NUREG/CR-0654

SAN D79-0324

RP

Nuclear Power Plant Fire Protection -

Fire-Hazards Analysis

(Subsystems Study Task 4)

Dennis L. Berry, Earl E. Minor

Printed September 1979

• Sandia Laboratories

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NUREG/CR-O654

SAND79-0324

RP

NUCLEAR POWER PLANT FIRE PROTECTION -

FIRE-HAZARDS ANALYSIS (SUBSYSTEMS STUDY TASK 4)

Dennis L. Berry

Earl E. Minor

Manuscript Submitted: January 1979

Date Published: September 1979

Sandia Laboratories

Albuquerque, New Mexico 87185

operated by

Sandia Corporation....

for the

U.S. Department of Energy

Prepared for

Engineering Methodology Standards Branch

Office of Standards Development

U.S. Nuclear Regulatory Commission

Washington, DC 20555

Under Interagency Agreement DOE 40-550-75

NRC FIN No. A-1080

3

ABSTRACT

This report examines the adequacy of existing fire-hazards

analysis methodology in the context of nuclear power plant

safety. By combining and simplifying a number of available

analysis techniques and by demonstrating the technical merit of

each technique, this report develops a conservative fire-hazards

analysis method which can be easily used by both designers and

regulators. As described, a suitable analysis method for

nuclear power plants should involve three phases. These are:

(1) a deterministic evaluation of passive fire barriers under

limiting fuel load and ventilation conditions; (2) a

probabilistic evaluation of supplementary fire protection

measures (e.g., suppressions systems); and (3) a subjective

evaluation of unquantified fire hazard conditions (e.g., cable

arrangements). An example of each analysis phase is provided.

5,
ACKNOWLEDQ4ENT

The authors acknowledge the contribution made to this

report by Mr. Bert Cohn of Gage-Babcock & Associates, Inc. Mr.

Cohn developed the probabilistic critical-path technique

described on pages 49 through 60, 71 through 74, and 79 through

82.

6
CONTENTS

Page

Summary 11

1. Introduction 13

Study Objective 13

Traditional Analysis Approach 13

Study Description 15

2. Analysis Methods Available 16

Subjective Analyses 17

Deterministic Analyses 20

Probabilistic Analyses 29

3. Most Suitable Analysis Method 37

Combining Available Analysis Techniques 37

The Technical Merit of Candidate Analysis Techniques 41

Simplifying the Candidate Analysis Techniques 62

4. Example Fire Hazards Analyses 78

Example Fuel Load and Ventilation Barrier Analysis 78

Example Critical Path Analysis 81

Example Qualitative Fire Scenario Analysis 84

5. Conclusions 86

6. Recommendations 88

Overall Analysis Sequence 88

Fuel Load and Ventilation Barrier Analysis Segment 89

Probabilistic Analysis Segment 89

References 91

APPENDIX -- Combined Induced Draft Plus Forced Ventilation 95

7
ILLUSTRATIONS

Figure Page

1 Illustration of Integrated Time-Temperature Curve

Concept for Equating Fire Severities 25

2 National Fire Protection Association Decision Tree 31

3 GSA Probability Chart for Fire Development Within

a Building 33

4 Typical Branches of a Fire-Hazards Analysis Fault Tree 35

5 Analysis Sequence for Each Nuclear Power Plant

Safety Area 39

6 Comparison of Models for Assessing Fire Severity

in Nuclear Power Plants 49

7 Critical-Path Diagram for Fire Development in a

Building 51

8 Event L, Automatic Fire Suppression Fails 61

9 Event T, Fire Barrier "Wearout" Failure 61

10 Typical Time-Temperature Response Curves for Different

Ventilation Rates 65

11 Typical Time-Temperature Response Curves for Different

Fuel Loads 65

12 Acceptable Fuel Load and Ventilation Combinations

Assuming Heat Loss Into Walls Only 68

13 Typical Temperature Limitation for Calculated Time/

Temperature Curves 69

14 Acceptable Fuel Load and Ventilation Combinations

Assuming Heat Loss Into Walls and Either 50% or 100%

of Hot Gases Removed 70.

15 Airflow Rate Induced by a Fire in a Room Having an

Opening of Height ho and Area Ao 71

16 Layout for Fuel Load/Ventilation Barrier Analysis

Example 79

8
TABLES

Table Page

I Preliminary Hazard Analysis 19

2 Hazard Inventory Method 21

3 Hazard Mode and Effect Analysis 21

4 Probability Values Corresponding to Figure 7 Events 56

5 Selected Physical Parameters 64

6 Critical Path Analysis Worksheet 74

7 Probability Values for Event H, Occurrence of

Meaningful Fire 77

8 Nuclear Power Plant Control Room CPM Fire Hazard

Analysis Worksheet 83

9 Example Qualitative Fire Scenario Analysis of a

Cabinet Fire 85

9

SUMMARY

An uncontrolled fire in a nuclear power plant can seriously jeopardize overall plant safety. Recognizing this, the Nuclear Regulatory

Commission has undertaken a broad program in fire protection research, a

portion of which focuses on the performance of fire-hazards analyses.

This report examines the adequacy of existing fire-hazards analysis techniques in the context of nuclear power plant safety.

Traditionally, nuclear power plant designers have used a combination

of fire codes, insurance agency requests, regulatory guidance,.and fire

consultant recommendations to assess the adequacy of fire protection

system designs. This assessment process, often referred to as a firehazards analysis, normally has required engineers to balance a perceived

level of fire risk against some degree of fire protection. Unfortunately,

decisions regarding what constitutes an appropriate level of fire protection often have depended upon the experience and judgment of the

engineer making the analysis, with the results being quite subjective.

A review of a large number of available analysis techniques has shown

that no one method can satisfactorily circumvent the subjective nature of

current fire-hazards analysis practice. Methods which .prove conservative

and technically sound are usually too complicated, while more easily

applied techniques lack technical merit. Fortunately, it ispossible to

develop a new analysis methodology which combines the best attributes of

conservatism and ease of application from several existing techniques.

By first performing a bounding deterministic evaluation under limiting fuel load and ventilation conditions, to establish the adequacy of

passive fire barriers, an analyst can eliminate a number of "safe" fire

conditions from further scrutiny. For those fire situations demonstrated

to be too severe for passive-barrier containment, a second evaluation can

be made to assess the probability that supplementary fire protection

11
measures (e.g., fire suppression) will be adequate. Finally, for those

cases for which the probabilistic analysis proves unacceptable or inconclusive, a subjective evaluation can be made in accordance with a standardized format and logic sequence. This approach emphasizes the use of a

conservative and technically sound deterministic method in preference to a

purely subjective evaluation, but it is recognized that--under certain

circumstances--a lack of either theoretical understanding or experimental

precedents may require that one rely on subjective judgment.

12
NUCLEAR POWER PLANT FIRE PROTECTION -

FIRE-HAZARDS ANALYSIS (SUBSYSTEMS STUDY TASK 4)

1. Introduction

Based on the need to support near-term regulatory and licensing objectives for nuclear power plant fire protection, the Nuclear Regulatory Commission (NRC) Office of Standards Development requested Sandia Laboratories to develop the underlying logic and technical bases associated with

four specific fire protection topics. The topics selected by the NRC were

fire ventilation, fire detection, fire barriers, and fire hazards analysis. Separate reports addressing the findings of the first three study

topics have been completed with recommendations for establishing suitable

technical bases. The fourth topic, fire-hazards analysis, is the subject

of this report.

Study Objective

Although it is important (1) to establish what levels of fire protection adequately reduce nuclear power plant fire risk to acceptable safety

levels and (2) to perform a fire-hazards analyses of a complete nuclear

facility, these evaluations fall outside the scope of this report.

Instead, the major objective of this study was limited to assessing the

adequacy of existing fire-hazards analysis techniques in the context of

nuclear power plant safety.

Traditional Analysis Approach

Among those knowledgeable about fire protection, the term "firehazards analysis" conveys a meaning which, in principle, is understood

universally, just as the term "marketing survey" is understood by most

real estate developers. To a fire protection engineer or insurance underwriter, a fire-hazards analysis traditionally involves the application of

13
basic fire protection principles and experience to particular conditions

existing within an area in question. Numerous factors (including fuel

load, suppression capability, detection reliability, fire brigade training, structural integrity, property value, life safety, fuel ignitability,

oxygen availability, and insurance costs) are evaluated by fire protection

engineers until, in their judgment, fire risk is balanced'by appropriate

fire protection measures.

Except in those cases where building codes are specific, decisions

regarding what constitutes an appropriate level of fire protection can

often depend upon the experience of the engineer performing the analysis.

Just as a marketing survey can range from liberal to conservative with

regard to both its technical basis and recommended course of action, a

fire-hazards analysis can be unjustifiably harsh or lenient depending upon

the particular experiences of the engineer performing the analysis. In

practice, of course, the consultation services of consistently liberal or

conservative fire protection engineers would not normally be sought,

because their fire-hazards analysis results soon would be recognized as

misaligned with accepted practice.

Unfortunately, the use of generally accepted practices in the performance and assessment of traditional fire-hazards analyses can lead to fire

protection decisions which lack a technical basis. The accepted practice

of using certain fire-retardant cable insulations and cable tray spatial

separations are two examples wherein correct technical bases later were

shown to be lacking.1 2 3 Another example to be addressed more thoroughly

in this report is the accepted practice4 of equating a one-hour fire to a

fuel load of 80,000 Btu/ft 2 .

Since, in the context of nuclear power plant safety, it may be

inappropriate to judge the adequacy of fire protection measures solely on

the basis of "accepted practice," we tried to quantitatively assess each

analysis situation by deterministically applying basic principles of fire

phenomenon. The use of probabilistic and subjective analysis methods was

limited to those cases where, because of limited state-of-the-art

knowledge, a detailed understanding of the fire phenomena is lacking.

14
Study Description

A viable fire-hazards analysis method for nuclear power plant

application

" Should be derived from, but not necessarily duplicate,

available and proven techniques

" Should be defendable in terms of conservatism and

technical basis

" Should be easily used by both designers and regulators.

krom these analysis criteria, at least two questions arise.

1. What available fire-hazards techniques, if any, can be

used to predict fire severity from a .knowledge of the

physical conditions existing in particular power plant

areas?

2. What available analysis techniques, if any, can be

applied consistently by designers and the NRC to yield

conclusions that are straightforward, believable,

conservative, and quantitative?

Answers to these questions are provided by this report for a large

number of candidate analysis methods, each of which has been applied previously by other fire protection analysts to various types of residential,

commercial, industrial, or nuclear power plant hazards. Since it was

found that all of the available analysis methods reviewed proved deficient in meeting at least one of the analysis criteria, it was decided to

select and combine from available analyses those attributes most responsive to the needs of nuclear power plant designers and regulators. It will

be shown that the analysis method which resulted from this approach relies

initially upon conservative assumptions and deterministic calculations of

15
fuel load and ventilation conditions to bound expected fire severity. If

such bounding conditions are found to be acceptable to plant safety, the

analysis is terminated. If, however, plant safety cannot be assured under

conditions of a conservative bounding analysis, other supplementary fire

protection measures (e.g., detection, manual suppression, and automatic

suppression) are evaluated in a probabilistic fashion to assess what level

of fire protection can be derived from these measures. If the results of

a probabilistic analysis also are found to be unsatisfactory or

inconclusive, a subjective analysis is finally performed.

When plant safety cannot be assured by a bounding analysis, it must

be recognized that, without a nuclear power plant safety systems analysis,_

it is difficult to judge what relative level of fire protection meets

overall plant safety objectives. This uncertainty arises from the fact

that some plant safety areas are of more importance to a safe shutdown

than other areas, and therefore these important areas require a higher

level of fire protection. As stated under "Study Objective," the

determination of what is "safe enough," from the standpoint of overall

nuclear plant fire protection safety, falls outside the scope of this

report.

2. Analysis Methods Available

As stated in the last section, it was concluded early in this study

that a viable fire-hazards analysis for nuclear power plant application

should (1) be derived from, but not necessarily duplicate, available and

proven analysis techniques; (2) be defendable in terms of being conservative and technically sound; and (3) be easily used by both designers and

regulators. With these criteria established, a large number of analysis

methods were reviewed. By limiting this review to only those methods

which have received at least some practical scrutiny, the first criteria

automatically is satisfied; only the second two criteria remain to be met.

This section examines how well these remainijng criteria are achieved by a

number of existing analysis techniques.

16
For ease of understanding, it proved convenient to assign each candidate analysis method to one of three categories depending on whether a

particular method was based on subjective judgments, deterministic calculations, or probabilistic logic. The analysis techniques falling within

these categories are addressed in the following sections.

Subjective Analyses

This technique bases decisions for establishing satisfactory levels

of fire protection strictly on the judgment of the individual performing

the analysis. In practice this category of analysis is the most widely

used, with successful precedents being set in the areas of fire insurance

and public safety. The primary advantage of a subjective analysis stems

from the flexibility and relative ease with which the analysis can be performed by a trained professional, while its main disadvantage is related

to the strong dependence of the analysis on the qualifications of the professional. To overcome this, some precision has been added to a few subjective analyses through the establishment of strict analysis formats.

However, it turns out that, even with improved formats, all subjective

analyses must rely solely upon experienced judgment to defend the degree

of technical merit and conservatism inherent in each fire protection

decision.

In performing a subjective analysis, an analyst develops conclusions

regarding which hazard situations are acceptable and which are not, but

the analyst in no way is required to demonstrate, either analytically or

empirically, that a particular judgment is both conservative and technically sound. While in some situations it is possible for a subjective

analyst to cite codes or standards based on calculations and testing, it

is more often the case that judgment alone must be invoked. This is particularly true in nuclear power plant fire-hazards analyses because few

existing fire codes or standards were written with nuclear plant safety in

mind. Fire protection measures which meet traditional monetary and life

safety objectives for insurance purposes may be totally inadequate for

meeting nuclear power plant safety goals.

17
The following sections describe several of the analytical methods

which were categorized as subjective for purposes of this study. In each

example, it can be seen that an analyst must judge whether a particular

combination of fire hazard and fire protection results in an acceptable

level of safety.

Preliminary Fire-Hazards Analysis Method -- This type of analysis

presents a tabular listing of what are believed to be the primary hazards

of concern, together with a qualitative estimate of the potential effects

of these hazards on safety systems and of the "best" method to control the

hazards. Table I shows a typical format for a preliminary fire-hazards

analysis based on examples presented in References 5 and 6. In practice,

the transition from a preliminary to a final hazards analysis of this type

occurs after a proposed plant design has been built, thereby permitting

some refinement of the column descriptions used in Table 1. This form of

preliminary and final hazards analysis lacks quantitative support for estimates of hazards, safety impacts, and control effectiveness.

Fire-Hazards Analysis Outline Method -- This analysis method identifies what steps should be taken in performing a fire-hazards analysis, but

it does not provide the technical basis for actually making the analysis.

Each step of this analysis method usually asks the fire protection analyst

to perform a task such as: list applicable codes and standards, develop

the design criteria for suppression requirements, or analyze available

backup protection systems. The method used for performing these tasks is

left to the discretion of the analysts. Examples of this methodology are

found in National Fire Protection Association (NFPA) Code 802, "Recommended Fire Protection Practice for Nuclear Reactors," and in the March 1978

draft of a proposed appendix to the American National Standard "Generic

Requirements for Light Water Nuclear Power Plant Fire Protection," ANSI/

ANS-59.4-1977. Although a step-by-step analysis procedure of this type

will help a designer formulate a comprehensive analysis outline, it will

not provide the technical guidance needed to accomplish the tasks of the

outline. A complete analysis method should include both a step-by-step

procedure outline and a technically defendable method for accomplishing

each step of the outline.

18
TABLE 1

Preliminary Hazard Analysis*

Primary

Hazard

Cables in

Head Access

Area

Plant

Operating

Conditions

Hazard

Initiation

Safety

System

Affected

Control

Rod

System

Potential

Effects

Failure to

Shut Down

Reactor

At Power Can of

Cleaning

Fluid

Level of

Hazard

Low--

Cables are

Fire Retardant and Rods

will Drop

Faster Than

Fire can

Disable Them

Method of

Control

Fire Retardancy and

Detection,

Followed by

Manual Suppression With

Portable

Extinguisher

Etc

*Example based on References 5 and 6.
Hazard Inventory Method -- A hazard inventory analysis is similar to

a preliminary fire-hazards analysis, except that in the inventory method

an attempt is made to quantify levels of fire hazard and fire protection.

Table 2 shows the format typically used. It can be seen from this table

that the hazard inventory method quantitatively describes fuel loads and

protective measures occurring in each fire zone. What the method lacks is

a logical or calculational means of correlating the types and amounts of

fuel in each zone with the numbers and kinds of selected detection and

suppression measures. In practice, an analyst simply lists the fuel load

and then states, without proof; that a certain combination of fire barriers, detection, and suppression will be adequate. Reference 7 illustrates the use of the hazard inventory technique.

Hazard Mode and Effects Analysis Method -- This technique is a refinement of the preliminary hazards analysis method discussed above, but still

lacks mathematical rigor. Unlike a preliminary hazards analysis which

views each fire zone as the basic element for study, a hazard mode and

effects analysis often starts at a more detailed level with subsystems or

components, as illustrated in Table 3. However, except for its depth of

coverage, a hazard mode and effects analysis provides little fire

protection information beyond that derived from a preliminary hazards

analysis. This is because both methods lack the ability to quantitatively

estimate fire hazards and fire protection effectiveness.

Deterministic Analyses

Unlike subjective analyses which base decisions on the judgment of

the analyst, deterministic analyses base satisfactory levels of fire protection on testing and modeling of fire phenomena in terms of measurable

parameters and physical theory. Typically, complications associated with

characterizing fire initiation, growth, propagation, and extinguishment

dictate the need for simplifying assumptions to approximate problem solutions. Often the degree of approximation and the level of complication

are conflicting measures of the practical usefulness of a given model.

If approximations are too crude, the resulting model may predict fire

phenomena which are totally incredible. If, on the other hand, few

20
TABLE 2

Hazard Inventory Method

Fire Area/

Zone

Maj or

Equipment

Reactor Coolant Pump

Fire Hazard

Material Quantity1-B Cable

Grease

55 ft 3

12 lb

Fuel Load

(Btu)

74.6 x 106

0.23 x 106

Passive

Detection

Smoke

Detectors

(4)_

Suppression

Sprinklers

(0.2 gpm/ft 2 ),

Hose Stations (2)

Oil 265 gal 150.7 x 106

Etc

TABLE 3

Hazard Mode and Effect Analysis

Failure

Mode Item Subsystem Function Mechanism

Effect

Immediate/

Ultimate

Overpressure

or

Underpressure

Accident

Conditions

Required

Fire in

Nearby

Equipment

High

Temperature

Instrument

Panel

Pressurizing Systeem

Maintain

Plant

Pressure

Indicate

Incorrect

Pressure

Hazard

Severity

Moderate:

Backup

Systems

for

Pressure

Relief

Neutralization

Measure

Automatic

Suppression

Etc
approximations are made, the resulting model may require very sophisticated computer calculations and a comprehensive data base to reach a

solution.

Because of the need to combine ease of calculation with believability

of results, most progress in the area of fire modeling has come from

efforts to describe the effect of limiting fire severities on barriers,

while attempts to characterize the mechanisms by which fires start,

spread, and are extinguished, have been only marginally successful. This

is understandable when one recognizes that minor variations in fuel type,

fuel configuration, ventilation, and suppression system design can drastically affect the progress of a fire. Similar to the way in which a successful campfire depends on the initiating fire source (a match alone vs

lighter fluid), the fuel arrangement (wood shavings vs a large log), and

the fuel's fire retardancy (damp wood vs dry wood), minor variations from

one room to the next and even variations within the same room can result

in fire hazards ranging from innocuous to severe. Because of this difficulty, the approach most often applied to describe fire growth has been

the use of scenario analyses which are deduced from a logical combination

of test information and speculation.

The following sections describe examples of both the scenario

analysis method for characterizing fire growth and the barrier analysis

approach for defining limiting fire severities.

Scenario Analysis Method -- A scenario analysis may be viewed as a

subjective analysis to which numbers have been added. For instance, an

analyst simply may state in a subjective analysis that the size of a

particular fire will be limited by its expected propagation rate and the

response of a fire brigade. In a scenario analysis of the same situation,

the analyst speculates what is believed to be a credible sequence of

events for fire initiation, growth, and extinguishment and, at each juncture in the scenario, the analyst justifies subsequent fire events on the

basis of related test data, calculations, or experience. It is this

detailed justification that distinguishes scenario analyses from subjective analyses.

22
Numerous examples of the scenario technique applied to nuclear power

plant situations are presented in Reference 8. As given there, a typical

fire scenario may begin with a fire starting in one of two redundant cable

trays in a particular plant location. By invoking test information on the

propagation rate of fires burning along cable trays, the minimal cable

tray separation distance to prevent propagation between trays, the

response time of fire detectors, the effectiveness of available suppression schemes, and the safety importance of the cable trays involved, an

analyst can ascertain whether a fire starting as described will jeopardize

overall plant safety.

Unfortunately, germane test information rarely is available when

developing a scenario, so it is necessary to make assumptions regarding

the impact of any conditions which differ from those used during testing.

These assumptions may involve differences in oxygen availability, fire

retardancy, fuel configuration, initial fire size, detector arrangment, or

suppression installation. Another uncertainty results from not knowing

whether the assumed scenario is, in fact, conservative. Alternate

scenarios can always be postulated which may or may not be more stringent

than the original case.

Despite these drawbacks, a scenario analysis represents an improvement over a subjective analysis, since in the latter case fire protection

conclusions are stated without reference to any technical bases except

perhaps the experience of the analyst involved. On the other hand, a

scenario analysis, as illustrated in Reference 9, provides a structured

thought process in which the fire protection analyst states and technically defends all assumptions and conclusions. When used in conjunction

with both deterministic and probabilistic data from closely related test

cases, a scenario analysis provides the best mechanism for judging fire

safety in those situations where total fire involvement cannot be

tolerated (e.g., certain power plant control and cable spreading rooms).

In cases where total fire involvement can be permitted (e.g., redundant

areas separated by passive barriers), an attractive alternative to a

scenario analysis would be to demonstrate barrier effectiveness under some

limiting, worst-case fire conditions. This approach is discussed next.
Barrier Analysis Method -- In this report the term "barrier analysis"

refers to analysis techniques which describe fire severity in terms of

total involvement of combustibles in a room and-in terms of total involvement effect on the room's structural integrity. Total involvement of

combustibles, often referred to as flashover, has been recognized by

numerous studies as the most severe condition for containing a fire.10-20

Also, room flashover was considered by the National Bureau of Standards

(NBS), as early as 1922, to be the design basis for judging barrier

effectiveness.12

Out of this background have come two general approaches for predicting fire severity and its impact on barriers. One approach characterizes

fire hazards as a function of fuel load, while the second defines fire

hazards in terms of fuel load and available air for combustion. Each of

these methods is discussed in the following sections.

Fuel Load Method. It seems logical that the more combustibles a

room has, the more difficult it will be to contain a fire within that

room. This reasoning forms the basis of the fuel load method for barrier

analysis.

As described in Section 6, Chapter 8 of the National Fire Protection

Association Handbook, 4 a fuel load analysis starts with adding up the

weight of combustibles in a room and converting the weight to energy content of the fuel per unit floor area (e.g., Btu/ft 2 ).' At this point, the

measured fuel load is compared to a linear fire-duration scale in which a

fuel load of 80,000 Btu/ft 2 corresponds to a fire of I hour's duration and

a fuel load of zero corresponds to a fire of zero duration. On this

basis, a measured fuel load of 240,000 Btu/ft 2 would be treated as a 3-h

fire.

The linear scale relating fuel load to fire duration was first proposed by the NBS in 1928 and was prompted by an effort to preserve the

usefulness of a time-temperature test curve for testing building materials

in furnaces. The test curve which had been adopted 10 years earlier (in

24
1918) by the American Society for Testing Materials (ASTM) is still in use

today. 2 To establish the relationship between the ASTM time-temperature

curve and the linear time-fuel load curve, NBS conducted numerous fullscale burn tests using cellulosic combustibles. From these tests NBS concluded that a fuel load of 80,000 Btu/ft 2 will produce a fire as severe as

a 1-h ASTM furnace test. By adopting this conclusion, it is assumed

implicitly that the time-temperature history of an actual fire is of no

consequence to structural integrity provided that the predicted fire duration, linearly calculated from the fuel load per unit area, does not

exceed the time limit to which the structures were furnace tested. In

other words, as long as the integrated, time-averaged temperature for the

predicted fire is equal to the integrated, time-averaged temperature for

the furnace test, both fire-severities are'assumed to be equal, regardless

of actual fire duration. Figure 1 schematically illustrates this

assumption.

Same Severity when A = A2

A 

2

4IJ

Time t = test furnace time limit t

t = actual fire duration

a limit

Figure 1. Illustration of Integrated Time-Temperature

Curve Concept for Equating Fire Severities

25
The ease with which an analyst can convert a measured fuel load into

a fire duration for purposes of selecting a standard, tested barrier has

resulted in wide acceptance and use of the fuel load analysis method.

Unfortunately, the method has a number of shortcomings which may render. it

inappropriate for nuclear power plant fire analysis.

1. The fuel load method ignores the effect of ventilation

conditions on fire duration and intensity. When the

linear scale relating fuel load to fire duration was

developed by NBS, test conditions simulated offices and

residences with open windows or doors to provide adequate air for combustion. Most safety areas of a

nuclear plant have no direct openings to the outside

and, therefore, the mechanisms by which air enters or

heat leaves the fire zone differ from those originally

tested.

2. The fuel load method ignores the detrimental effect

which hot, short fires may have on structures. By

using an integrated, time-averaged temperature, the

fuel load method equates hot, short fires to cooler,

long-lasting ones. Since structural steel and concrete

have maximum temperature limits beyond which degradation of strength is accelerated, 2 1 2 2 it may be

incorrect to equate fires on the basis of average

temperatures, especially in those cases where an actual

fire reaches temperatures above the standard ASTM

furnace time-temperature test curve.

3. The fuel load method ignores the effect of different

types of combustibles on fire severity. Since different combustibles burn at different rates whenever

sufficient air is present (Reference 4, Section 6), the

rate of heat release during a fire can vary depending

on what is burning. The linear scale developed by NBS

relating fire duration to fire load was based on tests

26
using cellulosic combustibles. Other materials, such

as the cable insulation and oil found in nuclear power

plants, can be expected to burn differently than cellulosics. Whenever a lack of sufficient air for combustion limits the rate of burning, differences between

combustibles become less significant. However, as

explained in item I (above), the situation of

ventilation-limited fires was not addressed in the NBS

testing.

From the above findings for this study, it was concluded that the

fuel load method for analyzing the adequacy of fire barriers lacks the

technical bases and' conservatism needed for nuclear power plant

application. Although the method is simple, the results lack credibility.

Fuel Load and Ventilation Method. The fuel load analysis method

discussed in the previous section treated fire severity as a one-parameter

problem--fuel load. Since simple fireplace drafts and dampers demonstrate

the influence of ventilation on fire severity, it is reasonable to view

limiting fire phenomena in terms of two parameters--fuel load and

ventilation. Recognizing this, many fire protection researchers have

attempted to measure or model the interrela-tionships between fuel load,

ventilation, and fire severity.10-20 Those efforts which have proven most

productive have taken cognizance of the following facts.

e Fully developed fires burn at limiting rates determined

by either (1) the kinetic limitations of combustion

which are functions of the available fuel surface area

(i.e., surface-controlled burning) or (2) the limitations of available oxygen for dombust-i6f- -I.e.,-

vent ilation-controlled burning).

27
* Total involvement of the combustibles in a room (i.e.,

after flashover) represents the most severe condition

for containing a fire with barriers.

o The time-temperature history of fire in a room must

satisfy basic laws of physics which equate the rate of

heat evolution to the rate of heat loss.

After applying these facts, a number of researchers found that fire

severities differed significantly from those predicted by the oneparameter fuel-load analysis method. 1 0 12 15 16 20 23 This is not

surprising since the addition of ventilation more closely approximates

real fire phenomena. Unfortunately, most of the combined fuel load and

ventilation analysis techniques developed to date have several major

deficiencies which make them unattractive for nuclear power plant fire

use.

" Most-of the techniques are cumbersome. An analyst often

has to apply first principles of fluid dynamics and heat

transfer to solve a particular problem.

" Many of the techniques treat ventilation limitations for

cases involving doors or windows which vent directly to

the outside. Situations involving forced ventilation

systems, rooms without windows, or doors connected to

hallways are rarely treated. These factors can limit

both the rate of combustion and the rate at which heat

is removed from a room.

* Few of the techniques relate predicted fire severity to

the conditions under which barriers are tested. As a

result, an analyst must judge what combinations of fuel

load and ventilation are acceptable for a given barrier.

In light of these deficiencies, it may appear that, although the

addition of ventilation technically improves the one-parameter fuel load

28
analysis technique, the improvement lacks practicality. To some extent

this conclusion is correct. However, under the "Most Suitable Analysis"

and "Example Fire Hazards" sections, we will demonstrate that a twoparameter barrier analysis method can be devised from available analysis

techniques, can be defended both technically and conservatively, and can

be easily implemented by both designers and regulators.

Probabilistic Analyses

Unlike subjective and deterministic analysis methods which describe

and interpret each fire event in physical terms, probabilistic analyses

make no effort to define numerically the temperatures, burn rates,

ventilation limits, suppressant concentrations, or any other parameters

characteristic of a fire. Instead, a probabilistic analysis establishes

satisfactory levels of fire protection on the basis of various forms of

graphs or logic charts which describe fire events in terms of their

probability of occurrence. The probability values used are typically

derived from historical data or by consensus.

There have been a number of different approaches to probabilistic

fire analysis, ranging from complicated methods which employ detailed

logic diagrams to less complicated methods which reduce fire event probabilistics to a few general categories. Similar to the deterministichazards analysis methods just discussed, the degree of simplification and

level of credibility associated with probabilistic analyses often 4re

conflicting measures of a particular method's usefulness. This point can

be illustrated by a review of several probabilistic analysis examples.

Statistical Method -- This method has been applied for years by

insurance underwriters. In practice, a fire-hazards analyst reviews

statistical data regarding the probability of fire occurrence and the

effectiveness of fire control measures under various conditions of fuel

type, fuel arrangement, structural design, suppression availability, and

detectfon-system installation. The analyst then-subjectively applies the

statistical data base to the particular fire-hazard conditions under

consideration.

29
For purposes of insuring against property losses, the statistical

method strikes an ideal balance between degree of simplification and level

of credibility. With relative ease, an analyst can statistically support

trade-offs between the level of fire risk and the cost of insurance.

Unfortunately, fire protection measures which have proven statistically

satisfactory for monetary and life safety insurance objectives may be

inadequate for meeting nuclear power plant safety goals.

National Fire Protection Association Decision Tree -- This approach

is presented in Section 6, Chapter 2 of the NFPA Handbook 4 and shown in

Figure 2. The method views fire events in a logical sequence leading to a

predefined fire objective for life safety and property protection. To

identify a suitable objective, the analyst must ask a number of questions

regarding the replacement value of the property and the wishes of the

property owner. Once a suitable safety objective has been established to

the satisfaction of the analyst and property owner, the NFPA decision tree

is used to analyze a logical set of fire events leading to the safety

objective. At junctures where an event may or may not occur (e.g., automatically suppress fire), the analyst subjectively estimates the

probability of the event occurring. These estimates are usually derived

from the analyst's experience or knowledge of probability data from

related historical events.

In comparison to the loosely defined Statistical Method discussed

above, the, NFPA decision tree represents an improvement in the overall

logic used to perform probabilistic analyses. As a result, fire protection decisions based on the NFPA decision tree are more credible, although

the decision-tree methodology is somewhat more tedious. However, because

subjective judgments are needed to define the suitability of each safety

objective and to establish the probability of each fire event occurring, a

NFPA decision-tree analysis conclusion may vary from liberal to conservative, depending on the experience of the- analyst and the wishes of the

property owner. Such variances are undesirable for nuclear power plant

fire-hazards analysis.

30
rh - LU TI

A *flOOII2,41

General Services Administration (GSA) Decision Trees -- Similar to

the NFPA method just discussed, GSA decision trees use a logic network to

define events leading to a predefined safety objective but, unlike the

NFPA approach, the GSA technique predefines the probability values to be

used at junctures in the decision tree. The GSA probability values originally were established by the consensus of several GSA fire protection

consultants through their combined knowledge of the fire event probabilities. By applying these established probabilities and by following the

logic of several generic GSA decision trees, an analyst can develop a

probability chart, similar to the one shown in Figure 3, to reflect the

progressively lower probability associated with increasing fire severity.

If the probability of a particular fire severity is judged by the analyst

or property owner to be unacceptable, steps are taken to upgrade overall

fire safety by improving selected portions of the fire protection system

(e.g., increased detection, reduced fire load, or improved separation).

Reference 24 presents an example of the GSA-analysis method.

0

a

20 .

20 -.

40

b

55 c d

": 60

.62 .64 .66 C. ele C.66 e

4. 80 \

0

90-- 90 - -- 95 "

o 09

Uws ws1ws2ws4 Wsn room r2 r4

Building Space Modules

Figure 3. GSA Probability Chart for Fire

Development Within a Building

33
Since GSA logic trees are more complicated than the NFPA decision

tree shown in Figure 2 and since the GSA method would require the development of a probability chart for each nuclear power plant safety area and

each fire protection system combination, it appears that the practical

usefulness of the GSA methodology for nuclear power plant design and

regulation is marginal, at best. When this is combined with questions

concerning the validity of the established probability values and the

acceptability of chosen safety objectives, it appears doubtful whether the

sophistication of the GSA technique is warranted.

Fault Tree Analyses -- Fault tree analyses differ from NFPA and GSA

decision trees primarily in terms of detail level and scope of coverage.

Instead of viewing only fire events, fault tree analyses consider detailed

component and system interactions from the standpoint of both fire occurrences and resulting safety system responses. Each fault tree is specific

for a given set of power plant arrangements and safety system functions.

As a result, considerable analysis effort is needed to carry out a comprehensive fault tree analysis of each nuclear power plant safety area.

Figure 4 presents a portion of a fire-hazards analysis fault tree developed for the Clinch River Breeder Reactor Plant Risk Assessment. 2 5 From

this figure, it is apparent that a large number of other safety-area fault

trees could be devised, each requiring the analyst to have a thorough

knowledge of fire event probabilities and safety system limitations.

Although a fault tree fire-hazards analysis would be based on proven

methodology 2 6 and would be technically defendable, it is doubtful whether

designers and regulators would consider the technique simple. For this

reason, it was decided to reject fire-analysis fault trees as viable

analysis alternatives for nuclear power plant design. This conclusion

does not exclude the use of fault trees as part of a generic evaluation of

fire protection in nuclear power plants. As indicated under "Study

Description," a generic analysis would be useful in establishing what

levels of fire protection adequately reduce nuclear power plant fire risk

to acceptable levels for overall plant safety. Since an evaluation of

overall fire risk lies outside the scope of this report, further consideration of fault trees is inappropriate here.

34
Failure of

Fire Protection

Water Distribution

* UCSR = Upper Cable Spreading Room

LCSR = Lower Cable Spreading Room

MCR = Main Control Room

Figure 4. Typical Branches of a Fire-Hazards

Analysis Fault Tree

35
LO

Figure 4. (Concluded)
3. Most Suitable Analysis Method

Under "Study Description" we listed three criteria for selecting a

fire-hazards analysis technique for nuclear power plant application. To

ascertain whether any presently available analysis methods can satisfy all

three of these criteria, in Section 2 we reviewed examples of three categories of analysis; no single analysis approach appeared to meet the

criteria. In all cases, those analyses which seemed technically sound

proved to be difficult to apply, while those which seemed readily implemented lacked technical conservatism. Because of this, it was decided

that the most suitable analysis method for nuclear power plant use would

be one which combines the best attributes of several complementing

analysis techniques.

Of the three criteria listed in "Study Description" for selecting an

analysis method, the most important from the standpoint of public safety

requires that the analysis be defendable in terms of conservatism and technical basis. If a combination of available analysis techniques can be

shown as conservative and technically sound, the only remaining task is to

simplify use of the techniques. Accordingly, the approach of (1) combining available techniques, (2) demonstrating their technical merit, and

(3) simplifying their application was selected for this study as the most

logical sequence to arrive at a straightforward yet credible analysis

method.

Combining Available Analysis Techniques

The categories of analysis addressed in Section 2 were termed subjective, deterministic, and probabilistic. Deterministic analyses are those

which describe fire events in physical terms (e.g., burn rates, temperatures, air requirements), while probabilistic analyses are those which

describe fire events in terms of their chance of occurrence. To some

extent, subjective analyses can be viewed as a combination of deterministic and probabilistic analyses, wherein an analyst judges both the expected physical severity of a fire and the probability' that available fire

protection measures will be adequate. Unlike most of the deterministic

37
and probabilistic methods shown in Section 2, however, subjective analyses

typically require no empirical or analytical justification for the conclusions reached by the analyst, and therefore it is difficult to demonstrate

the technical merit of a subjective analysis. Similarly, it is often

difficult to demonstrate the degree of conservatism and basis for many

deterministic or probabilistic analysis decisions, primarily because of

incomplete supporting data.

One way to circumvent this problem is through the use of a deterministic analysis to place a conservative upper limit on fire severity in each

.safety area of a nuclear power plant. If such bounding conditions are

found to be acceptable to. overall plant safety, further analysis is unwarranted. If, however, plant safety is found to be jeopardized under the

bounding conditions, a probabilistic analysis would be needed to account

for supplementary fire protection measures not considered deterministically. Only in those cases where a lack of theoretical or empirical information precludes the use of either a deterministic or probabilistic analysis

should it be necessary to base fire safety conclusions solely on the

subjective judgment of a fire protection analyst. Figure 5 schematically

shows the resulting relationship between the three analysis categories.

To carry out a fire-hazards analysis according to Figure 5, a technically defendable and easily executable combination of deterministic, probabilistic, and subjective analysis methods must be specified. To do this,

candidate analysis methods can be chosen from those reviewed in Section 2

by selecting a method from each analysis category which combines technical

merit with the least complexity. Candidates selected on this basis are

presented next.

Deterministic Analysis Candidate -- For this category, a barrier

analysis technique using conservative assumptions for fuel load and ventilation appears most promising. This is because simpler deterministic techniques, such as scenario analyses or fuel load barrier analyses, cannot be

easily demonstrated as conservative. Also, other more complicated analysis methods which attempt to describe the mechanisms by which fires start,

propagate, and are extinguished have been only marginally successful.

38
Figure 5. Analysis Sequence for Each Nuclear

Power Plant Safety Area

39
Probabilistic Analysis Candidate -- For this category, an analysis

technique which combines the numerical rigor of the GSA decision trees

with a simpler logic format appears most practical. Less complicated

methods, such as statistical analyses or NFPA decision trees, are not well

enough defined in terms of either format or numbers to be consistently

applied by power plant designers or regulators. On the other hand, faulttree analyses require an extreme level of detail which cannot be justified

in view of the meager statistical base available.

Subjective Analysis Candidate -- As presented in Section 2, subjective analyses constitute all techniques which rely solely on the experience and judgment of a fire protection analyst. Because it is difficult

to justify the adequacy of this approach, the analysis sequence shown in

Figure 5 has relegated subjective analyses to a position following both

deterministic and probabilistic analyses and, therefore, subjective

analyses come into play only when deterministic and probabilistic methods

prove inadequate.

In this context, subjective analyses still have some value in defining problems such as monetary risk, but their value to nuclear safety is

limited tO case-by-case assessments. Because of this limitation, a complicated subjective analysis format is unjustified. Instead, a more reasonable approach would be to allow an analyst to recommend fire protection

measures on the basis of a qualitative fire scenario. The fire scenario

could be developed in a conservative manner dictated by a predefined

analysis outline such as the type discussed in Section 2. By proceeding

in this way, the analyst will be forced to document the basis for each

fire protection decision in a manner which can be scrutinized by nuclear

safety regulators.

40
The Technical Merit of Candidate Analysis Techniques

Figure 5 presented a proposed sequence for carrying out a nuclear

power plant fire-hazards analysis in a way which, by being technically

conservative and easily executed, emphasizes the best attributes of three

complementary analysis categories. We then presented what appeared to be

the most promising analysis technique for each of the three analysis

categories:

Analysis Category Candidate Technique

Deterministic Analysis Barrier analysis using

conservative limits for

fuel load and ventilation

Probabilistic Analysis Decision tree analysis

applying the numerical

rigor of GSA decision

trees in a more simplified

format

Subjective Analysis Qualitative fire scenario

analysis following a prescribed analysis outline

According to the criteria established in this report for assessing

the viability of a fire-hazards analysis, the technical merit of any candidate analysis technique should be demonstrated in terms of its conservatism and technical basis. The following sections will demonstrate this

for the three analysis candidates.

Fuel Load and Ventilation Barrier Analysis Method -- On the basis of

considerable fire research,10-20 it is more realistic to view fire phenomena in terms of two parameters, fuel load and ventilation, rather than to

use only fuel load. Furthermore, as has been mentioned, total involvement

of combustibles in a room (after flashover) represents the most severe

condition for containing a fire with barriers; the actual level of post-

-fl-ashover severity is a function of either fuel-surface-controlled burning

or ventilation-controlled burning. Mathematically these facts can be

expressed in general terms as follows:

41
Rate of heat

buildup in a room

Rate of heat Rate of heat lost

produced by - from the room

the fire

(1)

To solve this equation, a number of assumptions and physical theories

must be cited to formulate each term in the equation. Since this has been

done in many publications,10-20 only the resulting expressions and underlying assumptions will be presented in this report.

First Term

8 Tf

Rate of heat buildup in a room = pfVCpf

where

Pf = density of fire gases in the room

Cpf = heat capacity of fire gases in the room

Tf = temperature of gases in the room

t = time

V = volume of the room.

Second Term

Rate of heat produced = RPAH_ for surface--controlled

by the fire burning,

where

R = Rate of pyrolysis when combustion is limited

by combustion kinetics of the fuel surface

AHc = Net heat release-upon fuel combustion,

or

Rate of heat produced = MairrAHC for ventilationby the fire controlled burning

42
where

Mair = rate of air entering the room for combustion

r = stoichiometric mass ratio of fuel to air for combustion.

Third Term

Rate of heat Heat lost Heat lost by Heat lost

lost from by radiation + convection + through openings

the room to walls to walls by radiation

Heat lost

+ through openings

by mass flow

Heat lost by a-A T 4-Twý radiation to walls W w f i

-L+

f w

where

a Stefan-Boltzmann constant

Aw = area of the walls

Twi = wall inside surface temperature

Ef = emissivity of the fire gases

Ew = emissivity of the wall.

Heat lost by

convection to = UA W(Tf - Twi)

walls

where

U = convective heat transfer coefficient..

Heat lost through(Ti abA - T

openings by radiation = I'

43
where

A = area of the opening

To = air temperature outside the room

E0 = emissivity of the air outside the room.

Heat lost through

openings by mass

flow = MfCpfTf - 4airCPaTo S

where

Mf = rate of fire gases leaving the room

CPa = heat capacity of air outside the room.

Substituting these terms into Eq. (1) yields:

aTf

PfVCpf at

R_4HC (for surface-controlled burning)

or

MairrAHC (for ventilation-controlled

burning)

- UA (Tf - Twi) -

- MfCpfTf + MairCPaTo (2)

Assuming that numerical values or functional relationships can be found

for all the physical parameters in Eq. (2), six undefined variables

remain: Tf, Rp, Mair, Twi, To, and Mf. This implies that five more

expressions beyond Eq. (2) are needed. If To is assumed to be known and

44
constant in any particular problem and if R is known from experimental

information, only three more expressions are required to relate Tf, Mair,

Twi, and Mf.

One relation is the following unsteady-state heat flow equation

written for the room walls:

pwCw EDt ax 8 x•f 3

With the boundary conditions

Tw(xO) To

aTw. (T4 4)

-k a' o-(fT Twi

w ax U (T f1 + 1 I

Ef E w

8Two T4 T 4)

-kw - = U(Tw o - T + ( L 0 1 -- 

ww 0

where

Pw =density of the wall

Cpw heat capacity of the wall

Tw= temperature at any time, t, or distance, x, within the wall

Two = outside wall temperature

kw = thermal conductivity of the wall

A second relation is the following steady-state mass balance on the

room:

45
Rp for surface

controlled burning

or M f - Mair (4)

Mairr for ventilationcontrolled burning

The third and final relationship needed for Tf, Mair, Twi, and Mf

describes Mair in terms of the physical characteristics of the room under

consideration. Obviously, in a room having shut openings with only forced

ventilation airflow, Mair is simply the ventilation system flow rate. On

the other hand, if airflow into the room results from natural convection

through openings, Mair must be related to the gas temperature in the room

through buoyancy and fluid dynamic expressions. This has been done in

Reference 27 to yield the following expression:

1/2

Pf

M a =i (Ao~ho) 2/3 Cd•/-- -,o- p•O (5)

0 + p 0 (1 + r)

Pf

where

h,.= the- length- of the opening

Cd = fluid dynamic discharge coefficient (0.68)

g = gravity acceleration

po = density of entering air

By solving Eqs. (2) through (5) simultaneously, it is possible to

find the temperature of fire gases in a room as a function of time for

specified room dimensions, fuel characteristics, wall construction, and

air and gas properties. In practice, a solution is found only after a

number of simplifying assumptions have been made which are shown as

conservative and technically sound. Assumptions typically involved are

46
I. The temperature, Tf, of the hot gases within a burning

room is assumed to be uniform with no spatial variations. This assumption is based on observations that

postflashover fire conditions are characterized by

large-scale turbulence which mixes hot gases to the

point of neutralizing temperature gradients. 1 5 - 1 7 23 27

Except where cool air is being drawn into the

combustion zone in the immediate vicinity of the floor,

Tf can be assumed as independent of horizontal or

vertical displacement.

2. The heat capacity, density, and thermal conductivity of

the walls are assumed functions of temperature only and

not functions of distance inside the wall. This means

that the wall can be represented as a homogeneous

slab.

2 8

3. The heat capacity and density of the fire gases inside

and leaving the room are assumed the same as those for

air under the same temperature conditions. This assumption is prompted by a lack of information regarding the

exact composition of the fire gases. If it is

recognized that 79% of air entering a combustion zone

leaves as uncharged nitrogen and that the density and

specific heat of flue gases are comparable to air at

the same temperature and pressure conditions, the

validity of this assumption is justified.29

With these assumptions and some experimental values for Hc, , Ef,

EwS and U, Eqs. (2) through (5) can be solved with the aid of a computer.

This has been done in References 15, 23, and 27 for a number of cases

involving combustible loadings, ventilation conditions, and construction

materials characteristic of residential and commercial buildings. For

nuclear power plants, similar calculations can be carried out by selecting

physical parameters representative of power plant conditions. However,

for a nuclear power plant barrier analysis to be conservative, additional

47
adjustments to the governing equations, beyond those involving physical

parameters, must be made.

In typical residential and commercial buildings, windows and doors

provide direct openings between a burning room and the outside environment. As a result, heat is lost from a burning room through the radiation

and convection of heat from hot gases within and leaving the room and by

the introduction of cool air into the room. These heat loss mechanisms

prompted the inclusion of the following terms in Eq. (2):

TAT4 _T4

+ Mair Cp a - MfCpfTf ,and -- 1 f 0

(cool air) (flow heat loss) E + E

(opening radiation

heat loss)

Nuclear power plant safety areas usually have no windows or doors

opening to the outside. Instead, safety areas connect to a maze of hallways and stairways which may lead to other internal plant areas before

reaching the outside. Because this situation has not been modeled generically, it is difficult to predict how closely iVapproximates cases where

windows and doors open directly outside. What can be stated is that less

heat will be lost from the nuclear power plant area than predicted by

Eq. (2). Since the amount of actual heat loss reduction cannot be readily

predicted, a conservative approach is to assume that no heat is lost from

the fire area by the entrance of cool air, by radiation outward through

openings, or by outflow of hot gases. This is equivalent to saying that

heat is lost only through convection and radiation into the walls of the

room.

For the above assumption to be conservative, a second assumption must

be made relating to the effect of internal partitioning on airflow to the

fire zone. Although it is conservative to consider that a lack of doors

and windows will hamper heat loss from a fire area, it is not conservative

to assume that a lack of direct openings to the outside will reduce

48
burning rates by limiting airflow. Accordingly, for purposes of calculating airflow to a nuclear power plant fire, assume that all openings lead

directly to the outside..

Figure 6 presents a ranking of the overall conservatism inherent in a

fuel load and ventilation barrier analysis that incorporates the above

assumptions for heat loss and air inflow. Such an approach will be conservative and will approximate actual fire severities more closely than

either instantaneous or surface-controlled burn rate analyses which are

more conservative, but less realistic. Obviously, as ventilation rates

approach surface-controlled burning limitations, the distinction between

the surface-controlled burning and ventilation-controlled burning disappears. It should also be recognized that, if a nuclear power plant

ventilation scheme can be shown capable of removing a specified amount of

heat during a fire, such a scheme would warrant reduced conservatism in

the fuel load and ventilation barrier analysis model. These points will

be taken up again under "Simplifying the Candidate Analysis Techniques."

Increasing

Conservatism

Combustion of all fuel

instantaneously, assuming

heat loss into walls only

Combustion of all fuel at

surface-controlled burn rate,

assuming heat loss into walls

only

Combustion of fuel at

ventilation-controlled

limit, assuming heat loss

into walls only and airflow

unrestricted by internal < Actual Severity for

partitioning Ventilation-Controlled

, ~Fires

Combustion of fuel at

ventilation-controlled limit,-

assuming room openings

communicate directly with

the outside Decreasin Con~servatis

Figure 6. Comparison of Models for Assessing

Fire Severity in Nuclear Power Plants

49
As part of this study, fire severity calculations were made for

several representative nuclear power plant safety areas using the fuel

load and ventilation barrier analysis technique just discussed. A computer program, 2 7 30 originally developed for residential and commercial

application, was used to make the calculations; however, it was modified

to limit heat losses to only radiative and convective heat removal by the

walls of a burning room. It was found from the calculations that a wide

range of fire severities (as characterized by time-temperature fire histories) can be predicted, depending upon fuel load, ventilation availability, and heat loss wall area. Unfortunately, the significance of the

predicted severities and the usefulness of-this computer program are both

dubious for nuclear power plant designers and regulators. In particular,

the following questions must be resolved:

* How can different fuel loads, ventilation rates, and

room configurations be related to fire severity for easy

application by designers and regulators?

9 What criteria should be used to ascertain whether a

conservatively predicted time-temperature fire history

is acceptable from the standpoint of barrier integrity?

These two questions are addressed later in the context of simplifying

the fuel load and ventilation barrier analysis method for nuclear power

plant application. Before this is done, however, the technical merit of

the candidate probabilistic and subjective analyses will be demonstrated.

Modified Decision-Tree Analysis Method -- In those instances where a

fuel load and ventilation barrier analysis proves inconclusive or predicts

barriers alone to be inadequate, Figure 5 suggests that a probabilistic

analysis should be done to ascertain whether other supplementary fire

protection measures will meet fire safety objectives. The review of existing probabilistic analysis methods in Section 2 was the basis for a conclusion that an analysis technique which combines the numerical rigor of the

GSA decision tree with a more simplified logic format appears most

promising.

50
One analysis method which does this was proposed in 1976 at the

annual meeting of the Society of Fire Protection Engineers by John

Campbell and Bert Cohn from the firm of Gage-Babcock & Associates, Inc.

The proposed technique is called a critical-path technique and is shown

schematically in Figure 7. In principle, the critical-path method follows

a fire event from initiation to the point at which the fire is either

extinguished or breaches the barrier surrounding the fire area being

analyzed.

Figure 7. Critical Path Diagram for Fire

Development in a Building

51
Return to

Fire Uv Penetrates Ign tio Fe Barie In Niext Space

Figure 7. (concluded)

52
As shown in Figure 7, the critical-path analysis starts with paths

radiating out from a point representing the initial contact of a source of

ignition with a source of fuel. Possible outcomes at this point are the

igniting source may be too brief or of too low an intensity; there may be

inadequate fuel or the fuel may be too difficult to ignite; somebody may

notice the igniting flame or spark and remove the fuel from the danger

zone before it can ignite; the fuel may ignite and the fire may progress

to an incipient state. As the fire progresses from incipiency to the next

stage, its progress can be interrupted by insufficient fuel, manual extinguishment, or automatic suppression, each of jhich is represented by a

different path. As before, one of the paths represents the continuing

fire which now is referred to as a "meaningful" fire, because it is well

established in a fuel supply. Using similar logic, subsequent fire events

can be traced through Figure 7.

In comparison to the logic of the other probabilistic analyses

reviewed in Section 2, the critical-path method is much simpler; there are

only 21 individual fire events for which probabilities must be defined.

As a result, implementation and acceptance of the technique is greatly

facilitated. However, the technical basis and conservatism provided by a

critical-path analysis remains a function of the individual probability

values assigned to each fire event; the usefulness of any probabilistic

analysis method results from the assignment of conservatively reasonable

probabilities. If realistic probabilities are-ignored and instead all

event probabilities are defined as either zero (never occurring) or one

(always occurring), the resulting analysis would be classified as

deterministic. Such an approach ignores the fact that no event occurs"

with 100% assurance, especially those events involving people or automatic

systems. Recognizing this, at this point it was decided to present a

discussion of the technical basis and degree of conservatism that can be

assigned to probability values on the. critical-path diagram.

The events shown in Figure 7 were analyzed by Gage-Babcock & Associates, Inc., starting with the probability of fire being initiated when a

source of ignition contacts a source of fuel. This depends on (I) whether

fuel is available at the point of ignition and how easy it is to ignite,

53
and (2) whether someone notices the ignition and prevents the incipient

fire. Automatic detection and automatic extinguishment of a fire at this

stage is rare and was not considered.

As a conservative assumption the probability of an ignition source

contacting the fuel was set equal to i, because it is difficult to demonstrate a lesser probability for this event. Similarly, the condition of

fuel being always available in the area under consideration and easy to

ignite was also assigned a probability value of 1; thus, any ignition

source is considered adequate to sustain a fire in an easily ignitable

fuel. If there were absolutely no fuel in a space or no reasonable way to

ignite it, a probability of zero would apply. However, because some transient burnable material may be introduced into such a pristine space, a

probability of .01 was assigned to the possibility that a transient fuel

supply will be available at the time of ignition.

For the situation where someone notices ignition and prevents an

important fire, it was estimated that for spaces attended at all times the

ignition would be discovered in the fuel supply in 99 cases out of a hundred. If the space is seldom attended, the probability of discovering the

initial ignition was estimated as essentially 0. For conditions in

between these extremes, it was reasoned that the vast majority of ignitions occur as a result of people being present. Thus, a probability of

.9 was assigned to the discovery of the ignition even when a space is

attended only 1/3 of the time.

Although the above probabilities for discovering a fire are capable

of being refined, it is doubtful whether better information is available

at this time. Statistics for this event normally are not recorded because

fires that do not progress beyond the incipient stage are so insignificant

that they are not likely to be reported or even remembered.

In a manner similar to that just described for the occurrence of an

incipient fire, the probabilities for other events shown in Figure 7 were

estimated. Each estimate was derived by Gage-Babcock & Associates,. Inc.

from available statistical or test data or from the experience and

54
judgment of fire protection engineers at .Gage-Babcock. Table 4 summarizes

the results of this effort.

For each probability shown in Table 4, it was necessary to visualize

events at particular stages of fire development so that a valid estimate

of the probability of success or failure could be made. As an example,

automatic sprinklers have a statistical success rate in excess of 96%;

success is defined as the sprinklers operating to extinguish or control a

developed fire. However, in practice there is a very low probability that

a sprinkler will operate while a fire is still in the incipient stage.

Accordingly, it is estimated that there is only a 20% chance that sprinklers will operate and control a fire while the fire still is small enough

to be extinguished with a hand-held fire extinguisher. Beyond this stage,

the probability that the sprinklers will operate approaches rapidly the

96% figure usually cited. These operating characteristics of sprinklers

are well known and have been reflected in the probability values selected

for Figure 7.

Undoubtedly, the probabilities of failure or success assigned to

events in Table 4 can be challenged. However, it is anticipated that differences of opinion will be relatively small, and even large differences

of opinion will produce only minor effects on final results. One example

of a typical office occupancy was calculated where the probability assigned to successful suppression of a small fire with hand extinguishers was

changed from .95 to .50. Although the assumed probability of success for

this event was cut almost in half, the probability of total room involvement changed by only .00001%. To two significant places, the probability

for total room involvement remained 1.5 x 10-5.

Although some may argue that it would be better to abandon the use of

a probabilistic analysis until better statistical fire data are available,

such an argument ignores two facts. By estimating probabilities on the

basis of the best information available and by applying probabilities in a

systematic manner, the practice of individual analysts making unsupported

or irrational subjective judgments regarding fire protection requirements

is avoided. This represents significant progress in the area of

5.5
performing fire-hazards analyses. The critical-path diagram shown in

Figure 7 and the probability figures presented in Table 4 constitute a

simplified format for accomplishing a probabilistic analysis.

TABLE 4

Probability Values Corresponding to Figure 7 Events*

Values for Event D, Probability of Incipient Fire

Event B, Combustion Succeeds

Event C2 , Combustion Suppressed B C2

Fuel always available and very easy to ignite 1.0 .5

Fuel always available and easy to ignite .1 .9

Fuel always available but *difficult to ignite .01 .99

Fuel transient and easy to ignite .01 .99

Event C1 , Combustion Discovered C1

Space attended at all times .99

Space attended most of the time .95

Space attended 1/3 of the time .90

Space seldom attended .0

Values for Event H, Probability of Meaningful Fire

Event El, Fire is Discovered El

Space attended at all times .90

Space attended most of the time .20

Space attended 1/3 of the time .01

Space seldom attended .00

Event E 2 , Fire is Automatically Detected E2

Early warning fire detection, thorough

coverage of room/space .90

Early warning fire detection, minimal

coverage of room/space .45

Rate-of-rise heat detection, standard coverage .40

Fixed-temperature heat detection (including

auto. sprinklers), standard coverage .20

*The numerical values shown in Table 4 and Figures 8 and 9

by Gage-Babcock & Associates, Inc. (Bert Cohn).

were provided

56
TABLE 4 (Contd)

E3 .E4

Events E3 and E4 If Fire If Fire is Automatically

Fire Manually Suppressed Is Detected and Response

Discov- Normally is Within --

ered 1 min 3 min 10 min >10 min

1. Standard installation of fire extinguishers suitable for the hazard .95 .95 .90 .20 .05

and personnel trained in their use

2. As 1., but substandard coverage .85 .85 .80 .20 .05

3. Improper fire extinguisher type:

Class B or C ext for Class A fire .80 .80 .70 .10 .02

Class A ext for Class B fire .50 .50 .40 .10 .02

Class A ext for Class C fire .80 .80 .70 .20 .05

4. No.training in extinguisher use .50 .50 .45 .15 .04

Event F, Fuel Available

Fuel available throughout the space .98

Fuel available in much of the space .80

Fuel available in half of the space .50

Fuel available in some of the space .10

Transient fuel only .05

Event C, Automatic Fire Suppression Fails

Standard Automatic Fire Extinguishing System Installed -

Thorough coverage, early-warning actuation .11

Thorough coverage, rate-of-rise actuation .21

Thorough coverage, fixed-temperature actuation .61

Area coverage, early-warning actuation .56

Area coverage, rate-of-rise actuation .61

Area coverage, fixed-temperature actuation .81

Substandard System or Installation .90

No Automatic System 1.00

57
TABLE 4 (Contd)

Values for Event M, Probability of Total Room Involvement

Event Ii, Fire Discovered

Space attended at all times

Space attended most of the time

Space attended 1/3 of the time

Space seldom attended

I1

.90

.20

.01

.00

Event 12, Fire Automatically Detected

Use .99 for any standard installation of automatic

Fire detectors or sprinklers (with waterflow alarm).

Use lower value for installation not conforming to standards.

13 14

Events 13 and 14 If Fire If Fire is Automatically

Fire Manually Suppressed Is Detected and Response

Discov- Normally is Within --

ered 1 min 3 min 10 min >10 min

1. Standard installation of fire extinguishers suitable for the hazard .95 .95 .90 .20 .05

and personnel trained in their use

2. As I., but substandard coverage .85 .85 .80 .20 .05

3. Improper fire extinguisher type:

Class B or C ext for Class A fire .80 .80 .70 .10 .02

Class A ext for Class B fire .50 .50 .40 .10 .02

Class A ext for Class C fire .80 .80 .70 .20 .05

4. No training in extinguisher use .50 .50 .45 15 .04

Event J, Fuel Continuity Favorable

Va lues of (Average Fire Load)3/2

Room Height

Ceiling Flame Over 1, Over 2, Over 5, Over 10,

Spread Rating 1 or Less Up to 2 Up to 5 Up to 10 Up to 20 Over 20

25 or Less .001 .05 .10 .25 .50 .90

26 to 75 .01 .10 .25 .50 .90 .99

76 to 200 .05 .25 .50 .50 .99 .999

201 to 400 .10 .50 .90 .99 .999 .999

Over 400 .25 .90 .99 .999 .999 .999

58
TABLE 4 (Contd)

Event K, Oxygen Available

Area of Potential Air-Intake Openings

(Windows Doors, Ducts, Louvers, Etc)

As Percentage of Total Wall and Ceiling Area

Under 3% 3% to 10% Over 10%

Average Fire

Load (PSF)

Under 7

7 to 15

16 to 30

Over 30

.95

.90

.50

.20

.99

.95

.90

.50

1.00

.99

.95

.90

Event L, Automatic Suppression Fails

(See Figure 8)

Event N, Closure Element Fails

Fire Resistance of Closure Element N

None: no closure or combustible closure

Noncombustible but nonfireresistant

(glass, aluminum)

Noncombustible and fireresistant

(wired glass, steel)

10-min. fire-resistance rating

20-min. fire-resistance rating

30-min. or greater fire-resistance rating

1.00

.99

.97

.70

.40

.10

NOTES:

1. A "closure element" is a portion of the enclosing

walls, floor, or ceiling (or floor-ceiling

assembly), and includes doors, dampers, windows,

cable penetration seals, etc.

2. Multiply N value by 0.2 if there are no

(combustible) materials normally on the

proximity to the closure element.

ignitable

outside in

3. Multiply N value by 0.1 if closure element is

inside a steel duct.

59
TABLE 4 (Contd)

Event RI, Fire Discovered

If Fire Room or Adjoining Spaces are -

Attended at all times

Attended most of the time

Attended 1/3 of the time

Seldom attended

.999

.98

.80

.50

Event R2 , Fire Automatically Detected

Area automatic fire detection or automatic sprinklers

in space communicating with fire room: .80

Instrumentation which would give overheat or other trouble

signals likely to be interpreted as a fire signal: .40

None of the above: .00

Event R3 , Fire Manually Suppressed

'Standard' Fire Department or Equivalent Fire Brigade Response -

Within 3 min. .50

Within 10 min. .40

In more than 10 min. .30

Fixed water extinguishing system installed in space under

consideration; manually controlled from outside immediate

fire area .70

Event S, Oxygen Available

Area of Potential Air-Intake Openings

(Windows Doors. Ducts, Louvers Etc)

Average Fire As Percentage of Total Wall and Ceiling Area

Load (PSF) Under 3% 3% to 10% Over 10%

Under 7 .95 .99 1.00

7 to 15 .90 .95 .99

16 to 30 .50 .90 .95

Over 30 .20 .50 .90

Event T, Fire Barrier "Wear Out" Failure

(See Figure 9)

60
I I I I I I I I I I I I i I

Standard installations of automatic firesuppression sstems in spaces housing the

class of combustibles indicated

Curse .\ wet-pipe automatic sprinklers

(Class X and iigh-flashpoint Class 13),

foam (Class A and B), carbon dioxide,

7 balon, dry chemical (Class B).

Cuise B dry-pipe or preactlion sprinklers

(Class 1, bigh-flashpoint Class B), halon

6 (Class A).

Curse C wet-pipe sprinklers (low flashpoint V,

Class B), carbon dioxide, dry chemical

"51 (Class N).

!Z Cure D) dr*.'-pipe or preaction sprinklers

(low-flashpoint Class B)

~40 N

3'c

30-

20

I 

I I I

001 002.005 01 02 0 9 10 .20 30 40 50 60 .70 80 .90 95 08 .W) 99 919 A] \X

lIT 1I,

Figure 8. Event L, Automatic Fire Suppression Fails

80 I'ire Rtesistan~e Ratings

Ptr \STNI EI11 for \%alls and

,'loor-Ceiling \ssemblies

Per \9 I'l7 L152 lor Doors, Daniptrs.

70-

60

* 50

40

310

.001.002 005.01 02 05 10 .20 30 40 .50 .60 .70 80 .90 95 98 99 WlIS.n'g \I \I

Nil\ T

Figure 9. Event T, Fire Barrier "Wearout" Failure

61
Qualitative Fire Scenario Analysis Method -- Figure 5 suggests that a

subjective analysis should be tried only after efforts to demonstrate fire

safety by deterministic or probabilist~ic methods have failed. This is

because subjective analyses are not easily shown as technically sound and

conservative. Consequently, we concluded that, whenever subjective

analyses are used, a predefined analysis outline for postulating fire

scenarios should be followed on a case-by-case basis.

Although following an analysis outline and postulating fire scenarios

does not guarantee that an analyst will be technically correct and conservative, the approach does assure that each analysis' is pursued in the same

manner and that each analysis decision is defended and available for

scrutiny. In addition, by following an analysis format which has been

developed through the consensus of a number of fire protection authorities, there is reasonable assurance that decisions will be based on the

most appropriate and best available fire protection codes and standards.

Beyond this, however, little can be said to support the tech-nical merit

of the qualitative fire scenario analysis method.

In the preceding pages the technical merit of three candidate firehazards analysis techniques have been considered: (1) fuel load and

ventilation barrier analysis, (2) modified decision-tree (critical-path)

analysis, and (3) qualitative fire-scenario analysis. From the standpoint

of technical merit, it was shown that the barrier analysis is preferable

for nuclear power plant use. Despite this, there are a-number of situations where a barrier analysis is inadequate and where a critical-path

analysis or even a qualitative fire-scenario analysis must be used. For

example, redundant safety equipment may occupy the same safety area and

thereby preclude the use of barrier analysis for a flashover fire in the

entire room. In this case, a critical-path analysis would be needed.

Simplifying the Candidate Analysis Techniques

For any fire-hazards analysis technique to be worthwhile, it must be

usable by those individuals responsible for designing and regulating the

installation of fire protection systems. If an analysis method is too

62
complicated, mistakes will be made in its application and interpretation.

It would be unreasonable to require an analyst to solve a set of unsteadystate heat balance equations for each safety area of a nuclear power plant

or to require an analyst to develop probability values for each criticalpath analysis. Accordingly, it was decided as part of this study that the

three chosen analysis techniques should be reduced to more simplified

formats. The results of this effort are presented in this section.

Simplified Fuel Load and Ventilation Barrier Analysis -- Equations

for those physical parameters of fire phenomena which govern the severity

of fire under conditions of full room fire involvement have been presented. When solved, the equations predict the time-temperature history of a

fire as a function of three variables--fuel load, ventilation rate, and

room size. Of course, to reduce the total number of variables to three,

other parameters (such as heat capacities, emissivities, thermal conductivities, and densities) must be defined. This can be done with relative

ease, however, by applying conservative values of published data which

most closely approximate nuclear power plant conditions and by noting that

the actual values selected for some parameters are often of little

consequence to the problem results. For example, flame emissivity values

can range from 0.3 to 0.9 with only a 20°C difference in the resulting

fire-gas temperature; wall heat capacity values can vary by a factor of 4

with virtually no difference in fire-gas temperature after 1 h. 2 7 On this

basis, the physical parameters listed in Table 5 were selected to help

calculate time-temperature histories for various nuclear power plant fuel

loads, ventilation rates, and room sizes. Figures 10 and 11 show typical

resulting time-temperature curves which were calculated on the basis of

the assumptions listed in Section 3.

63
Table 5

Selected Physical Parameters

WALLS (Concrete):

Density

Heat Capacity

Thickness

Emissivity

Convective Transfer Coefficient

Thermal Conductivity

FUEL (Cable Insulation):

Net Combustion Heat

Surface Pyrolysis Limit

Fraction of Pyrolysates

That Burn

Stoichiometric

Fuel-to-Air Ratio

PW =

Cpw =

1920 kg/m 3 (Ref. 31)

1052 J/kg-K (Ref. 31)

0.202 m

e w = 0.65

U = 5.0 (Tf - Tw )i/ 3 j/s-m2-K

(Refs 28 aný 30)

Iw 1.-878 J/s-m-K @ 273 K

0.609 J/s-m-K @ 1473 K

(assumed linearly variable

-Ref. 31)

Hc = 25.9 x 106 J/kg fuel burned

(Ref. 3)

RP =kg Fuel (Ref. 3)

600 s

- 0.7 (Ref. 27)

14.8 kg fuel (assuming complete

kg air combustion of

polyethylene to

CO2 plus H20)

AIR AND FIRE GASES:

Emissivity

Heat Capacities

f = 0.9 (assuming a smoky fire)

CPa = functions of temperatures as

shown in Refs. 30 and 32.

64
= 25.4 kT/s air

= 1493 m-

= 9615 kg

'=" 700

= 600

4j

500

• 400

Figure 10.

Typical Time-Temperature

Response Curves for

Different Ventilation Rates

0 30 60 90 120 150 180

Time (min)

Figure 11.

Typical Time-Temperature

Response Curves for

Different Fuel Loads CO,

14

04

Ventilation = 25.4 k /s air

Heat Loss Area = 1493 mFuel Load = 9615 kg

Ventilation = 25.4 kg/s air

Heat Loss Area = 1493 m2

",, Fuel Load = 3490 kg

Time (min)

65
In reviewing Table 5 and Figures 10 and 11, several points should be

noted.

" The combustible used for the example calculations was

cable insulation. This selection covers the bulk of

combustibles found in nuclear power plant safety areas,

when supplemented by a separate treatment of fuel oil

combustibles.

" Cable properties and surface-controlled burn rate limits

in Table 5 were derived from cable burn test data measured at Sandia Laboratories. 3 This information establishes an upper burn rate limit for conditions of open

burning with unrestricted air availability. However, a

straightforward calculation can show that, on the basis

of stoichiometric air requirements alone, surfacecontrolled burning under conditions of total involvement

will not occur in most nuclear power plant areas because

of airflow restrictions. Such air restrictions cause

total-involvement burning to occur at a rate less than

the surface-controlled limit observed during open burning. Viewed another way, one may assume that a portion

of a room is burning at its surface burn limit instead

of the total room burning at an air-restricted limit.

From the standpoint of calculating fire severity timetemperature histories, either viewpoint is equally valid

but, since ventilation phenomena are more easily treated

mathematically, it is convenient to treat room-burn

phenomena as ventilation-controlled total involvement.

* Figures 10 and 11 show how changes in fuel load, ventilation rate, and room size affect the resultant timetemperature history. This is understandable, considering how each variable enters into the governing heat

balaiice equations, Eqs. (2) through (5). To simplify

these results, it was decided to eliminate one variable

66
by normalizing both the fuel load and ventilation rate

with respect to room size. Practically, this means that

fuel loads and ventilation should be determined in terms

of the heat-loss area of a room and not the floor area

of a room.

Although time-temperature histories can be calculated for various

combinations of normalized fuel load and ventilation, a method is needed

to relate the normalized variables in a simplified manner that does not

require a computer for solutions by designers and regulators. Such a

method should identify what numerical values of each variable are satisfactory for design purposes.

Since a basic premise of the fuel load and ventilation barrier

analysis requires that barriers be adequate under conditions of a worstca'se fire, the criteria for design should be

" A barrier shall not be exposed during a fire to a higher

temperature than the one to which it was originally

tested.

" A barrier should not be exposed to a fire longer than

the time for which it was originally tested.

In other words, a time-temperature history for a room calculated with

the conservative heat balance Eqs. (2) through (5) should never exceed the

temperature or test duration of the standard time-temperature curve specified in Reference 2 for barrier and barrier-element testing. For various

conditions of fuel load and ventilation, the above criteria can be applied

to ascertain which combinations are acceptable for a particular barrier

rating. Figure 12 shows the results of such an effort.

The diagonal lines in Figure 12 correspond to fire durations on the

basis of a selected duration and a stoichiometric air balance. For a

specified ventilation rate, increasing fuel load increases fire duration--

it takes longer to burn more fuel.

67
'40

4) Xo400- (0

350 NC. Duration-Limit

Curves

• 300.

0

, 250

a) "= 200-

LWTemperature-Limit .10

'4 Curve

100

0

-1 50 S0)

4405 10 15 20 25

Air Rate (kg/s - m2 of heat loss area) x 10-3

Figure 12. Acceptable Fuel Load and Ventilation Combinations

Assuming Heat Loss Into Wall Only

The curved line in Foigure 12 corresponds to combinations of fuel load

and ventilation rate whose time-temperature histories just reach but do

not exceed the standard time-temperature curve (see Figure 13). For a

specified fuel load, increasing ventilation rates cause the fire temperature to exceed the standard time-temperature test curve. If a nuclear

power plant safety area has a fuel load and ventilation rate combination

lying to the left of the temperature-limit curve and lying to the right of

the governing fire-duration line, the fire barriers surrounding the room

should withstand total-room fire involvement of all fuel within the room.

68
0 bUU . Calculated LypLcal

'• 700 Curve

: 600

s 500

S400

300

200

100

1 2 3 4- 5

Time (h)

Figure 13. Typical Temperature Limitation for

Calculated Time/Temperature Curves

As was pointed out in Section 3, the equations used to develop

Figure 12 contain a number of conservative assumptions. One assumption of

major importance involved the limitation of heat loss to convection and

radiation to the walls of the room. If, through ventilation system

design, the removal of heat from a burning room can be assured, higher

combinations of fuel load and ventilation rate can be tolerated without

exceeding the standard time-temperature test curve. Figure 14 shows cases

where additional heat is removed by venting 50 and 100% of the hot gases

produced during a fire. The difference between Figures 12 and 14 is

striking.

In order to use Figures 12 and 14, a designer must be able to relate

a roomss actual fuel load and ventilation conditions to the coordinates of

the figures. For fuel load, this can be done by adding up the weight of

combustibles in a room and converting this to fuel combustion energy per

69
unit area of wall and ceiling. The room floor area is conservatively

excluded, because cool air entering the fire zone will tend to enter along

the floor; also the fuel itself will shield the floor from the hot gases

in the room.

SDuration-LIimit

, 900 Curves

-• 800 00% Hot Gas

0) Removal

M 700 Temperature1Limit Curve CD

o 600

w 500

450% Hot Gas Removal

o 400 Temperature-Limit.

Curve

300

200

0

1-1 100-0mi

10 20 30 40 50 60 70

Air Rate (kg/s - m2 of heat loss .area) x 10-3

Acceptable Fuel Load and Ventilation Combinations

Assuming Heat Loss Into Walls and Either 50% or 100% of

Hot Gases Removed

Figure 14.

For ventilation rates, three situations exist:

I. If a room has only forced ventilation with either

sealed or small openings, the ventilation system flow

rate can be used directly in Figures 12 and 14.

2. If a room has only fire-induced natural ventilation

through door or vent openings, Eq. (5) in Section 3 can

be used. Unfortunately, since the density terms in

Eq. (5) are functions of temperature and since the

70
variation of temperature in a burning room requires

solution of the other equations in Section 3, the use

of Eq. (5) for design purposes is unattractive. An

alternate approach derived from Eq. (5) is presented in

Figure 15.

14

12

0

8

6

4-

0

Figure 15.

A (m2)

Airflow Rate Induced by a Fire in a Room

Having an Opening of Height ho and Area Ao

This figure, relating fire-induced ventilation rates to

opening size, was calculated by conservatively noting

that the density square root term in Eq. (5) has a

maximum value of 0.2144 for air entering at a temperature of 200C. 2 7 Recognizing this, Eq. (5) can be

reduced to

Mair = 0.522 Ao fo kg/s

71
This equation was used to produce Figure 15.

3. If a room has both fire-induced natural ventilation

through openings and forced ventilation, it is theoretically possible to account for the interrelationship

between these two effects by recognizing that both

natural and forced ventilation are related to the

static pressure of gases within a room. For natural

ventilation, which results from a temperature-induced

buoyancy gradiant between the inside and outside of a

burning room, an increase in room static pressure will

tend to reduce the flow of induced draft air entering

the room. Similarly, an increase in room static

pressure will tend to reduce the flow of forced draft

air being blown into the room by a ventilation fan.

Since a balance must be reached between the room

static pressure and the flow of air and hot gases into

and out of the room during a fire, it is possible to

relate these phenomena through fluid dynamic and material balance equations for specified fan characteristics and opening sizes. The Appendix has done this for

an example case involving a room with one door open and

-a forced ventilation fan in operation. Using the

relationships in the Appendix, a designer may use known

ventilation system and fan characteristics to relate

ventilation flow to the static pressure in a room.

Once this static pressure is found, corresponding fireinduced flow can be calculated and the sum of the two

flows may be used in Figures 12 or 14 to determine the

expected fire severity.

In practice, however, a conservative and simpler

approach involves the direct addition of forced and

fire induced airflows by assuming that the flows do not

interact. As described in the Appendix, the flows of

72
both forced ventilation and fire induced ventilation

will tend to reduce because of their combined effects

on room static pressure during a fire. By assuming no

interaction between the two flow mechanisms, the resulting flow obtained by addition will be conservative.

By using either Figure 12 or 14 together with a knowledge of a safety

area's wall and ceiling area, fuel load, and ventilation characteristics,

a designer or regulator can determine whether fire barriers alone will

contain a total-involvement fire within the area in question. Application

of this technique to a power plant area is the first example in Section 4.

Simplified Critical-Path Analysis -- Earlier in this section Figure 7

showed a critical-path diagram describing the logic of fire initiation,

growth, and extinguishmuent. At each logic juncture, there exists a

probability that certain subsequent fire events will occur, and Table 4

proposed a number of probabilities for use with the critical-path diagram.

To further simplify the critical-path analysis method, it was decided

to develop the analysis worksheet shown in Table 6. This worksheet combines event probabilities according to the critical-path analysis logic in

a manner which can be easily applied in combination with the probabilities

presented in Table 4.

For purposes of a nuclear power plant fire-hazards analysis, the use

of Table 6 by analysts can be made most consistent by precalculating representative probability values for a number of similar power plant areas.

Because of its overwhelming importance to the analysis results, the probability of Event H (occurrence of a meaningful fire) was chosen to be

precalculated for a number of plant safety areas. To limit the analysis

variation possible, several assumptions were made to group similar design

conditions into common categories.

73
TABLE 6

Critical Path Analysis Work Sheet

I. PROBABILITY OF INCIPIENT FIRE (Event D)

D : B(l - CIC 2 )

D = [ ](I - [ ][ ]) =

Ii. PROBABILITY OF MEANINGFUL FIRE (Event H)

E = (1 - E1 E 3 )(1 - E2 E4 )

E = (1 - [ ]M ])(I - 1 IL I) =

H = D-E.F.G

H = [ ][ I[ IL I = : _

III. PROBABILITY OF TOTAL ROOM INVOLVEMENT (Event M)

I : (1 - 1113)(I - 1214)

I = (1 - [ ][ ])(H - [ I[ ]) =

Fire Load/Height Ratio [Average Fire Load] 3 /2.

Room Height

[ lbs/ft 2 ] 3 / 2

[ ft]

M = H.I.J-KL

M I[ L L I H =

IV. PROBABILITY OF FIRE COMMUNICATING TO ADJOINING SPACE (Events Q and U)

Q (before Event M); U (after Event M)

Q = H.N if significant fuel source normally is present in the fire

room directly under or adjoining the closure element.

Q = M-N otherwise.

Multiply N by factor as given in Notes.

Q I H I =

R = (0 - RiR3 )(1 - R2 R3 )

R = (I - L IL ])(l - H I I) 1)

U = M.R.S.T Multiply by 0.2 if there are no ignitable (combustible)

materials normally on the outside in proximity to the room enclosure.

Otherwise, use 1.0.

U = [ I[ I[ IL ][I.0 or 0.2]

74
It was assumed that typical values for H probabilities could be

obtained by limiting the probability entries for Events B through G to not

more than three sets of conditions. In addition, some of the event

probabilities for Events B through G can be combined to yield more generic

categories of events for which probabilities can be assigned. Proceeding

in this manner, the following categories and events were determined:

Combustibility: Low

Moderate

High

Probability

.001 (BXF)

.01

.1

.90 (E 1 ) .99 (CI)

.05 .90

.00 ..00

Room Attended:

Response Time (when

room is unattended):

Automatic Detection:

Combustion Suppressed:

At all times

Over 1/4 of time

1/4 or less of time

Under 3 minutes

3 to 10 minutes

Over 10 minutes

.90

.20

.05

(E 3 & E4)

Early-warning type

Heat-actuated type

None

Fuel available,

easy to ignite

Fuel available, difficult to ignite

Fuel transient

.45 (E2 )

.20

.00

.90 (C2 )

.99

.99

.80 (G)

.40

Automatic Suppression: Area sprinklers

Special hazard

suppression system

In arriving at the above probabilities, it was assumed that in every

nuclear power plant being considered, portable fire extinguishers of the

proper type and size will be distributed throughout the plant in the

required numbers and that personnel will receive training in their use.

It was also assumed that any automatic fire-detection or fireextinguishing system installed in the areas under consideration will

conform to recognized standards.

75
Using the above event probabilities, Table 7 was prepared by following the logical relationship for Event H shown in Table 6. By selecting

the occupancy most representative of a particular safety area being analyzed, a designer can determine directly the probability that a meaningful

fire will occur (Event H) and then proceed with a determination of subsequent fire events (e.g., fire manually suppressed, etc) on a case-by-case

basis. Application of this method is the second power plant area example

in Section 4.

Simplified Qualitative Fire Scenario Analysis Method -- In Section 3

we stated that, even though subjective analyses are not readily defended

in terms of conservatism or technical merit, it is often necessary to conduct a subjective analysis whenever deterministic and probabilistic

approaches prove inappropriate. In those instances where a subjective

analysis must be followed, we concluded that a qualitative fire scenario

would be the best candidate if it is based on either NFPA 802, "Recommended Fire Protection Practice for Nuclear Reactors," or the March 1978 draft

proposed appendix to the American National Standard "Generic Requirements

for Light Water Nuclear Power Plant Fire Protection," ANSI/ANS-59.4-1977.

For outlining the many diverse factors affecting satisfactory fire protection, both the NFPA 802 and the ANSI/ANS documents are reasonably complete. However, neither guideline helps an analyst to develop the underlying reasoning needed for a scenario analysis.

One way to overcome this shortcoming would be through the use of a

fire scenario logic chart. Such a chart could lead an analyst from beginning to end of a fire in a step-by-step sequence. It was found that, of

the logic sequences available, the one least complicated for a subjective

analysis has been described already in Section 3 as part of the criticalpath probabilistic analysis (Figure 7).

76
TABLE 7

Probability Values for Event H, Occurrence of Meaningful Fire

PrbhAhb Jry of Eint H

1 2 3 4 5 6

No Auto. Heat Det. Early Warn.

Typical Occupancies Combustibility Attended Response Protection or Spklrs. Fire Det.

Low All Times Under 3 min 3.8 x 10-6 3.1 x 10-6 2.2 x 10-5

Control Room, Small

(Constant Surveil lance) Mod 1.2 x 10-4 1.0 x 10- 4 .7 x 10-4

Control Room, Parts

Not Under Surveillance

Offices

Supply Rooms

Machinery, Elec.

Equipment Rooms

Storeroams, Cable

Spreading Rooms

Paint, Oil Storage

High 2.1 x 10-3 1.7 x 10-3 1.2 x 10-3

Low Over 1/4 Under 3 min 1.0 x 10-4 .85 x 10-4 .62 x 10-4

of time

Mod .0057 .0047 .0034

High .018 .015 .011

Low Over 1/4 3-10 min 1.1 x 10-4 1.0 x 10-4 1.0 x 10-4

of time

Mod .0059 .0057 .0054

High .019 .018 .017

Low Over 1/4 Over 10 min 1.1 x 10-4 1.1 x 10-4 1.1 x 10-4

of time

Mod .0060 .0060 .0058

High .019 .019 .019

Low 1/4 or less Under 3 min .0010 .00082 .00060

of time

Mod .010 .0082 .0060

High .10 .082 .060

Low 1/4 or less 3-10 min .0010 .000%, .00091

of time

Mod .010 .0096 .0091

High .10 .096 .091

Low 1/4 or less Over 10 min .0010 .00099 - .00098

of time

Mod .010 .0099 .0098

High .10 .099 .098

Multiply probability of event H by 0.8 if space is protected with a normal

automatic sprinkler installation; multiply by 0.4 if hazards are protected

with a special hazard system using fast-acting fire detection (smoke, flame,

or rate-of-rise). Multiply by 0.32 if both. -4
By qualitatively following Figure 7, an analyst would be forced to

consider.a number of logical fire event outcomes and, at each logic juncture, the analyst would have to defend subjectively any assumptions

affecting subsequent fire progress. In this way, each fire protection

decision would be derived from a consistent thought process which qualitatively considers the most likely sequence for a fire's development. By

combining the scenario logic for specific fire events with the generic

analysis outlines provided in NFPA 802 and the ANSI/ANS 59.4 document,

subjective analyses could be performed in a repeatable manner that

provides a documented basis for design and regulatory decisions. The

application of this method is the third example situation outlined in

Section 4.

4. Example Fire Hazards Analyses

Because in this study we have tried to develop a fire-hazards analysis that can be carried out in a straightforward fashion, it is important

to demonstrate that the three analysis methods selected can be easily

executed. To do this, examples of fire-hazards analyses of three typical

nuclear power plant areas have been completed using the analysis

techniques described in the last section.

Example Fuel Load and Ventilation Barrier Analysis

For this example, a cable spreading room adjacent to a control room

will be considered. Since the fuel load and ventilation barrier analysis

stipulates limiting conditions of total room involvement, this spreading

room must be one of two redundant spreading rooms serving the control

room. In this analysis, it will be assumed that the adequacy of a fire

barrier between the spreading room and the control room is to be evaluated. Figure 16 illustrates the situation.

78
Redundant Spreading Room1

P111111171171111111117711 111

Control Room

Fire Barrier

.- Being

Evaluated i

Fan Example Spreading Room

DoorC )pening 1 • I

Figure 16. Layout for Fuel Load/Ventilation

Barrier Analysis Example

The following parameters are selected as typical:

Spreading Room Size = 35 x 25 x 5 m

Total Fuel = 10 000 kg insulation

Net Combustion Heat = 25.9 x 106 J/kg

Size of One Open Door = 1 x 2.5 m

Normal Forced Ventilation = 4.3 kg/s at 00 C and 1 atm

From these parameters, the heat loss area is calculated as 1475 m2

and, from Figure 15, the fire-induced ventilation airflow for the selected

door opening is about 2.1 kg/s, yielding a combined airflow of 6.4 kg/s.

In terms of the units used in Figure 12, the normalized airflow is 4.34 x

10- 3 kg/s-m2 .

79
For the fuel, the normalized fuel load can be determined

10 000 kg x 25.9 x 10 -6- = 175 x 106 -•J

1475 m m

In terms of more familiar units, the airflow rate and fuel load in

this example convert to:

standard ft 3

Air Rate = 10 500 stin rain (SCFM)

Fuel Load = 26 000 Btu

ft floor area

From Figure 12, the above airflow rate and fuel load result in a fire

where the duration is about 5-1/2 h and where the temperature severity

never exceeds the standard time-temperature test curve.

Since a 5-1/2 h fire rating lies beyond the range of 1 to 3 h for

most commercially available barriers, one or more of four corrective

options should be invoked. The options are:

1. Reduce the fuel load to about 90 x 10 6 J/m2 (13 300

Btu/ft 2 ) floor area.

2. Insure that the fire is extinguished so that it burns

no longer than the selected barrier rating.

3. Increase the air rate to between 8.3 x 10-3 and 60 x

10-3 kg/s-m2 (20 000 and 145 000 SCFM) and

simultaneously guarantee removal of all hot gases from

the burning room (see Figure 14).

4. Increase the air rate to between 8.3 x 10-3 and 12 x

10-3 kg/s-m2 (20 000 and 29 000 SCFM) without

guaranteeing hot gas removal (Figure 12).

80
The selection of an appropriate, option depends upon the particular

case in question. For an existing power plant,- Option 2 is most attractive; but, for a newly designed plant, Options 1, 3, or 4 may be more

appropriate. In any event, the analysis technique has shown the conditions of fuel load and ventilation under which a barrier alone can be

expected to contain a fully developed fire. This information is of

significant value to both designers and regulators in determining what

additional fire protection measures, if any, are needed to supplement

passive fire barriers.

Example Critical-Path Analysis

For this example, a control room will be considered. By using the

worksheet (Table 6).derived from the critical-path logic diagram

(Figure 7), it is possible to proceed with the control room analysis in

one of two ways. The analyst may either start with Step I of the worksheet, "Probability of Incipient Fire," or the analyst may start with Step

III, "Probability of Total-Room Involvement." If it is decided to start

with Step I, event probability values from Table 4 must be combined to

calculate the probability of a meaningful fire (Event H) for use in subsequent worksheet calculations. If Step III is chosen as the starting

point, the probability of Event H may be selected from the precalculated H

values shown in Table 7. In this example, the results of both techniques

will be shown.

In order to proceed, a number of assumptions are necessary to sufficiently define the control room arrangements under consideration. These

assumptions are:

" The room is attended at all times and is small enough to

be observed throughout by operators in attendance.

" The room combustibles are spread throughout the room for

an average fuel load of 10 kg/m 2 floor area (2 lb/ft 2 ).

81
" The room is enclosed with fire-resistant construction

rated at 2 h'

" The room has area smoke detectors at the ceiling level.

" The room has an adequate number of fire extinguishers of

the proper type and personnel are trained in their use.

* The room interior finish is noncombustible.

" The plant has a fire brigade capable of responding to

the control room within 3 min.

" The room ceiling height is about 3.5 m (12 ft).

Using these assumptions, Table 8 was developed for the control room

by selecting appropriate probability values from Table 4 or Table 7. -From

Table 8 it can be seen that the probability of a meaningful fire in a

control room is about 10-4 or 1 chance in 10 000 that the contact of an

ignition source to moderately ignitable fuel will result in a meaningful

fire. As indicated in the table, this probability estimate could be

deduced from precalculated H values in Table 7 or from a recalculation of

H as shown in Table 8. From the standpoint of total control room fire

involvement, Table 8 indicates a probability of less than 10-9.

By comparing this result with the probabilities for total room fire

involvement in other plant areas, an analyst can determine, on a relative

basis, which areas are overly or inadequately protected. In addition, an

analyst may readily review Table 8 for each plant area to determine which

portions of fire safety systems (fire retardancy, barriers, suppression,

etc.) should be modified to give a consistent level of safety.

82
TABLE 8

Nuclear Power Plant Control Room

CPM Fire Hazard Analysis Work Sheet

I. PROBABILITY OF INCIPIENT FIRE (Event D)

D : B(0 - CIC 2 )

D = [ ,/ 1(1 - [ .qq][ *q ]) = .0109

II. PROBABILITY OF DEVELOPING FIRE (Event H)

E = (1 E1 E3 )(I - E2 E4 )

E = (1 - [.qO ][ [ .,/.'][ 44]) = .0•30.0')(1 

H = D.E.F.G r H =0.00072 from Table 7

for moderate corbustiH = [.oIE[.Od03][.0 [ /,10] =.00t0fe . bilitv control room

,I. occupancv.

III. PROBABILITY OF TOTAL ROOM INVOLVEMENT (Event M)

I (l - I113)(1 - 1214)

1 (1 [ .9o ][ .qr])(.l ][ ) -DOW .

Fire Load/Height Ratio = lAverage.Fire Load] 3 /2

Room Height

[ ' lbs/ft 2 ] 3 /2

[ /z- ft]

M = H.I.J-K.L

M = P.o0w][.03][ ,ol][ 01 ,4 ][ ,-,0 ] -- 4 /X-/0

IV. PROBABILITY OF FIRE COMMUNICATING TO ADJOINING SPACE (Events Q and U)

Q (before Event M); U (after Event M)

Q = H.N if significant fuel source normally is present in the fire

room directly under or adjoining the closure element.

Q =M.N otherwise.

Multiply N by factor as given in Notes.

Q = [.o'][ .97 ][ ,A ] = /.7x /o0

R = (I -RiR 3 )(I - R2 R3 )

R (1 (- [,4f7][.,4"O ])(l - [. ,e0 ][,4"o ]) =,o

U = M-.R-S.T M ultiply by 0.2 if there are no ignitable (combustible)

materials normally on the outside in proximity to the room enclosure.

Otherwise, use 1.0.

U .I0/1[ .,o][ 3 .%r ][,oo/][l.O or 0.2] 4 /0

83
As indicated in Section 1 of this report, the significance of fire

involvement probabilities cannot be determined on an absolute safety basis

without a knowledge of the importance of the affected safety area and

without a definition of what constitutes an acceptable level of fire risk.

These uncertainties, together with uncertainties regarding the selected

probability values for detection response, barrier effectiveness, and

oxygen depletion, preclude deriving any absolute fire safety conclusions

from Table 8.

Example Qualitative Fire Scenario Analysis

In the section titled "Simplifying the Candidate Analysis Techniques," we stated that an uncomplicated fire scenario analysis could be

carried out by combining the logic of the critical-path analysis

(Figure 7) with the generic analysis outlines provided in NFPA 802 and

ANSI/ANS 59.4. Since the generic analysis portion of this approach

primarily provides a' prosaic method for identifying potentially hazardous

areas and for organizing fire protection information associated with.these

areas, only an example of the qualitative critical-path analysis portion

will be discussed in this section. For an example of the generic analysis

outline, any of several current nuclear power plant safety analysis

reports may be consulted.

As explained earlier, a qualitative fire scenario analysis should be

invoked only after the conclusions of deterministic and probabilistic

analyses prove unacceptable or inconclusive. One example would be the

analysis of a fire within a cabinet containing two redundant electrical

circuits. By following the logic shown in Figure 7, a scenario analysis

describing the most likely progress of a cabinet fire can be performed so

that, at each juncture in the fire development, the required technical

basis for predicting subsequent fire progress is clearly stated. An

example of this is shown in Table 9.

It can be seen that the scenario analysis in Table 9 clearly indicates the need for additional technical basis in the form of testing.

Without this information, an analyst must conclude that a cabinet fire

84
involving non-IEEE 383 tested wiring may detrimentally affect more than

one redundant circuit within the same cabinet. Any other conclusion would

be based on conjecture without supporting merit.

TABLE 9

Example Qualitative Fire Scenario Analysis

of a Cabinet Fire

Event

Ignition Source

Contacts One

Redundant Circuit

incipient Fire

Likely Outcomes

No incipient fire occurs

if IEEE 383 wiring is

involved

Incipient fire occurs

if non-IEEE 383 wiring

is involved

Fire discovered and

manually suppressed

before redundant circuit

is damaged

Fire automatically suppressed before redundant

circuit is damaged

Meaningful fire occurs

in one circuit

Basis

The use of wiring

qualified to IEEE 383

(Ref. 1), requires

very large and prolonged ignition

sources to initiate

a fire (Ref. 3)

Testing must demonstrate this capability

Testing must demonstrate this capability

Without the testing

called for in the preceding statements this

outcome is unavoidable

Testing must demonstrate this from the

standpoint of heat and

corrosive combustion

products

Without the testing

called for above, this

outcome is unavoidable

Meaningful Fire

Occurs in One

Circuit

Fire is contained by

,separating barrier or•

distance within the

cabinet, so that the

redundant circuit

remains unaffected

Redundant circuit is

unacceptably degraded

by the fire

85
Although the results of this example scenario analysis are inconclusive, they clearly indicate design weaknesses requiring further scrutiny.

Such findings are of importance to both designers and regulators in determining obvious fire protection deficiencies.

5. Conclusions

A viable fire-hazards analysis methodology for nuclear power plant

application

" Should be derived from, but not necessarily duplicate,

available and proven techniques

" Should be defendable in terms of conservatism and

technical merit

" Should be easily used by both designers and regulators.

Applying these criteria, the authors reviewed a large number of candidateanalysis techniques, each of which has been used by other fire protection

analysts for various types of residential, commercial, industrial, and

nuclear plant fire hazards. It was found that all of the available analysis methods reviewed proved deficient in meeting at least one of the analysis criteria and, therefore, it was decided to select and combine from

available techniques those analysis attributes most responsive to the

needs of nuclear power plant designers and regulators.

In Section 3 we developed what appears to be the most suitable analysis method for nuclear power plants. The method can be summarized as

follows:

* First: A bounding deterministic evaluation is made

under limiting fuel load and ventilation conditions to

establish the adequacy of passive fire barriers.

86
" Second: If the results of the bounding deterministic

evaluation prove unacceptable for plant safety, a

probabilistic analysis is performed to assess the

effectiveness of supplementary fire protection measures

(e.g., fire suppression).

" Third: If the results of the probabilistic analysis

prove unacceptable or inconclusive, a subjective

analysis is carried out using a standard format and

logic sequence.

To apply this methodology, an analysis technique was developed, technically defended, and simplified in Section 3 for each of the above analysis segments. Section 4 then demonstrated the usefulness of the analysis

procedure.

On the basis of this study and a review of the fire-hazards analyses

performed to date for several nuclear facilities, it is concluded that

improvements can be made in most of the analysis techniques presently

used. These improvements are important in eliminating the lack of both

conservatism and technical merit inherent in mai traditional analysis

approaches.

The analysis methodology suggested in this report (Section 3) represents the first analysis procedure derived with the specific objective of

ensuring conservative, yet credible, analysis results for achieving

nuclear power plant fire safety. As a result, the objective of plant

safety has remained unclouded by the monetary incentives typically associated with many traditional.analyses performed for insurance purposes.

Within the state of the art, the analysis approach suggested in this

report has been shown as conservative and technically sound, while simultaneously being easily applied by designers and regulators.

87
6. Recommendations

As explained in the previous section, the most suitable fire-hazards

analysis methodology for nuclear power plants appears to be a sequential

technique made up of three segments termed deterministic, probabilistic,

and subjective. Although we concluded that this approach combines the

best attributes of several existing analyses for technical merit and ease

of application, a number of recommendations should be cited regarding the

limitations and potential areas of improvement of the overall analysis

sequence and of each analysis segment.

Overall Analysis Sequence

The fire-hazards analysis procedure presented in this report enables

an analyst to evaluate bounding fire conditions and to compare the relative effectiveness of various fire protection alternatives. The methodology does not provide guidance for determining what nuclear power plant

areas actually require fire protection or what levels of fire protection

are sufficient to meet overall plant safety objectives. Since, as explained in the introduction of this report, the answers to these questions lie

beyond the scope of this study, two recommendations are appropriate.

1. Before any of the fire-hazards analysis procedures

developed in Section 3 are applied to a nuclear power

plant area, an evaluation should be made to ascertain

the relative safety importance of the area being

analyzed. Preferably this evaluation should be based

on a systems analysis of the area in question. The

only areas or area combinations requiring a thorough

fire-hazards analysis are those whose systems are shown

to be (a) important to accomplishing a safe plant

shutdown, (b) functionally jeopardized by a fire, and

(c) backed up by no other system elsewhere.

2. Before any fire protection scheme is adopted, a minimum

fire safety objective should be established. One way

88
to do this would be through the designation of a maximum acceptable probability for fire risk.to public

safety, while another way would be through the use of

common-mode or single-failure criteria, similar to

those used for other postulated design-basis accidents

(i.e., redundant systems philosophy).

Fuel Load and Ventilation Barrier Analysis Segment

The fuel load and ventilation barrier analysis technique developed in

Section 3 was based on general principles of fire phenomena, up to the

point at which the technique was simplified for easier application. At

this stage, a number of assumptions were made to demonstrate the usefulness of the analysis methodology and, although the assumptions were chosen

to represent certain typical power plant conditions, some other situations

are recommended for examination.

1. Cable insulation was selected as the combustible for

the bounding barrier analysis. Other combustibles,,

such as diesel oil and lubricatingoil, should also be

considered.

2. Reinforced 8-in. concrete walls were chosen to represent the barriers for analysis. Other barriers, such

as block walls or'panels having different physical

properties, should also be analyzed.

Probabilistic Analysis Segment

In addition to the recommendation above for establishing an acceptable overall fire-risk probability objective, several recommendations for

carrying out the probabilistic analysis are appropriate.

1. Actual tests should be run to refine the probability

values for automatic detection system effectiveness.

This program should focus on defining what effect

89
nuclear power plant conditions have on detection

probabilities derived from nonnuclear applications.

2. The probability values associated with room flashover

(Event J - fuel continuity interrupted) should be modified to account for the rate of fire development. Even

at the same average fuel load, concentrated combustibles will promote flashover more quickly than dispersed

combustibles, because the rate of fire development is

often enhanced by concentrated fuel conditions.

3. The probability that a fire will penetrate a fire barrier depends on both the tested rating of the barrier

in comparison with actual fire conditions and the

probability that all portions of the barrier function

as tested. As part of the deterministic fuel load and

ventilation barrier analysis, the probability of barrier failure was taken as unity when a barrier is

exposed to a fire more severe than rated test conditions. However, no probability values have been

assigned to barrier failures which restart from random

failure of barrier elements such as doors, dampers, and

penetration seals. These failures will occur with a

finite probability even though an actual. fire severity

lies below the conditions used for testing.

Probabilities are needed to establish the chance for

this type of failure to determine what numbers and

types of openings degrade a "passive" fire barrier to a

point at which active fire-protection measures are more

reliable.

Subject to the recommendations outlined in this section, the firehazards analysis procedure .developed in this study appears to be the best

approach available for evaluating fire protection effectiveness in the

context of nuclear power plant safety.

90
References

I. IEEE Standard for Type Test of Class IE Electrical Cables, Field

Splices, and Connections for Nuclear Power Generating Stations, Std.

383-1974.

2. Standard Methods of Fire Tests of Building Construction and

Materials, ASTM E 119-76.

3. Cable Tray Fire Tests, SAND77-1125C, Sandia Laboratories,

Albuquerque, NM, July 1977.

4. Fire Protection Handbook, Fourteenth Edition, National Protection

Association, Boston, MA, 1976.

5. E. S. Cheaney and A. J. Coyle, System Safety Analysis of a Commercial

Vessel, Battelle Columbus Laboratories ReportBATT-G-2955-0001, OH,

1977.

6. Clinch River Breeder Reactor Plant Preliminary Safety Analysis

Report.

7. Diablo Canyon 1 and 2 Nuclear Reactor Safety Analysis Report, Docket

Number 50275-377.

8. I. B. Asp, E. A. MacDougall, and R. E. Hall, Design Base Fires in

Nuclear Power Plants, Brookhaven Laboratories Report BNL-NUREG-23392,

Upton, NY, October 1977.

9. C. D. Coulbert, "Energy Release Criteria for Enclosure Fire Hazard

Analysis - Parts I and II," Fire Technology, Vol. 13, No. 3 (pp 173-

184) and No. 4 (pp 316-324), August and November 1977.

10. S. E. Magnusson, and S. Thelandersson, "A Discussion of Compartment

Fires," Fire Technology, Vol. 10, No. 3 (pp 228-246), August 1974.

11. R. S. Alger and S. J. Wiersma, Ship Fire'Characteristics Part 1 -

Sealed Compartments, Naval Surface Weapons Center Report NSWC/WOL TR

76-125, Dahlgren, VA, November 1976.

12. V. Babrauskas and R.. B. Williamson, Post-Flashover Compartment Fires,

University of California-Berkeley Report UCB FRG 75-1, Berkeley, CA,

December 1975.

13. A. F. Robertson and D. Gross, "Fire Load, Fire Severity, and Fire

Endurance," Fire Test Performance ASTM STP 464, American Society for

Testing and Materials (pp 3-29), 1970.

14. R. Baldwin and P. H. Thomas, "Passive and Active Fire ProtectionThe Optimum Combination," Fire Technology, Vol. 10, No. 2 (pp 140-

146), May 1974.

91
15. T. T. Lie, "Characteristic Temperature Curves for Various Fire

Severities," Fire Technology, Vol. 10, No. 4 (pp 315-326), November

1974.

16. T. Z. Harmathy, "Designers Option - Fire Resistance or Ventilation,"

6th C.I.B. Congress, Budapest, 1974.

17. T. E. Waterman, Scaling of Fire Conditions Supporting Room Flashover,

Illinois Institute of Technology Research Institute Report M 6170,

Chicago, IL, December 1967.

18. T. Z. Harmathy, "Design of Buildings for Fire Safety - Parts 1 and

2," Fire Technology, Vol. 12, No. 2 (pp 95-100) and No. 4 (pp 219-

236), May and August 1976.

19. P. H. Thomas, "Some Problem Aspects of Fully Developed Room Fires,"

Fire Standards and Safety ASTM STP 614, American Society for Testing

and Materials (pp 112-130), 1977.

20. T. Z. Harmathy, "A New Look at Compartment Fires - Parts 1 and 2,"

Fire Technology, Vol. 6, No. 3 (pp 196-217) and No. 4 (pp 326-351),

August and November 1972.

21. U. Schneider, "Behaviour of Concrete Under Thermal Steady State and

Non-Steady State Conditions," Fire and Materials, No. 1 (pp 103-115),

1976.

22. J. K. Green, "Some Aids to the Assessment of Fire Damage," Concrete,

Vol. 10, No. 1 (pp 14-17), 1973.

23. K. Kawagoe, Estimation of Fire Temperature - Time Curves in Rooms,

Building Research Institute Paper No. 29, Japan, October 1967.

24. H. E. Nelson, The Application of Systems Analysis to Building

Firesafety Design, General Services Administration Paper.

25. Clinch River Breeder Reactor Plant Risk Assessment Report, Appendix

10, Science Applications Report, San Francisco, CA.

26. Reactor Safety Study, U. S. Nuclear Regulatory Commission Report WASH1400/NUREG-75/014, Washington, D. C., October 1975.

27. V. Babrauskas, Fire Endurance in Buildings, University of California -

Berkeley Report UCB FRG 76-16, Berkeley, CA, November 1976.

28. H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, 2nd

Edition, Oxford Press, 1959.

29. Steam--Its Generation and Use, Babcock & Wilcox, New York, NY, 1975.

92
30. V. Babrauskas, COMPF - A Program for Calculating Post-Flashover Fire

Temperatures, University of California - Berkeley Report UCB FRG 75-

2, Berkeley, CA, January 1975.

31. E. E. Minor and D. L. Berry, Task 3 - Fire Barriers - Nuclear Power

Plant Fire Protection Subsystems, SAND78-1990, Sandia Laboratories,

Albuquerque, NM, (to be issued).

32. Chemical Engineers Handbook, 4th Edition, McGraw-Hill, New York, NY,

1963.

93

APPENDIX

Combined Induced Draft Plus Forced Ventilation

The flow through a room opening as the result of pressure within the

room can be expressed as

C dPIi 1A-I)2 

P1 - P2 - 2RT1 vl (see Figure A-i) ,(A-)

where

T, = the average temperature inside the room

P1 = the pressure of gases inside, the room averaged vertically over

the opening

P2 = the pressure of air outside the room averaged vertically over

the opening

W, = the molecular weight of gas inside the room

Cd = the opening discharge coefficient, usually around 0.7

R = the universal gas constant

v, = the average velocity of gases through the opening as shown in

Figure A-I.

It is possible to express P1 and P2 as

s Pi1Wlghl

P, = P 1 2RT g+ (A-2)

P = p + P2 W2 gh1 (A-3) 2 .. 2RT

95
where

s P1 the static pressure of gases at the top of the opening inside

the room

g = the acceleration of gravity

P2 = the static pressure of air at the top of the opening outside the room

W2 = the molecular weight of air outside the room

hl the vertical portion of opening through which ventilation system

flow is forced in combination with induced draft flow from the

room

T2= the temperature of air outside the room.

ThX

T1

Forced SVenting

v Component h1 1 +

Induced h

Draft 0

Component_

Induced

Draft h2

v 2 Component

Y AF I -q

Figure A-i. Combined Case of Induced Draft

Plus Forced Ventilation Air Supply

Substituting Eqs. (A-2) and (A-3) into (A-i) for P1 and P2:

s 2RT

I \2RT 1 - Wlghl/

S( 2RT 2

22RT 2- W2ii

= P~~(2RTRi I~h 1

C 2

2RT 1

96
Solving for vI:

( 1/2

2RT1 P 2RT Wlgh 1

I andCd Ps 1)W 2 2 V1 1--2RT l - W gh 1

For a given T2 , P 2 W 2 and Cd'

Vl fl(Tl, PI, h,, and WI)

where

f, = the functional relationship shown in Eq. (A-4).

(A-4)

In a similar way v 2 can be expressed as

CdP 2W2 2

P2 -P 2 2RT2 v 2

where

P P s

2 2

P, 1 I

(2RT 2 )

2RT2 -2W 2 ghI - W2 g (h 0 - hi)

2RTI - 2 WlghI- Wlg (h 0 - hl)

or

2RT Ps) Tl, 2 1 _( 1

2 C W Ps v IV-1

d 2 2 \ý2/

2RT- 2W2 ghI - W2 g

2RT1 - 2WighI- Wig

1/2

(h 0 h

1=-il)

(A-5)

97
For a given T2s P2, W2, ho, and Cd,

v 2 = f2 TV1 PS hi, and W)

where

f 2 is the functional relationship shown in Eq. (A-5).

If, on the basis of assumption 3 on page 47, the molecular weight of

the room gases is taken to be equal to the molecular weight of air, W1 can

be eliminated as a variable. Also, T1 can be eliminated as a variable for

design purposes by selecting a conservative average temperature

representative of the standard time-temperature test curve. As a result

of these assumptions,

vI = fIPis hI ,

v2 f 2 (P, hl)

To solve these equations involving four variables, two more relationships

are needed.

One relation can be derived from known characteristics of the ventilation system, relating the discharge backpressure of the ventilation system

fans to their flow characteristics. Such information is expressed usually

as pressure-flow curves supplied by fan manufacturers, or in mathematical

terms for the room shown in Figure A-I:

F = q(Ps + P1) 1

98
where

q = a functional relationship characteristic of a particular fan

Aps = the ventilation system pressure drop to the room

P1 = the pressure in the room

F = the volumetric flow rate of the fan at the discharge

temperature and pressure of the fan.

If it is noted that, for typical room pressure conditions and for reasonably insensitive fan performance, P1 can be taken as equal to PS, F may be

.expressed as

F q(AP + P) s 1 (A-6)

Unfortunately Eq. (A-6) has introduced another variable F, so that two

more equations are still needed.

One equation can be found by

report in terms of the pressures,

shown in Figure A-I. Assuming W1

1

v2 3 d 0a

rewriting Eq. (5) in the body of the

temperatures, heights, and velocities

equals W2 , the resulting equation is

\1/2

(A-7)

A final relation can be expressed as an overall material balance,

assuming W1 = W2 . This gives

Mass of Air Entering + Mass of Fuel Burning = Mass of Gases Leaving,

99
or

(r +1) [) j + 2h 2 h h,)o w P 2] v h 1Iw oP 1

where

TV = the ventilation system temperature

wo = the width of the opening.

Solving this equation for F:

Vh w P "2 Vho - hl)w~

F = 11o1 0 2_2__(-8 1 Ti (r + 1) T2 P s+ AP

Equations (A-4) through (A-8) can be solved simultaneously for Vl,

v2, F, PI, and hl. To effect a solution, Eq. (A-5) should be rearranged

as follows:

V 2dCW

2RT- Wgh) [I 2RT2 ] ( 2) - 2RT2 + W gh(

h=2 - ( 1, ) (A-9)

One procedure for solving these equations:

1. For a particular opening size (wo and h specified),

pick a value of P1 and solve Eq. (A-7) ?or v 2 .

2. Substitute v2 into Eq. (A-9) and solve for hI.

3. Substitute hI into Eq. (A-4) and solve for v1 .

4. Substitute hl, vl, and v2 into Eq. (A-8) and solve for

F.

100
5. Compare the calculated F to the F value obtained from

Eq. (A-6) for a particular AP, and the P1 assumed in

Step 1.

6. Assume a new P and repeat Steps 1 through 5 until the

F values calculated in Steps 4 and 5 agree.

By converting v 2 and F into mass flow rates of air, it is possible to

produce theoretical curves relating the mass flow of air from forced

ventilation and induced-draft ventilation to the static pressure in the

burning room. Once a designer, through trial-and-error solution, finds

values of ventilation Uystem flow rate and room static pressure which

satisfy the theoretical curves and the corresponding pressure/flow characteristics of the ventilation system fan, the resulting static pressure

can be used to find the induced draft flow. The forced and fire-induced

draft flows then can be added for use in Figures 12 and 14 to predict fire

severity.

Unfortunately, in most cases it would be impractical to use the theoretical curves relating static pressure to flow rates, primarily because

of the sensitive nature of the fire-induced draft airflow rates to slight

changes (approximately 1-Pa range) in static pressures. To compound this

problem, a designer seldom knows ventilation system pressure drops and fan-

-performance characteristics to-an acc-uracy-gTeat-er than-abe---50--P-a

As a result of these practical limitations, the only reasonable

approach available for conservative calculation of combined airflow rates

is to assume that forced draft and fire-induced draft air components can

be added without interaction. This approach is conservative, since a

forced-ventilation system blowing air into a room will tend to increase

the room's static pressure and thereby reduce the fire-induced draft

airflow. At the same time, changes in forced-draft flow as a result of

static pressure changes will be insignificant because of the overwhelming

effects of ventilation system pressure drop and fan performance.

101

NRC FORM 335 1. REPORT NUMBER (Asi5gned by DDC)

(7-77) U.S. NUCLEAR REGULATORY COMMISSION

BIBLIOGRAPHIC DATA SHEET NUREG/CR-0654

4. TITLE AND SUBTITLE (AddVolume No, ifappropnate) 2 (Leave blank)

Nuc1ear Power P1-ant-Fire-Protection - Fife Hazards

Analysis (Subsystems Study Task 4) 3 RECIPIENT'S ACCESSION NO

7. AUTHOR(S) 5 DATE REPORT COMPLETED

Dennis L. Berry, Earl E. Minor MONTH I YEAR

__September 1979

9 PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) DATE REPORT ISSUED

Sandia Laboratories MONTH 7YEAR

Albuquerque, New Mexico 87185 6 (Leave blank)

8 (Leave blank)

12 SPONSORING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) 10 PROJECT/TASK/WORK UNIT NO

Engineering Methodology Standards Branch

Off-ice of Standards Development

U. S. Nuclear Regulatory Commission Fin No. A-1080

Washington, D. C. 20555

13. TYPE OF REPORT PERIOD COVERED (inclusive da.tes)

Final Technical

15. SUPPLEMENTARY NOTES 14 (Leave blank)

16 ABSTRACT (200 words'or less)

This report examines the adequacy of existing fire-hazards analysis methodology in the

context of nuclear power plant safety. By combining and simplifying a number of available analysis techniques and by demonstrating the technical merit of each technique,

this report develops a conservative fire-hazards analysis method which can be easily

used by both designers and regulators*. As described, a suitable analysis method for

nuclear power plants should involve three phases. These are: (1) a deterministic

evaluation of passive fire barriers under limited fuel load and ventilation conditions;

(2) a probabilistic evaluation of supplementary fire protection measures (e.g.,

suppression systems); and (3) a subjective evaluation of unquantified fire hazard

conditions (e.g., cable arrangements). An example of each analysis phase is provided.

17 KEY WORDS AND DOCUMENT ANALYSIS 17a DESCRIPTORS

Fire

Fire Hazards Analysis

17b IDENTIFIERS/OPEN-ENDED TERMS

18 AVAILABILITY STATEMENT . 19 SECURITY CLASS (This report) 21 NO OF PAGES

Unclassified

20 SECURITY CLASS (Thispage) 22 PRICE

Unlimited Unclassified s

NRC FORM 335 (7-77)

NRC FORM 335 1. REPORT NUMBER Assigned by DDC)

(7.77) U.S. NUCLEAR REGULATORY COMMISSION1

BIBLIOGRAPHIC DATA SHEET NUREG/CR-0654

4 TITLE AND SUBTITLE (Add Volume No., if appropriate) 2 (Leave blank)

Nuclear Power Plant Fire Protection - Fire Hazards

Analysis (Subsystems Study Task 4) 3 RECIPIENT'S ACCESSION NO

7 AUTHOR(S) 5 DATE REPORT COMPLETED

Dennis L. Berry, Earl E. Minor MONTH [YEAR

September 1979

9. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) DATE REPORT ISSUED

Sandia Laboratories MONTH 19YEAR

NewMexco 718 6 ePtamber 1979 Albuquerque, New Mexico 87185 (Leave blank)

8 (Leave blank)

12 SPONSORING ORGANIZATION NAME AND MAILING ADDRESS (include Zip Code;

10 PROJECTITASKIWORK UNIT NO

Engineering Methodology Standards Branch

Office of Standards Development 11 CONTRACT NO

U. S. Nuclear Regulatory Commission Fin No. A-1080

Washington, D. C. 20555

13 TYPE OF REPORT PERIOD COVERED (Inclusive dates)

Final Technical

15. SUPPLEMENTARY NOTES 14 (Leave blank)

16 ABSTRACT P200 words or less)

This report examines the adequacy of existing fire-hazards analysis methodology in the

context of nuclear power plant safety. By combining and simplifying a number of available analysis techniques and by demonstrating the technical merit of each technique,

this report develops a conservative fire-hazards analysis method which can be easily

used by both designers and regulators. As described, a suitable analysis method for

nuclear power plants should involve three phases. Thesesre: (1) a deterministic

evaluation of passive fire barriers under limited fuel lTad and ventilation conditions;

(2) a probabilistic evaluation of supplementary fire protection measures (e.g.,

suppression systems); and (3) a subjective evaluation of unquantified fire hazard

conditions (e.g., cable arrangements). An example of each analysis phase is provided.

17 KEY WORDS AND DOCUMENT ANALYSIS 17a DESCRIPTORS

Fire

Fire Hazards Analysis

17b IDENTIFIERS/OPEN-ENDED TERMS

I8 AVAILABILITY STATEMENT 19 SECURITY CLASS (Thsreportl 21 NO OF PAGES

Unclassi fi ed

20 SECURITY CLASS (Thispage) 22 PRICE

Unlimited Unclassified s

NRC FORM 335 (7-77)