NUREG/CR-4230

Probability-Based Evaluation

of Selected Fire Protection

Features in Nuclear Power Plants

Prepared by M. A. Azarm, J. L. Boccio

Brookhaven National Laboratory

Prepared for

U.S. Nuclear Regulatory

Commission
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States

Government. Neither the United States Government nor any agency thereof, or any of their

employees, makes any warranty, expressed or implied, or assumes any legal liability of responsibility for any third party's use, or the results of such use, of any information, apparatus,

product or process disclosed in this report, or represents that its use by such third party would

not infringe privately owned rights.

NOTICE

Availability of Reference Materials Cited in NRC Publications

Most documents cited in NRC publications will be available from one of the following sources:

1. The NRC Public Document Room, 1717 H Street, N.W.

Washington, DC 20555

2. The Superintendent of Documents, U.S. Government Printing Office, Post Office Box 37082,

Washington, DC 20013-7982

3. The National Technical Information Service, Springfield, VA 22161

Although the listing that follows represents the majority of documents cited in NRC publications,

it is not intended to be exhaustive.

Referenced documents available for inspection and copying for a fee from the NRC Public Document Room include NRC correspondence and internal NRC memoranda; NRC Office of Inspection

and Enforcement bulletins, circulars, information notices, inspection and investigation notices;

Licensee Event Reports; vendor reports and correspondence; Commission papers; and applicant and

licensee documents and correspondence.

The following documents in the NUREG series are available for purchase from the NRC/GPO Sales

Program: formal NRC staff and contractor reports, NRC-sponsored conference proceedings, and

NRC booklets and brochures. Also available are Regulatory Guides, NRC regulations in the Code of

Federal Regulations, and Nuclear Regulatory Commission Issuances.

Documents available from the National Technical Information Service include NUREG series

reports and technical reports prepared by other federal agencies and reports prepared by the Atomic

Energy Commission, forerunner agency to the Nuclear Regulatory Commission.

Documents available from public and special technical libraries include all open literature items,

such as books, journal and periodical articles, and transactions. Federal Register notices, federal and

state legislation, and congressional reports can usually be obtained from these libraries.

Documents such as theses, dissertations, foreign reports and translations, and non-N RC conference

proceedings are available for purchase from the organization sponsoring the publication cited.

Single copies of NRC draft reports are available free, to the extent of supply, upon written request

to the Division of Technical Information and Document Control, U.S. Nuclear Regulatory Commission, Washington, DC 20555.

Copies of industry codes and standards used in a substantive manner in the NRC regulatory process

are maintained at the NRC Library, 7920 Norfolk Avenue, Bethesda, Maryland, and are available

there for reference use by the public. Codes and standards are usually copyrighted and may be

purchased from the originating organization or, if they are American National Standards, from the

American National Standards Institute, 1430 Broadway, New York, NY 10018.
NUREG/CR-4230

BNL-NUREG-51878

RP

Probability-Based Evaluation

of Selected Fire Protection

Features in Nuclear Power Plants

Manuscript Completed: February 1985

Date Published: May 1985

Prepared by

M. A. Azarm, J. L. Boccio

Brookhaven National Laboratory

Department of Nuclear Energy

Upton, NY 11973

Prepared for

Division of Engineering

Office of Nuclear Reactor Regulation

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

NRC FIN A3710

-iiiABSTRACT

A probabilistic approach for the evaluation of major fire protection

measures in nuclear power plants is described. The methods developed are applied to two representative fire areas -- one similar to a cable routing room

and the other typical of a diesel generator room. The fire areas chosen for

application, the fire scenarios described, and the various fire-damage states

specified in the two illustrative examples are used to evaluate those fireprotection guidelines which deal with automatic/manual fire detection and suppression systems, rated barriers, divisional separation, drainage systems, dampers, and fire rating of electrical cables. Tabular results are presented,

which reflect the relative merits of these systems/features in terms of conditional probabilities of achieving various room-damage states. The conclusions

drawn and the lessons learned through the course of this study are discussed,

and the areas that may need further investigation are identified.
-ivCONTENTS

ABSTRACT. .................. ................................................. 1 1 1

LIST OF TABLES ....... ... . ................................................. vi

LIST OF FIGURES .......................................................... viii

EXECUTIVE SUMMARY ...... ........ ......................... .... ......... t

1. INTRODUCTION ............................................................ 1

1.1 Background ................................................ ...... 01

1.2 Objective ................ .......... 0...0........... 0.. 00..... ..

1.3 Scope ...................................................... ...... 3

1.4 Approach ....................................... 000000 . ... ... . .4

2. GENERAL APPROACH ....................................................... 6

2.1 Description of Representative Fire Areas .......................... 6

2.2 Standard Review Plan (SRP) Guidelines Proposed

for Evaluation ............................ 0.0...................... 8

3. BASIC METHODOLOGY ............ ............. ................ .. ... .. 11

3.1 Introduction ...................................................... 11

3.2 Deterministic Fire Enclosure Models ............................... 12

3.3 Deterministic/Probabilistic Analysis of Barrier Failure ........... 14

3.4 Probabilistic Analysis of Detection/Suppression ................... 18

4. REPRESENTATIVE CABLE-ROUTING ROOM ANALYSIS ............................. 23

4.1 First-Stage Fire Growth ........................................... 24

4.1.1 Detection Time ............................................. 27

4.1.2 Suppression Time ........................................... 29

4.1.3 Probability for Room-Damage State(s) ....................... 31

4.2 Second-Stage Fire Growth ....... ............................ .32

4.2.1 Determination of Bounds on Burning Rates ................... 35

4.2.2 Determination of Cable-Damageability Times ................. 37

4.2.3 Suppression Analysis in Second-Stage Fire Growth ........... 42

4.3 Miscellaneous Factors ............................................. 45

4.4 Ranking of Fire Protection Measures in Cable-Routing Room ......... 46

4.5 Qualitative Appraisal of Flame-Retardant Coating and

Rated Barriers .................................................... 51
CONTENTS (Cont'd)

.Page

5. REPRESENTATIVE DIESEL GENERATOR ROOM ANALYSIS .......................... 53

5.1 First-Stage Fire Growth .......... ......

5.2 Second-Stage Fire Growth ........................... 57

5.3 Conclusions (Diesel Generator Room)............................... 61

6. LIMITATIONS OF THE RESULTS/METHODS ................................... 66

7. SUMMARY OVERVIEW AND FUTURE RECOMMENDATIONS...; ........................ 78

REFERENCES............ . ................................................... 72

APPENDIX A: Parameters Used for the Analyses ............................... A-1
-viTABLES

Page

Table S-1 - Fire Protection Features/Systems Considered .................... xii

Table S-2 - Probabilities of Room-Damage States for Various Fire

Protection Design Features (Cable-Routing Room) ................ xiii

Table S-3 - Probabilities of Room-Damage States for Various Fire

Protection Design Features (Diesel Generator Room) ............. xiv

Table 1 - Response-Time Cumulative Distribution Function for

Automatic Detection System ....... ........ ...... e28

Table 2 - Successful Sprinkler Operation ................................. 30

Table 3 - Discretized Cumulative Distribution Function for

Fire Growth and Suppression .................................... 33

Table 4 - The Probability of First-Stage Fire Growth ..................... 34

Table 5 - The Burning Rates for Various Cable Trays in the

Representative Cable-Routing Room .............................. 37

Table 6 - The Timing for Cable Damageability for the Lower Bound

Burning Rate ................................................... 41

Table 7 - The Timing for Cable Damageability for the Upper Bound

Burning Rate .............................................. 41

Table 8 - Upper and Lower Bounds on the Timing of Cable Damageability and the Associated Lognormal Parameters ................ 43

Table 9 - Probability of Successful Suppression in Second-Stage

Growth for IEEE-Qualified Cable ............... .. ........... 45

Table 10 - The Timing and Probability of Cable Damageability in

Second-Stage Fire Growth for Cables That Are Not IEEE Qualified ................................................. 47

Table 11 - Conditional Probabilities of Achieving First- and SecondStage Fire Growth Based on Various Fire Protection System/

Features Implemented (Cable-Routing Room) ...................... 49
-viiTABLES (Cont'd)

Page

Table 12 - Reduction Factors That Can Be Credited for Various Fire

Protection'Measures at Different Cable Tray Configurations

for Estimating the Probability of Second-Stage Growth .......... 50

Table 13 - The Fire Protection Measures Considered for the Analysis

of the Representative Diesel Generator Room .................... 61

Table 14 - The Modified Reduction Factors (Importance Factors) for

Diesel Generator Room .......................................... 64

Table 15 - Probabilities of Room-Damage States for Various Fire

Protection Design Features (Diesel Generator Room) ............. 65

Table A-1 - The Values for Physical Parameters Used in the Study ........... A-2
-viiiLIST OF FIGURES

Page

Figure

Figure

Figure

Figure

Figure

1.

2.

3.

4.

5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

A schematic diagram of the representative cable-routing room .... 8

The effectiveness of automatic suppression system ............... 21

Manual fire suppression model.................................. 22

Estimated range of burning rate predicted by COMPBRN ............ 34

The target temperature vs time for the second-stage

fire growth (lower bound of burning rate) ........................ 39

The target heat flux vs time for the second-stage

fire growth (lower bound of rurning Rate ........................ 39

The target temperature vs time for the second-stage fire

growth (upper bound of burning bate) ............................ 40

The target heat flux vs time for the second-stage fire

growth (upper bound of burning rate) ............................ 40

Region of integration for Eq. 4.15 .............................. 44

The cumulative probability distribution for first-stage fire growth ..................................................... 55

The cumulative distribution function for the average room

temperature during post-flashover phase .............. ........... 59

Fragility of rated barrier vs the duration of flashover

phase for the representative diesel generator room .............. 59

Event tree constructed for the various fire scenarios in

diesel generator rooms ........................................ 63
-ixEXECUTIVE SUMMARY

This document describes a method which can be used to assess from a probabilistic point of view the relative merits of the NRC guidelines for fire protection. These guidelines are contained in Section 9.5.1 (Fire Protection) of

the Standard Review Plant (SRP). The approach embodies a hybrid selection of

physical models for fire propagation determinations and probabilistic models for

active and passive fire-mitigation system reliability. The intention of this

study is to investigate the practicality and usefulness of implementing probabilistic risk assessment techniques as an adjunct to the present fire-safety

review process.

Two examples, employing the method described in this document, are presented. In each case, the fire areas chosen, the fire scenarios prescribed, and the

fire-damage states specified in each example are designed to evaluate those fire

protection features in the SRP which deal with automatic/manual detection and

suppression systems, ventilation and exhaust systems, rated barriers, spatial

separation of redundant divisions, curb/dike and drainage systems, and electrical cable rating.

For each of the two fire areas analyzed (a cable-routing room and a diesel

generator room), tabular results reflect the relative merits of these systems/

features. This is accomplished by determining the conditional probabilities of

achieving prespecified room-damage states resulting from a fire. Accordingly,

given a particular enclosure geometry, combustible fuel loading, and initiating

fire and considering the combination of fire protection features and systems

that are "designed into" the area of concern, coupled with a measure of system

reliability and fire vulnerability, one obtains a quantitative basis for firesafety evaluation.

In Sections 2 and 3, this general approach and basic methodology are further elaborated. Since evaluation of all the SRP Guidelines would not have been

practical, judgment was used to limit those that can be addressed. This
judgment was tempered by the in situ combustible loading prescribed for each of

the two fire areas. For example, the basic approach for the cable-routing room

stresses the relative protection provided by spatial separation of redundant

cable divisions; whereas for the diesel generator room, the emphasis is on the

effectiveness of barriers in preventing the spread of the fire. In either case,

the deterministic fire propagation and growth models used are considered stateof-the-art "zone" models which have been modified (see Section 3) to account for

forced ventilation and spatial dependency of fire-induced gas dynamic

parameters.

Probability distribution functions are assigned to several dependent fire

parameters, e.g., fire initiation and growth times, and energy release rate.

These are coupled to the transport models to judge the effectiveness of active

and passive fire protection and fire mitigating systems. For example, Section 3

describes a probabilistic analysis of aerosol fire detection and water suppression systems. Correlation laws for detector response time, expressed as a function of detector spacing, room geometry, and energy release rate of the fire,

are incorporated with a probabilistic function of detector reliability to assess

detection time probabilistically as a function of fire growth. Likewise, empirical curves are used to assess the effectiveness of water suppression systems.

In Section 4, the method described is used to probabilistically evaluate

fire protection systems and features typically used in a cable-routing room. In

this case, the prespecified, or ultimate room-damage state (UDS) is defined as

the failure of all redundant divisions. Locations of these divisions, relative

to each other and to the distance below the enclosure ceiling, are the two basic

parameters investigated. Table S-i lists the fire-protection features and systems considered in this example as well as those for the diesel generator room

example. Six of a total of 15 fire-protection measures are indicated along with

a code identifier. Combinations of these identifiers are itemized in Table S-2

to indicate what protection systems/features are in place (or not in place) for

the cable-routing room. Along with these combinations are the estimated probabilities for achieving the prespecified UDS. Pairwise comparison of the results
-xi -

for each case provides a rationale for evaluating a specific fire protection

feature. For example, comparison between Case 1 and Case 4 indicates an orderof-magnitude reduction In UDS probability due to installation of an automatic

water sprinkler system.

In Section 5, the method described is used to probabilistically evaluate

fire protection systems and features typically used in a diesel generator room.

In this example, the UDS is defined as failure of enclosure barriers and/or inoperability of dampers and doors. Accordingly, this example stresses the modeling employed to assess the conditional probability of containing the fire within

the area of origin. Table S-3 summarizes the results obtained in this example.

In this case, as compared to Table S-2, the results illustrate what fire protection systems/features are in place (or not in place) and what is the conditional

probability for the fire to spread beyond the diesel generator room. Pairwise

comparison provides a rationale for evaluating general system features. For instance, the effectiveness of a 3-hr barrier (Case 3) compared to a 1-hr barrier

(Case 1) indicates the former is 40 times more effective than the latter in preventing further growth.

The conclusion of this evaluation, as to the usefulness of the approach, is

contained in Section 6. For this limited study and only for the fire scenarios

considered, it is shown that an automatic water suppression system, the use of

qualified cables, and trained fire brigades are the more important protection

features for the cable-routing room example. For the second example, a 3-hourrated barrier, automatic operation of doors and dampers, and an effective suppression system were most important.

It must be stressed that these results are fire-scenario and enclosure-room

specific. The prime purpose of this document is to describe a methodology that

may be potentially useful in deterministic fire safety reviews and evaluating

fire protection guidelines.
-xiiTable S-1

Fire Protection Features/Systems Considered

Identifier System/Feature

1 Automatic Detection System (Aerosol Detectors)

2 Automatic Suppression Systems (Sprinklers; Total Flooding Halon)

3 Automatic Doors/Dampers

4 Electrical Cable; Proper Rating/Installation

5 Qualified vs Nonqualifled Cables

6 Manual Fire-fighting Equipment Availability and Staff Familiarity

7 Cable Tray Location (within uniformly stratified layer)

8 Fire Brigade; adequate training and plant familiarity

9 High Capacity Drainage System

a 1-hr rated barriers; including doors and penetrations

b 2-hr rated barriers; including doors and penetrations

c 3-hr rated barriers; including doors and penetrations

d Cable Tray Location (within nonuniform region); Separation - 10 ft

e Cable Tray Location (within nonuniform region); Separation - 20 ft

f Cable Tray Location (within nonuniform region); Separation - 30 ft
-xii -

Table S-2

Probabilities of Room-Damage States for

Various Fire Protection Design Features

(Cable-Routing Room)

First-Stage(a) Second-Stage (b)

Case Design Identification Growth Growth

No. Code Probability Probability

1 4/5/1/2/6/7/8* 1.5(-2) 5.7(-3 +

2 4/5/1/2/6/7/- 1.5(-2) 1.5(-2)

3 4/4/1/2/-/7/8 2.2(-2) 8 .4(-3)

4 4/5/1/-/6/7/8 1.5(-1) 5.7(-2)

5 4/5/-/2/6/7/8 1.9(-2) 7.4(-3)

6 4/-/1/2/6/7/8 2.2(-2) 1.2(-2)

7 -/5/1/2/6/7/8 4.5(-2) 1.0(-2)

8 4/5/1/2/6/d/8 1.5(-2) 1.0(-2)

9 4/-/1/2/6/d/8 2.2(-2) 1.8(-2)

10 4/5/1/2/6/e/8 1.5(-2) 6.9(-3)

11 4/-/1/2/6/e/8 2.2(-2) 1.5(-2)

12 4/5/1/2/6/f/8 1.5(-2) 5.8(-3)

13 4/-/1/2/6/f/8 2.2(-2) 1.2(-2)

*Design identification as described in Table S-1.

+5.7(-3) B 5.7(10)-3.

(a)First-Stage Growth is defined as failure of one redundant shutdown cable

division.

(b)Second-Stage Growth is defined as failure of all redundant shutdown cables,

i.e., the Ultimate Room-Damage State (UDS).
-xivTable S-3

Probabilities of Room-Damage States

for Various Fire Protection Design Features

(Diesel Generator Room)

First-Stage(a) Second-Stage(b)

Case Design Identification Growth Growth

No. Code Probability Probability

1 1/2/3/c/8/9* 3. 0(-2) 6.6(-4)+

2 1/2/3/b/8/9 3.0(-2) 9.6 (-3)

3 112/3/a/8/9 3.0(-2) 2.7(-2)

4 1/-/3/c/8/9 0.33 1.0(-3)

5 1/-/3/b/8/9 0.33 0.1

6 1/-/3/9/8/9 0.33 0.3

7 -/-/3/c/8/9 0.66 2.0(-3)

8 -/-/3/b/8/9 0.66 0.2

9 -/-/3/a/8/9 0.66 0.6

*Design identification as described in Table S-1.

+3.0(-2) = 3.0(10)-2.

(a)First-Stage Growth is defined as the fire involvement of the diesel fuel.

(b)Second-Stage Growth is defined as the fire propagation beyond the original

fire enclosure, i.e., the Ultimate Room-Damage State (UDS).
-1-

1. INTRODUCTION

1.1 BACKGROUND

The posture of the Nuclear Regulatory Commission (NRC) on nuclear power

plant design and operability, as it has evolved over the years, has been grounded on traditional engineering practices supplemented by additional safety measures and analyses to ensure a sufficiently conservative approach to plant safety. To protect against various preconceived accident conditions, designers and

plant operators were introduced to such concepts as the single-failure criterion

and defense-in-depth. The success of this regulatory approach has been generally confirmed by studies on plant risk which have indicated that accidents within

the design basis contribute a small portion of the risk.

However, the current regulatory structure is not without its problems. Because our understanding of the normal and various upset states is incomplete and

because of the tendency to analyze plants, system by system, regulatory practices have been developed that are not always well integrated with each other.

This can result in an uneven coverage of the safety issues, difficulty in assigning priority to new safety issues as they arise and in determining the

actual level of risk that is attained, and to some extent, misinterpretation of

certain safety requirements. In fact, as noted by Ernst and Murphy1 , conservative limits that have been instituted make it "difficult to relax requirements

after further knowledge of plant behavior is acquired." They go on to note that

Probabilistic Risk Assessment (PRA) techniques can offer the regulator a realistic, integrated description of the plant that is useful in the analysis of many

safety issues. These techniques allow the regulator to focus attention on the

uncertainties due to our lack of knowledge, which can sometimes provide vital

information even if the level of risk is not well defined.

A case in point is the rule making issue dealing with fire protection.

Here, a defense-in-depth posture has also been adopted. A balanced approach, as

envisioned by the NRC, through general criteria2 and specific requirements,3,4

is based upon multiple layers of active and passive protective measures, such as
-2-

detection, suppression, barriers, and spatial separation of redundant safetyrelated systems. The combination of guidance contained in Appendix A to Branch

Technical Position (BTP) 9.5-1, as implemented by the staff in their fire protection review programs for operating reactors, and the requirements set forth

in Appendix R to IOCFR50 define the necessary conditions for demonstration of

compliance with General Design Criterion 3 of Appendix A to 1OCFR5O. Currently,

fire protection requirements for new nuclear plants (those that commenced operation before January 1, 1979) are delineated in the Standard Review Plan (SRP),

Section 9.5-1, "Fire Protection," Revision 3, July 1981.5 This revision now incorporates all the technical requirements found in Appendix R which was appended

to IOCFR50 to resolve several fire protection issues still undetermined since

fire protection programs 6 were evaluated by the staff after the Browns Ferry

fire.

However, in SECY-83-269, which summarizes the licensees' fire protection

exemption requests to Appendix R requirements, the staff's disposition of those

requests, and any generic issues that may be raised by these requests, Dircks 7

requested that the present guidelines should be revised to incorporate recent

clarifications and to delete any requirements that are not applicable to new

plants.

1.2 OBJECTIVE

Reflecting the increasing emphasis and use of probabilistic-techniques in

the assessment of fire risk at nuclear power plants, the NRC has embarked on a

program for implementing the use of such techniques as an adjunct to their deterministic safety review process. Since PRA techniques can offer the regulator

a realistic, integrated description of the plant, the objective of this phase of

the overall program is to assess from a probabilistic point of view the safety

advantages associated with each of the fire protection guidelines contained in

Section 9.5-1 of the SRP. The purpose of the study is to develop an approach

which can be used to respond to the request made in SECY-83-269.
-3-

1.3 SCOPE

Specifically, this study incorporates existing deterministic fire models

with existing reliability techniques to evaluate some of the major SRP guidelines. Since it was deemed impractical to evaluate all the guidelines, judgment was used to limit the number of guidelines addressed. Those that have been

stressed in this report deal with the effectiveness of automatic/manual detection and suppression systems, ventilation and exhaust systems, rated barriers,

spatial separation of redundant divisions (the so-called 20-ft rule), curb/dike

effectiveness, drainage systems, and electrical cable fire-resistant rating.

Two examples employing the method described in this report are presented.

For each of the two fire areas analyzed (a cable-routing room and a diesel generator room), the results reflect the relative merits of various fire-protection

systems and features.

For example, given that an enclosure, containing say two redundant safe

shutdown divisions, is a priori specified with a specific fire load, geometry,

and ventilation, the methodology employed herein attempts to answer the following questions:

1. Given an initiating fire, of a prespecified size, what will be the conditional probability that one of the two redundant trains is affected?

2. What will be the conditional probability that the ensuing fire, without

fire suppression, will damage the remaining division if that division

had been separated from the first by a) 10 ft, b) 20 ft, c) 30 ft, or

d) any spatial separation; and, what will be the effect of (1) the vertical distance between the initiating fire and these divisions, and (2)

the vertical distance.between the enclosure ceiling and these

divisions?
-4-

3. How much would this overall conditional probability change if each

division were comprised of IEEE-353* rated cabling as compared to nonIEEE-383-rated cables?

4. How could manual and/or automatic detection and suppression methods

change this probability, and what would be the effect of these methods

during the various fire growth stages?

1.4 APPROACH

At the outset it must be emphasized that although the methodology employed

to address these questions is somewhat general and implements what are construed

as state-of-the-art deterministic fire models and probabilistic techniques with

existing reliability data bases, the results presented are specific to the fire

scenario selected and should not be taken out of context. Also, for these cited

examples, only a few of the SRP guidelines have been addressed. Relative evaluation of each guideline, even for the few fire scenarios studied, is beyond the

present scope of this project.

Accordingly, we start by selecting two representative fire areas, viz., a

typical cable-routing room and a diesel generator room. Because of the relative

fire loads in each of these areas, the ensuing study will emphasize the relative

fire protection provided by fire barriers surrounding the latter enclosure,

while analysis for the former enclosure will draw attention to the relative fire

protection provided by spatial separation of redundant divisions. A hybrid

deterministic/probabilistic approach is developed using existing "zone" models

for assessing fire growth, coupled with existing reliability data on detection/

suppression systems. Fire propagation and growth are based upon those unit models employed in COMPBRN8 in conjunction with enclosure fire/radial jet-flow modeling as described by Cooper 9 -,I. The latter model, which not only accounts

for those fire plume and uniformly stratified zones within an enclosure fire but

also considers the interaction of the fire plume with the enclosure ceiling, is

*IEEE - Institute of Electrical and Electronics Engineers.
-5-

needed to investigate the effect of divisional separation. The former enclosure

model, being basically a two-zone model, viz., fire plume and stratified hot

layer, cannot be used to make assessments on fire protection provided by spatial

separation. Certain aspects of the COMPBRN code and its attendant physical models have been used, however, in determining subsequent fire growth. Uncertainty

bounds on the predicted results are established using bounding analysis and the

results of past enclosure fire tests.

Coupled to these deterministic models for determining fire initiation,

propagation, and growth are probabilistic analyses performed to judge the effectiveness of manual and automatic detection/suppression systems during various

fire-growth stages. The results are then combined to estimate the probability

of various room-damage states as a function of the implementation (or lack

thereof) of specific fire-protection guidelines.

Further details of the overall methodology are provided in Section 2.

Section 3 describes the specific approach taken. It also includes the rationale

used in selecting the two representative fire areas investigated, a general description of the fire hazards in each area, and an identification of the specific SRP Section 9.5.1 guidelines under study.

Results from applying the methodology described in Sections 2 and 3 are

discussed in the next two sections. Further discussions are provided regarding

the assumptions made which may temper the conclusions.

Section 6 makes recommendations for improving the methodology for relative

appraisal of fire protection features specified in current guidelines and also

provides a general discussion on probabilistic fire analysis.
-6-

2. GENERAL APPROACH

This task entailed 1) a definition of two representative fire zones in

nuclear power plants which comply with the SRP Guidelines, 2) an estimation of

the probability of a fire damaging various components in these zones based upon

a prescribed initiating fire, and 3) after sequential removal of the various

fire-protection features, a reestimation of the probability of damage states in

order to determine the effect of these fire protection measures. The following

section describes the area layout, the fire loading, and the size of the two

rooms selected for this study. Damage states depend on fire scenario as well as

on what equipment in a given fire area should be protected from fire stressors.

As such, specific damage states are defined in Section 4 for the two fire zones

analyzed. Since evaluation of all SRP Guidelines would have been impractical,

judgment was used to limit the number of SRP Guidelines that can be addressed in

this study. This is discussed in Section 2.2

2.1 DESCRIPTION OF REPRESENTATIVE FIRE AREAS

The-two representative fire zones selected have been based on the following

considerations:

1) The equipment housed in these rooms is typical of equipment found in

nuclear power plants.

2) The fire load and the size of the rooms are also typical of those in

nuclear power plants.

3) The fire scenarios in these rooms address the potential for failure in

redundant safe shutdown divisions.

4) The layout of the rooms is rather simple to be compatible with the

assumptions employed in the existing computer codes used.
-7-

The first enclosure basically typifies a cable routing room in the auxiliary building, with electrical cabling as the major fire hazard but considered as

a low fire loading relative to the other room under study. The other room represents a diesel generator room containing fuel oil and lube oil as the major

fire hazards. Each room is described below.

Room 1 - This room, 15 m long, 12 m wide, and 6 m high, contains Divisions

I and III power cables and control cables associated with shutdown method A

along one side. Divisions II and IV power cables and the control cables associated with shutdown method B are located along the opposite side of the room.

The other components in the room are assumed unimportant for fire hazard analysis. A simplified schematic diagram of the room is given in Figure 1. The

cable trays are assumed to be 0.61 m wide (24 in.) and 15 m long. The trays

containing power cables are assumed to contain only one layer of 24 single-conductor No. 2 cables. The trays containing control cables are assumed to be

filled with two layers of 7 conductor No. 9 cables, totaling 52 per tray. The

various other considerations for this room are:

Rating (Insulation/

Jacket) : (IEEE rated (EPR/Hypalon); IEEE nonrated (PE/PVC)

Ventilation : Forced ventilation (6 room changes per hour)

Geometry : With openings; without openings

Auto. Detection : With smoke detectors (spacing 3 m); without detectors

Auto. Suppression : With automatic sprinkler systems; without automatic

sprinkler system

Manual Suppression: With trained fire brigades and standby hose stations

Barriers : 1-, 2-, 3-, 4-, 5-hr ratings
-8-

EI IiIII IV

II IV

A B

E

12m

Figure 1. A schematic diagram of the representative cable-routing room.

Room 2 - The configuration for the representative diesel generator room is

12 m wide, 15 m long, and 7.6 m high. A common wall separating the two diesel

generators in the plant is rated for 3-hr. The generator unit, centered within

the room and set over an integral fuel supply day tank, with an additional day

tank mounted on a wall, occupies an area measuring 3 x 10 m. The room contains

nothing else except for a switching panel and a small number of cables running

in metal conduit.

The 1100 gal of diesel fuel in the two-day tanks (max. inside storage

allowed by SRP Section 9.5.1 guidelines), in addition to 250 gal of lube oil,

are the major combustibles for this area. The small quantities of electrical

cable located within control panels and conduits are not considered.

2.2 STANDARD REVIEW PLAN (SRP) GUIDELINES PROPOSED FOR EVALUATION

In accordance with General Design Criteria 3 and 5, the purpose of the fire

protection program is to provide assurance, through a defense-in-depth design,

that a fire will not interfere with shutdown functions or significantly increase
-9-

the risk of radioactive release to the environment. A defense-in-depth design

basically entails a) prevention of fires from starting; b) quick detection and

suppression of those fires that occur, putting them out quickly, and limiting

their damage; and c) plant safety systems designed such that a fire that starts

despite the fire prevention program and burns for a considerable time despite

the fire suppression activities will not interfere with the performance of

essential plant safety functions.

The guidelines specified in Section SRP 9.5.1 which provide this defensein-depth posture can be divided into five generic groups:

1) administrative controls and design provisions to prevent the existence

of transient combustibles and undesirable ignition sources;

2) early detection of fire by means of reliable automatic detection systems and routine fire inspections;

3) early suppression of fire by means of reliable automatic suppression

systems, experienced fire brigades, reliable manual suppression systems, safe removal and containment of liquid fuel spills, etc.;

4) passive protection of the components associated with redundant shutdown

methods to ensure the performance of essential plant safety functions

by means of separation and rated barriers; and

5) preventing propagation of smoke, products of combustion, heat, and

toxic chemicals from suppression agents to the areas where the manual

actions for bringing the plant to safe shutdown are required. This is

done by means of dampers, rated penetrations, automatic closed doors,

etc.

An item-by-item evaluation of all the SRP Section 9.5.1 Guidelines cannot

be done at this time because of time limitations and lack of data and proper

modeling. Thus, the SRP Guidelines found in Sections C.1, to C.4, which basi-
-10-

cally deal with fire protection requirements, administrative controls, fire

brigade organization, and quality assurance, etc., have not been evaluated in

this study. However, the following are addressed on a generic basis and are

contained within Section C.5 of the Standard Review Plan Section 9.5.1:

1. IEEE-383 qualified cablings (C.5.e.3).

2. Barrier effectiveness and penetrations (C.5.a.1, C.5.a.2, C.5.a.3,

C.5.a.4, C.5.b.2).

3. Separation (C.5.b.2).

4. Automatic suppression and detection system (C.6).

5. Ventilation and dampers (C.5.f).
-11-

3. BASIC METHODOLOGY

3.1 INTRODUCTION

To evaluate the NRC requirements regarding protection against fire events,

two interrelated approaches are taken: fire dynamics and fire scenarios. The

former describes fire phenomena in quantitative terms; the latter provides generalized descriptions of actual or hypothetical, but credible, fire incidents.

Combining these two approaches into a tractable analysis entails 1) identification of various fire growth stages (fire scenario), 2) determination of the time

required to reach each stage (fire dynamics/deterministic modeling), and 3) estimation of the likelihood of achieving each stage (fire dynamics/probabilistic

modeling).

Definitions of the fire scenarios are made consistent with the layout of

the safety equipment in the room. For example, if a room contains two redundant

trains of safety cables, first-stage fire growth may be defined as fire involvement (or loss) of one division, whereas second-stage fire growth implies the

loss of both divisions.

To determine the likelihood of achieving each fire growth stage, a probabilistic approach is used concomitantly with the deterministic models. Specifically, probabilistic models are used for detection and suppression systems,

both manual and automatic. The successful operation of these systems depends

on 1) the actuation mechanism's responsiveness to fire symptoms, and 2) the

suppression mechanism's capability of limiting the fire growth at each growth

stage. The time required for successful suppression is, therefore, compared

with the time it takes to achieve the various stages of fire growth. Also taken

into account are the related uncertainties needed to estimate the probabilities

of reaching various room damage states., The interacting mechanism between the

energy absorption rate due to suppression and the energy release rate as the

fire progresses was not evaluated in this study and is at present beyond the

state-of-the-art.
-12-

Finally, a fire barrier analysis is employed when the particular fire scenario (e.g., a high combustible fuel-load area) indicates the possibility of a

barrier failure.

3.2 DETERMINISTIC FIRE ENCLOSURE MODELS

Deterministic enclosure fire-growth models are employed to address the

following: ignition, flame spread, flame growth, maximum burning intensity, and

subsequent product of combustion migration. Zone model approaches are employed,

notably those developed by Siu 8 and Cooper 9 "10 . Although their governing

equations are readily solved with the use of a computer, these zone models do

not lend themselves to generalized solutions which can be displayed "once and

for all" by charts, graphs, or tables. Nevertheless, for specific fires [i.e.,

a fire of a specified energy release rate, Q(t)], solutions for arbitrary enclosure height and area and for arbitrary fire elevation can be obtained.

Siu's fire growth and fire propagation models are embodied in the computer

code, COMPBRN. The fire scenario consists of a fire located at a distance H below the ceiling of an enclosure of area A. The ambient conditions in the enclosure are described by the density, Po, the specific heat at constant pressure,

Cp, and the absolute temperature, To. The fire is located at an elevation,

A, above the floor (i.e., the total height of the enclosure is H + A). Depending upon the combustible involved (in situ or transient), algorithms in COMPBRN

can be used to determine the total energy release rate of the fire, Q(t). In

COMPBRN, Siu uses a basic two-zone model, viz., a homogeneous, stably stratified, hot gas layer (upper gas layer) above a relatively quiescent and cool

ambient air environment (lower gas layer) which is laterally entrained and mixed

with the fire plume gases during its ascent to the ceiling. Since the modeling

assumes that the gases in the upper layer are fully mixed at every instant, perturbations in the gas dynamic process propagate with infinite speed within the

upper layer. Therefore, such effects as thermal plume/ceiling interaction do

not become manifest and are masked by the modeling simplifications employed.
-13-

Cooper employs a three-zone model, namely, an additional zone, above the

upper gas layer which accounts for the formation of a turbulent, laterally expanding ceiling jet brought about by the interaction of the fire plume with the

ceiling. Adding another zone, by definition, entails reckoning with another

characteristic length scale, viz., the width of the ceiling jet, 6, and therefore additional modeling complexity. The thickness of the jet flow is, however,

determined using models derived by Alpert.1 1 To evaluate those fire protection

requirements that deal with spatial separation, this complexity, although still

formulated by Cooper in a simple, mathematically tractable fashion, is indeed

necessary.

At this juncture, the basic methodology mainly employed in determining fire

growth in enclosures for the particular fire scenarios under investigation can

be summarized as follows:

1. COMPBRN algorithms, where feasible, are used to predict the early

stages of fire growth.

2. At the later stages of fire growth, Cooper's three-zone model is used

to predict the enclosure environment and its impact on other combustibles located in upper layers. The bounds of heat release rates are

determined on the basis of a ventilation-controlled burning model. To

account for energy radiated from the fire zone and energy absorbed by

the internal bounding surfaces, an empirical loss coefficient is used.

Uncertainties in the method stem primarily from 1) the uncertainty in the

input parameters, 2) the uncertainty in defining the amount and location of the

fuel that initiates the fire (pilot fire), 3) the systematic uncertainty resulting from the lack of complete understanding of fire behavior, and 4) the simplification required to develop the physical models and to make the mathematical

models numerically tractable. It is beyond the scope of this study to attempt a

systematic appraisal of all these sources of uncertainty. Hence, the uncertainty bounds, determined in various stages of this study, are judgmental.
-14-

3.3 DETERMINISTIC/PROBABILISTIC ANALYSIS OF BARRIER FAILURE

The generally accepted method for rating fire barriers is based on their

survivability period when exposed to the thermal impact from standardized tests.

If the structural element has a fire resistance which meets the required time of

fire duration (rating), adequacy of the design is presumed.

To define equivalent fire severity of the ASTM (American Society for Testing and Materials) standard furnace test, NBS (National Bureau of Standards)

concluded that the fuel load of 80,000 Btu/ft 2 of floor area will produce a fire

impact (i.e., gas temperature) as severe as 1-hr ASTM furnace test. However,

the method of testing and the definition of equivalent severity have a number of

shortcomings which may render standardized test results inappropriate for

nuclear power plant fire barrier analysis. For example:

(a) The equivalent furnace severity fuel load method neglects the actual

time-temperature curve in the fire area. The actual time-temperature

curve of a specific fire area is a function of many parameters such as

ventilation rate, pyrolysis rate, and different types of combustibles,

room geometry, and size, etc. 1 2

(b) The standard test ignores the actual loading of the load-bearing

structures and the effect of temperature on the mechanical properties

and design capacities of the structural elements.

Admittedly, the traditional classification system is convenient and may

provide a reasonable design for most situations. However, in nuclear power

plants, the need to minimize the fire risk necessitates the development of a

systematic procedure based on the specific characteristics of a fire area, taking into accountithe associated uncertainties. Such methodologies known as

limit state design analyses have been the subject of several studies. 13 -14

Although this subject has been understood theoretically, the complete application to actual fire analysis has not been fully performed.' In fire barrier design one should be concerned with assuring that the barrier will meet certain
-15-

functional requirements. These can be expressed in terms of limit states with

respect to 1) load bearing capacity, 2) thermal insulation, and 3) integrity

during fire exposure. Implicit in the third limit state could be the effectiveness of barriers in resisting the spread of smoke and toxic gases. In a deterministic sense adequacy is achieved when the resistance, R, to a particular fire

stress (smoke, temperature, etc.) is greater than the barrier capacity, S, i.e.,

if R > S, design functionality has been achieved. However, in a general sense,

fire engineering design is nondeterministic and one must recognize the impossibility of absolute compliance with a preset goal. Idealistically, performance

has to be described and measured in probabilistic terms. Thus one is concerned

with the probability of "failure," Pfail, which is indicative of the probability that R < S or Pfail = P (R-S<O). Verification that a particular functional requirement is achieved must then begin with an appropriate heat exposure

model(s) and heat capacity model(s), recognizing that uncertainties are encountered because of a) intrinsic randomness, b) uncertainties with respect to the

physical model(s), and c) uncertainties with respect to the stochastic model.

A number of available deterministic fire barrier analyses were reviewed to

find an easily implemented approach, which addresses some of the concerns previously discussed and which can be readily coupled to the probabilistic approach. An approach for barrier analysis which represents a compromise between

accuracy in real fire environment simulation and practicality of implementation

was developed on the basis of the following assumptions:

1. Structural damage will occur during the post-flashover phase of a fire

scenario. Although the pre-flashover phase conditions may likewise

lead to serious structural damage, they are neglected.

2. The criterion used for the structural failure is defined as the product

of the average gas temperature and the exposure period (baking time).

The structural failure is then determined by comparison of this factor

with the equivalent ASTM furnace time/temperature curve.

3. The activation of either an automatic or manual suppression system

before flashover is assumed to prevent flashover from occurring.
-16-

Estimating the temperature signature during the flashover phase is based on

an expression which accounts for the conservation of mass and energy within a

control volume (the fire enclosure). The pressure change in the room is assumed

to be small, and the assumption of an ideal gas is implemented. Hence, the conservation of mass equation is

t

Pav + fo + dt = pfV (3.1)

where Pa and pf are, respectively, the global densities of air at temperatures Ta and Tf; n; is the mass loss rate of the fuel which is not necessarily equivalent to the burning rate of the fuel; ma is the mass flow rate of

air entering through the ventilation system and is assumed to be constant; me

is the mass flow rate of gas leaving the room through the exhaust. It will be

shown later that me is a function of Tf and its time rate of change, Tf.

The volume of the room is given by V, the gas temperature in the room is given

by Tf, and Ta is the temperature of air outside the room.

With the assumption of an ideal gas and taking the derivatives of the above

with respect to time, yields

e + la + (moT a /Tf )Tf ,(3.2)

where mo is the initial mass of the air in the room (-paV).

The conservation of energy can be expressed as

t d (T) QB (Tt) " Q. (T,t) (3.3)

where QB is the heat release rate from burning Cp is the heat capacity of
-17-

the air at constant pressure (assumed constant), and QL is the amount of heat

lost by conduction through the walls and radiation through the openings.

With the assumption of an ideal gas, manipulations yield

Tf=Tf -8T~ (3.4)

where

a [(Q'QL) + C T /E; C T) (3.5) B460+la Cpa opa

[ ma + m]/m0o Ta - (3.6)

The solution of the above equation (assuming weak time dependency during flashover) with the initial condition of Tf(t=O) = Ta becomes

Tf = Ta {exp(at)/[1-(S/a)Ta (1-exp(at)fl} . (3.7)

The expression for a is treated as a random variable based on the distribution

of QLDeriving constitutive relations for the heat loss, QL, entails a degree

of complexity not warranted for this preliminary study. Indeed, models and

analyses are available which can take into account such parameters as wall

thickness and its physical properties, convective heat transfer on the unexposed

side as well as on the exposed side, which is, of course, related to the enclosure room geometry. The degree of sophistication needed, however, was not compatible with the zonal approach currently employed. In this regard, using engineering judgment and the insights gained from results presented in References 9

and 10, QL is assumed to be lognormally distributed with upper and lower

bounds at the 5 percentiles to be 0 . 8 QB and 0. 4QB respectively. This implies a 90% probability that the value of QL is between these two values.
-18-

Thus the average room gas temperature, Tf, is given by

Tf = (1/0t)tn[(BT a/1 ) exp(at)] , (3.8)

which for large times (i.e t+-) becomes

tim f=/=[(QBQL) + ma Cp Ta] [(ma + m) C (3.9)

The above two equations are used in Section 5 to determine the average

gas temperature (Tf) and the probability of structural failure for the representative diesel generator room.

3.4 PROBABILISTIC ANALYSIS OF DETECTION/SUPPRESSION

Early detection and fire suppression are two of the most important considerations that should be taken into account in performing an adequate fire probabilistic analysis. In state-of-the-art probabilistic models the probability of

successful suppression and detection is usually estimated from existing data in

nuclear power plants. 15- 16 However, the data used apply primarily to manual

suppression and do not totally reflect the effectiveness of automatic suppression. In addition, the data are aggregated such that the type and severity of

the fire at the time of suppression initiation are not known. The inadequacy of

data and the lack of modeling of fire propagation during suppression usually result in unsupported, conservative estimations of equipment damage states. Other

studies, such as those by Gallucci 1 7 and Levinson,1 8 provide a more systematic

approach to the probabilistic analysis of detection and suppression systems. In

those two studies, emphasis was placed on system operability rather than on the

time the systems respond, which is a requisite parameter in this study.

The approach taken consists of two steps: establishment of the response

time of detection/suppression system, and to evaluate their associated availabilities and effectiveness.
-19-

In determining the availability of the automatic detection system (smoke

detectors are considered for this study), a binominal distribution is assumed to

represent the probability that m detectors out of a total of n detectors are inoperable, i.e.,

Pf(m/n) = (n!)/[m!(n-m)!] P• (1-Pm)n-m (3.10)

where Pf (m/n) defines the failure of m-out-of-n detectors, and Pf defines

the demand failure of a given detector. Deterministic response time is probabilistically related to successful detection using the above distribution (or an

approximated form thereof) in connection with a relation which describes the

increase in detector spacing (or coverage area) as a function of the number of

operable detectors, viz.,

1/2 WD {m/n} = (A/n-m)1/ (3.11)

detector spacing, WD, is deterministically related to detector response in the

following way.

To estimate the response time of detectors, a model, 19 developed by Newman,

is used in which the response time (tR) of a smoke detector is composed of two

separate times: a transit time (tt) for the smoke front to reach the detector,

and a detection or actuation (alarm) time (tD), i.e.,

tR = tt + tD.

If the convective fraction, Qc, of-the overall heat release rate, QB,

can be approximated by a power law in time, i.e., by Qc = atP, then Newman

shows that

tt = K (1.1WD/H + 0.38) (3.12)

which relates smoke transit time to ceiling height, H, and detector spacing,

WD. The other parameters in the above equation are defined as
-20-

K E (A ge/H4 ) 1/(3+p) (3.13)

A =gpaTaC , (3.14)

with g the acceleration due to gravity and the parameter a reflecting the fire

growth rate.

For a typical smoke detector arrangement on a smooth ceiling, Newman derives the following correlation

tD = (Qc/H)- 1 (45WD + 8.0) (3.15)

for the expected value of tD.

Having an expression for tt and tD (and therefore tR) as a function

of room geometry, detector spacing, and fire properties, ventilation is taken

into account by using the following expression:

tRf /tR =1.0 + 2.1 (H4 / c )1/3 " (3.16)

where tR,f is response time in a ventilated enclosure and tv is the time required for volumetric change of the enclosure air and is equal to the ratio of

the enclosure volume to the volumetric flow through the ventilation per unit

time.

Thus, given a value of Pf and n, [1-Pf(m/n)] determines the probabilistic distribution for (n-m) available detectors. Equation (3.11) then determines

the probabilistic distribution of coverage area which, when incorporated in the

above correlations, provides one with a distribution function for detector

response time.

For the effectiveness of an automatic suppression system, Figure 2 from

Reference 12 is used. These curves basically represent the probability of successful control of fire in an enclosure with a standard coverage vs the fuel

load per unit area of the floor.
-21-

The probability of successful operation of an automatic suppression system,

in addition to its effectiveness, is incorporated through the reliability data

given in Reference 18.

rd i-nstallations of automatic firesuppre ssion systems in spaces housing the

class of combustibles indicated:

Curve IN: wet-pipe automatic sprinklers

(Class A and hign-flashpoint Class B); foam (CasA -dIl carbon dioxide.

7-halon, dryv chemical (Class 11).

CreI:dr-y-pipe or preaction sprinklers

(Class A, (,igl,-fLashpoint Class B); halon

60o (Class Al.

Curve C; wvet -pipe sprinklers (low flaahpoint

Class I10; carbon dioxide, dr~y chemical

(Class A).

C:urve D: dry-pipe or preactien sprinklers

(Inw- lashtpolint Class Il).

400

Nil\%

Figure 2. The effectiveness of automatic suppression system

(reproduced from Reference 12).

The time required to detect and suppress a fire manually is considered

probabilistically for two special cases (1) the automatic detection and suppression systems are unavailable and (2) the room is unoccupied. With regard to

Case (2), the fire can be indirectly sensed manually through its impact on the

operability of equipment located within this unoccupied area. It was assumed at

this facility that the impact of the fire on equipment would be sensed by the

control room operator. In this case, the probability of detection is assumed to

be an increasing function of time after the equipment malfunctions. The time
-22-

required for manual detection depends on the type of safety components affected

by fire and their locations in the plant. It is assumed that the operator will

dispatch the plant staff to those locations where the malfunction has occurred.

The fire will then be visually detected by plant staff, and minor mitigation

actions may take place. The bounds for the time required to detect the fire

manually is determined from the data in the literature. 1 8

Once the fire is detected, the probability that manual suppression will

bring the fire under control can be estimated from the curve in Figure 3. This

curve, developed from existing fire data in nuclear power plants, 1 6 basically

indicates the probability of successful manual suppression vs available time.

The manual suppression phase of an enclosure fire is therefore not modeled

deterministically in this study.

The conclusions of this study regarding enclosure damage states are drawn

from the comparison between the fire growth time (or stages in fire development)

and time required for successful suppression. The following sections describe

how this concept is applied by way of two examples.

1.0

0

a,

IU.

C u 

15 30 45 60

Time Available to Suppress Fire (minutes)

Figure 3. Manual fire suppression model.
-23-

4. REPRESENTATIVE CABLE-ROUTING ROOM ANALYSIS

Two stages of fire growth are defined for this analysis. The first stage

indicates loss of one of the redundant shutdown divisions; the second stage requires the fire to propagate to the other redundant shutdown division. The time

required for the fire to propagate to a first-stage fire growth is determined

with the COMPBRN computer code. The uncertainties associated with the size and

location of the initiating fire, the ignition criterion for the cables, and the

other unit models in the COMPBRN code are qualitatively discussed and evaluated.

The time required to reach a second-stage growth is determined by Cooper's enclosure fire model. The uncertainty in second-stage fire growth results largely

from the wide uncertainty associated with the estimated burning rate.

To determine the likelihood of achieving each fire growth stage, a probabilistic approach is used concomitantly with the deterministic models. The

operability, effectiveness, and response time of detection and suppression systems at each stage of fire growth are estimated through probabilistic analyses.

The comparison between the fire growth time and time required for successful

suppression is used to estimate the probability of achieving each fire growth

stage.

To evaluate the relative importance of various fire protection measures,

the change in probabilities of the room-damage states corresponding to growth

stages, with or without a given fire protection measure, is estimated. Finally,

the relative importance of various fire protection measures is estimated in the

form of reduction factors in the probability of reaching each fire growth stage.

At the outset, the quantitative results obtained for the probabilities of

various room-damage states in this representative room must be tempered with the

existing limitations found in both probabilistic and deterministic fire analyses. However, since this study concentrates on evaluation by relative comparisons of various fire protection systems/features, it is expected that these

limitations will not severely affect the conclusions drawn.
-24-

4.1 FIRST-STAGE FIRE GROWTH

For this analysis a combustible liquid spill is considered as the fire initiator. The spill is assumed to be located directly under one of the redundant

divisions. The time required to achieve what is termed as first-stage growth is

based upon when the three cable trays in one side of the cable routing room

(Figure 1) become damaged.

Defining electrical cable damageability due to fire and establishing trends

that are a function of physical parameters which define the dynamics of the fire

and the surrounding environs have been the subject of many experimental studies

conducted by the Factory Mutual Research Corporation (FMRC) and Sandia National

Laboratories. 20 -24 Damageability in these studies is defined as a change in the

properties of a cable which cause impairment to its normal function. Four categories of electrical cable damageability have been defined2 1 : cable insulation/

jacket degradation (generation of combustible vapors), auto-ignition, piloted

ignition, and electrical failure. Damageability is usually quantified in terms

of 1) the critical heat flux at or below which damage is not expected to occur,

and 2) the energy required to sustain the damage process. This latter quantity

is simply defined as the product of the difference between the externally applied heat flux and the critical heat flux and the time required for the cable

to reach the critical heat flux. For example, Tewarson 2 3 reports that the critical heat flux measured for a particular sample of EPR/Hypalon cable is 19

kW/m 2 ; the critical energy is 1420 kJ/m2 ; and the critical temperature calculated for cable ignition is 488 0C, which is based upon a simple radiative heatflux model.

This temperature is used in this example to define the ignition temperature, Tig, used in the ignition model in the COMPBRN I computer code, from

which the time to achieve first-stage growth is derived. In COMPBRN I, the fuel

elements (in this particular case EPR/Hypalon electrical cables) are modeled as

homogeneous semi-infinite slabs. To find the time to ignition, tig, the heat

conduction equation is solved. By assuming a constant heat flux at the cable
-25-

boundary (q;") and neglecting radiative and convective heat losses, a closedform solution is obtained showing the timewise variation of fuel element surface

temperature (Ts), viz.,

SII

t = (,/4a)[k(Ts - To)/qo] , (4.1)

where a and k are respectively the element's thermal diffusivity and conductivity. Substituting Tig for Ts into the above expression, the ignition

time, t*, is determined.

Employing this ignition model, and considering as the initiator a fire from

a 3.81-1 (1 gal) spill of fuel oil, 0.3 m (1 ft) in diameter, COMPBRN I calculates that 330 sec are required to ignite the cable tray located 3 m directly

above the liquid pool-spill fire. The other two cable trays, located 2 m

further above, require an additional 90 sec. Appendix A presents the input data

used for this example.

Judgmentally, a time of 420 sec to achieve a first-stage growth seems

rather short. This is understandable when one considers that the ignition model

[Eq. (4.1) does not consider the radiative and convective losses from the fuel

element. In fact, the above equation indicates that for a given ignition temperature (Ts = Tig) the fuel element can ignite no matter how small the external heat flux is.

Toward the end of this study, the authors became aware of an improved version of COMPBRN I* which incorporates these added features. But, a closed form

solution to the heat conduction equation [like Eq. (4.1)] does not exist. The

fuel element temperature is found numerically in COMPBRN II.** An equilibrium

point is established where the heat losses from the fuel element balance the

heat gains in which case the surface temperature of the fuel levels off. If

*Chung, G., Siu, N., and Apostolakis, G., COMPBRN II: Code Description and

Simulation of Experiments, UCLA-ENG-8404, March 1984.

**M. Kazarians (Pickard, Lowe, and Garrick) - private communication.
-26-

this temperature is below Tig, then by definition the cables would not ignite,

but of course they could be degraded (for example, by melting). The results*

indicate that a larger size (0.84 m instead of 0.3 m) would be needed to ignite

the cables. However, although losses are accounted for, the time required to

reach first-stage growth is reduced (270 sec) owing to an increased flame height

resulting from a larger-diameter spill.

One must understand, however, that although an improved heat transfer model

is employed, accurate modeling of ignition time and temperature is highly complex. Before ignition, polymers undergo several phase transitions. No well-defined melting point exists, since transition of a solid polymer to a viscous

liquid can consist of several phase transitions before subsequent evolution of

combustible vapors. These vapors must mix with air in proper stoichiometry for

ignition to occur. Also, the temperature of the surrounding air is crucial for

either auto or piloted ignition. Experiments performed to establish some

measure of the critical heat flux to cables usually employ a thermal radiative

source to simulate the heat flux from a fire. The predominant heat transfer

mechanism in these experiments is through radiation. Thermal energy transferred

to the surrounding gas environment is through convection processes occurring in

the vicinity of the heated cable and not, as in a real fire, through large-scale

convection resulting from the initiator fire plume. Thus, the experiments differ from the actual case in the sense that the cable sample is not engulfed by a

hot gas plume and, hence, the convective heat transfer to the sample is not simulated. Judgmentally, the actual critical heat flux is expected to be lower

than those established from small-scale, radiative-type experiments.

Because of uncertainties associated with existing fire models, empirical

correlation for flame and plume characteristics, cable properties, and the possibility of various types and sizes of pilot fires, it is judgmentally assumed

that the timing for first stage fire growth would be between 4.5 and 7.5 min.

Therefore, the timing for the first-stage fire growth is considered probabilistically by a lognormal distribution with 5 and 95 percentiles of 4.5 and 7.5

min, respectively.

*M. Kazarians (Pickard, Lowe, and Garrick) - private communication.
-27-

4.1.1 Detection Time

To estimate the probability of successful detection and its associated timing, both possibilities, i.e., automatic and manual detection, are considered.

The following are summary discussions on each of the possible detection

mechanisms:

(a) Automatic Detection System - The room is assumed to be equipped with

ceiling-mounted smoke detectors spaced every 3 meters. Using the data found in

Reference 18, the mean value for failure of a smoke detector per demand is

assumed to be 0.13. For the room under consideration (180-mi2 ceiling area), a

total of 20 smoke detectors are presumed to be installed. Such a dense Installation of smoke detectors may not be typical in nuclear power plants, and it may

be more applicable to fire test facilities. However, the sensitivity of final

results to the number of smoke detectors installed is minimal.

The relation between failure of smoke detectors and their response time is

based on the average coverage of the available detectors, i.e., if all the 20

detectors are operable the spacing Is 3 m. However, if we assume 10 detectors

are not operable, then the average spacing for the remaining detectors is about

4.2 m. In this section, we apply the probability distribution laws and correlations indicated in Section 3.4.

The failure of m out of n detectors can be estimated from the binomial distribution [Eq. 3.10) or approximately by an equivalent Poisson distribution,

i.e.,

Pf(m/n) = (n) Pf (1 p)n-m (4.2)

where Pf (m/n) = failure of m detectors out of n, and Pf = failure of a

detector per demand. For large n and small Pf, this equation can be approximated by

Pf(m/n) = e Im/m! M (4.3)

where P = n-Pf"
-28-

The associated coverage area for the case in which m detectors have failed

is simply

WD(m/n) = (A/n.-m) 1 /2 9 (4.4)

where A is the ceiling area and (n-m) is the number of available detectors.

Using the equations given in Section 3.4 for detector response time (in

seconds) and the following information: po = 1.2 kg/m3 , Cp = 1008 J/kg *K,

g = 9.81 M/s2 , = 126 kW, To = 3000K, Qc = 126 kW, and H = 6 m, the relation between response time and the number of operable detectors is

tR = 215 (n-m),1/2 + 30.8 . (4.5)

The above equations are used to drive the cumulative probability distribution for response time of the smoke detectors, as shown in Table 1. Since the

probability distribution function calculated for the detector response time has

a very low spread, a point estimate of 1.5 min (90 sec) will be used for the

rest of the analysis.

Table 1

Response-Time Cumulative Distribution Function

for Automatic Detection System

tR(sec) Pdf Cdf

78.8 6.2x10- 2 6.2x10-2

80.1 0.193 0.253

81.5 0.25 0.505

82.9 0.217 0'722

84.6 0.14 0.862

86.3 0.07 0.932

98.9 0.0028 0.977
-29-

(b) Manual Detection of Fire - The room under consideration is assumed to

be normally unoccupied [case (2) in Section 3.4]. Hence, manual detection of

the fire would be due to a component failure signal or would occur randomly by

plant staff. The timing for observing an abnormality in the plant due to a fire

in this area is estimated from the electrical failure of the nearest cable tray

to the transient fuel. From the results discussed in the preceding Section,

this is estimated to be between 3 and 6 min. However, after the abnormality is

detected, a plant staff member would be sent to assess the cause. The time required for plant personnel to reach the affected area is judged to be 3 to 5

min.1 8 Hence, manual detection of this fire may take between 6 and 11 min. A

lognormal distribution with 5 and 95 percentile corresponding to 6 and 11 min,

respectively, is therefore used in this case for the time required to detect the

fire manually.

4.1.2 Suppression Time

There are three possibilities for extinguishing the fire before it reaches

first-stage growth:

(a) Automatic Suppression/Automatic Detection - In this scenario the fire

will be detected and suppressed automatically. A water sprinkler system is considered as the only means for automatic suppression in this room. The probability that the sprinkler system will fail to operate can be estimated from existing data summarized in Table 2. A failure probability in the range of 0.01 to

0.1 for sprinkler systems is indicated. Similar failure data from Reference 18

for Halon systems indicate a failure probability of 0.0536 to 0.142. Hence a

range of failure probabilities from 0.01 to 0.15 is assigned for the inoperability of automatic suppression systems.

The estimated effectiveness of the automatic suppression system is based on

the curves presented in Reference 12 (Figure 2 of this report.) The probabilities for successful suppression vs the fuel load per unit floor area for the

various types of suppression systems are given by these curves. The equivalent

heat of combustion per pound of fuel used for generating these curves is not
-30-

Tabl e 2

Successful Sprinkler Operation

Data Source Rate of Success, % Time Period

NFPA 95.8 1892-1924

NFPA 96.2 1925-1970

U.K. Fire Statistic 91.7 1965-1969

IRI (Industrial Risk Insurers) 91.06-99.35 1973-1977

U.S. Navy 94.8 1966-1970

Australia 99.76 1886-1968

FM 76-91 1970-1972

known. However, in the same reference, curves are shown indicating the effectiveness of rated barriers, under the assumption that a fuel load of 80,000

Btu/ft 2 is required to fail a 1-hr-rated barrier. Hence, the equivalent heat of

combustion per pound of fuel for generating these curves is expected to be about

3.01,MI (2857 Btu). The fuel load estimated for the representative cable-routing room is about 15,000 MJ and the floor area is 180 m2 . This corresponds to

an equivalent fuel load of about 2.6 lb/ft 2 . The failure probability for successful suppression from the curves is therefore about 1.0 x 10-3.

From the above discussion, it can be concluded that the failure of a suppression system to extinguish the fire during first-stage fire growth is between

0.01 to 0.15 and is dominated by the failure of a suppression system to actuate

rather than its delivered suppressant density.

(b) Automatic Detection/Manual Suppression - The response time of an automatic detection system, as discussed previously, is estimated to be about 1.5

min. It Is also, assumed that it takes 3 to 5 min for plant personnel to reach

the fire area and observe the extent of the fire. If the fire Is small, It is
-31-

assumed that it can be extinguished with a portable extinguisher. For the personnel to find the portable extinguisher and use it properly, Reference 18 estimates a 2-mmn time limit of 2-mmn. Hence, it takes between 6.5 and 8.5 min before the fire can be extinguished manually. As noted before, the timing for the

first-stage fire growth is lognormally distributed with the bounds between 4.5

and 7.5 min. Using the lognormal distribution function for successful manual

suppression with the 5 and 95 percentile of 6.5 to 8.5, one can then estimate

the probability of successful manual suppression.

(c) Manual Suppression/Manual Detection - The estimated timing for manual

detection is estimated based on lognormal distribution with 6 and 11 mmn as the

two bounds. The same discussion for manual suppression/automatic detection also

applies here. Hence, the 5 and 95 percentile bounds of lognormal distribution

in this case would be 8 to 13 min, respectively.

4.1.3 Probability for Room-Damage State(s)

The room damage state (RD1) is considered as loss of one redundant safe

shutdown division. This, by definition, corresponds to a first-stage fire

growth. Its probability can be functionally expressed as the probability that

the time required to reach RD1 is less than the suppression time, i.e.,

P(RD1) = P[(tRD1 - t supp) < 0) (4.6)

or

P(RD1) = i [1-P(ts<t)0 P[tgc (t,t+dt)] dt

0 (4.7)

fo [ s'Ft(t')] ftg (t') dt
-32-

where P(ts<t) = Fts(t) is the probability of successful suppression before

time t, and P[tge(t,t+dt)] - ftg(t)dt is the probability that the time required for first-stage fire growth is between t and t+dt.

The integration in the above equation is obtained by means of a discretized

probability distribution function. Table 3 gives the discretized points of the

cumulative probability distribution for the growth and suppression of the fire.

As previously discussed, the failure probability for automatic detection/

automatic suppression is not sensitive to the timing of first-stage fire growth.

It is defined by a probability distribution with the bounds at 0.01 to 0.15, and

it shall be treated differently. To simplify the calculations, a point estimate

equivalent to the mean plus one standard deviation is conservatively used for

this case. Table 4 provides the probability of the first-stage fire growth

based on various protection measures.

4.2 SECOND-STAGE FIRE GROWTH

For this stage, we consider the propagation of fire from one redundant division to the other. To calculate the timing required for second-stage growth,

Cooper's fire enclosure model is used, requiring as input the burning rate from

the first stage. For this particular example, the burning rate was determined

on the basis that only the three cable trays on one side of the room (one shutdown division) are fully involved. Two case runs were performed. In the first,

the maximum value for the heat of combustion of electrical cables was used in

conjunction with the liquid transient fuel that initiated the fire. In the

second, an average value for the heat of combustion for the cables in addition

to a small amount of transient fuel considered just sufficient to ignite the

cable tray nearest to the initiating fire was used.

The burning rates resulting from these two runs are depicted in Figure 4.

In our judgment these burning rates are unrealistically high. Ostensibly, this

is due to the following inadequacies within the COMPBRN computer code:
-33-

Table 3

Discretized Cumulative Distribution Function

for Fire Growth and Suppression

Time f (tg) F(tAD+tMS) F(tMD+tMS)

4.5 0.05 £

5.0 0.059 C £

5.5 0.37 C £

6.0 0.60 0.004

6.5 0.79 0.05 £

7.0 0.9 0.24 0.01

7.5 0.95 0.57 0.02

8.0 0.986 0.84 0.05

8.5 0.995 0.95 0.11

9.0 0.998 0.994 0.21

9.5 0.999 0.999 0.32

10.0 1. 1. 0.45

10.5 1. 1. 0.58

11.5 1. 1. 0.80

f(tg) - Cumulative distribution function for the timing of first-growth

stage.

F(tAD+tMS) - Cumulative distribution function for the timing of automatic

detection/manual suppression.

F(tMD+tMS) - Cumulative distribution function for the timing of manual

detection/manual suppression.
-34-

Table 4

The Probability of First-Stage Fire Growth

Automatic Detection/Automatic Suppression* 0.10

Manual Detection/Manual Suppression 0.94

Automatic Detection/Manual Suppression 0.89

Automatic/Manual Suppression and

Automatic/Manual Detection 0.068

* The failure probability for automatic suppression system with the bounds of

0.01 to 0.15 is fitted to lognormal distribution with associated normal parameters of (-3.25, 0.824) or lognormal parameters of (0.054, 0.053). The value

indicated is mean plus one standard deviation of lognormal distribution.

I - High Burning Rate

II - Low Burning Rate 108

4.

.0

Time (Sec.)

Figure 4. Estimated range of burning rate predicted by COMPBRN.
-35-

1. An unrealistically high horizontal flame spread is predicted.

2. No feature is built into the computer code to distinguish the transition from surface-controlled to ventilation controlled burning.

To avoid this difficulty and because of inherent characteristics of the

probabilistic assessment which deals with the uncertain elements rather than

accurate ones, an alternative approach for estimating the bounds on the burning

rates is implemented. This is discussed in the following section.

4.2.1 Determination of Bounds on Burning Rates

For the purpose of probabilistic fire analysis, given the lack of accurate

estimates for the enclosure heat generation rate (burning rate), it has been decided to define the enclosure heat release rate for the second stage of fire

growth as a pulse with duration of WB (minutes) and amplitude of QB (kW).

Although other shapes such as Maxwellian, Gamma, Exponential, etc., could have

been used for the enclosure heat release rate, however, the calculations are

much simpler if the time characteristic of the burning rate is defined as a

pulse.

The bounds on the burning rate can be translated to a range of values for

WB and QB. One simple relation that can be established between QB and

WB is to equate their product to the total heat of combustion available during

second-stage fire growth. That is, for each cable tray, the following equation

can be established:

WBiQBi = Qtl for the tth cable tray . (4.8)

The bounds for the heat of combustion of EPR/Hypalon are estimated to be

between 2.96x10 7 J/kg and 1.7x10 7 J/kg which provide the bounds on a per tray

basis. To estimate the average amplitude of the fire, it is conservatively assumed that all the oxygen entrained up to the flame height is consumed. Knowing

the mass of oxygen entrained, the heat release rate can be determined on the
-36-

assumption that 1 kg of oxygen (burning with a hydrocarbon) would produce 12.5

MJ of energy. 2 2 Hence, the mass of air entrained into the plume (up to the

flame height) establishes the heat release rate through the following plume

equations:

p= 0.21 p0 (gz) 1/ 2 z2 (Qz) 1/3 , (4.9)

( / po Cp TO (gZ) 1/ 2 (Z) 2 (4.10)

= (12.5) (0.21) (p02 /Pa) (4.11)

where P02 and Pa are the densities of oxygen and air at 3000K, respectively,

and po is the density of the air in fire environment. Using standard thermodynamic data, manipulations yield

Q = 3.15 (Ta/To)2/3 (1-dr)1/2 Z5/2 . (4.12)

To estimate the flame height (Z), we consider a right-angle circular cone

where the area of the flame base is equivalent to the area of the cable tray. A

semi-apex angle of 300 is used for estimating flame height. The calculated

equivalent radius for each of the cable trays is equivalent to 1.7 m. This

yields a flame height of about 0.98 m. Inserting this value for Z in the above

equation gives the following expression

= 3.0 (T/T0 ) 2/3 (1.X 1/2 • (4.13)

which is then used to. estimate the burning rate (in MW) of each of the cable

trays.

The value of the local gas temperature (To)is unknown. However, COMPBRN

computer runs, for first-stage fire growth, indicate that at the end of this
-37-

stage the hot layer thickness and temperature are 1.4 m and 7570 K, respectively.

Accordingly, the lower tray of the second division is not within the hot gas

layer. The upper bound for the heat release rate can be estimated by assuming

the value of To for the two cable trays at the 5-m elevation to be 757 0K where

the cable tray at the 3-m elevation is 300 0K. For the lower bound, all cables

are assumed to be in the hot gas layer. With Xr for EPR/Hypalon cabTes prescribed (Ar = 0.62), the bounds on the average burning rate for each cable

tray can be estimated using the above expression. The pulse width for the burning rate of each cable tray is established using these bounding values in conjunction with the upper and lower bounds for the heat of combustion of the EPR/

Hypalon cables. The estimated results for the ranges of the burning rates and

durations are given in Table 5. This approach, although simplistic, seems to

reasonably envelop the possible heat release rates for this representative room.

Table 5

The Burning Rates for Various Cable Trays

in the Representative Cable-Routing Room

Characteristic of

Cable Tray Total heat of combustion Heat Release Rate (QW)

No. Height Lower Bound Upper Bound Lower Bound Upper Bound

1 3 m 1.77(+3) MJ 3.08(+3) MJ (0.83 MW, 35 min) (1.85 MW, 27 mtn)

2 5 m 1.30(+3) KJ 2.20(+3) M4 (0.83 MW, 26 min) (1.0 MW, 36 min)

3 5 m 1.30(+3) K) 2.20(+3) MJ (0.83 MW, 26 min) (1.0 MW, 36 min)

4.2.2 Determination of Cable-Damageability Times

The enclosure fire analysis is used to estimate the local gas temperature

and the convective heat flux to the cable trays. The cable tray surface temperature is estimated by means of a one-dimensional heat conduction model for a

slab with a thickness equivalent to cable outside diameter. The convective heat
-38-

transfer coefficient is assumed to be 2 kW/m2 0C for a cable tray located inside

the hot layer. If the cable trays are within the radial jet flow region, the

convective heat transfer coefficients are estimated on the basis of Cooper's

model. The radiation heat transfer to a cable tray. having a separation distance

greater than 3 m from the fire axis is assumed to be negligible. Three divisional separation distances are considered in this study, viz., 3, 6, and 9 m

from the fire axis. For trays inside the hot layer but below the jet layer,

separation effectiveness cannot, by definition, be assessed. Figures 5 and 6,

respectively, depict the target temperature and heat flux as a function of time

for the lower burning rate scenario, Similarly, the results for the case with

the upper bound burning rate are given in Figures 7 and 8.

The estimated burning rates used for exercising the Cooper's model for the

second-stage fire growth do not account for the interaction between the hot

layer environment and the burning of the cables. The burning rate for the

cables engulfed in the hot layer is governed by the amount of oxygen available

in the hot layer. Lower oxygen concentration in the hot layer is expected to

yield lower burning rates. Assuming that the oxygen concentration is Inversely

proportional to gas temperature, a cutoff hot layer gas temperature of 9000C is

assumed to yield zero burning.

A single cable-damageability criterion could not have been identified for

EPR/Hypalon cables. Therefore, the cable-damageability criterion used in this

study is either a cable surface (target) temperature of 440 0C or an integrated

surface heat flux between 1800 and 4000 kJ/m 2 , depending on the size of external

heat flux. Tables 6 and 7 present the associated timing for cable-damageability

criteria vs separation distance. A point estimate for the timing of cable damageability for each case is calculated on the basis of the mean of three estimated timings. The lower and upper bounds calculated in this manner are then

used as 5 and 95 percentile of a lognormal distribution. These results are

given in Table 8.
-39-

I

IIIII

Hot Layer Temp.

Target in jet flow 3m away

Target in jet flow 6m away

Target in jet flow 9m away

Target in hot layer

1300

1

41

CL

300

2 4 6 8 10 12

Figure 5. The target temperature vs time for the second-stage

fire growth (lower bound of burning rate).

I

II

IV

Target in jet layer 3m distance

Target In jet layer 6m distance

Target In jet layer 9m distance

Target in hot layer

g

4,

re

0D

0

Time (minutes)

Figure 6. The target heat flux vs time for the second-stage

fire growth (lower bound of burning rate)
-40-

IV

III

IV

Hot layer temperature.

Target in Jet layer 3m

Target In Jet layer 6m

Target in Jet layer 9m

Target in hot layer

1300

a

wL

4.,

II.

300

4

Time (minutes)

Figure 7. The target temperature vs time for the second-stage

fire growth (upper bound of burning rate).

I

II

Ill

IV

Target

Target

Target

Target

in

in

in

in

jet layer 3m distance

jet layer 6m distance

jet layer 9a distance

hot layer

11

0

Figure 8.

II

The target heat flux vs time for the second-stage

fire growth (upper bound of burning rate).
-41-

Table 6

The Timing for Cable Damageability for the Lower Bound Burning Rate

Distance Temp. Integrated Heat

From Criteria Flux Criteria

Fire Axis >440 0C 1800 kJ/m2 4000 kJ/m2 Mean Time

(W) Location (min) (min) (min) (min)

3 J.L* 9 4 9 7.3

6 JL* 18 6 14 12.7

9 J•L* 32 11 19 20.5

>3 HL** 32 12 19 21.

*J.L - Jet Layer

•*HL - Hot Layer

Table 7

The Timing for Cable Damageability for the Upper Bound Burning Rate

Distance Temp. Integrated .Heat

From Criteria Flux Criteria

Fire Axis >440 0C 1800 kJ/m2 4000 kJ/M2 Mean Time

(m) Location (min) (min) (min) (min)

3 JL* 6.5 3.5 6. 5.3

6 JL* 12. 6. 11. 9.7

9 J.L* 21. 8. 11.4 13.4

>3 H.L** 21. 9. 15.4 15.1

•JL - Jet Layer

•*HL - Hot Layer
-42-

4.2.3 Suppression Analysis in Second-Stage Fire Growth

Beyond first-stage growth, the only mechanism assumed to be effective in

extinguishing a fire is manual suppression by the fire brigade using standby

hoses. From the discussion given in Section 4.1.2, it can be assumed that manual detection is achieved with a high probability (an approximate probability of

1) by the end of first-stage fire growth. The time required to bring the fire

under control is estimated from the data given in Reference 15. The suppression

success probability distribution as a function of time is generated on the basis

of the Weibull distribution function with parameter (a.,) equal to (0.615,

13.5), respectively. However, the suppression curve generated does not differentiate between the two stages of fire growth. Hence, to estimate the successful suppression in second-stage growth, a conditional probability distribution

describing the successful suppression in this stage given failure to suppress in

the first stage is needed. This conditional probability can be formally expressed as

P(t 1 < slt l +t2)

P(ts t+ t2 ItI ti 2 (4.14)

where t, is the time required to reach the first stage fire growth [distributed

lognormally by fl(tl)3, t 2 is the time required to reach second stage fire

growth [distributed lognormally by f 2 (t 2 )], and ts is the time required for

successful suppression [given by a Weibull distribution denoted by fs(ts)].

Theregion of integration for the fixed value of ts5 and the joint distribution function of t, and t 2 is shown in Figure 9. Hence, for all values of

ts (assuming that t, and t 2 :are independent variables), the conditional probability for successful suppression is given by the following expression:
-43-

Table 8

Upper and Lower Bounds on the Timing of Cable Damageability

and the Associated Lognormal Parameters

Parameters of Lognormal

Distance - Distribution**

From Fire Lower Bound Upper Bound x v

Axis (m) Location* 5% 95% (W) (a)

3 J.L 5.3 min 7.3 min 6.6 2.3

(1.83) (0.34)

6 JL 9.7 min 12.7 min 11.4 3.1

(2.4) (0.27)

9 J.L 13.4 min 20.6 min 18.3 9.1

(2.8) (0.47)

>3 H9L 15.1 min 21. min 19.2 6.7

(2.98) (0.34)

*JL: Jet Layer; H.L: Hot Layer

**The parameters of lognormal distribution are V and a. The mean (x) and

variance (v) are estimated on the basis of the following expressions.

2

= e +a/2

v= e2l+ 2[(ea -1)]
-44-

ti

ts

t 2

ts

Figure 9. Region of fintegration for Eq. 4.15

P(ts -ý t1+t2~~ It2 ti

t f0

ts=O tl =0

j 0 jSt

0 tl=O

fj(tl)[1-F2 (ts-t 1 ))dtj fs(ts)dts

(4.15)

fl(t)dt fs(t)dt

The above equation can be simplified if the average values of tj and t 2 Instead of their distributions are used. In this analysis, the ranges for tj and

t 2 determined in the previous section are comparatively much smaller than the

range for ts. Hence, for the first order approximation, the values of tj and

t 2 are assumed fixed and equal to the mean of their distributions given by tj*

and t 2 *, respectively, which simplifies the above expression, viz.,

P(t s -ý t1+t2 it s > tj

) 2 1 fs(t)dt

ts =t *

f *fs(t)dt ts=t1* a (4.16)

Using this expression, the probability for failure to suppress during

second-stage fire growth (before cables are damaged) is given in Table 9.
-45-

Table 9

Probability of Successful Suppression in Second-Stage Growth

for IEEE-Qualified Cable

Distance Probability

From Fire of Failure

Axis (m) Location* tl* t 2 * Suppression

3 JoL 6.7 6.6 0.69

6 J.L 6.7 11.4 0.46

9 J.L 6.7 18.3 0.39

>3 HoL 6.7 19.2 0.38

*J.L - Jet Layer; H.L - Hot Layer.

4.3 MISCELLANEOUS FACTORS

The purpose of this section is mainly to discuss the impact of IEEE qualified vs non-IEEE-qualified cables on fire safety. In addition, the importance

of using proper by rated cables and splices for reducing the fire frequency is

evaluated. The three major parameters for cables are

Cs - mass loss rate per rate of energy absorbed (g/kJ),

Etd - total energy required for damageability (kJ), and

Hf - actual heat of combustion (kJ/g).

If it is assumed that a constant portion of the heat release rate is absorbed by

the fuel to sustain the fire, then the value of Cs multiplied by Hf is an

important measure for estimating the transition probability from a small incipient fire to a sustained one. The value of Eid basically determines how quickly damage occurs under a fire environment. This is especially important for the

evaluation of second-stage fire growth times.
-46-

To differentiate or ascertain the effect of IEEE rating, nonqualified PE/

PVC cables are compared with qualified EPR/Hypalon cables. The value of "Cs"

for EPR/Hypalon is 0.17 g/kJ where for PE/PVC it is 0.22 g/kJ. The values of

Hf for EPR/Hypalon cables are between 29.6 and 17.4 kJ/g, whereas for PE/PVC

cables they are between 30.8 and 22.1 kJ/g. Using the average value for Hf,

it is expected that the frequency of self-ignited IEEE qualified cable fires is

lower, by a factor of 1.5, than IEEE-nonqualified cables.

The probability for reaching second-stage fire growth when the cables are

not IEEE qualified is estimated on the assumption that-the burning rate after

first-stage fire growth is independent of the cables' characteristic parameters

(see Section 4.2.1). Hence, the timing and the probability of second-stage fire

growth can be calculated from the results given in previous sections modified

for the energy damageability index of PE/PVC. The energy damageability indexes

for EPR/Hypalon and PE/PVC cables are between 1800 and 4000 kJ/m and 530 and

1000 kJ/m2 , respectively, depending on the size of external heat flux. 20

°21

The final results for PE/PVC cables are given in Table 10.

The fire data from past experience in nuclear power plants16 indicate that

the use of-proper splices and rated cables can reduce the frequency of selfignited cable fires by a factor of 3. This is an important reduction In cable

self-ignited fires that has to be accounted for in fire analysis. Although it

may change as more data becomes available, a factor of 3 reduction is credited

in this study for proper rating and splices of the cables.

4.4 RANKING OF FIRE PROTECTION MEASURES IN THE CABLE-ROUTING ROOM

The analyses performed thus far facilitate the probabilistic ranking of

those fire protection measures deemed important for this.type of a room. Here,

emphasis is on the following:

1. electrical design of cables such as proper rating and properinstallation of splices,

2. use of IEEE-qualified cables,
-47-

3. automatic detection,

4. automatic suppression,

5. accessibility of portable extinguishers and standby hoses, and also the

plant staff's familiarity with the fire-emergency and fire-fighting

procedures.

6. 10-ft separation,

7. 20-ft separation,

8. 30-ft separation,

9. proximity of cable tray to ceiling, and

10. trained, well-drilled fire brigades familiar with the plant layout.

Table 10

The Timing and Probability of Cable Damageability

in Second-Stage Fire Growth for Cables that are not IEEE Qualified

Distance Lower/Upper Probability of

From Fire Bounds on Second-Stage

Axis (m) Location* Timing ()lo)x Growth

3 J-L (3., 4.5) 4.** (1.3, 0.42) 0.8

6 J-L (5.9, 9.2) 8. (2.0, 0.47) 0.68

9 J.L (9.6, 15.3) 13.7 (2.5, 0.49) 0.52

>3 H-L (10., 15.3) 13.5 (2.5, 0.45) 0.53

*J.L - Jet Layer, H.L - Hot Layer.

**The numbers in the parenthesis are the lower and upper bounds, and the number

outside the parenthesis is the mean of the timing for the second-stage growth

and are expressed in minutes.

xThe parameters of the lognormal distribution are defined by v and a. For

more information, refer to Table 8.
-48-

Table 11 gives the probability of first- and second-stage room damage

states for various combinations of the above fire protection measures. Inclusion of all possible combinations of fire protection would make the table unmanageable, and thus the following considerations are taken into account to limit

the table's size without loss of information:

(a) If manual suppression (Item 10 above) is removed from the analysis,

then the probabilities of first-and second-stage growth would be the

same. This stems from the assumption that second-stage growth can be

prevented only by fire brigades (manual suppression).

(b) If the cables are not in proximity of the ceiling (Item 9), namely,

if they are located in hot layer region, the change in separation distance (Items 6,7,and 8) would not change the probability of either

first- or second-stage growth.

(c) The importance of automatic detection/suppression system and proper

cable rating/splices (Items 3, 4, and 1) would affect only the probability of first-stage fire growth.

(d) In general, Items 1 to 5 would affect the probability of first-stage

fire growth, and Items 6 to 10, in addition to Item 2, would affect

the probability of second-stage fire growth.

Given the above considerations, a total of 13 cases have been analyzed with

the results depicted in Table 11. These results are summarized in Table 12

which Identifies the important fire protection measures and their associated reduction factors that can be implemented to fire frequency of a room to estimate

the probability of second-stage fire growth. The reduction factor basically indicates a multiplicative change in the probability of ultimate room damage state

when a specific fire protection feature is implemented.
-49-

Table 11

Conditional Probabilities of Achieving First- and

Second-Stage Fire Growth Based on Various Fire

Protection Systems/Features Implemented

(Cable-Routing Room)

Design Identification First Stage Growth Second Stage Growth

Case No. Code (c) Probability (a) Probability (b)

1 4/5/1/2/6/7/8 1.5(-2) 5.7(-3)

2 4/5/1/2/6/7/- 1.5(-2) 1.5(-2)

3 4/5/1/2/-/7/8 2.2(-2) 8.4(-3)

4 4/5/1/-/6/7/8 1.5(-1) 5.7(-2)

5 4/5/-/2/6/7/8 1.9(-2) 7.4(-3)

6 4/-/1/2/6/7/8 2.2(-2) 1.2(-2)

7 -/5/1/2/6/7/8 4.5(-2) 1.0(-2)

8 4/5/1/2/6/d/8 1.5(-2) 1.0(-2)

9 4/-/1/2/6/d/8 2.2(-2) 1.8(-2)

10 4/5/1/2/6/e/8 1.5(-2) 6.9(-3)

11 4/-/1/2/6/e/8 2.2(-2) 1.5(-2)

12 4/5/1/2/6/f/8 1.5(-2) 5.8(-3)

13 4/-/1/2/6/f/8 2.2(-2) 1.2(-2)

(a) First-Stage Growth is defined as failure of one shutdown cable division.

(b) Second-Stage Growth is defined as failure of all redundant shutdown

cables, i.e., the Ultimate Room-Damage State (UDS).

(c) The design identification code indicates what fire protection system/

features are in place. The following identifier codes are used in this

study:

Identifier System/Feature

1 Automatic Detection System (Aerosol Detectors)

2 Automatic Suppression Systems (Sprinklers; Total Flooding Halon)

3 Automatic Doors/Dampers

4 Electrical Cable; Proper Rating/Installation

5 Qualified vs Nonqualified Cables

6 Manual Fire-fighting Equipment Availability and Staff Familiarity

7 Cable Tray Location (within uniformly stratified layer)

8 Fire Brigade; adequate training and plant familiarity

9 High Capacity Drainage System

a 1-hr-rated barriers; including doors and penetrations

6 9-hr-rated barriers; including doors and penetrations

c 3-hr-rated barriers; including doors and penetrations

d Cable Tray Location (within nonuniform region); Separation - 10 ft

e Cable Tray Location (within nonuniform region); Separation -20 ft

f Cable Tray Location (within nonuniform region); Separation - 30 ft
Table 12

Reduction Factors That Can be Credited For Various Fire Protection Measures

at Different Cable Tray Configurations For Estimating The Probability of

Second-Stage Growth

EARLY SUPPRESSION OVERALL*

DISTANCE AUTOMATIC PROPER MANUAL & TRAINED FIRE TRAINED FIRE REDUCTION FACTOR

SEPARATION FROM SUPPRESSION ELECTRICAL AUTOMATIC BRIGADES & BRIGADES &

DISTANCE CEILING SYSTEM DESIGN DETECTION IEEE QUALIFIED NON-QUALIFIED IEEE NON-

(ft) (ft) (ITEM 4) (ITEM 1) (ITEM 5) CABLES CABLES QUALIFIED QUALIFIED

> 10 > 4 10 3 1.4 4.5 2.1 189 88.2

> 10 < 4 10 3 1.4 2.25 1.25 94.5- 52.5

> 20 < 4 10 3 1.4 3.25 1.47 136.5 61.7.

> 30 < 4 10 3 1.4 3.9 1.9 163.8 79.8

*For the fire scenarios simulated, the conditional probability of a fire reaching the ultimate damage

state (UDS) can be expressed by

P(UDSjIfire) = (7)/ fi =/ft

fi's are the reduction factors associated with various fire protection features.

ft is the overall reduction factor.

I

U,
-51-

4.5 QUALITATIVE APPRAISAL OF FLAME-RETARDANT COATING AND RATED BARRIERS

The important fire protection measures for preventing specific room damage

states under specific fire scenarios have been identified for the representative

cable-routing room. This is done probabilistically from the existing modeling

and data. However, several important aspects of fire protection measures have

not been included in this study. These are discussed in the following.

Flame-retardant coatings - The primary purposes of flame-retardant coatings

are to prevent the ignition of electrical cable insulation and to stop the propagation of the flame along the cable bundle. This is done through various processes. First, an ablative process consisting of dehydration and other endothermic chemical reactions, coupled with the formation of a carbonaceous char,

reduces the heat transmission to the protected cable. Second, the carbon char

and the inorganic components of coating form a surface of high emissivity that

results in the radiation of a significant amount of heat away from the protected

cable. Third, the fire-retardant additives in the coating form products in the

fire which inhibit the combustion process and minimize the flammability of

materials in the vicinity of the cable.

If the first-stage fire growth is considered as the damageability of one

division of safe shutdown cables directly above the pool fire rather than their

ignition, then the use of flame-retardant coatings would not affect the probability of first-stage growth significantly. However, its effect on second-stage

fire growth would be significant because of expected reduction in burning rate.

This effect has not been considered in this study. Some potential disadvantages

of coated cables may need further study: 1) reduction in cable amperage, 2)

aging due to radiation or harsh environment (effective age of 10 years in normal

environment is specified by a coatings manufacturer), and 3) the coating can be

removed by water (inadvertent suppression).

Rated Barriers - The primary purpose of rated barriers is to separate the

divisional cables in the room by means of passive noncombustible obstacles.

This may be a 3-hr rated wall separating the east and west sides of the
-52-

representative cable-routing room, or a blanket-type barrier which is wrapped

around the divisional cables required for one train of safe shutdown systems.

Considering that the cable routing room is a low fuel load area, it is safe to

assume that the barrier would be effective at least for the duration of time for

which it is rated. Regardless of the type of barriers (a rated blanket or

wall), it is our judgment that the first-stage fire growth probability will not

change. However, the probability of second-stage fire growth could be drastically reduced. The probability of second-stage fire growth for the case that a

3-hr-rated wall separates the two zones within the cable routing room would be

most influenced by the probability of a connecting door being open, or the failure of ventilation dampers to close. For the blanket type of rated barrier, the

probability of second-stage fire growth would be strongly influenced by the possibility of burning in a hot layer in the vicinity of the blanket. The effectiveness of the rated blankets cannot be estimated properly given the existing

modeling.
-53-

5. REPRESENTATIVE DIESEL GENERATOR ROOM ANALYSIS

In a typical diesel generator room, several scenarios of fire initiation

and progression can be envisaged. According to a review of reported incidents

as documented in Licensee Event Reports (LERs) for the period of 1978 to 1982,

most of the fires in, this area are caused by ruptures in the fuel or lube-otl

lines. A likely fire-initiation scenario would then be the subsequent ignition

of lube oil issuing from a ruptured line (e.g., 0.5-in.-diam break) fin the form

of an atomized spray. The ignition source considered is the hot dieseT exhaust

ducting (exhaust temperature -1O00OF). This, of course, presupposes that the

diesels are running. For this particular study, a loss-of-offsite-power (LOOP)

initiating event is assumed to be in progress.*

Two stages of fire growth are considered: one deals with the eventual involvement of the day tank (having an assumed 550-gal capacity); the other assumes the invoTvement of total engine fuel and oil inventory (I100 gal of fuel

oil and 250 gal of lube oil). For the first, involvement of fuel stored in the

day tank is through radiative heat transfer from the initiating (pilot) tire to

the day tank surfaces. This pilot fire is, again, caused by a fuel mist generated by a break in a high pressure fuel line (85 to 100 psig) settling onto (or

interacting with) hot diesel surfaces (the exhaust duct). Subsequent involvement of the engine fuel located within the day tank is due to the- eventualI evaporation and escape of this engine fuel (through tank venting and possible

rupture) as a result of the radiative thermaT energy being absorbed.. Secondstage growth simply considers that the total fuel inventory in the room is. suddenly and readily available and burns under ventilation-limited conditions.

The fire protection features qualitatively and quantitatively analyzed in

this example include automatic detection systems, Halon flooding systems, operation of doors and dampers, rated barriers of I-, 2-, or 3-hr rating, drainage

systems, and fire brigades.

*Discussions regarding the quantification of subsequent and probable room-damage

states, given this plant-initiating event, form a requisite factor for the observations made and conclusions drawn therefrom.
-54-

To assess the various fire protection features required in the SRP guidelines now under evaluation, an additional characteristic time besides those

identified previously must be defined and quantified. Thus, in addition to

times associated with barrier failure (termination of second-stage fire scenario

for this example), and various forms of detection and suppression activities,

one has to define a time, t*, when the engine fueT in the day tank becomes involved. This is discussed in the following section on first-stage fire growth.

5.1 FIRST-STAGE FIRE GROWTH

Without exact knowledge of the fuel spray dynamics and the amount of atomized fuel that interacts with hot, surfaces, a probability distribution is ascribed to the size of the pilot (or initiating) fire. The size of the fire,

Qp (in megawatts), is assumed lognormallj distributed with values of 2 and 12

MW, respectively, at the 5 percentile lower and upper bounds.

Considering further that 46% of this energy is released in the form of

thermal radiation (Xr=0.46), the radiative heat flux absorbed by the 550-gal

capacity day tank (the target) can be assessed. This tank, cylindrical in

shape, 2 m in length (L), and 0.6 m in radius (R), is assumed located, generally, a distance, d, from the radiating pilot flame (the source). Subsequent fire

involvement of the fuel within the day tank occurs at a time, t*, when the fuel

temperature reaches its evaporation threshold (Tevaporation=4 7 0 °F). Conservatively assuming that all this radiative energy is absorbed by the liquid fuel

within the day tank, conservation of energy principles coupled with constitutive

radiant-flux laws indicate that

t* 15.3(10" 3 )/Qp (sec) (5.1)

A separation distance (source to target) of 3 m is assumed. This equation was

derived by integrating the following energy equation, viz.,

d/dt (mC pT) = Qtr = Ft+p Qp Xr = [(L)(R)/nd 2 ]QpXr (5.2)
-55-

where Qtr is the radiative heat transmitted to the day tank as a result of pilot fire of Qp MW; m is the mass of day tank fuel (m=6148 lb for 550-gal capacity); and Cp is the specific heat of the fuel assumed linearly proportional

to the fuel temperature, T. The radiative view factor, Ft.,.p, is obtained considering a cylindrical-shaped vessel for the target and a radiating point source

located a distance, d, away. This expression, viz., Ft+p = [(L)(R)/Hd 2 ] assumes that the distance between target and source is much greater than the

radius of the target. With the pilot fire size considered as lognormally distributed, the probability of t*, i.e., the probability that t* is less than some

prespecified time, is shown in Figure 10.

Fire-mitigating actions, such as detection and suppression, and probabilities associated with reaching this first-stage fire growth are then dependent

upon the probability that these actions are taken before t*.

1.

0.8

t 0.6

S 0.4 E

Time (minutes)

Figure 10. The cumulative probability distribution for first-stage fire growth.
-56-

On the basis of the possible fire protection features available in this

zone, the probabilities for successful detection and suppression of the fire,

before it reaches first-stage growth, are estimated for the following three different scenarios of extinguishment: a) automatic detection and suppression, b)

automatic detection and manual suppression, and c) manual detection and

suppression.

(a) Automatic Detection and Suppression - Detection time is determined

using the same approach discussed in Section 4 for the representative cablerouting room. The parameters needed are the ventilation rate, room height, convective heat-release rate, and detector spacing. For this example, the corresponding values of these parameters are 18 room changes per hour, 7.6 m, 2.6 MW,

and 3 m, respectively. The convective heat-release rate is based on the median

value of the lognormal distribution for the size of the pilot fire. This results in an estimated detection time of -5 min. Suppression is provided by

automatic operation of a total flooding Halon system. During first-stage fire

growth, this system is assumed to be effective in extinguishing the fire in a

short period of time (less than a minute). However, the functional reliability

associated with successful suppression depends on the reliability of the automatic Halon system as well as of both rated doors and dampers to close on

demand. Failure of doors or fire dampers to close on demand reduces the effectiveness of the total flooding features of this suppression system.

The probabilities assigned to such failure are not available at the present

time. Therefore, the probability of each of these failures is assumed to be the

same as for the failure of a motor-operated valve to close on demand which, from

the WASH-1400 Reactor Safety Study, is 1.OxlO- 3 .

The failure probability of an automatic Halon suppression system is estimated to be 8.7x10-2 based on the geometric mean of upper and lower bounds reported in Reference 18. Hence, the probability for reaching first-stage growth

is estimated to be 8.9x10- 2 for the case where failure of one damper and one

door to operate is of concern.
-57-

(b) Automatic Detection and Manual Suppression - The response time of

automatic detection systems is estimated, using the same approach given in Section 3.4, to be -5 min. Assuming that it takes 3 to 5 min for the plant staff

to reach the fire area and report the extent of fire, the fire in this scenario

could not be extinguished by hand-held extinguishers. Hence, manual fire suppression will primarily be accomplished by the fire brigade using other firefighting equipment. The probability for manual suppression of the fire before

it reaches first-stage growth is thus based on the cumulative probability distribution functions given in Figures 2 and 9. Here the initiation time for suppression is the sum of detection time (5 min) and the time required for the

plant staff to reach this zone (3 to 5 min). This time period is conservatively

assumed to be 10 min. With this information, the probability of a fire reaching

first-stage growth is 0.33.

(c) Manual Detection and Suppression System - Manual detection of the

early stages of a fire in the diesel generator room during a loss of offsite

power transient is considered to have a low probability. The fire will be

detected manually with high probability only if the diesel generator stops running, since the plant staff would then be dispatched to investigate the cause.

The time required for detection of fire, based on the above scenario, is judgmentally estimated to be about 30 min from the initiation of fire. From the

results presented in Figures 2 and 10, the estimated probability for manual

suppression of the fire before it reaches the first-growth stage, given this

detection time, is 0.66.

5.2 SECOND-STAGE FIRE GROWTH

Second-stage fire growth is defined as a fire starting in one of the diesel

generator rooms, propagating to the other diesel generator room, and disabling

the redundant unit. Three possible scenarios are considered for the occurrence

of second-stage growth: a) propagation of the fire through the rated barriers,

b) propagation of the fire through the open dampers and doors, and c) failure of

the redundant unit because of independent or common-mode failure.
-58-

(a) Second-Stage Propagation Through Rated Barriers - For this large fire

(1100 gal of fuel oil and 250 gal of lube oil), the burning rate is expected to

be controlled by the amount of oxygen available. Given the ventilation rate of

18 room changes per hour and the room size to be 40 ft W x 50 ft L x 25 ft H,

the estimated heat release rate is 22 MW, assuming that 12.5 MJ of energy are

generated for each kilogram of oxygen consumed.

The amount of fuel evaporated during a post flashover phase is taken to be

approximately 0.061 kg/n 2 sec on the basis of information in Reference 8. The

area available for the fuel spill is calculated by. subtracting the area occupied

by the diesel generator unit from the floor area. The value of mn is thus

-9.4 kg/sec. Using the ratio of total mass of fuel to the equivalent mass

loss rate of fuel due to burning, and assuming the heat of combustion to be

about 4.67x10 7 J/kg, the maximum duration of post-flashover burning is

-2.7 hr, although the actual duration is expected to be much less because of

the unburned vapor fuel lost through the ventilation system. Figure 11 presents

the cumulative probability distribution function for the average room temperature during post-flashover fire, based on the approach discussed in Section 2.1.

The estimated probability distribution for the failure of 1-, 2-, and 3-hourrated barriers vs the duration of fire are given in Figure 12. The criteria and

the approach for estimating the failure probability of a rated barrier are discussed in Section 3.3.

The failure probability of a rated barrier can be determined from these

curves, if the duration of flashover phase is known. However, the duration of

the flashover phase cannot be simply determined for a ventilation-limited fire

because of lack of modeling for evaluating the amount of unburned fuel that may

leave the room through the ventilation ducts and the interactive effect of firefighting activities on controlling the fire. In addition, the problem is more

complex if one considers the high probability for the recovery of. loss of offsite power (the reason why the diesels were operating) within the first few

hours. Given the above concerns and for the sake of comparison, of the equivalent duration of flashover phase is 1.5 hours. Hence, the failure probabilities
-59-

a

L)

C

0

2

I'.

C

C

4,

2

.0

1.

4'

a

S

'U

A

1.0

0

300 S00 700 900 1100

Average temperature (OC)

1300 1500 1700

Figure 11. The cumulative distribution function for the average room

temperature during post-flashover phase.

1

S

I-2

U

h.

1.

El

S.

I.

'U

0

4,

Durationof Flashover (hr.)

Figure 12. Fragility of rated barrier vs. the duration of flashover

phase for the representative diesel generator room.
-60-

for 3-, 2-, and 1-hr-rated barriers, assuming 1.5 hr for the duration of flashover phase, are estimated to be l.OxlO- 3 , 0.31, and 0.9, respectively.

(b) Second-Stage Propagation Through Open Dampers and Doors - As discussed

previously, the probability of doors or dampers failing to close automatically

when required is about 2.Ox10- 3 . Given this failure, it is conservatively assumed that the automatic suppression system would not be effective. Therefore,

the probability of fire propagation to the redundant diesel generator unit due

to failure of automatic doors and dampers is conservatively estimated to be

2.Ox10- 3 .

(c) Second Room Damage State Due to Independent or Common Cause Failure of

Diesel Generators - The estimated probability for the failure of both diesel

generator units in the last two cases (a, b) assumes a fire initiated in one

unit propagates to the redundant unit. However, the failure of both diesel generators can also take place without the need for the fire to propagate from one

unit to the redundant unit. This can happen as the result of independent or

common-cause failure of the second unit when the first unit is disabled by a

fire.

The average probability of independent failure of a diesel generator unit

is estimated for the nuclear industry2 3 to be approximately 2.5xi0- 2 . The data

for estimating the probability of the second diesel generator catching fire,

given the first unit has been failed because of fire, are not available. However, it is known that the overall conditional probability of the second diesel

unit failure given that the first has already failed for various failure modes

is about 0.15. Thus, this type of failure for both diesel units is expected to

dominate the others previously discussed if the enclosure walls are rated for

3-hr (cases a and b). However, since this scenario of failure is not totally

dependent on the impact of fire, it is not included in the quantitative results

of this study. The discussion here is intended to emphasize the importance of

non-fire-related failure of redundant diesel generators compared to fire-related

failures.
-61-

5.3 CONCLUSIONS (DIESEL GENERATOR ROOM)

For the representative diesel generator room as described, and based on the

analysis performed, some of the important fire protection features given in

Table 13 are addressed. The importance of the drainage system (Item 7) that it

reduces the duration of fire in second-stage growth by removing the spilled

fuel. For example, if the drainage system capacity (average flow rate) is about

50 gpm, the duration of the flashover phase is expected to be reduced by 25%.

The proper analysis for the quantitative evaluation of the drainage system

effectiveness cannot be made at the present time. However, it is included in

the list in Table 13 to indicate the relative importance of this system based on

our qualitative judgment.

Table 13

The Fire Protection Measures Considered for the Analysis

of the Representative Diesel Generator Room

1 - Automatic Detection System

2 - Automatic Suppression System (Halon Flooding System)

3 - Automatic Doors and Dampers

4 - 1-Hr-Rated Barriers Including Doors and Dampers

5 - 2-Hr-Rated Barriers Including Doors and Dampers

6 - 3-Hr-Rated Barriers Including Doors and Dampers

7 - High-Capacity Drainage System

8 - Trained Fire Brigades

In general, this study is indicative of an important conclusion in regard

to the adequacy of the SRP Section 9.5.1 guidelines for protecting the redundant

diesel generator units from a single-exposure fire. It is shown here that in

nuclear power plants for which the diesel generator units are separated by 3-hrrated barriers, and each diesel generator room is equipped with automatic suppression and detection system, the probability of both diesel generator units
-62-

being damaged by a single-exposure fire is comparatively much smaller than the

probability for non-fire-related failures. It should also be noted that these

observations are based on an accident scenario initiating with the loss of offsite power and the possibility for a diesel generator to catch fire subsequently. The recovery of offsite power within the period of fire propagation

plays an important role in deriving and interpreting results for these rather

specific scenarios.

Finally, a ranking scheme similar to the one used for the cable-routing

room cannot be devised here. To estimate the reduction factors, certain conditions must be met. For example, in a cable-routing room, if the redundant

cables are separated by 20 ft, are located in proximity of the ceiling, and are

IEEE qualified, then the probability of second-stage growth can be estimated by

multiplication of the failure probabilities for automatic, early manual, and

late manual suppressions. This multiplicative nature allows us to define the

reduction factors which, in most cases, are the reciprocal of the failure

probabilities.

To examine whether the same type of multiplicative rule exists for the representative diesel generator room, an event tree depicting the possible fire

scenarios for reaching the first and the second-stage growth is constructed in

Figure 13. The size and the number of the branches in this event tree are

reduced by eliminating the highly improbable scenarios. For a well designed

diesel generator room, consisting of 3-hr-rated barriers and an automatic

detection/suppression system, the probability of second-stage growth can be

expressed by

P(SSG) = P(EMS)[P(UDC)P(SSGR) + P(DDC)] , (5.3)

where the notations of the events are described in the Figure 13, and the-bar

over the event DDC (dampers and doors closed) indicates "failure to operate."

The above expression is not a simple multiplication. The main reason foe this

is the dependence between the failure of doors and dampers to close and the effectiveness of the automatic suppression system.
-63-

Auto Auto Suppres- Dampers & Early Manual First Stage second stage

Initiation Detection sion System Doors Closed Suppression Growth Has I hr 2 hr 3 hr growth has

of Fire Available Actlvlatlon As Required Effective Reached barrier barrier barrier reached

I AD AS DOC EMS FSGR IHBR 2HBR 3HeR SSwt

Figure 13. Event tree constructed for the various fire scenarios in diesel

generator rooms.
-64-

Hence, in order to define reduction factors somewhat similar to those employed for the cable-routing room, the following expression has been used:

Prob. (SSG; when system Sj is removed)

J= Prob. (SSG; when system Sj is not removed) 9 (5.4)

where SSG represents the first stage growth.

The values for fj's for various fire protection systems are given in

Table 14. In addition, the probabilities for first- and second-stage growth for

a number of typical fire protection designs are given in Table 15.

Table 14

The Modified Reduction Factors (Importance Factors)

for Diesel Generator Room

Rating of Fire Protection Features from Table 13

the Walls

hr 1 2 3 4 5 6 7

3 3.03 1.52 500.00 --- --- 46.50 3.03

2 20.80 10.40 34.40 --- 3.20 --- 3.02

1 22.20 11.10 12.20 1.14 .- --- 2.97
-65-

Table 15

Probabilities of Room-Damage States

for Various Fire Protection Design Features

(Diesel Generator Room)

First-Stage(a) Second-Stage(b)

Case Design Identification Growth Growth

No. Code Probability Probability

1 1/2/3/c18/9* 3.0(-2) 6.6(-4)+

2 1/2/3/b/8/9 3.0(-2) 9.6(-3)

3 1/2/3/a/8/9 3.0(-2) 2.7(-2)

4 1/-/3/c/8/9 0.33 1.0(-3)

5 1/-/3/b/8/9 0.33 0.1

6 1/-/3/9/8/9 0.33 0.3

7 -/-/3/c/8/9 0.66 2.0(-3)

8 -/-/3/b/8/9 0.66 0.2

9 -/-/3/a/8/9 0.66 0.6

+3.0(-2) 3.0(10)-2.

(a)First-Stage Growth is defined as the fire involvement of the diesel fuel.

(b)Second-Stage Growth is defined as the fire propagation beyond the original

fire enclosure, i.e., the Ultimate Room-Damage State (UDS).

*The design identification code indicates what fire protection system/features

are in place. The following identifier codes are used in this study:

Identifier System/Feature

1

2

3

4

5

6

7

8

9

a

b

c

d

e

f

Automatic Detection System (Aerosol Detectors)

Automatic Suppression Systems (Sprinklers; Total Flooding Halon)

Automatic Doors/Dampers

Electrical Cable; Proper Rating/Installation

Qualified vs Nonqualified Cables

Manual Fire-fighting Equipment Availability and Staff Familiarity

Cable Tray Location (within uniformly stratified layer)

Fire Brigade; adequate training and plant familiarity

High Capacity Drainage System

1-hr rated barriers; including doors and penetrations

2-hr rated barriers; including doors and penetrations

3-hr rated barriers; including doors and penetrations

Cable Tray Location (within nonuniform region); Separation - 10 ft

Cable Tray Location (within nonuniform region); Separation - 20 ft

Cable Tray Location (within nonuniform region); Separation - 30 ft
-66-

6. LIMITATIONS OF THE RESULTS/METHODS

This study has presented a method and rationale that can be used to evaluate the relative importance of certain fire protection features found in nuclear

power plants., It has considered certain design features required for mitigating

the consequences of particular fire scenarios in two predefined enclosure configurations. This analysis, although performed conservatively, by no means envelops all the varieties of enclosure configurations found in nuclear power

plants. Various sources of uncertainty have been discussed, some qualitatively

and some quantitatively. It should be noted, however, that these cautionary

notes in no way discredit or reduce the importance, applicability, and the

potential usefulness of the method described. Rather our intent is to caution

against misinterpretation of the results and their extrapolation to other fire

scenarios.

Before an attempt is made to use the results tabulated in Tables 11, 12,

14, and 15, and the conclusions drawn therefrom are applied to other fire safety

situations, the following must be considered:

a) Assure that the distribution and the heat of combustion for the in situ

combustibles are not drastically different from those considered herein.

b) Assure that the ceiling height and the floor area of the actual room

are comparable to those of the representative room.

c) Examine the fire protection features in the room of concern and assure

their compatibility with those modeled in the analysis.

Conceivably, by usingthe probabilistic results of this study (or applying the

method specifically) and a parallel deterministic assessment of the particular

fire-protection system, the regulatory body may make recommendations about

whether or not additional fire-mitigation features are required.
-67-

It should also be noted that until the approaches described are further

used and the results investigated, i.e., additional room geometries, in situ

combustible loadings, and fire scenarios, the results presented thus far cannot

be summarily used to evaluate the relative merits of the fire protection guidelines found in Section 9.5-1 of the SRP. Indeed, those fire protection features

and systems that have been addressed can be analyzed with the method described

on a case-by-case basis.
-68-

7. SUMMARY OVERVIEW AND FUTURE RECOMMENDATIONS

In a general sense, fire engineering design is nondeterministic. Some

level of risk is virtually unavoidable. Historically, fire safety requirements

and design criteria have been written without actually stating their objective

safety levels and even without any analytical measurement of the objectives involved. Indeed, a defense-in-depth philosophy is generally implemented through

varying combinations of active and passive fire protection features. Admittedly

the traditional classification of, say, fire barriers is convenient and may provide a reasonable design basis for certain types of structures and occupancies,

the risks of which are well experienced. But the design becomes questionable in

cases where the exposure and/or the structural response, as well as the associated uncertainties, may seriously affect the impact from fire, in turn affecting

the performance of other systems. Safety or economy may be affected even more

if conditions governing the frequency of fire or the exposure of a structure and

its internals to a fire differ from average conditions and are thereby not

adequately accounted for. For these reasons there is an urgent need to evaluate

the levels of safety inherent in present nuclear power plant fire protection

regulations. Thus, to design in and appraise "fire safety," performance has to

be described and measured in probabilistic terms.

Idealistically, the essential components of a probability-based fire protection/fire safety review methodology include the following:

(1) Analytical modeling of relevant processes, verification of the models,

their validations, and accuracies; determination of critical design

parameters.

(2) Formulation of functional requirements, expressed either in deterministic or probabilistic terms.

(3) Determination of design parameters.
-69-

(4) Verification by reliability analysis that the choice of safety factors

leads to safety levels which are consistent with expressed functional

requirements.

The major objective of this study has been to use existing physical and probabilistic fire models to develop an integrated methodology through which the relative importance of various fire protection features can be investigated. A

limited study has been performed which provided insights regarding the relative

importance of several active and passive fire protection systems and identified

where future implementation of somewhat more detailed modeling efforts is

warranted.

On the basis of this study, which entails a hybrid fire scenario and fire

dynamics approach applied to two representative fire areas in nuclear power

plants, we conclude that an overall general ranking of fire protection systems

and measures cannot be fully determined. However, for a given enclosure and for

a given fire scenario, a relative ranking for each is listed (Sections 4.4 and

5.3). These two lists have been developed on the basis of investigating what

each fire protection feature or combinations thereof contribute in reducing the

probability for fire-induced failure of redundant shutdown equipment. Indeed,

importance ranking of fire-protection systems with respect to some risk measure

can be different depending upon the level (and the measure) of application,

viz., component level, system level, fire area level, accident sequence frequency level, etc. In this study, the measure used was the conditional probability of achieving various fire-damage states in specific fire areas.

It has been shown that 20-ft separation between redundant safe.shutdown

cables does not provide passive protection equivalent to a 1-hr-rated barrier.

However, the level of fire protection probabilistically achieved by combinations

of the various fire protection systems investigated is indeed assuring.

Finally, several areas in both deterministic and probabilistic fire analysis which are in need of further investigation can be identified. The lack of

knowledge and accurate modeling in these areas is the prime contributor to the
-70-

overall uncertainty of the results. The following provides summary discussions

on some of these deficiencies and recommendations for improving the existing

models.

1. Deterministic analysis: the need for improving the existing deterministic computer codes for the modeling of an enclosure fire is Inevitable. The major area of concern which may require immediate improvement is the model used for estimating'the burning rate via ignition and

flame spread algorithms. In addition, a built-in mechanism in the

models to account for the transition of a surface-controlled to a

ventilation-controlled burning is of great importance. Understandably,

other areas of deterministic analysis need improvement as well. In

most cases, these improvements yield themselves to a complex mathematical model such as the use of three-dimensional field models instead of

two- or three-zone stratified models. The degree of model complexity

may be reduced to a more moderate level by taking the importance of the

various phenomena into consideration by means of sensitivity or perturbation analysis.

2. Probabilistic risk analysis: fire propagation and component damagea-

•bility are best described by time-dependent probabilistic analyses.

The overall plant risk caused by fire-initiating events shall be estimated-via time-dependent probabilistic models as well. For a component

which is required to perform its safety function within the first halfhour of an .accident, any fire-induced failure of the component beyond

that time is of no concern. Therefore, plant risk may be evaluated

properly if and only If the timing of an accident progression is to be

compared with the timing of fire propagation. Such time dependent

-probabilistic analyses have yet to be determined.

In addition, suppression and detection models need more realistic treatment. The probabilistic models implemented for these systems depend heavily on

engineering judgment and data with minimum effort devoted to the physical modeling of the process. Such a heavy reliance on data requires more reliable data
-71-

properly classified to perform accurate estimations. Such data sources are not

available at this time. Therefore, combined efforts consisting of improving the

existing data sources and developing the required physical models for suppression and detection systems are recommended.
-72-

REFERENCES

1. Ernst, M. L. and Murphy, J. A, Regulation of Nuclear Power Plants, Mechanical Engineering, Vol. 106, No. 11, November 1984.

2. Appendix A. General design criteria for nuclear power plants, 10 CFR Part

50, General Design Criterion 3, "Fire Protection."

3. Branch Technical Position 9.5-1 (TTP 9.5-1), Guidelines for Fire Protection

for Nuclear Power Plants Docketed Prior to July 1, 1976.

4. Appendix R, Fire protection program for nuclear power facilities operating

prior to January 1, 1979, 10 CFR Part 50, Licensing of Production and

Utilization Facilities.

5. NUREG-0800, Standard review plan, Section 9.5.1, Fire Protection Program,

Rev. 3, July 1981.

6. Hanauer, S. H., et al. Recommendation Related to Brown's Ferry Fire Report

by Special Review Group, NRC Report NUREG-0050, February 1976.

7. SECY-83-269, Memorandum from W. J. Dircks to the Commissioners, July 5,

1983; Subject: FIRE PROTECTION ROLE FOR FUTURE PLANTS (SECY-82-267).

8. Siu, N. 0., Probabilistic Models for the Behavior of Compartment Fires,

School of Engineering and Applied Science, University of California, prepared for U.S. Nuclear Regulatory Commission, NUREG/CR-2269, August 1981.

9. Cooper, Leonard Y., The Development of Hazardous Conditions in Enclosures

with Growing Fires, U.S. Department of Commerce, NBSIR 82-2622, December

1982.

10. Cooper, Leonard Y., "Convective Heat Transfer to Ceilings Above Enclosure

Fires," U.S. Department of Commerce, presented at 19th Symposium (International) on Combustion, pp. 933-939, Philadelphia, Pa., The Combustion

Institute, 1982.

11. Alpert, R. L., Turbulent ceiling-jet induced by large-scale fires, Combustion Science and Technology, Vol. 1, pp. 197-213, Factory Mutual Research

Corp., Norwood, MA 1975.

12. Berry, D. L. and Minor, E. E., Nuclear Power Plant Fire Protection FireHazard Analysis (Subsystems Study Task 4), NUREG/CR-0654, September 1979.

13. Magnusson, S. E. and Petterson, 0., Rational design methodology for fire

exposed load bearing structures, Division of Structural Mechanics, Lund

Institute of Technology, Fack 725, S-220 07 Lund 7 (Sweden), Fire Safety

J., Vol. 3, 227-241 (1980/81).
-73-

14. Magnusson, S.E. and Petterson, 0., Functional approaches - an outline," CIB

Symposium Fire Safety in Buildings: Needs and Criteria, Publication 48,

CIB Proceedings, pp. 120-145, Amsterdam, June 1977.

15. Fleming, K. N. et al., A Methodology for Risk Assessment for Major Fires

and its Application to an HTGR Plant, GA-A15402, July 1979.

16. Azarm, M. A. et al. A Preliminary Review of the Limerick Generating Station

Severe Accident Risk Assessment, Volume I: Core Melt Frequency, NUREG/CRBNL NIJREG Draft, Brookhaven National Laboratory, August 15, 1983.

17. Gallucci, R.H., A Methodology for Evaluating the Probability for Fire Loss

of Nuclear Power Plant Safety Functions, Ph.D. Thesis submitted to the

Graduate Faculty of Rensselaer Polytechnic Institute, Department of Nuclear

Engineering, May 1980.

18. Levinson, S. H., "Methods and Criteria for Evaluation of Nuclear Reactor

Fire Protection Alternatives and Modifications," Ph.D. Thesis submitted to

the Graduate Faculty of Rensselaer Polytechnic Institute, Department of

Nuclear Engineering, December 1982.

19. "Fire Tests in Ventilated Rooms - Detection of Cable Tray and Exposure

Fires," prepared by Factory Mutual Research Corp., EPRI NP-2751, February

1983.

20. Tewarson, A., Rhan, M. M., and Steciak, J., Combustibility of Electrical

Wire and Cable for Rail Rapid Transient System, Vol. 1: Flammability, prepared by Factory Mutual Research for U.S. Department of Transportation,

DOT-TSC-UMTA-83, 4, 1, May 1983.

21. Lee, J. L., "A Study of Damageability of Electrical Cables in Simulated

Fire Environments," prepared by Factory Mutual Research Corp., EPRI

NP-1767, March 1981.

22. Tewarson, A. et al. Experimental Evaluation of Flammability Parameters of

Polymeric Materials, Factory Mutual Research, FMRC J. I. 1A6R1, February

1979.

23. Tewarson, A., Damageability and Combustibility of Electrical Cables, EPRI/

FMRC Seminar, Factory Mutual Conference Center, Norwood, Mass., December

9-11, 1981.

24. Cline, D.D., von Riesemann, W. A., and Chauez, J. M., Investigation of

Twenty-Foot Separation Distance as a Fire Protection Method as Specified in

1OCFR5O, Appendix R, NUREG/CR-3192, prepared by Sandia National Laboratory,

SAND-83-0306, October 1983.

25. Battle, R. E. and Campbell, D. J., Reliability of Emergency AC Power Systems at Nuclear Power Plants, Oak Ridge National Laboratory, NUREG/CR-2989,

July 1983.

A-1

APPENDIX A

PARAMETERS USED FOR THE AKALYSES

The nominal values for major physical parameters used for the fire analyses

are listed here. In cases where the nominal values are not well defined,

acceptable ranges are generally determined. To avoid repetition, the specific

parameters of a fire area such as ventilation rate, dimensions of the room,

etc., have not been included in the list. In addition, data used for the probabilistic analysis such as reliability and effectiveness of ffre.detection and

suppression systems are determined fn concert with the associated fire zone.

Therefore, those data are not included here but are part of the main body text.

of this report.
A-2

Table A-1

The Values for Physical Parameters Used in the Study

1. Properties of Air at Temperature T=300K

Density :

Specific Heat

Ideal- Gas Assumption :

2. Properties of EPR/Hypalon Cables

Heat of Combustion :

Fraction of Radiation

Heat Release Rate:

Density :

Specific Heat

Thermal Conductivity :

Surface-Controlled Specific

Mass Loss Rate :

Specific Mass Loss Rate Radiation

Augmentation :

Pilot-Ignition Temperature

Auto-Ignition Temperature

Total Energy of Damageability

3. Properties of PE/PVC Cables

Heat of Combustion :

Fraction of Radiation Heat

Release Rate :

Density :

Specific Heat :

Thermal Conductivity :

Surface-Controlled Specific

Mass Loss Rate :

Specific Mass Loss Rate Radiation

Augmentation :

Pilot-Ignition Temperature

Auto-Ignition Temperature

Total Energy of Damageability

and p=1 atm

p = 1.18 kg/m3

Cp = 1004.8 J/kg°K

pT p = P'TJIp

1.7x10 7 < Hf < 2.96xi07 (J/kg)

=xr 0.62

p = 1436 kg/m3

C = 1600. J/kg- 0 K

K = 0.1 W/m-*K

0 < mo < 0.003 (kg/m2 -s)

Cs = 0.17x10- 6 kg/j._m2

Tp= 440°C

Ts= 488°C

1800 < Eid- 4000 (Kj/m 2 )

2.21x10 7 < Hf < 3.08x10 7 (J/kg)

.r = 0.56

p = 1715 kg/m3

C = 1632.7 J/kg-*K

K = 0.08 W/m-*K

0 < mo < 0.003 kg/m2 -s

Cs = 0.22x10- 6 kg/J-m2

Tp= 430*C

Ts= 478°C 2

530 < Eid < 1000 (kJ/m2)
A-3

TABLE A-1 (Cont'd.)

4. Properties of Fuel Oil

Density :

Specific heat (T in OK)

Boiling Temperature :

Surface Controlled Specific

Mass Loss Rate :

Heat of Combustion

Fraction of Radiation Heat

Release Rate:

p = 900 kg/m3

C = 1716.5 + 3.54 T J/kg-°K

TB a 244*C

m0 -

Hf =

0.06k kg/n 2 sec

4.67x10 7 J/kg

5. Additional Data

Mass Ratio of Oxygen to Air

Heat of Combustion for 1 kg of

Oxygen Burning with Hydrocarbons

Density of Oxygen (Ideal Gas

Assumption) at Temperature T=300K

Fraction of Heat Lost by Conduction

Through Walls and Radiation Through

Openings :

Xr 0.4

a =0. 21

Hf 12.5 MJ

p(02) = 1.3 kg/ni3

0.4 <. Ic < 0.8

Ac(average) = 0.6

NRC FORM 335 1. REPORT NUMeER (AwInedby ODCJ

41el! U.S. NUCLEAR REGULATORY COMMISSION NUREG/CR-4230

BIBLIOGRAPHIC DATA SHEET BNL-NUREG-51878

4. TITLE AND SUBTITLE (Add Volume Na., iappropriate) 2. kLeave blanki

Probability-Based Evaluation of Selected Fire Protection 3. RECIPIENTS ACCESSION N.

Features in Nuclear Power Plants

I. AUTHOR(S) 5. DATE REPORT COMPLETED

M. A. Azarm and J. L. Boccio MONTH J YEAR

February 1985

9. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (include Zip Codel DATE REPORT ISSUED

Department of Nuclear Energy MONTH YEI 4 t8

Brookhaven National Laboratory May 1985

Upton, New York 11973 6. Leave. blank)

S. (Leave blankl

12. SPONSORING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Codel 10. PHOJECTITASK/WOHK UNIT NO.

Division of Engineering

Office of Nuclear Reactor Regulation 11. FINNO.

U.S. Nuclear Regylatory Commission

Washington, D.C. A-3710

13. TYPE OF REPOHT PERIOD COVERED 0lnclusIve dates)

Technical

1b. SUPPLEMENTARY NOTES 14. (Leawe olank)

16. ABSTRACT I200 words o,rle" A probabilistic approach for the evaluation of major fire protection

measures in nuclear power plants is described. The methods developed are applied to two

representative fire areas -- one similar to a cable routing room and the other typical

of a diesel generator room. The fire areas chosen for application, the fire scenarios

described, and the various fire-damage states specified in the two illustrative examples

are used to evaluate those fire-protection guidelines which deal with automatic/manual

fire detection and suppression systems, rated barriers, divisional separation, drainage

systems, dampers, and fire rating of electrical cables. Tabular results are presented,

which reflect the relative merits of these systems/features in terms of conditional

probabilities of achieving various room-damage states. The conclusions drawn and the

lessons learned through the course of this study are discussed, and the areas that may

need further investigation are identified.

I/. KEY WO0IiUS AND DOCUMENT ANALYSIS Idla. UESCIPTOHS

Fire Protection Systems, Reliability Analysis, Fire Risk Analysis, Fire Modeling

I li. IDENTIFIERS,'OPEN-ENOED TERMS

18. AVAILABILITY STATEMENT 19. SECURITY CLASS (This reportl 21. NO OF PAGES

Unclassified

Unlimited 120 SECURITY CLASS (rh,s pad,) 22 PRICE

I lb NRC FORM 335 flit ll




UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, D.C. 20555

OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300

FOURTH CLASS MAIL

POSTAGE & FEES PAID

USNRC

WASH. D.C.

PERMIT No. 0-67

2

C

C,

CA) 0

0

w

w

IimW

cm r

>--

!Zn0

m

0

2