(0C NUREG/CR-6476

SAND96-2633

Circuit Bridging of Components

by Smoke

Prepared by

T. J. Tanaka, S. R Nowlen, D. J. Anderson

Sandia National Laboratories

Operated by

Sandia Corporation

Prepared for

U.S. Nuclear Regulatory Commission
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NUREG/CR-6476

SAND96-2633

Circuit Bridging of Components

by Smoke

Manuscript Completed: October 1996

Date Published: October 1996

Prepared by

T. J. Tanaka, S. P. Nowlen, D. J. Anderson

Sandia National Laboratory

Albuquerque, NM 87185

C. Antonescu, NRC Project Manager

Prepared for

Division of Systems Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code W6051

Abstract

Smoke can adversely affect digital electronics; in the short term, it can lead to circuit

bridging and in the long term to corrosion of metal parts. This report is a summary of the

work to date and component-level tests by Sandia National Laboratories for the Nuclear

Regulatory Commission to determine the impact of smoke on digital instrumentation and

control equipment. The component tests focused on short-term effects such as circuit

bridging in typical components and the factors that can influence how much the smoke

will affect them. These factors include the component technology and packaging,

physical board protection, and environmental conditions such as the amount of smoke,

temperature of bum, and humidity level. The likelihood of circuit bridging was tested by

measuring leakage currents and converting those currents to resistance in ohms.

Hermetically sealed ceramic packages were more resistant to smoke than plastic

packages. Coating the boards with an acrylic spray provided some protection against

circuit bridging. The smoke generation factors that affect the resistance the most are

humidity, fuel level, and bum temperature. The use of CO2 as a fire suppressant, the

presence of galvanic metal, and the presence of PVC did not significantly affect the

outcome of these results.

111 iii NUREG/CR-6476

Table of Contents

List of Figures.................................................................................. vii

List of Tables .................................................................................... VII

Acknowliedgmnents.......................................................... ......... Vill

List of Acronyms ................................................................................ ix

Introduction....................................................................................... 1

Circuit Bridging Tet on Components ...................................................... 3

Overview........................................................................................... 3

Components Tested............................................................................... 4

Chip Packages .................................................................................. 8

Comb Patterns .................................................................................. 8

Optical Isolators............................................................................... 10

Memory Chips ................................................................................ 10

Scenarios Tested ................................................................................. 10

Fuel Level ..................................................................................... 11

Burn Temperature..............................................................................1

Humidity....................................................................................... 11

Suppression (CO2) and Galvanic ............................................................ 12

Other Measurements.......................................................................... 12

Test Matrix ....................................................................................... 12

Results ............................................................................................ 13

Resistance Measurements....................................................................... 13

Scatter Diagrams .............................................................................. 15

Parameter Modeling .......................................................................... 18

Summary of Factors Important to Change in Resistance ................................. 22

Optical Isolator Chip............................................................................. 23

Memory Chips ................................................................................... 23

Summiary of Results............................................................................ 28

Technology and Packaging ..................................................................... 28

Protection......................................................................................... 28

Significant Factors in Determining Circuit Bridging......................................... 29

Conclusions...................................................................................... 30

References ................................................................................... 31

Appendix A Defining Credible Smoke Exposure Scenarios........................... A-1

Appendix B A Review of Smoke Damage to Electrical Equipment ................ B-i

v NUREG/CR-6476
Appendix C Preliminary Tests of the Impact of Smoke on Digital I&C

Equipment .............................................................................................. C-1

Appendix D Smoke Exposure Conditions for Experimental Digital Trip

System ..................................................................................................... D-1

Appendix E Impact of Smoke on Advanced I&C: Selection of Sample

Components and Test Matrix ............................................................... E-1

NUREG/CR-6476 Vi v
List of Figures

Figure 1. Combustion chambers (quartz cylinders) underneath the exposure

cham ber ......................................................................................................... 5

Figure 2. Exposure chamber of Lexan on top of combustion chamber ...................... 6

Figure 3. (a) Chip mounting board. (b) Comb pattern board ..................................... 7

Figure 4. Setup for smoke exposure test ...................................................................... 9

Figure 5. The PC unit covered with housing for the test ............................................ 9

Figure 6. Switching test circuit ....................................................................................... 11

Figure 7. Exposed chip-mounting boards from different tests .................................... 15

Figure 8. Exposed comb pattern boards ...................................................................... 16

Figure 9. Plot of resistance against time for comb pattern (test 23) ........................... 17

Figure 10. Logl0(R) during smoke production compared with pretest levels for a 160-

V com b pattern ............................................................................................ 17

Figure 11. Logl0(R) after exposure compared with pretest for 160-V comb patterns ....... 18

Figure 12. Comparison of average resistance during smoke production and after

venting ........................................................................................................ 19

Figure 13. Pareto analysis of significant factors .......................................................... 21

Figure 14. Optical isolator delay from test 27 ............................................................. 24

Figure 15. Optical isolator rise time from test 27 ........................................................ 24

Figure 16. Optical isolator delay from test 5 ............................................................... 25

Figure 17. Optical isolator rise time from test 5 .......................................................... 25

List of Tables

Table 1 Cable Fuel Weights, Total (g) ..................................................................... 12

Table 2 Test M atrix .................................................................................................. 14

Table 3. Optical Isolator Measurements ................................................................... 26

Table 4. Memory Chips and Optical Isolator .......................................................... 27

vii vii NUREG/CR-6476
Acknowledgments

The authors would like to acknowledge the following people who have contributed to this

experimental work: John Garcia, D. Michael Ramirez and Edward Baynes for their hard

work in setting up the experiments, building and maintaining equipment, and cleanup at

Sandia National Laboratories; David W. Palmer, Mike Knoll, and Richard S. Flores of

Sandia National Laboratories and Kofi Korsah of Oak Ridge National Laboratories for

their contributions and consultations on digital electronics; and Ruth Haas and Ellen

Walroth for their contributions to improving the report. This work has been sponsored by

the USNRC Office of Research under the project management of Christina Antonescu,

FIN W6051. Stephen J. Martin and Allen Camp are thanked for their encouragement and

support.

NUREGICR-6476 viviii
List of Acronyms

PTA Comb pattern A with 160-V bias

PTB Comb pattern B with 50-V bias

PTC Comb pattern C with 5-V bias

PTD Comb pattern D with 0-V bias except 5 V during measurement

LCC Ceramic leadless chip carrier

CDIP Ceramic dual-in-line package

CFP Ceramic flat package

TOC Transistor outline can

TTL Transistor transfer logic (0- to 5-V logic)

PDIP Plastic dual-in-line package

SOIC Plastic small outline integrated chip package

PLC Plastic leaded chip carrier package

ix ix NUREG/CR-6476

Introduction

Smoke can adversely affect the electronics in telecommunications and computer

equipment. In a fire, smoke can spread easily and quickly, hence it may cause equipment

failure over a widespread area. Many of the instrumentation and control systems in

nuclear power plants are based on old technologies and replacements are no longer being

manufactured. Instead, digital (microprocessor-based) technologies are being proposed.

As a result, the U.S. Nuclear Regulatory Commission (USNRC) has determined that the

potential for smoke damage to digital systems warrants investigation. As a part of an

ongoing assessment of environmental qualification guidelines to determine their

applicability to microprocessor-based equipment, a study of the impact of smoke was

initiated. A user-need letter to the Office of Nuclear Regulatory Research included a

request to specifically study the effects of smoke along with other environmental stressors

to confirm the adequacy of the qualification process for advanced instrumentation and

control (I&C) systems.

Consequently, an overall program to confirm the qualification process for digital I&C

systems was initiated. The program has been coordinated through Oak Ridge National

Laboratory (ORNL), which has performed susceptibility tests for electromagnetic

interference (EMI), temperature, humidity, and smoke (conducted with the author) on an

experimental digital system (Korsah et al., 1996). ORNL has also investigated likely

designs for future digital safety systems (Korsah et al., 1994). The overall program

includes a study by Brookhaven National Laboratory (BNL) to determine the risk

sensitivities of environmental stressors. To determine the risk sensitivities of selected

environmental stressors on the safety systems, BNL studied the reliability of digital

electronics in different environments and their relative contribution to the power plant

risk (Hassan and Vesely, 1996).

In order to assess the impact of smoke on digital I&C systems, Sandia National

Laboratories (SNL) is conducting studies and tests on digital I&C equipment exposed to

smoke. This research includes background studies on the smoke scenarios that are

postulated to occur in nuclear power plants, smoke damage to electrical equipment, and

typical microprocessor equipment that should be tested. The letter reports to the USNRC

on these studies are included as Appendices A, B, and E in this report. Two series of

preliminary smoke exposure tests have also been conducted: one on an active multiplexer

board and the other on ORNL's experimental digital safety system. These results are

included as Appendices C and D of this report. The results from all of the appendices are

briefly summarized below. The main objectives of this report are to consolidate these

previous results and to report on the first series of component exposure tests.

"Defining Credible Smoke Exposure Scenarios" (Appendix A) describes the scenarios

that are postulated to occur in a nuclear power plant. It is difficult to define the exact

nature of smoke because it is so dependent on the fire conditions and the surrounding

environment. The scenarios are defined in terms of factors that are of primary importance

for generating smoke in different areas of a plant. These include configuration of fuel,

quantity of fuel, the type of fuel, the size of the fire, and the duration of the smoke

exposure. Other factors are fire suppression agents and humidity levels. This report has

formed the basis for the smoke exposure tests. Because the scenarios are defined in terms

of factors in smoke production rather than the smoke products, they are easier to

reproduce.

"A Review of Smoke Damage to Electrical Equipment" (Appendix B) surveys the public

literature on the results of accidental fires, controlled fire tests, and smoke corrosivity

I 1 NUREGICR-6476
tests. Accidental fires in the telecommunications industry and in the U.S. Navy prompted

this study of smoke corrosivity because much of the damage in these fires was caused by

smoke. Although there are detailed reports on the results of accidental fires, the

uncontrolled nature of the events makes it difficult to draw conclusions on how electronic

equipment functioned during a fire and what the levels of smoke exposure were. There

were few controlled fire tests that monitored the functioning of electronic equipment

while it was exposed to smoke. Most of the testing identified was concerned with

salvage of equipment after fires rather than performance during a fire. Despite the

interest in the effects of smoke on electronics by the telecommunications industry and the

U.S. Navy, there are no existing military specifications for smoke survivability; however,

they do exist for temperature and humidity survivability.

"Preliminary Tests of the Impact of Smoke on Digital I&C Equipment" (Appendix C)

reports the results of exposing an operating multiplexer board to smoke. The board

configuration was designed by ORNL, as was the software to test functionality, and it is

similar in function to equipment that is likely to be used for advanced I&C safety

systems. The multiplexer board was connected to a personal computer (PC), which was

outside the smoke chamber, through a serial communications port. The multiplexer's

task was to measure the current output from a similar board that was placed outside the

smoke chamber and to report digitized current output values to the controlling PC. The

multiplexer board was exposed to a relatively high smoke level produced by burning

three different types of cable-ethylene propylene rubber, chlorosulfonated polyethylene,

and polyvinyl chloride (PVC). A new multiplexer board was used for each test.

The multiplexer boards performed flawlessly during the exposures to the first two types

of cable smoke, but the program that ran the measurements halted several times during

the smoke exposure with PVC. The program indicated that communication errors

occurred between the PC and the multiplexer board. The program could be restarted and

the multiplexer continued to work correctly for the next month. The intermittent failures

suggest that the electronics could be experiencing circuit bridging by the smoke, causing

intermittent shorts.

"Smoke Exposure Conditions for an Experimental Digital Trip System" (Appendix D)

describes the smoke conditions that were used to expose the ORNL experimental digital

trip system. The results are described in a report by ORNL (Korsah et al., 1996) and a

paper (Tanaka et al., 1996). There were eight smoke exposures for three different

modules of the experimental digital trip system, which consisted of computers linked by a

fiber optics network. A host computer that was not exposed to smoke logged the state of

a reactor trip channel. Multiplexer boards similar to those tested and reported in

Appendix C provided the input signals to the computers. The smoke affected the

communications on the network, causing temporary error states for lighter smoke

exposures, and failures for some of the heavier exposures. Because of the cost of the

system, the modules were exposed to smoke several times and cleaned between

exposures. After the first exposure, error states occurred even after the modules were

cleaned, leading to some uncertainty in the overall results. These tests showed that

smoke will affect a digital safety system, but it is unclear how much exposure will result

in failures.

Results from both preliminary smoke tests (Appendices C and D) showed that, in

addition to the corrosion of metal contacts that can occur weeks to months after exposure,

computer systems fail during smoke exposures. The failures were intermittent in many

cases and primarily affected the communications between computers or between

computers and multiplexer boards. Communication links between computers or between

a computer and a multiplexer board are especially vulnerable to the effects of smoke

NUREG/CR-6476 2
because these components are typically made so that the connectors can be rapidly

connected and thus they are not highly protected. Digital communications depend upon

high-speed variation in voltages in the output circuitry. If any connector pin is

momentarily shorted, the communication may be interrupted. Analysis of the failures

indicates that the smoke was bridging contacts that should have been electrically isolated.

All of these preliminary tests were performed on systems that included several operations

such as voltage measurement (analog measurement), conversion to digital output, and

digital communications. By testing systems, all possible failure points are exposed, but

the tests are complicated and expensive because it is difficult to analyze the errors in a

large system. Ideally, an entirely new system should be used for each smoke exposure,

but this is expensive for a large system. It is easier to test smaller subsections of the

systems, such as components or circuits, but such tests can only be useful if the expected

failure mechanisms are identified and monitored. The balance of this report describes the

results of one series of component tests.

Circuit Bridging Tests on Components

Overview

Since circuit bridging by smoke was isolated as an early failure mechanism, it is

important to study factors in smoke generation that affect circuit bridging. The SNL

program is based on two series of tests to explore this question. The first series of tests

has been completed and is the focus of this report. "Impact of Smoke on Advanced I&C:

Selection of Sample Components and Test Matrix" (Appendix E) summarizes the tests

and the equipment that were chosen for studying component reliability and the tests that

are planned for studying functional board reliability. Results of the first series of tests are

discussed in the following sections. The second series of tests on functional circuits is

under way.

Experience with shorts in the preliminary tests indicated that intermittent pathways

develop between contacts as a result of smoke. Thus the objective of the componentlevel tests was to determine the smoke-related factors that are important in causing

failures from shorts. Components were exposed to smoke, and responses indicative of

the formation of leakage paths that would lead to shorting or loss of function were

measured. Leakage of current indicates a drop in resistance. In these tests the leakage

currents are expressed in terms of resistance for seven chip packages and four comb

patterns with different voltages. Loss of function for the optical isolator chip was

determined by changes in the amplitude, delay, and rise time of an output pulse. The

functionality of memory chips was measured before and after the smoke exposures.

It is known from surface insulation resistance tests that current leakage increases at high

temperature (85 °C) and humidity (85% RH) (Iman et al., 1995). Typicalyretest

resistance measurements are >10 2 ohms, which drop to approximately 10 ohms within

hours after introduction of environmental conditions. This experiment raised two issues

in the measurement of current leakage for chip packages and comb patterns: (1) given the

temperature and humidity levels for these tests, the levels to which the resistance will

drop are unknown, and (2) the expected resistance levels for the components exposed to

smoke are unknown. Because of these issues, the circuit bridging tests had no predicted

output and no predetermined pass or failure limits for the measurements. Instead, smoke

generation factors were varied to determine which factors had significant impact on the

electrical characteristics of components and leakage between contacts.

3 3 NUREG/CR-6476
The factors that were tested included the amount of fuel, the presence of PVC in the fuel,

the burning temperature, the humidity, introduction of CO,, and the presence of galvanic

material. Some of the factors that were varied directly affected the smoke production;

however, humidity, CO, and galvanic metal were included because they were likely to

interact with the smoke and affect electronic components. High humidity is known to

affect electronics and is commonplace after a fire if sprinklers or water-based fire

extinguishers were used. CO2 is another common fire suppressant. It has been proposed

that CO, may affect electronics by suddenly cooling the electronics and cracking the

bonds. It has been included in some of the smoke exposures to determine if it would

cause problems for the electronics. Zinc in galvanized metals has been known to

combine with chlorides that are frequent components of smoke. ZnCl2 is hygroscopic and

will absorb water from the air. The ZnC12-water mixture forms a syrup, which can drip

onto components under the right conditions.

The test objects included seven chip packages, four comb patterns at different voltages,

optical isolators, and 16 K memory chips. To determine whether coatings or housings

had any effect on the chips or comb patterns, some of these test components were coated

with an acrylic spray or housed in a computer case.

The tests were similar to the smoke exposure tests that were conducted for the digital

safety system for ORNL (Appendix D). The components to be exposed were placed in a

Lexan smoke exposure chamber which was connected by stainless steel chimneys to the

combustion chamber underneath the exposure chamber (Figures 1 and 2). Smoke was

produced by heating the fuel with radiant heat lamps. The smoke was produced over a

period of 15 minutes to an hour and the components were exposed to the smoke for

approximately 1 hour. Then the smoke was vented and the exposure chamber was

opened, exposing the components to the controlled temperature and humidity of the

environmental chamber. The components were monitored before each smoke exposure

and at least every 5 minutes throughout the test for 24 hours from the beginning of the

exposure. During this monitoring period the components were either in the controlled

environmental chamber or in the sealed smoke exposure chamber.

This experiment was not a standardized test and no baselines for acceptable or

unacceptable performance exist. However, the test and the current leakage measured for

the various chip packages, comb pattern bias conditions, and control parameters have

yielded a considerable amount of information that can be used to analyze the effect of

smoke on electrical components.

Components Tested

The components tested consisted of seven chip packages (Figure 3a), four comb patterns

at different voltages, an operating optical isolator chip, and two 16-K memory chips. The

chip packages included a ceramic leadless chip carrier (LCC, Ul in Figure 3a), a ceramic

dual-in-line package (CDIP, U2), a ceramic flat package (CFP, U3), a transistor outline

can (TOC, U4), a plastic dual-in-line package (PDIP, U5), a small-outline integrated

circuit (SOIC, U6), and a plastic leaded chip carrier (PLCC, U7). The ceramic packages

and the TOC were empty packages while the PDIP and SOIC both contained four hex

inverter circuits. These components were chosen because they typify modem

microprocessor electronics. They represent both surface and plated-through-hole

mounting schemes and standard patterns used to measure soldering process quality and

surface insulation resistance.

NUREGICR-64764 4
r.__

Figure 1. Combustion chambers (quartz cylinders) underneath the exposure chamber. Fuel (pieces of

cable) was placed in the center of the chamber inside aluminum foil-lined stainless steel trays and burned

by lamps immediately above the quartz chamber. Load cells to measure the weight of the fuel are located

beneath the quartz chambers and are connected by a rod.

5 NUREG/CR-6476
Figure 2. Exposure chamber of Lexan on top of combustion chamber. The Lexan is opaque because the

chamber is filled with smoke. The lower right quartz lamp is emitting heat to burn fuel.

NURIEG/CR-6476 6
Figure 3. (a) Chip mounting board. (b) Comb pattern board.
Chip Packages

The objective of the chip package experiments was to measure the changes in resistance

due to smoke between contacts in components. A chip mounting board was designed that

allowed the leakage current between adjacent conductors to be measured while 5 V of dc

was supplied to the contacts. There were between 7 and 44 contacts for the seven chips.

(Appendix E describes these packages.) Since any pair of contacts may be bridged, as

many contacts as possible were monitored.

When plastic chip packages are made, the chip and pins are manufactured first, and then

the plastic package is molded around the electronics. For metal or ceramic packages, the

package is fabricated first, and then the chip is mounted within the package. The easiest

packages to monitor were the four empty chip packages; every other conductor was

connected by paths on the printed circuit board. The three plastic packages contained

integrated chips; therefore, only adjacent conductors that were independent of one

another were monitored. The leadless plastic chip carrier (PLCC) was especially

complicated, because it contained an 8-bit A/D converter with serial interface with 1370

field effect transistors. Both through-hole and surface-mounted components are

represented in this list of chip packages. The LCC, CFP, SOIC, and PLC are surfacemounted packages and the CDIP, PDIP, and TOC are through-hole-mounted packages.

For the empty packages, all of the even-numbered pins were connected to a 5-Vdc supply

and all of the odd-numbered pins were connected to ground. For the hex inverters, the

power and ground pins were allowed to float, the inputs to the inverters were connected

to 5 V, and the outputs were connected to ground. The PLC chip has 11 analog input

pins; alternate input pins were connected to the power and ground while the chip was

powered with 5 V. The chip-mounting boards were connected by ribbon cables and card

edge connectors to the instrument measuring current leakage, which was located outside

of the environmental chamber.

For each smoke exposure, four chip-mounting boards in different configurations were

tested (Figure 4). Three boards were mounted inside the smoke exposure chamber; these

boards were either bare, coated with an acrylic spray, or housed in a personal computer

chassis (Figure 5) with an operating fan (i.e., a typical chassis "muffin" fan). A bare

control board was located inside the environmental chamber, but outside the smoke

exposure chamber. The boards were placed in position approximately 1 hour before the

test in most cases. The card edge connectors were wrapped in black electrical tape to

prevent smoke from corroding the connectors.

The resistances were measured with a Keithley 617 multimeter and two HP

switcher/controllers. The multimeter, used as an ammeter, was switched between the

chip packages and comb patterns. The switching was controlled so that the chip packages

always had 5 V between the pins. The ammeter was switched into a circuit for 3 seconds

before each measurement to allow it to reach to a stable condition. To ensure that the

power supplies were not overloaded by chip package shorts, limiting resistors were

included in the circuit.

Comb Patterns

Surface insulation resistance was measured on comb patterns on Institute for

Interconnecting and Packaging Electronic Circuits (IPC) B-24 printed circuit boards.

The IPC-B-24 boards (Figure 3b) contain four identical comb patterns. The comb teeth

were 0.4 mm wide with 0.5-mm spaces between opposing teeth. The resistances of the

NUREGICR-64768 8
Figure 4. Setup for smoke exposure test. The bare and coated chip-mounting boards and comb patterns

are on left and right sides, respectively. The PC chassis with the cover removed is located in the center of

the exposure chamber. The bare control board, located outside of the chamber, is draped over the ladder at

the right rear.

Figure 5. The PC unit covered with housing for the test. The discoloration is the result of earlier tests

since the units were reused.

9 NUREG/CR-6476
comb patterns were measured similarly to those of the chip packages except that the

voltages were different. (One hundred sixty volts were applied to comb pattern A, 50

Vdc to pattern B, and 5 Vdc to pattern C.) Pattern D was grounded except during

measurements, when 5 Vdc was applied. The different voltages allowed evaluation of the

effect of electric field strength on circuit bridging during and after smoke exposure.

As with the chip boards, four boards were included for each test: a bare board, an acryliccoated board, a board housed in the PC chassis, and an unexposed control board (see

Figure 4). The boards were connected to the power and measurement circuit in the same

manner as the chip boards and were measured by the same ammeter.

Optical Isolators

A functioning 6N138 optical isolator, which provides a coupling for electrical circuits

with a direct electrical connection, was included in all of the smoke exposures. This

device operates by using a light-emitting diode and a photodetector that are built into a

plastic DIP body. The 6N138 is a low-input current, high-gain optocoupler built for

transistor-transistor logic (TTL) applications. The test circuit consisted of a square wave

input pulse, and the resistors and capacitors necessary to provide a switching test circuit

as shown in Figure 6 (Hewlett-Packard, 1993).

For each exposure, a new optical isolator chip was placed in a socket containing the

supporting circuit. The input and output wave form parameters, pulse rise time,

amplitude, and delay from the input pulse were measured on a digital oscilloscope. A

starting wave form was recorded, and if the values of rise time, delay, or amplitude varied

by more than 5% from the starting values, a new wave form was recorded.

Memory Chips

Two package types for 16-K memory chips were exposed during each test: a plasticpackaged commercial chip and a ceramic-packaged chip developed at Sandia National

Laboratories. The two chips had identical circuitry. They were subjected to a potential

of 5 V across their voltage supply and ground pins, but were not operated during the

exposure. Standard functional tests were performed on the chips before and after the

tests.

Scenarios Tested

The factors that were varied for the smoke exposures were fuel amount, presence of

polyvinyl chloride as part of the fuel, burn temperature, humidity, addition of CO,. and

presence of galvanic metal. Fuel load, burn temperature, and humidity factors were either

at a high or low level. The levels were chosen to span a range for a credible fire. For

CO2 and galvanic metal, the material was either present or absent. Because of the high

number of parameters, every possible permutation was not performed; instead, only

conditions that corresponded to likely scenarios in plants were produced.

A fire condition matrix was generated according to likely smoke scenarios, and tests 1 to

15 followed this procedure until CO2 and galvanic metal were determined not to be

detrimental to the components. The experimental design matrix was then modified so

that PVC could be included as a parameter and other combinations of factors could be

tested, such as the combination of high burn temperature and high fuel load. In all, 27

tests were performed.

NUREG/CR-6476 10
Figure 6. Switching test circuit. (Adapted from Hewlett-Packard Optoelectronics Designer's catalog.)

Fuel Level

Two standard amounts of cable were used as fuel in these tests as the high and low levels:

3 g and 100 g of plastic insulation and jacket material. The fuel consisted of a mixture of

cables that are typical of nuclear power plants. The exact amount of plastic that was

available for burning is not known because the plastic was not stripped from the cable

conductors and measured directly; however, the mass loss of the fuel was measured for

each test. The proportion of each type of cable was determined by the number of plants

that use the cable as reported by EPRI in their study, "Low-Voltage EnvironmentallyQualified Cable License Renewal Industry Report; Revision 1" (Bustard and Holzman,

1994). Table 1 shows the total mass of the fuel samples used to produce smoke for the

tests.

Burn Temperature

The cable material was burned at either high (50 kW/m2 ) or low heat fluxes (25 kW/m2 ).

At low heat flux the cables just smoldered, but for high heat flux tests the cables were

ignited if possible with a butane pilot light. For all high-heat flux bums, at least part of

the burn took place with a flaming fire; however, the flame did not last the full time that

the radiant lamps were on, presumably because the amount of oxygen available in the

combustion cell had dropped.

Humidity

High (75% RH) and low (<20% RH) humidity levels were controlled by an

environmental chamber that housed the smoke exposure chamber. The humidity was

always set before the test, but the humidity within the smoke exposure chamber was not

controlled during the smoke exposure since the chamber was entirely enclosed to control

the corrosive smoke. After the smoke was vented, the smoke exposure chamber was

opened and the environmental chamber controlled the humidity and temperature. The

humidity level before the test can affect the resistance of the circuit bridging

measurements before the smoke is introduced.

11 NUREG/CR-6476
Table 1. Cable Fuel Weights Total (g)

Cable Name Insulation Jacket Low Fuel High Fuel High Fuel

no PVC with PVC no PVC

Rockbestos Firewall III FRXLPE CSPE 1.8 38.6 57.0

Anaconda Flameguard lkv EPR CSPE 1.0 17.3 30.4

Brand Rex XLPE XLPE CSPE 20.0

Okonite Okolon EPR CSPE 16.3

Kerite HTK 0.8 16.7 23.3

Rockbestos Coax (le) 0.5 13.8 15.6

Raychem XLPE XLPE 0.4 9.9 10.2

Dekoran Dekorad EPDM CSPE 0.7 11.7 23.3

BIW EPR CSPE 8.11

Kerite FR 1 7.0

PVC PVC PVC 14.4

Suppression (C02) and Galvanic

Additional effects were added by flooding the exposure chamber with CO. from a fire

extinguisher after the fuel was burned and adding a piece of corrugated galvanic roofing

material above the test articles. When the CO2 was added, 60% of the volume of the

smoke exposure chamber was filled with CO2, a standard percentage of the volume that

automatic fire suppression systems will flood a room with in the case of a fire. This

corresponds to 2.5 lb of CO2 in the smoke exposure chamber.

The galvanized roofing material was included in some of the tests to simulate industrial

environments. Zinc in the galvanized coating could combine with chlorides in smoke to

form ZnC12, which readily absorbs water from the atmosphere to form syrupy deposits.

In past fires (Reagor, 1992), the presence of galvanic metal has increased damage to

electronics because droplets of ZnClI and water have deposited on the electronics. The

roofing material was cut so that it was above all of the test samples, but did not cover the

chimney areas of the chamber. This metal piece was suspended approximately 1 foot

above the test samples and covered almost the entire smoke exposure chamber.

Other Measurements

Temperature, humidity, smoke deposition, smoke optical density (turbidity), and fuel

mass loss were measured, and soot samples were chemically analyzed. Descriptions of

the methods used for these measurements can be found in Appendix D under smoke

environment measurements. The results of these measurements are presented in

Appendix F.

Test Matrix

The test matrix for all exposures performed is presented in Table 2. A value of I

indicates a high level for fuel, burn, or humidity. A value of 1 for PVC, suppression, or

galvanic indicates the presence of these conditions. Results of the tests were evaluated in

terms of these bipolar indicators rather than individual values. Test numbers were

assigned in order of the test performance.

NUREG/CR-6476 12
Results

Resistance Measurements

Smoke exposure caused changes in resistance in all components and comb patterns.

Figure 7 shows exposed chip-mounting boards from several different tests. The upper

left board is a control board (unexposed to smoke) from test 25, the upper right board is

the coated board from test 16, the lower left board was in the PC chassis during test 25,

and the lower right board was in the PC chassis during test 17. During test 25 the soot

from the fan tended to form clumps, fly off the fan, and deposit on the boards housed

inside the chassis. The lower right-hand board from test 17 experienced very nearly the

same environment as the lower left-hand board from test 25, differing only in that the fuel

mixture for test 17 contained PVC while that for test 25 did not.

The comb pattern boards shown in Figure 8 are arranged according to the same tests as

the boards in Figure 7, with soot discoloration more evident on these lighter colored

printed circuit boards. Although the effects of different voltages are not evident in these

reproductions, the higher voltage comb patterns (160 and 50 Vdc) collected more soot

than the lower voltage patterns (5 and 0 Vdc). The voltage on the 160-Vdc comb pattern

was sufficiently high that when large pieces of soot landed on the bare board, sparks were

observed.

The resistances of 44 components (7 types of chips and 4 combs in each of 4

configurations: bare, coated, in housing, and control) per test were measured for a 24-

hour period. In this period, approximately 450 measurements were made on each

component.

The pretest values of resistance were expected to be the same between chips of the same

type; however, this was not the case. In addition to differences caused by pretest

humidity, the resistances of similar components were also different.

Figure 9 shows the plot of log,0 [resistance(R)] against time of the four 160-V comb

patterns for test 23. This plot can be used to draw some conclusions about the differences

between the surface insulation resistance of the board that was bare and exposed to

smoke, the coated board that was exposed to smoke, the bare board that was mounted in a

PC chassis, and the control board, which was not exposed to smoke. The control board

had very high resistance throughout the exposure; although some variation was evident,

all values were quite high and changes were insignificant compared with the boards

exposed to smoke. The coated board and the board housed in a PC chassis behaved

similarly; they both started at high resistance values and then their resistance values fell

by 3 or 4 orders of magnitude when the smoke was produced. The bare board had the

lowest resistance of all of the boards.

Resistances measured in these circuit bridging tests varied over many orders of

magnitude, from a maximum of almost 1 petaohm (10' ohms) to a minimum of 1

milliohm (10' ohms). This large range was easier to analyze in terms of logarithms of

resistance. By analyzing the data in terms of logarithms of resistance rather than

resistance itself, more emphasis was placed on the smaller values of resistance than

would have been the case if the data were analyzed in terms of just resistance.

Henceforth, all discussion in this report is in terms of the log, 0(R).

13 NUREG/CR-6476
Table 2. Test Matrix

Test No. Fuel PVC Bum Suppression Galvanic Humidity

1 0 0 1 0 0 1

2 0 0 0 0 0 1

3 0 0 0 0 0 1

4 1 1 0 1 0 1

5 1 1 0 0 0 1

6 0 0 1 0 1 1

7 1 1 0 0 1 1

8 0 0 1 1 0 1

9 0 0 0 0 1 1

10 0 0 0 1 0 1

11 0 0 1 1 1 1

12 1 1 0 0 0 0

13 1 1 0 0 1 0

14 1 1 0 1 0 0

15 0 0 1 0 0 1

16 1 1 0 0 0 1

17 1 1 1 0 0 1

18 1 0 1 0 0 1

19 0 0 1 0 0 0

20 0 0 0 0 0 0

21 1 0 0 0 0 1

22 0 0 0 0 0 0

23 0 0 1 0 0 0

24 1 1 0 0 0 0

25 1 0 1 0 0 1

26 1 1 1 0 0 1

27 1 0 0 0 0 1

NUREG/CR-6476 14
Figure 7. Exposed chip-mounting boards from different tests. Upper left: control. Upper right: test 16.

Lower left: in PC chassis for test 25. Lower right: in PC chassis for test 17.

Scatter Diagrams

Log,,(R) was plotted against time from all 27 tests, but the plots are not reproduced here

because of the large number (27 x 11=297 in all). Instead, scatter plots have been

developed to condense the data into fewer pages. For each component, the log, 0(R) at

pretest, the log,1o(R) averaged during the smoke production, and an average log,1o(R)

starting 2 hours after the smoke exposure for 24 hours were determined for each test.

Scatter plots of these averaged resistances (on a log scale) are presented in Figures 10, 11,

and 12 for the 160-V comb pattern for all tests. All of the points represented by the solid

circles are bare patterns, all of the squares are coated patterns, all of the triangles are

patterns placed inside the PC housing, and all of the asterisks are control patterns.

The averaged values that are plotted in the scatter diagrams represent a varied number of

points. In many cases the pretest value was only one measurement, whereas the aftersmoke value represents the average of as many as 430 data points. Analysis of each data

point as a function of time would have been an overwhelming task; therefore, all of the

analysis henceforth deals with the averaged values of pretest, during smoke, and after

smoke.

15 15NUREG/CR-6476
Figure 8. Exposed comb pattern boards. Arranged as in Figure 7.

Figure 10 compares log1 o(R) during smoke production with pretest levels. Values located

along a line between (0,0) and (15,15) correspond to patterns whose resistance did not

change because of the addition of the smoke. It had been expected that all of the points

represented by * (control) would be located along this diagonal because these patterns

were not exposed to smoke. As shown by the scatter plot, many of the control patterns

did not change value significantly, but some of the resistances increased, as indicated by

their position above the diagonal line. The points that are below the line indicate that the

resistance fell as a result of the smoke. Note that the pretest values range between 8 and

15 while the logj0(R) during smoke exposure ranges between 3.5 and 15. The range of

resistances for the presmoke patterns is much smaller than the resistances during the

smoke exposure.

Figure 11 compares the log10(resistance) pretest with the average after the smoke was

vented (2 to 24 hours after the smoke exposure began). In this figure it is interesting to

note the clump of solid dots (bare board) at approximately 10'0 ohms along the diagonal line. This indicates that the resistances of the bare boards reverted back to their starting

NUREG/CR-6476 16
1E+15

1E+14

1E+13

1E+12

1E+11

1E+10

1 E+09

IE+08

1 E+07

1 E+06

1 E+05

~ *~. * .:.. :.Control . *

44I'

Chasi

n V I I I I I

0. 10. 20. 30. 40. 50.

Time(hrs)

60. 70. 80. 90.

Figure 9. Plot of resistance against time for comb pattern (test 23).

16.00

14.00

12.00

10.00

8.00

6.00

4.00

2.00

*0 Coated

Chassis

Ix Control -l 22I12

0.00 4e-'

0.00 2.00 4.00 8.00 8.00 10.00

Pretest

12.00 14.00 16.00

Figure 10. Log1 o(R) during smoke production compared with pretest levels for a 160-V comb pattern.

17 NUREG/CR-6476
16.00

14.00-A.

12.00

10.00 o Bare

aD Coated

8.00 A Chassis • x Control & 0 •Series5

6.00 g0 •

4.00

2.00

0.00IIIIII

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 11. Log,0(R) after exposure compared with pretest for 160 V comb patterns.

value after the smoke was vented. The dots were not as tight on the previous figure. In

general, most of the resistance measurements were higher after the smoke was vented

than they were when the smoke was being produced, indicating some recovery.

Figure 12 shows the comparison of the average resistance during the smoke production

with the average after the venting period. In this figure a large number of points are

located above the diagonal, indicating that the resistance was lower during the smoke

exposure than during the venting period. For these boards, after the smoke exposure, the

resistances recovered to some extent. For the values below the diagonal, the resistances

became worse as time went on even though the smoke was vented from the chamber. It

is interesting to note that the boards that had lower resistance values (higher degradation)

during the smoke exposure did not recover as well as those that had higher resistances

(lower degradation) during the smoke exposures. This would indicate that, for lighter

smoke exposures, the electronics may recover from the smoke exposure, whereas for

heavier smoke exposures, they may not. Similar figures for all of the component

packages and comb patterns are included in Appendix F, figures 1 to 33.

Parameter Modeling

The scatter plots provide a good intuitive grasp of the effects of smoke in general

throughout these tests and show the differences among different protective measures for

the boards, but in order to determine which of the smoke generation, environmental

condition, or board condition factors were most significant, variances in resistance were

analyzed using general linear models. The effect of these different factors is determined

by assessing the contribution of each factor to resistance in the linear models.

NUREG/CR-6476 18
16.00

14.00

12.00

10.00

I10 Coated I

8.00 a Chassis IX Control I•Ser5Ies

6.00

A

4.00

htAM&&AD 0 a 0

2.00

0.00I I I I

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 12. Comparison of average resistance during smoke production and after venting.

Equations were derived from the averaged logi0(R) values that model resistance of each of

the eleven components and comb patterns at pretest, during smoke exposure, and after

smoke exposure. These 33 models (11 components and combs at 3 periods each), are

presented in Appendix F, Table 4. The models indicate the factors that had an influential

effect on the resistances of these test samples.

These tests involved seven factors: fuel level, PVC, burn temperature, humidity,

suppression, galvanic metal, and board condition (bare, coated, housed in chassis, and

control). General linear models use indicator variables for the factors. For these tests

two-level factors were used; therefore indicator variables of values, 1 for high or present

levels, and 0 otherwise, were used. (See Table 2 for the values of the indicator variables

for each test.) The four board conditions are also represented by four two-level indicator

variables, one each for bare, coated, or chassis. For example, if bare = 1, then all other

board condition indicators such as coated or chassis must be 0. All three board

conditions set to 0 indicates the control board.

General linear models use stepwise least-squares regression analysis of these indicator

variables to produce equations for the resistance value. The final model is a linear

combination of the indicator variables. The model determines the best combination of

significant factors; coefficients for factors that are insignificant are dropped from the

equation. Cross products of up to three factors at a time, representing third-order

interaction effects, were included in the model. For example, the combination of high

fuel loads and a bare board can result in more significant changes in resistance than either

a bare board or high fuel load considered separately. The mathematical equivalent of the

combination of a high fuel load and a bare board would befuel x bare. If the

19 19 NUREG/CR-6476
combination of high fuel and a bare board is more significant than either high fuel or bare

board alone, a coefficient for this combination of factors appeared in the model. This

modeling process determines the most significant factors influencing test results.

The following empirical general linear model shows the relationship between resistance

and the significant factors for the transistor outline can (TOC) for all conditions before

smoke exposure:

logl 0(R) = 12.27- 4.61 x humidity

+ 1.09 x chassis

+3.82 x bare x humidity

+3.05 x coated x humidity

This model includes a constant value that was based on the control board, and

coefficients that are added or subtracted from the constant depending on whether any of

the factors are in their "high" state (indicator variable value = 1). The constant

corresponds to the resistance of the control TOC for low humidity.

This model can be used to determine a value for the control TOC in high humidity. In

this case, the contributions from bare, coated, and chassis would be 0 because for the

control board, the indicator variables of bare, chassis, and coating are 0, and only the

contribution due to high humidity, -4.61, would remain because humidity would be 1.

The model would give a value of:

logl0[R(fQ)] = 12.27 -4.61 = 7.66

When all of the models are analyzed on an individual basis, they may yield confusing

information. For example, for the ceramic leadless chip carrier during pretest, the

control, bare, and chassis boards are all exposed to the same environments. Table 4 in

Appendix F shows that the values "before" the test are different, as noted by the value - 4.04 under the column marked "bare." This large variation between the different

conditions of the ceramic leadless chip carrier is again modified for the case of a high

bum temperature by the value +4.28 under BxBare, and by the value -4.81 for the highhumidity, high-bum temperature case (BxHxBare).

To obtain an overall understanding of the most influential effects, a Pareto analysis was

performed (Figure 13) on all 11 components taken as a whole. The height of the column

indicates the number of times that each factor or combination of factors appears in the 33

models. A separate column was graphed for each period that was analyzed: presmoke,

during smoke, and after smoke. Single occurrences of any factor were not included in the

graph. Since there were 7 chip components and 4 comb patterns, 11 is the maximum

possible number of times that a factor or combination of factors could appear in the

models. Figure 13 does not indicate whether the factor will increase or decrease the

resistance, it merely indicates whether the factor is significant. Significant factors

appearing most frequently in the different resistance models had more influence on the

overall test results.

Not only do the models provide information on significant factor effects, they also

provide insight into the experimental test conditions, the conduct of the experiment, and

data variability. For example, the constant term corresponding to the control board at the

NUREG/CR-6476 20
7

o Presmoke

* During Smoke

* After Smoke

6

5-

4-

3

2 1 1 l 

6! s 

1 

TX 

L I 

U> 

11 I. 

Re I~ 

.1 1 

*I b b 

11 

I* 

1+ 1IL D I [I11 

*

1 1 1

bp 0

0 A 21~0

Factors

Figure 13. Pareto analysis of significant factors.

21 NUREG/CR-6476
low or absent levels varies widely-the 5-V comb pattern is particularly low, as can be

seen in Appendix F.

The models also show the four different boards to be significantly different from each

other before the smoke exposure began, which was unexpected. The coated boards were

sprayed with an acrylic coating, so they may have been different, but differences in the

bare, chassis, and control boards are probably the results of variation in the testing

circuitry. A low-resistance measurement could be caused in two ways, leakage across the

chip or comb pattern, or leakage in the measurement circuit such as shorts in the

connectors or cables. Although the circuits were checked before testing began, they were

not checked regularly between tests. Stray wires or solder may affect the leakage current

measurements because the leakage currents for these components are very low to start

with.

Another issue brought out by the models is the identification of some exposure factors as

being significant at pretest, before the exposure begins. For example, in the presmoke

160-V comb model, coefficients for burn andffuel should not be included because the

amount of fuel and the burn temperature should not have any effect on leakage before the

fuel is ignited. The analysis should not have allowed these factors to enter the models.

Their inclusion in some of the models indicates that they account for some of the

variation in the response variables. Further investigation into the inclusion of these

spurious factors at pretest was not possible due to insufficient sample size and lack of

repetition.

Summary of Factors Important to Change in Resistance

The coefficients for all of the components were considered together to determine which

factors were the most significant during the different periods of ihe exposures. The

factors that were significant before the smoke exposures were the type of board and the

humidity level. The humidity was applied to all boards prior to the test, and the high

level of humidity lowered the resistance. The type of board (bare, coated, or chassis) also

was significant, but unlike humidity, the values of the coefficients were not consistently

negative or positive, possibly reflecting the variation in test circuitry or in the boards

themselves.

During the smoke exposure, more factors became significant for resistance. These factors

included burn temperature, amount of fuel, a combination of fuel level and humidity, and

a combination of burn temperature and galvanic metal. In the modeling of the

relationships of significant factors, these additional factors appeared as products with

board type, indicating that the control boards were not influenced by these factors. The

"high" or "present" state for all of these factors tended to lower the resistance (increase

degradation).

The same factors that were significant during the exposure were significant for resistance

after the smoke exposure: type of board, humidity, burn temperature, fuel, and a

combination of fuel and humidity. The presence of galvanic metal was not significant for

the after-smoke resistance.

The factors that appear to be most significant overall are humidity, fuel level, and bum

temperature. Humidity appears alone, which is reasonable since the humidity was

applied to all boards, including the control board, but fuel level and burn temperature

NUREG/CR-6476 22
appear as products with board condition or with humidity and board condition. The

condition of the board strongly affected how much these factors influenced the resistance.

For example, the combination of fuel and humidity had a larger effect on bare and chassis

boards than on the coated boards after the smoke exposure. Also, the combination of fuel

and humidity was stronger than the effect of fuel or humidity alone, indicating a

synergistic effect on the circuit bridging.

Optical Isolator Chip

The amplitude, rise time, and delay of the output of the optical isolator was recorded once

for each scan of the resistance measurements. The recorded measurements were averaged

for 16 consecutive traces by a digitizing oscilloscope and recorded with the resistance

measurements by the computer. When the change in the amplitude, rise time and delay

were large enough, the wave form was also recorded. The following equation, which in

effect averages the changes of all of the measured parameters, provides the criterion for

recording a wave form:

where A is the amplitude, t, is the rise time, td is the delay time, and A indicates the

change in the corresponding value from the last recorded wave form.

The values of rise time and delay are plotted in Figures 13 to 16 for tests 27 and 5 and

illustrate how the rise time and delay are affected by smoke. Both of these tests had high

fuel humidity, high fuel loads, and low burn temperature. During test 27, the optical

isolator ceased pulsing during the smoke exposure, resulting in the rise time and delay

values approaching infinity. The optical isolator in test 5 did not quit functioning, but

some jitter in rise time is shown. Such jitter may affect the performance of the

communications between digital systems, but the reliability of those systems will depend

upon the particular circuits that are involved. Table 3 presents some of the measurements

taken during the smoke exposures. The columns that start with "l'" ratio, rise, or delay

are pretest values. Note that the pretest ratios generally dropped as the tests progressed, presumably due to some degradation in the chip-mounting fixture that was reused, with

new chips, in each test. The same was true in general for the pretest rise and delay times.

The average values represent averages during the entire test and the "STD" values are the

standard deviations. No statistical evaluation of these data has been performed.

Memory Chips

The 16-K memory chips were tested both before and after the smoke exposures, but were

not tested during the exposures, though 5 V was applied during the exposures. Table 4

lists the pass/failure (1 stands for pass and 0 stands for failure) of each chip. More

failures occurred for the plastic-bodied chips than the ceramic-bodied chips. The

hermetic seal on the ceramic-bodied chips may have increased their reliability. The

optical isolator output is also presented in the table in terms of passing or failing. The

criterion used for the optical isolator was whether the chip continued to put out a signal.

The optical isolator and both memory chips were dual in-line packages (DIPS), so

comparisons can be made between the failures of these chips and the resistance measured

for the plastic DIP on the bare chip mounting board for particular tests. The modeling

equation for a bare plastic DIP was used to derive the resistances in Table 4. The failures

23 23 NUREGICR-6476
1.OE-4

9.0E-8

8.OE-5

7.OE-5

6.OE-5

5.OE-5

4.OE-5

3.OE-5

2-0E-5

1.OE-5

O.OE+O I I

0 5 10 15 2D 25

Time (Hr)

Figure 14. Optical isolator delay from test 27.

ID.E-A

9.OE-5

8.OE-5

7.OE-5

6.0E-6

5.OE-5

4.OE-S

3.OE-5

2.OE-6

1.OE-5

0.0E40

0 5 10 15

Time (Hr)

25

Figure 15. Optical isolator rise time from test 27.

NUREG/CR-6476 24
1 E-04

9E-05

8E-05

7E-05

6E-05

5E-05

4E-05

3E-05

2E-05

1E-05

OEE00

Figure 16.

1 E-04

9E-05

8E-05

7E-05

6E-05

5E-05

4E-05

3E-05

2E-05

I E-05

OE+00

0 5 10 15 20 25

TIme (HO

Optical isolator delay from test 5.

0 5 10 15 20 25

Time (HO

Figure 17. Optical isolator rise time from test 5

25 NUREG/CR-6476
Table 3. Optical Isolator Measurements

Test 1 ratio ave ratio STD ratio 1V rise ave rise std rise I' delay ave delay STD delay

I 1.00E+00 0.992 1.32E-02 3.79E-05 3.60E-05 2.30E-06 l.llE-05 1.IIE-05 1.21E-07

2 9.92E-01 0.992 1.27E-02 3.61E-05 3.71E-05 2.19E-06 1.19E-05 1.22E-05 1.47E-07

3 1.OOE+00 0.992 9.78E-03 3.77E-05 3.59E-05 2.24E-06 1.09E-05 1.1OE-05 1.22E-07

4 9.92E-01 0.989 1.11E-02 3.39E-05 4.05E-05 2.06E-06 1.07E-05 1.28E-05 3.79E-07

5 9.84E-01 0.995 8.34E-03 3.90E-05 4.41E-05 2.40E-06 1.27E-05 1.34E-05 5.18E-07

6 9.76E-01 0.981 9.95E-03 4.64E-05 4.75E-05 2.23E-06 1.53E-05 1.55E-05 1.71E-07

7 9.84E-01 0.985 1.OOE-02 4.48E-05 4.73E-05 2.39E-06 1.49E-05 1.56E-05 1.73E-07

8 9.84E-01 0.976 1.03E-02 5.04E-05 5. 1E-05 2.47E-06 1.77E-05 1.80E-05 1.36E-07

9 9.59E-01 0.973 6.41E-03 4.90E-05 5.00E-05 1.28E-06 1.79E-05 1.77E-05 1.81E-07

10 9.59E-01 0.978 6.64E-03 4.48E-05 4.73E-05 1.72E-06 1.57E-05 1.58E-05 1.51E-07

11 9.84E-01 0.979 9.83E-03 5.43E-05 5.46E-05 2.56E-06 1.81E-05 1.87E-05 1.31E-07

12 9.76E-01 0.991 1.lOE-02 3.98E-05 4.22E-05 1.68E-06 1.42E-05 1.43E-05 1.43E-07

13 9.76E-01 0.993 8.94E-03 4.02E-05 4.26E-05 1.60E-06 1.40E-05 1.43E-05 1.56E-07

14 9.92E-01 0.990 7.61E-03 4.18E-051 4.OOE-05 1.33E-06 1.38E-05 1.39E-05 1.41E-07

15 9.59E-01 0.749 3.80E-01 4.79E-05 3.05E+36 1.71E+37 1.77E-05 3.05E+36 1.71E+37

16 9.75E-01 0.638 4.35E-01 5.64E-05 1.23E+37 3.27E+37 1.86E-05 1.26E+37 3.30E+37

17 9.59E-01 0.962 6.16E-03 5.13E-05 5.34E-05 2.70E-06 1.92E-05 1.95E-05 3.90E-07

18 9.59E-01 0.967 1.04E-02 5.12E-05 5.33E-05 2.96E-06 1.91E-05 1.93E-05 3.71E-07

19 9.59E-01 0.970 1.07E-02 5.20E-05 5.47E-05 3.33E-06 1.94E-05 1.95E-05 1.97E-07

20 9.59E-01 0.971 9.84E-03 5.15E-05 5.51E-05 2.88E-06 1.94E-05 1.95E-05 1.83E-07

21 9.43E-01 0.960 4.59E-03 5.42E-05 5.34E-05 2.OOE-06 2.03E-05 1.98E-05 3.12E-07

22 9.84E-01 0.979 8.38E-03 5.65E-05 5.55E-05 2.35E-06 1.90E-05 1.93E-05 2.30E-07

23 9.59E-01 0.977 9.37E-03 5.50E-05 5.57E-05 2.83E-06 1.96E-05 1.95E-05 1.92E-07

24 9.75E-01 0.975 8.97E-03 5.38E-05 5.54E-05 2.25E-06 1.93E-05 1.94E-05 2.51E-07

25 9.19E-01 0.947 1.15E-02 5.10E-05 5.43E-05 3.02E-06 2.OOE-05 2.02E-05 4.79E-07

26 9.59E-01 0.855 2.54E-01 5.35E-05 2.38E+36 1.52E+37 2.OOE-05 2.17E+36 1.45E+37

27 9.67E-01 0.778 2.81E-01 5.26E-05 2.62E+36 1.59E+37 1.92E-05 2.62E+36 1.59E+37

of the optical isolator and memory chips occurred during tests where log(R) < 10 in all

cases except for test 15; however, failures did not occur every time log(R) < 10.

[Log(R)=3 in these measurements corresponds to shorting of at least one pair of contacts.

The test setup did not allow for measurement of resistance below 100 ohms.] Although

NUREG/CR-6476 26
Table 4. Memory Chips and Optical Isolator

TestNo.Calulaed og R) Cramc Mrn lasic em. Optical

TestNo.og 1Caculted R)Jraic Mm 1Pastc Mm~j Isolator

1 12 1 1 1

2 12 1 1 1

3 12 1 1 1

4 3 1 0 1

5 3 1 1 1

6 12 1 1 1

7 3 1 0 1

8 12 1 1 1

9 12 1 1 1

10 12 1 1 1

11 12 1 1 1

12 13 1 1 1

13 13 1 1 1

14 13 1 1 1

15 12 1 1 0

16 3 1 1 0

17 3 1 0 1

18 3 1 0 1

19 13 1 1 1

20 13 1 1 1

21 3 1 1 1

22 13 1 1 1

23 13 1 1 1

24 13 1 1 1

25 3 0 0 1

26 3 1 1 0

27 3 1 0 0

the chips package measurements indicated that the contacts of the plastic DIP shorted for

certain tests, the optical isolator and memory chips did not always fail on the same tests.

The chip package measurements were more sensitive because they monitored all pairs of

contacts rather than only those required for a chip to operate. The chip packages were

also placed in slightly different locations in the smoke exposure chamber than the

memory and optical isolator chips (both were on the right side of the chassis as shown in

Figure 4), exposing the different chips to different amounts of smoke and in different

orientations. No measurements were made of amount of smoke or soot in the immediate

vicinity of these chips, so no conclusions can be drawn regarding the effect of different

locations in the exposure chamber. Also, no thermocouples were located close enough to

the chip boards to resolve differences between them.

27 27 NUREGICR-6476
Summary of Results

Technology and Packaging

Digital I&C components are available with many different technologies and packages.

Metal and ceramic packaging, which are hermetically sealed, tend to be more reliable

than plastic packages. The data from the 16-K memory chips show that the ceramic

packages were more robust than the plastic packages in a smoke environment.

Differences between ceramic and plastic packages were not evident from the resistance

measurements; therefore, the higher failure rate of plastic packages may be due to

penetration of the plastic by humidity rather than shorting of contacts with soot. Most

common digital electronics, however, use inexpensive plastic packages. Hermetically

sealed packages are significantly more expensive and are not typically available unless

used for military applications.

The voltages at which the digital electronics operate vary according to the digital chip

technology. Resistance measurements on the comb patterns indicate that patterns are

affected by smoke more if the applied voltage is higher. The higher voltage lowers the

resistance before the smoke is applied and continues to produce more leakage current

throughout the exposure. Visually, soot tends to accumulate more around the high

voltage patterns, and the 160-V pattern was observed to be arcing during the smoke

exposure.

Two hex inverter chips were included on the chip mounting board to determine the

difference between an SOIC package and a DIP. The data indicated no appreciable

difference between these two packages: when the fuel was high, both packages shorted;

when the fuel was low, the resistance dropped slightly for both packages, and then

recovered. Little difference was observed among all of the chip packages. A reason for

not being able to determine differences may be that the low fuel loads were too low to

cause much change in resistance, while for a high fuel load, virtually all of the packages

shorted.

These tests measured leakage currents that determined the change in resistance between

contacts on components. Loss of resistance can cause problems in many components and

circuits; for example, if the resistance between a contact to supply voltage and one to an

input signal drops, a false signal may be received by the device input from the voltage

supply. The likelihood of this happening depends on many factors such as the amount of

smoke, location of the contacts, and humidity level. In general, loss of resistance

between contacts will cause serious problems in any digital circuit. The exposures of

several different circuits will be tested next in this project.

Protection

Bare boards were highly affected by smoke while coated boards seemed to be less

affected. Housing the boards in a PC chassis, which contained a fan, only minimally

protected them. Visually, some of the boards that were placed within the chassis looked

worse than those that were unprotected because clumps of soot were deposited on the

board. For the high fuel tests, however, virtually all components in the-PC chassis were

shorted in these situations and little difference could be observed in the resistance

measurements.

NUREG/CR-6476 28
Significant Factors in Determining Circuit Bridging

Several observations can be made from the component tests; the most significant factors

are humidity, fuel level, and burn temperature. High humidity has been shown to affect

the surface insulation resistance of printed circuit boards in environmental testing (Iman

et al., 1995). Other fire corrosivity tests that used comb patterns also showed that the

resistance of exposed comb patterns is highly affected by humidity (Caudill et al., 1995).

As more fuel is burned, resistance drops. Smoke from plastic leaves a film, which is

black and powdery if the plastic is burned in the flaming mode with adequate oxygen.

The film is white and oily if produced without a flame and black and oily if produced

with a flame but the flame extinguishes due to lack of oxygen. For low fuel loads, the

resistance generally dropped during the smoke exposure but recovered after the smoke

was vented. This recovery was not often the case with the high fuel loads; once the

circuit was shorted, it never recovered.

The burn temperature ( flaming vs. smoldering) did not affect the resistance as much as

the humidity or fuel amount. The burn temperature would be expected to change the

smoke products in two ways: different chemical products can be produced at different

temperatures, and the mass loss rate of the fuel is slower if the fire is smoldering.

Chemical analyses for Cl, Br and SO 4 show a low correlation between burn temperature

and the amount of these chemicals in the soot (Appendix F). Analysis of the amount of

fuel that was burned shows that neither the burn temperature nor the fuel level alone

determines the percentage of fuel that is burned. If a large amount of fuel was burned at a

low temperature, approximately 20% of the fuel was consumed, while if a small amount

of fuel was burned at either temperature or a large amount of fuel was burned at high

temperature, approximately 40-50% of the fuel was burned. These results are presented

in Appendix F.

The circuit bridging tests showed that the synergistic effects of smoke and humidity are

higher than for humidity alone. This observation is supported by the frequency with

which the modeling equations contain the combination of fuel level and humidity

coefficients. If each factor, fuel and humidity, reduced the resistance in an additive way

instead of synergistically, the effect of these two factors could be adequately modeled by

each factor alone.

PVC (included as part of the fuel load on the high fuel tests only) showed very little

effect on resistance. Only a small proportion of the fuel was PVC in these tests and, with

the high fuel load, many of the components shorted without PVC. There is also little

correlation between the Cl deposition found by chemical analysis and the presence of

PVC. Although the other cable materials did not contain PVC, some had high

proportions of Cl and Br, which are typically used as fire retardants. No other cable

materials were singled out for study in these tests.

The addition of CO2 as a fire suppressant did not affect the resistance adversely,

supporting results from the ORNL smoke exposures. The addition of CO2 may be

beneficial to the electronics by cooling the room and blowing away some of the soot

deposition. These findings are supported by tests on the effect of CO2 on computers

(Arvidson and Persson, 1993).

The galvanic metal was expected to trap Cl in the form of ZnCI2, form a thick liquid, and

drip onto the electronics (Reagor, 1992). Although a greasy film formed on the galvanic

metal that was suspended in the smoke exposure chamber, the film never accumulated

29 NUREG/CR-6476
enough water to drip. Instead, the metal piece formed a surface upon which some of the

smoke deposited. The overall effect was to reduce deposition on the surface of the

electronics and reduce the negative effect of the smoke. On some high-humidity tests,

water appeared to be collecting at the base of the PC chassis. This water did not affect

any of the components. Overall, these tests were found to be of an inadequate scale to

properly assess the importance of this factor.

Resistances of control board components varied during the smoke exposures. It is not

clear if factors other than humidity affected these boards. Other possibilities include

EMI. For the functional circuit tests planned for fall 1996, several "dry" runs are

planned. These runs will include tests of humidity only and tests in which the radiant

heat lamps are turned on but no fuel will be available to create smoke.

Some of the resistance values were lower than expected even before the smoke was

applied. The electrical circuitry had leakage currents that were not resolved before

testing. Although most of these problems were resolved before testing the circuits,

leakage remained in a few. This indicates circuits should be carefully tested before each

smoke test.

The card edge connectors and sockets for the functional chips were not changed between

tests although they were protected by electrical tape. These connectors were still affected

by smoke and were replaced once during the test series. Accumulated smoke can cause

cur-rent leakage paths over time with repeated exposures.

Conclusions

The tests described in this report studied factors that could possibly affect circuit bridging

in typical digital electronic components. The factors that were studied can be divided

into three categories: component technology and packaging, circuit board protection with

coatings or enclosures, and smoke generation factors. Several conclusions may be drawn

from the results of these circuit bridging tests and the previous tests described in the

appendices to this report. These conclusions include:

1. Smoke causes circuit bridging in components. Circuit bridging increases leakage

currents and can cause failures because stray currents cause errors in digital

circuitry.

2. Factors that affect circuit bridging are the amount of smoke, humidity level, and

burning mode.

3. Although surface deposits cause some circuit bridging, the presence of smoke in

the air also causes increased leakage.

4. The effect of circuit bridging on digital I&C equipment depends on the particular

components and circuits. Component packaging (plastic, ceramic, or metal) and

technology (CMOS, FET, or fast Schottsky) are only a few of the factors that

determine the likelihood of survival of a circuit or component.

5. Conformal coatings add some protection to circuits.

NUREG/CR-6476 330
6. Mechanical protection may also protect circuits, depending on the presence of a

ventilation fan. A fan may draw more smoke into the electronics.

Smoke may have a very destructive impact on digital electronics, producing immediate

circuit bridging and, after several weeks, corrosion of metal parts. The component tests

focused on circuit bridging in typical components and the factors that can influence how

much the smoke will affect them. These factors include the component technology and

packaging, physical board protection, and environmental conditions such as the amount

of smoke, temperature of bum, and humidity level. Hermetically sealed ceramic

packages were less affected by smoke than plastic packages. Coating the boards with an

acrylic spray provides some protection against circuit bridging. The most significant

factors were humidity, fuel level, and bum temperature. The use of CO2 as a fire

suppressant, the presence of galvanic metal, and the presence of PVC did not

significantly affect resistance measurements. The next series of tests will include a study

on functional circuits and will be reported in a separate NUREG/CR.

References

Arvidson, M., and H. Persson. 1993. Koldioxids (CO2) paverkan pa elektronik -

slackning med laga koncentrationer. [Carbon Dioxide Suppression Systems - Risk of

Damage to Electrical Equipment.] Swedish National Testing and Research Institute, SR

Report 1993:20.

Bustard, L., and P. Holzman. 1994. Low-Voltage Environmentally Qualified Cable

License Renewal Industry Report; Revision 1, Electric Power Research Institute, Palo

Alto, CA.

Caudill, L. M., J. T. Chapin, R. B. Comizzoli, et al., 1995. "Current State of Fire

Corrosivity Testing: Preliminary Electrical Leakage Current Measurements," in Proc.

International Wire & Cable Symposium, pp. 432-437.

Hassan, M., and W. E. Vesely. 1996. Risk-based Prioritization of Environmental

Stressors Which Can Affect Advanced Digital I&C Equipment in Nuclear Power Plants,

Technical Report L-1908, T6, Brookhaven National Laboratories, Upton, NY.

Hewlett-Packard, 1993. Optoelectronics Designer's Catalog: U.S.A., Hewlett-Packard

Co. 5091-4573E.

Iman, R. L., R. V. Burress, D. J. Anderson, et al., 1995. "Evaluation of Low-Residue

Soldering for Military and Commercial Applications: A Report from the Low Residue

Soldering Task Force," Sandia National Laboratories, Albuquerque, NM.

Korsah, K., R. L. Clark, and R. T. Wood. 1994. Functional Issues and Environmental

Qualification of Digital Protection Systems of Advanced Light-Water Nuclear Reactors,

NUREG/CR 5904, Oak Ridge National Laboratories, Oak Ridge, TN.

Korsah, K., T. J. Tanaka, T. L. Wilson, and R. T. Wood. 1996. Environmental Testing of

an Experimental Digital Safety Channel, NUREG/CR 6406, Oak Ridge National

Laboratories, Oak Ridge, TN.

31 NUREG/CR-6476
Reagor, B., 1992. "Smoke Corrosivity: Generation, Impact, Detection, and Protection,"

Fire Sciences, v. 10, pp. 169-179.

Tanaka, T. J., K. Korsah, and C. E. Antonescu. 1996. "Preliminary Studies on the Impact

of Smoke on Digital Equipment," in Proc.Twenty-Third Water Reactor Safety

Information Meeting, U. S. Nuclear Regulatory Commission, Office of Nuclear

Regulatory Research, Washington, DC, pp. 85-99.

NUREG/CR-6476 32
Appendix A

Defining Credible Smoke Exposure Scenarios

Prepared by:

Steve Nowlen

Sandia National Laboratories

September 1994

Abstract

This letter report defines smoke threat scenarios which are likely to be experienced by

advance electronic circuit components during a fire event in a nuclear power plant.

Smoke is, in general, a very poorly understood material. The direct definition of the

composition of the smoke likely to be generated during any given fire event is not

possible based on the current state of knowledge. Hence, for this report, the approach

taken is to identify and define those factors which are considered of primary importance

to the generation of "representative" smoke, rather than attempting to define the exact

nature of the smoke itself. This definition includes the consideration of the "quantity," "quality," and duration of the smoke exposure. Smoke quantity is defined in terms of the

ratio between the mass of fuel burned during a given fire scenario and the volume of air

into which the fire products will be distributed. Smoke quality is defined as those aspects

of the threat scenario which will influence the chemical composition of the smoke. These

include the materials likely to be burned, the anticipated modes of burning, and

synergistic interactions with transport surfaces and fire suppressants. The duration of the

exposure considers both the duration of the component exposure to the actual smokefilled environment, and the post-exposure period during which continue exposure to the

accumulated deposits would be expected. The smoke threat scenario definitions provided

represent a range of conditions which are expected to encompass the likely smoke threat

scenarios for nuclear power plant fires.

NUREGICR-6476A- A-1
Table of Contents

I. Introduction ...................................................................................................................... 4

A . Objective ............................................................................................................ 4

B. Scope of D efinition ............................................................................................... 4

HI. Equipm ent Threat Issues ............................................................................................. 5

A . O verview ............................................................................................................ 5

B. D irect Corrosive Attack ........................................................................................ 5

C. Circuit Bridging Faults .......................................................................................... 7

D . Contact Fouling ....................................................................................................... 7

E. Degradation of Fine M ovem ent ............................................................................. 8

F. Interactions at Elevated Tem peratures ................................................................... 8

III. Characterization of Sm oke Quantity ......................................................................... 9

A . Introduction .......................................................................................................... 9

B. Sm all In-Cabinet Electrical Control Panel Fire ...................................................... 9

1. General Scenario Description .......................................................................... 10

2. Quantity of Com bustible M aterials Involved ........................................................ 11

C. Large Electrical Control Panel Fire ....................................................................... 12

1. G eneral Scenario D escription ........................................................................... 12

2. Quantity of Com bustible M aterials Involved .................................................... 14

D . General Fire Area Scenarios ................................................................................. 15

1. G eneral Scenario Description .......................................................................... 15

2. Quantity of Com bustible M aterials Involved ................................................... 16

E. Sm oke Spread from an Adjacent Fire Area ........................................................... 17

F. Summary of Scenario Fuel Mass to Air Volume Ratios ...................................... 17

IV. Identification and Characterization of Critical Smoke Quality Parameters ............ 18

A . Overview ................................................................................................................... 18

B. Type of M aterials Burned .................................................................................... 19

1. Com bustible G ases ............................................................................................ 19

C. Liquid Fuels .............................................................................................................. 20

1. Transient Fuels .................................................................................................. 20

2. In situ Solids ...................................................................................................... 20

3. M aterial Identification Conclusions ................................................................. 22

D . Com bustion M ode ................................................................................................ 23

E. Suppressant Interactions ...................................................................................... 24

1. W ater-Based Suppressant Interactions ............................................................ 24

2. Carbon D ioxide Suppressant Interactions ......................................................... 24

3. D ry Chem ical Powder Interactions .................................................................... 25

4. H alon Suppressant Interactions ........................................................................ 25

F. Transport Interactions ........................................................................................... 26

G . Sum m ary of Sm oke Quality Insights .................................................................... 26

V . Duration of Sm oke Exposure .................................................................................... 27

A . Overview ................................................................................................................... 27

B. Sm oke Purge Tim e ............................................................................................... 28

C. Post-Fire Recovery Tim es .................................................................................... 29

D . Transport/Exposure D uration Interactions ........................................................... 29

E. Sum m ary of Exposure Duration Tim es ............................................................... 30

A-2 NUREG/CR-6476
VI. Summ ary of Exposure Characterization Insights ................................................... 30

VII. References .................................................................................................................. 32

NUREG/CR-6476 A-3
I. Introduction

A. Objective

The objective of this report is to define smoke exposure scenarios representative of those

likely to be encountered in the event of a fire within a nuclear power plant. These

scenarios are expected to be used as the basis for testing nuclear power plant equipment,

and in particular advanced types of integrated circuit and fiber optic based components,

for smoke damage vulnerability.

B. Scope of Definition

In the definition of smoke exposure scenarios, the factors considered to be of primary

importance are those which will determine the quantity, quality, and duration of the

smoke exposure. Quantity is associated with both the gross amount of smoke generated

and with the dilution of that smoke into the affected air volume. Quality is related to the

physical nature of the smoke itself, and in particular, to its chemical composition. The

duration of the exposure will include consideration of the fire growth and suppression

periods, as well as the post-fire recovery times. A number of parameters can be identified

which will influence these aspects of the smoke exposure environment. This study has

attempted to identify and characterize those parameters that will be of primary influence.

It must be recognized at the outset that the current state of knowledge regarding the

nature of smoke is relatively poor. This primarily affects those issues associated with

smoke quality. In fact, even the definition of what constitutes smoke varies. Smoke is

often considered synonymous with soot, that is, the carbonaceous particulate formed as a

part of the combustion process. However, in this study a more liberal definition of smoke

will be used. That is, smoke will be defined in a broader sense to include both soot and

any other combustion products that have the potential to become surface deposits, or to

interact with surface deposits, during a fire. This definition will also include the

consideration of chemical changes induced through interactions with enclosure surfaces

during the process of smoke transport, and though interactions with fire suppression

agents.

Under this fairly broad definition, smoke will be composed of a wide variety of chemical

species, and a definition of a smoke threat scenario must include some consideration of

this chemical composition. To the extent possible, expected chemical species and

interactions will be discussed as documented in both experimental research and

information based on actual fire experience. However, it is well beyond the scope of this

study to provide a full definition of the anticipated chemical composition of surface

deposits.

A-4 A-4 NUREG/CR-6476
Instead, the definition of smoke threat will be addressed primarily through the source of

the smoke, rather than through the smoke itself. That is, rather than defining the exact

composition of the smoke, this study will define smoke by identifying (1) the type and

mass of materials most likely to be the source of smoke, (2) how smoke might be

generated and transported to the site of the component, and (3) how long the smoke

exposure is likely to last. These factors correspond to the quantity, quality, and duration

of the smoke exposure as defined above. It is expected that by reproducing these factors

in testing, the chemical composition of the smoke generated will be representative of that

which would be encountered in an actual fire incident, even though the exact composition

of the smoke may remain poorly defined at the outset of testing. It is expected that

testing will include a variety of diagnostic measures to assess the actual nature of the

smoke generated and hence will greatly improve our overall understanding of smoke

generation and characteristics.

II. Equipment Threat Issues

A. Overview

Before considering those aspects of a fire that are important to the definition of credible

smoke threat scenarios, it is useful to consider the mechanisms by which a device might

be disabled by smoke. In particular, this project is interested in the impact of smoke on

the performance of advanced types of electronic components. Such devices are unique in

comparison with older-style electromechanical devices that have typically been the focus

of past fire vulnerability investigations [1,2,3]. The factors that make advanced circuit

components unique include the compact nature of the devices, the nature of the individual

components which comprise an overall device, and the nature and sensitivity of the

device's operation.

Because the advanced electronic devices are unique in their construction and operation,

unique modes of component failure are to be expected. The following sections identify

those mechanisms which are expected to represent important potential modes of failure

for integrated circuitry-based devices and systems.

B. Direct Corrosive Attack

One of the principal means by which smoke can threaten the operability of electronic

equipment is through direct corrosive attack on the metallic surfaces of the components.

In general, such attack has not been considered a short-term concern. That is, for the

older style electromechanical devices most commonly used in current reactor

applications, the relatively massive nature of the components implies that extensive levels

of corrosion would be required to compromise component functionality. The time frame

required for such extensive corrosion was considered inconsistent with the short time

NUREG/CR-6476A- A-5
frames typically considered risk important (fire risk is generally dominated by short-term

scenarios). Hence, direct corrosive attack has not been considered as a potential

contributor to plant fire risk.

However, when advanced electronic circuits are considered, a potential does exist for

short-term corrosive attack to compromise circuit performance, and hence, to represent a

significant contributor to plant fire risk. For example, circuit miniaturization efforts are

resulting in smaller circuit path traces being used in the fabrication of printed circuit

boards. For these small circuit traces, even a relatively limited and short-term corrosive

attack, particularly if the attack is in the form of "pitting" corrosion rather than general

surface corrosion, may result in the short-term loss of circuit functionality. Hence, for

advanced circuit board-based devices, direct corrosive attack is of potential concern.

In general, fire-induced corrosion has been attributed primarily to acidic combustion

products (acid gases). In fact, the German DIN 57 472 Part 813 standard simply makes a

direct measurement of acid gas content of material combustion products and assumes a

direct correlation between acid gas content and corrosive potential [4]. In a similar

fashion, the French CNET test will also provide corrosion assessments that are directly

correlated to the acid gas content of the fire products (due to the high moisture content of

the test cell and to the induced condensation of moisture directly onto the measurement

target) [5].

For the purposes of this project, a somewhat broadened view of the corrosive attack issue

is appropriate in order to support realistic assessment of threats to components. For

example, experience in the telecommunications industry [6] has shown that interactions

of the acidic smoke from a cable fire with the galvanized surfaces of common airhandling ductwork can introduce galvanic salts into the smoke stream. These galvanic

salts readily deposit onto component surfaces, resulting in direct and aggressive corrosive

attack.

Another important consideration is the potential that fire suppressants might accelerate

the rate of corrosion through synergistic interactions with the smoke deposits. One such

synergistic effect is clearly anticipated in the case of water-based suppressants. Acid

gases are generally very hygroscopic. Moisture condensation onto a component surface

will act to "scavenge" acid gases from the airborne smoke onto the components, and will

promote the formation of acid on the components. As a second example, consider that

many nuclear plants utilize carbon dioxide fire suppression systems in either a roomflooding or manual distribution mode. The presence of high concentrations of carbon

dioxide in combination with moisture can lead to the formation of carbonic acid deposits,

and carbonic acid can be an aggressive corrosion agent. This is hypothetical only, there

is no known experience.

Interaction of a fire with Halon may also be of concern because Halon has a relatively

high bromine content. Thermal breakdown of the Halon will result in the generation of

acidic HBr. However, it should also be noted that the importance of Halon as a fire

suppressant agent is decreasing rapidly due to concerns that fluorocarbons are attacking

A-6 A-6 NUREG/CR-6476
the Earth's ozone layer. Halon is one of the products targeted for removal from use as a

result of the international Montreal Accords on the use of fluorocarbons. Hence, it is not

recommended that extensive investigations of Halon interactions be undertaken at this

time.

C. Circuit Bridging Faults

The deposition of smoke onto a component carries a potential for the creation of

electrically conductive short circuit or circuit bridging paths [1]. Such paths may be

formed at any point on a component, but would be of particular concern where smaller

physical separation distances are present. For advanced electronic circuits, small

separations will be quite common. In particular, closely packed circuit traces on the

surface of a printed circuit card would be vulnerable to bridging unless protective

coatings are applied to these traces. Also, the "pin outs" or connection pins associated

with various integrated circuitry devices (chips, transistors, resistors, capacitors, selector

switches, etc.) are typically quite close together and hence will also be vulnerable to

circuit bridging failures.

It is likely that scenarios involving water used as a fire suppressant will be the most

aggressive environment for circuit bridging faults. As discussed above, the formation of

a condensation droplet on a printed circuit card will likely act as an agglomeration point

for soot particulates and acid gases. Such a droplet could easily bridge several exposed

contact points on closely packed printed circuit boards such as those typically used in the

construction of advanced electronics. Such failures have been noted in prior experimental

studies [1].

Attack by "dry" smoke in the absence of water sprays is less likely to result in circuit

bridging failures. However, as discussed further below, test experience with cable fires in

particular has revealed that even in the absence of water sprays, cable fire smoke can be

heavily laden with tar-like substances which may also act as circuit bridging agents [1].

Another mechanism for circuit bridging faults derives from the observation that smoke

interactions with galvanized air handling ducts can result in the formation of galvanic

salts. The deposition of these salts onto a printed circuit card may also induce circuit

bridging failures. Such deposits have been observed in recently documented

telecommunications industry fire events [6]. It is currently unclear whether such deposits

resulted in circuit bridging faults during these events.

D. Contact Fouling

Another potential mechanism for electronic component failure due to smoke deposition

would be contact fouling. That is, the deposition of smoke particulates onto open contact

pairs may result in degraded contact resistance or contact fouling. Such a failure

mechanism is, perhaps, of relatively minor concern for integrated circuitry devices

NUREG/CR-6476A- A-7
because of the more typical use of solid-state switching devices. However, the potential

for contact fouling should be considered should a device include open, unprotected

contact pairs in its design.

E. Degradation of Fine Movement

In past testing [1], it has been observed that the deposition of smoke onto a device can

result in the degradation of fine mechanical movements. In particular, one mode of

failure identified for a strip chart recorder exposed to the smoke from a cable fire was the

fouling of the mechanical indicator pen, preventing proper operation.

In the case of advanced circuitry components, this mechanism is likely to be of relatively

minor concern. In general, such components would be expected to be less dependent on

fine mechanical movements, but rather, would be primarily dependent on the operation of

solid state devices. However, in certain cases vulnerabilities might exist. In particular,

output or indicator devices associated with such systems may incorporate such fine

mechanical movements in their operation. Degradation of these outputs may constitute

failure of the system.

F. Interactions at Elevated Temperatures

In general, the older-style electromechanical devices that have been used in current

reactor designs have relatively high thermal damage limits. In comparison, advanced

electronic circuits are much more sensitive to elevated temperatures, and may experience

a loss of function at relatively modest temperatures. Many of the fire scenarios likely to

be considered important with regard to smoke damage will involve relatively modest

elevated temperatures.

In particular, note that during a fire event a loss of the normal enclosure HVAC air flow

is likely. That is, most ventilation supply and exhaust systems will be designed to shut

down during a fire event to contain the spread of smoke and fire beyond the room of

origin. Such control may be exercised either through mechanical dampers actuated by a

fusible link, or through the ventilation control system based on actuation of smoke

detectors either in the room of fire origin or in the ventilation duct work. Hence, a loss of

enclosure ventilation is a likely occurrence in the event of a fire, and manual recovery of

the system may take a significant period of time. This will likely lead to an increase in

the general room temperature for even a relatively small fire.

Given the likelihood that room temperatures will be increased for even a small fire, it is

important to consider the potential for interactions involving a combination of smoke

exposure and modest temperature increases. In particular, exposure to modest

temperature increases may place advanced electronic devices in a generally more fragile

operational state and hence may make the components more susceptible to other forms of

degradation, including smoke-induced damage in a synergistic manner.

A-8 NUREG/CR-6476
III. Characterization of Smoke Quantity

A. Introduction

The first step in the process of defining credible smoke threat scenarios for advanced

electronic equipment is to define the quantity of smoke to which a component is likely to

be exposed. In this context, quantity refers to the amount of smoke per unit volume one

would expect a component to become immersed in. This will vary depending on the type

of fire scenario one is attempting to represent. Hence, the definition of the "quantity" of

the smoke exposure considered appropriate will be approached by first defining those

general fire threat scenarios that might lead to such exposures. Based on the types of fire

threat scenarios that are considered relevant, many other aspects of the expected fire

threat can be defined. In particular, this would include factors such as the types of

materials burned, the amount of material burned, mode of combustion, suppression

interactions, and likely transport interactions. There are many additional factors which

could also be included in this list (e.g. configuration of combustible material and air

supply.) However these are considered the most significant.

There are any number of possible fire scenarios that might be considered threats to

nuclear power plant operations. However, in the context of the current study, only a

limited subset of these scenarios is likely to be of significant concern. In particular, this

study is concerned with the potential exposure of advanced electronic components to

smoke. Hence, only those areas which will house such equipment, or which might lead

to a smoke exposure for that equipment (e.g., adjacent rooms), will be of interest. This

narrows the range of relevant fire scenarios which must be considered.

There are four general fire scenarios that could result in the exposure of plant equipment

to smoke. Each of these four scenarios would lead to a somewhat different smoke threat.

These general fire scenarios are described in the following sections. These descriptions

include a discussion of the likely progress of each fire event, and other important fire

parameters.

Also presented is a discussion of the total amount of material likely to be burned in each

scenario. This value is then coupled to an assessment of the volume of air into which the

combustion products are likely to be distributed. Hence, the final assessment of smoke

"'quantity" is given as a mass of combustible materials burned per unit volume of affected

air.

NUREG/CR-6476A- A-9
B. Small In-Cabinet Electrical Control Panel Fire

1. General Scenario Description

This scenario would involve a very limited fire which remains confined to a small subset

of an overall electrical control panel. In this case, it is postulated that the fire would have

virtually no impact on the environment within the overall fire area, but that modest heat

input and significant amounts of smoke might be generated and contained within the

electrical control panel itself. Thus, those components housed within the same electrical

control panel but not involved directly in the burning process would be exposed to a

combined mild-to-moderate thermal exposure and smoke deposition environment.

The most likely source of such a fire would be a self-ignited electrical fire initiated by the

failure of a component within the cabinet. Hence, the fuels which might burn would be

those associated with the construction of electrical components, and potentially, a limited

quantity of instrumentation and control wiring. For fires of this type, the early stages are

likely to be characterized by overheating of a component which might experience an

extended period of smoldering combustion. This might be followed by a brief period of

open flaming. The fire might be extinguished naturally (self-extinguishing) or through a

fire suppression agent.

In most cases, fires of this type would be expected to be short lived. That is, if the fire

remains confined to a small subset of the available fuel, then that fuel will be quickly

exhausted, and even barring suppressant intervention, self-extinguishment would be

expected in a short time. (By definition, a cabinet fire that grows to involve a significant

fraction of the available in-cabinet fuel load would be classified as a large cabinet fire.

This scenario is presented in Section 3.3 below.)

It is somewhat difficult to establish bounds on fuel involvement associated with a "small"

fire. During the SNL cabinet fire test program [7,8], preliminary testing to assess the

impact of various cabinet and fuel loading parameters on fire development often noted

that on the order of 5-15% of the available fuel would be consumed during fire tests for

which very limited fire growth was observed. For tests in which more extensive fuel

involvement was noted, it was also noted that continued fire growth would occur and

eventually virtually all of the available fuel would be consumed. That is, a threshold type

of behavior was noted.

Based on actual in-plant experience, much smaller fires than those observed in the SNL

test program have been experienced [12]. In some cases, self-extinguishing fires have

been observed that involved only a single electrical component (e.g., a relay or single

printed circuit card). While no actual value of the quantity of fuel burned in these events

was given, a lower limit on fuel involvement will be established as 1% of the available

cabinet fuel load. Hence, these values of 1-15% will be used as upper and lower bounds

on fuel involvement for the small cabinet fire scenario.

A-10 A-1O NUREG/CR-6476
Also note that in most of the limited-involvement SNL tests, the fire would typically

continue for no more than 15 minutes before self-extinguishing. If an initial period of

smoldering combustion were included, then a maximum time interval on the order of 30

minutes during which actual combustion takes place might be expected.

If a fire suppression agent is involved, it may be applied though a fixed general area

system, a fixed local system, or through manual intervention. If manual suppression

methods are employed, the most likely approach would be application of a hand-held

extinguisher. Since this scenario involves an electrical equipment fire, the most likely

suppression agent would be CO, or dry chemical. Water might also be used to suppress

such a fire, but only if the particular area were protected by a fixed fire suppression

system triggered by either area or in-cabinet smoke detectors. Note that both water and

dry chemicals would be less likely agents given current plant practice. (Even a crosszoned detection system would be likely to actuate in this situation.) In this case, the

components might also experience an increase in humidity to essentially saturation

conditions, but would not likely be subject to direct sprays.

Plant areas subject to this type of fire would be any area that houses electrical control

panels. Of particular interest to the current study, this would include the main control

room and the remote shutdown areas. Also of potential interest might be areas

immediately surrounding the containment structure in which one might find electronic

multiplexing equipment.

2. Quantity of Combustible Materials Involved

In this scenario, the fire is postulated to remain confined to a small subset of a single

electrical control panel. Hence, the quantity of material burned will be relatively small.

However, the smoke is also assumed to remain largely confined to the fire cabinet itself,

and hence, the volume of air into which the combustion products are distributed is

similarly small.

In general, there are three types of control panels used in the nuclear industry. These are

vertical, benchboard, and console type cabinets [7]. A wide variety of cabinet sizes are

used, depending on the particular application. In general, a typical single electrical

control panel may have a volume ranging from about 30 to 300 cubic feet. Much larger

cabinets may also be encountered, although beyond this limit, it is relatively common to

employ some form of internal partitioning. Cabinets may also have either open back or

closed back designs. For the purposes of this scenario, it will be assumed that the cabinet

has a closed back. In this case, the cabinet will typically be equipped with open air flow

louvers which allow for the natural ventilation of the cabinet.

The fuel loadings contained within a control panel may also vary widely [7]. Some

cabinets, such as switchgear, motor control center, and breaker cabinets, will have

minimal combustible fuel loads. However, those panels associated with instrumentation

NUREG/CR-6476 Al A-11I
and control will typically have much higher fuel loads due to the concentration of control

and instrumentation wires and components within the cabinet.

In the SNL cabinet fire test program [7,8], fuel loads were calculated on the basis of the

heat content of the available fuel per unit of floor area in the cabinet (MJ/m& or Btu/ft2).

The fuel loadings that were tested were based on an extensive review of industry

practices. The maximum fuel loads for control panels were determined to be 258 MJ/md

(22,700 Btu/ft2 ) with a typical load determined to be 170 MJ&m2 (15,000 Btulft2).

For the purposes of this program, it is of greater interest to express these values in terms

of the fuel mass per unit volume of enclosed cabinet space. These values can be "renormalized" by assuming two values. First, the "typical" heat of combustion for a cable

insulation material used in the SNL study was 30 kJ/g. Second, a typical cabinet was

identified in that same study as having a height of approximately 2.3 m (7.5'). Using

these values, one can calculate that the maximum fuel load in a control panel would be

approximately 3.7 kg/m3 (0.23 lb/ft3

) and a typical fuel load would be approximately

2.5 kg/m3 (0.15 lb/ft3).

As discussed above, in the scenario involving a limited in-cabinet fire, it is assumed that

from 1 to 15% of this fuel will be consumed in the fire. Using these values the range of

fuel burned per unit volume of affected air space (in this case, the cabinet air volume) can

be determined. For a cabinet with the maximum fuel load, values ranging from 0.038 to

0.56 kg/m3 (0.0024-0.035 lb/ft3) are obtained. For the typical fuel load, values ranging

from 0.026-0.38 kg/m3 (0.0016-0.023 lbs/ft3) are obtained. Hence, the range of fuel

loadings recommended for testing to simulate this scenario is from 0.026 to 0.56 kg/r 3

(0.0016-0.035 lb/ft3).

Note that in calculating these values the effects of natural circulation through the cabinet

have been ignored. Such circulation would be expected to dilute the smoke to some

extent, although the rate of dilution is unknown. The values obtained assuming that no

dilution takes place would represent a conservative upper bound value.

C. Large Electrical Control Panel Fire

1. General Scenario Description

In the second scenario of interest, it is postulated that a large fire occurs within an

electrical control panel, but that the fire remains confined to that one panel. This fire

would be the type experienced in certain of the fire tests performed by SNL as a part of

an experimental investigation of electrical control panel fires [7,8]. This type of scenario

has been postulated as representing the most severe fire that might be experienced in the

main control room, for example [9]. Such fires might also be experienced in other plant

areas as well [12].

A-12 A-12 NUREGICR-6476
In this scenario, it is postulated that a fire has been ignited through either a small transient

fuel source or through an electrical fault. However, in contrast to the small cabinet fire

scenario, it is postulated that the fire grows beyond the initial ignition source, and

eventually consumes all of the combustible materials within the cabinet in the fire. In the

SNL tests, growth times from the onset of open flaming to peak fire intensity of

approximately 10 to 15 minutes were experienced for fires of this type. Total fire duration

of from 15 to 30 minutes were recorded before all of the available fuel was consumed.

(No fire suppression attempts were employed in these tests.)

In this scenario, the most important aspect of the smoke exposure question will be

associated with the equipment located in the balance of the fire area, rather than that

housed within the burning cabinet itself. That is because it is postulated that a relatively

large fire has occurred, it would also be postulated that all of the equipment within the

fire source cabinet would be destroyed by direct thermal exposure. Hence, the smoke

exposure question would be associated with the balance of the equipment within the

affected fire area. This equipment might experience only a modest increase in

temperature, but may experience significant and extended smoke exposures [9,10].

In this scenario, the most important aspects of the fire will be those associated with

growth of the fire and fully developed fire stages. Ignition may involve initial smoldering

followed by open flaming, but in terms of the overall fire area exposure, the smoke

generated during this stage will be relatively unimportant. Once open flaming begins and

the fire begins to grow, however, the enclosure environment will rapidly deteriorate, and

a dense layer of smoke is likely to form in the room as a whole. In testing, it was

observed that the smoke layer descended to the floor of the test enclosure within

approximately 6-12 minutes of ignition [10].

Once the fire becomes fully developed, it is also likely that a period of oxygen-limited

combustion may be experienced. This would generally result from local ventilation

effects associated with the burning cabinet itself, rather than global oxygen depletion

within the overall fire area. This effect may be particularly pronounced for cabinets

which have closed doors so that only limited natural ventilation opening are available.

Hence, in the fully developed stages of the fire, a relatively inefficient combustion

process may be experienced, resulting in the generation of significant amounts of soot

and other unburned pyrolysates.

The methods likely to be employed in the suppression of such a fire would depend on the

area in which such a fire occurred. For example, if the fire occurs within the main control

room, then manual suppression by operators using hand-held extinguishers or by the fire

brigade is the most likely response. Fixed fire suppression systems are very rarely

installed in the main control room. For other plant areas, either manual response or fixed

fire suppression system response may occur. Hence, these fires would be subject to the

full range of potential suppression agents (see further discussion of suppressant

interactions in Section 4 below).

NUREG/CR-6476 A1 A-13
2. Quantity of Combustible Materials Involved

The large cabinet fire scenario involves a fuel source quite similar to that discussed in

Section 3.3.2 above, except that a full involvement of the cabinet is expected. In

particular, the control panel fuel loads which have been described above would also apply

to this scenario. The maximum fuel loading which was cited in the SNL study [7,8] was

a total fuel heat content of 1.53x10 6 kJ (1.47x10 6 Btu). Again using a nominal cable heat

of combustion value of 30 kJ/g, this heat content value corresponds to a fuel mass loading

of 51 kg (112.2 lb). Using a similar procedure, the minimum fuel loading utilized in any

test which was intended to represent a full control panel mockup was approximately 25

kg (55 lb). (Note that other tests did involve lighter fuel loads, but that these tests were

intended to assess specific aspects of cabinet fire growth behavior and did not represent

full cabinet mockups.) These values will be used to represent the minimum and

maximum amounts of material which would be expected to burn under the conditions set

forth for this fire scenario.

The second aspect of this fire scenario which must be quantified is the volume of space

into which it is expected that the combustion products would be dispersed. For this

scenario, the smoke is assumed to remain confined to the room that houses the burning

cabinet. Unfortunately, the volume contained in a typical nuclear plant fire area can vary

widely. The range of consideration can be reduced somewhat by considering only those

areas likely to be of interest in the context of this scenario. This would include the main

control room, auxiliary shutdown area, switchgear rooms, and potentially cable spreading

areas (many cable spreading rooms will include various electrical panels).

In 1985 Brookhaven National Laboratory (BNL) performed a study to assess the typical

size and configuration of nuclear power plant enclosures.' Three types of generic

enclosures were identified; the vault, corridor, and bay areas. Of these, the vault

configuration is of interest here because it included those types of areas identified as

relevant to the current study immediately above. Based on a statistical sampling of 80

vault-type rooms at 8 different power plants, BNL determined that the mean

configuration of an enclosure of this type was a rectangular room measuring

24.4x12.2x6. 1 m (80x40'x20') (LxWxH). Therefore, the volume of a typical enclosure

would be 1816 m3 (64,000 ft3

).

Using this room volume as representative, a range of fuel mass to air volume ratios can

now be calculated. Based on consumption of the minimum cited cabinet fuel load from

the SNL tests, a value of 14 g/m3 (8.7x10- lb/ft3) is obtained. Using the maximum full

cabinet fuel load cited yields a value of 28 g/m3 (1.7x 103 lb/ft3).

1

This information was gathered as a part of an NRC-sponsored fire model development and validation

effort undertaken during the 1985-1987 time frame. The information was communicated by BNL to the

USNRC and SNL through internal memoranda, and through presentations at working group meetings.

SNL retains copies of this documentation.

A-14 NUREG/CR-6476
Note that in developing these fuel mass to air volume ratios, it has been assumed that the

ventilation system will not dilute the smoke exposure. This assumption is appropriate for

general plant areas where the normal ventilation system would typically be shut down

automatically upon fire detection. However, this assumption would not reflect the

conditions that would prevail in the main control room where the ventilation system

would likely be placed in a full exhaust, emergency smoke purge mode of operation. In

this case it would be appropriate to assume that some dilution of the smoke would occur.

A typical control room ventilation system will provide a smoke purge mode ventilation

rate of approximately 10 room air changes per hour [11]. Hence, for a 15-30 minute fire

scenario, it would be appropriate to assume that the smoke would be diluted by a factor of

approximately 2.5-5. Incorporating these values into the above estimates of fuel mass to

air volume ratio yields control room scenario values of from 2.8 g/m3 (1.7x 10-4 lb/ft3

) to

11.2 kg/m3 (7.0x104 lb/ft 3).

D. General Fire Area Scenarios

1. General Scenario Description

In addition to the large electrical control panel fires, one might also expect that significant

room-wide smoke exposures could also occur as a result of a more generalized fire within

a fire area. In this case, the most likely materials to be involved would be electrical

cables (see further discussion of likely fuel sources presented below). Fires may also

involve an initial transient fuel or electrical panel fire which then spreads to nearby

electrical cables.

The parameters of a fire of this type would be quite similar to those discussed above for a

large electrical panel fire. Differences might include somewhat larger and longer lasting

fires, and higher enclosure temperatures. Thus, a more severe smoke exposure

environment might be experienced. As with the large electrical panel fires, the full range

of fire suppression systems utilized by the industry may come into play in such a fire. In

particular, the use of water for a more generalized fire would be much more likely than in

the case of the large electrical panel fire.

Most any fire area in the plant would be subject to fires of this type [12]. The one

exception would be the main control room. That is, the main control room is unique in

several ways which will significantly reduce the likelihood of a large generalized area fire

occurring. First, the main control room is continuously manned, and hence, very rapid

fire detection and intervention times are expected. This will also imply that a transient

fuel source will be very effectively controlled and limited. Second, high-energy electrical

equipment is not typically housed in the control room, and hence, the number of potential

high energy fire sources is very limited. Based on these factors, the occurrence of a large,

generalized fire in the main control is not considered likely, and hence, this scenario

would not generally be considered applicable to the main control room.

NUREG/CR-6476 A-15
2. Quantity of Combustible Materials Involved

In this case, the quantity of combustible materials involved would include both the

transient combustibles if such fuels are involved, and the in situ fuels. A wide range of

fuel involvement levels is possible. Recall that this scenario is associated with general

area fires which are of sufficient size to present a smoke exposure threat to the entire fire

area. The objective here, then, is to establish reasonable upper and lower bounds on

significant general area fire fuel consumption values. Hence, very small fires will not be

considered because they are unlikely to produce sufficient quantities of smoke to

represent a threat to an entire fire area.

As a rough estimate, the minimum fuel involvement for a significant general area fire will

be assumed to correspond to the minimum fuel loads cited above for the full-simulation

cabinet fire tests. Recall that the minimum full-simulation cabinet fuel load evaluated in

the SNL cabinet fire test program was 25 kg (55 lb). Note that the peak heat release rate

measured by SNL for this particular test was approximately 840 W [8]. However, in the

cabinet fire test program, a significant fire intensity enhancement occurred due to the

containment of some of the fires' beat output within the cabinet and radiative feedback

from the heated cabinet surfaces [7,8]. In an open fire configuration, the same fuel

configuration would not benefit from the enhanced feedback, and might be expected to

yield a peak heat release rate on the order of 500-600 kW, a reasonable lower bound

intensity for significant fires.

For the upper bound, it would be appropriate to consider a fire that might involve a stack

of cable trays, perhaps in addition to a electrical panel fire as an initiation source. Hence,

as an upper bound, one might consider a fire involving the maximum full-simulation

cabinet fire loads cited above, in addition to supplemental fuel from overhead cable trays.

The amount of overhead cable fuel that might become involved in such a fire could,

again, vary widely depending on the plant-specific conditions which prevail. As a rough

estimate, it will be assumed that an additional cable tray fuel load equal to that available

in the cabinet might become involved. Hence, the upper bound fuel consumption will be

set at twice that cited above for the most heavily loaded of the full-simulation cabinet fire

tests. This would correspond to 102 kg (224 lb) of fuel.

In the assessment of the typical enclosure volume into which the fire products might be

distributed, the same arguments presented above would also directly apply to this

scenario. Hence, a typical enclosure of the type of interest here will be assumed to have a

total volume of 1816 m3 (64,000 ft3). Based on these values, the lower bound on fuel

mass to air volume ratio for the significant general area fire scenario would be the same

as the lower bound cited above for the large cabinet fire in the absence of ventilation

dilution effects, 14 g/m 3 (8.7x 104 lb/ft3). The upper bound value would correspond to 56

g/m3 (3.4x 103 lb/ft3).

Note that these values include no consideration of ventilation system diluting effects. In

this case, the scenario is not considered applicable to the control room, and hence,

A-16 NUREG/CR-6476
ventilation shutdown would be expected. No additional dilution due to ventilation effects

would be recommended for this scenario.

E. Smoke Spread from an Adjacent Fire Area

The final fire threat scenario of potential interest would involve the spread of smoke from

one fire area into an adjacent fire area. The most likely paths for such spread would be

transport through ventilation ductwork (should the fire dampers not close), or transport

through doorways that might be opened for manual fire fighting and post-fire recovery

access.

In this fire scenario, the fire source could be either the large cabinet fire or the general

area fire as discussed above. However, in this case the smoke exposure would involve a

lower density of smoke, and a reduced thermal threat. Fire suppressant interaction would

also be less important because the adjacent area components would not be subject to

direct application of fire suppressants unless a spurious actuation of a fixed fire

suppression system in the adjacent area were to occur. Components might be subjected

to very high levels of humidity, however, if water is used to fight the fire in the adjacent

space.

For this scenario, a significant dilution effect would be expected. That is, as the smoke

spreads from the room of fire origin, it would mix with the fresh air in the adjacent space.

Under conditions in which both rooms became equally involved in the spread of smoke, a

doubling of the affected air volume would occur (assuming the rooms are essentially the

same size). Hence, dilution of the general enclosure fuel mass to affected air volume

ratios cited above by a factor of 2 would be appropriate. This would yield values ranging

from 7 g/m3 (4.3x10 4 lb/ft3) to 28 g/m3 (1.7x10 3 lb/ft3).

F. Summary of Scenario Fuel Mass to Air Volume Ratios

For each of the fire scenarios identified, a range of values on the anticipated fuel mass

consumed to affected air volume ratio has been provided. Table I summarizes these

results.

Note that by far the most severe ratios are associated with the localized effects within an

electrical control panel due to a small electrical fire occurring within that cabinet (the

small cabinet fire scenario). This results primarily from the fact that a very small volume

of space (that within the cabinet only) is assumed to be involved in the distribution of

smoke. Hence, relatively large fuel mass to affected air volume ratios are predicted,

despite the fact that as little as 5% of the cabinet fuel load is assumed to burn.

In contrast, the least severe exposures are those associated with a large cabinet fire

occurring within the main control room. In this scenario, it is postulated that the smoke

generated by the burning of a single control panel is distributed throughout the full room

NUREG/CR-6476 A1 A- 17
Table 1. Summary of recommended fire scenario fuel mass to affected air volume ratios.

Scenario (affected area) Fuel mass to affect air volume ratio

Lower Bound Upper Bound

Small Cabinet Fire (in-cabinet effects) 26 g/m3 560 g/m 3

1.6x10.& lb/ft3 3.5x 10.2 lb/ft3

Large Cabinet Fire Control Room 2.8 g/m 3 11.2 g/m3

(room effects) 1.7x 10" lb/ft3 7x10 4 lb/ft3

General Areas 14 g/m3 28 g/m3

8.7x 104 lb/ft3 1.7x 10 3 lb/ft3

General Area Fire (room effects) 14 g/m3 56 g/m3

8.7x 104lb/ft3 3.4x 10"3 lb/ft3

Adjacent Area Fire (room effects) 7 g/m3 28 g/m3

4.3x 104lb/ft3 1.7x 103 lb/ft3

air volume and that the ventilation system would continue to operate in an emergency

smoke purging mode. The high rates of air exchange associated with this mode of control

room ventilation system operation would significantly dilute the smoke exposure, even

though a smoke layer would still be expected to form.

The difference between the lower and upper bounds for all of the cases cited represents a

factor of 200. That is, the overall lower bound cited is 2.8 g/m3 (1.7x10-4 lb/ft3) and the

overall upper bound cited is 560 g/m 3 (3.5x10 2 lb/ft3). This wide range of values reflects

the wide variability in the types of fire scenarios which might be encountered, and is

considered representative of actual plant conditions. As a result, it is recommended that a

very wide range of fuel mass to test enclosure air volumes be evaluated to ensure that this

wide range of anticipated behavior is assessed.

IV. Identification and Characterization of Critical Smoke

Quality Parameters

A. Overview

The issue of smoke quality is the issue of the chemical composition of the smoke,

including the changes in composition that might take place during the processes of

generation, transport, and deposition. As discussed above, the definition of smoke

quality will be significantly affected by the general lack of knowledge regarding the

chemical nature of smoke. Hence, in this study, smoke quality will be addressed through

the definition of the source of the smoke and through identification of potential transport

and suppressant interactions that might alter the nature of the smoke exposure.

A-18 A-18 NUREG/CR-6476
Consistent with this approach, those parameters which are considered critical to the

proper characterization of smoke quality are identified in the following subsections. In

particular, the following parameters are discussed in detail below:

- the type of material burned,

- the mode of combustion experienced,

- suppressant interactions, and

- transport interactions.

B. Type of Materials Burned

Perhaps the most critical parameter in the determination of smoke quality will be the

nature of the material(s) which is(are) actually burned. A variety of combustible

materials are likely to be encountered in a nuclear power plant fire. The smoke generated

by different materials will be inherently different. That is, while virtually all the

materials of interest will produce some quantity of carbonaceous soot, the balance of the

smoke chemical composition will vary widely. The range of potential chemical

combustion products will depend on the chemical composition of the burning material(s).

In general, the combustible materials of interest in a nuclear power plant can be divided

into four fuel categories; namely, combustible gases, liquid fuels, solid transient fuels,

and solid in situ fuels.

1. Combustible Gases

Combustible gases are often present in a power plant. The most common would be

hydrogen which is present as both a component coolant (particularly as a turbine-bearing

coolant), and as a by-product of the reactor operation (through dissociation of water).

Many fire events have involved combustible gases, and particularly hydrogen fires and

explosions [12].

However, in terms of the generation of smoke and the threat to plant equipment

represented by that smoke, the importance of combustible gases is minimal because these

fuels tend to be very clean burning and introduce no exotic chemical species into the

combustion products. Hence, while such fires may generate significant quantities of

smoke, the smoke generated by the combustible gases themselves will generally be

composed of relatively "clean" carbonaceous soot. Such soot will not likely result in a

significant threat to electronic equipment because of the minimal potential for corrosive

attack or creation of electrical short circuit paths. Some difficulties with contact pair

fouling and restriction of fine mechanical movement can be anticipated, but as discussed

above, these degradation modes are not considered critical for advanced electronic

components.

The importance of combustible gases as a fuel is much more significant if they are a

source of fire exposure for the other in situ fuels that might become involved in such a

fire. Even issues of fire suppressant interactions are of secondary concern because

NUREGICR-6476 A1 A- 19
virtually all of the fires that have involved combustible gases have been controlled by

simply isolating the source of the gas [12]. The remaining concerns would center on any

other local fuel source, as discussed below, which had become ignited during the initial

gas fire. The smoke threat to electrical equipment represented by the direct combustion

of gaseous fuels is not considered significant and will not be discussed further.

2. Liquid Fuels

Liquid fuels would include such items as lubricating and cooling oils, cleaning solvents,

and diesel fuel. These items are commonly used in pumps, hydraulic-operated

equipment, diesel-driven motors, and other equipment requiring lubrication. Fires

involving such fuels are relatively common for plant areas housing such equipment

(generally due to leakage problems) [12]. These types of materials are generally

composed of relatively simple chemical species, primarily hydrocarbons. Hence, the

smoke generated by fires involving such fuels will be composed largely of carbonaceous

soot. The only other significant chemical species that might be evolved would be

associated with any additives included in the material's formation. Such additives are

relatively uncommon for most fuels of interest and hence invasive chemical compounds

are not likely to be produced during their burning. Based on these observations,

conclusions similar to those cited above for gaseous fuel sources would apply to liquid

fuels as well, and no further consideration will be given to such fuel sources.

3. Transient Fuels

Solid transient fuels would include general trash, paper waste, wood, plastics, cloth, etc.

These fuels can generally be found in small quantities in most plant areas at any given

time. However, plant fire protection procedures typically will include administrative

controls that actively limit the extent to which such items are allowed to accumulate

within critical plant areas. While the smoke from transient fuel fires may contain a

variety of chemical species (depending on the exact materials present at the time of the

fire), fires involving such fuels will generally be limited in both extent and duration. Due

to the limited availability of such fuels, a fire that remains limited to transient fuels is

unlikely to produce sufficient quantities of smoke to result in short-term equipment

damage problems. Because of these factors, conclusions similar to those cited above for

gaseous fuels would also apply to transient fuels. In this case, the transient fuels would

be considered of secondary importance, and would be of interest primarily for the extent

to which they represent a potential fire exposure source for the other in situ fuel items.

4. In situ Solids

The final, and by far most important, category of fuels is the in situ solid fuel elements.

Of these materials, the majority will be in the form of cable insulation and jacketing

materials. Several factors combine to support a conclusion that cable insulation and

jacketing materials will represent far and away the most important materials to be

considered in assessing a smoke threat to electrical equipment. Of secondary interest will

A-20 A-20 NUREG/CR-6476
be the materials used in the construction of the other types of electrical components

(including phenolics and other types of hardened plastics).

A variety of cable insulation materials are commonly employed in the nuclear industry.

Each of these materials is likely to introduce unique constituents when burned. Hence, it

is expected that testing might be expected to involve both individual materials assessed

on a relative basis, and the use of combinations of materials to assess potential synergistic

effects. Cable insulations popularly applied in the nuclear industry include materials

based on the following compounds [13,14]:

- Cross-linked polyolefin (XLPO) (including the more

specific class of cross-linked polyethylene (XLPE)) - Polyvinyl chloride (PVC)

- Ethylene-propylene rubber (EPR)

- Chlorosulfonated polyethylene (CSPE or Hypalon)

- Teflon

- Silicone and silicone/rubber compounds

Virtually any significant fire scenario in a nuclear power plant will involve cable

insulation materials to some extent. In particular, those fire scenarios expected to result

in the exposure of advanced electronic components are likely to involve the burning of

cables. This would include control room and remote shutdown area fires, and fires

associated with electrical penetration areas of the containment structure where signal

processing and multiplexing systems are likely to be located [15]. Hence, cable

insulation and jacketing materials are considered the single most important class of fuel

to be considered in this program. Most of the other materials used in plant construction

are noncombustible.

The importance of cables as a threat to electronic equipment also derives from the nature

of the smoke produced in a cable fire. Today cable insulation and jacketing materials are

generally composed of either rubber-based, silicone-based, or polymer-based materials.

Typical production processes for multiconductor cables will also include some type of

physical binding material such as nylon or cellulose strands or a cellophane-type

overwrap. In the formulation of cable insulations, a wide range of chemical species are

added for use as plasticizers, aging performance enhancers, and fire retardants. Fires

involving cable insulations are particularly "dirty" in comparison with fires involving

other types of fuels because these various chemical species are also liberated during the

fire. Of particular concern, a variety of acid gas species are likely to be produced during

any such fire due to the wide use of halogen additives [chlorine (Cl), fluorine (F),

bromine (Br), iodine (I), etc.] as fire retardant agents. The extent of chemical production

is dependent on the specific chemical composition of a particular cable.

For example, polyvinyl chloride (PVC) materials are composed of as much as 40% by

weight chloride. This chloride is released during a fire, and much of it eventually

becomes bound to the soot particulates in the form of soluble chloride ions [1]. Upon

contact with water, a hydrochloric acid (HCl) solution is readily produced. While PVC is

NUREGICR-6476 A2 A-21
no longer popularly used as a cable insulation for new applications, it is commonly

present in most areas of nuclear power plants constructed prior to 1980. (PVC fell out of

favor in the U.S. nuclear industry as a result of the 1975 Brown's Ferry fire and the

ensuing regulatory actions.)

For other cable insulation types, other chemical species will be of predominant concern.

It is expected that these acidic combustion products will play a key role in the problem of

smoke damage to advanced types of electrical equipment. This role will likely involve

both direct corrosive attack on the components and the formation of electrical current

leakage or shorting paths as discussed above.

The presence of these varied chemical species can also lead to interactions of the smoke

with other materials in the plant. For example, experience in the telecommunications

industry [6] indicates that the interaction between acidic smoke from a cable fire and

galvanized ventilation ducts can result in the formation of galvanic salts, which can in

turn be deposited on electrical equipment. This has been identified as the source of

significant nonthermal fire damage in this industry. (This issue is discussed further

below as associated with transport issues.)

Further, fires involving cable insulation materials are typically low-efficiency fires. That

is, due to the nature and the arrangement of fuel, the fires produced are inefficient in

converting fuel to heat. This means that the combustion process results in the release of

many intermediate combustion products (see further discussion of burning mode effects

below). These products are often composed of relatively massive molecules that

agglomerate easily, accelerating the process of surface deposition. Hence, the

accumulation of tar-like deposits has been noted in cable fire tests [ 13].

The final class of in situ solid fuel which is also of potential interest would be those

materials used in the construction of other types of electrical equipment. In particular,

those materials used in the construction of such items as printed circuit boards, relays,

switches, indicators, plastic cable routing devices, wire ties, and cable splicing connectors

are also generally composed of a variety of polymeric materials. These materials are also

likely to produce a relatively aggressive form of smoke when burned, although relatively

little information on the burning of these materials is currently available.

5. Material Identification Conclusions

Based on these considerations, it is concluded that, far and away, the most significant

threat of smoke damage to electrical equipment will be associated with the burning of

cable insulation and jacketing materials. This conclusion is based on both the dominant

role cables play in the overall plant combustible fuel load, and on the nature of the

combustion products expected. Of secondary importance are other materials use in the

construction of other types of electrical equipment.

A-22 A-22 NUREG/CR-6476
C. Combustion Mode

When materials bum, that burning can take place in different ways. In particular, burning

has classically been divided into open flaming and smoldering modes of combustion.

Open flaming is generally the more efficient mode of burning and is typically associated

with more volatile fuels, larger fires, and higher temperature fires. Smoldering is

generally a less efficient mode of burning, and is often associated with less volatile fuels,

lower temperature exposures, and either the very early or late stages of a fire event.

Smoldering combustion will also dominate the early stages of an electrical fire initiated

through component failure.

However, in the consideration of the smoke threat, it would also be useful to consider the

impact of oxygen availability on fire burning. That is, fires can also be classified in terms

of fuel surface controlled and oxygen-limited fires. In a fuel surface-controlled fire, the

rate of combustion is limited only by the quantity of fuel that has actually been ignited.

In oxygen-limited burning, the fire intensity will be limited to that which the available

oxygen can support. In general, a fuel-surface controlled fire is associated with more

efficient burning. Oxygen-limited burning may occur during the later stages of a fire, and

a decrease in combustion efficiency will typically result. Oxygen-limited burning

conditions may prevail as a result of either limited availability of oxygen in the fire area

as a whole, or due to more localized ventilation limitations. For example, a closed

electrical control panel in which a significant fire occurs is likely to involve oxygenlimited burning even though the room as a whole may be well ventilated.

Both of these factors, flaming versus smoldering and fuel surface versus oxygen-limited

burning, are expected to affect the type of smoke generated during a fire. For example,

the overall efficiency of the fire will determine the extent to which combustible

compounds are actually oxidized. Fires with low burning efficiencies will result in many

unburned pyrolysates being generated. This can be a very important factor because these

unburned pyrolysates tend to be relatively heavy (dense) compounds and will quickly

deposit onto the available surfaces, including the electronic components. In a highefficiency fire, the products of combustion will be more heavily dominated by gaseous

species such as CO, CO, and water vapor. One would also expect the particulate in a

high-efficiency fire to be dominated by carbonaceous soot, which is likely to remain

airborne much longer.

The effects of open flaming versus smoldering may also be important beyond the

question of combustion efficiency. Under open flaming conditions, it would be expected

that virtually all of the combustible material would be burned over a relatively short

period of time, and much more of the carbonaceous material would be liberated as

airborne gases and soot. This would imply that all of the chemical species likely to be

generated would be generated simultaneously making subsequent chemical interactions

more likely. Under smoldering fire conditions, the material will burn much more slowly

with the most volatile compounds liberated first. A larger fraction of the carbonaceous

material would likely remain as char as well. Unfortunately, the more volatile

NUREG/CR-6476 A-23 A2
compounds include the plasticizing and flame retardant additives that are likely to result

in significant production of acid gas.

As a result of these anticipated differences, it is recommended that investigations be

undertaken to assess the impact of burning mode on the smoke threat. That is, each mode

of combustion is likely to result in a somewhat different smoke threat, and it is not clear

which mode might be most significant. Controlled tests should be performed to explore

each mode to ensure that the most significant mode of combustion is identified and

understood.

D. Suppressant Interactions

In most fire events, some form of active fire suppression is likely to come into play. The

most commonly applied suppressants in the nuclear industry are water, water-based

foams, and carbon dioxide (CO2). Each of these types may be used in either hand-applied

devices (hose streams, fire extinguishers, and hand-held CO, discharge nozzles) and in

fixed fire suppression systems (area coverage and directed spray systems for the waterbased systems and room flooding or local discharge systems for CO,). Also commonly

encountered are hand-held fire extinguishers that use dry chemical powder. This is a fire

suppression agent often used to fight electrical equipment fires. Each of these systems

represents a unique potential for interactions involving combustion products. To a much

lesser extent, Halon systems may also be used.

1. Water-Based Suppressant Interactions

For the water-based systems, the obvious effect will be to increase the general humidity

levels within the fire area to near saturation levels. The suspended droplets can act as

agglomeration points for the soot in the air and hence can lead to an increased rate of soot

deposition on exposed surfaces. Moisture droplets will also form on the various exposed

surfaces, including the surfaces of the advanced electronic components. Due to the

highly hygroscopic nature of acid gas molecules, the deposited moisture will then act to

scavenge acid gas molecules from the environment, creating liquid acid deposits. These

acidic deposits can cause direct corrosive attack, and can create short circuits and circuit

bridging faults. Hence, the interactions with moisture are expected to be a general

increase in the rate of soot deposition, and the formation of liquid acid deposits.

2. Carbon Dioxide Suppressant Interactions

For CO2 systems there are three potential modes of interaction that might be observed.

First, the discharge of a CO2 system can introduce very high levels of static electric

charge on surfaces that are not fully grounded. Such static charges may induce very large

current surges, which could easily destroy solid-state chips (note that even relatively mild

A-24 A-24 NUREG/CR-6476
static charges can cause failures in various types of solid-state chips). This potential may

be enhanced by smoke deposits, which can partially bridge circuit paths. Second, the

discharge of CO2 will introduce a pronounced and very abrupt cooling effect. This would

be expected to lead to water vapor condensation and the formation of moisture droplets,

or even frost, as surface deposits. Third, the increased concentration of CO2 gas could

result in the formation of carbonic acid in surface moisture droplets. Carbonic acid is an

aggressive, albeit short-lived, corrosion agent that might aggressively attack circuit

components. This aggressive attack, when combined with smoke, might increase the

vulnerability of circuit components to damage.

3. Dry Chemical Powder Interactions

Dry chemical powder is a suppression agent which is commonly used to fight electrical

equipment fires. It is one of the most commonly encountered fire suppressant agents

used for hand-held extinguishers. Hence, its use in fighting smaller electrical fires is

quite likely.

Dry chemical powder is generally considered an aggressive corrosion agent. Often,

electrical equipment sprayed with dry chemicals will be destroyed in a relatively short

time if it is not cleaned. Hence, it is likely that dry chemical powder may introduce a

significant threat to electronic equipment in and of itself. In addition, it may introduce

synergistic interactions with the smoke deposition, which might accelerate the rate of

component degradation.

It is recommended that some tests be performed to assess the potential for synergistic

effects involving fire-fighting powders. However, because the powders themselves

represent a potential threat to electronic equipment, it may be necessary to test such

materials in the absence of smoke to determine whether synergistic effects with the

smoke exposure are significant. Hence, testing in both the presence and absence of a

smoke exposure is recommended.

4. Halon Suppressant Interactions

To a lesser extent Halon is also used as a fire suppressant by the nuclear industry. Halon

is a fluorocarbon compound with a particularly high Br content. It is well known that

when it is discharged into a high-temperature environment, or brought into contact with

high-temperature surfaces, Halon will break down, releasing this Br as an acidic gas.

Hence, an increased acid attack potential could be introduced by Halon release. In

addition, Halon discharge will result in a pronounced localized cooling effect, though not

as severe as that expected for CO2 discharge. However, it is not recommended that an

experimental investigation of Halon and smoke interactions be undertaken at this time.

Halon has been identified as an ozone threat, and hence, it being phased out of all use in

response to international accords on the protection of the ozone layer. It is anticipated

that the use of Halon in nuclear power plants will become more and more rare in the near

NUREG/CR-6476 A-25
future, and the importance of Halon as a fire suppressant will be largely negated. It

should be noted that efforts to find a replacement for Halon are being pursued by various

industrial concerns. Should such replacements be brought to the market and find

application in the nuclear industry, then the new materials should be reviewed to

determine whether an investigation of potential adverse effects might be warranted.

E. Transport Interactions

The transport of smoke from the fire source to the component of concern is an important

aspect of the smoke quality definition. This transport may take place through simple

passive spread of the smoke within the fire cabinet or fire area, or through the local

ventilation and air-handling systems. There is one important potential interaction

associated with this transport process which should be considered. That is, chemical

interactions may take place between the smoke and the various surfaces involved in the

movement of smoke from one location to another.

In many fires involving cables in communications facilities [6], it has been observed that

the acidic smoke will interact with galvanized metals, resulting in the formation of

galvanic salts. These salts are hygroscopic and in a humid environment, can attract water

from the air, forming a liquid zinc chloride solution. This solution can run or drip onto

electronics. Once these deposits form on a component, an aggressive and debilitating

attack will begin. In a nuclear power plant, extensive galvanized metals will typically be

present. This will certainly include the cable trays and conduits, and may also include

certain of the electrical control panels themselves. Ventilation air handling ducts are also

typically constructed from galvanized sheet metal.

Testing should be performed to assess this interaction. It is recommended that sheets of

galvanized metal be place within the test enclosure during some of the controlled tests.

Placement of metal platens in the general smoke environment should be sufficient to

simulate interactions remote from the fire source, such as those that might occur with airhandling ducts. It is also suspected that elevated temperatures might significantly

increase the rate of these interactions. Hence, it is recommended that some of these tests

be performed with the metals in close proximity to the actual fire source in order to

simulate situations in which, for example, the burning cables are housed in galvanized

cable trays or in close proximity to galvanized conduits.

F. Summary of Smoke Quality Insights

The issue of smoke quality has been approached by defining those factors considered

important in determining the nature of the smoke to which components would be exposed

in a nuclear power plant fire. It is anticipated that by reproducing these factors in the

laboratory, a representative smoke exposure will result, even if the a priori knowledge of

the smoke composition remains poor.

A-26 NUREG/CR-6476
The first factor identified was the type of materials that are likely to be burned. Cable

insulation and jacketing materials were identified as the single most important material

based on both their dominance of the in situ plant combustible fuel loads and on the

nature of the smoke produced when these materials are burned. Of secondary importance

were other types of polymeric materials used in the construction of general electrical and

electronic equipment. Other fuels, including combustible gases, liquid fuels, and

transient fuels, were considered of minimal concern based on the limited quantities

present in a plant and/or based on.the relatively "clean" nature of the smoke such fuels

produce (i.e., gases and liquid fuels in particular will generally produce a smoke

composed primarily of carbonaceous soot).

A second factor identified was the mode of burning. It was noted that factors particularly

associated with the efficiency of the fuel burning are expected to significantly affect the

nature of the smoke produced. Incomplete combustion products are expected to result in

more massive smoke particles, more varied chemical species, and faster deposition onto

exposed surfaces. It was recommended that testing be performed to encompass a range of

fuel burning conditions including smoldering, open flaming, and oxygen-limited

combustion.

Suppressant interactions were also identified as a potential issue. In particular, likely

synergistic interactions associated with water-based suppressants, CO2, and PKP powder

were discussed. Testing to assess the importance of these interactions is recommended.

Finally, it was noted that experience in the telecommunications industry has indicated

that the interaction of acidic combustion products with the surfaces of galvanized metals

can result in the generation of galvanic salts that can be readily deposited onto exposed

surfaces. Based on the wide use of galvanized metals, it is recommended that testing be

performed to assess this issue.

V. Duration of Smoke Exposure

A. Overview

In considering the duration of smoke exposure, two time periods are relevant. First, the

period of time required to detect and suppress the fire, and to then remove the smoke

from the fire environment will determine the time during which the component is directly

exposed to the smoke-filled environment. Second, the extent and timing of post-fire

equipment recovery actions will determine the extent to which a post-fire equipment

exposure threat will continue. Each of these two time periods presents the potential for

equipment damage.

In terms of the direct impact on fire risk, the earlier period, that period during the fire

itself, will be of primary concern. In general, the dominant fire risk scenarios are

relatively short lived and take place during a 1 hour or less [9]. Hence, the short-term

NUREG/CR-6476 A2 A-27
equipment damage questions are of primary concern. However, longer term damage

questions should also be considered. That is, one of the questions that should be

addressed as a part of this program is the potential that smoke deposition might lead to

component failures at some time after the fire. The following subsections discuss various

aspects of the exposure duration question during each of these time periods

B. Smoke Purge Time

In the first case, the time required to purge the smoke from the environment, a wide range

of exposure duration times should be expected. These will be directly related to the

magnitude of the postulated fire event, and would be significantly affected by such

factors as the availability of detection and fixed fire suppression systems.

The shortest time frames expected for smoke purging would be associated with the small

cabinet fires in which the only actions required to remove the smoke would be opening of

the cabinet doors and suppression of the fire. In this case, the postulated fire remains

quite small and hence no significant smoke accumulation in the general area is assumed.

Once the cabinet has been opened, the smoke will dissipate and be handled by the normal

ventilation system. Times on the order of 15-30 minutes would be considered typical for

such scenarios [12].

The time to purge smoke from the environment would increase as the magnitude of the

fire threat increased. For the fire scenarios involving a large cabinet fire and the resulting

general area smoke exposure, times on the order of 1 to 2 hours might be expected to

fully purge the environment of smoke for general plant areas [8,9]. This includes the

time required for manual suppression and for manual recovery of the ventilation system.

If the fire is postulated to occur in the main control room, somewhat shorter time frames

would be expected, on the order of 30-60 minutes, because faster intervention and

continued ventilation system operation are expected.

For scenarios involving generalized room fires and the potential for the spread of smoke

to adjacent areas, times of anywhere from 30 minutes to 4 hours might be anticipated. In

this case, a very broad range of fire suppression and ventilation recovery times can be

expected given the variety in fire threats, fire detection capability, and fire suppression

availability which could be encountered [121.

In summary, the minimum time frame required to purge smoke from the environment in

the event of a fire is expected to be on the order of 15 minutes. This would correspond to

the scenarios involving small in-cabinet fires which are very quickly detected and

suppressed. The maximum time required to purge smoke is considered to be on the order

of 4 hours. This assessment is based largely on past fire experiences and includes an

assumption that manual recovery of the ventilation capability will be required. This

would also correspond to the most severe and generalized of the fire scenarios postulated.

It is recommended that electronic components be tested to encompass this range of actual

smoke exposure durations.

A-28 A-28 NUREG/CR-6476
C. Post-Fire Recovery Times

Once a fire has been suppressed and smoke has been purged from the environment, a

potential still remains that some equipment failures might occur at some time after the

fire. Clearly, the extent and timing of post-fire equipment recovery actions will play a

critical role. In particular, studies have shown that immediate intervention to reduce

ambient humidity levels can minimize the extent of corrosion damage [16]. It has also

been shown that much equipment can be cleaned and restored to operation if such efforts

are begun shortly after the fire (within on the order of 24 hours of the fire event) [16].

However, it is currently unclear to what extent plant operating procedures provide for

such actions to be taken in the nuclear industry.

Hence, it should be expected that an extended period of exposure to smoke deposits may

be experienced with only minimal post-fire intervention. This could lead to subsequent

unanticipated equipment failures, which might result in triggering of a plant transient.

However, it must also be recognized that, based on the results of past fire risk

assessments, the most important fire scenarios from a plant risk perspective occur during

time frames of roughly one hour or less [9]. Hence, the issue of longer term exposures

will be more important to the question of general post-fire equipment reliability, and

considered of secondary concern. Nonetheless, it is recommended that the smoke

exposure testing include an assessment of equipment performance over a minimum

period of 24 hours.

It is also recommended that, should the scope of work allow, certain of the exposed

components be monitored for extended periods of time (on the order of a month) after the

exposure. This would provide an additional assessment of the long-term failure potential

of smoke-exposed equipment. This approach could also be used to assess the

effectiveness of various component cleaning procedures.

D. Transport/Exposure Duration Interactions

It should be noted that there is a direct relationship between the issues of exposure

duration and smoke transport. That is, the duration of the smoke exposure may be

directly affected by the manner in which the smoke reaches the component of interest. If,

for example, the component is contained within an electrical panel in which a small fire

occurs, then much of the smoke may remain contained within the cabinet for extended

periods. If the component is affected by a more generalized smoke layer developed

during a larger fire impacting a full room, similarly long residence times should be

expected. In contrast, certain fire scenarios may involve only minimal exposure times if,

for example, a component is located directly in the path of the fire plume from a

relatively small fire. In this case, smoke exposures lasting no longer than the fire itself

may result.

NUREG/CR-6476 A-29
The second aspect of the smoke transport/exposure duration issue is that smoke is a

dynamic material whose properties will change with time. The smaller smoke particles

will agglomerate into larger particles over time, and the larger particles will deposit more

quickly than the smaller particles. Also, it has been observed in testing [1,17] that the

acid gases generated in a cable fire will become bound to the soot particles in relatively

short times (on the order of minutes). Thus, the deposition of soot becomes a mechanism

for the deposition of acid products as well. In this case, it is important that the smoke

exposure be of sufficient duration to simulate these smoke "aging" effects.

It is recommended that testing be performed to simulate a range of exposure durations. In

the ASTM smoke corrosivity standard that is being used as a guide to construction of the

smoke test chamber [18] it was specifically intended that a sample of smoke would be

captured and held for an extended period in order to assess corrosive smoke potential. A

standard smoke exposure period of 1 hour is used. This exposure is expected to represent

a reasonable duration for many fire scenarios. However, the period of exposure can be

varied by the experimenter with virtually no effort. It is for this reason that this particular

standard has been identified as the preferred test "model" for use in this program.

E. Summary of Exposure Duration Times

The recommended range of smoke exposure durations has been addressed in two parts.

First is the time period during which the component is actually exposed to a smoke-filled

environment. It is recommended that the actual smoke exposure duration used in testing

should be varied between 15 minutes and 4 hours to encompass the fire scenarios

identified as of interest. Second, the time after exposure to the smoke-filled environment

during which the accumulated smoke deposits would continue was assessed. A minimum

monitoring time period of 24 hours of functionality is recommended. In addition, it is

recommended that some of the components be periodically monitored for functionality

for an extended time after exposure. Periodic testing of the exposed components for an

extended time (preferably several months) would provide an assessment of the likelihood

that component failures could occur a significant amount of time after a fire event. If the

scope of work allows, it is also recommended that the effectiveness of component

recovery (cleaning) efforts be assessed in this way as well.

VI. Summary of Exposure Characterization Insights

In attempting to characterize credible smoke threat scenarios to which advanced

electronic components might be exposed, a number of factors were considered. In

general, the definition that has been presented is based on the characterization of three

aspects of the smoke threat; namely, the "quantity", "quality" and duration of the smoke

exposure.

A-30 A-30 NUREG/CR-6476
"Quantity" was defined in terms of the quantity of smoke per unit volume of affected air

to which a component might be exposed. In practice, the definition was based on an

assessment of the mass of fuel that would be consumed in the fire scenarios of primary

interest, and on an assessment of the size of the air volume into which the resulting fire

products would become distributed. The resulting fuel mass consumed to affected air

volume ratios ranged from 2.8 g/m3 (1.7x10 4 lb/ft3) to 560 g/m3 (3.5x10 2 lb/ft3). The

lower bound value was associated with a postulated large control panel fire within the

main control room, and included an assumption that the smoke would be diluted by

continuous operation of the ventilation system in an emergency smoke purge mode. The

upper bound value was associated with a small cabinet fire that consumed 15% of the

available fuel in a single control panel, and an assumption that all of this smoke would

remain confined within the fire panel. The assumption was that the rest of the

components in the fire source panel might not be damaged by direct involvement in the

fire, but that they would be exposed to the smoke from the fire.

The question of smoke "quality" refers to the actual composition of the combustion

products, and in particular, the chemical composition of the smoke. It was noted that the

general state of knowledge regarding the chemical composition of combustion products

for most materials is quite poor. In particular, most such efforts have focused on the

characterization of toxic fire products rather than on fire products in general. Hence, the

approach taken for this study was to define the types of materials that would be expected

to burn in a nuclear plant fire, and the interactions that might alter the chemical

composition of the materials that would eventually be deposited onto the electronic

components of interest. The one fuel material identified as of overriding interest was

cable insulation. Of secondary importance were other materials used in the construction

of electrical and electronic equipment. Of more limited interest were transient fuels (e.g.,

trash, wood, various plastics) and other in situ fuels (e.g., lubricants, solvents, and

flammable gases). It was also noted that various modes of combustion can be expected,

including in particular smoldering, open flaming, and oxygen-limited combustion.

Important interactions were identified that involve fire suppressants (in particular CO2,

water, water based foams, and fire fighting powders) and transport of the smoke (in

particular interactions with galvanized metal surfaces).

The final question, that of exposure duration, was addressed in two parts. First, the actual

time during which a component might be exposed to a smoke-filled environment was

assessed. The range of times cited was from 15 minutes to 4 hours. It is recommended

that the exposure testing encompass this range of exposure durations. Second, the time

associated with continued exposure to the accumulated deposits in the absence of

additional smoke exposure was considered. It was recommended that a minimum

postexposure monitoring period of 24 hours be allowed for all tests. It was also noted

that periodic longer term monitoring (on the order of monthly testing) of some of the test

samples would also be prudent. This long-term monitoring would help to assess the

potential that components might be damaged by long-term corrosive attack well after a

fire event has been experienced. If the scope of work allows, the effectiveness of

component recovery (cleaning) measures could also be assessed by long-term monitoring

of components.

NUREG/CR-6476 A3 A-31
VII. References

1. Screening Tests of Representative Nuclear Power Plant Components Exposed to

Secondary Fire Environments, SAND86-0394, NUREG/CR-4596, Sandia National

Laboratories, June 1986.

2. Investigation of Potential Fire-Related Damage to Safety- Related Equipment in

Nuclear Power Plants, SAND85-7247, NUREG/CR-43 10, Sandia National Laboratories,

November 1985.

3. Prioritization of Reactor Control Components Susceptible to Fire Damage as a

Consequence ofAging, SAND93-7107, NUREG/CR-6103, Sandia National Laboratories,

January 1994.

4. "Corrosivity Test Methods for Polymeric Materials. Part 3- Modified DIN Test

Method," Journal of Fire Sciences, V12, No. 2, pp. 155-174, March/April 1994.

5. "Corrosivity Test Methods for Polymeric Materials. Part 2- CNET Test Method,"

Journal of Fire Sciences, V12, No. 2, pp. 134-154, March/April 1994.

6. Hinsdale Central Office Fire Final Report, Joint report of the Office of the State Fire

Marshal (Illinois) and the Illinois Commerce Commission Staff, March 1989.

7. An Experimental Investigation of Internally Ignited Fires in Nuclear Power Plant

Control Cabinets, Part I - Cabinet Effects Tests, SAND86-0336, NUREG/CR-4527/V1,

Sandia National Laboratories, April 1987.

8. An Experimental Investigation of Internally Ignited Fires in Nuclear Power Plant

Cabinets, Part 1 - Room Effects Tests, SAND86-0336, NUREG/CR-4527N2, Sandia

National Laboratories, October 1988.

9. Fire Risk Scoping Study: Investigation of Nuclear Power Plant Fire Risk, Including

Previously Unaddressed Issues, SAND88-0177, NUREG/CR-5088, Sandia National

Laboratories, December 1988.

10. Enclosure Environment Characterization Testing for Base Line Validation of

Computer Fire Simulation Codes, SAND86-1296, NUREG/CR-4681, Sandia National

Laboratories, March 1987.

11. A Review of Regulatory Requirements Governing Control Room Habitability

Systems, SAND84-0978, NUREG/CR-3786, Sandia National Laboratories, August 1984.

A-32 NUREG/CR-6476
12. Users Guide for a Personal-Computer-Based Nuclear Power Plant Fire Data Base,

SAND86-0300, NUREG/CR-4586, Sandia National Laboratories, August 1986.

13. A Summary of the USNRC Fire Protection Research Program at Sandia National

Laboratories; 1975-1987, NUREG/CR-5384, Sandia National Laboratories, December

1989.

14. Aging, Condition Monitoring, and Loss-of-Coolant Accident (LOCA) Tests of Class

1E Electrical Cables, SAND91-1766/1-3, NUREG/CR-5772, Sandia National

Laboratories, August 1992 (report in three volumes).

15. Functional Issues and Environmental Qualification of Digital Protection Systems of

Advanced Light-Water Nuclear Reactors, ORNLiTM-12164, NUREG/CR-5904,

Brookhaven National Laboratory, April 1994.

16. "Damage Management," Fire Surveyor, June 1992, pp. 20-23.

17. "Transport and Decay of Hydrogen Chloride: Use of a Model to Predict Hydrogen

Chloride Concentrations in Fires Involving a Room-Corridor-Room Arrangement," Fire

Safety Journal, 16 (1990), pp. 33-52.

18. "Standard Test Method for Measuring the Corrosive Effect of Smoke fro the Burning

or Thermal Decomposition of Materials and Products," ASTM E05.21.70, Draft #2,

November 1992.

NUREG/CR-6476 A-33

Appendix B

A Review of Smoke Damage to Electrical Equipment

July 27, 1995

Revised July 26, 1996

A letter report for the USNRC Office of Research

Prepared by:

Tina J. Tanaka

Components & Structures Safety & Reliability Department

Sandia National Laboratories

NRC FIN W6051

Abstract

This report reviews literature on smoke damage to electrical equipment. Smoke can damage

electrical equipment because soot and acidic gases can corrode metals, coat contacts, and cause

shorts. Smoke damage to electronics is not limited to immediate failures, but also includes longterm failures. Some smoke damage statistics are available from accidental fires, but detailed

information on long-term damage from accidental fires is not readily available because best

practices for fire recovery include immediate cleaning of electronic equipment to halt corrosion.

Detailed information on long-term damage from smoke is available from controlled fire tests and

has also been correlated with damage due to ionic substances in the air. Nuclear power plant

operators should be aware of both the short-term and the long-term failures that can result from

smoke exposure.

NUREG/CR-6476B- B-1
Introduction

This report summarizes information available in the public literature on smoke damage from

both actual fires and smoke damage testing. A related subject, corrosion of electronic

components and assemblies, is also discussed here because the contamination produced by a fire

can be compared to long-term contamination from a polluted atmosphere.' Other aspects of

smoke damage considered include cleaning of smoke-exposed equipment, and methods used to

monitor the amount of corrosion that equipment experiences in order to predict the equipment's

useful lifetime. These aspects will be touched on briefly.

Assuming that all fires cannot be prevented, minimization of smoke damage to electrical

equipment may be approached in four ways: (1) regulate the type of fuel available to ensure that

the smoke will be noncorrosive, (2) control the smoke propagation so that the electronics will not

be exposed, (3) protect the electronics with conformal coatings or otherwise encase it to prevent

smoke deposition, and (4) immediately following a fire, take aggressive measures to clean and

recover smoke-exposed components. Some industries use combinations of smoke damage

prevention and equipment recovery. Once electrical equipment has been exposed to smoke, it is

important to determine whether the equipment must be replaced or can be cleaned. This report

includes findings by other researchers on the amount of chemical deposition that can be tolerated

by electrical equipment.

Smoke Damage Reported from Accidental Fires

This section reviews the smoke damage information available in the public literature from the

nuclear power, telecommunications, shipboard, and other industries. Reports on fire events in

the nuclear power industry are unique in that the reports concentrate on root causes of the fire

and actions by personnel during the fire rather than on the total cost and personal injury, as in

other industries. As a result, fire damage reports from the nuclear power industry have little

information on the type of damage to equipment experienced or on the long-term effects of a fire.

This singular viewpoint by the nuclear power industry reflects its emphasis on public safety

rather than economics.

Nuclear Power Industry

Two databases on fire events in nuclear power plants are available: the EPRI database,2 which

includes fire events dating from 1965 to 1988,and the Sandia National Laboratories (SNL)

database,3 which includes fire events from 1965 to June 1985. The information reported in these

databases was obtained from several sources, including the licensee event reports (LERs),

American Nuclear Insurers, nuclear power experience, power reactor events, and several

probabilistic risk assessments. The main function of these databases is to provide information on

the frequency, severity, locations, and ignition sources of fires in nuclear power plants for risk

assessment analyses. Although these databases include a field to list the damaged equipment, the

B-2 B-2 NUREG/CR-6476
reporting of such information tends to be sparse and the mode of damage is not given. Damage

reported in these databases was mainly the result of direct effects such as burned equipment.

The LER database does not reveal any specific information on equipment damaged by smoke or

the synergistic effects of smoke and other chemicals introduced during fire suppression. Smoke

damage probably occurs, but because the LER is more related to the root cause of a problem

rather than the resulting damage, not much analysis of the damage is reported.

Nuclear industry surveys contained little information on smoke damage to electrical equipment.

Smoke damage is probably a factor in the failure of equipment after a fire; however, little

attention has been paid to the failure mechanisms of electronics in a fire.

Telecommunications Industry

The telecommunications industry currently leads the nation in the investigation of smoke damage

to electrical equipment. Several telephone central offices have been severely damaged by fire

and smoke.4'

5

'

6 Studies of these events have revealed that smoke can account for up to 95% of

the total equipment damage.' The most extensively studied central office fire, in Hinsdale,

Illinois, in 1988, resulted in lost communication for 42,000 customers for several days, although

the fire was confined to cables in an area about 6 feet in diameter and several punch card bins

directly below the fire source. Included in the loss of communication was contact with the fire

station and the FAA air traffic control center for O'Hare airport.

Smoke induced failures were observed within 6 hours at the Hindsdale fire. Although much of

the switching equipment was functional after the fire, the equipment was badly corroded and had

to be replaced in a matter of weeks. The switching system used electromechanical switches

(relays). Much of this damage arose from acid gases, produced primarily from the burning of

polyvinyl chloride (PVC) cable insulation. Burning PVC generates hydrochloric acid because

PVC is 35% chlorine by weight. Hydrochloric acid attacks electronics by forming metal salts

that both weaken the metal structure and erode the conducting metal. The hydrochloric acid

corroded the switch contacts, allowing them only 1 month of life after the fire.

Bellcore (Bell Communications Research) has been a leader in the studies on smoke damage for

the telecommunications industry.6'

7 Reagor from Bellcore reports that the level of chloride attack

is the most important parameter in determining if equipment should be cleaned. The expected

total amount of zinc chloride (a metal salt) that a piece of equipment will accumulate on its

surface from the environment over a lifetime is 30 to 60 gLg/in 2. In contrast, fires can contribute

between 30 and 600 jig/in2 in a matter of minutes to hours. Bellcore research has shown that if

contamination levels are below 200 jig/in 2, the equipment can be restored easily, and at levels

between 200 gg/in2 and 600 jig/in 2, it can also be restored with some effort, but levels above 600

jig/in 2 are uneconomical to salvage.

Comizzoli and Sinclair, at AT&T Bell Laboratories, have studied corrosion of electronic

components and assemblies.8 '

9

" 0 Comizzoli proposed that the corrosion of metal contacts on

electronic equipment such as integrated circuits is accelerated by the combination of exposure to

atmospheric contaminants and an environment above a critical relative humidity of about 60%.

NUREG/CR-6476B- B-3
Atmospheric contaminants include dust and acid gases such as sulfur dioxide, nitrogen oxides,

and chlorides. Above the critical relative humidity, dust and moisture combine to form

electrolytic solutions. These solutions cause electrical leakage and corrosion on the component

or assembly. Comizzoli also proposed some test methods to determine how well the electronic

components are protected by assembly designs and coatings. These tests involved controlled

application of ionic particles by a forced air system.

The oil fires in Kuwait offered Comizzoli et al." an opportunity to study the effects of oil smoke

on electronics. The Kuwait oil fire experiments showed that H2S from the burning oil was a

significant contributor to corrosion of silver and copper coupons and to contact resistance of pin

and socket connectors. Coupons and connectors that were housed in filtered areas showed far

less corrosion. These results suggest that electronic equipment should be protected by either

coating the electronics with materials that will not allow the penetration of ionic substances and

humidity or controlling the air flow so that smoke will not come into contact with the electronics.

The telecommunications industry has emphasized the reduction of fuels that produced corrosive

smoke as a method to limit smoke damage to their equipment. For example, most telephone

companies now require that cable suppliers for the telecommunications industry manufacture

cables that produce less corrosive smoke. As a result, both the plastics industry' 2 and Factory

Mutual Research Corporation (FMRC) have become heavily involved in testing the relative

corrosivity of the smoke from different materials. The efforts of the plastic manufacturers to

produce standards for testing the corrosivity of materials are documented in a later section.

Most research into the effects of smoke on telecommunications systems has concentrated on the

long-term effects of smoke exposure, such as corrosion, rather than short-term effects, such as

circuit bridging. This may be a result of the amount of attention paid to the Hinsdale fire where

the power was turned off before the fire could be extinguished; it was this power loss that led to

the most extensive loss of function. During the fire, some smoke exposed switching equipment

failed, but little research has been devoted to prevention of immediate failures of the electrical

equipment. Instead the telephone industry has concentrated on reducing the losses from

corrosion after the fire. Although the telephone industry stressed the need for continuous service,

they can tolerate a temporary loss of service. The need for continuous service at a nuclear power

plant is much more critical.

Shipboard Fires

Shipboard fires have caused extensive loss of life and damage to equipment. Damage in the

British Navy in the period 1988 to 1989, for example, included 279 fires of different sizes and

amounted to £121K in losses.'3 Pivotal fire events leading to changes in the cable material

specifications for the British navy were fires started by missiles during the Falkland Islands war

in 1982.'4 The HMS Sheffield and HMS Ardent were both hit with missiles and sank because the

heavy black smoke from PVC cabling and linoleum prevented sailors from extinguishing the

fires. PVC (a halogenated compound) is considered inherently flame retardant because it is

approximately 35% chlorine by weight. However, given enough heat, PVC will burn and

produce black sooty smoke and large quantities of hydrochloric acid. Halogenated compounds

(those compounds which include fluorine, chlorine, bromine, and iodine) and Halon suppress

B-4 B-4 NUREG/CR-6476
fires by action of the halogen ions. Halogen ions react with hydrogen faster than oxygen,

preventing the oxygen from reacting with the fuel, thus extinguishing the fire. 5 In the process of

extinguishing the fire, however, toxic and corrosive acid gases form. Since smoke from

halogenated cables (such as PVC) is especially black and toxic, the British Navy has eliminated

PVC cables from their ships.

Fire-resistant cables that are nonhalogenated have been developed in the past 10 years. These

cables are doped with metal hydrates instead of halogens. Aluminum trihydrate and magnesium

hydroxide are two fillers that reduce the flammability of polyethylene, but do not contain

halogens. The flame retardancy is the result of (a) a smaller proportion of polymer, (b)

absorption of heat by the metal hydrates, and (c) release of water upon decomposition.' Cables

filled with metal hydrates produce less smoke, are less corrosive, and are less toxic than

halogenated cables.

The U.S. Navy has studied fire and smoke production for many years,'7.'8 and they also changed

their specifications as a result of changes made by the British Navy. The older specifications on

cables, such as MIL-C-915, included physical characteristics such as tensile strength, elongation,

and bending endurance. The newer specifications, MIL-C-24640 (1984) and MIL-C-24643

(1986), also include measurements of acid gas production, halogen content, smoke index, and

toxicity index. The smoke index and toxicity index are related to Naval Engineering Standards

borrowed from the British Navy. The smoke index measures the optical density of smoke, while

the toxicity index measures the amount of toxic gases produced by burning cables. Halogens are

no longer allowed in the cable jacket, which is the outermost layer of a cable. Research funded

by the U. S. Navy includes an assessment of the toxicity and the optical density of smoke from

shipboard materials.' 9 Work performed after 1985 includes studies on smoke damage to

structural materials such as steel that may be used in bulkheads.20 These studies included smoke

corrosivity tests from different types of cables. Current studies include an investigation of

potential damage to electrical equipment from Halon and its replacement fire suppressants.2

Other Fire Events

Factory Mutual is the leading insurance company organization doing research on fire and smoke

damage. FMRC claims that the effects of fire on a computer system, even a small smoldering

fire, can be extensive over a period of time, even if the fire is not located in the same room.

According to FMRC, computer rooms have been damaged more often by fires originating

outside the rooms than by fires originating within the room.2 Unfortunately, the bulk of FMRC

data related to these findings is proprietary, and hence, was unavailable.

Smoke Exposure Testing

This section discusses failure modes of electrical equipment in a smoke environment, equipment

survivability tests, and smoke corrosivity tests. Smoke-generated failures are specific to the

equipment exposed, and while some failure modes are very quick, others, such as corrosion, are

slower. Equipment survivability tests in this context are tests of the reliability or performance of

NUREG/CR-6476 B-5
equipment in a smoke-filled atmosphere. Equipment survivability tests have generally been

performed during full-scale fires and using equipment that is typical of systems that will be

exposed to smoke. No standard tests exist for equipment survivability in a smoke-filled

environment. In contrast, smoke corrosivity tests are small-scale material tests that measure how

corrosive the smoke from a particular material is to a specified metal target. Manufacturers of

cable and building materials developed these tests to prove that smoke from their cables is

benign. These tests provide only a relative measure of the corrosivity of combustion products

under very specific test conditions and the results cannot be used directly to predict equipment

damage.

Failure Modes

Most smoke failure modes are similar to failures caused by environmental pollution, but they

take place more quickly. In atmospheric pollution, dust and small particles can coat equipment

over periods of years and cause damage and failure. This damage occurs because of corrosion of

metal contacts, bridging of conductor circuits, and formation of nonconducting surfaces on

electrical contacts. Dust can damage moving parts, such as in mechanical timers and relays,

interfering with movement. Pollutants can also obscure optical equipment. The deposition of

smoke from a fire can be the equivalent of decades of dust in a normal environment. During a

fire, the environment is not only very polluted by smoke, but often the addition of humidity and

fire suppressant chemicals produces harmful synergistic effects. These conditions can produce

catastrophic failures for electrical equipment over relatively short periods (minutes to hours).

Smoke produces many problems for humans; it is toxic and obscures vision. For electronic

equipment, these qualities are not usually important, except in the case of optical equipment.

Because these qualities are very important for humans, however, standard tests have been

developed for measuring both smoke obscuration23 and toxicity. These tests are material tests,

that is, the various materials that will be the fuel are tested rather than any equipment. No studies

have been published on the effect of smoke on optical equipment.

Smoke damage to electrical equipment through corrosion of metal, especially from plastics such

as PVC, has been known for several decades.24 The development of standards to test materials

for smoke corrosivity has only been actively pursued since the mid-1980s. Present and proposed

smoke corrosivity tests concentrate on measuring the loss of metal through corrosion, although

corrosion is not the only failure mode for electrical equipment in a smoke environment.

Recently, several investigators in the field of smoke damage (including SNL) have proposed

experiments" that will also measure insulation resistance between contacts on a printed circuit

board. Soot deposits can reduce the insulation resistance by providing a conductive path

between contacts or circuit traces. Loss of insulation resistance can lead to undesired current

paths (leakage currents) or circuit bridging. Circuit bridging as a failure mode can be as

destructive as corrosion, but will occur on a much shorter time scale. No standards have been

developed to measure this effect.

The formation of nonconducting deposits on contacts can also lead to circuit failure. For

mechanical relays and switches, this can be an important failure mode. No standard test for the

B-6 B-6 NUREG/CR-6476
effect of smoke from different materials has been developed to investigate this failure mode;

however, tests on switchgear 16 and sockets11 show that smoke has a significant effect on contact

resistance. This particular failure mode is not considered relevant to digital circuits because

these circuits do not generally use open contact pairs as switching devices.

Failures due to circuit bridging, changes in contact resistance, loss of mechanical motion, and

optical obscuration can occur immediately from smoke exposure, while metal loss failures will

take place slowly over a period of weeks to months following the fire. Delayed failures are a

problem that may be overlooked by plant operators and that can seriously affect reliability and

plant safety. The most important failure mode is highly dependent on the type of electrical

equipment that is exposed to smoke. Studies of the reliability of electrical equipment in a smoke

environment must include equipment that is typical of the equipment under study and a smoke

environment that is characteristic of the type of environment that can occur in a fire. Some

failures, such as a change in contact resistance and inhibition of motion, are not considered

relevant to digital equipment. Material tests such as those published by the ASTM are important

to qualify material for use in a plant, but are no substitute for testing electrical equipment in the

smoke environment.

Smoke Exposure to Electrical Equipment

In 1986 Jacobus26 performed full-scale fire tests and smoke exposures on electrical equipment.

The test articles (equipment considered to be typical of that used in nuclear power plants) were

tested in a room that contained a burning electrical cabinet. Jacobus tested twenty-four switches,

thirteen meters, five relays, two strip chart recorders, two electronic counters, one power supply,

one power amplifier, and one oscilloscope amplifier. Some of the equipment was tested with

protective covers on, while other equipment was tested with the covers off. The protective

covers significantly reduced particulate accumulation. The components that were most strongly

affected were those with mechanical moving parts, such as chart recorders. Circuit boards on the

electronic counter also failed because of leakage currents. Switches and relays experienced

problems due to high contact resistance at low voltage levels. Nuclear-qualified cables and PVC

cables provided the fuel for these experiments.

Bridger et all 6performed two full-scale tests on switchgear using standard switchgear wire and

new low-emission switchgear wire. The switchgear included pushbuttons, indicator lights,

ammeters, relays, and switches. Bridger monitored the equipment before the test, immediately

after the exposure, 7 days after the exposure, and 3 weeks after the exposure. The low-emission

wire, which contained no halogens, caused less change in contact resistance in the equipment.

The residue from the low-emission wire was also easier to clean than that from the standard wire.

DuPont has tested three commercial 386 PCs using smoke from a dynamic combustion

instrument called a cone calorimeter.2 7 Commercially available utility programs tested the PC for

functionality during and after the smoke exposure. Three different cables provided the smoke for

these tests. Of the three tests, two PCs failed and one survived. DuPont considers these tests

important as far as illustrating the potential for damage from smoke originating from different

materials; however, a PC is much too complicated as a test object to reveal the precise causes of

failure.

NUREG/CR-6476B7 B-7
Smoke Corrosivity Testing

Low smoke corrosivity requirements by the telecommunications industry have produced great

interest in smoke corrosivity testing. Early tests have been performed using a full-scale fire, that

is, the fire is contained in a room about the size of a small office or larger, and the smoke

exposure samples are contained in an adjacent comparable room. Since full-scale testing is not

very economical, standards committees have concentrated on developing small-scale tests that

are supposed to simulate the effects of a real fire. The controversy that develops over use of a

small-scale test is: does the small-scale test really produce smoke has the characteristics of a fullscale fire and are the exposure conditions typical of those expected in real fires? Hydrochloric

acid gases, which cause corrosion, combine quickly with water. As a result, these gases may not

be well distributed throughout a room after a fire.", 29 In a fire, soot is also not well distributed;

the heavier deposits are closer to the fire. The extent to which factors such as these are

accounted for in the various standard tests is questionable.

Since the mid-1980s there have been many proposed test standards to measure the corrosivity of

smoke from solid materials. These tests measure either smoke acidity or metal loss. Chapin has

compiled a list of smoke corrosivity tests that are currently in use or proposed by organizations

around the world3"' (see Table 1). These tests only assess one failure mode (corrosion) either

directly, as in the last four methods on Table 1 that measure loss of metal by measuring

resistance, or indirectly by measuring the acidity of the smoke as in the first eight tests listed in

the table. Note that acid gas generation tests consist of percolating smoke through distilled water

and then measuring the resulting acidity with a pH meter or by other chemical analysis. Since

many halogenated materials produce acidic gases when burnt, and the acidity of the gases is

assumed to determine how corrosive they are, the acidity measured by these tests gives some

indication of how corrosive the smoke may be.

The early work on smoke corrosivity concentrated on measuring corrosion through the loss of

metal from solid metal coupons. The loss of metal was measured in either of two ways: (1)

measuring the weight gain as a function of time -- this measures the additional weight of oxygen

due to formation of rust (metal oxides),31,20 or (2) sandblasting off all of the corroded metal and

then weighing the metal loss. 2 In either case, the change in weight is very small compared with

the weight of the original piece of metal; thus, the measurements must be done very accurately.

Other uncertainty is added because for the first technique other materials may add to the weight.

With the sandblasting technique, some noncorroded metal may be lost in the sand-blasting

process.

Instead of measuring weight changes, metal corrosion can be measured by measuring resistivity.

On the basis of this idea, corrosivity probes have been developed which are made up of thin

metal traces on printed circuit boards of a standard thickness. Instead of a simple trace, however,

a two-legged bridge is formed; one leg is protected by epoxy and the other leg is exposed and

allowed to corrode. The change in resistance of the exposed leg determines the metal loss, and

hence the rate of corrosion. The reason that two legs of a bridge are needed is that as temperature

fluctuates, the resistance of a single trace may fluctuate; however, if a bridge formation is used,

temperature effects common to both legs are eliminated. Currently both the ASTM standards

developed by the D9 and E5 committees use this corrosion probe (manufactured by Rohrback) as

B-8 B-8 NUREG/CR-6476
Detafled Summary of Smoke Corrosivity Test Methods

seanmp Cisnbflon Host source Hoeaitb Condlsine Center

Chamb.er -,p Csisgory Stendard Probe Cosyoqlonliteenmsseas fereaoss aoflit-ssts Llseatlans,

AcIdGasGeneralton [EC754-1,1994-01 Test on Gases Evolved Duing Mateualtrom Saco lab

(dynamic method. Combustion of Materials thoa sable, 0.5-1.0 Oet

stationary ftae*) Cables, Pait I Ido pieces

AcId Gas Genetraon SS002-1

(dynamie method,

stationary urnaes)

Acid Gas Gaerod•n 8.3 6245: Part I

(dynamie method,

stationary furnace)

Acid Gas Generaton CSA C22.1 No. 0.3-

(dynamic method, M1085-.se 4.31

trav4ng firnace)

Acid Gas Generaton SAA AS 1640.5.3

(dynasic metad

stationary frnace)

Test on Gases Evolved Cuing Matedaldrom t lSes tube Combustlon of Materials from cable, 0.5-1.0 gu i

Cablee, Pad I Ndo places

Gases Evolved During Matersal tfo Suesa tub

Celson of Matertals from cable. 0.5-1.0 god

Cables, Past 1: Oclerrdnion of Into piece.

Amoui t of Halogen Add Gas

Evoked During Combustion of

Polymsrst Matedals Taken tem

Cable.

Teat to Detoensdie Add Ges Mbtesial tolson $Ik tube

Evolutios sable, S00 mg. WO

MathodsoyTeethforectriw Mitsslt from Slica tab.

Cables. Cords and Condtinors. cable. 00 sg

Pad 5: FiPe Teats - Pas 5.3::

Deblssisailon of iasdoun of

Halogen Add Gas Evoked

During Combusion of Polymeric

Matedals Taken tosm Cables

Test an Gases Evolved During Material from Slea tlie

Combustion of hiartasl soIr snae, 1000 l ng

Cable., Past 2

Test on Gases Evolved Duing matertal frnm SHea tube

Codsil of ilatesals them Cable. 10o mg

Cables, Part 2

Tub. funaes Stationary ftrnae, het dry air 20

ramped tros 23-SM00 In e/on

40 min then mald. 20

inri

Tab furnace Statlonary tensae, hoat dry aIr 20

rMmped trom 23-00Ch In nVMRiA

40 ndn Owen mald. 20 rain

Tubs buarsce Stationary taunse, hest dry air 20

ramped tivi 2340010C In vdnm2l

40 edn Owen wnt. 20

Mh

2 water tlrs 220 Thtrast w4th HN03.

ad 0.14N NOH AGN03, FOe44S04,

2 water trape 220 Tlrtse with 16403.

mf 0.IN NaOH ApW3, Fe? IW$04.

1*44SCN

2 water tnig 220 Titrate W4t l4'103.

d 0.IN NnOH AOW, FP.144SO4.

1444SCN

Amount of halogen add

expressed as mng H1I/5

Amount of halogen add

esyreesed is se HCVg

Amoutr of halogen add

expressed asmg Hr"g

Matedals condaining

fOuotsde orhalogen

acid'> 5 mg/g. not

suitable for 15ro

halogen materials

Materials erdantiing

&olde or halogen

acid > 5 mgfg. not

sultable for zser

halogen materials

Materials cotainidng

Oiuside or halogen

addS S ssg/g, not

sultable foraro

halogen materlale

Tube tense, 800C,6.1ovee, dryar. 100 #ld 3 water p. pH 5 t11tate ith 0.1NNaOH Percentage of sold ps Require. tof add simple hested S adn, is 7. add Sol 0.IN vokved (besed on WN gas beforehand sin to heat deposits Iaf111 ofadd)

Tubs tuene. 8O0 for 10 minutsd dry air S Ail pH by rtgion

Acid Gas Generation IEC 754-2

and Conductiiy

(dynmand method.

efationery tnsace)

Acid Gas GeUroo BS OM602-2

and ConduethAly

(dynarnc method,

stationary furnace)

Tube tumac. preheat ,93SCfbr30 dryasr, 100didin I or 2 wash botls pHH ad conductivity dnrotes (15-3O h) Nvd lO10001pH S. 7

Tube futne. preheat> 935CfM 30 dry air, 100 elmdn I or2 wash bottee pH nd conductivity

ildioe. (Vol 1000lO ) pH S7

3 runa; pH. mean, stdov.

cover, conduceivity;

p1*4.3 and sond! 10

3 runw; pP, mean. stdev.

siver, conductivtly;

p1"h4.3 and sond 4 10

microslarnens

AcdGasGeneraion SASAS88003" 3 to Methods ofTe for Electbcal

and Conductivity becone AS 1600.5.4) Cables, Cords and Conductors:

(dynatte method. Dtessinatlon of IsOsit of

stationary furnace) Halogen Add Gas Evolved

During Comibusetion of Polyseicel

Material Taken trom Cables

CN9T (statc metho•) ISOIDIS 11907 Part 2. Plastics.- Smoke Generation - Mateial tfem 20erP oM -jaied 800hCf- ISO$e

a Jan 1991 Determination of the Conrosvity cable, 600 ing with seals, ndwome wnie

of Fire Eglue.ts Past 2.: Stati gromiie inbed with heated to SOC

Method 100 mg PE wahs wre

wrapped around

dumher

Dry ai, Iliected PWB 34 tracs 52 hitation resistance of

with water to a mtin ig. 0.3 m PW evory S0 e for I

specifc RH (90or wide, 17 edron how

40o60%) tiJck; cooled 10( . I,-due 6caie

Perentd cairoolvity

adusted to 40C, 8 slush

OiNS3436Travelng ISOfDIS11907Pwt3 Pllestls. SmokheGeneration- MatelriaIaOm Si =oube t Tubsounc 6000C, mot0melddrve aial 100+5 5h C1CTPWU Satr Insulationresistance1.3 %changelnr dis4ls ty, Tubs Fumsce eter•isnstion of tie Corosivoty cable, 9 p, v 0.4 pass himce over Ioled a 100 and 24 la satar exposu ReaP (dynarrse method) of Fire ELtueds. Part 3.: poll 400um x 15 sample, 20 "in durtion duing exposure at 23C 7520%RH

Dynamic method siox2mm Cons Calolmiter ISOIDIS 11907 Pad 4. StandardTesteMthdfor Inac cdableorll4 11 Her coricaheahter. Utl mass less ambletdair, RoIohbeCo Co on o lMeow"relsst•hn•efter Reportcalosrimlrydata (dynanic method) ASTI 0.5485 Determining the Corrosive plaques 100rs potycaibonate volable flux determinaton. test samping rate of 4S,0•OA 0 eoin exposre and sate fro mss goes and Effed of Comustiton Products square. end chanter. 105C from 10-100 70% of total mess loes. 4.5 Vns 24 hr elpoeuse to 23C. cosroson nine, Using the Cone Corrosimetor sealed, AL heated SS kWIM2 expose probe to e ssole ?S45% RH calcilated metal loss (1 wrapped. In frame transfer tuling fhr 60 rdin ir and 24 hrs) 1BS Smoke Chamber ASTMhE5.21.70 NsS Standard Tesa Method for Inat cable or Nc Radard Combustion .l IS irddutes 60% RH humidifed Rolhba• Coessea Close shufsier terS s5dn Plot metal lss for total (saticmethod) Measuiang the Coffosho Effects plaques SO min Heat Chamber. with load cel 4 ark I chamber CKS cosroolmoter heeIng ends, mese.,. ieposar. time and alfer

of Smoke homi •i•seg or square, '25 cal 200 Hier volme. quiels fR lamps 45.000A and shergIna heslrsance fo 24 hose. repost Thermal Decomposition of thick, A wrapped In combustionc eel at2000W end measabgil brdge 15 ndIntervals for l fr. ealebtry data; metal Matedals and Products 9r1s. chamber and 240V. fil 50 remove and esos. to Ios A,- (RrR.)Sf1005 S dhmtey kWhs2. sall 220 and 7S 5% RH for Is probe sa trom mf'

Igniter 24 Inr end rermese IR
does the French CNET test. Both ASTM standards suggest using the 2500-A and the 45,000-A

thick probes, however; Rohrback produces these in different ways. The "thick" and "thin"

probes never correlate very well; at times the thin probe will be corroded away, while the thick

probe does not show any corrosion at all.

Many workers in this field have compared the various international standards or proposed

standards.32,334 5,36 The most frequently compared techniques are the last four on Table 1, the

CNET method developed in France, the DIN 53436 method developed in Germany, the cone

calorimeter developed in the United States and the National Institute of Building Standards'

smoke chamber also developed in the Unites States. Factory Mutual also has their own testing

standards and has attempted to correlate large- and small-scale testing.37

Two methods of producing smoke for small-scale fire corrosivity tests have been developed: the

static method contains all of the air and fuel in an enclosed space, while the dynamic method

consists of a continuous air flow that is pumped past the burning material. While the dynamic

method ensures a more even burn, the target will not be exposed to all of the smoke. The static

method ensures that the target will be exposed to all of the smoke products because they are

contained, but the mode of burning is not very even.

The ASTM standard developed by the D9 committee, D5485, uses a cone corrosimeter to bum

the material and is a dynamic test.3' Air is pulled through the burning area with a fan at a

prescribed rate. A small amount (0.3%) of the smoke is pulled through a bell jar that contains the

corrosion target (Rohrback probe). Gases are pulled through the jar until 70% of the material is

burned up. The target remains in the bell jar for 1 hour. After this smoke exposure period, the

target is moved to an environmentally controlled chamber (20' C, 75% relative humidity), and

tested for 24 hours. One shortcoming of this test is that hydrochloric acid is one of the first

combustion by-products to be produced. Since smoke is pulled through the bell jar until a given

amount of fuel is burned, the hydrochloric gas may not be allowed to remain in the bell jar with

the target. That is, the early release HCl may be "flushed out" of the exposure by the continuous

flow of sample gas.

The ASTM standard developed by the E5.21.70 committee uses a radiant heat furnace and an

enclosed box.39 This is a static test in which all combustion products are confined to the exposure

chamber. The burning material is exposed to radiant heat for 15 minutes, and all of the smoke is

captured in a polycarbonate box that contains the target (Rohrback probe). The target remains in

the box with the smoke for a total of 1 hour. After this period, the target is moved to an

environmentally controlled chamber (20'C, 75% relative humidity) and tested for 24 hours. One

shortcoming of this test is that the fuel combustion process is not well controlled; the quartz

lamps provide radiant heat through a clear quartz combustion chamber that may become opaque

from soot. The advantage of this method is that all of the combustion products over the full

course of the burn are captured.

Material smoke corrosivity measurements are also made by measuring the acidity of gases

produced by the fire."" The DIN test 57 472 Part 813 Standard is one such test. In this test, one

gram of material is burned in a quartz combustion chamber using radiant heat lamps. A

continuous bottled air flow is maintained during the combustion and the gaseous burn products

B-10 B-10 NUREGICR-6476
are percolated through distilled water. The pH and conductivity of the distilled water are

measured with electrode probes. Changes in PH are correlated to the acidity of the combustion

products.

General Reliability Testing

Reliability testing for electronics has proceeded along a different path than that for smoke

corrosivity. Reliability testing has been performed on actual electronic parts such as connectors,

printed circuit boards, and chips, rather than metal blanks and resistive traces. The emphasis in

reliability testing has been on both metal loss and circuit bridging in both individual components

and assemblies. Although smoke corrosivity can be considered a subset of reliability testing in

general, the differences in approach of the two test communities are remarkable.

Accelerated Corrosion Testing

In "accelerated testing" the environments in which the components are tested are more harsh than

normal operating environments because the tester wants to accelerate the failure rate. Higher

temperature and relative humidity are the bases for most accelerated testing. For example, tests

on printed circuit board reliability are conducted at 850C and 85% relative humidity.4' Highly

accelerated stress tests (HAST) require high pressures, while the temperature and relative

humidity are maintained at 1400C and 85% relative humidity. 42

Ionic contaminants increase the corrosion of electronic components.9 The ionic contaminants can

be added in the form of a mixed gas or salt spray, and these are usually accompanied by elevated

temperature and humidity. 43 Bottled gases such as hydrogen sulfide or chlorine are added into

the test chamber to increase the corrosion rates. Nueberger" developed a detector to measure the

HCl distribution in the test chamber. The synergistic effect of humidity and ionic gases has

been found to be very important in determining the corrosion rate. Sometimes a "critical"

humidity, in the neighborhood of 60-70%, is defined to identify the onset of much faster

corrosion.

In general, the failure rate of electronics is dependent on the accumulation and type of deposition,

relative humidity, temperature, packaging, connector design, and bias voltage. In smoke

corrosion, the deposition rate of the contaminant is much higher than that experienced under

normal circumstances. The deposition occurring during a fire in a communications center in Los

Angeles, for example, was found to be equivalent to 9 years of deposition under normal

circumstances. 9 The bias voltage influences the corrosion process by encouraging dendritic

growth of conductors; therefore, it is important to apply typical voltages when testing for

reliability.

NUREG/CR-6476 B-11
Special Testing Chips

Most digital electronic devices have high reliability, but fail rapidly and without warning.

Emerson has developed "assembly" test chips that are more susceptible to environmental

problems and will start to fail earlier and more gradually than production chips. 37 These chips

can be used to indicate corrosion in accelerated tests and for tests of printed circuit board

assembly lines. They can also be used for highly critical applications to monitor the amount of

degradation of an electronic assembly. The assembly test chip may be used to indicate predicted

failure rates for critical digital assemblies. This represents a potentially promising approach to

nuclear plant applications as these chips might be used in actual components and might provide

early warning of impeding failures.

Surface Insulation Resistance

Surface insulation resistance (SIR) is a measurement of the resistance between two parallel

circuit traces. In practice SIR can be measured using a board with two interlaced electrodes that

form a comb. The resistance between the combs gives a measure of the amount of conductive

material that is deposited on top of the comb. These measurements have been used to ascertain

the quality of soldering on printed circuit boards. The combs may also be used to indicate circuit

bridging effects as a result of surface contamination or dendritic growths between comb

materials. The boards can also be used to assess the effectiveness of various coatings to prevent

such failures.

Summary

Very little information on the reliability of electronics has been gathered from accidental fires.

Most damage information from actual fires is anecdotal and not well documented. In a real fire,

no one stops to make careful measurements of the soot deposition or perform a chemical analysis

of the soot; instead, the immediate job is to put out the fire and clean up the equipment so it can

be salvaged. More information has been found from the study of controlled fires, but few tests of

the reliability of electronics in a controlled fire have been performed. The emphasis has instead

been on the relative corrosivity of the smoke produced by burning different materials.

Studies of corrosion on electronic equipment are more advanced than tests of the reliability of

equipment in a smoky environment. Reliability tests generally use mixed gases, high

temperatures, and humidity and hence are easier to reproduce than a consistent smoke

environment. The main problem that must be resolved is determining how to relate the corrosive

atmosphere to a smoke environment, and then, how to relate a simulated smoke environment to a

real fire. It is also important that smoke testing be expanded to include the use of more varied

targets in order to assess each of the potential failure mechanisms. To date, smoke exposure

testing has focused primarily on direct corrosive attack. Relatively little work on circuit bridging

faults has been undertaken.

13-12 B-12 NUREG/CR-6476
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NUREG/CR-6476 B1 B- 13
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27 Private communication with Robert Backstrom of Underwriters Laboratory, Northbrook,

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28 G. M. Galloway, and M. M. Hirschler, "Transport and Decay of Hydrogen Chloride: Use

of a Model to Predict Hydrogen Chloride Concatenation in Fires Involving a RoomCorridor-Room Arrangement," Fire Safety J. Vol. 16, pp. 33-52, 1990.

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a Large Apparatus. Part 2. Variables Affecting Hydrogen Chloride Decay," J. Fire Sci.

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30 T. Chapin, "Detailed Summary of Smoke Corrosivity Test Methods," Presented at

meeting April 27, 1995 at AT&T Bell Laboratories at Murray Hill, NJ.

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Generated During Shipboard Fires," Final Report, April 13, 1988, Naval Research

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32 S. L. Kessel, C. E. Rogers, and J. G. Bennett, Jr., "Corrosivity Test Methods for

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Products," 1st Fire and Materials International Conference, September 24-25, 1992,

Arlington, VA, pp. 201-212, Interscience Communications Ltd., London, Eng., 1992.

36 M. M. Hirshler, "Discussion of Smoke Corrosivity Test Methods: Analysis of Existing

Tests and of their Risks," Fire and Materials, Vol. 17, No. 5, 231-247, Sept/Oct 1993.

37 A. Tewarson, "Nonthermal Fire Damage," J. Fire Sci., Vol. 10, No. 3, pp. 188-242,

May/June 1992.

38 J. G. Bennett, Jr., S. L. Kessel, C. E. Rogers, "Corrosivity Test Methods for Polymeric

Materials. Part 4. Cone Corrosimeter Test Method," J. Fire Sci. Vol 12, No. 2, pp. 175-

195, Mar/Apr 1994.

NUREG/CR-6476 B-15
39 S. L. Kessel, J. G. Bennett, Jr., C. E. Rogers, "Corrosivity Test Methods for Polymeric

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40 G. C. Stevens, M. R. Dickson, M. G. Weller, "Acid Gas Emissions from Electrical

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B- 16 B-16 NUREG/CR-6476
Appendix C

Preliminary Tests of the Impact of Smoke on Digital I&C Equipment

Prepared by:

Tina J. Tanaka

Sandia National Laboratories

W6051

August 31, 1995

Abstract

This report presents the results of three preliminary smoke exposure tests on active digital

equipment. The smoke from three different types of cable was used to generate the test

environments. These tests show that intermittent failures of digital equipment can occur within

minutes of exposure to smoke. These failures are most likely caused by bridging of circuits by

soot. Although loss of metal due to corrosion is highly correlated to the failures exhibited in

these tests, it is not a likely short-term failure mode. Measurements such as temperature,

humidity, soot deposition, and chemical analysis of the soot and gases are also presented;

however, these measurements do not correlate well to the failures that occurred.

NUREGICR-6476c- C-1
Introduction

Objective

Sandia National Laboratories (SNL) has been tasked by the USNRC to investigate the impact of

smoke on the performance and reliability of digital instrumentation and control (I&C) equipment

which may be used in nuclear power plants. Studies on the failure of digital equipment in a

smoke atmosphere are rare, at least in part because smoke environments are not easy to quantify

and reproduce. Standardized tests have been developed by professional groups such as the

American Society for Testing and Materials (ASTM) and the National Institute of Standards and

Technology. However, at most, these tests have been developed to measure the relative

corrosivity of the smoke from various materials or to characterize the smoke output of materials.

These tests do not directly address the problems of electrical equipment operation which is the

subject of this effort. Because of the lack of a standard test method, SNL has adopted an

approach to testing that includes features from standardized smoke corrosivity tests that are

currently being developed by the American Society for Testing and Materials. This report

provides a description of preliminary experiments performed by SNL. The objective of these

preliminary smoke exposure tests was to explore and demonstrate a procedure for investigating

the impact of smoke on digital I&C equipment.

Scope

Three multiplexer units were exposed to three different smoke atmospheres. One unit failed,

while the other two performed without problems. This report describes the exposure method and

results of these tests. These tests were only preliminary in nature and a much broader follow-on

study is currently underway.

Smoke Exposure method

Smoke Exposure Environment

Smoke is a known hazard for electronic equipment; however, very few tests have been performed

to determine the reliability of electronic equipment in a smoke atmosphere. Smoke composition

can vary depending upon fire characteristics such as bum temperature, oxygen availability,

material burned, and whether the fire is smoldering or openly flaming. In order to produce

smoke in a standard and reproducible way for the purpose of these preliminary tests, the ASTM

draft corrosivity test standard produced by the E5.21.70 Subtask group was followed. This draft

standard was based on a standard toxicity test that has been in use for many years. The primary

measurement of this draft standard is the loss of metal from a corrosion probe as a function of the

material burned. Although the objective of this ASTM test (relative corrosivity) is different from

our objective of testing electronic equipment reliability in a smoke environment, the methods of

smoke production and the time of exposure to the smoke were adopted as the fundamental basis

for producing a "standard" smoke environment and exposure scenario.

The fuel was burned using radiant heat from a tungsten-quartz lamp aided by spark ignition

inside a cylindrical quartz combustion chamber. The smoke production and exposure equipment

is illustrated in Figure 1. The radiant heat lamps are adjusted so that a fixed heat flux level of 50

C-2 C-2 NUREGICR-6476
kW/m2 is produced at the fuel surface. The heat flux was measured with a Schmidt-Boelter

(thermopile) heat flux meter before each test to determine the amount of heat that is incident on

the fuel at the beginning of the test. Small variations in the positions of the lamps can affect the

heat flux that is incident on the sample. As smoke is produced, the quartz chamber becomes

coated with some soot and so the heat flux is reduced. No attempts were made to compensate for

this effect.

Nowlen' reported on the types of fires that take place at nuclear power plants and their typical

fuel load, where the fuel load is defined as the ratio of the weight of fuel consumed to volume of

air. The highest fuel load occurs in cabinet fires because of the limited volume of air in the

cabinet. Of course, if the fire is very large it will burn up all of the equipment in the cabinet, but

for a moderate fire, the equipment may survive the heat and flames but not the smoke. For these

preliminary smoke exposures, the fuel load was selected to simulate this high fuel load condition.

A fuel load between 26 g/m3 and 560 g/m3 corresponds to a small cabinet fire; therefore, a fuel

load of 75 g/m3 was chosen for these tests.

The volume of the exposure chamber was 0.2 mn3, so 75g/m3 requires 15 g of burnable fuel. Since

the most abundant fuel in a nuclear power plant is cable insulation and jacketing material, cables

were used to produce the smoke. The amount of cable material burned was determined by

stripping the insulation material from a sample cable piece and weighing the fraction of the total

cable weight that is made up of insulation. Typically, the insulation and jacketing materials

comprised 50 to 75% of the total mass of the cable. Lengths of cable which corresponded to 15 g

of insulation were loaded into the combustion cell on aluminum trays. The loaded trays were

weighed before and after the burn.

We expect that the type of fuel determines how destructive the smoke will be. In a power plant

there are several different types of cables used for instrumentation, power and control. In

addition, there are cables with insulation types that are no longer used for new applications, such

as as polyethylene and polyvinyl chloride (PVC), but which remain in place at the plants. For

these smoke exposures, three types of cable were burned, one type in each test. Test 1 used

Brand Rex cable made with cross-linked polyethylene (XLPE) insulation with a chlorosulfonated

polyethylene (CSPE) jacket. Brand Rex cable is the third most utilized cable supplier for nuclear

power containment.2 Test 2 used Anaconda Flameguard cable made with ethylene propylene

rubber (EPR) insulation and CSPE jacket. This is the most common combination of materials

used for insulation in containment.2 Test 3 used a Belden, non-nuclear qualified PVC jacketed

and insulated cable. Although PVC cables are no longer widely used in nuclear power

applications, they are still present in power plants and represent one of the most corrosive of

cable insulation materials. The weights of the fuel loaded in the trays are presented in Table 1.

Table 1. Table of Cable Materials used in Smoke Exposures

Test Cable Weight/in % plastic Sample Plastic weight

weight

I XLPE/CSPE 4.93 grin 54.6 27.54g 15.04g

2 EPR/CSPE 6.07g/in 76.0 20.25g 15.4g

3 PVC/PVC I 6.41g/in 57.4 26.42g 15.17g

NUREG/CR-6476C3 C-3
As the radiant heat lamps were turned on and the fuel heated, hot gases and smoke rose up the

30-cm long stainless steel chimney to the Lexan exposure chamber. The lamps exposed the fuel

material for 15 minutes as specified in the ASTM draft standard. During the entire 15-minute

period, a sparker, located 2.5 cm above the fuel, continuously sparked to provide an ignition

source for hot gases produced by the radiant heat lamps. After the 15-minute period, the lamps

were shut off, the chimney damper was closed, and a small mixed the smoke vapors. Since this

was a static smoke exposure, the smoke was not allowed to leave the exposure chamber for the

first hour of the test. The smoke chamber was sealed as well as possible to prevent smoke leaks.

To allow for the expansion of air and smoke vapors due to the initial heating and subsequent

cooling of the contained air volume, an empty plastic bag was placed over one of the ports. After

a total of one hour from the beginning of the test, the smoke was vented.

After the smoke exposure, the ASTM draft standard recommends that the target material (the

corrosion probe in the case of the standard) be placed in a controlled environment at 75%

humidity and 23'C for a 24-hour period after which the target is retested. This step simulates the

environment of a water suppressed fire. This would normally take place in an environmental

chamber; however, our environmental chamber was not completed at the time of these tests.

Hence, a variety of interim methods were attempted to control humidity with differing levels of

success.

In Test 1 we did not add any humidity for the first 24 hours so that the equipment remained at the

normal humidity for Albuquerque in the winter, below 20% relative humidity (RH). Since the

draft standard recommends holding the RH at 75%, we attempted to do this after the preliminary

24 hours by setting a beaker of hot water in the exposure chamber. However, this added too

much humidity. The test unit was left in the humid environment for three days. In Test 2 we

added humidity right after venting the smoke, using a portable hot steam humidifier. The

humidity was adjusted by opening doors and running fans, but there was no control over the

humidity overnight. In Test 3 we added humidity right after venting the smoke. This humidity

was added using a portable hot steam humidifier, but it in a more controlled way than for Tests I

or 2.

According to the draft standard, the RH of the sample should be maintained at 60% prior to the

test. The humidity inside of the box was measured before the lamps were turned on, but the

humidity sensor was removed after the initial reading to prevent damage to the sensor. The

humidity sensor was reintroduced to the smoke exposure chamber after the chamber was vented.

Due to the nature of the ambient environment prevailing at the time of the tests, a 60% RH was

not achieved in these tests.

Smoke Environment Measurements

The measurement of metal loss, while important in determining the relative corrosivity of the

smoke, is not the only parameter of interest in the smoke environment. Other standard

measurements of smoke, such as optical density, rate of fuel combustion, and time of ignition

and extinguishment, have been specified by the ASTM. We attempted to characterize the smoke

C-4 C-4 NUREG/CR-6476
environment by making these measurements, along with others such as temperature, smoke

deposition, and chemical analyses of gases and soot.

Corrosion Probe

The metal loss measurement is made with Rohrback/Cosasco probes that measure metal loss via

changes in resistance of a copper trace deposited on a printed circuit board. In the SNL tests

resistance was monitored with a high precision LCR (inductance, capacitance and resistance)

meter. The probe contains two copper traces of the same thickness and width: an uncoated trace

that is allowed to corrode and a reference epoxy-coated trace. The purpose of the two traces is to

separate the changes in resistance due to metal loss from changes due to temperature fluctuations.

Since the original intent of these probes was to measure atmospheric corrosion, the copper

probes normally sold by Rohrback/Cosasco (250 nm thick) are too sensitive (thin) and were

found to corrode away too quickly. The ASTM draft standard suggests using a thicker probe

(4500 nm) for greater longevity. The process used to deposit the copper traces on the thicker

probes differs from that used on the thin probes. For these tests both probe thicknesses were

used. Since changes in these measurements were expected to occur relatively slowly, the

measurements were made at one minute intervals during the first hour of the test, and at 10-

minute intervals during the rest of the test.

Load Cell

A commercial load cell (Single Point I, Interface Inc.) measured the loss of mass of the fuel as it

burned. The load cell was mounted under the quartz combustion cell as shown in Figure 1. The

fuel tray was mounted to the load cell through a hole in the combustion cell. The load cell

converts force by means of a strain gage into voltage and can measure weights up to three

pounds. Load cell output was recorded at five second intervals during the first 15-minute period

to determine how quickly the fuel was consumed. This measurement is included in the ASTM

draft standard.

Thermocouples

Temperature measurements were made throughout the test, in seven locations in the smoke

exposure chamber. Five Chromel-Alumel (type K) thermocouples housed in stainless steel

sheaths penetrated the walls and floors of the smoke chamber, (Figure 1). Two non-sheathed

type K thermocouples were welded to the surface of the quartz crystal microbalance cases (see

discussion in Section 0.) Although the temperature of the chamber was not controlled,

temperatures were measured to determine if any failures could have resulted from heat rather

than smoke. Temperatures were logged at five-second intervals while the lamps were on and at

one-minute intervals for the remaining time that the smoke was in the chamber. After the smoke

exposure chamber was vented, the temperatures were logged at 10-minute intervals.

NUREGICR-6476C- C-5
Turbidimeter

A turbidimeter measures the optical density of smoke by passing light through the smoke and

determining the loss of light intensity. Light from a high intensity bulb (white light) is focused

with a lens and then passed through a glass slide into the smoke exposure area. After crossing a

3.2 cm-section of the smoke chamber, the light passes through a second glass slide and into

another lens. The purpose of the glass slides is to keep smoke from fouling the optical

components of the turbidimeter. Beyond the second lens, a combination of prisms and filters

divide the light into three colors: red, blue, and infra-red. The optical throughput of each color is

then converted by photodiodes into an electrical signal. The voltage from the photodiodes was

recorded at five-second intervals while the radiant lamps were on, 1-minute intervals for the rest

of the first hour, and at 10-minute intervals after the smoke was vented.

The glass slides collected a coating of soot which should not be included in the smoke density

measurements since the measurement should only include the smoke that is suspended in the air.

The optical density of the smoke can be estimated from the difference in the turbidimeter output

before and after the smoke is vented.

Gas analysis

Gases for the analysis of the smoke were captured in two ways. A vacuum grab system captured

a 10 ml filtered sample of gas in a stainless steel bottle. For fuel samples that ignited (flaming

rather than smoldering), the gas sample was taken within a minute of the flame extinguishment.

If there were no flames, a grab sample was taken after the quartz lamps were turned off (15

minutes into the test). The samples were analyzed by a Tracor gas chromatograph. These

measurements provided the percentage of methane, carbon monoxide, nitrogen, carbon dioxide

and oxygen present in the sample. The amount of heat releasedwas estimated from the amount

of carbon dioxide and carbon monoxide in the air, or the amount of oxygen remaining in the air.

Samples of acids contained in the smoke were collected in a silica-gel-filled sorption tube. A

sampling pump pulled 20 ml per minute of smoke through the sorption tube during the entire

burn time (15 minutes). Analysis of the tube contents provided information on the relative

amount of chlorine present in the chamber. The materials that remained in the sorption tubes

were analyzed using ion chromatography following the procedure contained in NIOSH 7903.

Soot Deposition Measurement

Quartz crystal microbalances (QCMs) measured the mass deposition of the smoke as a function

of time. The purpose of this measurement was to determine the amount of soot deposited on

electronic equipment. Quartz is a piezoelectric material that undergoes shear deformation when

subjected to an electrical field. Conversely, if it is strained, it will generate an electrical field.

The QCM consists of a quartz crystal with electrodes to connect it to an electrical oscillator that

is tuned to the fundamental frequency of oscillation of the crystal. When material such as soot is

deposited on the surface of the crystal, the oscillation frequency decreases. The change in

frequency can be related to the additional mass on the quartz crystal. The frequency of

C-6 C-6 NUREG/CR-6476
oscillation is also dependent on the temperature of the crystal, so thermocouples were attached to

the QCM case to determine the temperature around the crystal (as noted in Section 0 above).

Two QCMs were used to measure the deposition, one in a horizontal face-up orientation and one

in a vertical orientation. The oscillation frequency was recorded at intervals of five seconds

while the heat lamps were on, and at one-minute intervals for the remaining first hour. After the

smoke was vented, the frequency was recorded at 10-minute intervals.

Soot Analysis

Ashless filter papers were used as a substrate to collect soot samples for chemical analysis. Two

filters were placed at the bottom of the smoke exposure chamber for each test to gather samples

of soot. These samples were submitted for chlorine analysis by ion chromatography and for

trace metals analysis by plasma-excited atomic emission spectroscopy (IPC). Ashless filters are

used because the analysis involves burning up the filter paper. Soot samples were also scraped

from a stainless steel sheet placed on the bottom of the smoke exposure chamber. The soot was

analyzed for total organic carbon.

Digital Equipment Description

The first component smoke tests at SNL have evaluated the affect of smoke deposited on the

Analog Devices® modules and backplanes. These components were selected by Oak Ridge

National Laboratory for testing because they incorporated operations needed for a reactor trip

system in a commercially available product.

Hardware

Analog Devices sells a series of plastic encased modules that can perform different functions:

measure voltage, measure thermocouple or resistive temperature device (RTD) output, and then

output current or voltage. All of these modules plug into a printed-circuit type board called a

backplane. The backplanes are powered with a 5 volt DC power supply. For these tests we

bought a 6BP04-2 backplane that can support up to four modules at one time. The backplane has

ports for an RS232 communication and an RS485 connection. The communication ports allow

the backplane to send and receive information from a computer to the various modules using

ASCII characters.

The modules are sold with a computer program that can configure and calibrate the modules.

Configuring the module includes determining the address of the module, the mode of function of

the module, and defining which port of the computer the module is located. The program can

also be used in conjunction with other calibrated equipment to calibrate the modules.

The Analog Output Module, model 6B21, can output a current between 0-20 mA or between 4-

20 mA. We configured it to output 0 - 20 mA. The module was set to use engineering units for

the output which means that the units are in milliamps (as opposed to a percent of span or a

hexadecimal output.) When assembled the module produced a current through a load resistor

NUREG/CR-6476C7 C-7
attached to the adjoining terminal strip on the backplane. The address of the output module was

configured to be 02.

To act as test equipment, three 6B 12 input modules were configured to measure between -20 to

+20 mA. The input modules were also configured to present the data in engineering units

(milliamps.) Since the module actually reads voltage rather than current, a precision load from

Analog Devices was placed across the terminal strip to act as the load for the output module. The

input modules were configured to be at address 01.

In order to allow the physical separation of the input and output modules needed for this test, two

separate backplanes were used, each requiring its own serial port. We used a Hauppage 386

personal computer with an additional 10 board to provide the serial ports (RS232). Thus the

input module was plugged into a backplane connected to the RS232 port called COMI and the

output module was plugged into a separate backplane that was connected to the RS 232 port

COM2. The mode of communications was 9600 baud, eight data bits, no parity, and one stop

bit.

A diagram of the hardware wiring is shown in Figure 2. The current from the output module was

determined by measuring the voltage (using the input module) across a precision resistor. The

input module was placed inside of the smoke chamber and the output module was placed outside

of the smoke chamber. During the smoke exposure the input module measurements were

compared to the output module readings. Ideally the measurements should match exactly.

Software

The software for assessing the performance of the test modules was developed by Kofi Korsah at

Oak Ridge National Laboratory in the form of a Turbo Basic program,. It has several menu

items to test the modules singly or to test the system as wired above. There are also options to

print out the data files to the screen or to a printer.

For the system test, the computer directs the output module to feed a known current to the input

module. The computer reads the actual output module current and reads the input module

measurements at one-second intervals. The computer then changes the output current and

repeats the readings. The current output starts at 0 and is incremented by +0.1 mA steps up to 20

mA. After the current output reaches 20 mA, the data from the previous 1400 readings are

recorded in a data file. The process is then repeated. Because the program only stores data after

1400 readings (approximately 20 minutes of data measurements), system failures can cause

significant loss of data.

Test Results

General Observations

Before each smoke exposure, the test units were assembled and tested to verify functionality.

The test program, MUXTEST5 was then run continuously throughout the smoke exposure and

post-exposure periods. The results of the multiplexer board exposures are listed in Table 2. The

XLPE/CSPE cable (Brand Rex) and PVC cable (Belden) both caught fire while the EPR/CSPE

C-8 C-8 NUREG/CR-6476
cable (Anaconda Flameguard) only smoldered. In fact, the PVC cable caught fire at 1:39

(min:sec), extinguished at 4:02, and reignited at about 5:00 and extinguished at approximately

8:00. The XLPE/CSPE cable ignited at 3:18 into the test and extinguished at 5:18.

Table 2. General Results of Smoke Exposure

Test Cable Equipment performance

I XLPE/CSPE No change

2 EPR/CSPE No change

3 PVC/PVC Intermittent failures

The smoke affected the equipment by halting MUXTEST5 rather than affecting the output data

values. Intermittent failures during Test 3 occurred three times. The first malfunction occurred

at 8:40 after the start of the test, while the radiant heat lamps were on and resulted in stopping

MUXTEST5. The computer indicated an error code and returned to the disk operating system

(DOS) prompt, indicating that MUXTEST5 had stopped. MUXTEST5 was restarted and the

data was normal. The next malfunction occurred at about 1:00:00 (one hour) when the chamber

was beginning to be vented. This malfunction did not stop the program, however the numbers in

the data file were unusual and the printout on the screen was shifted. At 1:20:00 humidity was

added to the environment. The next malfunction at 1:29:00 stopped MUXTEST5 and returned

the DOS prompt with the same error code as the first malfunction. Again, the program could be

restarted and the data was normal. The first and third failures are believed to be a

communications error, but since the there was no error trapping in the program to accommodate

these problems, the true nature of the failures are unknown. MUXTEST5 stopped again around

midnight, but that failure was probably due to problems in accessing the timer during the change

from one day to another.

The smoke-exposed equipment from Tests 1 and 2 both operated normally throughout and after

the smoke exposures. Although the equipment showed some visible soiling, this did not affect

the performance of the equipment. Steel parts that were exposed to Test 3 are especially

corroded after exposure.

To further explore failures in the equipment exposed during Test 3, the equipment was

periodically tested for one month after the smoke exposure. The equipment continued to operate

normally until humidity was added in the form of mist from a cool water mister. The mist

condensed on the backplane and caused the backplane to short. When the backplane began to

short, MUXTEST5 stopped again and produced the same error message as at 8:40 and 1:29:00

into the smoke exposure. This time, however, the equipment was permanently damaged and

would not restart.

NUREG/CR-6476C9 C-9
Comparison of Measured Values

Physical Measurements

Physical measurements of the smoke environment include the temperature, humidity, optical

density of smoke, mass deposition of soot, and loss of mass from the burning fuel. These

qualities help to identify the environment of the smoke exposure chamber.

The temperature was measured in seven locations in the smoke enclosure. The temperatures in

the exposure chamber varied depending on the position in the chamber. The heat in the chamber

was created by two sources, the radiant heat of the lamps and the heat from burning fuel. The

highest temperature was measured in the chimney. The second highest temperatures were

recorded at the top of the exposure chamber above the chimney. Temperatures in the middle of

the chamber, where the test equipment was located, were very close in value to temperatures at

the top of the chamber in the comer away from the chimney. The lowest temperatures were

located along the floor in the corner away from the chimney. These temperatures are presented

in Figure 3 for Test 1. Tests 2 and 3 had similar differences in temperature within the chamber.

After the first hour, when the chamber is vented to the atmosphere, there is little variation in the

temperature from location to location.

Temperatures in the exposure chamber were increased by addition of heat from both the radiant

heat lamps and the burning fuel. The heat from the burning fuel is a significant contributor to the

temperature of the chamber as shown by the Figure 4. Here both the PVC and XLPE/CSPE

cables, which were openly flaming, have higher temperatures than the EPR/CSPE cables which

only smoldered. The XLPE/CSPE cable showed an especially prominent peak at approximately

0.09 hours into the test. Since the PVC cable flame extinguished and then reignited, the peak in

temperature is not as evident. This peak is more prominent for temperatures measured nearer the

chimney than for those located away from the chimney as shown in Figure 3. For the locations

farther away from the chimney, the heat is more dispersed and does not have such a sharp peak in

time while the fuel was flaming.

The load cell information (Figure 5) shows the change in mass of the fuel as it is consumed. The

smoldering EPR/CSPE fire consumed less fuel than the two flaming fires. The rate of mass loss

is much higher while flaming than after the flames are extinguished.

The humidity measurements in the smoke exposure chamber were only valid after the smoke was

evacuated from the exposure chamber, about one hour and 20 minutes after the start of the smoke

exposure. As can be seen in Figures 6, 7 and 8, humidity was added to the smoke exposure

chamber after the first 24 hours for Test 1 and after the first hour and 20 minutes for Tests 2 and

3. Since one of the intermittent failures during Test 3 occurred nine minutes after the humidity

was first introduced, we can conclude that humidity was a contributor to the failure.

The optical density of the smoke includes the optical density of the deposit on the surface of

glass slides which protected the optical parts of the turbidimeter. The maximum optical density

for all of the tests was between 0.33 and 0.40/cm2 . However, the quality of smoke from Test 2

was very different from those from 1 and 3. Smoke from flaming fires tend to be very black

while the smoke from smoldering fires is white. The soot from flaming fires is black while the

C-10 NUREG/CR-6476
soot from smoldering fires is brown. While optical density is a standard measurement endorsed

by the ASTM, it did not seem to be particularly significant for these tests. Typically the smoke

was the most dense after the first five minutes of the bum, and then tended to clear up as the soot

settled out of the air. Figure 9 shows a typical output from the turbidimeter.

The soot deposition varied as a function of time, and as expected, the horizontally oriented probe

collected more soot than the vertical orientation as shown in Figures 10 through 12. The mass

deposition measurements were not continuous because of problems with shorting of the quartz

crystal oscillator circuit during some of the tests. Because the quartz crystal oscillator frequency

will change as the temperature changes, one thermocouple was attached to the case of each

oscillator. The temperature of these thermocouples never varied as much as the thermocouple

that was suspended in the middle of the exposure chamber, although they were placed very close

together. The temperature of the air fluctuates more rapidly than that of a stainless steel case due

to the differences in material thermal capacity; which accounts for the differences. The

temperature for which the oscillator circuit should actually be compensated is that of the quartz

crystal itself; however, these measurements were not made. The values of deposition in Table 3

were measured after the smoke exposure chamber was vented.

Table 3. QCM results

Test Vertical Deposition Horizontal Deposition

1 29 Jig/cm2 42 gig/cm2

2 17 Jig/cm2 182 Jig/cm2

3 17 Jig/cm2 47 Jtg/cm2

The deposition in the horizontal orientation was much higher for Test 2 than for Tests 1 and 3,

although the deposition in the vertical orientation was not significantly different from Test 3.

The soot was also much more tacky and black in the cases of Tests I and 2.

Chemical Measurements

The chemical measurements of the gases and material were performed at SNL. Ion

chromatography analysis of the filter paper and sorption tubes as well as the total carbon in a

sample of soot are included in Table 4. Analysis of metals from the filter paper is presented in

Table 5. The analyses of the vacuum grab samples by gas chromatography is presented in Table

6.

Table 4. Cl and C Chemical Analysis

Test Cable Filter Paper Sorption Tube % Carbon (wt)

Total Cl (14 cm2) Total Cl

1 XLPE/CSPE 0.45 mg 0.24 mg 13.4

2 11 EPR/CSPE 1.13mg 1 0.07 mg 1 37.6

NUREG/CR-6476 Ci C-11
3 1 PVC/PVC 0.35mg m 0.23mg 9.05

A surprising result of these analyses was that the amount of Cl from Test 2 was significantly

higher than for Test 3; however, the much higher proportion of horizontal deposition as shown in

Table 3 may account for the large amount of CI deposited on the filter. The amount of Cl on the

filter paper is 207 pg/in2 for Test 1, 520 pg/in 2 for Test 2 and 161 pg/in2 for Test 3. Reagor3 has

determined that the ability to salvage electrical equipment after a fire, as well as long-term failure

rates, are dependent on the amount of Cl contamination that is present. All of these values of CI

deposition are within the range of salvageable equipment according to Reagor (200 to 600

pg/in2.)

Table 5. Particulate Analysis (pg/14cm2)*

Test Zn Cu Ag Fe B P Pb Sb Sn

1 1.3 131 ND 3 8 ND 87 222 ND

2 0.5 29 ND 1 5 ND 1180 6 ND

3 0.7 119 6 3 ND 24 146 60 23

• ND means that the amount of metal was below the detectable limit of the analysis. The detection limit is

1 pg/filter.

The analysis of the particulate shows that some metals are deposited along with the soot.

Table 6. Analysis of Vacuum Grab Sample

Test CH, CO N-- 0 C-O

1 0.3% 1.0% 74.0% 15.0% 3.7%

2 0.1% 0.5% 73.4% 17.8% 2.3%

3 0.0% 0.5% 80.0% 18.3% 1.5%

Stand. Air 4 0.0% 0.0% 78.084% 20.946% 0.033%

The uncertainty is this measurement is estimated to be ±5% of the reading.

The vacuum grab samples were taken at different times in the test. For Test 1 the sample was

taken at 6:03 into the test, after the flames went out. For Test 2, the sample was taken at 15:30,

after the lamps were turned off because the fuel never ignited. For Test 3, the sample was taken

at 4:28, however, the fire started again at 5:00. Any calculations from the grab sample data from

Test 3 would be incomplete because the cable had not completed burning.

Since the fraction of 02 present after the test is smaller in Test 1 than in Test 2, we can assume

that more fuel was burned Test 1. In addition, the fuel mass loss was 12.8 g in Test 1 and 9.54 g

in Test 2. The heat released from the fuel may be calculated using the grab sample analysis.

This calculation is based on the percentages of 02, CO and CO2 before and after the burn as

described by Parker.5 The heat released from this calculation is 0.232 MJ for Test I and is

0.134MJ for Test 2.

C-12 C-12 NUREG/CR-6476
Corrosion Probe Results

Loss of metal measurement on corrosion probes may be the factor most closely related to the

failures of the test unit. Test 3 (PVC) cable was by far the most corrosive smoke and also was

the only test to have failures. Because of the intermittent nature of these failures, however, the

failures themselves were probably not due to loss of metal, but rather to circuit bridging by soot.

Perhaps the increased corrosivity of the smoke (which produces more metal loss) also encourages

circuit bridging. Figures 13 through 15 show the loss of metal for the corrosion probes for Tests

1, 2 and 3. The effect of adding humidity to the environment severely affected the corrosivity

probes: for both Test 1 and 3 the change in rate of metal loss for the thin probes is significant

after the humidity was raised.

There is very little correlation between the loss of metal for the thick and thin probes. The

processes for depositing the copper is different for the two types of probes so the structure of the

copper on the probes may be different. In general it was found that the thick probes are too

insensitive and the thin probes are too sensitive for our purposes. A probe thickness that is in

between 250 and 4500nm would be better for corrosivity testing.

Conclusions

Our tests show that smoke can cause failures in digital electrical equipment during the first 24

hours. These failures may be intermittent. These failures may be caused by intermittent shorts

from soot that bridges conductors in circuits. One explanation as to why the shorts observed in

these tests were intermittent is that when the short occurs, the current flow will cause the soot on

the current track to be burned off or to move. The equipment could then be restarted because the

current path for the short circuit would no longer exist. Corrosion also occurs, but it takes much

longer to cause failures. We conclude that the failures observed were in fact caused by circuit

bridging faults.

Humidity is a significant factor in producing failures. Humidity may add to the conductivity of

the soot as well as increasing the metal loss of conductors. Unfortunately, the humidity in a

smoke environment is difficult to measure since the smoke will damage the sensor and can

produce erroneous readings. Humidity control is important for standardizing any smoke

exposure tests. Future SNL tests are to be performed under closely controlled humidity

conditions.

When the physical and chemical measurements taken during these tests are compared, few show

significant differences between those exposures that resulted in equipment failure and those that

did not. The loss-of-metal measurement was the strongest indicator of failure for the digital

equipment although the suspected failure mode was not loss of metal (corrosion). A better

measurement would be to test directly for circuit bridging. One such test would be to measure

the insulation resistance loss between conductor traces as smoke is deposited. As insulation

resistance is lost, the likelihood of shorting will increase. Such tests are planned as a part of

follow-up efforts.

Needed improvements in testing software were identified in these tests. The testing software

should be robust enough to recover from intermittent failures. In these tests intermittent failures

NUREG/CR-6476 C1 C- 13
caused the testing software to stop which resulted in losing data from the computer and

interrupting the testing process routine. Determination of the time of failure could only be

estimated based on when the operator noticed that the program had failed. The software also

should include error trapping features which would allow errors to be detected and recorded, and

yet not cause the program to stop because of the smoke-induced equipment malfunctions. It was

found that the computer timer is reset at midnight, and since MUXTEST5 accesses the timer, the

program always failed at midnight.

In summary, the preliminary smoke exposure tests reported here show that intermittent failures

of digital equipment may occur within minutes of exposure to smoke. These failures are most

likely caused by bridging of circuits by soot. Although loss of metal due to corrosion is highly

correlated to the failures exhibited in these test, it is not a likely failure mode in such a short

period of time.

References

I Nowlen, S. P. Letter report on "Defining Credible Smoke Scenarios" dated September 30,

1994.

2 Bustard, L. and Holzman, P. "Low-Voltage Environmentally Qualified Cable License

Renewal Industry Report; Revision 1", TR-103841, Published by Electric Power Research

Institute, July 1994.

3 Reagor, B.T., "Smoke Corrosivity: Generation, Impact, Detection, and Protection", J. Of Fire

Sciences, Vol. 10, March/April 1992.

4 Weast, R.C., CRC Handbook of Chemistry and Physics, 69th Edition, p. F-155, 1988-1989.

5 Parker, W. J. "Calculations of the Heat Release Rate by Oxygen Consumption for Various

Applications," NBSIR 81-2427-1, March 1982.

C-14 C-14 NUREG/CR-6476
Appendix D

Letter Report to the USNRC

Smoke Exposure Conditions for Experimental Digital

Tip System

Prepared by

Tina J. Tanaka

Sandia National Laboratories

February 22, 1996

Revised July 26, 1996

NUREG/CR-6476D- D-1I
Smoke Exposure Conditions for Experimental Digital Trip

System

Abstract

In collaboration with Oak Ridge National Laboratory (ORNL), Sandia National

Laboratories (SNL) performed a series of tests exposing an experimental digital safety

system to smoke to determine the reliability of such a digital system in fires in nuclear

power plants. ORNL built the experimental digital trip system to serve as a representative

safety trip channel in environmental tests. Eight smoke exposures and a high temperature

test were performed, as well as a test with CO2, The smoke environments were specified

by the amount and type of fuel burned, the bum temperature, and the humidity level of

the environment after the bum. This report documents the physical and chemical

measurements of the environment to which the digital safety system was exposed.

D-2 D-2 NUREG/CR-6476
Acknowledgments

I would like to acknowledge the help of Kofi Korsah, Brian Swail, and Richard Wood

from Oak Ridge National Laboratory in providing a test unit, assisting in the exposures,

and documenting the test. They are to be especially commended for the excellent cleanup

of the smoke exposure unit. I would also like to thank D. Michael Ramirez for his

participation in the tests, in both setting up the equipment, helping to run the tests, and

being in charge of the cleanup after every test. This work has been sponsored by the

USNRC under the technical monitoring of Christina Antonescu.

NUREG/CR-6476D- D-3
Table of Contents

Introduction ..................................................................................................... 5

Project Objectives ...................................................................................................... 5

Scope of Study ....................................................................................................... 5

Experim ental Setup and Procedure .................................................................. 6

Test Equipment ....................................................................................................... .6

Sm oke Production ...................................................................................................... 7

Exposure of Electronic Equipment .............................................................................. 11

Sm oke Environm ent M easurements ........................................................................ 13

Temperature ...................................................................................................... 14

Fuel M ass Loss ................................................................................................. 15

Sm oke Opacity .................................................................................................... 15

Gas Analysis ..................................................................................................... 16

Soot Deposition ................................................................................................. 17

Soot Analysis ...................................................................................................... 17

Test Schedule ............................................................................................................... 17

Results of O RNL Exposures ........................................................................... 20

Test I ........................................................................................................................... 20

Test 2 ........................................................................................................................... 24

Test 3 ........................................................................................................................... 28

Test 4 ........................................................................................................................... 32

Test 5 ........................................................................................................................... 37

Test 6 ........................................................................................................................... 41

Test 7 ........................................................................................................................... 45

Test 8 ........................................................................................................................... 49

Conclusions .................................................................................................... 54

References ...................................................................................................... 55

D-4 NUREG/CR-6476
ORNL Experimental Digital System Exposures

Introduction

Project Objectives

The objectives of these tests were to expose an experimental digital safety system to

credible smoke environemnts. These exposures will help determine the failure modes of

a digital safety system in a smoke environment. The results will be added to information

from other environmental tests to determine the risk that can be expected from the use of

digital equipment.

Scope of Study

Test conditions simulated two generic locations within a nuclear power plant: the control

room, where the humidity is low and there are no sprinklers, and general plant areas

where the humidity may be higher and sprinklers could be used to suppress a fire. The

digital safety channel was exposed to three levels of smoke corresponding to: (1) a small

fire in a large room, (2) a small fire in the same cabinet as the safety equipment, or (3) a

medium fire in the same room as the safety equipment. These conditions are summarized

in Table 1.

Table 1. SimulatIon Conditions

Smoke Load Humidity Suppression Simulation Scenario

30% CO2 or none Control room - large fire located in separate

Low: cabinet from equipment

3 g/m 3 80% after Water General plant area - large fire located in

fire separate cabinet from equipment

30% None Control room-small fire located in the same

Medium: cabinet as equipment

26 g/m3 80% after Water General plant area - small fire located in the

fire same cabinet as equipment

High: 30% None Control room - medium fire located in the

same cabinet as equipment >200 g/m3 _________________________

This report presents the physical and chemical environmental measurements for the eight

smoke exposures that took place between May 30 and June 15, 1995 under the project

"Impact of Smoke on Advanced I&C System Hardware," FIN W605 1. Temperature and

humidity were also recorded between smoke exposures while the safety system was

exercised to determine whether the system was operating correctly (baseline runs). This

NUREG/CR-6476D- D-5
report only contains information on the smoke conditions, such as temperature, amount of

fuel burned, smoke deposition and chemical analysis. Information on the errors of the

experimental digital system is included in NUREG/CR 6406 [Korsah et al., to be

published].

Experimental Setup and Procedure

This section describes the test equipment, smoke exposure method, measurements, and

schedule. The tests were based on the ASTM E05.21.70 draft standard on corrosivity

[Kessel, et al., 1994], but we enlarged the smoke exposure volume, and added other types

of measurements. The enlarged volume accommodated rack-mounted equipment. In

order to produce enough smoke for the larger volume, four combustion cells were used

instead of one. Measurements besides those used on the corrosivity draft standard were

added so that the environment could be more fully characterized. These additional

measurements included chemical analysis of soot and gases, smoke optical density, mass

deposition rates, and temperature.

Test Equipment

The experimental digital trip system tested consisted of three main components: the

multiplexer and its computer (PRS/MUX), the digital trip computer (DTC), and the fiber

optic modules (FOMs). These components were individually exposed to smoke;

however, in order to determine the condition of the exposed component, they all needed

to operate and communicate with one another during the smoke exposure.

Communications between these components were monitored by a separate host computer

that was never exposed to smoke.

It is known from previous smoke exposures that in digital equipment communication

errors may result in intermittent failures rather than permanent failures (Appendix 3).

The software for this system was designed to wait for the correct response for a specified

period of time and then to retry the operation rather than halt it when an incorrect

response was received. Because of the critical safety issues during a fire, these

intermittent failures can be important for a nuclear power plant. The tests were designed

to study only short-term failures which would take place within 1 day of the smoke

exposure. Long-term failure modes (on the order of several days to weeks), which can

also be important, were not pursued because the test schedule did not allow for it. Such a

study may be important in the future in terms of what equipment can be reused after a

fire.

The equipment was exposed for 1 hour and then the smoke was vented. Equipment was

monitored for at least 12 hours after exposure and then cleaned. The cleaned equipment

was used for subsequent exposures. To determine if the cleaning process was effective,

the baseline operation of the equipment was monitored in a controlled environment for

several hours. The PRS/MUX was exposed to smoke four times while the DTC was

exposed to smoke three times and once to CO2. The FOMs were exposed to a smoke

D-6 NUREG/CR-6476
environment as well as high-temperature tests since earlier tests showed that they were

very sensitive to heat.

Smoke Production

No standard procedure exists to test the reliability of electrical equipment exposed to

smoke and smoke is a difficult environment to reproduce consistantly. When fuel bums,

it produces many different chemicals whose concentrations are dependent on the burning

conditions. Conditions such as the fire temperature, amount of oxygen available for

burning, whether the fuel is stacked tightly or spread out, and whether there is a flame or

only a smoldering fire can affect the type of smoke that is produced. The amount of

damage to the equipment may be affected by the temperature and humidity during and

after the fire. The soot that deposits on the equipment will be affected by the distance the

smoke travels from the fire to the equipment. In order to test the reliability of a digital

safety system, it must be exposed to a known, reproducible environment.

In these tests, the fire scenarios for different areas of a nuclear power plant were specified

(Appendix 1) in terms of (1) the ratio of fuel mass available to be burned to the volume of

air available defined as the smoke load, (2) the presence of humidity, and (3) addition of

fire suppression agents. The scenarios were selected from this set of possible smoke

conditions as shown in Table 1.

The smoke was produced in an enclosed volume following the basic protocol (with some

modification) of the American Society for Testing and Materials (ASTM) draft standard

on smoke corrosivity as developed by subtask group E05.21.70 [Kessel, et al., 1994].

These exposures are small-scale tests. No claims have been made by the ASTM that this

test models a true fire; however, the fuel-to-air volume, smoke duration, and humidity

levels were modified to simulate nuclear power plant fires.

The type of fuel burned to produce the smoke is a key factor in the amount of damage to

the electronic equipment. In a nuclear power plant, there are many sources of fuel for an

accidental fire, but the most abundant source is cable insulation. Many different qualified

cables are used for instrumentation and control. In addition, there are cables that are no

longer in use but are still in place. Because of the wide variety of available cables and the

limited scope of these tests, we burned a mixture of cable types rather than testing the

effect of individual cables.

The cables were selected from Table 3.4 in "Low-Voltage Environmentally-Qualified

Cable License Renewal Industry Report; Revision 1" [Bustard and Holzman, 1994],

which lists the ten most popular cable types used inside containments and the number of

plants that use these cables. From this table, the proportion of plants in which each of the

cables were used was determined. Common materials used for insulation and jacketing

for these cables include ethylene propylene rubber (EPR), chlorosulfonated polyethylene

(CSPE), neoprene, and cross-linked polyethylene (XLPE). The mass of a particular

cable's insulation and jacket material in the fuel mixture was proportional to the number

of plants that used the cable in containment.

NUREG/CR-6476D- D-7
A list of cable materials used in the smoke exposures for each of the tests is presented in

Table 2. The amount of fuel for each test corresponds to the conditions that were to be

simulated as presented in Table 1. Tests 1 to 4 correspond to a low smoke load, tests 5

and 6 to a medium smoke load, and tests 7 and 8 to a high smoke load. The weight of the

cable per length and percentage of plastic was measured by stripping the insulation

material from sample pieces. Typically the insulation material weighed 50 to 75% of the

total mass of the cable. After determining the length of cable required to obtain the

correct amount of fuel, the lengths were measured and cut for the fuel load. Rather than

just the insulation, the entire cable was placed in the combustion cell. The fuel was

loaded into small aluminum trays for ease of weighing before and after the exposure.

These measurements determined how much fuel was burned.

Small amounts of cable are difficult to cut and measure; therefore, for the small smoke

loads of 3 g/m3 , a reduced number of cables was used to produce the necessary amount of

fuel. The masses for cables of similar composition from the cable mixture were

combined and one cable was used to represent this combination. For the larger smoke

loadings, polyvinyl chloride (PVC) cable insulation and jacket materials were included.

Smoke from PVC contains hydrochloric acid and is very corrosive. Although few plants

use PVC cables today, PVC used to be a very popular cable insulation, and power plants

may still contain some of this material. Approximately 2% of the mass of the fuel

consisted of PVC for these larger smoke loads.

D-8 NUREG/CR-6476
Ch

Table 2. Fuel

Cable Name Insulation Jacket % plastic g/inch Fuel (grams)

Test #1 to 4 Test #5 and 6 Test #7 Test #8

Rockbestos Firewall III FRXLPE CSPE 56.7 5.07 1.87 10.49 139.49 40.66

Anaconda Flameguard lkv EPR CSPE 75 6.02 1.24 4.95 48.42 18.52

Brand Rex XLPE XLPE CSPE 54.6 4.93 5.26 71.33 20.01

Okonite Okolon EPR CSPE 60.9 5.73 4.33 53.41 16.62

Kerite HTK unknown unknown 73 11.1 1.42 4.83 44.92 17.52

Rockbestos Coax (le) unknown unknowq 58 1.38 0.53 3.64 47.66 13.68

Raychem XLPE XLPE NA 71 1.23 0.40 2.66 28.02 8.77

Dekoran Dekorad EPDM CSPE 54.2 2.74 0.80 3.37 45 8.93

B1W EPR CSPE 75.6 3.6 2.16 21.3 8.05

Kerite FR 1 65.6 2.13 1.81 21.04 6.92

PVC PVC PVC 56 6.25 1 1.34 14.72 4.62
The smoke production and exposure equipment is illustrated in Figure 1. Tungstenquartz lamps provided the heat for combustion. In order to produce flaming combustion,

ignition was aided by either an electrical spark or butane pilot flame. The cables would

not ignite without the spark or flame. The cylindrical quartz combustion chamber, 32 cm

long x 14 cm in diameter, contained the fuel during smoke production. The heat lamps

were placed outside of the combustion chambers, directed toward the fuel.

The heat flux was measured with a Schmidt-Boelter (thermopile) heat flux meter prior to

each test to ensure that smoke was produced in a consistent manner. Small variations in

the positions of the lamps can affect the heat flux that is incident on the fuel. The

measured heat flux included heat from the lamps as well as indirect heating from other

surfaces surrounding the test chamber which were heated by the lamps. As the

temperature of the surrounding surfaces rises, less power is needed from the radiant heat

lamps to obtain the same reading from the meter, thus the power needed to obtain a fixed

heat flux on the meter constantly decreases. To obtain a single power setting, a computer

controlled the lamps and measured the flux meter output to determine the power setting

needed to produce 50 kW/m2 at the fuel surface after 5 minutes of irradiation. This

corresponds to the radiant heat produced by a flaming fire. Although the heat flux

incident on the fuel could be reduced during a bum by coating the combustion chamber

with soot, the lamp power was not adjusted for this effect since the amount of reduction

would vary from test to test. The combustion cell was also cleaned thoroughly after each

test.

For small amounts of smoke, only one combustion chamber out of the four was used. For

medium or high smoke densities, two to four combustion chambers were used. The

smoke from the combustion chamber was conducted to the exposure chamber through

stainless steel chimneys. Each chimney had a motor-driven damper so that it can be

opened and closed remotely. The dampers were opened while the lamps were on, and

then closed when the 15- minute bum time was over. The dampers reduced the amount

of soot from one combustion chamber that could coat the surface of another chamber.

D-10 D-l0 NUREG/CR-6476
Results of Test 1

123 cm

92 cm

Exposure

Chamberhamber

4,

Load Cell

Combustion Ch mehambers

32 cm x 14 cm quartz cylinders

Figure 1. Smoke exposure unit

Exposure of Electronic Equipment

The smoke exposure unit is entirely housed in a walk-in temperature and hum-iditycontrolled environmental chamber. The temperature in the environmental chamber is

measured by a type T therrnocouple and controlled within about I _F. For most of the

tests this temperature was 75_ F. The humidity in the environmental chamber is

measured using a capacitive humidity sensor. The humidity is produced by an electrical

coil submerged in a tank of water within the environmental chamber, and the

environmental chamber reduces humidity by running the refrigeration and main heating

simultaneously. Because of the lag time in both producing and reducing humidity, the

humidity in the chamber is probably only controlled within 5% RH. For these tests the

humidity was set for 30% RH.

NUREG/CR-6476 D1 D-11I
Although the environmental chamber entirely surrounded the smoke exposure unit, the

temperature or humidity within the smoke exposure unit was uncontrolled while the unit

was closed up for the smoke exposure. After the smoke exposure, the smoke was vented

out of the exposure unit and environmental chamber.

Individual components of the experimental digital safety system were exposed to smoke

while the system was operating. In general, the equipment was exposed to smoke for a

total of 1 hour, after which the smoke was vented. The exposure time included the time

the smoke was being produced. The system was monitored for 24 hours after the

beginning of the smoke exposure. Given the limited time of measurements, only shortterm failure mechanisms were expected. Potential short-term failures include circuit

bridging, contact fouling, or loss of optical transmission. The effects of metal loss due to

corrosion would probably not cause failures during these short exposures.

The day after the exposure, the equipment was cleaned using spray degreasing

compounds. This process of cleaning the equipment for the next test exposure was

important because all the components were necessary to run the system. The intent was

to return the component to pre-exposure condition so that it could be reused as either the

exposure subject or as an unexposed part of the system during exposures involving other

components. The spray degreasing chemicals included Tech Spray Electronics

Cleaner/Degreaser and Chemtronics CFC-Free Electrowash 2000. Prior to each

exposure, the full system was subjected to baseline monitoring for several hours. This

was to determine the pre-exposure state. During the baseline tests, any errors as well as

the temperature and humidity of the equipment were logged.

The conditions of each of the eight smoke exposures varied according to the type of

environment that was to be simulated as shown in Table 2. The temperature and

humidity in a nuclear plant control room are well controlled, but in other areas in a plant

the humidity may be high because it is uncontrolled, or the fire may be extinguished with

water. For tests simulating conditions outside of the control room, humidity was added

during the smoke exposure.

To simulate the high humidity condition, steam was added to the smoke exposure

chamber after the fuel was burned. A standard amount of water, 34 g, was converted to

steam by heating it for 15 minutes in the combustion chamber. This allowed the digital

system to be exposed to smoke and steam at the same time. The amount of water added

to the exposure chamber was determined by calculating the volume of water needed to

produce 80% RH if the chamber was 80 F. The amount of water present in the chamber

given starting conditions of 30% RH and 75 F was subtracted from the water needed for

the humid condition.

Because of the corrosivity of smoke, an inexpensive humidity sensor with an uncertainty

of ±2% RH was placed in the smoke chamber with the equipment rather than using a

more expensive sensor with an output signal. This sensor had only a liquid crystal

readout, and could not transmit information to the computer. The humidity was recorded

manually at intervals during the tests.

D-12 D-12 NUREG/CR-6476
Results of Test 1

Carbon dioxide was added to test 4 after the smoke exposure. The CO, was added by

means of a fire extinguisher connected to the top of the exposure chamber. While the fire

extinguisher was being discharged, the extinguisher was weighed so that approximately

2.5 pounds of CO, was added to the chamber. This would correspond to filling the

chamber with CO, to 60% by volume. The effect was to lower the temperature in the

chamber and to remove much of the soot from the air. CO2 can also induce high static

charges.

Smoke Environment Measurements

The measurements presented in the results section of this report include both

measurements suggested by the ASTM draft standard and other standard measurements

of smoke, such as optical density, rate of fuel combustion, and time of ignition and

extinguishment. We have characterized the smoke environment by making these

measurements, along with others such as temperature, smoke deposition, and chemical

analyses. This section describes how the results were obtained. The measurements

performed are listed in Table 3 and procedures are described in this section.

Table 3. Measurements on Smoke Exposure Tests

Measurement Analysis

Baseline temperatures and smoke exposure temperatures

Temperature were statistically analyzed. The smoke exposure temperatures

were plotted.

The relative humidity of the environmental chamber was

Humidity statistically analyzed. Maximum values for the smoke

exposure chamber are given if available.

Total mass of fuel burned, weight of each cable type in fuel

Fuel mixture and time for 10 - 90% weight loss.

Graph of turbidity of smoke for red, blue, and IR wavelengths

Smoke Optical vs. time.

Density

Graph of mass deposition vs. time in horizontal and vertical

Soot Deposition orientations.

Grab sample: Proportion of atmospheric gases in chamber

Gas Analysis after fire. Sorption tube: analysis of acid gases

Filter paper: Chemical analysis of acids deposited on filter

Soot Analysis paper

NUREG/CR-6476 D-13
Temperature

Temperatures were measured in ten fixed locations in the smoke exposure chamber

throughout the test. The locations of the thermocouples are presented in Table 4. Two

nonsheathed type K thermocouples (Tql and Tq2) were welded to the surface of the

quartz crystal microbalance cases. T12 and T13 logged the temperature inside of the

experimental digital system that was being exposed to smoke. T17 logged the

temperatures near the computers located outside of the environmental chamber, such as

the host computer and the parts of the system that were not being tested. T18 and 19

logged the temperatures that were inside the environmental chamber but not in the smoke

exposure chamber. Temperatures were logged at 5-second intervals while the lamps were

on and at 1-minute intervals for the remaining time that the smoke was in the chamber.

After the smoke exposure chamber was vented, the temperatures were logged at 10-

minute intervals.

Temperatures were also recorded for the baseline equipment checks. The humidity

sensor houses a platinum resistive temperature detector (RTD) for use in calculating

relative humidity. This RTD was the only temperature monitored during the baseline

runs.

D- 14 D-14 NTREG/CR-6476
Results of Test I

Table 4. Thermocouple locations

TO Center of floor of smoke exposure chamber

TI Near lamp 1 in floor of exposure chamber

T2 Near lamp 2 in floor of exposure chamber

T3 Near lamp 3 in floor of exposure chamber

T4 Near lamp 4 in floor of exposure chamber

T5 Top of smoke exposure chamber

T6 Above lamp 1 in smoke exposure chamber ceiling

T7 Above lamp 2 in smoke exposure chamber ceiling

T8 Above lamp 3 in smoke exposure chamber ceiling

T9 Above lamp 4 in smoke exposure chamber ceiling

Tq I On vertical QCM case

Tq2 On horizontal QCM case

T 12 In experimental digital system

T13 In experimental digital system

T17 Laboratory temperature near computers

T 18 Floor of environmental chamber

T19 Floor of environmental chamber

Fuel Mass Loss

The fuel mass loss was measured in two ways: aluminum foil trays containing the fuel

were weighed before and after each test on a calibrated scale, and load cells measured the

loss of fuel mass during the burn at 5-second intervals. The load cells were mounted

under the quartz combustion cell as shown in Figure 1. The stainless steel fuel trays were

attached to the load cells by a shaft through a hole in the combustion cells. The foil trays

containing the fuel were placed on the stainless steel trays for simultaneous burning and

weighing. The load cell converts the weight into voltage by means of a strain gage. The

active measurement of fuel loss is recommended in the ASTM draft standard. Since the

scale used for these measurements was more accurate than the load cell, the final values

reported on the amount of fuel burned were determined from scale measurments.

Smoke Opacity

A turbidimeter measured the optical density of smoke by passing light through the smoke

and determining the loss of light intensity. The ratio of the initial intensity, I4, to the

NUREG/CR-6476 D-15
transmitted intensity through smoke, 1, is related to the turbidity (or extinction

coefficeint), ', and pathlength, 1. The ratios are related by the following equation [Van de

Hulst, 1957]:

I = 10 exp(-tl).

The ideal experiment would only measure the optical density of the smoke in the air, but

not that deposited on any surfaces. Because we wished to protect our optical components

from the smoke, glass slides were installed. To account for the loss of intensity due to

smoke on the protecting glass surfaces, we used two separate turbidimeters with different

pathlengths, assuming that the amount of smoke deposited on each glass slide would be

equal.

The turbidimeter was arranged as follows: Light from a high-intensity bulb (white light)

was focused with a lens and then passed through a glass slide into the smoke exposure

area. After crossing an open section of the smoke chamber, the light passed through a

second glass slide and into another lens which was isolated from the smoke exposure

chamber. The turbidimeter is well collimated A combination of prisms and filters

divided the light into three colors: red (0.6328 gt), blue (0.4579 g.), and infrared (1.060 gt).

The optical throughput of each color was converted by photodiodes into an electrical

signal. The voltage from the photodiodes was recorded at 5-second intervals while the

radiant lamps were on, at 1-minute intervals for the rest of the first hour, and at 10-minute

intervals after the smoke was vented. One turbidimeter had a pathlength of 1.9 cm and

the other had a pathlength of 3.2 cm through the smoke exposure chamber. The resulting

turbidity, t, can be calculated using this formula:

In F i. () i.,.

n[ I(6) 12(t)J

11 -12

where the subscripts I and 2 describe the two turbidimeters; I is the intensity measured

through the turbidimeters at zero time (before the smoke is added) and at any time t; and 1

is the respective pathlength for each turbidimeter.

Gas Analysis

Gases were captured in two ways-by a vacuum grab sample bottle and through a sorption

tube. A vacuum grab system captured a 10-ml filtered sample of gas in a stainless steel

bottle. These samples were analyzed by gas chromatography, which provided the

percentage of methane, carbon monoxide, nitrogen, carbon dioxide, and oxygen present

in the sample. (For reference, a standard air mixture contains 78% nitrogen, <21%

oxygen, <1% carbon dioxide, and <1% argon.)

Samples of acids contained in the smoke were collected in silica gel-filled sorption tubes.

A sampling pump pulled 20 ml per minute of air through the sorption tube during the

entire bum time (15 minutes). Analysis of the tube contents provided information on the

relative amount of acid gases ( chlorine, sulfur, and bromine) present in the chamber.

The materials that remained in the sorption tubes were analyzed using ion

D-16 D-16 NUREG/CR-6476
Results of Test 1

chromatography following the procedure contained in NIOSH 7903. Before the contents

of the sorption tube were measured, the instruments were calibrated for chlorine and

sulfur only. After all the samples were measured, bromine was found to be a significant

contributor for some tests. Since the procedure was not calibrated for bromine, only

estimates of the amount of bromine are reported. These estimates are the results of the

reported relative sensitivity of the procedure to chlorine and bromine.

Soot Deposition

The purpose of this measurement was to determine the amount of soot deposited on the

electronic equipment. Quartz crystal microbalances (QCMs) measured the mass

deposition of the smoke as a function of time. Quartz is a piezoelectric material that

undergoes shear deformation when subjected to an electrical field. Conversely, if it is

strained, it will generate an electrical field. The QCM consists of a quartz crystal with

electrodes to connect it to an electrical oscillator that is tuned to the fundamental

frequency of oscillation of the crystal. When material such as soot is deposited on the

surface of the crystal, the oscillation frequency decreases. The change in frequency can

be related to the additional mass on the quartz crystal. The frequency of oscillation is

also dependent on the temperature of the crystal, so thermocouples were attached to the

QCM case to determine the temperature around the crystal.

Two QCMs were used to measure the deposition-one in a horizontal face-up orientation

and one in a vertical orientation. The oscillation frequency was recorded at intervals of 5

seconds while the heat lamps were on, and at 1-minute intervals for the remaining first

hour. When the sparkers were used to ignite the fuel, the QCM data were noisier;

however, the frequencies recorded at the beginning of the smoke exposure before the

sparkers were started and late in the smoke exposure after the sparkers were turned off

indicate the amount of soot deposited during the exposure.

Soot Analysis

Previous studies have linked the amount of chlorine in smoke to equipment damage,

therefore it is important to determine the acidic compounds deposited on equipment

during a fire [Reagor, 1992]. Ashless filter papers were used as a substrate to collect soot

samples for chemical analysis. Two filters (4.25 cm diameter) were placed at the bottom

of the smoke exposure chamber for each test to gather samples of soot. The samples were

analyzed for chlorine, sulfur, and bromine by ion chromatography and for trace metals by

plasma-excited atomic emission spectroscopy. As in the sorption tube samples, the

procedure was originally calibrated only for chlorine and sulfur, but since a significant

amount of bromine was found in the samples, estimates of the amount of bromine are

reported using published relative sensitivities between chlorine and bromine. Ashless

filters were used because the analysis involves burning the filter paper.

Test Schedule

The smoke exposure schedule is presented in Table 5 and shows the type of exposure,

type of equipment (EUT), and conditions such as addition of fire suppression agents.

Since the TRP/MUX was most likely to be situated in general plant areas rather than the

NUREG/CR-6476 D1 D-17
control room, it is most likely to encounter higher humidity; thus water was added only to

exposures of the TRP/MUX. The DTC is most likely to be placed in the control room, so

CO2 was added to a test for the DTC. On June 6, two separate tests were performed-a

test to determine the effect of CO2 alone, and a smoke exposure with the addition of CO,.

Before each test the equipment to be used was installed in the smoke exposure chamber

and the environmental chamber was used to control the temperature and humidity for

baseline information. The baseline conditions for each test were recorded with the results

of that test.

The last test (8) on the FOMs was performed differently than the others. Because of the

anticipated problems due to heat on the FOMs, the environmental chamber was cooled so

that when the lamps were on they would not cause the FOMs to fail. Smoke was also

produced differently. A low smoke load was burned in lamp 1, and then the smoke was

allowed to remain in the chamber for 1 hour. After this, a second lamp with enough fuel

to provide for a medium smoke load when combined with the first lamp was burned and

the smoke was contained for 1 hour. For the last part of the test, lamps 3 and 4 were

burned to bring the total smoke load up to a high level, and then the smoke was contained

for 1 hour.

D-18 D-18 NUREG/CR-6476
Results of Test I

Table 5. Schedule of Smoke Exposures

Date Activity Smoke (g/m3) EUT Suppres- Exposure Time

sion

5/30/95 Smoke 2.8 TRP/MUX none 1 hour

5/31 Clean

6/1 Smoke 2.8 TRP/MUX water add H,0 immed. after bum

6/2 Clean

6/3 Smoke 2.8 DTC none 1 hour

6/5 Clean

6/6 Suppr. 0 DTC CO, add CO, only

6/6 Smoke 2.8 DTC CO, add CO. immed. after bum

6/7 Clean

6/8 Smoke 26 w/ PVC DTC none 1 hour

6/9 Clean

6/10 Smoke 26 w/ PVC TRP/MUX water add KIO immed. after bum

6/12 Clean

6/13 Smoke 320 w/PVC TRP/MUX none 1 hour 20 min.

6/14 Clean

6/15 Smoke 200 w/PVC FOM none 4 hours

6/16 Clean

Notes:

(1) No PVC in smoke, unless otherwise noted.

(2) Water suppression to TRP/MUX is not a direct spray of water. It is a humidity

increase in the area near the unit.

NUREG/CR-6476 D1 D-19
Results of ORNL Exposures

Test 1

Test name:

Test date:

Test time:

ORNLI

May 30, 1995

16:35:36 EDT

Baseline information: 4.3-hour baseline on May 30, 1995 beginning at 09:36:43 EDT.

Condition Minimum Maximum Mean SD

Temperature(C): 19.97 22.37 21.3 0.87

Relative humidity (%): 29.2 42.0 32.1 2.46

Cable Mixture:

Cable name Insulation Jacket % plastic Total g

Rockbestos Firewall III FRXLPE CSPE 56.7 2

Anaconda Flameguard lkv EPR CSPE 75 1.23

Kerite HTK unknown unknown 73 1.937

Rockbestos Coax (le) unknown unknown 58 0.59

Raychem XLPE XLPE 71 0.53

Dekoran Dekorad EPDM CSPE 54.2 0.923

Heat flux: 50 kW/m2 , 1 lamp only

Ignition: Sparkers

Lamp start time: 16:35:53 EDT

Flame start time: 16:37 EDT approx.

Flame off time: 16:38 EDT approx.

Mass loss of cables: 3 g

Time for 10-90% loss of cable mass: 2 min

Suppression: None

Sorption tube time: lhour

Sorption tube results:

Chloride Sulfate Bromide (estimate)

Front filter Back filter Front filter Back filter Front filter Back filter

0.023 mg 0.003 mg 0.013 mg 0.003 mg' 0.023 mg ND

Filter paper results:

Chloride Sulfate Bromide*

0.022 mg 0.003 mg 0.009 mg (estimate)

* The bromide values are estimated because the instruments were not calibrated.

D-20 D-20 NUREG/CR-6476
Results of Test 1

Grab sample results:

% Argon % N, % 0 % H, % CH, % CO, % %CO

0.8 78.9 20.0 0.0 0.0 0.2 ND

Temperature in chamber (_C):

TC Minimum Maximum Mean SD

TO 24.93 28.24 26.42 0.95

TI 24.52 30.23 27.74 1.31

T2 23.96 28.51 27.41 1.17

T3 24.01 30.09 27.58 1.61

T4 25.90 31.30 28.85 1.53

T5 22.58 26.78 25.02 0.99

T6 22.55 32.13 28.10 2.51

T7 22.63 25.98 24.68 0.81

T8 22.36 25.20 24.03 0.68

T9 22.92 25.08 24.16 0.55

Tq1 24.37 28.97 26.67 1.32

Tq2 24.50 28.89 26.67 1.26

T18 20.68 22.94 21.90 0.49

T19 20.61 23.04 21.90 0.51

NUREG/CR-6476 D-21 D2
Chamber Temperature (OC)

Test I

0

CL

E

34

32

30

28

26

24

22

20

32

31

30

29

28

27

26

25

24

0

0)

1mm

E

23 '

0.00 0.25 0.50 0.75 1.00

Time (hr)

D-22 NUREG/CR-6476
Results of Test 1

Smoke Optical Density and Accumulation

Test I

E

0

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

15

10

5

0

-5

-10

-15

CM

0

0

0.

CL

U)

Eu

-20 '

0.00 0.25 0.50 0.75 1.00

Time (hr)

NUREGICR-6476 D2D-23
Test 2

Test name:

Test date:

Test time:

ORNL2

June 1, 1995

11:48:49 EDT

Baseline information: 16 hour baseline run on May 31. 1995. starting at 18:42:06 EDT

Condition Minimum Maximum Mean SD

Temperature(_C): 22.5 23.2 22.9 0.16

Relative humidity %: 20.4 31.7 24.1 4.0

Cable Mixture:

Cable Name Insulation Jacket % plastic Total g

Rockbestos Firewall III FRXLPE CSPE 56.7 1.922

Anaconda Flameguard lkv EPR CSPE 75 1.336

Kerite HTK Unknown Unknown 73 1.159

Rockbestos Coax (le) Unkown Unknown 58 0.58

Raychem XLPE XLPE NA 71 0.384

Dekoran Dekorad EPDM CSPE 54.2 0.601

Heat Flux: 50kW/M2, 2 lamps (one for water)

Ignition: Sparkers

Length of flame: 2 minutes

Mass loss of cables: 2.8 g

Time for 10-90% loss of cable mass: 2 minutes

Suppression system: Water, 34 g boiled off after first 15 min cable bum

Maximum relative humidity in sensor inside chamber: 84%

Filter paper results:

Chloride Sulfate Bromide

0.029 mg 0.004 mg 0.007 mg Estimated

Grab sample results

I%Argon I%N, 1%O, I%H, I%CHA 1%CO, ICO

0.8 79.0 20.0 0.0 0.0 0.2 0.0

D-24 NUREG/CR-6476
Results of Test 2

Temperature in chamber .):

Variable Minimum Maximum Mean SD

TO 26.42 30.91 28.48 1.22

TI 25.22 32.37 29.09 2.0

T2 24.88 33.69 29.27 2.56

T3 24.93 32.64 28.49 2.04

T4 25.73 33.81 29.78 2.11

T5 23.67 29.55 26.87 1.65

T6 23.31 33.81 28.40 2.76

T7 22.89 33.61 26.97 2.91

T8 22.63 27.22 25.22 1.24

T9 23.26 27.56 25.59 1.14

T1O 25.81 31.91 29.05 1.65

Tq 1 25.71 31.72 28.92 1.64

Tq2 31.38 39.66 35.52 1.65

T13 30.60 37.32 34.55 1.58

T18 21.19 23.65 22.71 0.35

T19 21.02 24.08 22.51 0.48

NUREG/CR-6476 D-25 D2
Chamber Temperature (°C)

Test 2

36

34

c32

30 ~3O

8.28

C.

E 26

24

22

36

34

0

• 32

30

E 2 8

0

I- 26

I I I I * ' I ' ' ' I ' I ' I .I . . ' 

Bottom

TO

TI

T2

*.-- T3 ...... T4

24

0 5 10 15 20

Time (hr)

D-26 D-26 NUREG/CR-6476
Results of Test 2

Smoke Optical Density and Accumulation

Test 2

IE

(U)

*0

.0

1~

H

I

0

-1

0

0

CL

0 0

0.00 0.25 0.50 0.75 1.00

Time (hr)

NUREG/CR-6476 D2 D-27
Test 3

Test name:

Test date:

Test time:

ORNL3

June 3, 1995

11:15:46 EDT

Baseline Information: 16 hour baseline run on June 2, 1995, starting at 19:28:32 EDT.

Condition: Minimum Maximum Mean SD

Temperature (_C): 22.2 31.4 25.91 4.1

Relative humidity %: 26.0 34.4 30.2 1.1

Cable Mixture:

Cable Name Insulation Jacket % plastic Total g

Rockbestos Firewall Im FRXLPE CSPE 56.7 1.726

Anaconda Flameguard lkv EPR CSPE 75 1.054

Kerite HTK Unknown Unkown 73 1.219

Rockbestos Coax (le) Unknown Unknown 58 0.47

Raychem XLPE XLPE NA 71 0.396

Dekoran Dekorad EPDM CSPE 54.2 0.91

Heat flux: 50 kW/m2, 1 lamp

Ignition: Sparkers

Mass loss of cables: 2.63 g

Time for 10-90% loss of cable mass: 2.5 min

Suppression system: None

Sorption tube time: 15 min.

Sorption tube results:

Chloride Sulfate Bromide

Front filter Back filter Front filter Back filter Front filter Back filter

0.012 mg 0.002 mg 0.007 mg 0.004 mg 0.009 mg Est. ND

Filter paper results:

Chloride Sulfate Bromide

0.030 mg 0.003 mg 0.009 (Est)

Grab sample results:

% Argon % N, % 0 % 1 % CH, % CO, % CO

0.8 79.0 20.0 0.0 0.0 0.2 0.0

D-28 D-28 NUREG/CR-6476
Results of Test 3

Temperature in chamber (_C):

TC Minimum Maximum Mean SD

TO 25.49 29.21 27.73 0.84

TI 23.69 30.28 27.12 1.33

T2 23.91 29.92 27.07 1.24

T3 24.37 29.33 26.68 0.89

T4 24.52 30.84 27.76 1.72

T5 22.50 26.88 24.81 0.80

T6 22.67 32.54 26.26 2.45

T7 22.50 26.76 24.78 0.75

T8 22.02 25.90 24.25 0.60

T9 22.53 25.76 24.62 0.47

T1O 25.42 29.14 27.94 0.83

Tq 1 25.39 29.04 28.05 0.90

Tq2 34.08 39.75 37.72 1.03

T13 28.46 32.64 30.88 0.80

T18 20.03 23.16 22.38 0.42

T19 19.76 22.97 22.18 0.44

NUREG/CR-6476 D-29
Chamber Temperature (°C)

Test 3

0

0

I--

34

32

30

28

26

24

22

20

32

31

30

29

28

27

26

25

24

23

0

0)

I..

4-u

Cu L.

0)

0.

E

0)

I0 5 10 15 20

Time (hr)

D-30 D-30 NUREG/CR-6476
Results of Test 3

Smoke Optical Density and Accumulation

Test 3

1.5

E

Lon

1.0

0.5

0.0

-0.5

10

00%

0

0~

5

0

-5

-10 UL0.00 0.25 0.50 0.75 1.00

Time (hr)

NUREG/CR-6476 D3 D-31
Test 4

Test name: ORNL4

Test date: June 6, 1995

Test time: Two start times, 12:33:13 and 13:25:19 EDT

The first start was aborted after 1 minute because of failures in experimental

system due to EMI from sparkers. As a result sparkers were removed and test proceeded

without ignition.

Baseline information: 16.9 hour run on June 5, 1995 begining at 17:55:56 EDT.

Condition Minimum Maximum Mean SD

Temperature(_C): 22.14 22.91 22.60 0.18

Relative humidity (%): 29.79 32.74 31.19 0.79

CO2 only: This was a test to determine the effect of CO, on system.

2.6 lb of CO2 were added to exposure chamber.

Temperatures in exposure chamber (C

TC Minimu Maximu Mean SD

m m

TO 24.11 25.76 24.68 0.63

TI -10.56 23.89 18.40 8.7

T2 -11.25 23.57 18.44 8.9

T3 -10.24 23.89 19.24 8.6

T4 -3.30 26.17 21.75 6.8

T5 18.60 22.65 21.52 0.85

T6 17.31 22.87 21.71 1.2

T7 17.36 23.01 21.80 1.2

T8 17.14 22.72 21.62 1.2

T9 16.58 23.28 21.74 1.4

T12 21.80 29.16 24.71 2.3

T13 21.63 28.78 25.53 2.0

T18 21.90 22.77 22.26 0.26

T19 21.75 22.94 22.25 0.36

The following information refers to the smoke exposure portion of the test.

D-32 NUREG/CR-6476
Results of Test 4

Cable Mixture:

Cable Name Insulation Jacket % plastic Total g

Rockbestos Firewall III FRXLPE CSPE 56.7 1.803

Anaconda Flameguard lkv EPR CSPE 75 1.351

Kerite HTK 73 1.385

Rockbestos Coax (le) 58 0.497

Raychem XLPE XLPE 71 0.313

Dekoran Dekorad EPDM CSPE 54.2 0.777

Heat Flux: 50kW/M2, 1 lamp

Igniter: None.

Note: No flames, only smouldering on this test. Igniter of some type necessary for

flames.

Mass loss of cables: 2.8 g

Time for 10-90% loss of cable mass: 2 minutes

Suppression system: 2.48 lb. CO2added after lamps turned off (15 minutes after second

start of test)

Sorption tube time: 16 min

Sorption tube results:

Chloride Sulfate Bromide

Front filter Back filter Front filter Back filter Front filter I Back filter

0.018 mg 0.002 mg 0.007 mg 0.004 mg 0.008 (Est) ND

Filter paper results: None, filter paper was blown away.

Grab sample results:

I%Argon I%N, 1%0, O %H, I%CH I %CO, I%CO

0.5 51.0 12.1 0.0 0.0 36.4 ND

Temperature in chamber (_C)

TC Minimum Maximum Mean SD

TO 24.93 27.92 27.18 0.49

TI 1.169 29.36 26.25 1.06

T2 1.144 28.75 25.99 1.02

T3 1.531 29.43 25.91 1.04

T4 4.044 31.84 27.99 1.11

T5 19.88 25.90 24.09 0.48

T6- 19.74 29.28 24.39 1.09

T7 18.16 25.47 23.84 0.48

T8 18.57 25.10 23.66 0.40

T9 17.97 25.18 24.62 0.41

Tql 24.88 28.43 27.84 0.55

Tq2 24.79 28.36 27.76 0.56

NUREG/CR-6476 D-33
T12

T13

T18

T19

26.61

29.19

20.08

20.00

32.54

36.83

23.16

28.07

31.43

35.76

22.39

22.35 10.551

D-34 D-34 NUREG/CR-6476
Results of Test 4

Chamber Temperature (°C)

Test 4

30 ,. . . , , , ,

28 Top

0 26

I24

2 -22 T,

20 T,

18 -...... T!,

16

35

30 - 3Bottom

0 25

0

o 20/ 20 - TO

(U 1 5 • T1

T2 0-10 - T3

E ...... T4

j-- 5

0

0 1 2 3 4 5

Time (hr)

NUJREG/CR-6476 D3 D-35
Smoke Optical Density and Accumulation

Test 4

I

E

0

%%0

0

CM

E

0

40)

0

CL

Co

(U

-1

12

10

8

6

4

2

0

I I I I I I I I I I I a I I I I I I I

/f

I A

Horizontal

Vertical

,2

0.00

• . . I , , I I . , . I . . I

0.25 0.50 0.75 1.00

Time (hr)

D-36 D-36 NUREG/CR-6476
Results of Test 5

Test 5

Test name: ORNL5

Test date: 8 June 1995

Test time: 13:09:27 and 12:26:50 EDT, problem with program~and restarted after

15.5 minutes (after burning Lamp 1)

Baseline Information: 16-hour baseline run on June 7 1995 starting at 19:09:27 EDT.

Condition: Minimum Maximum Mean SD

Temperature (_C): 22.25 23.11 22.62 0.15

Relative humidity %: 27.32 35.09 30.15 0.81

Cable Mixture:

Cable name Insulation Jacket % plastic Total g

Rockbestos Firewall Ill FRXLPE CSPE 56.7 10.64

Anaconda Flameguard lkv EPR CSPE 75 4.96

Brand Rex XLPE XLPE CSPE 54.6 5.27

Okonite Okolon EPR CSPE 60.9 4.23

Kerite HTK 73 4.75

Rockbestos Coax (le) 58 3.69

Raychem XLPE XLPE 71 2.7

Dekoran Dekorad EPDM CSPE 54.2 3.54

BIW EPR CSPE 75.6 2.1

Kerite FR 65.6 1.81

PVC PVC PVC 56 1.4

Total I 1 1 45.09

Heat flux: 50 kW/m2, 2 lamps, 1 and 3

Ignition: Sparkers plugged in individually; problems with program, had to restart

BIGTEST (the program to run smoke system)

Mass loss of cables: 10.38 g from tray 1, 10.01 g from tray 3

Suppression system: none

Filter paper results: None.

NUREG/CR-6476 D-37
Grab sample results:

j%Argon I%N, 1%0 O %H, j %CHA % CO0 % CO

1.1 78.5 18.6 0.0 0.0 2.0 0.5

Temperature in chamber (_C):

TC Mean Minimum Maximum SD

TO 28.52 25.95 31.11 0.59

TI 30.05 24.45 34.56 2.26

T2 29.38 24.33 32.88 2.08

T3 29.99 24.28 36.32 1.58

T4 30.57 25.01 35.88 1.26

T5 27.12 22.87 30.91 1.52

T6 28.98 23.01 34.37 3.25

T7 26.13 22.94 28.58 1.17

T8 27.83 22.77 37.27 0.94

T9 26.06 23.18 29.14 0.65

T12 35.25 26.95 42.17 1.96

T13 32.99 27.83 36.95 1.43

T18 21.85 19.40 22.80 0.29

T19 21.64 19.11 22.87 0.41

D-38 NUREG/CR-6476
Results of Test 5

Chamber Temperature (°C)

Test 5

38 ' ' . * 

Top TI

36 opT

0" 34 T1

032'o S32.. TI

cc 30

Lc. 28

E

02624

22 1 1 1 1 1 1

38 1 1

36 Bottom T(

~T'

0 34

o_.0 - -T

a 32 --.

~30

aL28

E

1026

24

220 5 1 1

0 5 10 15 20

Time (hr)

NUREG/CR-6476 D3 D-39
Smoke Optical Density and Accumulation

Test 5

1.00

0.75

E

a) 0.50

0.25

0.00

-0.25

35

E

0)

0

CL0)

30

25

20

15

10

5

0

-5

-10 o0.00 0.25 0.50 0.75 1.00

Time (hr)

D-40 D-40 NUREGICR-6476
Results of Test 6

Test 6

Test name:

Test date:

Test time:

ORNL6

June 10, 1995

12:38:01 EDT

Baseline information: 17.25-hour run on June 9, 1995 starting at 19:04:25 EDT.

Condition: Minimum Maximum Mean SD

Temperature (_C): 22.58 23.77 22.91 0.20

Relative humidity %: 16.87 31.11 26.45 3.7

Cable Mixture:

Cable Name Insulation Jacket % plastic Total g

Rockbestos Firewall III FRXLPE CSPE 56.7 10.34

Anaconda Flameguard lkv EPR CSPE 75 4.94

Brand Rex XLPE XLPE CSPE 54.6 5.24

Okonite Okolon EPR CSPE 60.9 4.42

Kerite HTK Unknown Unknown 73 4.92

Rockbestos Coax (le) Unknown Unknown 58 3.58

Raychem XLPE XLPE NA 71 2.63

Dekoran Dekorad EPDM CSPE 54.2 3.2

BIW EPR CSPE 75.6 2.22

Kerite FR 65.6 1.79

PVC PVC PVC 56 1.27

Total 1 44.55

Heat flux: 50 kW/m2 , three lamps, 1 and 3 for fuel and 2 for water

Mass loss of cables: 9.93 g from lamp 1 and 10.04 g from lamp 3

Time for 10-90% loss of cable mass: Lamp 1, 4.3 min; lamp 3, 6.0 min.

Suppression system: 34 g water boiled off on lamp 2

Sorption tube time: 1 hour

Sorption tube results:

Chloride Sulfate Bromide

Front filter Back filter Front filter Back filter Front filter _]Back filter

0.514 mg 0.006 mg 0.152 mg 0.039 mg 0.270 mg Est. ND

Filter paper results:

Chloride Sulfate Bromide

0.057 mg 0.004 mg 0.062 mg Estimate

NUREG/CR-6476 D-41
Temperature in chamber (_ C):

TC Minimum Maximum Mean SD

TO 24.06 31.01 27.32 1.5

TI 23.65 35.25 29.87 3.3

T2 23.62 35.88 29.76 3.5

T3 23.48 35.98 29.94 3.8

T4 24.18 36.61 30.56 3.5

T5 22.26 32.08 27.51 2.8

T6 21.99 35.05 28.67 3.5

T7 22.09 36.61 27.42 3.5

T8 21.53 37.44 28.22 4.5

T9 22.04 30.38 26.55 2.2

T1O 25.25 35.78 30.27 3.0

Tq 1 24.79 35.81 30.06 3.1

Tq2 31.11 44.90 38.70 3.5

T13 32.37 51.05 41.55 4.3

T18 20.17 24.18 22.81 0.57

T19 20.00 24.54 22.57 0.72

D-42 NUREG/CR-6476
Results of Test 6

Chamber Temperature (°C)

Test 6

4 0 .. . . a.. i . , ,

38 Top

•. 36

.. 34

232

30 ......

(D 28

0L

E26

-- 24

22

201

3 8 .. . . . I . . . .

36 ' Bottom

Q 34

032

30 ......

cL28

E

-26

24

221

0 5 10 15 20

Time (hr)

NUREG/CR-.6476 D4 D-43
Mass Loss and Accumulation

Test 6

Ca

*0

E

0

4w

0

5

0

-5

-10

-15

-20

-25

-30

-35

-40

35

30

25

20

15

10

5

0

I I I II I I I I I I

Horizontal

I

J Vertical

I -5'

0. 0 0.5 1.0

Time (hr)

D-44 D-44 NURIEG/CR-6476
Results of Test 7

Test 7

Test name:

Test date:

Test time:

ORNL7

June 13, 1995

11:59:59 EDT

Baseline Information: 15-hour baseline run on June

No temperature and humidity information available

12, 1995 starting at 19:09:27 EDT.

Cable mixture:

Cable Name Insulation Jacket % plastic Total g

Rockbestos Firewall 11 FRXLPE CSPE 56.7 139.49

Anaconda Flameguard lkv EPR CSPE 75 48.42

Brand Rex XLPE XLPE CSPE 54.6 71.33

Okonite Okolon EPR CSPE 60.9 53.41

Kerite HTK unknown unknown 73 44.92

Rockbestos Coax (le) unknown unkown 58 47.66

Raychem XLPE XLPE NA 71 28.02

Dekoran Dekorad EPDM CSPE 54.2 45

BIW EPR CSPE 75.6 21.3

Kerite FR 65.6 21.04

PVC PVC PVC 56 14.72

Heat flux: 50 kW/m2, all 4 lamps (overloaded trays)

Mass loss of cables: Tray 1, 45.37 g (flaming); Tray 2, 52.66 g (flaming); Tray 3, 29.01

g (smoldering only); Tray 4, 33.09 g (smoldering only).

Time for 10-90% loss of cable mass: Lamp 1, 9:10 min; Lamp 2, 6:15 min; Lamp3, 8:02

min; Lamp 4, 9:09 min.

Suppression system: None

Sorption tube time: 1 hour, 20 minutes/

Sorption tube results:

Chloride Sulfate Bromide

Front filter Back filter Front filter Back filter Front filter Back filter

0.752 mg 0.007 mg 0.149 mg 0.042 mg 0.233 Est. ND

Filter paper results:

I Chloride Sulfate Bromide

0.742 mg 0.005 mg 0.547 mg Estimate

NUREG/CR-6476 D4-45
Grab sample results

%Argon I%N7, 1% O %H•, I%CH, I %CO, %CO

1.0 80.0 13.6 0.6 0.0 5.2 1.5

Temperature in chamber (_C):

TC Minimum Maximum Mean SD

TO 14.06 27.75 21.49 3.5

TI 14.02 31.91 25.47 5.0

T2 14.04 36.07 25.64 5.6

T3 13.80 31.76 24.32 5.1

T4 13.90 34.69 25.82 5.5

T5 12.56 30.48 23.17 4.9

T6 12.49 31.64 23.88 4.6

T7 12.85 44.87 24.56 6.5

T8 11.93 34.37 22.39 6.0

T9 12.34 34.03 21.74 5.4

T12 18.91 40.24 30.94 6.5

T13 18.55 42.70 32.18 5.9

T17 18.60 25.52 22.44 0.7

T18 9.898 18.99 14.40 1.6

T19 9.874 19.13 14.40 1.6

D-46 D-46 NUREG/CR-6476
Results of Test 7

Chamber Temperature (°C)

Test 7

0

0

4.'

Cu I0

a.

E

0

I50

45

40

35

30

25

20

15

10

40

35

30

25

20

15

10

0

CL

E

Bottom

TO

T1

-T2

T3

...... T4

11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 6 1

0 5 10 15 20 25

Time (hr)

NUREG/CR-6476 D4-47
Smoke Optical Density and Accumulation

Test 7

5

4

E

C,

%Now

3

2

1

0

-1

200

.00-

0

0~

0

150

100

50

01."•

0.0 0.5 1.0 1.5 2.0

Time (hr)

D-48 NUREG/CR-6476
Test 8

Test name:

Test date:

Test time:

ORNL8

June 15, 1995

12:53:28 EDT

Baseline Information: 17.5-hour temperature test for fiber optic modules on June 14,

1995, starting at 12:53:28. See temperature plot.

Cable Mixture:

Cable Name Insulation Jacket % Lamp 1 g Lamp 2 g Lamp 3

plastic _

Rockbestos Firewall III FRXLPE CSPE 56.7 1.94 8.91 29.81

Anaconda Flameguard lkv EPR CSPE 75 1.04 4.03 13.45

Brand Rex XLPE XLPE CSPE 54.6 0.00 5.41 14.60

Okonite Okolon EPR CSPE 60.9 0.00 4.47 12.15

Kerite HTK 73 1.02 3.99 12.51

Rockbestos Coax (le) 58 0.43 3.09 10.16

Raychem XLPE XLPE 71 0.30 2.30 6.17

Dekoran Dekorad EPDM CSPE 54.2 0.48 1.81 6.64

BIW EPR CSPE 75.6 0.00 2.05 6.00

Kerite FR 1 65.6 0.00 1.81 5.11

PVC PVC PVC 56 0.00 1.21 3.41

Heat flux: 50 kW/m& on all four lamps

Ignition: Sparkers only until ignited, no flame on lamp 4

Mass loss of cables: Tray 1, 2.43 g (flaming); Tray 2, 15.45 g (flaming); Tray 3, 23.14 g

(flaming); Tray 4, 23.28 g (smoldering).

Time for 10-90% loss of cable mass: Lamp 1, 2:27 min.; lamp 2, 5:26 min.; lamp 3, 6:20

min., lamp 4, 8:20 min.

Suppression system: None

Sorption tube time: 4 hours

Sorption tube results:

Chloride Sulfate Bromide

Front filter Back filter Front filter Back filter Front filter Back filter

0.124 mg 0.050 mg 0.012 mg 0.006 mg 0.075 mg Est. ND

Filter paper results:

Chloride I Sulfate Bromide

0.509mg 0.010mg 0.512 mg Estimate

NUREG/CR-6476 D-49
Temperature in chamber (_C):

TC Minimum Maximum Mean SD

TO 12.93 20.76 17.74 1.2

TI 12.66 24.84 18.49 2.6

T2 12.66 24.81 18.60 2.8

T3 12.49 25.69 18.87 3.2

T4 12.42 28.31 18.95 3.4

T5 11.54 24.69 17.40 2.8

T6 11.86 23.18 17.17 2.3

T7 12.20 29.53 17.92 3.6

T8 11.28 29.45 18.31 5.0

T9 11.74 31.81 17.82 4.6

Tql 13.87 26.42 19.39 2.8

Tq2 13.87 26.39 19.43 2.9

T12 18.48 30.60 25.23 2.3

T13 18.16 28.43 23.07 2.0

T17 19.13 26.10 23.29 0.86

T18 9.39 14.55 11.71 0.49

T19 9.29 14.50 11.64 0.53

D-50 D-50 NUREG/CR-6476
Chamber Temperature (°C)

Fiber Optic Module Temperature Test

35

0

CL

E

30

25

20

15

0

I-CL

E

0

10

40

35

30

25

20

15

10

0 5 10 15

Time (hr)

NUREG/CR-6476 D5 D-51
Chamber Temperature (°C)

Test 8

35

0

E

0

*6--

CL

E.

0.

30

25

20

15

10

30

28

26

24

22

20

18

16

14

12

10

0 5 10 15 20

Time (hr)

D-52 NUREG/CR-6476
Smoke Optical Density and Accumulation

Test 8

3

0

V.

2

1

0

-1

100

0

CL

(1)

80

60

40

20

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Time (hr)

NUREG/CR-6476 D5 D-53
Conclusions

The smoke exposures for the ORNL experimental digital system were sucessful in

providing a general indication of whether a digital safety system would perform under

smoke conditions. Three levels of smoke density were tested: low (approximately 3 g of

fuel/m 3

), medium ( approximately 20 g of fuel/m3), and high (more than 60 g of fuel/m3).

For high levels of smoke density, the fuel near the end of the exposure would not ignite,

but instead smouldered. This indicates that there was not enough oxygen to support open

combustion at this level. The grab sample from test 7 also supported this theory because

the level of CO was much higher than in any other tests and the level of oxygen was

down to only 13.6%.

Several fire suppression simulations were part of these tests. This included the addition

of humidity in the form of steam and CO2 from a fire extinguisher. When humidity was

added during the smoke exposure, the objective was to reach at least 80% RH. This was

accomplished using a simple calculation of the amount of water required in the chamber

to raise the humidity to this level given the temperature that was expected at the end of

the exposure. The addition of CO2 was accompanied with a test of CO2 alone from the

fire extinguisher. The CO2 had very little effect on the equipment, although the

temperatures in the exposure chamber dropped drastically. Filters that were placed in the

exposure chamber to determine the chemical composition of the soot were blown away

by the action of the fire extinguisher and were not analyzed.

The method of igniting the fire was changed partway through these tests because the DTC

was found to be too sensitive to the EMI generated by the sparkers. A butane lighter was

used instead. The lighter burned very cleanly and was used only for a short time to ignite

the fuel after it had been heated. The sparkers were used as a source of ignition when the

test equipment did not have failures from the EMI that they produced.

The results of this series of tests have shown that the SNL large smoke exposure chamber

can produce consistent smoke exposures as measured by the chemical products of gases

and soot. This is very important when considering the need for a standardized test and

for the upcoming circuit bridging series.

D-54 D-54 NUREG/CR-6476
References

Bustard, L., and Holzman, P., 1994. "Low-Voltage Environmentally Qualified Cable

License Renewal Industry Report; Revision 1," TR-103841, published by Electric

Power Research Institute, Palo Alto, CA.

Kessel, Stephen L., Bennett, James G. Jr., and Rogers, Charles E., 1994. "Corrosivity

Test Methods for Polymeric Materials. Part 1-Radiant Furnace Test Method [1],"

Journal of Fire Sciences, Vol. 12, No. 2, pp. 107-133.

Korsah, K., Tanaka, Tina J., and Wilson, T. L., to be published. "Environmental Testing

of an Experimental Digital Safety System," NUREG/CR 6406, Oak Ridge National

Laboratory, Oak Ridge, TN.

Reagor, Barbara T., 1992. "Smoke Corrosivity: Generation, Impact, Detection, and

Protection," Journal of Fire Sciences, Vol. 10, pp. 169-179.

NUREG/CR-6476 D-55

Appendix E

Impact of Smoke on Advanced I & C: Selection of Sample Components and Test

Matrix

A letter report to the USNRC, Office of Research

Prepared by:

Tina J. Tanaka

Components and Structures Safety and Reliability Department

Sandia National Laboratory

Albuquerque, New Mexico

July 27, 1995

Abstract

Smoke can be a serious threat to electrical equipment. A number of plants are starting to retrofit

their safety systems and may upgrade them to microprocessor based systems. Because of the

smaller size and density of the newer equipment, smoke may cause more widespread problems in

the replacement system than in the original equipment. As a result of the concern about the

effects of smoke in nuclear power plants, the Nuclear Regulatory Commission (NRC) sponsored

a study by Sandia National Laboratory on the impact of smoke on advanced instrumentation and

control (I&C) equipment. This report describes some of the equipment that SNL will expose to

smoke, and the specific scenarios that it will simulate for this effort. Three types of equipment

will be exposed: surface insulation resistance (SIR) boards, individual components (i.e., chips),

and functional circuit boards. The SIR test patterns imprinted on printed circuit boards will be

used to measure surface conductivity as a function of smoke deposition. Individual component

samples will be tested to determine the effect of smoke on various kinds of chip packaging.

Smoke obscuration of optical isolators will also be studied. For the functional board tests, SNL

will use a single standardized circuit board to assess the effects of smoke on functional circuits of

various types. To test individual components, SNL will vary the exposure conditions such as

burn temperature, fuel-to-air ratio, presence of galvanic metals, humidity level, and fire

suppression agents. The range of exposure conditions for the functional circuit tests will be

based on the results of the individual component tests. SNL will determine what exposure

conditions will cause functional boards to fail. Using this approach, the tests will cover both

variations in smoke exposure scenarios and variations in their effect on different types of

advanced digital instrumentation.

NUREG/CR-6476E- E-1
Acknowledgments

I would like to acknowledge the assistance of Dr. Kofi Korsah of Oak Ridge National

Laboratory, Dr. Mahbubul Hassan of Brookhaven National Laboratory, and Dr. David Palmer

and Dr. Dennis Anderson, both of Sandia National Laboratory, for educating me on digital

components and component reliability. Ms. Christina Antonescu has been invaluable in

procuring funding from the U.S. Nuclear Regulatory Commission and in providing advice on the

project direction. This work was supported by the U.S. Nuclear Regulatory Commission and

was performed at Sandia National Laboratories, which is operated for the U.S. Department of

Energy under Contract DE-AC04-94AL85000.

E-2 NUREG/CR-6476
Table of Contents

1. Introduction 4

2. Summary of Likely Failures in Microprocessor-Based Systems 4

3. Equipment to be Tested 6

3.1. SIR Boards 6

3.2. Integrated Circuits 8

3.3. Functional Board Tests 10

4. Schedule 11

4.1. Total Test Matrix 11

4.2. Schedule of Testing 12

5. Summary 12

6. References 13

NUREG/CR-6476E- E-3
1. Introduction

Smoke can be a serious threat to both digital and analog electrical equipment. Research by

telecommunications companies shows that in a fire, up to 95% of the damage to electronic

equipment results from smoke.' This threat also affects nuclear power plants. In many cases

nuclear power plants have had to upgrade their safety systems with microprocessor-based

systems because no analog replacement parts are available. The newer digital equipment is more

compact and can combine many functions, such as transmitting several different reactor

instrument readings over one cable, by using networking techniques. However, as a result of this

reduction in size and combination of functions, an environmental hazard such as smoke may

cause more widespread problems in a replacement digital system than it would in the original

analog system.

In a nuclear power plant, the first few hours after a fire are critical for plant safety (the time

required to achieve safe shutdown). Little information is available on the reliability of electronic

equipment that is exposed to fire and smoke for this critical period, although some data exist on

the reliability of electrical equipment in the long term (more than several days after the fire).

Short-term effects may include intermittent short circuits and may depend on conditions such as

humidity or the presence of fire suppression agents. There have been a few fire tests in which

operating electronic equipment was exposed to smoke, but there is no standard procedure for

such exposures. As a result of concern about the effects of smoke in nuclear power plants, the U.

S. Nuclear Regulatory Commission (NRC) began a program at Sandia National Laboratories

(SNL) in 1994 to study the impact of smoke on advanced instrumentation and control equipment

(I&C).

The types of digital equipment that can be used to retrofit safety systems can vary widely. In

order to limit time and effort in this project, SNL will test a limited set of typical digital

equipment. The smoke tests will expose three types of equipment: SIR boards with a specialized

test circuit, individual components (i.e., chips), and functional circuits. This report describes the

various components that will be tested and the likely failures anticipated. SIR boards and

individual components will be tested between August and December 1995. The tests of the

functional circuits are planned for 1996.

In a related effort, SNL and Oak Ridge National Laboratories (ORNL) are cooperating on testing

the effect of smoke on a simulated operating reactor trip system. ORNL designed, assembled

and programmed the system, certain parts of which were exposed to smoke under their program

on Advanced Instrumentation and Control. The tests took place in December 1994 and MayJune 1995 and the results will be published shortly.2

',3

2. Summary of Likely Failures in Microprocessor-Based Systems

When determining the type of equipment to test and the diagnostics required to assess

performance, it is important to determine the likely failure modes of a microprocessor system.

Based on a review of the literature and discussions with experts in digital circuits, four likely

modes of smoke damage have been identified: circuit bridging, contact fouling, corrosion, and

E-4 NUREG/CR-6476
Table 1. Summary of Anticipated Failure Modes of Electronic Components in a Smoke

Environment

Failure mechanism Failure cause

Corrosion Acid gases can combine with water to corrode metal

contacts and cause open circuits.

Shorts Soot can act as a circuit bridge between contacts and cause

leakage of currents and shorts.

Increased contact resistance Electrical contacts can be coated with soot , causing

increased contact resistance.

Mechanical binding Mechanical equipment, such as timers, may be impeded by

soot

Obscuration Smoke and soot may coat optical interfaces and reduce

optical transmission

Galvanic salt shorts Chlorine gases can combine with zinc from galvanized

surfaces to form ZnCl. This compound is hygroscopic and

conductive. When it combines with water it forms a syruplike liquid that can then drip onto electronics and cause

1 shorts.

hindering mechanical motion. Optical components may also fail because of obscuration. Failure

mechanisms from smoke are believed to be similar to those due to a polluted environment.

Although failures caused by pollution may occur over decades, the soot and smoke from a fire

can produce failures in minutes to weeks. Table 1 summarizes the types of failures anticipated

for electronic equipment. For advanced digital equipment, we expect failures primarily due to

corrosion and circuit bridging shorts. Contact fouling and mechanical binding are considered

unique to older analog equipment.

As a part of their collaborative work, in December 1994, ORNL and SNL exposed a small

microprocessor-based digital system to smoke.2 These tests provided information on the kinds of

failure modes that may be possible. In the tests, a host computer interrogated the condition of the

exposed digital system, but remained outside of the exposure. For one of the smoke exposures,

the interrogation program stopped, which indicated an error in the exposed system, but once the

system was restarted it then acted normally. Other failures occurred in the first 24 hours of

exposure, but also were intermittent. After exposure to smoke, the uncleaned system again

worked as designed. The system was periodically tested over 1 month without failure until

humidity was added with a cool-mist humidifier. The humidifier water condensed on the board

and shorted some of the contacts. This caused the same error that was seen during the smoke

test; however, in this case the equipment would not restart.

Based on this experience, we believe that during the smoke exposure, currents leaking from the

circuit bridging caused by soot probably caused a transient error which in turn caused the

NUREG/CR-6476E5 E-5
program to stop, but these shorts were of such limited duration that they did not destroy the

equipment. Shorts caused by circuit bridging were expected to take place during the smoke

exposure and the likelihood of such shorts is probably strengthened by the introduction of

humidity. This happened within an hour of the smoke exposure. Other effects, such as contact

corrosion, take place more slowly, perhaps over months. Since the time during and immediately

after a fire is so critical for a nuclear power plant if shutdown is required, SNL plans to mainly

investigate failure modes that are immediate (i.e., shorting and obscuration) rather than longer

term (i.e., corrosion).

3. Equipment to be Tested

Advanced microprocessor-based equipment can be classified in many ways, but all are based on

a printed circuit board populated with integrated circuits (ICs) or chips. ICs are designed using

various technologies: transistor-transistor logic (TTL), complementary metal-oxide

semiconductor technology (CMOS) and emitter-coupled logic (ECL). Within each of these

broad categories there as subcategories such as low-power Schottky (LS), advanced low-power

Schottky (ALS) and advanced Schottky (FAST), which are all TTL-based designs.4 The choice

of technology in a circuit is based on speed, power consumption, output, supply voltage, and

noise considerations.

The complexity of the IC determines the design and packaging of the final component. An IC by

itself is very small and delicate; it is the packaging that allows digital equipment to be handled

and assembled. As memory and logic chips become more complex, the density of

interconnections within the chip grows. The number of connecting pins on the final package also

grows with each advance in digital technology. These smaller, more densely packed connections

make the circuitry more susceptible to shorts or open circuits.

With respect to corrosive failures, the particular technology type, speed, or complexity of a chip

should have little effect; however, the way the ICs are packaged and mounted should affect

reliability greatly. Since the IC is encased in a package, the smoke will attack the package first.

Important considerations affecting corrosion include the metal or combination of metal contacts,

contact spacing, applied voltages, bonding techniques, and the material which protects the chip.5

In order to understand the relative importance of these aspects, SIR boards, ICs, and functional

boards will be exposed to smoke at SNL. The SIR boards and ICs will be exposed

simultaneously while the functional boards will be tested separately in 1996. The sections which

follow describe the test items.

3.1. SIR Boards

Soot from a fire can form a semiconductive layer on printed circuit boards. This layer can induce

current leakage between contacts on the boards and lead to immediate failure. One way to

measure the change in resistance of the surface of a printed circuit board is to measure the

surface insulation resistance (SIR). This is done with a dual comb pattern made of interlaced

E-6 E-6 NUREG/CR-6476
conducting tines (see Figure 1). A voltage across the comb pattern will create a field on the

surface of the board. To measure surface insulation, the current leakage between the comb

patterns is monitored. The Institute for Interconnecting and Packaging Electronic Circuits (IPC)

has developed several standard SIR boards and procedures for use in measuring surface

insulation resistance. Although SIR is not a standard measure for smoke corrosivity testing, a

collaborative effort of AT&T, DuPont and Underwriter's Laboratory (UL) is planning some

smoke exposures in 1995 using this technique to evaluate its use as a standard.

SNL will measure surface insulation resistance using one of these standard printed circuit boards

(IPC-B-24). The surface insulation resistance will be actively monitored during the smoke

exposure. In order to study the effect of various coatings, three boards will be tested

simultaneously in the smoke chamber while another will be monitored as a control board outside

of the smoke exposure chamber. The boards in the smoke chamber will include an uncoated

board, a conformally coated board, and a board enclosed in a metal box. The control board will

be at the same temperature and humidity conditions as the rest of the boards, but will not be

exposed to the smoke. The SIR will be measured at three different voltages: 5 V, 50 V, and 160

V. The 5-V measurement corresponds to the voltage found on many printed circuit boards. The

160-V measurement was chosen to match the fields on similar boards to be tested at UL. The

50-V measurement was chosen because it is in between the low and high voltage level and may

aid in determining the effect of different voltages on these circuits. Since the smoke deposition

can be affected by fields near the boards, the comb patterns will remain at the test voltage even

between measurements.

NUREG/CR-6476 E-7
Figure 1. Comb pattern

3.2. Integrated Circuits

Integrated circuits are generally contained in a package that allows for electrical connections but

which protects the IC from the environment. Many of these packages are made of either plastic,

ceramic, or metal bodies with copper contacts. The packages come in seven families: (1)

transistor outline (TO) can, (2) single-in-line package (SIP), (3) dual-in-line package (DIP), (4)

small-outline integrated circuit (SOIC) package, (5) chip carrier (this can be leadless or leaded),

(6) flatpack made of ceramic or metal, and (7) grid array.7 See Figure 2 for illustrations of some

of these chip packages. The first three families, the TO can, SIP, and DIP package, are mounted

to a printed circuit board by drilling holes in the board and soldering the connectors to the board

(through-hole mounting). The last four families are surface-mounted packages. The surfacemounted packages are connected by coating bonding pads on the printed circuit board with

solder paste, then placing the chips on the board and heating the board.

Newer surface mounting techniques allow many more connections per surface area than older

through-hole mounting schemes. Compact circuits not only allow for smaller packages, but also

improve the speed of a circuit. In a smoke environment, however, these smaller, more closely

packed connections can prove to be less reliable than the older through-hole mounted chips.

Conductors that are spaced more closely together may be easier to bridge with soot and smaller

conductors may corrode faster.

E-8 E-8 NUREG/CR-6476
SNL will investigate the effects of smoke on the chip-to-printed circuit board mounts by

measuring the resistance between leads on empty chip packages. Seven different chip packages

will be mounted on a printed circuit board and tested in four different configurations: exposed to

smoke, coated with a conformal coating and then exposed to smoke, packaged in a protective

box and then exposed to smoke, and unexposed to smoke. The seven chip packages are shown in

Figure 2.

Chip packages such as the ceramic dip, ceramic flat pack, ceramic leadless chip carrier, and TO

can are initially manufactured without an IC. Typically the IC is then bonded to the connections

inside the package and the package is sealed later. To test these chip packages, empty packages

(sealed but without an internal IC) will be mounted on a printed circuit board so that the

resistance between adjacent connectors can be measured during the smoke exposure. There will

be no connections inside the chip packages. Every other connector will either be connected to

ground or 5 V. The insulation resistance between the connectors will be monitored during the

smoke exposure. We expect that the insulation resistance will drop as the soot gets deposited,

indicating the possibility of circuit bridging or leakage currents.

Small Outline IC (SOIC)

Plastic

Dual-in-line (DIP)

Plastic

Dual-in-line (DIP)

Ceramic

Leaded Chip Carrier (PLCC)

Plastic

Transistor Outline Can (TO)

Metal

Leadless Chip Carrier

Ceramic

Figure 2. Chip packages used in chip mounting tests.

Flatpack

Ceramic

NUREG/CR-6476E- E-9
Plastic-bodied chips are manufactured by first connecting the IC to the connectors, then molding

the plastic body around the IC. It is difficult to obtain this type of chip package without an

interconnecting IC. Instead, SNL will use simple chips that contain several independent circuits

and will measure the resistance between adjacent connectors that are not on the same circuit and

thus should have no interconnection. One such type of chip is the 74/04 quad hex inverter

containing four independent circuits which should have no interconnection. We purchased both

a DIP- and SOIC-style hex inverter. These chips will also be mounted on the same printed

circuit board as the empty chip packages and will be subjected to the same tests.

Plastic leaded chip carrier (PLCC) bodies are only manufactured for complex integrated circuits.

The simplest PLCC circuit we were able to identify is an 8 bit A/D converter with serial interface

(this contains 1370 transistors!). This CMOS chip exhibits high resistance between contacts on

the input terminals when the chip is powered; therefore the chip will be powered while the

resistance between adjacent contacts is measured. This chip will be mounted on the same printed

circuit board as the empty chip packages and two hex inverter chips.

Chip packaging protects the IC from stresses due to vibration and chemical attack. Ceramic and

metal packages are hermetically sealed, that is, moisture cannot get into the package; however,

plastic chip bodies are permeable and exhibit less reliability in humidity tests." To study the

difference between plastic and ceramic packages, 16 K memory chips using both packages will

be exposed to smoke. The chips will be tested for functionality before and after the exposure.

During the smoke exposure, the chips will be powered with 5 V; however, they will not be

active. These tests should determine how smoke affects the integrity of the two kinds of

packaging.

Optical isolators are commonly used to isolate electrical signals from the rest of the circuit. The

electrical signal is converted to an optical signal (with perhaps a light-emitting-diode, LED) and

the optical signal is then converted back into an electrical signal with a photodiode. Electrical

signals generated on the photodiode side of the circuit are not allowed to pass through to the

other (LED) side of the circuit. Optical isolators are usually packaged the same way as

transistors, either in metal cans or in plastic sealed packages. Because of their rather unique

operation, unique failure modes are expected.

SNL will actively monitor the function of a 6N135 optical isolator. This isolator is sold in a

molded plastic DIP form. Previous tests showed that the smoke was the most dense within

minutes of the start of the test; therefore the performance of the isolator will be continuously

monitored during the smoke exposure test.

3.3. Functional Board Tests

The functional board tests will use a multifunctional board developed by the Low-Residue Solder

Task Force whose goal is to investigate ways to reduce the amount of ozone-depleting chemicals

used in the manufacture of printed circuit boards. 9 The task force is composed of experts in

printed circuit assemblies from the military and military contractors, Department of Energy, and

E-10 E-10 NUREG/CR-6476
the Environmental Protection Agency. This task force developed a multifunctional board to test

the effectiveness of various soldering processes. The board contains four different types of

functional circuits: high voltage and low current, low voltage and high current, high-speed

digital, and high frequency. Each of these circuits includes a mixture of through-hole and

surface-mount components. In many respects, the goal in testing a functional board that uses

different soldering and cleaning processes is similar to the goal of the SNL smoke tests. In both

cases, deposits on the board could affect their function.

SNL plans to assess the impact of smoke on these functional boards as the third type of smoke

exposure. By testing functional boards after the individual component tests, we will be able to

correlate the change in resistance to actual failures of functional circuits. The smoke exposure

conditions selected for these tests will be based on the results of the component tests.

4. Schedule

4.1. Total Test Matrix

The test conditions for a smoke scenario are affected by many factors, as discussed by Nowlen in

an earlier report."' These include the fuel composition, fuel-to-air ratio, temperature of bum,

humidity, presence of galvanic metals, and presence of fire suppression agents. To limit the

scope of the SIR board and individual component tests, we will allow only two conditions for

each characteristic; a high or low value, or the presence or absence of a chemical. In all cases the

fuel composition will be a mixture of the most common cable insulation and jacket materials that

are now in use at commercial nuclear power plants." This reduces the complexity of the tests

somewhat, but since we have identified five characteristics, each with two states, there are still

32 possible environments to test (25) . To increase the robustness of the test results, each test

should be performed twice, yielding 64 tests.

Some tests can be eliminated as not applicable to fires in a nuclear power plant. For example, a

high-temperature fire implies that the fire is quite large; in such a case, all of the equipment in

the cabinet where the fire originates would most likely be destroyed. Thus, the effects of a large

fire inside an equipment cabinet will not be tested. Using this reasoning, we eliminated tests for

a high fuel /air ratio that also includes high heat flux (note that a small fire with the smoke

confined to a single cabinet results in the highest fire/air ratio). In most cases the relative

humidity will be high following a fire. The only case in which it would remain low is if a small

fire were confined to a single panel in the control room. This eliminates all conditions where a

low fuel/air ratio can occur in conjunction with low humidity.

By considering only credible combinations of fire conditions, the total number of fire

environments can be reduced to 14 (see Table 2). For the sake of robustness, each test will be

performed twice (a total of 28). Each of these tests will include SIR boards, the chip mounting

tests, the chip packaging tests, and optical isolator tests.

NUREG/CR-6476 El E-11
Functional board testing will begin in 1996, but the test conditions are not yet determined. Each

of the functional board tests will be more complicated than the tests for the components, and

hence fewer tests will be conducted. The test matrix for the functional board tests will not be

defined until we have the results of the initial test series described here.

4.2. Schedule of Testing

Each smoke exposure test requires three days. The first day of each test sequence will include

setup and assembly of test parts. On the second day, the 24-hour smoke exposure test will begin,

and on the third day we will clean up the test chamber. The total testing period will require 84

testing days if there are no interruptions. We plan to start testing in August and finish by

December 22, 1995.

Testing of functional boards will begin in June 1996 and continue through September 1996.

This will allow time to evaluate the results of the individual components tests and procure the

functional boards.

5. Summary

This paper has described the test specimens and fire scenarios selected to study the impact of

smoke on digital equipment. Three types of equipment will be exposed to smoke: SIR boards,

individual components, and functional circuits. The tests of individual components are designed

to study the impact of smoke on circuit bridging, obscuration, and short-term corrosion. In these

tests, exposure conditions such as burn temperature, fuel-to-air ratio, presence of galvanic metals,

humidity level, and suppression effects will be varied. Standard surface-insulation-resistance

boards will indicate the amount of resistance lost as a function of smoke deposition. In 1996

functional circuits will be exposed to smoke for a limited number of exposure conditions as

determined by the individual components tests. These tests will enable correlation to be made

between insulation resistance loss and functional board failure. Overall these tests will provide a

comprehensive first evaluation of the effects of smoke environments on advance I&C being

planned for the next generation of power reactors.

Table 2. Smoke test matrix - SIR Board and Individual Components Tests

E-12 E-12 NUREG/CR-6476
6. References

1 Karydas, D. M., "Smoke Management Systems for the Telecommunications Industry," in

Network Reliability: A Report to the Nation, Compendium of Technical Papers Presented by

the Federal Communications Commission's Network Reliability Council, National

Engineering Consortium, Chicago, IL, June 1993, pp. G15-18.

2 Report to be published by SNL September 1995 on smoke exposures to multiplexer unit.

3 Report of system tests to be published by ORNL in 1995.

4 "Motorola Logic Families, Which is Best for You?", Gary Tharalson, Motorola Master

Selection Guide Rev .7, p. 3.1-3 - 3.1-6, 1994.

5 "Electronic Components and Assemblies: Reliability and Testing" by R. B. Comizzoli and J.

D. Sinclair, Encyclopedia of Applied Physics, Vol. 6, 1993 VCH Publishers, Inc. pp. 21-44.

6 "IPC Cleaning and Cleanliness Test Program: Phase 3 Water Soluble Fluxes Part I" Douglas

Pauls, September 30, 1992, Institute for Interconnecting and Packaging Electronic Circuits

(IPC). Presented at IPC 1992 Fall Meeting Oct. 12-15, 1992.

7 "Overview of Chip-Level Packaging," Barry C. Johnson, Electronic Materials Handbook,

ASM International Handbook Committee. pp. 398-407.

8 "Corrosion Criteria for Electronic Packaging: Part III-Corrosion by Package Family," Carl E.

Hoge, IEEE Trans. on Components, Hybrids, and Manufacturing Technology, Vol 13, no 4

p. 1105, 1990.

9 "Test Plan for the Evaluation of Low-Residue Soldering in Military Applications," Prepared

by the Low-Residue Soldering Task Force, R. Iman, Sandia National Laboratories, March 1,

1994.

10 "Defining Credible Smoke Exposure Scenarios," A letter report to the NRC, prepared by

Steve Nowlen, Sandia National Laboratories, Albuquerque, NM, September 1994.

11 "Low-Voltage Environmentally Qualified Cable License Renewal Industry Report; Revision

1," by L. Bustard and P. Holzman, TR-103841, Published by Electric Power Research

Institute, Palo Alto, CA, July 1994,

NUREG/CR-6476 E1E- 13

Appendix F

Component Test Results

This appendix is composed of tables and plots of information from the component tests

and includes the physical and chemical measurements of the smoke environment, scatter

plots of the resistance of the circuit bridging measurements, and coefficients that model

resistances from the circuit bridging measurements. These measurements may be useful

for comparing smoke exposure tests and for in-depth analysis of the results.

Table 1. Temperature and humidity during smoke exposures.

Test Ave Temp Min Temp Max Temp Ave Hum Min Hum Max Hum

1 25.1 22.9 30.8 70.9 61.4 76.8

2 24.3 21.4 28.7 73.2 64.8 77.2

3 24.4 21.3 27.3 71.2 61.5 75.6

4 26.2 -3.4 31.2 70.6 63.7 79.6

5 26.7 21.0 31.7 70.3 63.5 78.6

6 25.4 21.8 32.5 68.4 57.9 76.9

7 26.3 21.5 31.4 71.2 65.3 78.9

8 25.4 8.7 32.7 69.4 53.1 79.0

9 24.8 22.3 28.9 71.7 36.2 76.3

10 25.0 20.8 28.8 71.5 51.1 78.4

11 25.1 21.5 31.9 69.5 54.6 79.8

12 27.0 22.3 31.9 20.1 19.2 28.5

13 25.8 22.2 30.9 21.4 20.3 24.4

14 25.5 2.2 30.5 22.2 20.4 24.9

15 25.2 23.0 33.0 66.8 54.5 77.2

16 26.3 23.1 33.3 70.0 59.9 78.6

17 29.6 22.2 41.2 58.5 40.2 77.6

18 29.9 22.4 38.1 60.1 42.2 76.4

19 24.6 21.2 32.3 18.1 13.6 20.5

20 24.1 22.6 27.3 15.8 11.4 18.7

21 25.9 21.7 29.6 72.3 64.7 76.9

22 24.0 20.2 29.6 15.9 9.3 18.8

23 24.0 21.3 32.2 13.0 11.5 14.5

24 26.1 21.9 30.5 14.4 12.1 16.5

25 30.0 23.0 40.0 52.1 38.0 77.4

26 29.0 21.5 47.5 63.5 48.8 81.5

27 25.9 22.2 30.2 69.6 61.5 77.3

NUREG/CR-6476F- F- I
Table 1 presents the temperatures in the exposure chamber and the humidity that was

measured inside the environmental chamber surrounding the exposure chamber. The

temperatures in the exposure chamber were measured in 10 locations, 5 near the ceiling

of the chamber, and 5 near the floor. The average temperature presented in the table is

the average for the entire 24 hour test for all 10 locations. The locations and methods of

measurements are the same as those in Appendix 4. The very low temperatures that were

recorded for tests 4, 8, and 14 are the result of cooling by the addition of CO2. Humidity

was measured inside the environmental chamber, but outside of the smoke exposure

chamber throughout the 24-hour test. The smoke would have affected the accuracy and

lifetime of the humidity sensor, therefore, the humidity inside the exposure chamber

during the smoke exposure was not recorded. Immediately after the 1 hour smoke

exposure, the exposure chamber was vented and the air from the environmental chamber

was drawn into the exposure chamber, so the humidity of most of each test is included in

Table 1, but not the humidity during the actual smoke exposure.

Measurements of the fuel trays before and after the burn determined the amount of fuel

burned in grams. Although this does not indicate how much is deposited, it is a practical

measurement for reproduction of the effects. In the main body of this paper, the results of

the tests were evaluated in terms of high and low states of such factors as fuel load and

burn temperature. These factors determine how much smoke is deposited on the samples,

but are not direct measurements of the smoke deposition. Table 2 presents several

different measurements of how much smoke was deposited during the smoke exposure

tests; some of these measurements are direct while others are measurements of secondary

effects. Most of the smoke was deposited on horizontal surfaces, especially as the

amount of deposition increased. These measurements are included in this appendix so

that comparisons can be made with other experiments.

Quartz crystal microbalances (QCM) actively measured the smoke deposition in two

planes, vertical and horizontal, in pxg/cm'. A description of the QCMs is included in

Appendix 4. Mass densities greater than 200 pg/cm2 frequently caused the QCM to stop

oscillating; therefore, for most high fuel load tests, the QCMs did not provide a

measurement for the horizontal plane. Although the QCMs were not placed at the same

position in the chamber as the components, the QCM measurements are the most direct

measurement of deposition that was obtained in these tests.

The turbidimeter measured the smoke optical density in terms of the extinction

coefficient (turbidity). The turbidity in cm-' is presented for three wavelengths, IR (1.060

g), red (0.6328 g), and blue (0.4579 g). The larger extinction coefficients correspond to

more opaque smoke. The optical density is often measured on standard smoke tests of

materials and was included here for reference.

Many studies on corrosion and electronic equipment damage are based on the density of

chlorides, bromides and sulfates on surfaces. Threfore 4.25 cm circles of ashless filter

paper were placed in the smoke exposure chamber and were analyzed by ion

chromotography for chlorine, bromine, and sulfate. The results of these analyses are

presented in Table 2 in terms of milligrams of chemical per filter.

F-2 F-2 NUREG/CR-6476
(~1

Table 2. Smoke generation measurements.

Test No. Wt. Loss Hor. Vert. IR Red Blue Cl (norm) (CI) Bromide (Br) lim Sulfate (norm) (S) lim

____ QCM QCM lim

1 2.48 5.6 1.5 -- -- -- -- --.-- 

2 2.15 15 1.6 .. .. . 0.044 0.005 0.014 0.001 0.005 0.001

3 2.12 10.7 0.8 -- -- -- - -- --

4 30.8 >200 4.9 -- -- .-- -- -. --

5 31.47 >200 7.2 .. .. . 0.175 0.005 0.139 0.005 0.007 0.001

6 2.51 3.1 1.8 .. .. . 0.027 0.005 0.008 0.001 0.003 0.001

7 31.26 194 4 -- -- -- -- -- --

8 2.54 5.1 2.2 .. .. ........

9 2.1 10.7 0.7 .. .. ........

10 1.6 5.1 0.68 .. .. ........

11 2.15 3.5 1.1 ...-- -- -- -- --.

12 34.93 >200 2.4 .. .. . 0.098 0.005 0.216 0.005 0.005 0.001

hTJ
71P,'

Table 2. Smoke generation measurements (continued).

13 35.97 >200 2.13 -- 0.075 0.005 0.161 0.005 0.002 0.001

14 34.07 >200 11.5 -- -- -- -- -- --

15 2.53 7.6 1.2 -. -- .-- -- -- --

16 37.89 >200 4.1 0.17 0.21 0.87 0.113 0.005 0.256 0.005 0.003 0.001

17 69.58 >200 13.2 0.38 0.44 0.82 0.22 0.005 0.393 0.005 0.005 0.001

18 67.74 >200 -- 0.37 0.38 -- 0.193 0.005 0.409 0.005 0.003 0.001

19 2.56 1.54 0.97 -- -- -- 0.008 0.001 0.006 0.001 0.001 0.001

20 1.94 8.4 0.5 0.05 0.03 0.05 0.016 0.001 0.008 0.001 0.001 0.001

21 25.99 175 5.2 -- 0.116 0.005 0.195 0.005 0.002 0.001

22 2.3 11.5 0.64 .. .. . 0.012 0.001 0.011 0.001 0 0.001

23 2.48 1.7 1.7 .. .. . 0.011 0.001 0.01 0.001 0.001 0.001

24 34.66 >200 2.9 0.17 0.24 0.5 0.093 0.005 0.169 0.005 0.003 0.001

25 66.34 >200 8.2 0.41 0.55 0.45 0.207 0.005 0.371 0.005 0.003 0.001

26 67.43 >200 14.4 0.37 0.43 0.64 0.173 0.005 0.287 0.005 0.004 0.001

27 29.65 200 3.8 -- 0.16 0.48 0.103 0.005 0.239 0.005 0.002 0.001

I

4~
The amount of smoke that was produced was not only dependent on the amount of fuel,

but also upon the burn temperature. This point is illustrated in Table 3 where the

percentage of weight lost by the fuel (including copper conductors) can be compared with

the conditions under which the fuel was burned. The lowest percentages of weight lost

correspond to the conditions of high fuel and low burning temperature; the highest

percentages of weight lost correspond to a small amount of fuel and a high burn

temperature. Large amounts of fuel are slower to burn, especially if the burning

temperature is low. The burn temperature also influences how much of the available fuel

is released in the fire.

Table 3. Percentage of fuel burned.

Test No. Percent Fuel Burn

Wt. Loss

1 48.7 0 1

2 40.3 0 0

3 42.5 0 0

4 18.6 1 0

5 18.7 1 0

6 48.5 0 1

7 18.8 1 0

8 49.1 0 1

9 39.3 0 0

10 31.4 0 0

11 44.1 0 1

12 21.3 1 0

13 21.9 1 0

14 20.8 1 0

15 48.6 0 1

16 23.4 1 0

17 42.4 1 1

18 41.8 1 1

19 49.3 0 1

20 39.8 0 0

21 16.2 1 0

22 45.4 0 0

23 48.4 0 1

24 21.2 1 0

25 41.4 1 1

26 41.4 1 1

27 18.5 1 0

NUREG/CR-6476F- F-5
Figures 1 to 33 are scatter plots of the resistance for the 7 chips and 4 comb patterns

measured in these tests. These plots show how the chips and comb patterns as a whole

react to the smoke. Each chip or comb pattern has three plots of log,0 (R): pretest value

against during smoke, pretest value against after smoke, and during smoke against after

smoke. The pretest value was a single measurement; the values for during and after

smoke values were averages. Four symbols were used to depict the four board conditions

of bare (solid circle), coated (open square), chassis (open triangle), and control (*). A

line with a slope of 1 is included for reference. Points that fall on the line indicate no

change among the values that are plotted. These plots provide a general indication of

how the different components reacted to smoke during the various stages of the test.

LCC

S

C

* Bare

* Coated

A Chassis

x Control

-Slope I

0.00 2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00 16.00

Figure 1. Leadless Chip Carrier Log,0(R) Pretest vs. During Smoke

F-6 F-6 NUREGICR-6476
LCC

.1E

16.00

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00

0.00

0 Bare

0 Coated

A Chassis

x Control

-Series5

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 2. Leadless Chip Carrier Log1 ..(R) Pretest vs. After Smoke

LCC

I

0

a Bare

£ ,a Coated

£ &A AChassis

o Control a•-•Serles5

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 3. Leadless Chip Carrier Log,,,(R) During Smoke vs. After Smoke

NUREG/CR-6476F- F-7
CDP

E

U)

16.00-

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00 -'ý

0.00

* Bare

* Coated

* Chassis

x Control

- Slope 1

2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00 16.00

Figure 4. Ceramic DIP Logo(R) Pretest vs. During smoke

CDP

I

16.00-

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00 -

0.00

* Bare

o Coated

A Chassls

x Control

-Serles5

2.00 4.00 6.00 8.00 10.00 12.00 14.00

Pretest

Figure 5. Ceramic DIP Logo(R) Pretest vs. After smoke

16.00

F-8 F-8 NUREGICR-6476
CDP

16.00

IA flf

14 W

12.004

10.00+

I 8.00+

IFxx VXaA A

z

w K C

~mms p

* Bare

D Coated

A Chassis

z Control

-SerlesS

6.00+

4.00

2.00

U.UU4

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 6. Ceramic DIP Logo(R) During smoke vs. After smoke

CFP

16.00

I * Bare

* Coated

* Chassis

* Control

-81oe I

0.00r ICii

0.00 2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00 16.00

Figure 7. Ceramic Flat Pack Logo(R) Pretest vs. During smoke

NUREG/CR-6476 F-9
CFP

E

16.00-

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00 1

0.00

* Bare

* Coated

A Chassis

* Control

-Serdess

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 8. Ceramic Flat Pack Log1 o(R) Pretest vs. After smoke

CFP

I

C

16.00

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00

* Bare

o3 Coated

* Chassis

* Control

- Series5

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 9. Ceramic Flat Pack Logo(R) During smoke vs. After smoke

F-10 F-lO NUREG/CR-6476
TOC

iE

r.

0

D 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 10. Transistor Outline Can Logo(R) Pretest vs. During smoke

0 Bare

I Coated

* Chassis

* Control

-Slope 1

* Bare

I Coated

* Chassis

* Control

-SeresS5

TOC

I

I

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 11. Transistor Outline Can Log,,1(R) Pretest vs. After smoke

NUREG/CR-6476 F-11I
TOC

16.00.

I

E

(0)

14.00

12.00

10.00

8.00

6.00

4.00

2.00

A A

~~AA~ .:6 A 

* Bare

o Coated

A Chassis

z Control

-Series5

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 12. Transistor Outline Can Lg1 )(R) During smoke vs. After smoke

PLC

16.00.

14.00 -

12.00 +

10.001

C

8.00+

A

a

AA

.AA a. "O , , A .

* Bare

* Coated

A Chassis

x Control

- Slope 1

6.00 A

4.00

2.00

U0 ()F ..... .

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 13. Plastic Leaded Chip Carrier Log1 o(R) Pretest vs. During smoke

F-12 NUREG/CR-6476
PLC

I

e Bare

0 Ia Coated

0 •A Chassis

x Control

-Series5

A A A GO A 0a

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 14. Plastic Leaded Chip Carrier Log1 o(R) Pretest vs. After smoke

0.00

PLC

16.00

14.00

12.00

10.00

.3

* Bare

* Coated

A Chassis

x Control

-SerlesS

8.00

6.00

4.00

2.00

0.00 *-

0.00 2.00 4.00 6.00 8.00

During Smoke

10.00 12.00 14.00 16.00

Figure 15. Plasctic Leaded Chip Carrier Log,0(R) During smoke vs. After smoke

NUREG/CR-6476 F-13
SOIC

In

IM

E

(a

I

16.00-

14.00

12.00

10.00

8.00

6.00

4.00

2.00

0.00 ,

0 Bare

I Coated

A Chassis

* Control

-Slope I

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 16. Small Outline Integrated Chip Log10(R) Pretest vs. During smoke

$O1

i.00

.00..

!.00 1

1.00-

z0 z * Bare

a Coated

L.O0A 1.00 A Chassis

X Control

-Serdes5

.00

.00

2.0 •A AA B O*a•e•

I.00 III II

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 17. Small Outline Integrated Chip Log10(R) Pretest vs. After smoke

F-14 NUREG/CR-6476
SOIC

16.00

.3j

9 Bare

A Coated

8.00 A Chassis

z Control

-6Serles

6.00-

4.00.

2.00-

0.00I I I I

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 18. Small Outline Integrated Chip Log,0(R) During smoke vs. After smoke

PDP

16.00

14.00

12.00

10.00

E

C 8.00

a)

6.00

4.00

2.00

0 Bare

I Coated

A Chassis

z Control

-Slope

0.00 .*-

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 19. Plastic DIP Log11,(R) Pretest vs. During smoke

NUREGICR-6476 F1- 15
POP

16.00

14.00

12.00

10.00

I 8.00-

01

YA A,

ZUa

a " A AA

6.00

o Bare

o Coated

A Chassis

x Control

-SeilesS

16.00

4.00

2.00

0.00 2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00

Figure 20. Plastic DIP Log,o(R) Pretest vs. After smoke

PDP

I

* Bare

o Coated

& Chassis

x Control

-SerlesS

0.00 2.00 4.00 6.00 8.00

During Smoke

10.00 12.00 14.00 16.00

Figure 21. Plastic DIP Log,o(R) During smoke vs. After smoke

F=16 F-16 NUREGICR-6476
Comb Pattern, 160 V

S

0

t., c"

c

* Bare

o Coated

* Chassis

* Control

-Slope 1

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 22. Comb Pattern, 160 V, Log10(R) Pretest vs. During smoke

COMB PATTERN 160 V

I

E

16.00

14.00

12.00

10.00

8.00

6.00

4.00

2.00

* Bare o Coated

a Chassis

K Control

SerlesS

0.00 -K

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 23. Comb Pattern, 160 V, Loglo(R) Pretest vs. After smoke

NUREG/CR-6476 F-17
COMB PATTERN, 160 V

U) E

, •* Bare

a Coated

A Chassis x Control

S- Serdes5

D--

~AI&A AAO a 1 0

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 24. Comb Pattern, 160 V, Log1 o(R) During smoke vs. After smoke

Comb Pattern, 50 V

E

co

¢o

0

* Bare

a Coated

A Chassis

* Control

-Slope 1

0.00 I' I I I

0.00 2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00 16.00

Figure 25. Comb Pattern, 50 V, Logo(R) Pretest vs. During smoke

F-18 F-18 NUREGICR-6476
COMB PATTERN 50 V

16.00 .

14.00- S AlU Z •

12.00-

z

10.00. e Bare

0• a Coated

E 8.00- Chassis

* Iz Control

C -• Sertes5

6.00 -

4.00-

A A A AA A

2.00-

000I I

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 26. Comb Pattern, 50 V, Log,1 (R) Pretest vs. After smoke

COMB PATTERN, 50 V

16.00

14.00 %

0x

12.00 K

10.00- A * Bare

13 d Coated

) 8.00 a A Chassis

0 Control

Series5

.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smoke

Figure 27. Comb Pattern, 50 V, Log.o(R) During smoke vs. After smoke

NUREG/CR-6476 F- 19
Comb Pattem, 5 V

Ift An

14.00-

12.00-

10.00.

8.00. a 

6.00-

4.00-

2.00-

z

UO

za

A AN

* Bare

O Coated

A Chassis

* Control

-Slope 1

u11 nnP-i

0.00 2.00 4.00 6.00 8.00

Pretest

10.00 12.00 14.00 16.00

Figure 28. Comb Pattern, 5 V, Log1 o(R) Pretest vs. During smoke

COMB PATTERN 5 V

lIAlnn

I

14.00-

12.00

10.00

8.00

6.00.

4.00

2.00

01

CA,

AXI

AU

f A

* Bare

* Coated

A Chassis

* Control

-SeriesS

n no w~

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 29. Comb Pattern, 5 V, Log,0(R) Pretest vs. After smoke

F-20 F-20 NUREGICR-6476
COMB PATTERN, 5 V

IB

0.

e Bare

AIKo Coated

A Chassis

x x Control

A -Seriess

A A A

.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

During Smokce

Figure 30. Comb Pattern, 5 V, Log,0(R) During smoke vs. After smoke

Comb Pattern, ground

* Bare

A Chassis

z Control

-Slope I

E

Co

I,

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Figure 31. Comb Pattern, Grounded, Log,1 (R) Pretest vs. During smoke

NUREG/CR-6476 F-21
COMB PATrERN, GROUNDED

14.00 -

12.00 •

10.00 -

0

B

0A 8.00+

a I XIK A

Ile

* Bare o Coated

A Chassis

X Control

-Series5

6.00-

4.004

2.00+

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Pretest

Figure 32. Comb Pattern, Grounded, Logio(R) Pretest vs. After smoke

COMB PATTERN, GROUNDED

16.00

I

* Bare

* Coated

* Chassis

x Control

-Serdes5

0.00 2.00 4.00 6.00 8.00

During Smoke

10.00 12.00 14.00 16.00

Figure 33. Comb Pattern, Grounded, Log1 o(R) During smoke vs. After smoke

F-22 NUREG/CR-6476
Table 4 presents the coefficients for the equations from the general linear regression

analyses. The resistance responses that were modeled were those that were plotted in

Figures 1 to 33: log.0(R) at pretest, during the smoke production, and after the smoke

exposure. The coefficients in the table are for the factors determined to be significant.

Abbreviations have been used in this table to depict fuel level (F), bum temperature (B),

PVC (P), humidity (H), galvanic metal (G), and suppression (S). Combinations of terms

are indicated by F x B x G, for example. The column labeled R2 quantifies the percent of

the observed variation in the response variable that is explained by the model.

NUREG/CR-6476 F-23
Table 4. General Linear Modeling Results

71

Cornonent Parameter W Constant Bare Coated Chassis Bum PVC Humid Spsion rsFuelBxBare FxBare

Ceramic Before 0.77 13.16 -4.04 -1.62 4.28

Leadless During 0.82 10.67 1.66 -2.39 -3.69 4.28 -1.38

Chip Carrier After 0.83 12.57 -6.36 6.50 -5.13

Ceramic Before 0.60 12.76

DIP During 0.75 10.99 1.68 1.75

After 0.75 12.29

Ceramic Before 0.49 13.00 -2.19

flat pack During 0.90 14.59 -2.13 -1.95 -3.70 -2.26 -0.81 -3.17

After 0.86 13.13 -2.57 -4.55

Transistor Before 0.48 12.27 1.09 -4.61

Outline During 0.72 12.14 -2.88

Can After 0.88 11.83 1.53 1.88 -3.26

Plastic Before 0.26 12.20 -1.87 -1.22

leaded During 0.85 12.43 -4.10

Chip Carrier After 0.85 12.77 -4.07

Small Outline Before 0.43 12.56 -2.07

Integrated uring 0.85 12.43 -2.13

Chip After 0.90 13.01 -1.60

Plastic Before 0.28 12.84 -1.72

DIP During 0.59 12.85 -2.44

After 0.69 13.07 -1.50

Comb Before 0.37 13.15 -2.75 -2.24 -0.82

160 V During 0.88 13.34 -4.53 -3.08 -1.73

After 0.81 13.40 -3.84 -3.79 -3.08

Comb Before 0.67 12.61 2.03 -2.75 -2.10 1 -0.83

50 V During 0.90 13.77 -4.62 -4.12 -1.72 -7.61 -3.21

After 0.91 13.38 -4.15 -3.40 -7.50 1.30

Comb Before 0.85 8.90 4.44 3.38 -0.48

5 V During 0.91 8.40 5.00 3.96 -0.71 -5.55 -4.04

After 0.85 8.21 4.47 3.65 -6.27

Comb Before 0.93 11.76 -3.23 1.62 1.59

Grounded During 0.91 11.60 -3.23 1.52 1.581

_After 0.88 11.92 -3.61 1.23 1.391
Table 4. General Linear Modeling Results

71

LA

Corponent Parameter HxBare SxBare GxiieBxCoated FxCoated PVCxCoated HxCoated GxCoated BxChassis FxChassis

Ceramic Before -1.97

Leadless During -3.38 5.12 3.46 1.92

Chip Carder After -4.18 6.59:

Ceramic Before -2.72 -5.67 2.79

DIP During -4.41 -3.12

After -2.87 -7.06 3.08

Ceramic Before -2.26

flat pack During

_After -1.72

Transistor Before 3.82 3.05

Outline During 1.68 -1.66

Can After 2.01 3.14.

Plastic Before

leaded During -1.95

Chip Carrier After -2.68

Small Outline Before 2.17 1.51

Integrated During 1.11

Chip After .....

Plastic Before -1.65

DIP During -3.38

After -2.22

Comb Before -1.27

160 V During

After -1.57

Comb Before

50 V During -1.81 1.39

After

Comb Before

5 V During -3.03 -1.78 -2.97 -1.14 After -4.13

Comb Before

Grounded During

After -0.88 1
Table 4. General Linear Modeling Results

0~~

-4

Cornonent Parameter HxChassis GxChassis FxB BxH BxS FxH FxS SxG FxBxBare PxBxBare BxHxBare

Ceramic Before 2.06 -4.81

Leadless During

Chip Carrier After

Ceramic Before

DIP During -3.47

After -3.40 3.40

Ceramic Before -2.35 -1.52

fiat pack During -1.63 -2.01

After -1.76

Transistor Before

Outline During -1.24

Can After

Plastic Before

leaded During -1.13 -1.16 1.52

Chip Carrier After -0.88 -1.35

Small Outline Before 1.71 -2.19

Integrated During 2.56

Chip After 1.99 -1.09

Plastic Before -2.33

DIP During

After -2.51

Comb Before

160 V During -0.85

After

Comb Before

50 V During 2.16 3.77

After -0.69 2.29

Comb Before -0.93

5 V During 3.38

After -4.06 3.77

Comb Before -4.42

Grounded During

I_ lAfter
Table 4. General Linear Modeling Results

Component Parameter BxGxBare FxHxBare FxSxBare FxGxBare HxSxBare SxGxBare FxBxCoated PxBxCoated BxHxCoated

Ceramic Before -2.32 6.59

Leadless During

Chip Carrier After

Ceramic Before -2.84

DIP During -5.18

After -6.42

Ceramic Before

flat pack During

After -3.27 2.54

Transistor Before

Outline During -3.70 -2.78

Can After -9.11

Plastic Before

leaded During -2.73

Chip Carrier After -3.69 -3.17 2.95

Small Outline Before

Integrated During -6.71 -3.49

Chip After -6.89 -4.22!

Plastic Before

DIP During -5.58

After -6.06

Comb Before 1.85

160 V During -1.08

After

Comb Before

50 V During -1.99

After

Comb Before

5 V During 1.73 -3.13

After -3.55

Comb Before

Grounded During -1.76

After
Table 4. General Linear Modeling Results

00

Component Parameter BxSxCoated BxGxCoated FxHxCoated FxSxCoated FxGxCoated PxHxCoated SxHxCoated GxHxCoated

Ceramic Before

Leadless During

Chip Carrier After

Ceramic Before -9.76

DIP During -7.09 6.75 4.94 -3.44

After -8.34

Ceramic Before

flat pack During -2.29

After

Transistor Before

Outline During

Can After -7.77

Plastic Before

leaded During

Chip Carrier After

Small Outline Before

Integrated During

Chip After -4.43

Plastic Before

DIP During

After

Comb Before

160 V During -2.40

After -4.11 3.01 2.62

Comb Before

50 V During

After

Comb Before -4.89 1.28

5 V During -4.75 1.55

After _ -1.12

Comb Before -1.05

Grounded During

After -3.13
Table 4. General Linear Modeling Results

Component Parameter SxGxCoated FxBxChassis PxBxChassis BxHxChassis BxSxChassis BxGxChassis FxHxChassis FxGxChassis

Ceramic Before -5.53

Leadless During

Chip Carrier After -2.22

Ceramic Before 6.07

DIP During

After

Ceramic Before

flat pack During -2.93 After 4.12 -5.70

Transistor Before

Outline During -1.56 -3.36

Can After -1.41 -6.47

Plastic Before -3.21

leaded During -1.51 -3.25 -6.39

Chip Carrier After -8.07

Small Outline Before

Integrated During 4.84 -6.01 5.04 -7.83

Chip After 5.00 -5.27 4.99 -9.04

Plastic Before -5.03

DIP Dudng -2.96 After

Comb Before

160 V During -5.07 -8.59 After -9.77

Comb Before 2.20

50 V During -5.99 After -6.29

Comb Before 4.73

5 V During 5.40 After 1.79

Comb Before 4.47 4.15

Grounded During 2.04 After 3.63
Table 4. General Linear Modeling Results

71

Component Parameter PxHxChassis SxHxChassis SxGxChassis FxSxH [x~R

Ceramic Before -1.88 7.65

Leadless During

Chip Carier After

Ceramic Before

DIP During After __ _ _ _ _ ________ _ _ _ __

Ceramic Before

flat pack During 3.56

After

Transistor Before

Outline During

Can After

Plastic Before

leaded During

Chip Carder After

Small Outline Before 2.55 -1.93

Integrated During

Chip After

Plastic Before 9.63

DIP During

After -2.99

Comb Before

160 V During -1.51 8.56

After

Comb Before

50 V During

After

Comb Before 2.00

5 V During

After

Comb Before 0.63

Grounded During

After ON
NJRC FORM 35 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER

jz' 69) (Assigned by NRC, Add VaL, Supp. Rev.

1*kCM 1102, BIBLIOGRAPHIC DATA SHEET and Addendum Numbera Nny.)

j3201,3202

(See lgSbUCUOM 0- 7 fi ,,ve,,) NUREG/CR-6476

2. TITLE AND SUBTITLE SAND96-2633

Circuit Bridging of Components by Smoke

3. DATE REPORT PUBLISHED

MONTH j YEAR

October 1996

4. FIN OR GRANT NUMBER

W6051

5. AUTHOR(S) 6. TYPE OF REPORT

T. J. Tanaka, S. P. Nowlen, D. J. Anderson Technical

7. PERIOD COVERED Ondushe Dales)

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (f NRC, provide Division, Office orRagion, U.S. Nucear Regulatory Commision, and mailing addrasg Ioonacta,

provide name and mailing addressJ

Sandia National Laboratories

Albuquerque, NM 87185-0747

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (It"NRC, type Same as above', ff cornfracior, provide AlRC Division Off'te or Region, U.S. Nucleow Regulatoy Cornmiaion

and mailing address.)

Division of Systems Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

10. SUPPLEMENTARY NOTES

C. Antonescu, NRC Project Manager

11. ABSTRACT (200 words or less)

Smoke can adversely affect digital electronics; in the short term, it can lead to circuit

bridging and in the long term to corrosion of metal parts. This report is a summary of the

work to date and component-level tests by Sandia National Laboratories for the Nuclear

Regulatory Commission to determine the impact of smoke on digital instrumentation and

control equipment. The component tests focused on short-term effects such as circuit

bridging in typical components and the factors that can influence how much the smoke

will affect them. These factors include the component technology and packaging,

physical board protection, and environmental conditions such as the amount of smoke,

temperature of burn, and humidity level. The likelihood of circuit bridging was tested by

measuring leakage currents and converting those currents to resistance in ohms.

Hermetically sealed ceramic packages were more resistant to smoke than plastic

packages. Coating the boards with an acrylic spray provided some protection against

circuit bridging. The smoke generation factors that affect the resistance the most are

humidity, fuel level, and burn temperature. The use of CO, as a fire suppressant, the

presence of galvanic metal, and the presence of PVC did not significantly affect the

outcome of these results.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist res•archers in locating the report.) 13. AVAILABILJITY STATEMENT

Smoke unlimited

Electronic Equipment Damage 14. SECURITY CLASSIFICATION

Cable Fires (Tis palge)

Digital Equipment unclassified

(This Report)

unclassified

15. NUMBER OF PAGES

16. PRICE

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