guidance found in EPRI 

1011989, NUREG/CR-6850 [3] regarding counting for miscellaneous ignition source bins 

described in Chapter 6 of that reference. The relevant bins are (1) Bin 14 – Electric Motors, 

(2) Bin 21 – Pumps, (3) Bin 23 – Transformers, and (4) Bin 26 – Ventilation Subsystems. The 

general intent of the suggested changes was to ensure a higher level of consistency between four 

specific fire ignition source bins associated with electrical equipment (motors, pumps, ventilation 

subsystems, and transformers). The changes described in the chapter establish new criteria for 

eliminating certain motors and transformers from the counting process based on size (e.g., the 

associated component electrical power limits) or function. 

6.2 Resolution 

This section is divided into four subsections, one for each of the ignition frequency bins relevant 

to this chapter [NRC ADAMS Accession No. ML072840658]. 

6.2.1 Bin 14: Electric Motors 

For Bin 14, Electric Motors (Plant-Wide Components), the intent was to exclude motors 

associated with equipment counted in other bins. Specifically, the response to this FAQ includes 

the following guidance: 

Motors that are totally enclosed, including totally enclosed MOV drive motors, should be 

excluded from Bin 14 because the nature of the motor housing would prevent the extension of 

flames outside the motor casing. However, other motors (e.g., ventilated motors) do present the 

potential for fire spread outside the motor and should continue to be considered regardless of the 

motor application. Furthermore, the fire event database includes a small number of fire events 

involving MOV drive motors. Consequently, MOV drive motors that are not totally enclosed 

should be treated similarly to other general-use electric motors. 
Miscellaneous Fire Ignition Frequency Binning Issues (FAQ 07-0031) 

6-2 

The original guidance provided in Section 6.5.6 of the methodology for Bin 14 (page 6-16) can 

be clarified as follows: 

Bin 14 – Electric Motors (Plant-Wide Components): Electric motors associated with 

various plant equipment such as elevators, valves, etc., with the following clarifications: 

• This bin includes any electric motor with a rating greater than 5 hp unless the motor 

meets one (or both) of the two exclusionary provisions immediately below. The bin 

excludes motors with a rating of 5 hp or less regardless of motor application. 

• This bin excludes electric motors that are attached to equipment already identified 

and counted in other bins (i.e., reactor coolant pumps, air compressors, dryers, 

pumps, RPS MG sets, and ventilation subsystems). That is, motors associated with a 

piece of equipment counted as a part of another ignition source bin are not counted 

separately as motors, but rather, are considered an integral part of the larger 

equipment item (the pump, the compressor, etc.). 

• This bin excludes any motors, including MOV drive motors, which are totally 

enclosed regardless of the motor size. A totally enclosed motor is defined by the 

National Electrical Manufacturers Association (NEMA) as “a motor designed 

without air openings so there is no free exchange of air between the inside and 

outside of the enclosure but not necessarily air or water tight” (Reference: NEMA 

MG 2-201, Rev. 1, 2007, “Safety Standard and Guide for Selection, Installation, and 

Use of Electric Motors and Generators”). Specifically, motors meeting the following 

NEMA classifications are excluded from the motor counting process and are not 

considered as ignition sources: totally enclosed machines; totally enclosed nonventilated; totally enclosed fan-cooled; totally enclosed pipe-ventilated; totally 

enclosed water-cooled; and explosion-proof.” 

6.2.2 Bin 21: Pumps 

For Bin 21, Pumps (Plant-Wide Components), the clarification resulting from this FAQ indicates 

that excluding pumps associated with smaller (5 hp or less) hydraulic actuators is appropriate. 

The guidance relative to Bin 21 as provided in Section 6.5.6 is clarified as follows: 

Bin 21 – Pumps (Plant-Wide Components) and large hydraulic valves: For this 

methodology, it is assumed that above a certain size, fire ignition is the same for all 

pumps. Pumps with a rating of 5 hp or less are assumed to have little or no significant 

contribution to risk. The number of larger pumps (>5 hp) in all plant locations defined as 

“plant-wide” should be estimated. 

• This bin excludes small sampling pumps. 

• This bin excludes pumps with a rating of 5 hp or less. This bin includes pumps rated 

greater than 5 hp. 

• This bin excludes pumps associated with hydraulic actuators where the pump is rated 

5 hp or less. The bin includes pumps associated with larger hydraulic actuators where 

the pump is rated above 5 hp. 
Miscellaneous Fire Ignition Frequency Binning Issues (FAQ 07-0031) 

6-3 

6.2.3 Bin 23: Transformers 

As originally written, Bin 23, Transformers (Plant-Wide Components), did not establish a lower 

limit for the inclusion/exclusion of transformers. The response to the FAQ suggests that this bin 

only include transformers with a power rating above 45 kVA for dry-type transformers. The 

FAQ response is based primarily on a review of the combustible content of smaller transformers. 

Dry transformers with a rating of 45 kVA or less are expected to have insufficient content of 

combustible material to produce a challenging fire as defined in Chapter 6 of EPRI 1011989, 

NUREG/CR-6850. In the event of an insulation breakdown, significant electrical energy could 

be released (i.e., arcing inside the transformer). However, given the small amount of combustible 

materials within the transformer (mainly the varnish used to coat the windings), it is expected 

that a significant fire escaping the transformer’s housing is very unlikely. A potentially 

challenging fire is defined as one with the potential to spread beyond the bounds of the initiating 

component or to represent a direct threat to other plant equipment or cables. Small dry 

transformers would not appear to hold this potential. 

Note that the original wording of the ignition source bin in Chapter 6 of EPRI 1011989, 

NUREG/CR-6850 had already excluded “small lighting transformers.” The revision discussed 

immediately above will now exclude transformers with a rating of 45 kVA or less and will 

supersede the prior guidance. In order to avoid confusion and potential conflicts, the original 

wording associated with the exclusion of “small lighting transformers” has been deleted from the 

revised bin description provided below. The 45 kVA criterion applies to any dry-type 

transformers, including lighting transformers. 

The FAQ, however, concludes that the same arguments would not apply to oil-filled 

transformers. These transformers should be included in the count regardless of the power rating. 

Any oil-filled transformer has a sufficient combustible content (i.e., the oil) to represent a 

potentially challenging fire in the event of transformer failure. In practice, smaller, indoor-type 

transformers are generally not of an oil-filled design. The guidance relative to Bin 23 as provided 

in Section 6.5.6 is modified and clarified as follows: 

Bin 23 – Transformers (Plant-Wide Components): This bin nominally includes any 

indoor transformer that is not an integral part of a larger component. In particular, all drytype transformers with a rating greater than 45 kVA and all oil-filled transformers are 

included in this bin. Examples of transformers accounted for in this bin include: 

4160V/480V station service transformers attached to AC load centers; low voltage 

regulators; and 480V/208-120V auxiliary service transformers. The large yard 

transformers are not part of this bin. The number of indoor transformers should be 

estimated with the following clarifications: 

• This bin excludes control power transformers and other small transformers, which are 

subcomponents in electrical equipment. These small transformers are assumed to be 

an integral part of the larger component. 

• This bin includes all indoor, oil-filled transformers regardless of size. 
Miscellaneous Fire Ignition Frequency Binning Issues (FAQ 07-0031) 

6-4 

• This bin excludes dry-type transformers with a rating of 45 kVA or less. The bin 

includes all indoor dry-type transformers with a rating greater than 45 kVA. 

• This bin includes wall-mounted transformers, unless they satisfy the 45 kVA 

exclusionary criteria immediately above. 

6.2.4 Bin 26: Ventilation Subsystems 

For Bin 26, Ventilation Subsystems (Plant-Wide Components), the FAQ response indicates that 

some clarification is appropriate. In particular, the original intent in EPRI 1011989, NUREG/ 

CR-6850 was to exclude ventilation subsystems with very small motors consistent with Bin 14. 

In this case, the modification is simply a rewording of the existing guidance to make it more 

consistent with the other related bins. The guidance relative to Bin 26 as provided in 

Section 6.5.6 is clarified as follows: 

Bin 26 – Ventilation Subsystems (Plant-Wide Components): This bin includes 

components such as air conditioning units, chillers, fan motors, air filters, dampers, etc. A 

fan motor and compressor housed in the same component are counted as one component. 

The total number of ventilation subsystems should be estimated with the following 

clarification: 

• This bin excludes ventilation subsystems (e.g., fans, filter banks, or compressors) 

driven by an electric motor rated 5 hp or less. The bin includes any ventilation 

subsystem with an electric motor greater than 5 hp. 
7-1 

7 

BUS DUCT (COUNTING) GUIDANCE FOR HIGHENERGY ARCING FAULTS (FAQ 07-0035) 

7.1 Background 

FAQ 07-0035 requests clarification regarding the treatment of high-energy arc faults (HEAFs) 

specific to bus duct failures. Appendix M of the consensus methodology document (EPRI 

1011989, NUREG/CR-6850) provides guidance for the treatment of HEAFs in switchgear and 

load centers, but does not cover the treatment of bus duct fires. Further, while the document 

mentions bus ducts as a potential source of HEAF events, no fire event frequency or treatment 

guidance is provided. This FAQ is a split from FAQ 06-0017 (see Chapter 4). 

7.2 Resolution 

The resolution of FAQ 07-0035 [NRC ADAMS Accession No. ML091620572] states that 

review of the EPRI fire event database used in the development of EPRI 1011989, NUREG/ 

CR-6850 identified several events involving bus duct arc fault failures. This event set was 

supplemented by additional nuclear power plant (NPP) fire events identified by the NRC staff as 

potentially relevant to the bus duct arc fault question. All of the identified events were reviewed 

for relevance to the topic of this FAQ, and a final set of fire events to be used in calculating fire 

frequency was identified (see further discussion below). In addition, a public meeting was held 

(August 2007, ADAMS ML072560081) to discuss the team’s preliminary insights and to gain 

additional input from stakeholders. The proposed resolution is based on, and consistent with, all 

of the input received from these resources. 

7.2.1 Summary 

Intended scope of the bus duct analysis: 

The guidance provided here applies to any electrical power distribution bus associated with the 

equipment already identified in Appendix M of EPRI 1011989, NUREG/CR-6850 as subject to 

HEAF failures and fires. That is, the guidance applies to power distribution buses associated with 

switchgear, load centers, and motor control centers of 440 V or greater. The guidance also 

applies to bus ducts associated with turbine generator output (the iso-phase bus), unit main, 

auxiliary and start-up transformers, and the unit emergency generators and their associated 

power transformers. 

Classification of bus ducts by type: 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-2 

A review of bus duct physical configurations as typically used in power plant applications has 

resulted in a recommended practice that would first classify each bus duct as falling into one 

(and only one) of the following four general categories: 

Category 1: Nonsegmented or continuous bus ducts: A bus duct where the bus bar 

associated with each power phase is comprised of a single length of metal bar connecting two 

end-devices (e.g., terminating within a cabinet or at a specific piece of electrical equipment) 

with no intermediate junctions, transitions, or terminations along the length of the bus bars. 

Typically, the bus bars are wholly contained within a grounded metal enclosure that runs the 

full length of the distance between the termination points. 

– Nonsegmented bus ducts tend to be comparatively short (on the order of no more than 

12 feet in length) due to practical limits on the length of a single segment of bus bar. 

Examples: 

o A bus duct connecting a station service transformer to the associated switchgear 

cabinets where the transformer and switchgear are located in close proximity within a 

common fire area. 

o A bus duct connecting two separate banks of load cabinets located in close proximity 

(e.g., across an intermediate access walkway) and fed from a common power source. 

Category 2: Segmented bus ducts: A bus duct where the bus bars are made up of multiple 

sections bolted together at regular intervals (transition points). Here, the bus bars are 

contained within open-ended sections of metal covers that are bolted together to form a 

continuous grounded enclosure running the full distance between termination points. 

Segmented bus ducts are able to accommodate tap connections to supply multiple equipment 

termination points. 

– Segmented bus ducts tend to be longer in comparison to the nonsegmented bus ducts. 

Segmented bus ducts are used in cases where the required lengths and/or geometries 

make the use of nonsegmented bus ducts impractical. 

– The length of each segment may vary depending on supplier and installation details. 

– Segmented bus ducts tend to connect end devices that are remote from each other. 

Example: A segmented bus duct might be used to connect an oil-filled transformer 

located in an outdoor area to equipment (e.g., switchgear) located inside the plant 

buildings. 

Category 3: Cable ducts: A power conductor configuration that provides a function like a 

bus duct but uses a length of insulated electrical cable in lieu of metal bus bars. Cable ducts 

may be routed in a variety of ways, not necessarily within continuous runs of metal 

enclosures. 

– Cable ducts can be as long as, or longer than, a segmented bus duct because there is no 

practical limit to the length of cable that can be obtained and installed. 

– Cable ducts may be used in application conditions similarly to either a segmented or 

nonsegmented bus duct. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-3 

Category 4: Iso-phase bus ducts: A bus duct where the bus bars for each phase are 

separately enclosed in their own protective housing. The use of iso-phase buses is generally 

limited to the bus work connecting the main generator to the main transformer. 

7.2.1.1 Selection of Bus Duct Fire Scenarios 

A review of the experience base for all types of bus ducts revealed one common characteristic; 

namely, that all of the identified bus duct arc fault events occurred either at the termination point 

of the duct or at a transition point along the length of a segmented bus duct. With the exception 

of the iso-phase bus ducts (category 4), in those events occurring at the termination point, all had 

been included in the “high energy arc fault (switchgear and load centers)” or “catastrophic failure 

(transformers)” event sets for the end devices as fire ignition sources in EPRI 1011989, 

NUREG/CR-6850. Hence, these events are already accounted for in the methodology and are 

treated as originating in the end device. Because nonsegmented bus ducts (category 1) and cable 

ducts (category 3) have no transition points other than the terminations at the end device, no 

treatment of bus duct faults/fires independent from the treatment of fires for the end devices is 

required. That is, arc faults for these two categories of bus ducts, 1 and 3, are inherently included 

in the treatment of the end device, and no further treatment is needed. 

A review of available data indicates that events associated with iso-phase bus ducts (category 4) 

also manifest themselves at the termination points (i.e., the main generator or main transformer), 

but these events had not been included in the associated end device frequencies. The potential 

effects of the iso-phase faults also appear unique in comparison to the end device (transformer 

or exciter) fires as recommended in the existing guidance. Hence, some additional treatment for 

iso-phase bus duct faults occurring at the termination points is needed. For segmented bus ducts 

(category 2), a number of the identified fire events were manifested at bus transition points 

(a point where two segments of the bus duct are bolted together) rather than at the bus 

termination points. These events were generally attributed to loose bolted connections, to failed 

insulators, or to the accumulation of dirt/debris/contaminants in the bus duct. The key, however, 

is that the effects of the fault are manifested at transition points along the bus duct length. Fire 

scenarios for segmented bus ducts should, therefore, be postulated to occur at duct transition 

points (i.e., bolted connections). An alternate treatment is provided if the transition points cannot 

be easily identified based on external inspection. 

7.2.1.2 Ignition Source Counting Guidance 

Counting guidance: iso-phase bus ducts: For iso-phase bus ducts, there should generally be 

one iso-phase bus per unit (an iso-phase bus includes all three phases). If there is more than one 

iso-phase bus, simply count the total number of iso-phase buses per unit. For individual fire 

scenarios, the plant-wide frequency is applied (i.e., partitioned) equally to each end of each isophase duct counted. The plant-wide fire frequency and zone of influence are discussed below. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-4 

Counting guidance: segmented bus ducts: The analyst will need to choose between one of two 

recommended practices for counting segmented bus ducts as a fire ignition source. The choice 

will be dependent on whether or not the transition points can be identified based on an external 

visual inspection of the bus duct. 

Counting approach 1: If the transition points along the length of the segmented bus duct can be 

identified by external visual inspection, or based on plant electrical construction drawings, then 

count the total number of transition points. Note that transition point counting excludes the bus 

end termination points, which are considered a part of the end device for fire frequency purposes. 

Transition points may be identifiable based on visual observation or review of design drawings. 

Transition points for the bus bars may, or may not, correspond to junctions in the outer ducting 

that surrounds the bus bars. It is not intended that the protective duct be removed to identify 

transition points. 

However, industry feedback indicates that the joints or junctions in the outer ducting surrounding 

a bus duct cannot be assumed to correspond to junctions in the bus bars themselves without 

confirmation. A representative sample of plant applications should be inspected to ensure that the 

internal bus bar transition points and external duct junctions do in fact align with each other. 

Once the total count of transition points has been obtained, the plant-wide fire frequency is then 

partitioned to a specific location based on the number of transition points in the location of 

interest divided by the total number of transition points for the entire plant. 

Counting approach 2: If the transition points cannot be identified based on external visual 

inspection, or by plant electrical construction drawings, then the partitioning of fire frequency to 

a specific fire scenario is based on apportioning of the fire frequency equally along the length of 

the bus duct. Hence, the analysis must estimate the total length of segmented bus duct present in 

the plant under analysis. A “per linear foot” fire frequency can then be estimated by dividing the 

plant-wide fire frequency by the total length of segmented bus duct in the plant. 

That is, the fire frequency for a given fire scenario would be based on the ratio of the length of 

duct for which identified targets fall within the bus duct arc fault zone of influence (see 

discussion below for a definition of the zone of influence) to the total length of bus duct in the 

plant. A lower limit to the assumed fire frequency for any given fire scenario is also applied. 

That is, if the length of bus duct for which the identified target(s) fall within the zone of 

influence is less than 12 linear feet, then a minimum length of 12 feet should be assumed. This 

lower bound is based on the assumption that, lacking specific information on segment lengths, a 

nominal segment length of 12 feet should be assumed. Any single scenario is then assigned a fire 

frequency equivalent to that associated with one bus bar segment 12 feet in length (i.e., 

equivalent to one nominal transition point). 

Once the count and partitioning values are known, the next step is to develop fire scenarios for 

analysis. The development of fire scenarios is again expected to follow one of two potential 

approaches as outlined in the following discussions. The analyst should be aware that the second 

of these two approaches introduces a degree of uncertainty into the analysis. As noted above, arc 

faults generally occur at the transition points. When the actual location of the transition points is 

not known, the approach assumes that a fault might occur at any point along the duct length. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-5 

Hence, fire scenarios might be developed for locations where there is, in actuality, no junction 

point. By the same token, the approach partitions fire frequency equally along the length of the 

bus duct, whereas in reality faults would be more frequent at the actual (but unknown) transition 

points. It is recommended that in assessing analysis results, these observations be treated as a 

part of the uncertainty and sensitivity analyses, and that the analysis be refined for cases where 

risk-significant fire scenarios develop (i.e., by examining the bus duct to determine if any 

transition points are actually present in the segment of bus duct associated with a significant 

scenario). This is discussed further below. 

7.2.1.3 Anticipated Analysis Approach for Segmented Bus Ducts 

Approach for known transition points: If the transition points for the bus ducts are known, 

then the approach to analysis would focus on the development of scenarios at those known 

locations. Note that even when transition points are not known generally, certain locations will 

represent known transition points based on geometric factors. For example, if a horizontal duct 

changes direction (i.e., makes a flat or vertical turn) or changes elevation (e.g., a step), that 

geometric transition would represent a transition point for that bus duct. For known transition 

points, the analysis should look for fire PRA targets (i.e., fire PRA equipment and cables) within 

the zone of influence (described below) and postulate scenarios accordingly. 

Approach when transition points are not identifiable: In the case where transition points are 

not in known locations, the approach to analysis is similar but begins by assuming that a fault 

might occur at any point along the length of the bus duct. In this case, the analyst should trace 

the path of bus ducts through the plant and identify potential fire PRA targets within the bus-duct 

arc fault zone of influence (see definition below) at any point along the duct length. The 

development of fire scenarios would then depend on the relative length of bus duct for which an 

identified target set lies within the bus duct zone of influence. 

• Analysis Approach 1: Potential fire PRA targets are located within the zone of influence for a 

significant length of duct (greater than the nominal assumed segment length of 12 feet): In 

this case, an estimate of the scenario fire frequency can be based on the plant-wide fire 

frequency times the ratio of the length of duct (e.g., linear feet) for which scenario targets lie 

within the zone of influence to the total length of segmented bus duct in the plant. 

• Analysis Approach 2: A target set is identified but lies within the zone of influence for a 

limited portion of bus duct (i.e., less than the nominal assumed segment length of 12 feet): In 

this case, an initial analysis should assume that a fault occurs within that segment of the bus 

duct for which fire PRA targets might be impacted, however long it might be. The fire 

frequency assigned to the scenario is the minimum fire frequency value calculated based on a 

minimum 12 foot length of duct. 

Note that in either approach, the analysis can always be refined by examining the bus duct to 

determine if one or more transition points actually lie within the applicable bus duct segment. If 

no transition points are identified within that particular duct section, then a fault scenario need 

not be postulated and the scenario “goes away.” If one or more transition points are identified 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-6 

within a particular duct section, then the analysis can be refined based on the known locations 

(i.e., both the fire frequency and the impacted target set may be refined once transition points are 

identified). 

7.2.1.4 Fire Frequency and Frequency Partitioning 

Based on the review of the fire events database, Tables 7-1 and 7-2 list all of the identified 

“candidate” fire events used to estimate the frequencies for segmented bus ducts and for isophase ducts, respectively. The events listed were identified based on a review of the events in 

EPRI’s Fire Event Data Base (FEDB) and on a similar review of events identified in various 

NRC documents (information notices, inspection reports, LERs, and staff reports) as provided by 

the NRC staff. Each event that included a bus duct was reviewed to determine (1) if the event 

meets the general criteria associated with a “potentially challenging event” consistent with the 

treatment of other fire ignition sources and (2) if the event was indeed uniquely associated with a 

bus duct arc fault and consequential fire rather than some other fire ignition source bin. 

Table 7-1 

Segmented Bus Duct Fire Events Used in Frequency Calculations 

FEDB 

Incident 

No. 

Event 

Date 

Description 

195 4/15/80 Fire involved a supply bus located in a switchgear room. 

218 8/20/80 A short occurred on the bus work from the RAT to buses 1-1 and 1-2. This 

caused a reactor trip and turbine trip. Damage to equipment was limited to a 10 

foot section of the bus bar. In addition, insulation between the insulated bus bar 

supports experienced some cracking due to the force of the fault. Several 

nonsafety-related cables located in a cable tray adjacent to the bus experienced 

insulation failure as a result of this event. (NOTE: Information in FEDB has been 

supplemented with information provided in LER 88-001-00.)

575 3/19/87 A fault in a 6.9 kV feeder line to the in-house buses of a unit auxiliary 

transformer resulted in a fire and explosion outside the building. 

678 3/2/88 A section of the bus bar running from the mat to the bus switchgear was badly 

damaged due to insulation failure and a subsequent fault. During normal 

operations, a combination of insulation failure, debris accumulation, and 

possibly water resulted in an electrical fault in a main (4000 AMP) power bed 

bus bar. Degradation of the electric power feed resulted in a reactor trip. The 

fault was detected by a phase imbalance (differential current) alarm in the main 

control room and by reports of smoke in the turbine building basement. The fire 

brigade was called out. Deenergizing of the bus ended the fire. In addition to 

damage to the affected bus, several non-safety related cables located in a cable 

tray adjacent to the bus experienced insulation failure. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-7 

Table 7-1 (continued) 

Segmented Bus Duct Fire Events Used in Frequency Calculations 

FEDB 

Incident 

No. 

Event 

Date 

Description 

922 7/10/87 Insulation on a 4160 V bus bar failed. This condition resulted in a phase to 

ground fault, which caused extensive damage to the bus bar and a fire. Upon 

investigation, the equipment operator noticed smoke and fire coming from the 

vicinity of the electrical bus bar located on the eastern end of the 606' elevation 

of the turbine building. The bus fire terminated once the transformer was 

deenergized. A smaller fire was extinguished by the equipment operator when 

rags and rubber goods on a maintenance cart were ignited by the falling 

aluminum slag. 

2426 5/15/00 An electrical fault occurred on the 12 kV bus bars from UAT to non-vital

switchgear, resulting in a fire in the non-vital switchgear room. The fault

continued to be fed for 4-8 seconds by the decay of the main generator

electrical field during generator coast-down, contributing to catastrophic failure

of the bus bars. Security officers reported a fire at UAT 1-1, and operators

notified the fire brigade. The fire brigade arrived at the switchgear room and

determined that the fire was internal to the switchgear room and not associated

with UAT 1-1. Given the large amount of smoke, the fire brigade captain

requested off-site fire brigade support. The fire brigade extinguished a small fire

in the 12 kV bus duct with a carbon dioxide extinguisher within 17 minutes of

arriving at the switchgear room. Post-event inspection revealed that the center 

12 kV bus bar was missing for approximately 1 yard, with the two exterior bus

bars missing for approximately 6 and 9 inches. The bottom and top of the bus

duct was melted for several feet, along with sections of the duct work on the

perpendicular 12 kV bus sections at the tee connection. In addition, the fire had

burned an approximately 1 foot square hole in the bottom of a second 4 kV bus 

duct 4 inches above the 12 kV bus duct. Although the 4 kV bus bars and duct 

were covered with black soot, the only conductor damage was a small piece of 

metal missing from the center bus bar and one outer bus bar, which the 

inspectors considered to be indicative of a single phase-to-phase fault. The floor 

beneath the fault contained a large slag pile, and a great deal of metal had 

splattered on the face of 12 kV non-vital Switchgear D and E. However, no 

missiles penetrated the switchgear, and they remained energized throughout 

the event. Later inspection revealed no internal damage. Smoke patterns on the 

cabinet doors indicate that plastic components on the cabinet faces (e.g., gauge 

faces, identifier tags, indicator lamp housings) ignited and burned during the 

event.

Not in 

FEDB

5/18/83 Startup bus failed because of a phase B to phase C fault, which propagated to 

ground. Further investigation revealed several degraded areas in the bus 

insulation at the support blocks. (See NRC Information Notice 89-64 for 

information on this event.) 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-8 

Table 7-2 

Iso-Phase Bus Duct Fire Events Used in Frequency Calculations 

FEDB 

Incident 

No. 

Event 

Date 

Description 

792 7/15/88 Arcing and fire were observed at a 22 kV iso-phase bus duct due to damaged 

ground straps and a deteriorated gasket between the cover and the duct. 

929 10/9/89 Multiple ground faults caused by aluminum debris in an iso-phase bus duct 

started a chain of events that led to three separate fires: (1) an oil fire in the 'B' 

main power transformer, (2) a hydrogen fire under the main generator, and (3) a 

small oil fire in the generator housing. In addition to site fire brigade, off-site fire 

departments were contacted to assist. 

Not in 

FEDB

6/18/04 An electrical fault in the one phase of the iso-phase bus lead to a fire near the 

main transformer at Vermont Yankee.5 The fire also involved oil leaking from a 

flange on the main transformer itself and resulted in severe damage to the lowvoltage bushing box on top of the main transformer, to the generator PT cabinet 

in the turbine building, and to the iso-phase bus duct itself. 

The resulting set of relevant and potentially challenging events includes 7 events for segmented 

bus ducts and 3 events for iso-phase bus ducts. Note that, because of its significance, the June 18, 

2004, Vermont Yankee iso-phase fire event is included in the frequency calculations. All other 

frequencies were based on events prior to December 31, 2000. The iso-phase event set, therefore, 

extends beyond the period covered by the FEDB; hence, the number of plant reactor years was 

adjusted to reflect plant operations through mid-June 2004 for the iso-phase bus duct case only. 

Also, it was verified that no other iso-phase bus fires have been reported between January 1, 

2001, and mid-June 2004. The resulting plant-wide fire event frequency for segmented bus duct 

arc fault failures and iso-phase bus duct fires is characterized by the frequency distributions 

presented in Table 7-3. Note that in calculating fire frequencies, the number of plant reactor 

years is based on the entire U.S. fleet of light water reactors. That is, it has been assumed that all 

of the existing plants contribute to the bus duct fire frequency for both segmented and iso-phase 

events. 

Table 7-3 

Iso-Phase and Segmented Bus Duct Fire Frequency Distributions 

Frequency 

Bin 

Number 

of 

Events 

Total 

Reactor 

Years 

5th 

percentile 

50th 

percentile 

95th 

percentile 

Mean Variance

Segmented 

Bus Duct

7 2618 3.45E-05 8.51E-04 1.07E-02 3.27E-03 2.03E-02

Iso-Phase 

Bus Duct

3 2054 3.29E-05 6.79E-04 5.40E-03 1.85E-03 0.0801

 

5

 Reference: Licensee Event Report (LER) 50-064-2004-003-01, Revision 1, June 14, 2005. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-9 

Partitioning of these fire frequencies to specific locations for the segmented bus ducts should be 

performed in accordance with the counting guidance provided above. That is, each transition 

point will be assumed to have an equal fraction of the total plant-wide fire frequency. 

For the iso-phase bus, partitioning should assume that the likelihood of a fire is equal for each 

end of the bus (i.e., half of the frequency goes to the transformer end and half to the generator 

end) and that the initial fault is equally likely to occur in any one of the three phases (i.e., the 

fault initiates in one of the three phases, not all three concurrently). 

7.2.1.5 Estimating the Initial Fault Zone of Influence 

Zone of influence for segmented (non-iso-phase) bus duct fires 

The zone of influence for a segmented bus duct arc fault fire is considered unique from that 

assumed for electrical cabinets. The experience base illustrates that the bus duct events generally 

involved a pool of molten conductor and possibly burning insulation materials that forms within 

and then burns through the lower surface of the bus duct enclosure itself. This material spills out 

of the bus duct, may form a molten pool of metal on the floor or objects below, may splatter onto 

other nearby surfaces, and may ignite any combustible or flammable materials contacted. The 

recommended zone of influence is intended to reflect this experience base. For reference, one 

well-documented event considered prototypical of a bus duct fire occurred at Diablo Canyon on 

May 15, 2000.6

 Figures 7-1 and 7-2 provide photographs taken after this event (as provided in 

the cited inspection report—see footnote). Note the damage visible to the face panels on cabinets 

located below and to the side of the fault point. The photos show clear evidence (soot traces) that 

the surface-mounted components (e.g., the dial indicators, labels, switches, etc.) ignited and 

burned. Individual points of charring on the panel face are taken as indicative of impinging 

molten metal droplets. The exterior surfaces of cabinets on both sides of the access walkway 

directly below the primary faulting point were damaged. The fire did not extend to the interior of 

these cabinets. Surface damage occurred on cabinets on both sides of the aisle-way directly 

below the fault point, and extended to three adjacent panels on each side of the aisle. Based on 

the observed behavior, the recommended zone of influence for a segmented bus duct fire is as 

follows: 

• Assume that the effects of the bus duct fault will be manifested at a transition point (the fault 

point). Recall that failures at end point terminations are captured under the end point 

equipment. 

• The following zone of influence is assumed to originate from a point at the center of the bus 

duct at the assumed transition point location. 

– Assume that molten metal material will be ejected from the bottom of the bus duct below 

the fault point and will spread downward, encompassing the shape and volume of a right 

 

6

 References: (1) U.S. NRC Information Notice 2000-14, “Non-Vital Bus Fault Leads to Fire and Loss of Offsite 

Power,” 9/27/2000 and (2) U.S. NRC Diablo Canyon Inspection Report No. 50-275/00-09; 50-323/00-09, July 31, 

2000. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-10 

circular cone whose sides are at an angle of 15° from the vertical axis (a total enclosed 

solid angle of 30°). 

– Assume that molten metal material will also be ejected outwards and will spread within a 

sphere of 1.5-foot radius from the fault point. The fault point to be used when applying 

the 1.5-foot radial damage distance is the cross-sectional center of the bus duct. 

– The cone will expand (height allowing) to a maximum diameter of 20 feet. Beyond this 

point, the burning materials will fall straight downward in a cylindrical shape. Note that 

the maximum expansion zone for the cone (20-foot diameter) corresponds to a distance 

37 feet below the point of origin. 

– Assume that any exposed combustible or flammable materials within this cone-shaped 

and spherical zone of influence will be ignited. Combustible/flammable materials will not 

be considered exposed if they are protected by a fire-rated raceway wrap, conduit, or 

solid steel panels. Specific examples of the recommended treatment of exposed versus 

non-exposed materials are as follows: 

o The solid metal side panels of a cabinet will prevent ignition of the 

combustible/flammable materials inside the cabinet. 

o For cabinets with a solid steel top where all cable or conduit penetrations are sealed 

(e.g., consistent with the guidance provided in EPRI 1011989, NUREG/CR-6850, 

Chapter 11, with respect to the propagation of fires out of an electrical panel), molten 

material deposited on top of the cabinet will not burn through the panel top.7

o For cabinets with a ventilated top or unsealed cable or conduit penetrations, molten 

material deposited on top of the panel will penetrate into the panel and ignite the 

contents if the openings are within the zone of influence. 

o Open ventilation sections on cabinet side panels that are made up of an open mesh or 

screen section will allow the penetration of molten material into the cabinet if the 

openings are within the zone of influence. 

o For cabinet side panels or doors that include louvered ventilation openings where the 

louvers point downwards to the outside of the panel, molten material deposited on the 

surface of such panels will not penetrate into the cabinet. Cables in open-top cable 

trays will be ignited if they are within the zone of influence. 

o Cables in conduit will not be ignited by molten materials deposited on the outer 

conduit surface if the open ends of the conduit are located outside the zone of 

influence. 

o Cable in trays that are equipped with unventilated steel covers will not be ignited by 

molten metals falling from above. 

 

7

 Note that, relative to this particular point of guidance, it is the judgment of the authors that even the minimum 

thickness of a typical steel cabinet top panel as employed in practice by manufacturers will be sufficient to prevent 

burn-through of the molten material ejected from a bus duct. The guidance specifically excludes credit for aluminum 

panels. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-11 

o Cables in trays that are equipped with aluminum covers of any kind, or with 

ventilated steel covers, will be ignited by molten metals falling from above. 

o The first solid surface encountered by the material ejected from the bus duct will 

truncate the zone of influence along that line of travel. (Examples include: where the 

zone of influence intersects the floor, a sealed cabinet top, or a cable tray with a solid 

metal cover, it does not extend through that surface to other targets or flammable 

material beyond). 

• Damage and ignition within the initial zone of influence occurs at time zero (concurrent with 

the initial fault), but the ensuing fire can be assumed to develop over time from a point 

ignition origin (e.g., a cable tray should be assumed to ignite at one point, not over its entire 

exposed length). 

• Subsequent analysis of fire development, fire detection, and fire suppression response follow 

the same practices as applied to high-energy arc faults for switchgear and load centers. In 

particular, that manual fire brigade response curve applicable to high-energy arc faults for 

switchgear also applies to bus duct faults.

Figure 7-1 

Photograph of the point on the bus duct. 

The photograph shows where the arcing fault at Diablo Canyon was manifested (the fault 

point). Note that the tops of the cabinets to the left and right (the cabinets to the left are 

shown in Figure 7-2) are visible in the lower corners of this photograph. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-12 

Figure 7-2 

Photograph of the surface damage and burning. 

The photograph shows damage observed for the more heavily impacted cabinets on one 

side of the aisle-way below the fault point (the cabinet to the left as seen in Figure 7-1). 

Zone of influence for iso-phase duct fires 

For the iso-phase bus duct fires, it is recommended that the zone of influence should assume 

damage to any component or cable that would normally be considered vulnerable to fire damage 

(i.e., excluding items such as water-filled piping that would not normally be considered 

vulnerable to fire damage) located within a sphere centered on the fault point and measuring 

5 feet in radius. Any flammable or combustible material within this same zone of influence 

should be assumed to ignite. The recommended zone of influence is intended to cover both the 

initial fault effect and the potential burning of hydrogen gas8

 that may be released at low pressure 

from the bus casing upon rupture. An enduring fire (i.e., lasting beyond the initial fault) should 

be assumed consistent with the nature of any flammable or combustible materials present within 

the zone of influence and potential fire spread beyond the zone of influence. 

For the case of fires occurring at the main transformer termination points, the potential for 

involvement of the main transformer (and its oil) should be considered. In particular, the 

electrical lines will each penetrate the casing of the transformer, and this could allow the fire to 

spread to the transformer itself. Failure of the electrical penetration seals (e.g., melting of a 

rubber boot) could also create a path for oil leakage outside the transformer as was observed in 

the Vermont Yankee event. 

 

8

 Iso-phase bus ducts are generally filled with hydrogen gas at low pressure to enhance both cooling and electrical 

isolation. Upon rupture the hydrogen gas will leak from the duct, but neither a jet fire nor an explosion are 

anticipated due to the initial rupture. 
Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035) 

7-13 

The analysis should also consider the potential for involvement of additional hydrogen gas 

beyond that which will leak from the casing as a result of the initial fault. That is, the 

configuration of, and potential failure in, the hydrogen purge/fill system should be evaluated to 

determine if additional leakage of hydrogen gas is plausible. This assessment will require 

consideration of case-specific storage, piping, and valve arrangements. 

8-1 

8 

FIRE PROPAGATION FROM ELECTRICAL CABINETS 

(FAQ 08-0042) 

8.1 Background 

Text in Appendix G (Section G.3.3) of EPRI 1011989, NUREG/CR-6850 indicates that a fire in 

an unvented electrical cabinet does not propagate beyond the cabinet. More comprehensive 

language in Chapter 11 (Section 11.5.1.7.3) provides additional requirements on cabinet 

construction for those cabinets that do not propagate fires. In particular, Chapter 11 also requires 

that electrical cabinets have fire-sealed penetrations and be robustly secured for no propagation 

to occur beyond the cabinet. The bin 15 discussion in Chapter 6 provides information on 

definition of “robustly secured” as well. This clarification is not related to electrical cabinets for 

the purposes of high-energy arcing faults, as those faults counted in Appendix M for purposes of 

developing fire frequency are all assumed to breach the cabinet. 

The guidance provided in Appendix G for the screening of fire propagation from “unvented” 

electrical cabinets appears to conflict with the guidance provided in Chapters 6 and 11 of the 

main body. Clarification is needed. Portions of the text in Appendix G, Section G.3.3, were an 

unintended carryover from the original Fire PRA Implementation Guide (EPRI TR-105928) and 

were not modified to reflect the EPRI/RES team’s consensus. The alternate discussion provided 

in Chapter 11 represents the consensus positions. 

Appendix G (Section G.3.3) provides a general discussion of the effects of venting on fire 

development and fire propagation for electrical cabinets. In most regards, the discussions are 

correct. In particular, cabinet venting is important to the development of fires in an electrical 

cabinet. The point where the discussion deviates from the team consensus developed as a part of 

the methodology is where it discusses the potential for fire propagation outside the cabinet for a 

cabinet that is not vented. 

In order to achieve closure of this FAQ in a timely manner, the NRC developed a draft interim 

staff position, as discussed below [NRC ADAMS Accession No. ML092110537]. This position 

was developed using currently existing information, databases, and experimental results, and 

should not be seen as prejudicing the NRC’s view of future developments in this area. 
Fire Propagation from Electrical Cabinets (FAQ 08-0042) 

8-2 

8.2 Resolution 

The wording provided in Appendix G relative to the potential for fire spread beyond the 

boundaries of an unvented cabinet should be disregarded. The wording provided in Chapter 11 

relative to fire propagation from electrical cabinets is the intended and correct guidance. 

(Specific citations are provided below.) Specifically, those portions of the second paragraph in 

Section G.3.3 that read as follows should be disregarded: 

Electrical cabinets that are not vented do not propagate a fire. ... It is assumed that in the 

absence of other ventilation (other than those listed in Table G.3), penetrations will not 

allow sufficient air exchange to replace oxygen consumed by the fire, and an incipient 

fire will self-extinguish when there is no longer enough oxygen to support combustion. 

[Italics added for clarity.] 

Also, the final sentence of the third paragraph in Section G.3.3., which reads as follows, requires 

some clarification: 

... Therefore, air exchange through the top penetrations for typical NPP cabinet 

configurations listed above is not expected to be sufficient to support combustion. 

This latter discussion is correct but incomplete. The fundamental factor not addressed by the 

wording in both of these citations is that, once a fire starts inside an electrical cabinet, uneven 

heating of the cabinet side/top panels and door(s) will take place. This uneven heating can cause 

these elements to warp unless they are “robustly secured” as discussed in Chapters 6 (under the 

Bin 15 discussion) and 11. Warping will in turn create new openings for the passage of air into 

and out of the cabinet. The observation of this behavior and its impact on fire growth behavior 

was a major finding of the Mangs/Keski-Rahkonan (VTT Finland) tests which are also discussed 

(and cited) in Appendix G. 

In lieu of the wording from Appendix G, analysts should screen electrical cabinets for fire 

propagation potential based on the following guidance from Chapter 11 (Section 11.5.1.7.3, 

Step 7.a.3): 

In the case of electrical panels, the panel ventilation configuration and the latching 

configuration of the doors are important. If the panel contains open vents, either at the top 

or bottom of the pane, or if penetrations into the top or sides of the panel are not firesealed, fires can be assumed to be capable of spreading out of the panel to secondary 

combustibles. However, for un-vented cabinets, fire spread may be less likely. Fire 

spread out of the panel may still occur, unless the panel doors are attached and anchored 

at multiple points. Simple twist-handle style top-and-bottom door latches are not 

sufficient to contain a fire within a panel. Substantial warping of the door face may occur 

due to the heat of the fire. This can allow gaps to open in an otherwise un-vented panel. 

In contrast, fire spread is not considered likely given a weather-tight or waterproof 

cabinet construction where multiple mechanical fasteners secure panel access plates and 

where all penetrations into the panel are sealed. 
Fire Propagation from Electrical Cabinets (FAQ 08-0042) 

8-3 

As a point of clarification, it should be noted that in the above description on penetrations, the 

term “fire-sealed” was not intended to imply “fire-rated.” Rather, the intent was that penetrations 

into a cabinet would be sealed such that they would not readily allow for the passage of air. 

This clarification has the potential to impact the preliminary fire modeling and screening analysis 

for fire propagation from electrical cabinets (fire ignition source Bin 15). If the potential for fire 

propagation outside any given cabinet was dismissed based on the guidance provided in 

Appendix G, then the screening results for these cabinets, and these cabinets only, should be 

reconsidered based on the guidance provided in Chapter 11. 

This clarification should not require any reconsideration of the original fire ignition source 

counting results, provided the guidance in Chapter 6 was followed. 

9-1 

9 

MAIN FEEDWATER PUMP OIL SPILL FIRES (FAQ-08-

0044) 

9.1 Background 

FAQ 08-0044 was proposed by the Nuclear Energy Institute (NEI), through its NFPA 805 Task 

Force, to clarify the guidance from NEI 04-02, “Guidance for Implementing a Risk-Informed, 

Performance-Based Fire Protection Program under 10 CFR 50.48(c),” which in turn cited 

guidance on modeling oil spill fires as provided in EPRI 1011989, NUREG/CR-6850, 

“EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities,” as appropriate for use in 

developing fire probabilistic risk assessments (PRAs). The authors believed that the guidance on 

oil spill fires cited in EPRI 1011989, NUREG/CR-6850 did not specifically address such fires 

from main feedwater (MFW) pumps and that, when applied to MFW pump oil spill fires, could 

lead to misrepresentative estimates for both the ignition frequency and severity factor (in terms 

of size of oil spill) that could subsequently yield overly conservative estimates of risk when used 

generically in fire PRAs. Initial analyses were performed under the Memorandum of 

Understanding (MOU) between the Electric Power Research Institute (EPRI) and Office of 

Nuclear Regulatory Research (NRC-RES), but were not concluded prior to EPRI publication of 

EPRI 1016735, “Fire PRA Methods Enhancements: Additions, Clarifications, and Refinements 

to EPRI 1011989,” in December 2008 [7]. In order to achieve closure of this FAQ in a timely 

manner, the NRC developed a draft interim staff position, as discussed below [NRC ADAMS 

Accession No. ML092110516]. This position was developed using currently existing 

information, databases, and experimental results, and should not be seen as prejudicing the 

NRC’s view of future developments in this area. The NRC concurs with use of this analytical 

approach as an interim method for assigning frequency and severity of MFW pump oil spill fires. 

9.2 Resolution 

Early applications of EPRI 1011989, NUREG/CR-6850 guidance to the specific case of MFW 

pumps have led to anomalous results. Given the large quantity (several hundred gallons) of oil 

found in a typical MFW pump, even assignment of the “small fire” severity level (i.e., 10% of 

the oil) in EPRI 1011989, NUREG/CR-6850 leads the analyst to postulate a fire that is actually 

very severe, with a frequency higher than indicated by industry experience. Depending on the 

fire PRA components and cables near a MFW pump, the postulated fire scenarios can be risk 

significant. 
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-2 

The guidance mentioned above was not developed with high-volume oil systems in mind, but 

rather, was aimed primarily at smaller-volume pump lubrication systems. Revised guidance has 

been developed specifically for MFW pump oil fires to more accurately represent industry 

experience regarding such fires. 

Introduction and Problem Statement

Appendix E, Section E.3, of EPRI 1011989, NUREG/CR-6850 provides guidance for the 

treatment of oil spill fires as follows: 

The following steps are recommended for assigning the severity factor to scenarios 

involving oil spill fires: 

1. Determine the amount of oil that can be spilled in the room. 

2. Assign a severity factor of 0.02 to a scenario consisting of 98% or more of the 

amount of oil spilled and ignited. 

3. Assign a severity factor of 0.98 to a scenario consisting of 10% of the amount of oil 

spilled and ignited. 

Figure 9-1 provides a qualitative depiction of this guidance assuming that the amount of oil 

represents fire severity. The severity curve is divided into two parts: “small,” represented by 10% 

of the oil, and “large,” represented by 98% of the oil. 

Figure 9-1 

Oil fire severity factor guidance 
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-3 

Basis for the MFW Pump Oil Fire Revised Guidance

A similar partitioning approach toward fire severity is followed as documented in the original 

guidance, except that three severity levels are now defined instead of just two to reflect that (1) 

most MFW pump fire events had limited effect on the surrounding items and (2) none of them 

involved a large quantity of the pump lubricating oil. The three levels attempt to capture this 

experience and the potential for severe impact that can be experienced from an uncontrolled 

MFW pump oil spill. This is shown qualitatively in Figure 9-2 below. 

Figure 9-2 

Main feedwater pump oil fire severity factors 

The three severity levels are defined as follows: 

• Small fires are defined as fires involving small oil leaks leading to the ignition of oil-soaked 

insulation on piping and equipment. These fires will involve a fraction of the oil inventory 

smaller than assumed in the following fire sizes. Fire damage is expected to be limited to the 

MFW pump. 

• Large fires are defined as fire events involving a spill of 10% of the oil content (consistent 

with the small fire category in the original oil spill severity factor guidance as cited above). 

• Very large fires are fire events that involve a spill of up to 100% of the oil inventory (again 

consistent with the large fire category in the original guidance as cited above). 

Available fire modeling tools should be used to establish cable damage for large and very large 

fire sizes. 
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-4 

As described further below, all of the MFW pump fire events from the fire events database 

(FEDB), as referenced in EPRI 1011989, NUREG/CR-6850, fell into the category of small fires. 

None of these events included reports of damage to anything other than the burning insulation 

itself. However, some of the event reports state that open flaming was observed in the area of the 

burning insulation. These observations clearly indicate that some potential existed for fire spread 

to combustible materials or failure of fire PRA targets susceptible to fire damage had these been 

near the burning insulation. 

The intent of the recommended approach is to ensure that the potential for fire spread or fire 

PRA target damage is considered based on scenario-specific conditions. 

The next question that must be answered is the relative likelihood that, given a MFW pump oil 

leak/spill, the quantity of oil released would be equivalent to each of the three severity levels 

defined above (i.e., the severity factor or split fractions for small, large, and very large fires). 

This assessment is based on a review of the MFW pump oil fires identified in the FEDB. Table 

9-1 provides the main feedwater pump-related fire events in the FEDB that involved oil. Fifteen 

fire events are presented in this table. Of these 15 events, 12 events were labeled as 

“challenging” and three as “undetermined” in the original fire frequency analysis. 

Very large oil spills and fires have occurred, but only involving main turbine oil systems and not 

MFW pump oil systems. Given that larger oil spills have occurred in other plant lubrication 

systems, the possibility of such a spill from the MFW pump lubrication system cannot be entirely 

dismissed. Reviewing the event descriptions of all 15 MFW pump fire events, it was concluded 

that 10 events have sufficient information available to conclude that they were small fires that 

remained confined to the pump or nearby piping (e.g., oil-soaked insulation). In these 10 fire 

events, there was no reported damage to items above or adjacent to the location of the fire. 

Insufficient information is available to establish the severity of five of the events. However, it 

can be argued that if any of these five events were of the type classified here as either large or 

very large fires, then additional attention would have been expended in documenting the event 

(e.g., a more complete description would have been provided). Therefore, it can be inferred that 

these five events were also small fires. 

Since EPRI 1011989, NUREG/CR-6850 uses only at-power operation fire events for estimating 

the frequency of MFW pump fires, one may conclude that no large or very large MFW oil fires 

have taken place during power operation. Using the counting approach of EPRI 1011989, 

NUREG/CR-6850, there were 12 challenging and three undetermined events (counted as 0.5 

each) for a total of 13.5 fire events with no large or very large fires. From these statistics we can 

conclude that the conditional probability of a large or very large fire given a MFW pump oil fire 

is 0.5/14.5 = 0.034, or 3.4% (using Jeffreys’ approach). 

Thus, one can conclude that 0.034 (3.4%) represents the fraction of MFW pump oil spills/fires 

that would be considered large or very large per the definitions given above. As noted above, 

none of the MFW pump fire events could be labeled as a very large spill/fire. The recommended 

estimate recognizes that very large spills would occur, but with a lower probability than large 

spills, and assumes that 10% of the large or very large spills/fires would actually be very large 

spills/fires. This yields a net severity factor of 0.0034 (0.34%) for the very large fire case. 
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-5 

Summary of Recommended MFW Pump Oil Fire Revised Guidance 

The following steps are recommended for assigning the severity factor to scenarios involving 

MFW pump oil fires: 

7. Determine the amount of oil available in the system for the large and very large oil spill fires. 

8. The MFW pump oil fire plant-wide fire frequency remains unchanged. 

9. Assign a severity factor of 0.0034 (0.34%) to very large fires: scenarios involving 100% of 

the total oil inventory spilled and ignited. 

10. Assign a severity factor of 0.0306 (3.06%) to large fires: scenarios involving 10% of the 

total oil inventory spilled and ignited. 

11. Assign a severity factor of 0.966 (96.6%) for small fires: scenarios involving a leak that leads 

to a fire that only impacts the MFW pump. 

If either the very large or large fire scenario is found to be risk significant, the analyst may 

conduct a plant-specific analysis of the oil-containing devices/systems to identify various spill 

scenarios with the corresponding quantity of oil spilled and size of burning oil pool. 

Table 9-1 

FEDB Events Involving Main Feedwater Pump Oil 

Incident 

No. Date Description Challenging / Power 

Leak / 

Spill 

8 19-Aug-70 

Fire resulted when oil leaking from a pump-motor 

reservoir, through a temporary insulation crack, 

contacted hot pipe (520°F) and vaporized. This 

caused flashing when contacting ventilation air.

Challenging / 

At Power

Leak 

24 13-Sep-72 

A leak in the oil supply line to the feedwater 

pump soaked insulation on the feedwater supply 

line, and ignition occurred when oil came in 

contact with a hot pipe. Fire was quickly 

extinguished with dry chemical. As piping 

insulation was removed after fire (30 min), 

several flare-ups occurred, which again were 

quickly extinguished. Fire brigade response was 

prompt (< 5 min).

Challenging / 

At Power Leak

201 22-Apr-80 

A smoldering fire resulted from lube oil that 

leaked from a main turbine shaft-driven 

feedwater pump onto piping insulation. 

Challenging / 

At Power Leak
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-6 

Table 9-1 (continued) 

FEDB Events Involving Main Feedwater Pump Oil 

Incident 

No. Date Description Challenging / Power 

Leak / 

Spill 

476 26-Jun-85 

At 2126 hours on June 26, 1985, Waterford 3 

Steam Electric Station was at 91 percent reactor 

power when operations personnel received 

information of a fire in feedwater pump A. At 

2131 hours the control room was informed by 

personnel on the scene that the B feedwater 

pump, rather than the A pump, was on fire. Since 

the A pump was already secured, and since the 

steam generator water levels were decreasing, 

operation personnel tripped the main turbine and 

reactor. The fire was extinguished by plant 

personnel at 2136 hours. The fire was started by 

a small oil leak, and it was limited to a small 

portion of the outer wrapping of insulation on the 

feedwater piping.

Challenging / 

At Power Unknown

477 29-Jun-85 

On June 29, 1985 at 0957 the reactor was 

manually scrammed and the main turbine 

manually tripped due to a fire in the reactor 

feedwater pump 1B, while the remaining reactor 

feedwater pump was secured for maintenance. 

The fire was extinguished and the plant shut 

down in an orderly fashion.

Challenging / 

At Power Unknown

662 08-Jan-88 

While performing routine rounds, an equipment 

operator observed smoke being emitted from the 

2C Reactor Feed Pump (RFP) discharge piping. 

As it was initially suspected, an oil leak was 

causing the smoke. The fire was extinguished 

using fire extinguishers and fire water.

Challenging / 

At Power Leak

737 29-Jul-88 

As a result of maintenance activities, oil-soaked 

lagging and paper was ignited by the hot surface 

of Feedwater Pump A suction pipe.

Challenging / 

At Power

Leak

739 30-Jul-88 

Walking by Steam Generator Feedwater Pump 

A, plant personnel noticed smoke rising from the 

outboard pump bearing area, and immediately 

contacted the control room. The fire brigade 

commenced attack from the feedpump platform, 

but were unable to determine the effectiveness of 

the attack due to the steam rising from the pump 

casing. They redirected their attack from the 

bottom of the feed pump, putting the fire out in its 

incipient stage. 

Challenging / 

At Power Leak

824 13-Jul-92 

A fire was reported at the "B" reactor feed pump; 

the cause may have been due to oil-soaked 

insulation; the fire was extinguished in about 15 

minutes.

Challenging / 

At Power Leak
Main Feedwater Pump Oil Spill Fires (FAQ-08-0044) 

9-7 

Table 9-1 (continued) 

FEDB Events Involving Main Feedwater Pump Oil 

Incident 

No. Date Description Challenging / Power 

Leak / 

Spill 

961 11-Aug-91 

Event occurred on August 11, 1991. At 

approximately 08:55 a fire was reported in the 

turbine building. The plant was at power 

operation. The fire was reported to be 

approximately 18 minutes in duration. The fire 

was detected and extinguished by the fire 

brigade. This fire was extinguished by using 

water and carbon dioxide. The fire apparently 

originated from oil-soaked insulation resulting 

from a lubrication oil leak. The root cause of this 

event was turbine bearing lube oil leaking from 

the flange gasket of the ground brush assembly. 

This saturated the surrounding insulation with oil, 

which fueled the fire once ignited. A ground wire 

was pinched between the gasket and the flange, 

creating a leak path. 

Challenging / 

At Power Leak

1110 02-Feb-90 – Undetermined / 

At Power Unknown 

1240 11-Sep-76 

Oil-soaked insulation was smoldering. An air 

hose was being used to remove smoke from 

room to find its source. The air caused the 

insulation to flame. 

Undetermined / 

At Power Leak 

2183 13-Sep-76 – Undetermined / 

At Power Unknown 

2388 16-Dec-93 Ref. SOS 93-2116 Challenging / 

At Power Unknown 

2422 24-Aug-00 

Residual lubrication oil caught fire on the B 

reactor feed pump. The fire was in the area of 

the outboard pump bearing. The fire was 

controlled with portable extinguishers and 

completely extinguished using less than 200 

gallons of water from a hose line. 

Challenging / 

At Power Leak 

10-1 

10 

FIRE IGNITION FREQUENCY (FAQ 08-0048) 

10.1 Background 

FAQ 08-0048 was proposed by the Nuclear Energy Institute (NEI), through its NFPA 805 Task 

Force, to clarify the guidance from NEI 04-02, “Guidance for Implementing a Risk-Informed, 

Performance-Based Fire Protection Program under Title 10 of the Code of Federal Regulations 

Part 50 (10 CFR 50.48(c)),” which cited the fire ignition frequencies as provided in EPRI 

1011989, NUREG/CR-6850, “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power 

Facilities,” as appropriate for use in developing Fire Probabilistic Risk Assessments (PRAs). 

NEI believed that the fire events data cited in EPRI 1011989, NUREG/CR-6850 could be 

reinterpreted as supporting reduced frequencies, for the most part, for various fire bin ignition 

categories subsequent to approximately 1990. Initial analyses were performed under the 

memorandum of understanding (MOU) between the Electric Power Research Institute (EPRI) 

and Office of Nuclear Regulatory Research (NRC-RES), but were not concluded prior to EPRI 

publication of EPRI 1016735, “Fire PRA Methods Enhancements: Additions, Clarifications, and 

Refinements to EPRI 1011989,” in December 2008 [7]. 

10.2 Resolution 

EPRI 1016735 included the analysis completed to date for FAQ 08-0048, considered by the 

NRC as necessary but not yet sufficient to formally close out this FAQ. In order to achieve 

closure of this FAQ in a timely manner, the NRC has incorporated the EPRI-1016735 analysis 

into a draft interim NRC position, as discussed below [NRC ADAMS Accession No. 

ML92190457]. This draft interim position should not be seen as prejudicing the NRC’s view of 

future developments in this area. NRC-RES and EPRI, under the MOU, are updating the fire 

events database from EPRI 1011989, NUREG/CR-6850 to be inclusive through the year 2008, 

and establishing a process for subsequent periodic updating. Subsequently, NRC-RES and EPRI 

will be in a position to complete confirmatory analysis to update the fire bin ignition frequencies 

in EPRI 1011989, NUREG/CR-6850.9

 

Chapter 2, “Fire Ignition Frequency,” and Appendices A and B, “Statistical Methods” and “Fire 

Ignition Data and Results Details,” of EPRI 1016735, “Fire PRA Methods Enhancements: 

Additions, Clarifications, and Refinements to EPRI 1011989,” document the analysis performed 

by EPRI to revise, for the most part, the fire bin ignition frequencies cited in EPRI 1011989, 

 

9

 Upon successful completion, NRC-RES and EPRI will issue a joint report and eventually revise NUREG-CR6850, EPRI 1011989 to establish new frequencies for the fire bin ignition categories. 
Fire Ignition Frequency (FAQ 08-0048) 

10-2 

NUREG/CR-6850, for subsequent use in performing fire PRAs for NFPA-805 transitions, as per 

guidance in NEI 04-02, “Guidance for Implementing a Risk-Informed, Performance-Based Fire 

Protection Program under 10 CFR 50.48(c).” At the time of publication of EPRI 1016735 

(December 2008), NRC-RES, through the MOU with EPRI, was awaiting confirmatory analysis 

by EPRI that the revised fire bin ignition frequencies cited in EPRI 1016735 using the existing 

fire events data through year 2000 from EPRI 1011989, NUREG/CR-6850 could be considered 

applicable today. This confirmation would consist of analysis of more recent fire events data 

(i.e., subsequent to year 2000) to statistically verify that the revised frequencies subsequent to 

approximately 1990 could still be considered valid once data beyond year 2000 were 

incorporated. Since EPRI 1016735 was published prior to completion of this confirmatory 

analysis, the NRC was unable to reach closure of this FAQ through the formal process. 

Nonetheless, to achieve closure of this FAQ in a timely manner, the NRC establishes the 

following interim position with regard to the use of the new fire bin ignition frequencies in EPRI 

1016735, Chapter 2, Table 2-2, “Updated Bin Frequencies and Statistical Parameters (Individual 

Bins).” The NRC accepts use of these revised fire bin ignition frequencies for fire PRAs 

conducted for NFPA-805 transition for best-/point-estimate calculations of fire risk (core damage 

frequency [CDF] and large early release frequency [LERF]), including delta-risk values from 

plant change evaluations, with the following provision. The fire PRA, including plant change 

evaluations, must also evaluate the sensitivity of the risk and delta-risk results to evaluations 

performed using the current fire bin ignition frequencies10 in EPRI 1011989, NUREG/CR-6850, 

Chapter 6, “Fire Ignition Frequencies,” Table 6-1, “Fire Frequency Bins and Generic 

Frequencies,” and Appendix C, “Determination of Generic Fire Frequencies,” Table C-3, 

“Generic Fire Ignition Frequency Model for U.S. Nuclear Power Plants.”11 For those cases where 

the results from this sensitivity analysis indicate a change in the potential risk significance 

associated with elements of the fire PRA or plant change evaluations that affects the decisions12

being made (e.g., what is acceptable with the new frequencies from EPRI 1016735 might not be 

 

10 The sensitivity analyses should be performed for a fire ignition frequency bin using the mean of the fire ignition 

frequency bins contained in NUREG/CR-6850. Furthermore, sensitivity analyses only need to be performed for 

those bins characterized by an alpha from the EPRI 1016735 analysis that is less than or equal to 1. Note that an 

alpha value less than or equal to 1 is characteristic of a reverse-J shaped probability density function, i.e., the same 

shape as the non-informative prior distributions used in EPRI 1016735. This reverse-J shape is indicative of the 

large uncertainty in the bin fire frequency due to the sparsity of data for that bin, and therefore, the potential for 

significant changes should the post-2000 fire event data differ significantly from the 1991-2000 data. The required 

sensitivity analysis is, for the purpose of this interim solution, judged to provide an adequate indication of the effects 

on risk and delta-risk in such a case. Note also that a sensitivity analysis need not be performed for Bin 9 (Air 

Compressors); the alpha value in Table 2-2 of EPRI 1016735 appears to be in error. 

11 As modified to address any FAQs related to fire ignition frequencies that were closed out prior to issuance of 

EPRI 1016735, (e.g., FAQ 06-0017, “Guidance for Counting High-Energy Arcing Faults [HEAFs]” [ADAMS 

Accession No. ML072500300]), and any similarly related FAQs that may subsequently be closed out (e.g., FAQ 07-

0035, “Bus Duct Counting Guidance for High Energy Arcing Faults” [ADAMS Accession No. ML091620572]). 

12 The portion of the fire PRA supporting the decisions may be very focused for analyses associated with many fire 

protection features and systems. For example, a fire barrier may be subject to failure by a single type of fire ignition 

source or a fixed suppression system may be protecting a single type of ignition source. As a result, sensitivity 

analyses of all relevant ignition source bins may only require examination of a single (or few) ignition source bin(s) 

for a particular decision. 
Fire Ignition Frequency (FAQ 08-0048) 

10-3 

acceptable with the current applicable set from EPRI 1011989, NUREG/CR-6850), the licensee 

must address this situation by considering fire protection, or related, measures that can be taken 

to provide additional defense in-depth.13

Under the MOU, NRC-RES and EPRI are updating the fire events database from EPRI 1011989, 

NUREG/CR-6850 to be inclusive through the year 2008, as well as establishing a process for 

subsequent periodic updating. Therefore, NRC-RES and EPRI will be in a position to complete 

the confirmatory analysis for the new fire bin ignition frequencies in EPRI 1016735. Upon 

successful completion, NRC-RES and EPRI should jointly revise EPRI 1011989, NUREG/ 

CR-6850, to establish new frequencies for the fire bin ignition categories, eliminating the need 

for the specific sensitivity analysis cited here. 

10.2.1 Impact on Current Fire PRA and FAQ Implementation Plan 

1. The current PRA analysis should replace the frequency values used with the following for 

best-/point-estimate calculations of fire risk (core damage frequency [CDF] and large early 

release frequency [LERF]), including delta-risk values from plant change evaluations, with 

provision (2) below: 

Bin # Mean Frequencies 5% Bound 95% Bound Alpha Beta 

1 3.26E-04 1.28E-06 1.25E-03 0.5 1534

2 2.35E-03 2.75E-04 6.11E-03 1.5 639

3 2.34E-03 3.20E-04 5.89E-03 1.655 708.4

4 8.24E-04 4.23E-05 2.47E-03 1 1212.9

5 1.25E-03 2.64E-04 2.83E-03 2.32 1856.4

6 2.46E-03 5.00E-04 5.65E-03 2.235 906.73

7 4.81E-03 1.90E-03 8.80E-03 5.03 1045.1

8 5.04E-03 2.10E-03 9.02E-03 5.5 1091.1 

9 4.65E-03 1.83E-03 8.51E-03 0.5 1075.3 

10 1.18E-03 1.38E-04 3.07E-03 1.5 1271.9 

11 9.43E-04 4.84E-05 2.82E-03 1 1060.5 

12 1.32E-03 1.55E-04 3.43E-03 1.5 1137.8 

13 4.20E-04 1.65E-06 1.61E-03 0.5 1189.9 

14 3.41E-03 1.16E-03 6.61E-03 4 1173.6 

15.1 2.36E-02 5.36E-03 9.40E-02 0.453 19.16 

15.2 1.06E-03 1.24E-04 2.75E-03 1.5 1419.3 

16.1 1.27E-03 1.49E-04 3.31E-03 1.5 1200.4 

 

13 It is recognized that there is some degree of subjectivity in this determination, which may require consensus 

between the licensee and NRC on a plant-specific basis. Such a change need not necessarily result in a risk increase; 

A change in the risk profile (e.g., dominant fire scenarios, etc.) could also affect the decisions being made. Also, 

defense-in-depth measures should be commensurate with any potential increase in risk significance (e.g., as per RIS 

2005-07, “Compensatory Measures to Satisfy the Fire Protection Program Requirements” (ADAMS Accession No. 

ML042360547)). 
Fire Ignition Frequency (FAQ 08-0048) 

10-4 

Bin # Mean Frequencies 5% Bound 95% Bound Alpha Beta 

16.2 8.24E-04 9.66E-05 2.15E-03 1.5 1820.0 

17 1.18E-03 1.38E-04 3.07E-03 1.5 1271.9 

18 1.11E-03 1.30E-04 2.89E-03 1.5 1350.5 

19 1.24E-03 1.45E-04 3.22E-03 1.5 1212.9 

20 8.83E-03 2.02E-03 1.95E-02 2.5 283.19 

21 1.42E-02 8.81E-03 2.06E-02 15.5 1094. 

22 9.33E-04 3.85E-05 2.88E-03 0.92 985.87 

23 8.02E-03 4.18E-03 1.29E-02 9 1122.7 

24 3.65E-03 1.11E-03 7.38E-03 3.425 938.34 

25 8.28E-03 4.08E-03 1.37E-02 7.815 944.14 

26 6.12E-03 2.87E-03 1.04E-02 7 1144.1 

27 1.62E-03 1.90E-04 4.21E-03 1.5 927.84 

28 8.38E-03 4.06E-04 1.40E-02 7.5 894.67 

29 1.89E-03 5.84E-04 3.79E-03 3.5 1856.4 

30 9.78E-04 1.15E-04 2.55E-03 1.5 1534 

31 4.50E-04 1.77E-06 1.73E-03 0.5 1110.2 

32 5.44E-03 2.12E-03 1.00E-02 4.901 901.31 

33 2.10E-03 3.73-04 4.98E-03 2 951.96 

34 3.23E-03 8.81E-04 6.79E-03 3 927.84 

35 3.89E-03 1.20E-03 7.82E-03 3.5 899.78 

36 7.55E-03 3.52E-03 1.28E-02 6.91 915.35 

37 3.41E-03 9.71E-04 7.07E-03 3.15 922.51 

2. Note that sensitivities must be performed on the above frequencies for those cases pointed 

out in the earlier FAQ discussion (refer to second footnote in Section 11.2). 
11-1 

11 

CABLE TRAY FIRE PROPAGATION (FAQ 08-0049) 

11.1 Background 

FAQ 08-0049 was proposed by the Nuclear Energy Institute (NEI), through its NFPA 805 Task 

Force, to clarify the guidance on cable fire propagation provided in EPRI 1011989, NUREG/CR6850, for use in developing fire PRAs. The FAQ authors believed that the empirical cable tray 

fire propagation model in EPRI 1011989, NUREG/CR-6850 has led to conservative estimates of 

cable fire growth rates and unrealistically short room burnout times when used outside the Zone 

of Influence (ZOI)—i.e., outside the fire plume that extends above the ignition source. The 

purpose of this FAQ is to clarify (or emphasize) the limits of this model. 

In order to achieve closure of this FAQ in a timely manner, the U.S. NRC developed an interim 

NRC staff position, as discussed below [NRC ADAMS Accession No. ML092100274]. This 

position was developed using currently existing information, databases, and experimental results, 

and should not be seen as prejudicing the NRC’s view of future developments in this area. The 

staff’s interim position clarifies the limits of the cable tray fire propagation model provided in 

EPRI 1011989, NUREG/CR-6850. 

11.2 Resolution 

Predicting cable tray ignition, fire propagation (flame spread), and heat release rate is a complex 

endeavor due to the multiple phenomena that are involved. The NRC’s interim position is as 

follows: 

The empirical cable tray fire propagation model, as specified in EPRI 1011989, 

NUREG/CR-6850, when applied within the zone of influence above the burning fuel, 

provides times to ignition of horizontal cable trays, and angles of flame spread, 

immediately above the ignition source that are representative of the test results from 

Section 3.4 of NUREG/CR-5384, “A Summary of Nuclear Power Plant Fire Safety 

Research at Sandia National Laboratories, 1975-1987,” for use in fire PRAs when applied 

within the limits of those tests’ validity. The NRC recommends that the tray-to-tray fire 

spread rates and angles be used only in cases that are similar in nature to these tests upon 

which they are based, i.e., within the configurations specified in NUREG/CR-5384. Care 

must be exercised whenever applying this model. The experiment upon which this rule 

set in NUREG/CR-5384 is based included only cross-linked polyethylene (XPE) cables, 

which were constructed using thermoset materials that were not protected in any way. 

Therefore, if the configuration in question includes other than XPE cables, especially 

thermoplastic cables, that are not protected, a different rule set would be appropriate. 

Tests have shown that thermoplastic cables have higher heat release rates and lower 
Cable Tray Fire Propagation (FAQ 08-0049) 

11-2 

damage thresholds than XPE. However, if the configuration includes cables with 

flammability and electrical performance characteristics that are more robust than XPE 

cables, or cables that are protected in some way, this rule set may be conservative, also 

depending on the ignition source heat release rate, etc. 

An additional important aspect that needs to be considered to evaluate the application of 

the rule set is the separation distance between cable trays. The testing used separation 

distances of 8 inches of horizontal separation and 10.5 inches of vertical separation 

between cable trays. If the configuration in question has separation distances greater than 

these, the rule set may be conservative. 

The following discussion provides a means to understand the analysis of cable tray fires. In a 

very basic, first-order analysis, one can assume thermal damage occurs the instant the target 

reaches its minimum failure temperature. For example, if the target is an electrical cable and it is 

known that the cable (thermoplastic) fails at 400°F (204°C), the analyst can assume failure as 

soon as the cable is exposed to a 400°F (218°C) hot gas layer (the corresponding threshold for a 

thermoset cable is 625°F [329°C]). For ignition to occur, the cable insulation must be heated 

sufficiently to vaporize and form a flammable premixed system. We know, from thermal 

detector and sprinkler response, that all materials have a mass that must be heated before they 

can reach a target temperature. This thermal inertia is quantified as the response time index for 

detection and suppression devices. The same principle applies to electrical cables, however with 

a number of complications. For example, where is the cable located in the tray: top, bottom, 

against a side rail, or in the center of the cable mass? Does the cable have any fire retardant 

coating? If so, which brand? Cables that are thermally stable are difficult to ignite and exhibit 

higher ignition temperature. A cable or coated cable can ignite when its surface is hot enough to 

generate flammable gas, unless the level of oxygen available is insufficient for ignition. Then the 

gas can often accumulate elsewhere and burn later. 

These are just a sampling of the possible variables that complicate the thermal impact on cable 

tray fire propagation. Many of these factors are unknown, and could affect the time to the onset 

of ignition/gasification and flame spread of cable tray insulation, especially when considering 

locations elsewhere in the room (i.e., not immediately above the ignition source). These factors 

would tend to complicate the analysis and are not generally amenable to inclusion in a generic 

treatment. A detailed analysis would likely capture some of these aspects of the cable tray fire 

scenario. For example, EPRI 1011989, NUREG/CR-6850, Section H.1.5.2 (Tables H-5 and 

H-6), provides time to failure of thermoplastic and thermoset cable insulation when exposed to 

hot gas layer temperature. Beyond the configurations identified above, the NRC recommends 

that detailed fire modeling be performed. Zone fire models and computational fluid dynamics 

(CFD) codes are known to provide target thermal response modeling capabilities within or 

outside of the fire plume (zone of influence). A combination of engineering correlations and 

computer fire modeling can also apply to these types of scenarios. 
Cable Tray Fire Propagation (FAQ 08-0049) 

11-3 

The empirical tray-to-tray fire spread rule set is intended for use only in cases that are similar in 

nature to the test upon which it is based. The NRC recommends that the empirical cable tray fire 

propagation model, as specified in EPRI 1011989, NUREG/CR-6850, only be used to predict 

cable tray fire propagation within the following two configurations: 

Configuration 1: a single vertical stack of horizontal cable trays separated nominally in 

accordance with Regulatory Guide 1.75 [8] where the tray stack is located directly above 

a fire source. 

Configuration 2: two adjacent vertical stacks of horizontal cable tray separated nominally 

in accordance with Regulatory Guide 1.75 where both of the tray stacks being modeled 

are located directly above a fire ignition source such that both trays would be either fully 

or at least partially immersed in the fire plume. 

NRC recommends that these spread rate models, as applicable to the two configurations above, 

not be used outside the original zone of influence for the reasons previously cited. For NFPA 805 

applications, the licensee will need to develop and justify plant-specific, configuration-specific 

models if they are to be used outside the original zone of influence. These models will be an area 

of detailed staff review. 

12-1 

12 

LOCATION OF FIRES WITHIN ELECTRICAL CABINETS 

(FAQ 08-0043) 

12.1 Background 

Lessons from National Fire Protection Association (NFPA) 805 pilot plants and review of the 

Fire Probabilistic Risk Assessment (PRA) Standard indicate that the assumed locations for fires 

within electrical cabinets, as specified in NUREG/CR-6850, may be conservative. Conservative 

assumptions of cabinet fire locations can greatly affect the end results. 

NUREG/CR-6850, “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities, 

Volume 2: Detailed Methodology,” Section G.3.2, provides two possible approaches to the 

location for a fire within an electrical cabinet that is used for fire modeling calculations. In the 

introductory paragraph, the method suggests opening the cabinet to determine the location of the 

fire based on the location of the cable bundles. In the itemized bullets, the method suggests 

locating the fire at the top of the cabinet. 

In practical applications, the fire is always assumed to start at the top of the cabinet. This 

assumption is applied even for cabinets that are sealed on the top (without horizontal top vents or 

openings). However, the Fire Protection Significance Determination Process (SDP) (NRC 

Inspection Manual Chapter 0609, Appendix F), Page F-21, recommends assuming that the 

location of an electrical cabinet fire is one foot below the top of the cabinet. The basis for this 

guidance was derived from experimental observations, as discussed below. 

The NRC developed the interim position discussed below in order to achieve closure of this FAQ 

in a timely manner. This interim position was developed using currently existing information, 

databases, and experimental results, and should not be seen as prejudicing the NRC’s view of 

future developments in this area. Final endorsement of this position will be addressed through 

the next revision of either Regulatory Guide 1.205 or NUREG/CR-6850. 
Location of Fires Within Electrical Cabinets (FAQ 08-0043) 

12-2 

12.2 Resolution 

Testing14 has shown that early fire development begins heating the cables and other combustible 

materials within the cabinet. Once those internal combustibles get hot enough to generate 

significant pyrolyzates (combustion products), the fire will transition to oxygen-limited behavior 

(“oxygen-limited fire”) within the cabinet. Under these conditions, burning continues within the 

cabinet (cables develop deep-seated fire), but is limited by the availability of oxygen (i.e., air that 

flows into the lower sections of the cabinet through ventilation grills, gaps between cabinet 

panels, gaps around the cabinet doors, or gaps opened by warping of the cabinet panels and 

doors). Significant unburned pyrolyzates are also generated, which are then vented from the 

upper portion of the cabinet. 

As these unburned pyrolyzates leave the cabinet, they come into contact with ambient air 

(oxygen) and burn at or near the exit point (e.g., the upper ventilation grills or other openings). 

Hence, assuming that the fire origin is near the base of the upper vertical ventilation openings in 

an electrical cabinet is a reasonable predictor of typical cabinet fire location under large-scale 

burning conditions. In general, such vertical grills will typically be located within one foot of the 

electrical cabinet top so that the general application of the proposed “one-foot rule” from the Fire 

Protection SDP will be representative of the anticipated behavior for the vast majority of closedtop electrical cabinets. 

It is recommended that the assumed fire location used for screening and detailed fire modeling 

calculations for electrical cabinets sealed on the top (without horizontal top vents or openings) 

be one foot below the top of the cabinet. This assumption should be applied until detailed review 

of the cabinet contents can be performed for additional location determination, per Section G.3.2 

of NUREG/CR-6850. 

The assumed fire location for a fire within a cabinet that is not sealed at the top should be the 

top of the cabinet (see additional guidance below). As a point of clarification, it should be noted 

that in the above description on penetrations, “sealed at the top of the cabinet” was not intended 

to imply “fire-rated.” Rather, the intent was that penetrations into a cabinet would be sealed such 

that they would not readily allow for the passage of air. 

For vented cabinets, where the vent is either located on the side of the cabinet (vertical) or top of 

the cabinet (horizontal), the analysis can (as an alternative to the above assumed location) locate 

the fire at the uppermost vent. For example, if a cabinet includes two vertical vents on the side, 

the fire should be assumed to be located at the elevation of the upper vertical vent. Similarly, if 

there is a vent in the top corner of the cabinet, the fire should be assumed to be in the center of 

the corner vent. When two or more vents are located at the same height, then the analysis should 

conservatively assume the fire is located at the vent closest to any targets. If this last assumption 

 

14 Note that these insights are based on the same cabinet fire test programs referenced in NUREG/CR-6850, EPRI 

1011989; namely, those from Sandia National Laboratory/NRC, IRSN (Institut de Radioprotection et de Sûreté 

Nucléaire [Institute for Radioprotection and Nuclear Safety (France)]) and VTT (Valtion Teknillinen 

Tutkimuskeskus [Technical Research Centre of Finland]). 
Location of Fires Within Electrical Cabinets (FAQ 08-0043) 

12-3 

greatly affects the results, additional refinement may be needed either by more accurate 

modeling of the fire size expected to occur at each vent or by using the general guidance 

discussed above from NUREG/CR-6850. 

For cabinets that are neither vented nor considered sealed per FAQ 08-0042, the fire location 

would be assumed at the top of the door or opening that is expected to fail when fire damage 

occurs. If multiple doors or openings could possibly fail, the analysis should conservatively 

assume the fire is located at the uppermost door or opening. Additional refinement may be 

needed if this last assumption greatly affects the results. 

NUREG/CR-6850 should be revised to reflect the guidance provided above for the assumed fire 

location to be used for screening and detailed fire modeling for electrical cabinets. 

12.3 References 

1. Revision 0 to FAQ 08-0043, March 13, 2008, Accession No. ML081500507 

2. Revision 1 to FAQ 08-0043, December 4, 2008, Accession No. ML083540152 

3. NEI 04-02, Guidance for Implementing a Risk-Informed, Performance-Based Fire Protection 

Program Under 10 CFR 50.48(c), Revision 1, Accession No. ML052590476 

4. NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition (available through the Public Document Room or NFPA) 

5. Regulatory Guide 1.205, Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants, Accession No. ML061100174 

6. NRC Regulatory Information Summary 2007-19, Process for Communicating Clarifications 

of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During 

The Pilot Application of National Fire Protection Association Standard 805, Accession No. 

ML071590227 

7. NUREG/CR-6850 (EPRI 1011989), Accession Nos. ML050940183 (Vol. 1) and 

ML050940189 (Vol. 2) 

13-1 

13 

INCIPIENT FIRE DETECTION SYSTEMS (08-0046) 

13.1 Background 

The purpose of this interim position is to provide the current staff position for determining the 

probability of nonsuppression in fire areas that have installed incipient fire detection systems. 

FAQ 08-0046 was proposed by the Nuclear Energy Institute (NEI), through its National Fire 

Protection Association (NFPA) 805 Task Force, to seek additional guidance on modeling the use 

of incipient fire detection systems in fire probabilistic risk assessment (PRA) applications. The 

authors believed that insufficient guidance existed on modeling these systems in NUREG/CR6850 (EPRI 1011989), “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities.” 

Initial development of the additional guidance was performed under the memorandum of 

understanding (MOU) between the Electric Power Research Institute (EPRI) and the Office of 

Nuclear Regulatory Research (RES). These efforts were not concluded prior to EPRI publication 

of EPRI 1016735, “Fire PRA Methods Enhancements: Additions, Clarifications, and 

Refinements to EPRI 1011989,” in December 2008. 

Incipient fire detection systems have been used extensively in the telecommunications industry 

to minimize fire damage and limit interruption of service. A national consensus standard has 

been developed, NFPA 76, “Fire Protection of Telecommunications Facilities,” to address fire 

events in these high-value facilities. NFPA 76 classifies incipient fire detection systems as very 

early warning fire detection systems (VEWFDS)—systems that detect low-energy fires before 

the fire conditions threaten telecommunications service. 

In telecommunications service, VEWFDS have proven to be very effective in detecting fires in 

the incipient stage that originated in electrical and electronic cabinets and low-voltage electrical 

circuits (cable runs, junction boxes, termination cabinets, etc.). In fact, NFPA 76 essentially 

requires VEWFDS use in high-value areas such as main distribution frame equipment and other 

signal processing areas. 

In order to achieve closure of this FAQ in a timely manner, the NRC has developed an interim 

position, as discussed below. This position was developed based on the staff’s understanding of 

the VEWFDS detection equipment as well as how electrical and electronic equipment in nuclear 

power plants fail, and should not be seen as prejudicing the NRC’s view of future developments 

in this area. Final endorsement of this position will be addressed through the next revision of 

either Regulatory Guide 1.205 or NUREG/CR-6850. 
Incipient Fire Detection Systems (08-0046) 

13-2 

13.2 Resolution 

Applicability: This interim position applies to aspirating smoke detectors (ASD) installed as 

very early warning fire detectors as defined by NFPA 76 (2009 version), installed to monitor 

incipient degradation in electrical cabinets as discussed below. The position is based on the 

information on ASD VEWFDS available to the NRC staff. NFPA 76 requires that in order for a 

fire detection system to be considered a VEWFDS, it must meet two sensitivity criteria: It must 

be set up to provide Alert thresholds of at least 0.2 percent per foot obscuration (effective 

sensitivity at each port) and Alarm thresholds of at least 1 percent per foot of obscuration 

(effective sensitivity at each port). Licensees are free to propose the use of other technologies 

that meet these sensitivity requirements, but additional information/justification will be required. 

Spot-type detectors installed to meet the requirements of NFPA 72 may have been described as 

being capable of detecting fires in the incipient stage. In many cases the description of fire 

detection systems in licensee’s design and licensing basis documentation claims that the 

detection system can detect fires in the incipient stage. While this may be true to some extent, in 

order to obtain the credit described in this interim position, the detection system must be capable 

of meeting the more stringent requirements described in NFPA 76. 

Discussion: The current state of the art with respect to fire detection systems includes very 

highly sensitive detection systems designed to sense very slowly progressing degradation of 

electrical components before the flaming stage of fire occurs (incipient stage). There are 

numerous types of electrical components that exhibit this type of failure mode, many of which 

are used extensively in commercial nuclear power plants. Most low-voltage (~250 volts or less) 

electric and electronic components will degrade over a long period of time, with observable 

telltales that can be sensed by these sensitive detection systems. 

Examples of these include terminal strips, cables, interpanel wiring, electromechanical relays, 

transformers, switches, power supplies, amplifiers, bistables, controllers, manual-automatic 

control stations, indicators, gauges, and computers. In fact, a very high percentage of the 

electrical and electronic components inside cabinets in nuclear plants would be expected to 

exhibit this type of degradation. 

Industry Proposal: EPRI has developed a methodology to credit VEWFDS in Fire PRA 

quantification. EPRI Technical Report 1016735, “Fire PRA Methods Enhancements, Additions, 

Clarifications and Refinements to 1011989,” includes a discussion on crediting VEWFDS in 

Chapter 3, “Crediting Incipient Fire Detection Systems in FPRA Quantification.” There is also 

information on incipient fire detection systems in Appendix C, “Supplement for Crediting 

Incipient Fire Detection in FPRA Quantification.”

The EPRI report provides a good general overview of the concept of VEWFDS fire detection, as 

well as a good description of the types of fires that exhibit gradual degradation that is detectable 

using VEWFDS. 

The EPRI report proposes to apply this methodology to all electrical/electronic components with 

a voltage of equal to or less than 250VDC or 480VAC. 
Incipient Fire Detection Systems (08-0046) 

13-3 

The EPRI report also proposes to apply this methodology to various rotating equipment 

categories. Due to the variety of failure mechanisms related to mechanical/rotating equipment, 

the staff does not feel that application of the full risk reduction factors being considered for 

VEWFDS are appropriate for these components. Licensees that wish to credit risk reduction 

(beyond defense-in-depth) for VEWFDS that monitor rotating equipment must provide 

justification in the NFPA 805 License Amendment Request. The staff will consider each on a 

case-by-case basis. 

The EPRI approach utilizes an event tree to model the factors that could impact the effectiveness 

of the VEWFDS in preventing/mitigating a postulated event. The event tree provided by EPRI 

includes split fractions for: 1) the percentage of components that would be covered by 

VEWFDS, 2) the reliability/availability of the VEWFDS and 3) the percentage of time the preemptive actions of the first responders are successful. 

The NRC staff has reviewed the EPRI approach, and the NRC interim staff position represents 

changes to that approach. 

NRC Interim Staff Position: While the approach proposed by EPRI in report 1016735 provides 

a high-level approach to modeling a VEWFDS, there are several other issues that should be 

addressed and conditions applied to improve accuracy/realism. 

The first relates to the population of components that would exhibit incipient degradation. An 

additional factor should be added to the event tree to address the fact that a given electrical 

cabinet may have some percentage of components that may fail quickly and therefore not allow 

credit for incipient detection. Examples include electrical/electronic circuit boards that contain 

electrolytic capacitors, chart recorder drives, cooling fan motors, and mechanical timers driven 

by electric motors. 

The second relates to the rate of component degradation. The failure mechanisms that provide 

indication in the incipient phase may occur over extended time periods. This time period has a 

direct correlation to the effectiveness of the process since a longer degradation time would imply 

that the time period between detection of the degrading condition and a transition to a flaming 

fire would be longer, resulting in a higher probability of success that a fire will be prevented or, 

if not prevented, mitigated. However, although there is significant operating experience with 

VEWFDS, there is limited useful data documenting the duration of the incipient degradation 

time. As a result of the limited data, there is uncertainty related to the incipient degradation time. 

This uncertainty should be factored into the assessment of VEWFDS effectiveness. 

The third relates to the response to a VEWFDS alert/alarm. Since the proposed approach uses 

human intervention as the primary means of mitigating an impending fire, a factor should be 

inserted that allows this important part of the process to be accurately modeled. 

In order to prevent a fire, the response activity must also include actions to remove power from 

the failing component/subcomponent. As the event tree is currently constructed in the EPRI 

report, the actions to respond to the location and find the component/subcomponent have been 

addressed (essentially the “Operator Response”). The skill set needed to accomplish this is 
Incipient Fire Detection Systems (08-0046) 

13-4 

similar to that for responding to any other fire: the skill set corresponding to that of an operator 

or trained fire brigade member. However, once the component/subcomponent has been located 

and identified, another skill set will be required. This task (call it the “Technician Response”) 

requires someone knowledgeable in electrical/electronic circuits who can locate and read the 

appropriate drawings to determine how to remove power from the degrading device. In many 

cases, this activity is not a trivial exercise. It may involve researching numerous drawings 

(elementary, connection, interconnection, etc.) to properly locate the appropriate fuse, circuit 

breaker, or switch. In some cases, the required isolation device may not be in the same cabinet, 

the same row, or possibly even in the same room. 

If the actions to locate the device and remove power are not taken at an early stage, the 

effectiveness of the incipient process to prevent a fire is reduced. Note that this factor can be 

influenced by the rate of component degradation. If the degradation process occurs over a long 

time period, and is detected early in the process, the available time to prevent/mitigate a potential 

fire is much greater, allowing a higher probability of success. 

Based on these considerations, the event tree shown in Figure 13-1 is proposed for more 

accurately assessing the risk of fire assuming that a VEWFDS is installed. There is limited data 

from which the various factors can be derived. The EPRI report has cited a small number of tests 

that demonstrate the sensitivity of VEWFDS. The tests, however, do not address the duration or 

probability of the incipient degradation process that is a key factor in the true benefit of these 

systems. The NRC approach to dealing with this lack of data is addressed in the following. 

(1 - δ)

(1 - γ) (1 - ε1)

δ

(1 - β) ε1

(1 - ε2)

(1 - α) γ

ε2

β (1 - ε2)

λ

ε2

α (1 - ε2)

ε2 Fire Damage

End Point

No Fire

No Fire Damage to Targets 

Outside Cabinet

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

No Fire Damage to Targets 

Outside Cabinet

Fire Initiating Event

Fraction that Have 

an Incipient Phase 

Detectable by 

System

Detector System 

Availability and 

Reliability

Fire Damage

Successful Operator 

Response to Alert 

Technician Successful 

in Preventing Fire In 

Incipient Stage

Fire Suppressed

Figure 13-1 

Proposed event tree for assessing fire risk, assuming VEWFDS is installed 
Incipient Fire Detection Systems (08-0046) 

13-5 

Discussion of Branches of NRC Event Tree: At this time, this interim staff position, and the 

corresponding event tree, only apply to VEWFDS installed to monitor incipient fire conditions 

inside low-voltage (less than or equal to 250V) electrical cabinets. The branch points of the event 

tree are discussed in turn below. Each branch includes conditions that should be in place to 

obtain the credit listed. 

Fraction That Has an Incipient Phase Detectable by the System: For VEWFDS monitoring 

equipment inside electrical cabinets with a voltage of 250V or less, α, the factor for that 

percentage of components that do not exhibit incipient degradation, may be set to 0. 

To take credit for this value, only low-voltage (less than or equal to 250V) electrical cabinets 

may be included. In order to set this number to 0, the analyst must verify that the cabinet does 

not contain fast-acting components (such as electrical/electronic circuit boards that contain 

electrolytic capacitors, chart recorder drives, cooling fan motors, mechanical timers driven by 

electric motors, etc.) This assumption should be confirmed by inspection of the cabinet and 

adjusted if necessary based on the results of the inspection if there are components that would be 

fast acting. If fast-acting components are present, the event tree should include the branches 

addressing the Fraction that Has an Incipient Phase Detectable by System (α). For instance, if a 

cabinet contains 25 relays that would not be fast acting, along with a cooling fan and a motoroperated timer relay, the licensee could ratio the number of fast-acting components (2) to the 

total number in the cabinet (27) and come up with a value for α (α = 2/27 = 0.074). 

Where aspirated VEWFDS systems are used, the characteristics of the cabinet to be monitored 

must allow the use of an aspirated VEWFDS (aspirated systems would not function properly in a 

tightly sealed cabinet). 

In addition, in contrast to the EPRI position, 480 VAC cabinets and rotating equipment are also 

excluded. If licensees desire to credit VEWFDS on components with fast-acting, higher-voltage 

systems or components, or rotating equipment, additional factors should be included to address 

their higher probability of not exhibiting incipient behavior. 

Detection System Availability and Reliability: Success for this branch in the event tree means 

that the VEWFDS has issued an alert. β, the failure probability for this branch, can be 

determined using the process provided by EPRI in report 1016735 or set equal to 1E-02. 

The licensee should justify that their system is sufficiently similar to the systems evaluated in 

EPRI 1016735 when using this value for reliability. For example, EPRI 1016735 primarily has 

information on cloud chamber and laser aspirating detector systems. The use of other 

technologies should be justified to use the proposed value above. 

The system should be designed and installed by trained and qualified technicians to NFPA 76 

following appropriate vendor guidance, tested in accordance with an appropriate standard 

including appropriate vendor requirements, and maintained in accordance with manufacturer and 

code requirements. 
Incipient Fire Detection Systems (08-0046) 

13-6 

The system should pass the full vendor’s acceptance test and associated sensitivity testing, 

including any extended period of commissioning, prior to being placed in service. 

In addition to the regular functional testing required by NFPA 76 and any required preventive 

maintenance required by the vendor, the system should be tested and maintained in accordance 

with NFPA 72 and all vendor requirements (calibrated as required by the manufacturer). 

Most VEWFDS have the capability to provide two or more alarm levels. Alarms that are set to 

occur prior to the flaming stage are typically referred to as “Alerts,” and alarms that are set to 

occur when the device has entered the flaming or true fire stage are called “Alarms.” VEWFDS 

alert and alarm levels should be controlled through the licensee’s setpoint control program. 

Calibrations, such as rebaselining the alert and alarm levels, that reduce the sensitivity of the 

system should be evaluated to assure that the early detection function of the system is not 

compromised. Reductions in sensitivity should be considered in the fire PRA as a reduction in 

the system’s effectiveness. 

Testing and calibrations should be documented, and documentation should be maintained for the 

life of the plant. 

Successful Operator Responses to Alert: Success of this event implies that plant personnel 

have identified the cabinet that contains the source of the alert and have staged appropriately 

trained personnel (a qualified fire watch as is used for hot work or a “flash watch”) at that 

location, who are prepared to initiate fire suppression if an actual fire (e.g., open flaming) were 

to break out. γ reflects the likelihood that plant personnel fail to respond to an alert signal in a 

timely manner (i.e., prior to outbreak of open flaming within the source). γ, the probability of 

failure of the operator/fire brigade to respond to the alert and find the component, can be 

determined based on a human reliability analysis (HRA) or conservatively set to 1E-02 if the 

VEWFDS is addressable to multiple cabinets or to 5E-03 if the VEWFDS is addressable to an 

individual cabinet. The lower value recognizes that the cabinet affected is known and does not 

require additional investigation by the responders to identify the affected cabinet. 

The recommended value assumes that the VEWFDS provides at least one hour of warning prior 

to the actual outbreak of an open flaming fire. This value is considered conservative for an 

annunciator response when there is nothing else going on—since the alert occurs prior to any 

damage, it can be assumed there is no fire and no transient at this time. 

This number assumes that the operator response procedure directs the area and/or cabinet (if the 

VEWFDS is addressable to individual cabinets) to be investigated upon an alert from the 

VEWFDS. 

This number assumes that procedures would be in place to require establishment upon the 

annunciation of an alert of a qualified continuous “flash watch” (similar to that used to monitor 

hot work) until the potential for fire has been removed or until there has been a formal, 

documented evaluation of the event. (Note: One acceptable means of meeting this qualification 
Incipient Fire Detection Systems (08-0046) 

13-7 

requirement is to provide training in accordance with the requirements in NFPA 1081, “Standard 

for Industrial Fire Brigade Member Professional Qualifications,” Section 5.2.1, “Manual Fire 

Suppression.”) 

Effective methods must be established for locating the source of the incipient detection (portable 

VEWFDS, thermography, etc.) and the associated equipment must be dedicated for use, 

maintained in an operable condition, available on site at all times and appropriately staged to be 

rapidly accessed by first responders when needed. 

First responders are properly trained to respond to the incipient condition, identify the faulted 

cabinet, and suppress potential fires. Personnel using portable equipment to locate incipient 

degradation must be trained in its use, including on-the-job training such that they are familiar 

with the equipment, procedures for its use, and any limitations and/or precautions required. Also, 

adequate procedures exist, and the response process has been included within the scope of the 

fire brigade training and periodic drill process. 

Technician Successful in Preventing Fire in Incipient Stage: To simplify the analysis, δ, the 

factor for the probability of failure to remove power from the device once it has been located, is 

set to 1. This is done because of the difficulty in assessing the likelihood of successful 

prevention. 

This approach is taking credit for the fire watch only, as a surrogate for prevention. To be 

effective, the licensee must commit to procedures that require an appropriately trained fire watch 

to be in place until the problem has been resolved. Success in this approach is ultimately judged 

based on the ability to control the fire rather than suppress it. So long as the fire is prevented 

from growing significantly, the adverse consequences related to a large cabinet fire, and the 

associated fire growth due to secondary combustibles, are prevented. This is conservative, since 

in reality there would not be a fire contribution at all if the fire was prevented. In the case of fire 

prevention, the only impact on plant operation would be the unavailability of the component(s) 

in the cabinet for the duration of the repair. 

If a licensee desires to obtain more credit in this process, the more detailed NRC event tree may 

be used, including the branches with δ (with adequate and appropriate justification in the form of 

a detailed human reliability analysis). One way a licensee could achieve significant fire 

prevention credit would be to “prelocate” the isolation devices for all ignition sources within 

each cabinet in an effort to speed up the process. If such an effort was taken, additional credit for 

preventing fires could be allowed. This would need to include predetermining the isolation 

devices, conveniently displaying that information for use in response to VEWFDS alerts, training 

responders so that they could rapidly locate and operate the isolation device(s), and conducting 

drills to periodically demonstrate this ability. 

Fire Suppressed: There are two cases for this branch point. 

Success in the event following success in successful operator response to alert or alarm is that the 

“flash watch” stationed at the cabinet has successfully controlled the fire before it affects the 

target. ε1 represents the probability that, given success of event γ, the personnel staged at the 
Incipient Fire Detection Systems (08-0046) 

13-8 

cabinet responsible for the VEWFDS alert fails to promptly suppress the fire (i.e., quickly 

enough to prevent damage to PRA targets outside the cabinet) once open flaming does break out. 

ε1, the probability of “enhanced” nonsuppression, may be set to 1E-03. This is considered to be 

reasonable given the nature of the response required by a trained responder who is stationed at 

the location with the correct equipment. 

Success for the branches following failure in the successful operator response or detector 

unavailability is prevention of damage to the targets by the fire brigade. ε2, the probability of 

“normal” nonsuppression, should be taken from the Detection Suppression Event Tree in 

NUREG/CR-6850, Appendix P, using the electrical fire suppression curve for manual 

suppression as appropriate. Credit should be given as described in Appendix P for automatic 

detection and suppression (normal spot detectors and automatic suppression in the area) as well 

as delayed manual detection, manual actuation of fixed suppression, and manual suppression via 

the fire brigade. 

With the above simplifications, the event tree simplifies to the following: 

(1 - ε1)

(1 - γ)

ε1

(1 - β)

(1 - ε2)

λ γ

ε2

β (1 - ε2)

ε2

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

No Fire Damage to Targets 

Outside Cabinet

Fire Damage

Fire Initiating Event

Detector System 

Availability and 

Reliability

Successful Operator 

Response to Alert Fire Suppressed End Point

No Fire

No Fire Damage to Targets 

Outside Cabinet

Figure 13-2 

Simplified event tree 

Other General Considerations: Note that the staff plans to document the licensee commitments 

associated with the VEWFDS design, installation, testing, compensatory measures, and 

procedures for responding to the VEWFDS alert/alarms in each licensee’s license amendment 

request and reviewed and approved in the associated NRC Safety Evaluation, as applicable. 

Licensees that employ VEWFDS and model them in their fire PRA models that do not use the 

proposed values provided in this position should justify the various split fractions used in the 

plant-specific application and provide a characterization of the uncertainty on each of the split 

fractions and perform a sensitivity analysis to demonstrate robustness of the proposed position 
Incipient Fire Detection Systems (08-0046) 

13-9 

on acceptability of the plant change. The licensee should describe the operator response in 

sufficient detail for the NRC staff to understand how the human error probability (HEP) was 

determined. Regardless of how VEWFDS are modeled, licensees should provide a description of 

alarm response procedures, troubleshooting methods, and training of operators, maintenance 

personnel, fire watch standers, and fire brigade members. 

Based on the possible significant risk reduction being credited for VEWFDS installation, 

licensees should include in their fire protection program appropriate compensatory measures to 

address the unavailability/inoperability of the VEWFDS. These compensatory measures should 

be controlled through the use of a licensee-controlled process such as a Technical Requirements 

Manual or other defined process used to address fire protection program impairments that will 

ensure that the compensatory measures will be carried out. For licensees that plan to install 

VEWFDS as part of the NFPA 805 transition, the process for defining and controlling the 

compensatory measures should be described in the NFPA 805 License Amendment Request. 

Unless compensatory measures are evaluated to be equivalent to VEWFDS, such as continuous 

hot-work-type fire watch or use of a portable VEWFDS, an extended period of 

unavailability/inoperability may have a significant impact on overall plant fire risk. For example, 

an out-of-service period of four days would decrease the effectiveness of the system by an order 

of magnitude, based on the assumed availability factor. Even a day of unavailability would 

reduce effectiveness by a factor of about 3. 

Two additional factors in a performance-based approach are the implementation of the NFPA 

805 monitoring program and the fire PRA maintenance and update process. The staff expects 

licensees that implement VEWFDS to monitor the availability, reliability, and effectiveness of 

the VEWFDS so that, over time, more accurate and representative data may be used in the risk 

model. As required by NFPA 805, licensees are expected to set availability, reliability, and 

effectiveness targets and to take appropriate corrective actions when system performance does 

not meet the targets. Licensees are also expected to maintain their risk analysis current with the 

latest information. This includes consideration of new information from nuclear industry 

operating experience and external sources such as industry testing, research, data from other 

industries (such as the telecommunications industry), etc. While implementing the fire PRA 

maintenance and update process, if operating experience indicates that VEWFDS availability, 

reliability and effectiveness are not as high as currently modeled in the fire PRA, actions must be 

taken to update the analysis to reflect the new information. 

Licensees are cautioned that while the installation of VEWFDS to monitor critical control 

cabinets may significantly decrease fire risk and positively impact several of the fire protection 

defense-in-depth attributes (preventing fires from occurring, and rapidly detecting and 

suppressing those fires that do occur), consideration of defense-in-depth is a requirement of 

NFPA 805. Licensees are still required to demonstrate the ability to achieve the nuclear safety 

performance criteria assuming that a challenging fire impacts safe shutdown equipment. 

Depending upon the other defense-in-depth attributes for a given fire area, recovery actions 

and/or physical plant modifications may still be required to demonstrate the ability to meet the 

nuclear safety performance criteria. 
Incipient Fire Detection Systems (08-0046) 

13-10 

Deviations from the information provided in this position should be justified and, prior to credit 

in NRC regulatory activities, should be submitted to the NRC for review and approval. 

13.3 References 

1. Revision 0 to FAQ 08-0046, March 31, 2008, Accession No. ML081200120 

2. NRC Draft Interim Position on FAQ 08-0046, Accession No. ML091750338 

3. Resolution of stakeholder comments on the NRC Draft Interim Position, Accession No. 

ML093220197 

4. EPRI 1016735, Fire PRA Methods Enhancements, December 2008, Accession No. 

ML090290195 

5. NEI 04-02, Guidance for Implementing a Risk-Informed, Performance-Based Fire Protection 

Program Under 10 CFR 50.48(c), Revision 1, Accession No. ML052590476 

6. NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition (available through the Public Document Room or NFPA) 

7. Regulatory Guide 1.205, Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants, Accession No. ML061100174 

8. NRC Regulatory Information Summary 2007-19, Process for Communicating Clarifications 

of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During 

The Pilot Application of National Fire Protection Association Standard 805, Accession No. 

ML071590227 

9. NUREG/CR-6850 (EPRI 1011989), EPRI/NRC-RES Fire PRA Methodology for Nuclear 

Power Facilities, Accession Nos. ML050940183 (Vol. 1) and ML050940189 (Vol. 2) 
14-1 

14 

MANUAL NONSUPPRESSION PROBABILITY (FAQ 08-

0050) 

14.1 Background 

FAQ 08-0050 was proposed to clarify the guidance on manual nonsuppression probability 

provided in NUREG/CR-6850. This guidance required the separate consideration of fire brigade 

response time in manual nonsuppression analysis, despite its inclusion in much of the analysis. 

The purpose of this FAQ is to update guidance provided in NUREG/CR-6850, Appendix P, for 

the treatment of manual suppression and the fire brigade response. As a part of this update, a 

process has also been developed to adjust the nonsuppression analysis for scenario-specific fire 

brigade responses. 

The NRC developed the interim position discussed below in order to achieve closure of this FAQ 

in a timely manner. This interim position was developed using currently existing information, 

databases, and experimental results, and should not be seen as prejudicing the NRC’s view of 

future developments in this area. Final endorsement of this position will be addressed through 

the next revision of either Regulatory Guide 1.205 or NUREG/CR-6850. 

14.2 Resolution 

14.2.1 Introduction 

The suppression time of a fire is an important factor in the determination of the likelihood of 

fire-induced damage to a component. This time, labeled as Tsupp in this FAQ, is the time interval 

between when the fire is detected and when it is suppressed. Note that, depending upon the 

severity of the fire, the plant’s fire brigade may be called to respond. Figure 14-1 shows the 

conceptual relationship between Tdet (the time interval between the start of the fire and when the 

fire is initially detected), Tsupp, and Tfb, the time from fire detection until the fire brigade begins 

to apply suppressant agents.15

 

15 Note that Tdet, Tsupp and Tfb vary from fire to fire, and are treated statistically in the fire PRA. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-2 

Figure 14-1 

Conceptual relationship between fire detection time, suppression time, and fire brigade 

response time 

As discussed in NUREG/CR-6850, EPRI 1011989 (referred to elsewhere in this FAQ as 

NUREG/CR-6850 for brevity), the probability of nonsuppression by time t, Pns(t), is given by 

Pns(t) = Pr(Tsupp ≥ t) 

When used in computing the probability of fire-induced damage, t refers to the time available 

before damage to fire PRA targets occurs. Thus, in this application, t is replaced by the estimated 

time to damage minus the estimated time to detect the fire, i.e., <Tdamage> - <Tdet>. This 

difference represents the estimated time available to suppress the fire. Methods to compute 

<Tdamage> and <Tdet> are described in NUREG/CR-6850. 

Note also that this definition of time available to suppress the fire differs from NUREG/CR-6850 

in that it does not require an adjustment for Tfb (the fire brigade response time). That is, 

NUREG/CR-6850 defines the time available for manual suppression as the time to damage 

minus the time to detection minus the fire brigade response time, i.e., <Tdamage> - <Tdet> - <Tfb>. 

Under the revised approach, the fire brigade response time is already included in the distribution 

for Tsupp, as discussed below. 

Thus, the probability of fire-induced damage is given by: 

Pdamage = Pns(<Tdamage> - <Tdet>) = Pr(Tsupp ≥ [<Tdamage> - <Tdet>]) 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-3 

Two complications in the development of Pns(t) are the following: 

• Available data records for actual fire events are often incomplete or ambiguous regarding the 

detection time, suppression time, and brigade role and response time. 

• NUREG/CR-6850 does not provide guidance as to how generic nonsuppression probability 

distributions can be revised to reflect scenario-specific considerations (e.g., difficult-toaccess fire locations) that can affect the fire brigade response time. 

This FAQ resolution provides clarifying and revised guidance for the estimation of Pns(t). 

14.2.2 Solution 

This solution addresses the probability of nonsuppression for scenarios involving manual fire 

suppression (i.e., sequences D, E, H, I, L, and M in Figure P-1 of NUREG/CR-6850). In 

particular, this approach recognizes that manual suppression in these particular scenarios 

includes suppression activities by non-fire brigade personnel. Hence, there is some probability of 

manual suppression prior to arrival of the fire brigade. The solution is provided in two parts. The 

first part addresses cases where the fire brigade response time for the scenario being analyzed is 

judged to be comparable to the industry average. The second part addresses the process for 

making adjustments for cases where it is judged that the fire brigade response time distribution is 

significantly different from that underlying the events reported in the EPRI Fire Events Database. 

As a result of this FAQ, the two branch points in Figure P-1 that represent manual actuation of a 

fixed suppression system (MF) and the fire brigade response (FB) should be replaced by a single 

new branch point, manual fire suppression (MS). The manual nonsuppression curves and 

adjustment factor developed as a part of this FAQ can be used if no explicit credit is being taken 

for manual actuation of a fixed suppression system. In cases where this credit is being taken, a 

plant-specific analysis must be done. This plant-specific analysis must address the procedures 

and training for manually actuating a fixed suppression system, and explain how dependencies 

between manual actuation of a fixed suppression system and other manual suppression activities 

(e.g., manual suppression by portable extinguishers and hose stream) are addressed. 

The preceding discussion of MS replaces the NUREG/CR-6850 discussions of the MF and FB 

branch points from Figure P-1. 

(1) Industry-average response 

Figures 14-2 and 14-3 provide revised nonsuppression probability curves to be used when there 

are no scenario-specific factors that would tend to make the fire brigade response significantly 

different from the population of responses included in available data. Furthermore, the 

nonsuppression curves are used when the fire is not suppressed by prompt suppression, if 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-4 

applicable16 (i.e., “PS” fails), and not suppressed by automatic fixed suppression systems17 

(i.e., “AS” fails). These nonsuppression curves apply specifically to the new branch point MS, 

“manual suppression.” 

(2) Scenario-specific adjustments 

For cases where it is judged that the fire scenario being analyzed involves factors that will 

significantly affect fire brigade response (i.e., lead to a scenario-specific fire brigade response 

time more than 5 minutes different than the nominal fire brigade response time), the following 

approach may be used to estimate the impact of these factors on the probability of 

nonsuppression. 

Identify the scenario-specific factors expected to significantly affect Tfb , which is composed of 

the time for the fire brigade to reach the fire (after the fire is initially detected) and the time for 

the brigade to begin applying suppressants to the fire. Consider the location, accessibility, and 

type of fire; the location and condition of necessary equipment (e.g., hose stations) and material; 

and any special features of the fire location (e.g., proximity to sensitive equipment) that could 

affect the fire brigade’s decisions and actions. 

Document those factors judged to make the scenario unusual in comparison with fire scenarios 

more typical for the plant being analyzed. 

Estimate <Tfb-t> and <Tfb-s>, the mean typical and scenario-specific fire brigade response times,18

respectively, and document the basis (e.g., fire brigade response exercise results) for these 

estimates. 

Compute the probability of nonsuppression as follows: 

Pns(t) = Fns,i(t · Cs) = exp[-λ(t · Cs)] 

where Fns,i(•) is the exponential function for the appropriate nonsuppression curve from Figures 

14-2 and 14-3, λ is the corresponding mean suppression rate (1/time) from Table 14-2 below, 

and Cs is a scenario-specific adjustment factor: 

Cs = 1 - 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+

−

− −

− −

T T

T T

fb s fb t

fb s fb t

 

16 For example, a continuous fire watch during hot work activities. 

17 Automatic systems are unavailable, fail, or are assessed as not effective against the fire scenario. 

18 Recall that Tfb, even for a well-specified fire scenario, is a statistical variable. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-5 

Basis

(1) Industry-average response 

The adjustment to Figure P-1 to combine MF and FB into MS has been done to make the fire 

PRA treatment more consistent with the data. Since the manual nonsuppression curves generated 

with this FAQ credit both non-fire brigade and fire brigade suppression activities, the order of 

the top events in the event tree could be misleading. For example, non-fire brigade suppression 

activities, and even fire brigade suppression activities themselves, could very well come before 

manual actuation of a fixed suppression system. Therefore, there are potentially important 

dependencies between manual actuation of a fixed suppression system and the fire brigade that 

would have an impact on the available time that a fire brigade has to fight the fire, or the size of 

the fire that the brigade has to fight. 

The nonsuppression curves in Figures 14-2 and 14-3, and associated tabulated values in Table 

14-1, are based on a reanalysis of the 250 manual suppression fire events addressed in 

NUREG/CR-6850. Suppression rates for this reanalysis are provided in Table 14-2. This 

reanalysis provides a treatment of available data for fire duration and fire suppression times that 

is more consistent with the conceptual framework shown in Figure 14-1. It recognizes that 

manual suppression is a continuous activity that can begin once the fire is detected, rather than 

rely primarily on fire brigade suppression efforts. 

The nonsuppression curves from NUREG/CR-6850 and from this FAQ are each based on data 

provided in the EPRI Fire Events Database. This data is contained in Table 14-3. However, 

NUREG/CR-6850 uses, when possible, data entered in the “suppression time” field of the 

database. (For those events where suppression times are not provided, NUREG/CR-6850 uses 

the fire duration data19 entered in the EPRI Fire Events Database.) In this FAQ, the 

nonsuppression curves are based on data provided in the “fire duration” field of the database.20 

The recorded fire duration is the time from fire detection to extinguishment, and generally 

corresponds to Tsupp in Figure 14-1. 

As discussed earlier, the treatment in this reanalysis avoids the need to subtract the fire brigade 

response time from the available time to suppress the fire when estimating the damage 

probability. Thus, this treatment eliminates a conservatism inherent in NUREG/CR-6850 with 

respect to fire brigade response times. 

 

19 Fire duration and suppression time have specific database fields, and the times were taken from these fields. 

20 In a few exceptions, the duration times in the description of the event contradict the field. For those cases, the 

duration extracted from the event description is used in the analysis. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-6 

The nonsuppression curves respond to the uncertainty in the fire duration data in a manner 

consistent with that used in NUREG/CR-6850. Approximately 70 manually suppressed fire 

events in the EPRI Fire Events Database have duration data entered as a range (e.g., “16 to 30 

minutes” or “less than 5 minutes”). For such events, all points in the range are treated as being 

equally likely and the midpoint of the range is used in the numerical analysis, i.e., 23 min and 

2 min, respectively, for the examples. 

It should be noted that the analysis underlying Figures 14-2 and 14-3 has removed four fire 

events treated in NUREG/CR-6850. Three incidents (1176, 1345, and 2469 impacting the 

transient and welding nonsuppression curves) occurred in outside areas (2 events in a service 

building and 1 event in a steam generator construction area) and are outside the scope of this 

reanalysis. Incident 914 appears to be a duplicate of incident 495 based on similar event 

descriptions (with the exception that incident 914 is listed as an electrical fire in the suppression 

curve field and incident 495 is listed as an oil fire). Since the descriptions in the text clarify that 

the fire was an electrical fire, incident 495 was also removed. In addition to these four removals, 

one event, incident 821, was transferred from the analysis of electrical fires to that of oil fires. 

The associated description and data are more consistent with an oil fire. 

(2) Scenario-specific response 

The nonsuppression distributions shown in Figures 14-2 and 14-3 are derived from an analysis of 

events in which the fires were manually suppressed. Some (but not all) of these events were 

suppressed by the plant fire brigade, and so the nonsuppression distributions implicitly include 

the fire brigade response. The purpose of the adjustment described in the Solution portion of this 

FAQ is to address scenarios where the fire brigade response is expected to be very different from 

that included in the Figure 14-2 and Figure 14-3 curves. 

Although methods are available for estimating the contribution of the fire brigade response time 

to the overall fire duration (taking account of the uncertainties in the available data), such 

methods have not yet been fully tested with current data and incorporated into software tools for 

fire PRA practitioners. The FAQ solution uses a simple adjustment factor, labeled Cs above, that 

exhibits the following, appropriate trends as <Tfb-s> and <Tfb-t> change. 

If the scenario-specific fire brigade response is quicker than a typical response (i.e., <Tfb-s> is 

less than <Tfb-t>) then Cs > 1 (i.e., the effective time available for manual suppression is 

increased). 

If the scenario-specific fire brigade response is slower than a typical response (i.e., <Tfb-s> is 

greater than <Tfb-t>), then Cs < 1 (i.e., the effective time available for manual suppression is 

decreased). 

If the scenario-specific fire brigade response is the same as a typical response (i.e., <Tfb-s> equals 

<Tfb-t>), then Cs = 1 (i.e., there is no adjustment). 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-7 

(3) Correction factor 

The correction factor (Cs) is not based on a first-principles analysis. Rather, the factor was 

derived largely on an empirical basis in order to achieve the desired behavior. The objective was 

to create a relatively straightforward correction factor that adjusted the available time to reflect 

fire brigade responses that were either faster or slower than the typical case. Additional desirable 

characteristics of the adjustment include the following items. 

The magnitude of the adjustment should not be excessive for any cases. The objective for the 

interim position (e.g., pending additional validating research) was to allow for modest changes 

from generic values to reflect case-specific conditions. 

The adjustment factor should reflect that small differences in response time are generally more 

significant if the typical response time is small than if the typical response time is large. For 

example, a 5-minute difference in brigade response time is more significant to the overall fire 

behavior when the brigade response occurs within a 10–15 minute time frame than it is if the 

brigade response occurs within a 30–35 minute time frame. 

The adjustment factor should never be less than or equal to zero (that is, Cs>0 for all cases). If 

Cs≤0 for a given case, then no credit whatsoever would be given to manual fire suppression 

regardless of the time available before fire damage occurs. 

Even if the fire brigade response time exceeds the time available for suppression, the probability 

of nonsuppression should still reflect the potential that other plant personnel may intervene and 

suppress the fire. This characteristic is consistent with the intent of the original FAQ. 

The correction factor should work for all practical cases without the need for additional rule sets 

to limit application or to correct anomalous results. 

The form of the adjustment factor used derives from common temperature normalization forms 

used in heat transfer, especially when dealing with various conduction and convection problems 

where temperature differences tend to dominate the solution. This form does provide all of the 

desired characteristics listed above. 

Recall that the correction factor is defined as follows: 

Cs = 1 - 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+

−

− −

− −

T T

T T

fb s fb t

fb s fb t

The summation in the denominator may appear arbitrary, but is important because it acts to deemphasize small changes in larger numbers and prevents the correction factor from going to 

zero, at least for practical applications. Other potential formulations not utilizing the summation 

in the denominator (e.g., normalizing using just <Tfb-s > or <Tfb-t >) were generally found to be 

too volatile (yielding excessively large corrections) and tended to yield anomalous or 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-8 

unreasonable results for certain types of cases (e.g., yielding Cs≤0 for some cases). Other forms, 

such as a simple linear shift in the time available, also tended to yield anomalous results for 

certain types of cases, requiring the application of additional rule sets to correct such cases. 

The analyst may also note that the following is an equivalent numerical form for the correction 

factor: 

Cs = ( )( )⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

− + −

−

1/2 T T

T

fb s fb t

fb t

This alternate form illustrates that the correction factor can also be seen as the ratio of the typical 

response time to the average of the typical and case-specific response times. 

Insights from results

Figures 14-4 through 14-15 provide comparisons of the suppression curves from this analysis 

and from NUREG/CR-6850. First of all, the FAQ 50 curves typically provide comparable to 

slightly higher nonsuppression probabilities than NUREG/CR-6850. These curves are very 

similar since approximately one-half of the 250 suppression data entries in the database contain 

no entry for suppression time. Secondly, a “6850+10” curve was generated with the assumption 

of a 10-minute full fire brigade response. The results of this second comparison between the 

FAQ 50 curves and the “6850 + 10” curves demonstrate the potential degree of conservatism 

with the NUREG/CR-6850 approach of adding the fire brigade time to the suppression time. 

Insights from database review 

The reanalysis of the manual fire suppression events in the database highlighted that manual fire 

suppression is a more continuous process than the original NUREG/CR-6850 treatment 

provided. Unlike the NUREG/CR-6850 analysis, which assumed that manual fire fighting was 

largely a function of the fire brigade, manual fire suppression activities effectively begin as soon 

as a fire has been detected, and if needed, confirmed. Many of the fire events are suppressed 

before the fire brigade arrives in full. The revised analysis treats the overall process of manual 

fire suppression in a more continuous manner consistent with the actual response to a fire. 

Furthermore, for those cases where the database explicitly indicates that the fire brigade applied 

a hose stream, this database review confirmed that the duration data do not contain the time for 

detection. Approximately one-third of these events include information in the event description 

that allowed an independent confirmation for when the recorded duration began and ended. In 

those cases, the recorded duration began when the fire was detected by plant personnel or upon 

indications provided by alarms or failure of equipment. 

Finally, the time needed for confirmation of the fire after detection is rarely identified in fire 

events. A plant will generally confirm a detector actuation prior to sending out a full fire brigade 

to apply a hose stream. For those fires detected by fire watches or by plant personnel, 

confirmation may not be necessary. Note that nearly one-half of the entire 250 events in the set 

of suppression data identify plant personnel as the means of detection.
Manual Nonsuppression Probability (FAQ 08-0050) 

14-9 

14.3 References 

1. Revision 0 to FAQ 08-0050, May 13, 2008, Accession No. ML081200318 

2. NRC Draft Interim Position on FAQ 08-0050, Accession No. ML091660045 

3. Resolution of stakeholder comments on the NRC Draft Interim Position, Accession No. 

ML092180533 

4. NEI 04-02, Guidance for Implementing a Risk-Informed, Performance-Based Fire Protection 

Program Under 10 CFR 50.48(c), Revision 1, Accession No. ML052590476 

5. NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition (available through the Public Document Room or NFPA) 

6. Regulatory Guide 1.205, Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants, Accession No. ML061100174 

7. NRC Regulatory Information Summary 2007-19, Process for Communicating Clarifications 

of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During 

The Pilot Application of National Fire Protection Association Standard 805, Accession No. 

ML071590227 

8. NUREG/CR-6850 (EPRI 1011989), Accession Nos. ML050940183 (Vol. 1) and 

ML050940189 (Vol. 2) 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-10 

Table 14-1 Updated Numerical Results for Suppression Curves 

Time (min) 

T/G fires 

High-energy 

arcing faults 

Outdoor 

transformers 

Flammable 

gas 

Oil fires 

Electrical fires 

Transient fires 

PWR 

containment 

Welding 

Control room 

Cable fires 

All fires 

0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 5 0.883 0.947 0.836 0.881 0.684 0.602 0.531 0.687 0.392 0.189 0.446 0.714 

10 0.780 0.897 0.698 0.776 0.468 0.362 0.282 0.472 0.153 0.036 0.199 0.510 15 0.689 0.850 0.584 0.683 0.320 0.218 0.150 0.325 0.060 0.007 0.089 0.364 20 0.609 0.805 0.488 0.602 0.219 0.131 0.080 0.223 0.024 0.001 0.040 0.260 25 0.538 0.762 0.408 0.530 0.150 0.079 0.042 0.153 0.009 * 0.018 0.186 30 0.475 0.722 0.341 0.467 0.102 0.048 0.023 0.105 0.004 * 0.008 0.133 35 0.419 0.684 0.285 0.411 0.070 0.029 0.012 0.072 0.001 * 0.004 0.095 40 0.370 0.647 0.238 0.362 0.048 0.017 0.006 0.050 * * 0.002 0.068 45 0.327 0.613 0.199 0.319 0.033 0.010 0.003 0.034 * * * 0.048 50 0.289 0.581 0.166 0.281 0.022 0.006 0.002 0.024 * * * 0.035 55 0.255 0.550 0.139 0.248 0.015 0.004 * 0.016 * * * 0.025 60 0.226 0.521 0.116 0.218 0.010 0.002 * 0.011 * * * 0.018 65 0.199 0.493 0.097 0.192 0.007 0.001 * 0.008 * * * 0.013 70 0.176 0.467 0.081 0.169 0.005 * * 0.005 * * * 0.009 75 0.155 0.443 0.068 0.149 0.003 * * 0.004 * * * 0.006 80 0.137 0.419 0.057 0.131 0.002 * * 0.002 * * * 0.005 85 0.121 0.397 0.047 0.116 0.002 * * 0.002 * * * 0.003 90 0.107 0.376 0.040 0.102 0.001 * * 0.001 * * * 0.002 95 0.095 0.356 0.033 0.090 * * * * * * * 0.002 

100 0.084 0.337 0.028 0.079 * * * * * * * 0.001 

* A value of 1E-3 should be used 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-11 

Table 14-2 Original and Updated Mean Suppression Rates (λ) 

Original 

NUREG/CR-6850 Revised Analysis 

Suppression Curve 

No. of original 

events/revised 

events 

Original Total 

Suppression 

Time 

Original Mean 

Suppression Rate 

[/min] 

Revised Total 

Duration 

Revised Mean 

Suppression Rate 

[/min] 

T/G fires 21/21 749 0.03 846 0.025 Control room 6/6 18 0.33 18 0.33 PWR containment 3/3 23 0.13 40 0.075 Outdoor transformers 14/14 373 0.04 390 0.036 Flammable gas 5/5 195 0.03 197 0.025 Oil fires 36/36 404 0.09 474 0.076 Cable fires 5/5 21 0.24 31 0.161 Electrical fires 114/113 942 0.12 1113 0.102 Welding fires 19/18 99 0.19 106 0.188 Transient fires 24/22 199 0.12 174 0.126 High-energy arcing faults 3/3 239 0.01 276 0.011 All fires 24521 /246 3113 0.08 3655 0.067 

21 The “All Fires” nonsuppression analysis in the original NUREG/CR-6850 excluded events from the cable fire bin. Thus, the total number of events taken from 

the individual suppression analyses in the original NUREG/CR-6850 is 250; however, the number of events used in the “All Fires” curve is only 245. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-12 

Table 14-3 

List of Fire Events for Original and Revised Suppression Curves22 Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

398 9/7/1983 1983 Low Power Operation Cable 2 2 

510 2/1/1986 1986 Power Operation Cable 10 10 

681 3/9/1988 1988 Low Power Operation Cable 5 15 

2361 3/10/1986 1986 Power Operation Cable 2 2 

2425 3/1/2000 2000 Power Operation Cable 2 2 

485 8/24/1985 1985 Power Operation Containment (PWR) 15 24 

1041 7/11/1994 1994 Power Operation Containment (PWR) 6 14 

1488 10/21/1987 1987 Power Operation Containment (PWR) 2 2 

537 9/4/1986 1986 Low Power Operation Control Room 1 1 

659 12/30/1987 1987 Power Operation Control Room 1 2 

756 10/14/1988 1988 Low Power Operation Control Room 1 1 

928 3/1/1989 1989 Power Operation Control Room 1 2 

980 3/23/1990 1990 Undetermined Control Room 2 2 

2160 4/4/1996 1996 Low Power Operation Control Room 10 10 

238 1/24/1981 1981 Power Operation Electrical 5 30 

269 8/10/1981 1981 Power Operation Electrical 1 1 

352 11/3/1982 1982 Power Operation Electrical 5 5 

357 11/27/1982 1982 Power Operation Electrical 2 4 

388 6/19/1983 1983 Power Operation Electrical 4 4 

418 4/28/1984 1984 Low Power Operation Electrical 10 60 

469 5/2/1985 1985 Low Power Operation Electrical 1 11 

484 8/14/1985 1985 Power Operation Electrical 15 15 

490 10/11/1985 1985 Undetermined Electrical 11 11 

493 10/31/1985 1985 Power Operation Electrical 1 1 

498 12/3/1985 1985 Power Operation Electrical 10 10 

505 1/8/1986 1986 Low Power Operation Electrical 36 36 

513 2/19/1986 1986 Low Power Operation Electrical 6 6 

516 3/8/1986 1986 Low Power Operation Electrical 6 8 

518 3/22/1986 1986 Low Power Operation Electrical 1 1 

522 4/17/1986 1986 Low Power Operation Electrical 10 10 

529 6/22/1986 1986 Power Operation Electrical 1 1 

541 9/19/1986 1986 Power Operation Electrical 5 10 

 

22 Date, year, and mode of operation for each event are draft and have not been confirmed. This information is not 

relevant to the analysis, but provided as a modifier to the incident number. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-13 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

544 10/14/1986 1986 Undetermined Electrical 12 12 

551 12/16/1986 1986 Power Operation Electrical 1 1 

557 1/31/1987 1987 Low Power Operation Electrical 10 30 

572 3/14/1987 1987 Power Operation Electrical 3 8 

608 6/17/1987 1987 Low Power Operation Electrical 1 1 

611 7/2/1987 1987 Low Power Operation Electrical 12 12 

614 7/10/1987 1987 Power Operation Electrical 3 3 

625 9/17/1987 1987 Power Operation Electrical 1 14 

642 11/4/1987 1987 Power Operation Electrical 45 50 

644 11/10/1987 1987 Undetermined Electrical 10 10 

654 12/11/1987 1987 Power Operation Electrical 1 1 

656 12/17/1987 1987 Power Operation Electrical 25 30 

665 1/19/1988 1988 Low Power Operation Electrical 10 10 

667 1/28/1988 1988 Low Power Operation Electrical 7 7 

673 2/8/1988 1988 Low Power Operation Electrical 2 2 

708 5/10/1988 1988 Low Power Operation Electrical 3 8 

726 6/11/1988 1988 Low Power Operation Electrical 3 17 

735 7/21/1988 1988 Power Operation Electrical 2 13 

745 8/17/1988 1988 Power Operation Electrical 5 10 

755 10/5/1988 1988 Power Operation Electrical 2 3 

792 7/15/1988 1988 Power Operation Electrical 10 5 

821 12/22/1990 1990 Power Operation Electrical (to oil) 20 20 

876 3/8/1992 1992 Low Power Operation Electrical 6 6 

914 11/20/1985 1985 Low Power Operation Electrical 20 23 

922 7/10/1987 1987 Power Operation Electrical 3 3 

942 3/5/1989 1989 Power Operation Electrical 6 15 

977 1/19/1990 1990 Low Power Operation Electrical 9 9 

978 1/22/1990 1990 Low Power Operation Electrical 2 2 

1034 10/15/1996 1996 Power Operation Electrical 10 10 

1053 8/19/1989 1989 Power Operation Electrical 7 7 

1097 11/15/1986 1986 Low Power Operation Electrical 95 95 

1100 4/18/1989 1989 Power Operation Electrical 5 5 

1124 10/7/1986 1986 Undetermined Electrical 4 4 

1129 2/15/1989 1989 Low Power Operation Electrical 1 1 

1133 11/7/1989 1989 Undetermined Electrical 5 5 

1135 4/6/1990 1990 Low Power Operation Electrical 24 24 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-14 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

1137 6/7/1990 1990 Low Power Operation Electrical 1 1 

1139 7/9/1990 1990 Power Operation Electrical 3 3 

1141 9/10/1990 1990 Undetermined Electrical 5 5 

1142 12/19/1994 1994 Power Operation Electrical 10 10 

1160 9/27/1991 1991 Undetermined Electrical 5 5 

1163 2/29/1992 1992 Undetermined Electrical 5 5 

1173 2/20/1994 1994 Low Power Operation Electrical 12 12 

1213 2/9/1995 1995 Power Operation Electrical 5 5 

1262 11/2/1990 1990 Low Power Operation Electrical 3 3 

1264 10/3/1991 1991 Power Operation Electrical 1 1 

1270 10/12/1992 1992 Power Operation Electrical 1 1 

1276 7/25/1993 1993 Power Operation Electrical 35 35 

1335 3/3/1992 1992 Power Operation Electrical 7 7 

1337 3/31/1989 1989 Low Power Operation Electrical 9 9 

1339 6/28/1990 1990 Low Power Operation Electrical 29 29 

1487 4/17/1987 1987 Power Operation Electrical 2 2 

1489 10/26/1987 1987 Low Power Operation Electrical 2 2 

1491 6/11/1990 1990 Low Power Operation Electrical 2 2 

1501 10/11/1994 1994 Low Power Operation Electrical 2 2 

1504 8/15/1995 1995 Low Power Operation Electrical 10 10 

1509 11/23/1998 1998 Low Power Operation Electrical 1 1 

1511 3/19/1999 1999 Power Operation Electrical 2 2 

2127 5/25/1996 1996 Undetermined Electrical 2 2 

2161 7/10/1996 1996 Low Power Operation Electrical 2 2 

2179 1/12/1994 1994 Undetermined Electrical 22 22 

2190 1/8/1997 1997 Undetermined Electrical 45 45 

2191 3/7/1994 1994 Undetermined Electrical 2 2 

2211 2/13/1997 1997 Power Operation Electrical 10 10 

2219 3/21/1996 1996 Undetermined Electrical 2 2 

2227 3/2/1997 1997 Power Operation Electrical 2 2 

2236 10/22/1997 1997 Power Operation Electrical 10 10 

2251 1/16/1998 1998 Power Operation Electrical 10 10 

2255 1/11/1993 1993 Undetermined Electrical 2 2 

2269 10/31/1994 1994 Power Operation Electrical 2 2 

2272 11/19/1995 1995 Undetermined Electrical 10 10 

2273 9/25/1995 1995 Undetermined Electrical 2 2 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-15 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

2276 7/6/1995 1995 Power Operation Electrical 10 10 

2281 5/14/1998 1998 Power Operation Electrical 10 14 

2305 6/7/1998 1998 Undetermined Electrical 2 2 

2311 9/1/1999 1999 Power Operation Electrical 2 2 

2313 8/16/1999 1999 Power Operation Electrical 2 2 

2314 8/24/1999 1999 Power Operation Electrical 10 10 

2319 5/6/1999 1999 Undetermined Electrical 2 2 

2329 11/29/1992 1992 Power Operation Electrical 2 2 

2336 8/22/1990 1990 Power Operation Electrical 2 2 

2339 10/14/2000 2000 Power Operation Electrical 10 10 

2349 7/1/1998 1998 Power Operation Electrical 10 10 

2351 8/12/1997 1997 Power Operation Electrical 2 2 

2353 10/14/1996 1996 Power Operation Electrical 2 2 

2375 2/19/1999 1999 Power Operation Electrical 2 2 

2377 10/23/2000 2000 Low Power Operation Electrical 2 2 

2378 2/25/2000 2000 Power Operation Electrical 10 12 

2387 11/18/1993 1993 Power Operation Electrical 2 2 

2416 11/5/2000 2000 Low Power Operation Electrical 10 10 

2426 5/15/2000 2000 Power Operation Electrical 22 22 

2428 8/16/2000 2000 Power Operation Electrical 22 22 

2441 12/27/2000 2000 Power Operation Electrical 2 2 

2445 10/5/1987 1987 Undetermined Electrical 2 2 

2447 8/1/1987 1987 Undetermined Electrical 2 2 

2476 1/23/1989 1989 Undetermined Electrical 10 10 

433 7/20/1984 1984 Power Operation Flammable Gas 46 46 

512 2/17/1986 1986 Power Operation Flammable Gas 9 9 

528 6/19/1986 1986 Low Power Operation Flammable Gas 60 72 

1516 1/13/1998 1998 Power Operation Flammable Gas 20 10 

2356 8/31/1992 1992 Power Operation Flammable Gas 6023 60 

947 1/3/1989 1989 Power Operation High-Energy Arcing Faults 46 59 

2175 6/10/1995 1995 Power Operation High-Energy Arcing Faults 57 76 

2424 2/3/2001 2001 Power Operation High-Energy Arcing Faults 136 141 

260 6/30/1981 1981 Low Power Operation Oil 1 5 

 

23 Cited duration is 60+ minutes. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-16 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

262 7/14/1981 1981 Power Operation Oil 8 8 

263 7/16/1981 1981 Power Operation Oil 1 1 

266 7/24/1981 1981 Power Operation Oil 15 15 

296 1/9/1982 1982 Low Power Operation Oil 40 45 

476 6/26/1985 1985 Power Operation Oil 10 10 

477 6/29/1985 1985 Power Operation Oil 3 10 

495 11/2/1985 1985 Low Power Operation Oil (Deleted) 20 23 

508 1/25/1986 1986 Low Power Operation Oil 1 1 

524 5/10/1986 1986 Power Operation Oil 9 34 

535 8/13/1986 1986 Low Power Operation Oil 3 11 

559 2/8/1987 1987 Low Power Operation Oil 4 21 

566 3/1/1987 1987 Low Power Operation Oil 25 30 

662 1/8/1988 1988 Power Operation Oil 60 60 

710 5/10/1988 1988 Low Power Operation Oil 27 27 

736 7/24/1988 1988 Power Operation Oil 15 23 

737 7/29/1988 1988 Power Operation Oil 3 7 

765 11/27/1988 1988 Low Power Operation Oil 3 3 

811 4/17/1992 1992 Undetermined Oil 1 1 

824 7/13/1992 1992 Power Operation Oil 15 15 

875 5/27/1990 1990 Low Power Operation Oil 1 1 

961 8/11/1991 1991 Power Operation Oil 11 18 

1023 8/16/1993 1993 Power Operation Oil 10 5 

1108 6/6/1989 1989 Power Operation Oil 4 4 

1110 2/2/1990 1990 Power Operation Oil 5 5 

1263 3/8/1991 1991 Low Power Operation Oil 6 6 

1482 1/22/1986 1986 Power Operation Oil 10 5 

1483 3/13/1986 1986 Power Operation Oil 10 5 

1485 7/20/1986 1986 Low Power Operation Oil 10 5 

1506 2/24/1998 1998 Power Operation Oil 2 2 

1507 5/11/1998 1998 Power Operation Oil 2 2 

1514 10/9/1997 1997 Power Operation Oil 2 2 

2183 9/13/1996 1996 Undetermined Oil 45 45 

2345 11/3/2000 2000 Power Operation Oil 2 2 

2388 12/16/1993 1993 Power Operation Oil 10 10 

2422 8/24/2000 2000 Power Operation Oil 10 10 

368 2/16/1983 1983 Power Operation Outdoor Transformers 1 12 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-17 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

405 11/14/1983 1983 Power Operation Outdoor Transformers 40 40 

407 12/23/1983 1983 Power Operation Outdoor Transformers 120 120 

734 7/17/1988 1988 Power Operation Outdoor Transformers 2 2 

860 9/4/1992 1992 Power Operation Outdoor Transformers 27 27 

934 4/13/1986 1986 Power Operation Outdoor Transformers 120 120 

1033 6/23/1996 1996 Power Operation Outdoor Transformers 20 20 

1035 1/5/1999 1999 Power Operation Outdoor Transformers 15 15 

2283 6/23/1994 1994 Power Operation Outdoor Transformers 2 2 

2285 10/25/1994 1994 Power Operation Outdoor Transformers 10 10 

2331 7/19/1994 1994 Power Operation Outdoor Transformers 2 2 

2341 8/21/2000 2000 Power Operation Outdoor Transformers 2 2 

2407 10/18/2000 2000 Power Operation Outdoor Transformers 2 2 

2427 9/22/2000 2000 Power Operation Outdoor Transformers 10 16 

323 5/27/1982 1982 Power Operation Transient 20 20 

464 3/29/1985 1985 Undetermined Transient 5 5 

567 3/2/1987 1987 Power Operation Transient 4 4 

577 3/27/1987 1987 Power Operation Transient 5 5 

650 11/30/1987 1987 Power Operation Transient 1 1 

653 12/10/1987 1987 Power Operation Transient 10 15 

704 4/20/1988 1988 Power Operation Transient 8 10 

968 4/3/1989 1989 Undetermined Transient 8 8 

997 2/11/1992 1992 Undetermined Transient 5 5 

1050 1/1/1989 1989 Power Operation Transient 5 5 

1119 2/23/1989 1989 Power Operation Transient 1 1 

1128 3/10/1988 1988 Power Operation Transient 10 10 

1164 3/16/1992 1992 Undetermined Transient 10 10 

1171 4/13/1993 1993 Power Operation Transient 1 1 

1176 9/29/1994 1994 Power Operation Transient (Removed) 25 25 

1195 8/8/1990 1990 Power Operation Transient 1 1 

1345 2/6/1990 1990 Undetermined Transient (Removed) 7 7 

2253 4/1/1993 1993 Power Operation Transient 2 2 

2257 1/12/1994 1994 Undetermined Transient 2 2 

2262 7/2/1994 1994 Power Operation Transient 45 45 

2291 1/6/1993 1993 Undetermined Transient 2 2 

2386 11/13/1993 1993 Power Operation Transient 10 10 

2393 8/9/1995 1995 Power Operation Transient 2 2 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-18 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

2501 12/1/1999 1999 Power Operation Transient 10 10 

304 2/4/1982 1982 Power Operation Turbine Generator 10 20 

326 6/11/1982 1982 Power Operation Turbine Generator 30 45 

384 5/20/1983 1983 Power Operation Turbine Generator 18 20 

401 9/19/1983 1983 Power Operation Turbine Generator 2 5 

402 9/25/1983 1983 Power Operation Turbine Generator 1 1 

487 9/12/1985 1985 Power Operation Turbine Generator 30 35 

531 7/23/1986 1986 Power Operation Turbine Generator 3 8 

554 1/2/1987 1987 Power Operation Turbine Generator 95 95 

562 2/16/1987 1987 Power Operation Turbine Generator 45 45 

636 10/16/1987 1987 Power Operation Turbine Generator 6 8 

668 1/28/1988 1988 Power Operation Turbine Generator 217 217 

809 12/23/1989 1989 Power Operation Turbine Generator 4 44 

851 11/9/1991 1991 Power Operation Turbine Generator 15 15 

926 1/20/1989 1989 Power Operation Turbine Generator 14 20 

929 10/9/1989 1989 Power Operation Turbine Generator 160 160 

940 10/2/1987 1987 Power Operation Turbine Generator 16 25 

1024 12/25/1993 1993 Power Operation Turbine Generator 2 2 

1042 7/29/1994 1994 Power Operation Turbine Generator 9 9 

2124 6/15/1994 1994 Power Operation Turbine Generator 6024 60 

2229 8/1/1997 1997 Power Operation Turbine Generator 10 10 

2337 9/12/1991 1991 Power Operation Turbine Generator 2 2 

242 2/24/1981 1981 Power Operation Welding 2 2 

257 6/3/1981 1981 Power Operation Welding 3 3 

294 12/17/1981 1981 Undetermined Welding 0 0 

319 4/14/1982 1982 Power Operation Welding 2 2 

413 2/13/1984 1984 Power Operation Welding 0 0 

474 6/14/1985 1985 Power Operation Welding 3 5 

700 4/15/1988 1988 Power Operation Welding 10 15 

751 9/27/1988 1988 Undetermined Welding 10 10 

1095 9/8/1986 1986 Power Operation Welding 2 2 

1200 9/1/1992 1992 Power Operation Welding 1 1 

1201 10/5/1992 1992 Power Operation Welding 0 0 

 

24 Cited duration is 60+ minutes. 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-19 

Table 14-3 (continued) 

List of Fire Events for Original and Revised Suppression Curves Incident No. Date Year 

Mode of Operation Suppression Curve 

Fire Suppression Time 

(Tsupp) 

Original 

NUREG/ 

CR-6850 

Value [min] 

Revised 

FAQ 50 

Value [min]

1231 3/9/1993 1993 Undetermined Welding 1 1 

1232 1/25/1994 1994 Power Operation Welding 0 0 

1275 7/14/1993 1993 Undetermined Welding 27 27 

2126 7/22/1996 1996 Undetermined Welding 2 2 

2143 8/13/1993 1993 Undetermined Welding 2 2 

2188 3/5/1994 1994 Undetermined Welding 2 2 

2237 10/28/1997 1997 Power Operation Welding 22 22 

2469 7/14/1988 1988 Undetermined Welding (Removed) 10 10 

Summary: Non-Suppression Curves

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

PWR containment

control room

transient

welding

flammable gas

all fires

Figure 14-2 

Revised nonsuppression curves Part A 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-20 

Summary: Non-Suppression Curves

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

Turbine Generator

Electrical

High Energy

Arcing Faults

outdoor

transformers

Oil

Figure 14-3 

Revised nonsuppression curves Part B 

Containment curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-4 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-21 

Transient Curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-5 

Revised nonsuppression curves for individual groupings 

Welding Curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-6 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-22 

Turbine Generator Curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-7 

Revised nonsuppression curves for individual groupings 

Outdoor transformers curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-8 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-23 

Flammable gas curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-9 

Revised nonsuppression curves for individual groupings 

HEAF curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-10 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-24 

Oil curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-11 

Revised nonsuppression curves for individual groupings 

Cable curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-12 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-25 

Control room curve

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-13 

Revised nonsuppression curves for individual groupings 

Electrical curves

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850 + 10min

Figure 14-14 

Revised nonsuppression curves for individual groupings 
Manual Nonsuppression Probability (FAQ 08-0050) 

14-26 

All fires curve

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20 30 40 50 60 70 80 90

Time (min)

Probability

FAQ 50 Results

6850 Results

6850+10min

Figure 14-15 

Revised nonsuppression curves for individual groupings 
15-1 

15 

SPURIOUS OPERATION PROBABILITY (FAQ 08-0047) 

15.1 Background 

15.1.1 Purpose of FAQ 08-0047 

FAQ 08-0047 was proposed to clarify guidance on quantification of spurious actuation 

probabilities for independent vs. dependent combinations. NUREG/CR-6850 (EPRI 1011989) 

provided a calculational scheme for multiple spurious actuation probabilities that implicitly 

assumed independence. As such, it would overestimate the joint probability in cases where the 

spurious actuations exhibited dependence. Subsequently, technical exchange took place between 

the Task Force and NRC staff, documented in Revision 1 to FAQ 08-0047 (ADAMS Accession 

No. ML082770662). 

The NRC staff has reviewed the proposal to add this clarifying guidance on the discussion in 

NUREG/CR-6850 to NEI 04-02 as presented in FAQ 08-0047, Revision 1, and finds that nothing 

in this FAQ would prevent continued endorsement of NEI 04-02. Circumstances requiring 

guidance interpretation or new guidance are as follows. 

Lessons learned from pilot review of the Fire PRA Standard indicate confusing guidance within 

NUREG/CR-6850 on the determination of circuit failure probabilities (spurious operation 

probabilities) for components with multiple cables within a fire area. 

During the pilots, it was noticed that circuit analysts were basically assuming that many cables 

within a fire area could cause a spurious operation independently of the other cables affected by 

the same fire. However, under certain conditions, when the first cable is damaged (either from 

spurious operation or blowing the fuse in the circuit), the damage to the other cables does not 

affect the outcome, i.e., the likelihood of a spurious actuation of the component is not increased. 

Particularly, spurious actuation from the second cable contributes to increased spurious actuation 

probability of the component when the second cable has a separate power supply. NUREG/ 

CR-6850 doesn’t provide guidance for looking at the independence or dependence of cable 

failures, and the FAQ information below has been developed to provide this guidance. 
Spurious Operation Probability (FAQ 08-0047) 

15-2 

15.2 Resolution 

A potential issue has been identified that could lead to the development of highly conservative 

estimates of the conditional probability of spurious actuations in cases where a given fire 

scenario may lead to the failure of more than one cable capable of inducing the spurious 

actuation. In particular, NUREG/CR-6850, EPRI TR 1011989, contains a specific statement as 

follows: 

When more than one cable can cause the component failure mode of concern, and those 

cables are within the boundary of influence for the scenario under investigation, the 

probability estimates associated with all affected cables should be considered when 

deriving a failure estimate for the component. In general, the probabilities should be 

combined as an “Exclusive Or” function, as shown: (with corresponding equation). 

This statement appears twice in Chapter 10; namely in Section 10.5.3.1, page 10-9, bullet list 

item #3 (top of page) and in Section 10.5.3.1, page 10-11, bullet list item #4. 

This treatment assumes that the cable failures and corresponding circuit responses are 

independent. However, in various cases, the cable failures and the potential for specific circuit 

effects will not, in fact, be independent. In such cases, use of the “exclusive or” combinatorial 

approach would overestimate the overall spurious actuation likelihood. Clarification of this 

guidance is needed. 

15.2.1 Background 

One key consideration with respect to interpreting and clarifying the cited guidance is the 

manner in which the available test data were evaluated in the generation of the current estimates 

of spurious actuation likelihood. In that evaluation (e.g., see the expert panel report, EPRI 

1006961), circuit faults given cable failure were classified as either “spurious actuation” or “fuse 

blow.” That is, the panel did not explicitly consider potential intermediate modes of cable 

faulting, but looked at effectively the “bottom line” question; namely, did the cable’s failure lead 

to a spurious operation prior to loss of circuit power, or did circuit power trip prior to observation 

of a spurious actuation? 

To illustrate the potential for misapplication of the existing guidance, consider a case where 

either of two cables in the same fire area might cause spurious actuation of a motor-operated 

valve. This situation is not uncommon for areas such as motor control center (MCC) or 

switchgear rooms where one control cable runs from the control room to the switchgear or MCC, 

and a second control cable runs from the switchgear or MCC to the valve itself. In such cases, it 

is often possible to induce a spurious actuation due to faults in either cable. For this type of 

circuit, both of the control cables would typically be powered by a common control power 

transformer, likely located in the switchgear or MCC itself. 
Spurious Operation Probability (FAQ 08-0047) 

15-3 

If any fire scenario in the room could fail both cables, then application of the passage cited above 

would imply that the total probability of spurious actuation would be based on combining the 

two individual values. Taking 0.3 as an example of a typical spurious actuation conditional 

probability value (given cable failure), the combined probability would be (0.3+0.3–(0.3*0.3)), 

or 0.51. 

However, for this particular case, the actual spurious actuation behavior would be driven entirely 

by the first cable failure (regardless of which of the two cables actually fails first). Once the first 

cable fails, only two results are possible in the context of the spurious actuation likelihood 

values. Either a spurious actuation would occur, or a fuse-blow failure would deenergize the 

circuit prior to spurious actuation. In either case, failure of the second cable is irrelevant. In 

particular, for this type of case, a fuse-blow failure induced by the first cable failure would also 

deenergize the second cable. Thus the second cable would be unable to induce a spurious 

actuation, lacking the necessary control power energized source. Hence for this case, the correct 

spurious actuation likelihood would be that associated with the first cable to fail, or 0.3. 

In order for the “exclusive or” combinatorial approach to be appropriate, the different cables 

would need to be independently capable of actuating the component of interest. This might, for 

example, be the case when a single valve is controlled by more than one control circuit, each 

with its own independent power supply. For example, a pump outlet valve would typically be 

controlled by one circuit associated with operation of the pump itself, but might also be 

controlled by a second independent circuit if, for example, closure of the valve was necessary to 

the successful operation of another plant system (e.g., to prevent backflow through the pump 

given reorientation of the plant flow configuration). Another example of this independence test 

would be an air-operated valve that is controlled by two or more separate solenoid valves. The 

valve is controlled mechanically through the porting of air to/from the AOV actuator, which can 

be controlled by one or more SOVs. These SOVs may be provided with independent electrical 

circuits and cables. Since each solenoid/electrical circuit can independently cause the valve to 

actuate, the probability for spurious operation is the mathematic sum of the independent hot short 

probabilities (a blown fuse in the first solenoid circuit does not prevent a spurious actuation from 

a hot short in the second SOV circuit). 

However, in most cases, the circuit failure probability for a single cable should be used rather 

than adding the failure probability for each cable. A general exception, in addition to the specific 

examples above, is as follows: 

• Target cables that are powered from a separate/alternate power supply, typically powering a 

relay or other separate device, where a ground fault (single or multiple ground fault) or 

blown fuse would not affect the primary circuit, should be considered separately. These 

circuits, referred to as either auxiliary or “off-scheme” circuits, often provide automatic 

operation of the component from an instrument/control circuit. Failure of these circuits 

would not, in general, prevent operation of the component through either operator action or 

by spurious operation of the circuit. 
Spurious Operation Probability (FAQ 08-0047) 

15-4 

The following guidance is recommended for determination of the circuit failure probability for 

both initial screening and detailed FPRA analysis. The guidance applies for both intercable and 

intracable failures. 

15.2.2 Initial Circuit Failure Probability Determination 

1. For components with no auxiliary or off-scheme circuits (powered from a separate power 

supply), the circuit failure probability is assigned based on the limiting (highest) circuit 

failure probability for the cables within the fire area/compartment. 

2. For components with auxiliary or off-scheme circuits, the circuit failure probability is based 

on the sum of a) the limiting (highest) circuit failure probability for the primary circuit and b) 

the limiting (highest) circuit failure probability for the auxiliary/off-scheme circuit. The 

individual circuit failure probabilities should be combined using the “exclusive or” method. 

3. If circuit analysis has not been performed to determine if auxiliary or off-scheme circuits are 

present for the component, then the circuit failure probability is assigned based on an 

“exclusive or” sum of the two highest circuit failure probabilities for the component cables. 

The auxiliary or off-scheme circuits would not include a second/alternate power supply. 

Components with separate power supplies are not affected by hot shorts in the separate supply, 

since the required electrical separation requires isolation from the main supply. Therefore, only a 

single cable from an auxiliary or off-scheme circuit contributes to the increased spurious 

actuation probability. For example, if a swing pump can be powered from both the A train and 

the B train, hot shorts on the B train should not affect pump operation when the pump is 

physically connected to the A train power supply. The detailed circuit analysis should include 

this consideration when determining cables and circuits that can affect component operation. 

15.2.3 Detailed FPRA Circuit Failure Probability Determination 

Once a detailed fire scenario analysis is performed, where detailed circuit routing has been 

determined, the location of each circuit can be accounted for in the FPRA. In this case, the cable 

initially damaged determines the circuit failure probability. If the primary and auxiliary/offscheme circuit cable are contained within the same cable tray or are estimated to be damaged at 

the same time, the guidance above for initial circuit failure probability determination would 

apply. For other circuits, the following can be used: 

1. If the primary circuit cable is damaged first, then the circuit failure probability is assigned 

based on the initial cable damaged. 

2. If the auxiliary or off-scheme cable is damaged first, then the circuit failure probability for 

the component is determined from the “exclusive or” sum of circuit failure probabilities for 

a) the initial auxiliary/off-scheme circuit cable damaged, and b) the initial primary circuit 

cable damaged. 
Spurious Operation Probability (FAQ 08-0047) 

15-5 

Detailed circuit analysis and cable routing for the component should be performed to confirm 

application of the above guidance. When component circuits are determined to be unique, 

adjustments to the above guidance may be needed. For example, if spurious operation of a 

component requires circuit failures in two cables, then the model would need to be adjusted to 

account for damage to each of the various combinations of cables within the fire scenario. 

Another example would be if the component contained two independent auxiliary/off-scheme 

circuits, both powered from separate power supplies. In this example, the initial spurious 

operation probability assigned would be a sum of the three circuit failure probabilities, and the 

detailed analysis could include consideration for the location and damage time for each of the 

cables. Finally, if failure of the auxiliary/off-scheme circuit prevents spurious operation of the 

component, then the off-scheme circuit would be treated as if it was a primary circuit, and the 

component circuit failure probability (initially assigned) would be based on the limiting (highest) 

circuit failure probability. 

15.2.4 Summary 

It is recommended that the “exclusive or” combinatorial approach for spurious actuation 

probabilities only be applied in cases where multiple cables can cause the undesired component 

effect and the postulated cable failure modes and effects are found to be independent. In cases 

where the cables of concern are dependent, the likelihood of spurious actuation should be 

determined by the first cable failure only. If the spurious actuation probability is different for the 

different cables of concern (e.g., due to differences in the cable or routing configuration), the 

analysis can either determine which cable would likely fail first for the given scenario, or simply 

bound the individual cable values. 

16-1 

16 

HOT SHORT DURATION (FAQ 08-0051)

16.1 Background 

Introduction: 

The purpose of FAQ 08-0051 is to provide guidance for the probabilistic treatment of the 

duration of hot shorts and therefore spurious actuations in alternating current (AC) circuits, and 

discuss conditions under which this duration is to be applied. No guidance on the duration of hot 

shorts was provided in NUREG/CR-6850 (EPRI1011989), “EPRI/NRC-RES Fire Probabilistic 

Risk Assessment (PRA) Methodology for Nuclear Power Facilities.” 

Testing will be conducted on spurious actuations on direct current (DC) circuits in the 

summer/fall of 2009 and continue until spring of 2010. Once the duration data is collected and 

processed, an analysis will be performed to provide probabilities of hot short duration for DC 

circuits. 

In order to achieve closure of this FAQ in a timely manner, the Nuclear Regulatory Commission 

(NRC) developed a draft interim staff position, as discussed below. This position was developed 

using currently existing information, databases, and experimental results, and should not be seen 

as prejudicing the NRC’s view of future developments in this area. 

Discussion: 

According to NUREG/CR-6834, “Circuit Analysis - Failure Mode and Likelihood Analysis,” a 

hot short is defined as a “conductor to conductor short circuit in which one or more nonenergized, non-grounded conductors become energized due to the cable failure.” Hot shorts are 

important in fire PRA because of their ability to produce undesirable conditions for pumps and 

valves, e.g., spurious starting/stopping of pumps and repositioning or “sealing in” undesirable 

positions of valves. For example, a motor-operated valve (MOV) may be repositioned from a 

hot short, whereas an air-operated valve (AOV) may be sealed in from a hot short on the related 

solenoid- operated valve (SOV) control circuit. The duration of a particular hot short is important 

since a corresponding pump or valve may be unable to perform its credited function during the 

time it is undesirably positioned due to the spurious actuation. Furthermore, a hot short on the 

circuit can complicate the recovery of affected equipment since the recovery may be delayed 

until the hot short has cleared. In some cases, a hot short may be unable to clear. In those cases 

where the hot short can clear, recovery of equipment and its associated function may be allowed 

and may require manual actions. In other cases, such as the AOV/SOV case, clearing of the hot 
Hot Short Duration (FAQ 08-0051) 

16-2 

short may also clear the spurious actuation (e.g., an AOV may return to its desired state once the 

SOV hot short clears). 

When accounting for hot short duration in a fire PRA, the duration is paired with the occurrence 

of the spurious actuation. It should be emphasized that the probability that the hot short duration 

equals or exceeds a particular time, i.e. P(T≥ t), is used to characterize the likelihood that a hot 

short condition persists beyond the specified time. Therefore, in the context of an event tree, the 

failure path or downward branch would be characterized by P(T≥ t), and the complement, 1 - 

P(T≥t), is used to characterize the success path or upward branch. In practice, the duration time 

of interest would be a characteristic of the PRA scenario. 

For example, the spurious opening of a power-operated relief valve (PORV) might be riskimportant only if it persists beyond some time window. The available time window would need 

to be defined by the PRA accident sequences, in this example case, likely based on thermalhydraulic calculations. For a PORV control circuit, clearing of a hot short would typically allow 

the PORV to return to a closed state. Hence, the likelihood that a hot short induces a spurious 

actuation that persists beyond the available time window is a relevant consideration in the risk 

quantification. 

16.2 Resolution 

Figure 16-1 shows the probability of the duration of a spurious actuation from a hot short lasting 

greater than or equal to time, t, in minutes. The distribution in the figure is the complementary 

cumulative distribution function (CCDF) of a Weibull distribution, and of the form: 

P(T≥ t) = exp(- λ t β ) 

The λ and β factors for hot short duration are 0.963 and 0.579. 
Hot Short Duration (FAQ 08-0051) 

16-3 

Hot Short Duration

0.00

0.10

0.20

0.30

0.40

0.50

0 5 10 15

t (min)

P(T>=t)

Figure 16-1 

Durations of hot shorts 

Table 16-1 provides the corresponding probabilities of spurious actuation duration from a hot 

short lasting greater than or equal to time, t, in minutes. Due to the significant uncertainty with 

the duration probabilities, 0.01 is the minimum recommended probability to be used in fire PRA. 

Identifying a minimum is consistent with other probabilistic approaches, such as with the manual 

non-suppression probability. 
Hot Short Duration (FAQ 08-0051) 

16-4 

Table 16-1 

Probability of duration ≥ time (min) for hot shorts 

t (min) P(T≥t) 

0 1.00E+00 

1 3.82E-01 

2 2.37E-01 

3 1.62E-01 

4 1.17E-01 

5 8.67E-02 

6 6.60E-02 

7 5.12E-02 

8 4.03E-02 

9 3.22E-02 

10 2.59E-02 

11 2.11E-02 

12 1.73E-02 

13 1.42E-02 

14 1.18E-02 

15 1.00E-02 

>15 0.01 

It must be noted that the above hot short duration probabilities should not be used under certain 

conditions. These conditions are: 

1) The hot short duration probabilities should not be applied to spurious actuation of equipment 

caused by grounding of one or more conductors (i.e., for those spurious actuations not caused by 

hot shorts). For these cases, no credit should be given for hot short duration.

The hot short duration probability is based on circuits where spurious actuations are induced from 

hot shorts between an energized source conductor and a de-energized target conductor. When the 

insulation resistance between these two conductors is reduced as a result of fire- induced effects, 

then the target conductor becomes energized and sufficient current or voltage will cause the end 

device of the target cable to change states or actuate. 

Testing showed that cable hot shorts are a transient condition whose duration is limited by the 

eventual grounding of the damaged cable. In most cases, it is the actual clearing of the protective 

devices, caused by a single or multiple grounds that result in the hot short termination. For hotshort induced spurious operations, termination of the hot short can be correlated to termination of 

the hot short signal. In some cases, however, a spurious operation can be caused by a short to 

ground, which is generally a more likely initial failure mode than a hot short. For example, if 

multiple transmitter cables became grounded, the associated signal may register off scale high or 

low and may cause equipment to spuriously actuate. The grounding of the cable conductors in 

such cases will generally not clear the spurious actuation, since this is the most electrically stable 
Hot Short Duration (FAQ 08-0051) 

16-5 

state for the failed circuit. As such, the hot short duration probability is not applicable to these 

circuit configurations. 

2) The hot-short duration probability should not be applied if the spurious actuation produced by 

the hot short would not clear once the cable is grounded. For this case, no credit should be given

for hot short duration. A review should be completed to ensure that clearing the hot short will 

clear the spurious actuation, including identification of the device (e.g., fuse or circuit breaker) 

that would clear the hot short given cable grounding

During the EPRI/NEI25 and CAROLFIRE26 testing, the eventual cable grounding resulted in a 

blown fuse that stopped the hot short (i.e., the circuit transitioned from a transient hot short 

condition to its electrically stable state for the damaged condition). A review of numerous valve 

control circuits resulted in the identification of some cables that if eventually grounded, might 

not result in a blown fuse or hot short cessation. In these example circuits, cables routed to an 

Auxiliary Shutdown Panel Transfer Switch, where the transfer switch did not indicate the valve 

position, would not cause the fuse to blow if an eventual ground occurs. These particular circuits 

would, when grounded, result in the valve transferring position from open to closed and back to 

open, repeatedly. It is likely the valve motor operator would eventually burn out (depending on 

circuit design, it is also possible that the valve overloads would heat up and open), given this 

scenario. However, the valve position when the valve motor failed would not be predictable. In 

the specific case of MOVs, for example, mitigation of the hot short and opening of the circuit 

fuse would leave the control circuit de-energized, but would also leave the valve in whatever 

position it assumed as a result of the original hot short (i.e., a MOV will not automatically return 

to its pre-failure state). 

The hot short duration probability is based on test results where grounding of the cable (source, 

target conductors) caused protective devices (i.e. fuse or circuit breaker) to clear and de- energize 

the circuit resulting in termination of the spurious actuation (i.e., relay actuation). Some circuits 

have been identified where grounding of the cable will not de-energize the control circuit. These 

types of circuits are not bounded by the testing results and associated hot short duration 

probability. As such, use of the hot short duration probability is unacceptable for these types of 

circuit. Furthermore, if clearing of the hot short will not cause the end device to return to its prefailure condition, then the analysis should not assume that the spurious actuation will clear given 

that the hot short clears. The available tests did not include explicit treatment of such cases. All 

of the available AC spurious actuation tests were based on motor contactor relays as the target 

device. These relays would close given a hot short but open given hot short clearing. The 

behavior of these control relays, however, may not reflect the behavior of the end device (e.g., 

the actual MOV itself) which could be left in a faulted state even after the control circuit is deenergized. Therefore, the hot-short duration probability should not be applied if the spurious 

actuation produced by the hot short would not clear once the cable is grounded. 

 

25 EPRI TR-1003326, "Characterization of Fire-Induced Circuit Faults," Dec. 2002; 

26 NUREG/CR-6931-V1, "Cable Response to Live Fire (CAROLFIRE) Volume 1: Test 

Descriptions and Analysis of Circuit Response Data," April 2008. 
Hot Short Duration (FAQ 08-0051) 

16-6 

3) Given a short to ground on an auxiliary or “off-scheme” circuit, credit for recovery of a 

spurious actuation needs to include a functional circuit analysis demonstrating the effect of a 

short to ground on the auxiliary circuit. For cases where functional circuit analysis cannot 

demonstrate recovery, no credit can be given for hot short duration since the short to ground 

will not clear by itself.

Hot shorts from auxiliary or “off-scheme”27 circuits that are powered from a separate power 

supply should be reviewed for the impact of grounding the circuit. The spurious operation may 

not stop when the ground occurs and the hot short on the auxiliary circuit terminates. The 

functional impact depends on the circuit design, whether a “seal-in” contact is in the circuit, and 

the component failure mode once the auxiliary circuit has grounded. 

It should be noted that a constraint exists on recovery where the hot short duration may be 

applicable. Regardless of whether the hot short duration is applicable, credit should not be taken 

for functional recovery of electrical control functions for circuits that become inoperable as a 

result of a spurious actuation. Manual operation or recovery of the end device (e.g., manual 

operation of a valve or a manual pump start/stop) may be possible, but analyses must be 

performed to demonstrate the feasibility of the manual operation and, as appropriate, the 

reliability of these actions should be factored into the fire PRA.

MOV and certain pump control circuits generally cannot be electrically recovered once their 

control circuit goes to ground. Almost all MOVs fail-as-is, and cannot be remotely operated once 

the cable failure mode transitions from a hot short (spurious actuation) to ground. Credit for 

manual operation of the MOV locally (for restoration of the MOV) should consider that the hot 

short might not have cleared. Additionally, credit for local MOV operation should consider the 

circuit design to ensure that prolonged operation of the MOV motor (i.e., torque and limit 

switches do not deactivate the motor) does not damage the valve stem or actuator (local 

operation may no longer be possible if significant stem or actuator damage occurs).28 Larger 

pumps that use medium voltage or low voltage power circuit breakers as their “on/off” control 

device will generally fail as-is once a hot short on the breaker’s control circuit eventually clears 

and power to the control circuit is lost due to the control circuit fuse clearing. 

Restoration of control functions failed by fire will likely require significant repair actions. 

Manual operation may be plausible, but may also be compromised by persistent faults, including 

loss of control power faults, on either the primary portions of the control circuit or on off-scheme 

circuits (e.g., loss of a permissive signal might prevent pump start). 

4) The spurious operation duration probabilities should not be used for DC Circuits.

 

27 "Off-scheme" circuits are circuits that have common connections to the control circuit of interest, but do 

not perform the primary control functions. Off-scheme circuits may, for example, be associated with 

certain auto-start or auto-stop functions, provide permissive signals that allow system actuation, be using 

system status indications to generate permissive signals for other circuits, or simply monitor system status. 

28 See, e.g., Information Notice 92-18, “Potential for Loss of Remote Shutdown Capability During a 

Control Room Fire.” 
Hot Short Duration (FAQ 08-0051) 

16-7 

The application of hot short duration data obtained from testing of AC circuits is currently not 

considered applicable to DC circuits. Future review and analysis of the DC-circuit testing 

(currently ongoing) may determine if the AC hot short duration data is applicable and, if so, to 

what extent. 

Basis for analysis:

The statistical analysis was performed using the entire set of NEI/EPRI test data and 

CAROLFIRE test data. The duration data analyzed for this FAQ comes from a set of 

experiments designed to emphasize coverage of potentially important characteristics and not to 

develop sample statistical representations of field conditions. For purposes of this interim 

solution, it is judged adequate to treat the duration data using a simple statistical approach. 

Insufficient data exists to justify splitting the data into factors that would affect hot short 

duration. Thus, no examination of factors was performed in this analysis.29

Probability plots were constructed for typical candidate distributions (Weibull, Lognormal, and 

Exponential) and parameters estimated using ordinary least squares regression. Because of the 

better fit, the Weibull distribution was selected. The R-squared value corresponding to the hot 

short duration is 0.99, and confirms, along with examination of the predicted versus observed 

data, that the Weibull distribution provides a reasonable fit to the data sets. Table 16-2 includes 

the NEI/EPRI and CAROLFIRE data. 

 

29 A research task to consider probability of spurious actuation duration is being discussed. This task will 

likely examine durations and occurrence of hot shorts via expert elicitation, which is appropriate for the 

duration data since it is limited from a statistical perspective. 
Hot Short Duration (FAQ 08-0051) 

16-8 

Table 16-2 

Duration data (min) from NEI/EPRI and CAROLFIRE tests 

NEI/EPRI 

data CAROLFIRE 

Index duration Index data duration 

1 7.6 1 0.58 

2 0.9 2 0.02 

3 0.4 3 0.1 

4 2.1 4 0.02 

5 0.4 5 0.25 

6 0.8 6 0.4 

7 1.9 7 0.08 

8 1.9 8 7.62 

9 0.4 9 0.18 

10 0.4 10 0.1 

11 1.9 11 3.82 

12 1.9 12 5.78 

13 1.4 13 0.18 

14 1.4 14 1.63 

15 2 15 0.27 

16 2 16 1.77 

17 0.9 17 3.77 

18 0.9 18 1.05 

19 0.2 19 1.05 

20 0.1 20 0.48 

21 2 21 2.03 

22 0.1 22 0.32 

23 0.1 23 0.77 

24 0.1 24 0.03 

25 3.4 25 3.85 

26 0.7 26 0.7 

27 0.9 27 0.73 

28 1.1 28 0.52 

29 5.7 29 0.4 

30 5.2 30 0.55 

31 0.1 31 0.3 

32 0.1 32 0.15 

33 0.1 33 0.02 

34 0.1 34 0.02 

35 8.2 35 0.02 

36 4.7 36 0.02 

37 10.1 37 0.22 

38 1.1 38 0.02 

39 1.1 39 0.02 

40 0.1 40 0.02 

41 0.3 41 0.02 

42 0.1 

43 2.1 

44 11.3 

45 8.1 

46 0.2 

47 3.2 
Hot Short Duration (FAQ 08-0051) 

16-9 

16.3 References: 

1. FAQ 08-0051, Revision 0, December 4, 2008, ADAMS Accession No. ML083400188. 

2. NEI 04-02, Guidance for Implementing a Risk-Informed, Performance-Based Fire Protection 

Program Under 10 CFR 50.48(c), Revision 1, ADAMS Accession No. ML052590476. 

3. NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition (available through the Public Document Room or NFPA, 

Quincy, MA). 

4. Regulatory Guide 1.205, Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants, ADAMS Accession No. ML061100174. 

5. NRC Regulatory Information Summary 2007-19, Process for Communicating Clarifications 

of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During 

The Pilot Application of National Fire Protection Association Standard 805, ADAMS 

Accession No. ML071590227. 

6. NUREG/CR-6850 (EPRI 1011989), ADAMS Accession Nos. ML050940183 (Vol. 1) and 

ML050940189 (Vol. 2). 

7. NUREG/CR-6834, ADAMS Accession No. ML032731464. 

8. NRC Information Notice 1992-18, ADAMS Accession No. ML031200477. 

9. NUREG/CR-6931, Available through the NRC public web site. 

10. EPRI TR-1003326, Characterization of Fire-Induced Circuit Faults, ADAMS Accession No. 

ML023500265. 

17-1 

17 TRANSIENT FIRE GROWTH RATE (FAQ 08-0052) 

17.1 Background 

FAQ 08-0052 was proposed on the treatment of transient fuel fires provided in NUREG/ 

CR-6850 (EPRI 1011989), “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power 

Facilities.” The guidance was not explicit in two regards. First, the guidance does not explicitly 

state which of the available manual fire suppression reliability curves (the nonsuppression 

probability curves) should be applied to the case of transient fires in the main control room; the 

choices being the transient fire curve or the main control room (MCR) curve. Second, the 

guidance discusses fire modeling assumptions related to time-dependent fire growth profiles in 

general terms and in the specific case of electrical cabinet fires, but it does not discuss fire 

growth times for trash fires, one of the most commonly postulated transient combustible fires. 

The purpose of this FAQ is to clarify and update guidance provided in NUREG/CR-6850 

(EPRI 1011989), Appendices G and P, for the treatment of transient fires in terms of both 

manual suppression and time-dependent fire growth modeling. 

17.2 Resolution 

17.2.1 Nonsuppression Curves for Transient Fires in the MCR 

NUREG/CR-6850, EPRI 1011989, Appendix P, provides fire nonsuppression curves that 

characterize manual fire fighting effectiveness as a function of elapsed time since fire detection. 

Separate curves are provided for transient fires and for MCR fires. The guidance does not 

explicitly state which of these two fire nonsuppression curves should be applied to the case of a 

transient fire in the MCR. The recommended practice is that all fires occurring in the MCR, 

including transient fires, should be treated using the MCR fire nonsuppression curve. 

This recommended guidance reflects (1) that application of the general transient fire 

nonsuppression curve would not be accurate because the MCR is continuously occupied; 

(2) that in developing the nonsuppression curves, the MCR curve included all reported MCR 

fires regardless of the fire source; and (3) the intent of the original authors of the RES/EPRI 

consensus methodology. 

17.2.2 Transient Fire Growth Rates/Times 

NUREG/CR-6850, EPRI 1011989, Appendix G, discusses in general terms the treatment of fires 

using a time-dependent fire growth profile. Specific fire growth time (time from ignition to peak 
Transient Fire Growth Rate (FAQ 08-0052) 

17-2 

fire heat release rate) recommendations are made for electrical cabinet fires. The guidance does 

not, however, address estimating the fire growth rate/time of transient fires. 

The original intent of the authors of the RES/EPRI consensus method was that the use of a timedependent fire growth profile is appropriate for any fire modeling case where a basis for such a 

profile can be established. It was not intended that this treatment be limited to cabinet fires. 

Hence, use of a time-dependent fire growth profile for transient fires, given adequate basis for 

the assumed growth time, is both appropriate and consistent with the intent of the original 

guidance. 

Transient fires represent a wide range of potential fire sources, from liquid fuels (e.g., solvents) 

to solid fuels of various sizes, types, and configurations. It would be inappropriate to assume a 

single fire growth time for all transient fire sources. Rather, the fire growth time should reflect 

the nature of the transient fuel package that is assumed to be present in the fire scenario. 

Recommendations for appropriate fire growth times for three specific and common transient fire 

sources are detailed below. The following recommended fire growth times are the times from 

fire ignition to peak fire heat release rate. The fire growth profile is assumed to follow the classic 

“t-squared” curve as discussed in Appendix G of NUREG/CR-6850, EPRI 1011989. The three 

case recommendations are as follows. 

Common trash cans (i.e., plastic or metal receptacles up to 33 gallons in size intended for 

temporary trash collection) that contain routine types of refuse (paper, plastics, and other solid 

materials) may be assumed to grow from zero to peak heat release rate in 8 minutes. This 

guidance is based largely on two experimental programs that have tested transient fuel packages 

of this type. The details are provided in Appendix A. 

Common types of plant trash (paper, plastics, and other solid materials) that are contained in 

plastic trash bags but that are not contained within a plastic or metal receptacle may be assumed 

to grow from zero to peak heat release rate in 2 minutes. This guidance is based largely on one 

experimental program that has tested transient fuel packages of this type. The details are 

provided in Appendix B. 

Transients associated with spilled solvents or other combustible or flammable liquid fuels should 

be assumed to reach peak intensity immediately upon ignition. This guidance is the most 

conservative possible approach but also reflects the fact that fires associated with a spill of a 

liquid fuel would, in fact, grow to peak intensity in a very short period of time. 
Transient Fire Growth Rate (FAQ 08-0052) 

17-3 

It may be appropriate to assume a fire growth period for other types of transient fuels and, as 

noted above, it is the intent of the original guidance to allow for use of a time-dependent fire 

growth profile wherever appropriate. If a time-dependent fire growth profile is assumed for other 

types of transient fuel packages (e.g., packing material, boxes, wooden pallets, etc.) then the 

analyst should provide a basis for the assumed growth behavior that includes consideration of 

applicable fire test data and reflects the nature of the transient fuel package being considered. 

17.3 References 

1. Revision 0 to FAQ 08-0052, May 13, 2008, Accession No. ML081500500 

2. NEI 04-02, Guidance for Implementing a Risk-Informed, Performance-Based Fire Protection 

Program Under 10 CFR 50.48(c), Revision 1, Accession No. ML052590476 

3. NFPA 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric 

Generating Plants, 2001 Edition (available through the Public Document Room or NFPA) 

4. Regulatory Guide 1.205, Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants, Accession No. ML061100174 

5. NRC Regulatory Information Summary 2007-19, Process for Communicating Clarifications 

of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During 

The Pilot Application of National Fire Protection Association Standard 805, Accession No. 

ML071590227 

6. NUREG/CR-6850 (EPRI 1011989), Accession Nos. ML050940183 (Vol. 1) and 

ML050940189 (Vol. 2) 

18-1 

18 

REFERENCES 

1. Performance-Based Standard for Fire Protection for Light Water Reactor Electric Generating 

Plants, National Fire Protection Association, Brainard, MA, 805 Standard, 2001 Edition. 

2. U.S. Nuclear Regulatory Commission, 10 CFR 50, RIN 3150-AG48, “Voluntary Fire 

Protection Requirements for Light Water Reactors Adoption of NFPA 805 as a RiskInformed, Performance-Based Alternative,” Final Rule, June 2004 (Volume 69, Number 

115). 

3. EPRI 1011989, NUREG/CR-6850, “EPRI/NRC-RES Fire PRA Methodology for Nuclear 

Power Facilities,” September 2005. 

4. NEI 04-02, “Guidance for Implementing a Risk-Informed, Performance-Based Fire 

Protection Program Under 10 CFR 50.48(c),” Revision 1, September 2005. Accession No. 

ML052590476. 

5. Regulatory Guide 1.205, “Risk-Informed, Performance-Based Fire Protection for Existing 

Light-Water Nuclear Power Plants,” May 2006. ADAMS Accession No. ML061100174. 

6. ASME/ANS RA-Sa–2009, “Standard for Level 1/Large Early Release Frequency 

Probabilistic Risk Assessment for Nuclear Power Plant Applications (Addenda to 

ASME/ANS RA-S-2008),” February 2009. 

7. Fire PRA Methods Enhancements: Additions, Clarifications, and Refinements to EPRI 

1011989. EPRI, Palo Alto, CA: 2008. 1016735. 

8. Regulatory Guide 1.75, “Criteria for Independence of Electrical Safety Systems” Revision 3, 

February 2005. Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory 

Commission. 

9. Fire Events Database and Generic Ignition Frequency Model for U.S. Nuclear Power Plants. 

EPRI, Palo Alto, CA: 2001. 1003111. 

10. “Experimental Investigation of Internally Ignited Fires in Nuclear Power Plant Control 

Cabinets: Part II, Room Effects Tests.” NUREG/CR-4527 Volume 1, January 1988. 

11. Characterization of Fire-Induced Circuit Faults: Results of Cable Fire Testing. EPRI, Palo 

Alto, CA: 2002. 1003326. 

12. S. Nowlen, F. Wyant, Cable Response to Live Fire (CAROLFIRE) Test Report Volume 1: 

General Test Descriptions and the Analysis of Circuit Response Data, NUREG/CR-6931/V1, 

SAND2007-600, Sandia National Laboratories, Albuquerque, New Mexico, March 2007. 

13. NRC Inspection Manual Chapter 0609, App. F – Fire SDP. 

14. Gallucci, Raymond HV, PhD, Probabilistic/Statistical Examination of Cable Hot Short 

Durations Due to Nuclear Plant Fires, ANS PSA-08, Knoxville, TN, September 2008. 

A-1 

A 

SUMMARY OF EXPERIMENTAL INSIGHTS FOR TRASH 

FIRES 

There have been at least three studies that have experimentally evaluated fires involving trash 

receptacles (trash cans). The guidance in this FAQ resolution relies primarily on two of these 

three studies, the third being discounted as nonrepresentative to nuclear power plant (NPP) 

applications. 

The first study was conducted at the University of California, Berkeley, Lawrence Berkeley 

Laboratory (LBL) [A-1]. This report describes a series of fire tests involving unconfined bags of 

trash and trash in commercial plastic trash receptacles. The difficulty with this particular test set 

lies in the nature of the “trash” used. The plastic bags were filled with either “eucalyptus duff,” 

which is to say, leaves and twigs gathered from under local eucalyptus trees, or with polystyrene 

cups, paper cups, and “fluffed up paper towels.” The trash cans were filled using empty pint-size 

wax/paper milk cartons obtained from the campus cafeteria. Many of the milk cartons were 

opened at both ends and then stacked in several layers filling the receptacle. The open ends of the 

carton were placed facing up/down. These open cartons were then loosely filled with shredded 

pieces from additional milk cartons. 

These fuel packages are clearly not typical of the types of general refuse one might expect to find 

in a NPP. They had a clear tendency to maximize both fire growth rates and peak fire intensity 

given both the rather flammable nature of the fuels and the fact that the fuel configuration 

maximized the fuel surface area. As a result, great care should be exercised when applying these 

tests to NPP situations. In the case of the trash can fires, two other quality data sets of more 

direct relevance are available, so the LBL tests are discounted as nonrepresentative of NPP 

applications. 

The second study was performed as a part of the early Nuclear Regulatory Commission (NRC) 

Fire Protection Research Program and involved a series of tests designed to characterize a range 

of transient fuel fire source packages that might be found, in particular, in the control room of a 

NPP [A-2]. This included testing of both large (30 gallon) and small (typical office-size) plastic 

trash receptacles filled with crumpled paper. The results of particular interest are those for fuel 

package 4 (tests 7 and 8) and fuel package 5 (test 9). The most relevant results for these tests 

are reproduced in Figures A-1 through A-3, which illustrate the heat release rate profiles for 

tests 7–9. 
Summary of Experimental Insights for Trash Fires 

A-2 

Figure A-1 

Heat release rate for NUREG/CR-4680 test 7, small trash can fire 

Figure A-2 

Heat release rate for NUREG/CR-4680 test 8, small trash can fire 
Summary of Experimental Insights for Trash Fires 

A-3 

Figure A-3 

Heat release rate for NUREG/CR-4680 test 9, large trash can fire 

Fire intensity in all three tests follows a similar general trend with respect to the heat release rate 

profile. In the case of test 7, the trend is observable but generally less pronounced because this 

fire remained at a relatively low intensity throughout. The behaviors of primary interest are those 

trends that occur on a time scale of minutes rather than the shorter-term variability that occurs on 

a time scale of seconds or tens of seconds. Note that in each case, the fires show a rapid rise in 

fire intensity over the first 2–4 minutes, reflecting fire spread across the top surface of the paper 

filling the trash can. The general trend of increasing fire intensity continues as the fire spreads 

deeper into the crumpled paper filling the trash can. This continuation of the initial growth stage 

lasts for several additional minutes (up to 7–15 minutes), at which time an initial peak in fire 

intensity is observed. After this initial peak intensity is reached, the fires subside somewhat. 

During this subsidence period the trash can itself was observed to begin melting and to collapse 

in upon itself. Once the trash container itself ignites, fire intensity grows relatively quickly, 

reaching a second higher peak after 15–33 minutes. Overall, the behaviors of primary interest in 

each of the three tests are the initial peak reached in 7–15 minutes and the second larger peak 

reached in 15–33 minutes. In interpreting these test results, weight is given primarily to the 

observed time to reach the initial peak in fire intensity, which for these three tests were 7, 8, and 

13 minutes, respectively. 

The third study of interest [A-3] was performed at NIST and involved two experiments 

“conducted to help characterize the potential hazard from ignition of nominal 136 L (30 gal) 

trash containers made from high density polyethylene (HDPE) and loaded with cellulosic 

debris.” The trash containers tested are illustrated in Figure A-4, and are quite similar to fuel 

package 5 as tested in NUREG/CR-4680, with the exception of the trash loading. 
Summary of Experimental Insights for Trash Fires 

A-4 

Figure A-4 

Photograph of the trash receptacle tested in NISTFR-4018 

In the case of the NIST tests “each trash container had 10 kg (22 lbs) of debris ‘typical’ of a 

construction site. The debris consisted of cut pieces of 2" X 4" lumber, sawdust, cardboard, 

paper, and cups, food wrappers, and paper bags from a fast food restaurant.” Again, the results of 

direct interest to this FAQ are the heat release rates. These results are reproduced in Figure A-5. 

Figure A-5 

Heat release rate test results for the two tests documented in NISTFR-4018 
Summary of Experimental Insights for Trash Fires 

A-5 

The NIST tests show a somewhat similar behavior to the SNL tests in terms of the initial fire 

growth. The first of the NIST tests (trash container 1) actually shows a steady climb in fire heat 

release throughout the test; the fire had not yet reached a true peak when it was suppressed at 

about 800 seconds (about 13 minutes). For the second test (trash container 2) a peak heat release 

rate is observed at about 550 seconds (about 9 minutes), followed by a period of relatively steady 

burning at a somewhat reduced intensity. 

Overall, the five available tests indicate fire growth times that range from about 7 to 13 minutes. 

Based on these test results, the recommended practice for this type of fuel source, general refuse 

in a plastic trash receptacle, is to assume a fire growth time of 8 minutes. 

It is also recommended that the same result be extrapolated to fires involving general refuse in a 

metal trash receptacle of similar size. This is because both of the cited studies noted that the 

initial fire growth behavior is actually associated with fire spread through the refuse and that the 

plastic trash receptacle became involved in the fire at a somewhat later time (i.e., after 8 or more 

minutes). Hence, similar initial growth behavior is likely to apply to metal trash receptacles, even 

though neither of the cited studies actually tested this type of receptacle. 

It should be noted that a final study of potential interest is a National Bureau of Standards (NBS, 

now NIST) literature review completed in 1985 [A-4]. This review did not involve the 

conducting of any new tests, but did consider the results of prior test programs, including both 

the LBL and SNL tests cited here. There is one figure in particular presented in this report that is 

worthy of some discussion, namely Figure 11. This is a recommended heat release rate versus 

time profile that was developed to bound all of the observed trash fire curves. This particular 

figure shows an initial growth to peak of one minute. This initial growth rate is dominated by 

other types of trash fires and is not characteristic of the trash can fires. Hence, the application of 

this curve to a trash receptacle (trash can) fire would be conservative in comparison to the 

recommended practice developed here. 

References 

A-1. Von Volkenburg, D. R., et al., “Towards a Standard Ignition Source,” Lawrence Berkeley 

Laboratory, LBL-8306, October 1978. 

A-2. S. P. Nowlen, “Heat and Mass Release for Some Transient Fuel Source Fires: A Test 

Report,” NUREG/CR-4680, SAND86-0312, Sandia National Laboratories (SNL), 

October 1986. 

A-3. D.W. Stroup and D. Madrzykowski, “Heat Release Rate Tests of Plastic Trash 

Container,” National Institute of Standards and Technology (NIST), NISTFR-4018, 

April 24, 2003. 

A-4. B.T. Lee, “Heat Release Rate Characteristics of Some Combustible Fuel Sources 

in Nuclear Power Plants, National Bureau of Standards (NBS),” NBSIR 85-3195, 

July 1985. 

B-1 

B 

SUMMARY OF EXPERIMENTAL INSIGHTS FOR TRASH 

BAG FIRES (FAQ 08-0052) 

This appendix provides insights relative to the fire growth characteristics of trash bag fires 

as unique from the behavior of a fire involving trash confined to a trash receptacle (a trash 

can fire). The available tests potentially relevant to trash bag fires are documented in two 

reports [B-1, B-2]. 

The first report is the Lawrence Berkeley Laboratory (LBL) report cited and discussed in 

Appendix A of this report. The difficulties associated with the fuel loading in these tests have 

been described in Appendix A. Given the atypical nature of the fuel loading, care must be 

exercised in extrapolating these results to a more representative nuclear power plant (NPP) 

transient fuel load. Given that this is the only known study to report results for an actual trash 

bag fire, the results will be considered. The trash bag test results from the LBL study are 

summarized in Figures B-1 and B-2. 

Figure B-1 

Results of the LBL tests involving a trash bag filled with eucalyptus duff 
Summary of Experimental Insights for Trash Bag Fires (FAQ 08-0052) 

B-2 

Figure B-2 

Results of the LBL tests for trash bags filled with cups and paper towels 

Figure B-1 compares the results of the first trash bag test to a test involving a small (6.6 liter or 

about 1.5 gallon) trash can. The trash bag fire grew to peak intensity in just 2 minutes. One 

interesting result here is that the trash bag fires grew in intensity more quickly than did 

nominally similar trash can fires. Figure B-2 illustrates the results of three tests involving one, 

two, and three trash bags, respectively. Note that the fires reached peak intensity in 1–2 minutes, 

again emphasizing the relatively rapid rate of fire growth compared to trash can fires. The 

explanation for this difference is that the trash bags alter two key factors. First, in the trash bag 

fires there is a relatively larger exposed fuel surface area compared to the trash can fires. In the 

trash bag fires, the fire is free to spread over the entire surface of the trash bags, whereas initial 

fire growth in the trash can was mainly associated with spread only over the upper surface of the 

trash within the trash can (e.g., rather than over the outer surface of the trash can itself). Second, 

the trash bag fires have freer access to oxygen in comparison to the trash can fires. For the trash 

can a two-way flow of oxygen into and then fire gases out of the trash can must be established 

(again given initial fire spread through the trash rather than the trash can itself). The trash bags 

create a more classical fire plume development pattern with cold air and oxygen flowing in from 

the sides and fire gases flowing upward above the fuel source. The trash bag configuration is 

more conducive to rapid fire development. 

The only other effort that has explored transient fuel sources other than trash receptacles (trash 

cans) is the previously cited NUREG/CR-4680 [B-2]. This test program did not involve trash 

bags, but did include testing of a transient fuel package made up of stacked computer paper 

(15 lbs or a 3" stack) and crumpled paper (1.5 lbs) in a 12"x16"x12" cardboard box (fuel package 

3, fire test 5). Figure B-3 illustrates this fuel package. This particular fuel package is roughly 

similar in nature to a trash bag fire in that the cardboard became involved in the fire relatively 
Summary of Experimental Insights for Trash Bag Fires (FAQ 08-0052) 

B-3 

quickly and the box did not appear to substantially inhibit air flow to the burning fuel. The heat 

release rate result for this test is illustrated in Figure B-4. For the SNL test, the time to peak fire 

intensity was approximately 4 minutes. 

Figure B-3 

SNL fuel package 3—box with crumpled paper 

Figure B-4 

The heat release rate profile from SNL test 5 involving fuel package 3 

Overall, it is concluded that the LBL test results showing peak heat release rates being reached 

within 1 minute in two cases and 2 minutes in the third likely exaggerate the early rate of fire 

growth as compared to more typical trash fire configurations. However, the SNL tests showed a 

fire growth time of 4 minutes for a roughly similar fuel package. Both test efforts illustrate that 

trash fires occurring outside of a trash receptacle (trash can) will grow more rapidly than will 

fires involving similar fuels that are confined within a trash can. 
Summary of Experimental Insights for Trash Bag Fires (FAQ 08-0052) 

B-4 

Given the available test results, the recommended general practice for the case of a trash bag fire 

(i.e., general refuse collected into a plastic bag but not contained within a trash receptacle) is 

based on a blending of the SNL and LBL test results. For NPP applications, recommended 

practice for trash bag fires is to assume a fire growth time of 2 minutes. 

As a final note, the SNL tests did involve two other small transient fuel source packages (fuel 

packages 1 and 2). However, both of these packages involved use of a solvent (1 qt of acetone) 

as a part of the fuel loading. The ignition of this solvent dominated the early fire behavior for 

these tests. These fuel packages are not considered representative of typical solid refuse (trash) 

that would exclude flammable liquids. Hence, these tests have not been considered in developing 

the transient fuel fire growth times discussed here. 

References 

B-1. Von Volkenburg, D. R., et al., “Towards a Standard Ignition Source,” Lawrence Berkeley 

Laboratory, LBL-8306, October 1978. 

B-2. S. P. Nowlen, “Heat and Mass Release for Some Transient Fuel Source Fires: A Test 

Report,” NUREG/CR-4680, SAND86-0312, Sandia National Laboratories (SNL), 

October 1986. 
BIBLIOGRAPHIC DATA SHEET

(See instructions on the reverse)

NRC FORM 335

(9-2004)

NRCMD 3.7

U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER

 (Assigned by NRC, Add Vol., Supp., Rev.,

 and Addendum Numbers, if any.)

NUREG/CR-6850

Supplement 1

3. DATE REPORT PUBLISHED

MONTH

September

YEAR

2010

4. FIN OR GRANT NUMBER

2. TITLE AND SUBTITLE

Fire Probabilistic Risk Assessment Methods Enhancements

5. AUTHOR(S)

J.S. Hyslop, J. Kratchman, N. Siu, C. Afshar, B. Najafi, F. Joglar, Y. Li, D. Henneke, M. Warner

6. TYPE OF REPORT

Technical

7. PERIOD COVERED (Inclusive Dates)

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division, Office or Region, U.S. Nuclear Regulatory Commission, and mailing address; if contractor,

Division of Risk Analylsis, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, type "Same as above"; if contractor, provide NRC Division, Office or Region, U.S. Nuclear Regulatory Commission, 

Division of Risk Analysis, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555

provide name and mailing address.)

and mailing address.)

10. SUPPLEMENTARY NOTES

NRC Project Manager, J.S. Hyslop; EPRI Project Manager, K. Canavan

11. ABSTRACT (200 words or less)

This report documents the interim guidance of the industry and U.S. Nuclear Regulatory Commision (NRC) on several issues

from the National Fire Protection Association (NFPA) Standard 805 Frequently Asked Questions (FAQs) Program arising from

the use of NUREG/CR-6850, EPRI 1011989 "EPRI/NRC Fire PRA Methodology for Nuclear Power Facilities." The FAQ

program was established by the Office of Nuclear Regulatory Regulation (NRR) to support the NFPA 805 pilot plants' transition

to the voluntary rule, 10 CFR 50.48(c), and is documented in Regulatory Issue Summary RIS 2007-19, "Process for

Communicating Clarifications of Staff Positions Provided in Regulatory Guide 1.205 Concerning Issues Identified During the Pilot

Application of National Fire Protection Standard 805." In general, the FAQ solutions improve our knowledge about these

respective issues, and the result is more realistic estimates of fire-induced core damage frequency.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist researchers in locating the report.)

Fire, Fire Risk, Risk Analysis, Probabilistic Risk Assessment, PRA, Risk-Informed Regulation, Fire

Protection, National Fire Protection Association Standard 805, NFPA 805, NFPA 805 Frequently Asked

Questions Program, NFPA 805 FAQ Program

14. SECURITY CLASSIFICATION

13. AVAILABILITY STATEMENT

unlimited

(This Page)

unclassified

(This Report)

unclassified

15. NUMBER OF PAGES

16. PRICE

NRC FORM 335 (9-2004) PRINTED ON RECYCLED PAPER

Electric Power Research Institute (EPRI), 3420 Hillview Avenue, Palo Alto, CA 94303

Science Applications International Corporation (SAIC), 1671 Dell Avenue #100, Campbell, CA 95008

GE Hitachi Nuclear Energy Americas LLC, 3901 Castle Hayne Road, Wilmington, NC, 28402

Electric Power Research Institute (EPRI), 3420 Hillview Avenue, Palo Alto, CA 94303