NUREG/CR-5079

SAND87-3139

Experimental Results

Pertaining to the Performance

of Thermal Igniters

Manuscript Completed: September 1989

Date Published: October 1989

Prepared by

Michael K. Carmel

Sandia National Laboratories

Albuquerque, NM 87185

Prepared for

Division of Engineering and Systems Technology

Office of Nuclear Reactor Regulation

U.S. Nuclear Regulatory Commission

Washington, DC 20555

NRC FIN A1301

ABSTRACT

This report summarizes the results of various experimental programs

regarding the performance of thermal igniters for the deliberate ignition of

hydrogen in light water reactors. Experiments involving both premixed

combustion and combustion with continuous hydrogen injection are reviewed.

Combustion characteristics examined include flammability limits of hydrogen:air

and hydrogen:air:steam mixtures, combustion pressure rises, combustion

completeness, flame speeds, and heat transfer aspects. Comparisons of igniter

type and igniter reliability under simulated reactor accident conditions are

included. The results of the research programs provide a broad data base

covering nearly all aspects of hydrogen combustion related to the performance

of deliberate ignition systems.

iii

CONTENTS

Page

1. INTRODUCTION ............ ............................... I

1.1 Deliberate Ignition Systems. .. ........ . . ... ........... .... 1

1.2 Igniters ................ ............................... 1

1.3 Experimental Research Programs ........... .................... 2

2. SNL FITS TESTS ................ ............................ 7

2.1 Facility Description ............ .......................... 7

2.2 Flammability Limits ........ ............... . ... .......... 7

2.3 Combustion Pressure Rises ............ ...................... 11

2.4 Heat Transfer Characteristics . . . ........... .............. .. 11

3. SNL VGES TESTS ............ .................. .. ......... 17

3.1 Facility Description ............ ......................... 17

3.2 Combustion Pressure Rises ............ ...................... 17

3.3 Effect of Igniter Type ............ ........................ 21

3.4 Flame Speeds ............ .......................... .... 21

3.5 Effect of Diluent Gases .......... ....................... .. 21

3.6 Effect of Aqueous Foams . .. ........ . ................... 25

3.7 Effect of Water Sprays ....... . ........ .................... 25

4. NTS TESTS ................ ............................... 27

4.1 Facility Description ............ ......................... 27

4.2 Combustion Completeness .......... ....................... .. 27

4.3 Combustion Pressure Rise ....... ....................... .. 27

4.4 Effect of Igniter Location ....... ...................... .. 32

4.5 Burn Characteristics in Continuous Injection Tests .............. 32

4.6 Burn Duration During Premixed Tests ........ ................. 35

4.7 Heat Transfer Characteristics ........ .................... .. 35

5. LLNL HYDROGEN IGNITER EXPERIMENTAL PROGRAM ....... .............. 39

5.1 Facility Description ............ ......................... 39

5.2 Flammability Limits and. Combustion Completeness ..... ........... 39

5.3 Combustion Pressure Rises .......... ........... . ......... 43

5.4 Flame Speeds ......................................... .... .43

5.5 Glow plug Reliability .......... ........................ .. 43

6. WHITESHELL TESTS ............... ........................... 47

6.1 Facility Description ............ ......................... 47

6.2 Ignition Limits ........ . ........... .................... 47

6.3 Combustion Completeness .......... ....................... .. 52

6.4 Combustion Pressure Rises ............ ...................... 52

6.5 Condensation Experiments ........... ....................... 52

7. ACUREX TESTS ............ ............................. ... 59

v
7.1 Facility Description.... ... . .. .. .. 59

-7.2 Burn Characteristics in Premixed Tests. ................ 59

7.3 Effect of Igniter Location ............. ..................... ... 61

7.4 Effect of Steam, Spray, and Fog . . ......... . ............ ...... 61

8. FENWAL TESTS ................ ............ ................. ... 65

8.1 Facility Description ............ ......................... ... 65

8.2 Combustion Pressure Rises ........... ............. .......... .65

8.3 Burn Characteristics in Premixed Tests ...... ................ ... 65

8.4 Burn Characteristics in Continuous Injection Tests . ............ 69

9. 1/4-SCALE MARK III CONTAINMENT HYDROGEN COMBUSTION PROGRAM ...... .... 71

9.1 Facility Description ............... ......................... 71

9.2 Approach of the 1/4-Scale Tests ................................... 71

9.3 Results of the 1/4-Scale Tests ........ .................... ... 73

-10. SUMMARY ............................................ ........ 75

10.1 Flammability Limits ............ ......................... ... 75

10.2 Combustion Pressure Rises .......... .................. .... .. 75

10.3 Combustion Completeness .................................... ... 76

,10.4 Effect of Igniter Type and Location .......... ................. 76

AO,.5 Flame Speeds ................ ........................... ... 77

210.6 Heat Transfer Characteristics ........ .................... ... 77

710.7 Combustion Characteristics of 1/4-Scale Tests ...... ............ 77

10.8 Scaling Considerations ............ ............. ........... 78

11. REFERENCES ....................... ...... ............... ... 79

,r .

S.

vi
LIST OF FIGURES

Figure Page

1.1 Schematic of General Motors AC 7G glow plug and assembly ........... 3

1.2 Schematic of.Tayco Model 193-3442-4 thermal igniter and assembly . , 4

2.1 Schematic of the FITS vessel .... ..................... 8

2.2 FITS hydrogen:air:steam flammability data with fans off ........ 9

2.3 FITS hydrogen:air:steam flammability data with fans operational . , 10

2.4 Normalized peak combustion pressure for the cold- and hot-wall

hydrogen:air burns in the FITS vessel ....... ....... .......... 12

2.5 Normalized peak combustion pressure for the hydrogen:air:steam burns

in the FITS vessel as a function of the hydrogen concentration in

hydrogen:air only ............................... 13

2.6 Ratio of the global radiativet tootal energy deposition for the

hydrogen:air burns in the FITS vessel ..... ................ ... 14

2,7 Ratio of the global radiative to total energy deposition for the

hydrogen:air:steam burns in the FITS vessel as a function of the

hydrogen concentration in hydrogen:air only .......... . ....... 15

3.1 Schematic of the VGES chamber ........ ... .................... 18

3.2 Schematic of the VGES chamber for water spray tests ... ......... .. 19

3.3 Normalized peak pressure for VGES hydrogen:air burns .... ......... 20

3.4 Upward flame speed values in VGES chamber ..... .............. .. 22

3.5 Downward flame speed values in VGES chamber ...... ............. 23

3.6 Mean pressure derivative (AP/At) for VGES tests ..... ........... 24

4.1 Schematic of Nevada Test Site facility ....... ................ 28

4.2 Combustion completeness for NTS premixed combustion tests . .. . .... 29

4.3 Normalized peak pressure for NTS premixed combustion tests ......... 30

4.4 Gas pressure profiles for three NTS premixed tests comparing the

effect of sprays and fans .... ...................... 31

4.5 Gas pressure profiles for three NTS continuous injection tests

comparing the effect of sprays and fans .......... ............. 33

4.6 Gas pressure profiles for three NTS continuous injection tests

comparing the effect of nozzle size ...... ................. .. 34

4.7 Burn duration for NTS premixed combustion-tests ... .......... ... 36

4.8 Peak radiative and total heat flux for "standard" NTS premixed

combustion tests ... . .......... ................. ...... 37

4.9 Peak radiative and total heat flux for steam-laden NTS premixed

combustion tests .......................................... .* 38

5.1 Schematic of LLNL facility ............................ 40

5.2 Combustion completeness for LLNL dry tests .............. 41

5.3 Combustion completeness for LLNL steam tests .... ............. ... 42

5.4 Combustion pressure rise for LLNL dry tests ...... ............. 44

5.5 Combustion pressure rise for LLNL steam tests ...... ............ 45

6.1 Schematic of Whiteshell test vessel ............. ..... 48

6.2 Ignition limits for Whiteshell tests with the GM AC 7G glow plug

igniter (fans off) ........... .......................... ... 49

6.3 Ignition limits for Whiteshell tests with the GM AC 7G glow plug

igniter (fans on) ..... ..... .......................... ... 50

6.4 Ignition limits for Whiteshell tests with the Tayco helical igniter. . 51

6.5 Combustion completeness for Whiteshell dry tests ..... ........... 53

6.6 Combustion completeness for Whiteshell steam tests ............. ... 54

6.7 Normalized peak combustion pressure for Whiteshell dry tests ........ 55

6.8 Normalized peak combustion pressure for Whiteshell steam tests . ... 56

vii
6.9 Ignition limits for Whiteshell tests in steam condensing atmosphere 57

7.1 Schematic of Acurex test facility .................... ... 60

7.2 Effect of igniter location in Acurex continuous injection tests . 62

7.3 Effects of steam injection and water sprays in Acurex continuous

injection tests ........ ....... ........................... 63

7.4 Effects of steam injection and water fog in Acurex continuous

injection tests ........ ....... ........................... 64

8.1 Schematic of the Fenwal test facility ........ ................ 66

8.2 Normalized peak combustion pressure for the hydrogen:air:steam burns

in the Fenwal vessel as a.function of the hydrogen concentration

in hydrogen:air only .... ........... ... ................... 67

8.3 Combustion characteristics of hydroqen:air (dry) burns .... ........ 68

9.1 Schematic of the 1/4-scale test facility .... ............... .. 72

viii
LIST OF TABLES

Table Page

1.1 Scope of Thermal Igniter Experimental Programs ...... ............ 5

3.1 Igniter Threshold Values for VGES Tests ..... ... ............... 26

7.1 Results of Acurex Premixed Tests ......... .................... 59

ix

ACKNOWLEDGEMENTS

This work was supported by the U.S. Nuclear Regulatory Commission under

the direction of A. Notafrancesco. I would like to express my appreciation to

S.E. Dingman, A. Notafrancesco, R.E. Hogan, L.S. Nelson, J.E. Kelly, T.D.

Brown, and F.E. Haskin, who reviewed the original draft of this document and

who provided constructive changes and suggestions, which have been incorporated

in this work.

xi

1. INTRODUCTION

The combustion of hydrogen during postulated core uncovery accidents in light

water reactors can be a threat to the integrity of the containment system and

safety equipment of these plants. One of the methods used to mitigate the

effects of hydrogen combustion is deliberate ignition of the hydrogen by

igniters in containment. The systems are designed to burn hydrogen at low

concentration rather than allowing it to accumulate to more threatening levels.

Many experimental programs have been conducted to address the performance of

igniters under varying conditions. This report serves to summarize the

conclusions of those programs regarding the effectiveness of deliberate

ignition systems.

1.1 Deliberate Ignition Systems

Deliberate ignition systems are currently utilized in BWR Mark III and PWR ice

condenser plants. The medium-volume (i.e. 3-4x10 4 Mn3) containments that these

plants employ have design pressures of about 100 kPa, or 15 psig (although

detailed analyses predict much higher ultimate failure pressures). The

integrity of these containments could be threatened by the pressure transient

resulting from the deflagration (subsonic flame burn) or detonation (supersonic

combustion wave) of rich mixtures of hydrogen. They could, however, survive

combustion of lean mixtures, which produce lower pressure rises'. The

deliberate ignition systems are designed to promote burning of lean rather than

rich hydrogen mixtures.

The effectiveness of igniters as ignition sources in these systems can be

evaluated by the characteristics of the combustion processes they initiate.

Measurements of the peak pressure of combustion, resultant flame speed, and

completeness of combustion are useful in determining igniter performance. Also

relevant to effectiveness under severe accident conditions are qualities such

as performance in the presence of water vapor or microfog and operability

during station blackout scenarios [1].

1.2 Iginiters

Devices most widely used as ignition sources in current deliberate ignition

systems are thermal (hot surface) igniters. Thermal igniters provide a

transfer of thermal energy to provide ignition, and require electrical input

for operation. Other ignition sources that have been considered include spark

plugs, pilot flames, plasma jet igniters [2,3], and catalytic igniters [4].

This report will only address the performance characteristics of thermal

igniters.

Figures 1.1 and 1.2 show the two types of thermal igniters currently installed

in deliberate ignition systems [5]. These figures also show the igniter

assemblies, which may vary among plants. Figure 1.1 shows the General Motors

AC Model 7G glow plug, a cylindrical igniter with a design operating voltage of

12 Vac and a power requirement of about 95 watts. Figure 1.2 shows the Tayco

Model 193-3442-4 helical igniter, with a design operating voltage of 120 Vac

and a power requirement of about 525 watts. The assemblies include sheet metal

shields to prevent direct impingement of containment spray droplets. These

assemblies are distributed throughout the containment in strategic locations.

The igniters are to be energized immediately following the start of an accident

1
and to remain on as long as emergency operating procedures prescribe (usually

until safe shutdown of the unit).

1.3 Experimental Research Programs

Numerous research programs conducted since 1980 have provided data on hydrogen

combustion and the performance of igniters to determine the effectiveness of

deliberate ignition systems. These include studies of the combustion

characteristics of hydrogen burns initiated by igniters, flammability limits of

hydrogen:air and hydrogen:air:steam mixtures, avid hydrogen mixing processes.

Several programs have investigated the effect of water sprays on igniter

performance. Others have studied the effect of steam condensation on igniter

effectiveness. Most research programs reported heat transfer characteristics

of hydrogen combustion and resultant flame speed data.

This report summarizes findings relevant to the performance of thermal igniters

from eight major hydrogen combustion research facilities. These are:

(1) The Fully Instrumented Test Site (FITS)

(2) The Variable Geometry Experimental System (VGES)

(3) The Nevada Test Site (NTS)

(4) The Hydrogen Igniter Experimental Program at Lawrence

Livermore National Laboratory (LLNL)

(5) The Whiteshell Nuclear Research Establishment (WNRE)

(6) Tests conducted by the Acurex Corporation

(7) Tests conducted by Fenwal Incorporated

(8) The 1/4-Scale Mark III Containment Hydrogen Combustion Program

Chapters 2 through 9 of this report review each test program individually, and

Chapter 10 serves to summarize the important conclusions regarding deliberate

ignition obtained from these test programs.

Table 1.1 outlines the general scope of the research programs reviewed here.

All but two of the test programs involved small-scale testing, performed in

experimental vessels with less than 20 m3 free volume. The two large-scale

programs were the NTS tests performed in a 2000 m3 spherical vessel, and the

1/4-Scale tests performed in a 1/4-scale model of a Mark III containment. All

but one of the programs (the 1/4-Scale tests) examined pre-mixed combustion of

hydrogen:air or hydrogen:air:steam mixtures, and four of the programs

investigated combustion with the continuous injection of hydrogen and steam

into the test volume. In general, all the experimental research programs

reviewed here provide fairly comprehensive studies of hydrogen combustion

phenomena and yield results that are relevant to the design of deliberate

ignition systems.

2
,a

3.08

CABLE

Figure 1.1 Schematic of General Motors AC 7G glow plug and assembly

from Reference 5).

(taken

3
-HEATING

ELEMENT

Figure 1.2 Schematic of Tayco Model 193-3442-4 thermal igniter and assembly (taken from Reference 5).

4
TABLE 1.1 - Scope of Thermal Igniter Exerimental Programs

FITS VGES NTS LLNL WNRE Acurex Fenwal 1/4-Scale

L"

Small-scale

Large-scale

Pre-mixed 0 0 0 0 0 0 0

Dynamic .(Continuous

Injection)

Igniter Type

Igniter Location 0

Steam 0 0 0

Diluent Gases 0

Flammability Limits 0

Varied Initial

Temperature "

Sprays _________

Condensation 0 0

Fans 0 0 0 0 • 0 0

Aqueous Foam 0
6
2. SNL FITS TESTS

A series of 239 hydrogen:air:steam combustion experiments sponsored by the NRC

in 1983 was performed in the 5.6 m3 Fully Instrumented Test Site (FITS) vessel

located at Sandia National Laboratories in Albuquerque, New Mexico [6]. These

experiments addressed combustion characteristics and flammability limits of

combustible atmospheres that might occur inside containment during a severe

accident. The data obtained in these premixed combustion tests is useful in

benchmarking computer code simulations of hydrogen:air:steam combustion and

containment response modeling.

2.1 Facility Description

Figure 2.1 shows a schematic of the FITS facility. As shown, the facility is a

cylindrical vessel 3.4 m (11.2 ft) tall and 1.5 m (4.9 ft) in diameter, with a

design working pressure of 2.38 MPa (345 psia) and 2.86 cm (1.13 in) thick

steel walls. The upper head of the vessel is removable and there are twentyfive port penetrations used for instrumentation and gas feedthroughs. Burns

were initiated with both spark plug and glow plug igniters located in the lower

portion of the vessel. Two pneumatic fans were used to ensure proper mixing

before every experiment and during the turbulent mixing tests.

2.2 Flammability Limits

The hydrogen:air:steam flammability results are shown in Figures 2.2 and 2.3

with air partial pressures of -83 kPa (12 psia) and an initial temperature of

O-110C. Figure 2.2 shows results for the quiescent (fans off) tests, while

Figure 2.3 shows results for the turbulent (fans on) environment. In these

figures, an observation is classified as "no burn" if no appreciable pressure

rise was detected (generally << 6 kPa). An observation is classified as a

"marginal burn" if the measured overpressure was less than 10% of the Adiabatic

Isochoric Complete Combustion (AICC) calculated overpressure for the same

initial conditions, while those measuring more than 10% of the AICC

overpressure are classified "burns". These figures indicate that any

combustible hydrogen:air mixture is inerted in the FITS vessel with -52% steam.

A correlation of this data was developed to describe the three-component

flammability limit over the entire range of flammability [6]:

%Steam = 100 - %H2 - 37.3exp(-0.007%H2) - 518.Oexp(-0.488%H2) (2.1)

where %Steam is the volume percent of steam that will inert a

hydrogen:air:steam mixture containing %H2 volume percent hydrogen and

(100-%Steam-%H2 ) volume percent air.

7
4 C-LEVEL PORTS

4 E-LEVEL PORTS

6 F-LEVEL PORTS

2 G-LEVEL PORTS

Figure 2.1 Schematic of the FITS vessel (taken from Reference 6).

8
z

0

0

0e

80

70-

60-

50-

40-

30-

20-

10-

0

INITIAL CONDITIONS,

FANS OFF

NOT TANK (0383 K) PAIR - 83 kPa

0

0

U

NO BURN

0

BURN

o eo.Oe

0

o e~cgc

0 4WD

0 0 5 0 

40(v a 0@e~ 0 e0

0

I

10 2

20 330 40 50 60

% STEAM

' BURN

MARGINAL BURN

NO BURN

Figure 2.2 FITS hydrogen:air:steam flammability data with fans off (taken

from Reference 6).

9
z

0

0

ae

0-

80-

70-1

60-

50 -

40-4

30-

20-

10-

0

INITIAL CONDITIONS,

FANS OPERATIONAL

HOT TANK 0383 K) SPAIR" 83 kPo

00

0

0 0 ft

NO BURN

000 0

00060

BURN

00 00

0 0 0 0 0

0 0 00 0

06)O CPO4 0 0

08 0oO 0%.0

4

Ji

0

I

10 2

20 I

30 0

40 550 60

% STEAM

0 BURN

MARGINAL BURN

. NO BURN

Figure 2.3 FITS hydrogen:air:steam flammability data with fans operational

(taken from Reference 6).

10
2.3 Combustion Pressure Rises

The combustion peak pressure was measured for each of the hydrogen:air and

hydrogen:air:steam burns. Figure 2.4 shows the normalized peak pressure (ratio

of peak-to-initial pressure) as a function of the initial hydrogen

concentration for both the quiescent and turbulent hydrogen:air burns in coldwall and hot-wall tests. Figure 2.5 shows the same data for the

hydrogen:air:steam burns as a function of hydrogen concentration in

hydrogen:air only. Cold-wall burns are those hydrogen:air tests performed at

ambient preignition temperature (-300 K), while hot-wall burns refer to those

performed at elevated temperatures (-385 K).

The normalized peak combustion pressure was observed to increase with

increasing hydrogen concentrations of up to -30%, at which point it decreased

with further increases in hydrogen concentration. The presence of steam tended

to decrease the normalized peak pressure when compared to equivalent

hydrogen:air burns (i.e., burns with equivalent hydrogen-to-air ratio). A

comparison of the experimentally measured peak combustion pressure to the AICC

calculated peak pressure indicated that relatively complete combustion (>95%)

occurred for hydrogen concentrations greater than -10%, while less complete

combustion was observed for hydrogen concentrations approaching the lean

flammability limit.

Figures 2.4 and 2.5 also demonstrate that turbulence increased the extent of

combustion of the lean burns (where buoyancy governs flame propagation), while

having less effect on the rich burns. The results show that burns initiated at

ambient temperatures (cold-wall) resulted in more severe combustion

environments than did burns at elevated temperature (hot-wall) with the same

hydrogen-to-air ratio, since fewer moles of hydrogen and oxygen were available

as the gas densities decreased.

2.4 Heat Transfer Characteristics

The experimentally measured pressure signatures were used to infer the global

(spatially averaged) postcombustion heat transfer characteristics through use

of the data reduction program SMOKE [7]. These results show that convection

dominated the time-integrated heat transfer of the lean (<10%) hydrogen:air

burns, while radiation was slightly more prevalent for the hydrogen:air burns

near stoichiometry. Radiation was the dominant cooling mechanism for

hydrogen:air:steam burns with moderate quantities of steam (<20% by volume),

due to the increase in bulk gas emittance. For higher steam concentrations

(>-30%), the total energy deposition to the vessel walls decreased slightly

compared to equivalent hydrogen:air burns and radiation and convection were

equally important mechanisms, due to the reduced combustion severity and lower

post-combustion gas temperature. Figures 2.6 and 2.7 show the ratio of

radiative to total postcombustion energy deposition for the hydrogen:air and

hydrogen:air:steam burns, with the ratio for the hydrogen:air:steam burns again

plotted as a function of the hydrogen concentration in hydrogen:air only.

11
10

9

a-i

S

7

6

5

4

3

2

I

0

0 10 20 30 40 50 60 70 80

"°V/ HYDROGEN

ej

a10

9

8

7

6

5

4

3

2

1

n

. .I. . . . . I t . . I . . . I . . I " I , I . .

0 0

A0

£6o

160

HOT WALL BURNS

66 o a, FANS ON

a - FANS OFF

V

0 10 20 30 40 50

"'/. HYDROGEN

60 70 80

Figure 2.4 Normalized peak combustion pressure for the cold- and hot-wall

hydrogen:air burns in the FITS vessel (taken from Reference 6).

12
I0

I .n...eL.qI4.a.fl...qfla...l 9 IUKDLC, I WqjrqIIiR4 *DRY* BURNS

* '0-20% STEAM

W * 20-301 STEAM

3040X STEAM

30-40% STEAM

6

4 4 £

o .,*. voO o ° 4 4L* 00 T

N 3 *0y OT 0 00

. 9, < 2 004 0 o, *to

0 L I 6

0 10 20 30 40 50 60 70 sO

X HYDROGEN IN HYDROGENAIR ONLY

7 QUIESCENT CONDMONS

"ADRY"BURNS

1A 0-201 STEAM

S6a20-301 STEAM

0IL9 30-40X STEAM

h - '40X STEAM

< 54

0 0

0 0

0 0 0

O 20 0 0

0 10 20 30 40 so 60 70 so

X HYDROGEN IN HYDROGENAIR ONLY

Figure 2.5 Normalized peak combustion pressure for the hydrogen:air:steam

burns in the FITS vessel as a function of the hydrogen

concentration in hydrogen:air only (taken from Reference 6).

13
z

0

P

0.

CL

w

z

wU

OR

0

0

0i Ce

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 10 20 30 40 50 60 70 80

% HYDROGEN

Figure 2.6 Ratio of the global- radiative to. total energy deposition for the

hydrogen:air burns in the FITS vessel (taken from Reference 6).

14
0-A

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

I I I~~ ~ ~ * I 

kOa

A C0 0 0°6 0 0 an

a V AV

,&a0v V+

a bV 0 0 +

0 0 0 0

7 + +

+ - "DRY' BURNS

AM >0-20% STEAM

o m 20-30% STEAM

7 m 30-40X STEAM

o m >40% STEAM

I'll I If 1111 .-

0 10 20 30 40 50 60 70 80

% HYDROGEN IN HYDROGEN:AIR ONLY

Figure 2.7 Ratio of the global radiative to total energy deposition for the

hydrogen:air:steam burns in the FITS vessel as a function of the

hydrogen concentration in hydrogen:air only (taken from

Reference 6).

15
16
3. SNL VGES TESTS

The 5.1 en Variable Geometry Experimental System (VGES) combustion chamber at

Sandia National Laboratories was used extensively for studies of closed volume

deflagrations in hydrogen:air mixtures in the absence of steam [8]. The NRC

sponsored this experimental program in 1981 to examine the effects on

hydrogen:air combustion of varying particular parameters:

• hydrogen concentration

" pre-combustion gas motion

• pre-combustion gas pressure

• igniter type

" igniter location

" concentration of additional diluent gas (N2, C02 )

" presence of an aqueous foam

• presence of water sprays

3.1 Facility Description

The VGES combustion chamber is a cylindrical tank 4.27 m (14 ft) in length and

1.22 m (4 ft) in diameter. The bottom end is buried in the ground with only

the flanged top end exposed. The top is covered by a 0.61 m (2 ft) high

removable dome. The first 11 test series and over 100 experiments utilized the

VGES tank in the configuration shown in Figure 3.1. Latertest series

examining the effect of water sprays used the configuration shown in

Figure 3.2.

The muffin-type fans in Figure 3.1 are used to mix the contents of the tank

before and after hydrogen addition and also before postburn gas sampling.

Figure 3.2 shows a muffin fan mounted at the same level as the igniter to

produce a horizontal gas flow across the igniter. This flow is set up to

simulate a containment environment, where airflows are caused by the

entrainment of the containment atmosphere by downward falling drops during the

operation of the water spray system.

Three types of igniters were used during the first series of VGES experiments--

an exposed 300-W photoflood lamp filament, a standard GM AC 7G cylindrical glow

plug, and a 30-J raised spark-gap consisting of two copper wires 2 mm (0.05 in)

apart. During the water spray series, the GM AC 7G glow plug was studied along

with a Tayco Model 193-3442-4 helical igniter.

3.2 Combustion Pressure Rises

Results of the dry tests show that peak combustion pressures increased rapidly

with hydrogen concentration from 5% to 8%. The normalized peak pressure data

(ratio of the peak-to-initial pressure) are plotted in Figure 3.3. Also shown

are data for the adiabatic isochoric complete combustion calculation at ambient

conditions. The figure shows that measured pressures were substantial

fractions of the theoretical maximum pressures for hydrogen concentrations

above 7%. The falloff of the data from the adiabatic values above 10% hydrogen

was attributed to tests conducted with reduced air pressure or increased

initial mixture temperature, which both cause slight reductions in peak

17
GROUND

LEVEL

GAS INLET

& SAMPLING*

DYNAMIC

(b)

ERMOCOUPLES +

(a)

* Mounted in vessel head

+ Arrayed along several

vertical lines

Figure 3.1 Schematic of the VGES chamber, (a) elevation view and (b) top

view showing access ports.

18
GROUND

LEVEL

GAS INLET

& SAMPLING

,DYNAMIC

POWER

(b)

OCOUPLES

(a)

• Mounted in vessel head

+ Arrayed along several

vertical lines

Figure 3.2 Schematic of the VGES chamber for water spray tests,

(a) elevation view and (b) top view showing access ports.

19
9

8

7

6

0

x 5

4

3

2

I

0 5 10 15" 20 25 30

HYDROGEN CONCENTRATION (VOLUME%)

Figure 3.3 Normalized peak pressure for VGES hydrogen:air burns (taken from

Reference 8).

20
pressure at lower hydrogen concentrations. The disparity is present because

the adiabatic values were calculated assuming ambient pressure and temperature.

Pre-combustion gas motion was very important for hydrogen concentrations below

10%. In general, burns with the fans on produced higher peak pressures for

hydrogen concentrations below -8% due to an increase in combustion completeness

and energy release rate. Reducing the pre-combustion gas pressure had the

effect of slightly decreasing the normalized peak pressure and mean pressure

derivative over the range tested. This is due to a reduction in the number of

moles available for combustion and indicates a reduction in the chemical

release rate and the completeness of combustion.

3.3 Effect of Igniter Type

Variations in igniter type were found to be unimportant. All three igniters

used during the experiments proved reliable in igniting the hydrogen

concentrations tested. Igniter location was important only for quiescent

mixtures with less than 8% hydrogen. In these cases, the combustion

completeness was lowered when the igniter was raised because the flame

propagated only upward, leaving unburned hydrogen at the bottom of the vessel.

3.4 Flame Speeds

Flame speed values, obtained from thermocouple arrival-time data, are shown in

Figures 3.4 and 3.5 for both upward and downward speeds. Higher flame speeds

were attained in burns with the fans on, especially with hydrogen

concentrations below -10%. These figures also show that upward flame

propagation is faster than downward propagation. Figure 3.6 shows the

calculated mean pressure derivative (AP/At) as a function of hydrogen

concentration. This value increased with increasing hydrogen concentration and

was larger for the "fans on" burns. The figure indicates that the chemical

energy release rate did not tend to significantly increase in quiescent

mixtures until hydrogen concentrations were above -8%, while the release rate

for the "fans on" burns tended to increase for hydrogen concentrations above

-6%.

3.5 Effect of Diluent Gases

Tests conducted with extra nitrogen added to hydrogen:air mixtures at ambient

temperature indicated no significant differences in the burn characteristics

when compared to tests performed with similar hydrogen:air molar ratios (after

N2 addition). The addition of nitrogen does, however, dilute the fraction of

hydrogen in the mixture and produce burns indicative of the actual hydrogen

concentration. The peak combustion pressure is reduced with the addition of

nitrogen when compared to hydrogen:air mixtures with the equivalent

hydrogen:air molar ratio, and a mixture can be inerted if enough nitrogen is

added.

The addition of CO2 had the same effect as N2 on hydrogen:air combustion in

reducing the peak pressure, burn velocity, and pressure derivative. The

increased heat capacity of C02 , however, made it more effective in reducing

21
40

35

30

25

W 20

15

~10

5

0

0 5 10 15

% HYDROGEN

O FANS OFF

* FANS ON

20

Figure 3.4 Upward flame speed values in VGES chamber (taken from

Reference 8).

22
20

~15

10

0

0 5 10 15

% HYDROGEN

20

0- FANS OFF---

N-- FANS ON

Figure 3.5 Downward flame speed values in VGES chamber (taken from

Reference 8).

23
30

14J

H)

25

20

15

10

5

0

0

00.

.0

0

5 1

0'

0 .0 15 20 25

VOLUME PERCENT HYDROGEN

o FANS OFF

* FAN S ON

Figure 3.6 Mean pressure derivative (AP/At) for VGES tests (taken from

Reference 8).

24
peak combustion pressure compared to N Results of these tests tend to

indicate that -54% CO2 will inert a hydrogen:air mixture. Comparisons of

combustion with hydrogen:air:C0 2 mixtures and similar hydrogen:air:steam

mixtures from FITS [6] and Whiteshell [15,16] tests tend to indicate that CO2

and steam have comparable combustion mitigation effects.

3.6 Effect of Aqueous Foams

A series of VGES tests studied the effects of hydrogen combustion in aqueous

foam. For hydrogen:air mixtures with less than -15% hydrogen, filling the tank

with 620:1-expansion foam caused marked reductions in the peak combustion

pressures and temperatures. The pressure rise times were similar between tests

with and without foam at the same hydrogen concentrations. However, with

higher hydrogen concentrations (>15%), the pressure rise time was greatly

reduced in tests with foam (-9 ms) compared to tests without foam (-70 ms).

The pressure waves and flow generated by the accelerated flames in the foam

tests caused severe damage to the foam generator and fan, even though no

appreciable difference in peak pressure was observed compared to the no-foam

tests.

3.7 Effect of Water Sprays

Another series of VGES experiments studied the ability of helical (Tayco) and

cylindrical (GM) igniters to ignite lean hydrogen:air mixtures and to operate

at elevated temperatures when subjected to water spray fluxes and/or gas flows

that simulate the conditions in containment during the operation of the water

spray system. The initial experiments were performed with a Water spray

equivalent to that anticipated in the upper compartment of a PWR ice condenser

containment (37.3 L/m2 .min (0.915 gal/ft2 min)). It was found that the helical

igniter would ignite a lean, 6.5% hydrogen:air mixture at a surface temperature

of 575°C (1066°F), while the cylindrical igniter needed a surface temperature

of 721°C (1330°F) to ignite the same mixture.

This test series showed that when either igniter was exposed to unshielded

water spray, there was a threshold spray flux- (L/m2 .min) above which it would

not operate properly at elevated temperature. This threshold flux was

substantially lower for cases where the igniter was cold when first exposed to

the spray than for cases where the igniter was hot when it was first exposed.

The cold igniter cases refer to tests performed with the igniter and the water

spray solenoid energized simultaneously. The hot cases are those in which the

igniter was turned on and allowed to heat up for at least 100 s before the

water spray system was activated. This allowed the surface of the igniter to

reach its plateau temperature by the time the solenoid valve was actuated.

Heat transfer on the igniter surface is reduced since the surface is above the

film boiling temperature, and the igniter is allowed to remain hot at a higher

spray flux than for the cold cases. The threshold spray flux in both hot and

cold cases was also sensitive to both the igniter geometry and operating

voltage. For fluxes below the threshold, the surface temperature of the

helical igniter was normally more strongly affected by impinging water sprays

than was the cylindrical igniter.

Horizontal gas flows without sprays at velocities between 10 and 20 m/s cooled

both igniter types to temperatures below which they would satisfactorily ignite

lean (6%) hydrogen:air mixtures. Table 3.1 summarizes the spray flux and

25
airflow threshold values for both igniters. Measurements showed that the tip

of the cylindrical igniter was cooled less strongly by the airflows than were

its side walls. When exposed to water spray fluxes and airflows

simultaneously, the surface temperature of the helical igniter was highly

sensitive to both, while the surface temperature of the cylindrical igniter was

determined primarily by airflows.

A horizontal sheet metal spray shield placed above either igniter protected it

against cooling by water spray fluxes equivalent to that expected in a PWR ice

condenser containment, and insures satisfactory igniter operation for

horizontal airflows up to 5.6 m/s.

Table 3.1 Igniter Threshold Values for VGES Tests

Helical

Igniter

(120 Vac)

Cylindrical

Igniter

(14 Vac)

Surface temperature

required for ignition

Threshold spray

flux - cold igniter

Threshold spray

flux - hot igniter

Threshold air flow

(at 0.083 MPa)

575"C 7210C

-16 L/m2 .min

-24 L/m2 .min

-39 L/m2 .min

>-53 L/m2 .min*

-13 m/s

-7 m/s (@12 Vac)

-20 m/s

-10 m/s (@12 Vac)

-12 m/s Wal l

Tip

* The highest spray flux tested was -53 L/m2 .min.

26
4. NTS TESTS

A series of tests to investigate hydrogen combustion in a large-scale vessel

was conducted by EG&G at the Nevada Test Site (NTS) in 1983. These tests,

funded jointly by the Electric Power Research Institute (EPRI) and the Nuclear

Regulatory Commission (NRC), studied hydrogen mixing and ignition processes as

well as survivability of safety-related equipment in postulated degraded-core

accident hydrogen burn environments. Both premixed and continuous injection

combustion tests were performed with mixtures of hydrogen, air and steam.

4.1 Facility Description

The NTS tests were performed in a 15.85 m diameter, 2048 m3 spherical vessel

(hydrogen dewar), shown schematically with support systems in Figure 4.1. The

test series consisted of forty separate experiments in two general classes.

Twenty-four of the experiments were premixed hydrogen:air:steam combustion

tests, with hydrogen concentrations ranging from 5 to 13% by volume and steam

concentrations from 4 to 40% [9,10,11]. The second class of tests involved the

continuous injection of hydrogen into air:steam mixtures to study the effects

of diffusion flames [9,10,12]. Ignition was initiated by GM AC 7G glow plug

igniters located in various places throughout the volume to investigate

variations in igniter location. Several of the premixed and continuous

injection tests also evaluated the effect of spray systems and fans on

combustion rate and post-combustion gas cooling.

4.2 Combustion Completeness

Figure 4.2 shows the combustion completeness for the premixed tests as a

function of initial hydrogen concentration. It shows a direct relationship

between the initial concentration of hydrogen and the fraction of hydrogen

burned. Deviations from the trend were primarily the result of variations in

igniter location, with combustion completeness lower for tests with igniters

positioned in the center and top of the vessel, where downward flame

propagation is limited. Variations in steam content and the presence of sprays

tended to have a lesser effect on the combustion completeness. The figure

demonstrates that complete combustion occurred in the NTS vessel for nearly all

burns with initial hydrogen concentrations greater than 7-8%, and the glow plug

could effectively ignite lean hydrogen mixtures as low as 5.2% (provided steam

concentrations were low).

4.3 Combustion Pressure Rise

Gas pressure profiles were obtained for each of the NTS tests. Figure 4.3

shows the normalized peak pressure (ratio of peak-to-initial pressure) for each

of the pre-mixed burns. In the range tested, the normalized peak pressure

increased with increasing hydrogen concentration, with the values for tests

with steam (>10%) generally lower than those for the "standard" tests

(-5% steam) at equivalent hydrogen concentrations (i.e., burns with equivalent

hydrogen-to-air ratio). Steam acted as a heat sink and an inert diluent in the

combustion mixture, which tended to lessen the peak gas temperatures and

pressure excursions.. Fans and sprays were effective in increasing the

27
Spray control system

Water sJPPI

' header

H2 control system

H2 tube trailer

Figure 4.1 Schematic of Nevada Test Site facility (taken from

Reference 12).

28
120

(I)

E

0

0

°:0

I..

E

0

U

100

80

60

40

~0

20

0 2 4 6 8 10 12 14 16 18 20

Hydrogen Concentration in Hydrogen:Air Only (%)

Figure 4.2 Combustion completeness for NTS premixed combustion tests.
a,

(0

0~

:3

-0

(0

-N

E

I0

a,

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0

0

a

x

5% Steam

> 10% Steam

Sprays On

Fans On

DA

0 w

03

wCD 0

0P 03

A

0

0

a

I I I I I I I

0 2 4 6 8 10 12 14 16 18 20

Hydrogen Concentration in Hydrogen:Air Only (%)

Figure 4.3 Normalized peak pressure for NTS premixed combustion tests.
2.5

C')

0-

N

E

0

z

2.0

1.5

1.0

Cu

I-'

0.5

0 50 100 150 200

Time (seconds)

Figure 4.4 Gas pressure profiles for three NTS premixed tests comparing the

effect of sprays and fans.
turbulence and enhancing the rate of combustion for lean mixtures of hydrogen.

Figure 4.4 compares pressure profiles from three 6% hydrogen combustion tests.

As the figure shows, sprays were found to promote more turbulence and more

rapid combustion than were fans. The same trend was observed in the continuous

injection tests, as Figure 4.5 demonstrates. In this figure, the pressure

profiles from three tests with nearly identical source parameters are

presented. These tests were conducted with a I m diameter diffuser source and

nominal flowrates of 1.8 kg/min hydrogen and 27 kg/min steam. The figure shows

that fans significantly reduce the peak gas pressure and temperature due to an

increase in the heat transfer coefficient, but that sprays are clearly the most

effective in enhancing the heat transfer.

4.4 Effect of Igniter Location

The NTS tests demonstrated the advantage of having more than a single igniter

to initiate combustion. Igniter location was important in the combustion of

lean (<-8%) hydrogen mixtures, since downward flame propagation is limited in

these cases. In two premixed quiescent cases where there was a single igniter

at the top of the vessel, local burning was observed around the igniter, but

the flame was quenched before it could propagate downward to initiate a global

vessel burn. Other cases demonstrated that igniters distributed throughout the

vessel, as they would be in reactor containments, could effectively ignite

combustible mixtures of hydrogen. Ignition occurred when the local

concentration around the igniter reached the lean flammability limit for that

mixture.

4.5 Burn Characteristics in Continuous Injection Tests

The ignition behavior observed most frequently in the continuous injection

tests was slow ignition and diffusion flame burning, as opposed to fast

ignition (deflagration) which occurred in the premixed tests. The ignition

behavior was determined by both the fluid motion induced by the hydrogen-steam

source and the location of the igniters. For slow ignition, the gas pressure

and temperature rises within the vessel were produced entirely by diffusion

flame. Figure 4.5 shows this behavior for the three continuous injection tests

discussed previously. The slow initial rise in pressure due to gas compression

by the injected mixture was followed by a more rapid pressurization around 160-

300 sec. as combustion was initiated and the entire plume was ignited. The

pressure decrease around 600-750 sec. occurred after the oxygen concentration

dropped, extinguishing the diffusion flame.

Three continuous injection tests were carried out to determine the effect of

the source nozzle size on the combustion characteristics. Three different

source diameters of I m, 0.13 m, and 0.038 m were tested with nominal flowrates

of 1.9 kg/min hydrogen and 28 kg/min steam. Figure 4.6 demonstrates that

decreasing the nozzle size increased the heat transfer coefficient and reduced

the peak pressure. The smaller nozzle diameter for a fixed flowrate caused an

increase in the jet momentum, resulting in increased bulk gas motion and

boundary layer dissipation.

32
cc

cc

0)

I

0

0

0

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0

Time (s)

Figure 4.5 Gas pressure profiles for three NTS continuous

comparing the effect of sprays and fans (taken

Reference 13).

injection tests

from

33
0

CO

(0

C0

0

0

C"

q

0

C;

0.0 200.0 400.0 600.0 800.0 1000.0

Time (s)

Figure 4.6 Gas pressure profiles for three NTS continuous injection tests

comparing the effect of nozzle size (taken from Reference 13).:

34
4.6 Burn Duration During Premixed Tests

The NTS tests demonstrated that flame spherical propagation speed is not

meaningful for representing large volumetric burns with low hydrogen

concentration (5%-8%). For burns at these low concentrations, the flame would

rise through the vessel in a turbulent ball of fire, its upward motion

dominated by the buoyant effect of the heated mixture. Upon reaching the top

of the vessel, the flame would proceed to burn around the sphere, eventually

reaching the lower regions. Due to these observed differences in vertical and

horizontal flame propagation speeds in the NTS vessel, a more accurate

representation of large scale combustion phenomena was the burn duration,

defined as the time interval from combustion initiation to the time of peak

pressure. Figure 4.7 demonstrates the effect of steam concentration, fans, and

sprays on burn duration for varying initial concentrations of hydrogen. The

figure shows that the burn duration decreased as the hydrogen concentration was

increased. This can be attributed to the turbulence in the flames themselves,

which increase as hydrogen concentration is increased. The figure also shows

that, as noted before, sprays were more effective than fans in inducing

turbulent mixing.

4.7 Heat Transfer Characteristics

Global estimates of the radiative and total postcombustion heat transfer were

inferred from the pressure-measurements. The comparative peak radiative and

total heat flux results are plotted in Figures 4.8 and 4.9 for both the "standard" (-5% steam) and steam-laden (>10%) tests. These show that the

relative importance of radiative heat transfer increases with initial hydrogen

concentration. Radiative transfer becomes the more important energy exchange

mechanism in the "standard" tests for hydrogen concentrations above 8%. This

mechanism does not become dominant in the steam-laden tests until hydrogen

concentrations are greater than 9-10%, since steam acts as a diluent in

reducing the peak temperatures.

35
60

50

i i iI •I

0

o 5% Steam

o >10 % Steam

A Sprays On

x Fans On

(0

C

0

-0

(0

0

40

30

20

0

0

w~

0

0 10

0

6) 0 0

0

1 911

0 2 4 6 8 10 12 14 16 18

Hydrogen Concentration in Hydrogen:Air Only (%)

20

Figure 4.7 Burn duration for NTS premixed combustion tests.
12

10

0 Peak Total Heat Flux

a Peak Radiative Heat Flux

I I I I I I

0.

0

0

r1

8

6

4

0

0 0

-41

an

2

0

0

01 0

I I I I

0 2 4 6 8

Hydrogen Concentration in Hydrogen:Air Only

10 12

Figure 4.8 Peak radiative and total heat flux for "standard" NTS premixed combustion tests.
30 I I I I I I

25

o Peak Total Heat Flux

- a Peak Radiative Heat Flux

x Fans On

C

X

03

x.

r

20

15

10

8

U

0

0 0

0 0

0

I w 0 I n I I I I 0

0 2 4 6 8 10 12 14 16 18 20

Hydrogen Concentrotion in Hydrogen:Air Only (%)

Figure 4.9 Peak radiative and total heat flux for steam-laden NTS premixed combustion tests.
5. LLNL HYDROGEN IGNITER EXPERIMENTAL PROGRAM

The NRC sponsored the Hydrogen Igniter Experimental Program at Lawrence

Livermore National Laboratory (LLNL) in 1980-81 to evaluate the use of glow

plugs as deliberate ignition sources in hydrogen:air:steam environments

[13,14]. The purpose of the work was to better understand the influences of

steam and fog on ignition and flame propagation in lean hydrogen mixtures.

5.1 Facility Description

The LLNL program consists of 100 premixed combustion tests conducted in a 0.5 m

diameter by 1.5 m long compressed air storage tank. The tank has 0.5 cm thick

walls, a working pressure of 1.4 MPa, and a free volume of 0.3 m3 . The test

facility, shown schematically in Figure 5.1, includes a steam generator, compressed air and hydrogen supplies, and a 15 cm electric fan to study the

influence of mixture circulation on burn characteristics. The ignition source

for the experiments is the GM AC 7G glow plug, which can be placed at various

elevations in the center of the vessel. The primary data recorded from the

tests includes temperature, pressure and gas concentration measurements.

5.2 Flammability Limits and Combustion Completeness

Dry (hydrogen:air) combustion tests were conducted with hydrogen concentrations

from 4% to 16% by volume. Additional tests were performed with stoichiometric

(29%) mixtures. The results show that burns could be achieved for hydrogen

concentrations as low as 6%, with the transition to complete combustion

occurring at concentrations-between 8% and 9%. Concentrations of 5% hydrogen

could be ignited if the fans were used to circulate the mixture. Figure 5.2

shows the burn completion of the dry tests as a function of the initial

hydrogen concentration for both the "fans off" and "fans on" tests. Neither

the glow plug nor a one-joule spark source was capable of initiating

detonations in stoichiometric mixtures of hydrogen and air.

Steam tests were conducted with hydrogen concentrations from 8% to 12%, for

steam concentrations of up to 50%. Combustion occurred consistently for steam

concentrations as high as 40%. Figure 5.3 shows the burn completion as a

function of initial hydrogen concentration for the steam tests. The mixtures

with steam concentrations of 50% could not be ignited due to the presence of

suspended water droplets.

Several condensation-type tests were conducted at 50% steam and 10% hydrogen

where the steam was allowed to condense slowly while the glow plug remained

activated. Combustion was not indicated by discrete pressure rises in most of

the tests, even though hydrogen was being consumed. This could have been the

result of thermal recombination or limited combustion. The presence of

condensation water droplets in the vessel mixture could also have suppressed

pressure rises.

39
Gas

vent

Glow plug conductor H2 fill bottle

Compressed air supply

O

10.6 ft 3

pressure vessel

Steam

bypass

MOO

Condensate

drain

Figure 5.1 Schematic of LLNL facility (taken from Reference 14).
I I I * I I * I I I

100 N 0 0

.801-

it60

C

0

P

0

i

I

0

0

00

8

20 H 0

0

0 Quiescent mixture

0 Circulated mixture

I , I , I , I I I

4 6 8 10

Initial hydrogen fraction (%)

12 14 16

Figure 5.2 Combustion completeness for LLNL dry tests (taken from

Reference 14).

41
10% Steam fraction X denotes circulating mixture

0 20% Steam fraction test

0 30% Steam fraction

o 40% Steam fraction

a 50% Steam fraction

100 A

80

0

"E- 60-

0

0

40 0 0 0 aR 40- 00

20 0 0

0 0

I * I I I I I

6 7 8 9 10 11 12 13 i4 15

Initial hydrogen fraction (%M

Figure 5.3 Combustion completeness for LLNL steam tests (taken from

Reference 14).

42
5.3 Combustion Pressure Rises

Combustion pressure rise for the dry tests is shown plotted as a function of

initial hydrogen concentration in Figure 5.4. The corresponding adiabatic

isochoric complete combustion (AICC) value is also shown in this figure. Burns

with the fans on produced higher peak pressures for hydrogen concentrations

below 8% due to an increase in combustion completeness, while both approached

the theoretical maximum peak pressure for concentrations greater than 8%.

The combustion pressure rise for the steam tests is shown in Figure 5.5. The

presence of steam tended to decrease the pressure rise when compared to

equivalent dry burns (i.e., burns with equivalent hydrogen-to-air ratio), with

the 10-30% steam tests more closely resembling the dry tests than those with

40-50% steam. Circulation of the hydrogen:air:steam mixtures with the fan

increased the peak pressure and burn completeness as in the dry tests.

5.4 Flame Speeds

The pressure and temperature history data from many of the dry tests and steam

tests was analyzed to determine flame speeds during the combustion events. The

results show that the average speed for a test increased with initial hydrogen

concentration. Speeds were generally greater for flames propagating upward,

and also greater for ignition in the center of the chamber rather than on the

bottom.

5.5 Glow plug Reliability

The GM AC 7G glow plug consistently ignited mixtures at surface temperatures

between 7006C and 8000C, with the higher temperatures necessary to ignite steam

mixtures, and showed no appreciable deterioration throughout the series of

tests.

43
90

80 -

70 -

60

=050

"- 40

30

20 --

10 --

10

0 -. 4

Figure 5.4

4 6 8 10 12 14

Initial hydrogen fraction (%)

16

Combustion pressure rise for LLNL dry tests (taken from

Reference 14).

44
60 I I I I I I I I

50t0

0

0

0

n

10% Steam fraction

20% Steam fraction

30% Steam fraction

40% Steam fraction

50% Steam fraction

X denotes circulating mixture

40 -- A 11

CL

I

4% 0

a 30 -

CI

B 201-

10- 0

0 ft GO MDM

I I I I I I I I

6 7 8 9 10 11 12 13 14 15

Initial hydrogen fraction (%)

Figure 5.5 Combustion pressure rise

Reference 14).

for LLNL steam tests (taken from

45
46
6. WHITESHELL TESTS

The Electric Power Research Institute (EPRI) sponsored a series of over 300

premixed hydrogen combustion tests at the Whiteshell Nuclear Research

Establishment in Pinawa, Manitoba [15,16]. The program was intended to confirm

the effectiveness of a deliberate ignition system in controlling hydrogen which

might be released to containment during a postulated degraded core accident.

Experiments were performed to determine the hydrogen:air:steam concentration

regimes in which thermal igniters would be effective, for both hydrogen-lean

and hydrogen-rich mixtures.

6.1 Facility Description

The Whiteshell tests were conducted in a 17-liter quasi-spherical vessel with a

pressure rating of 4 MPa. Figure 6.1 shows a schematic of the test vessel with

the components used in the experiments. A penetration on one of the

hemispherical walls is used for gas injection and sampling. Instrumentation is

provided to measure static and transient pressures as well as vessel and gas

temperatures throughout the tests. Flame visualization an.d photography can be

achieved through a pair of 10 cm diameter viewports. A small fan with a 7.4 cm

diameter blade is available for experiments involving turbulent mixing.

Two types of igniters were tested in the Whiteshell experiments -- the GM AC 7G

cylindrical glow plug and the Tayco Model 193-3442-4 helical igniter. Both

have been utilized in deliberate ignition systems installed in BWR Mark III and

PWR ice condenser containments.

6.2 Ignition Limits

Figures 6.2 and 6.3 show the ignition limit results for the glow plug igniter,

for both the quiescent (fan off) and turbulent (fan on) tests. Figure 6.4

shows results for the helical igniter, which was only tested in the lean

hydrogen concentration range. In these figures, a mixture was classified "no

ignition" if an ionization gap probe located at the top of the vessel was

unable to detect a flame arrival. "Marginal ignition" was defined for burns

resulting in less than a 12 kPa pressure rise, while "ignition" indicated burns

resulting in greater than a 12 kPa pressure rise. Also plotted in Figures 6.2

and 6.3 for comparison is the correlation of ignition limit data from the FITS

tests (Equation 2.1). This equation correlates both quiescent and turbulent

flammability data.

The tests showed that both the GM glow plug and the Tayco igniter could ignite

dry hydrogen:air mixtures containing more than 5.5% hydrogen under quiescent

conditions. Dry hydrogen concentrations as low as 4.5% could be ignited under

turbulent conditions. Mixtures containing as much as 55% steam were ignited in

both quiescent and turbulent tests. For the hydrogen-rich tests, where only

the GM glow plug was tested, the results indicate turbulence had less effect on

the ignition limits than in the lean tests. The glow plug could ignite dry

hydrogen:air mixtures containing as much as 78% hydrogen in both quiescent and

turbulent tests. The igniter surface temperature required for ignition was

higher in the hydrogen-rich tests but not significantly affected by steam

concentration.

47
/

I

I

/I

I

\

/

ELECTRICAL

WIRES

10 CM

Figure 6.1 Schematic of Whiteshell test vessel (taken from Reference 17).

48
I I I II I I I I I I I

80

o

FANS OFF

0

A

0

Ignition

Marginal

No Ignition 70

60

0

CQ) C)

C:

0.

()

CY).

0-o

T,

50

40

30

20

10 1

C)

I\

t0

0 13 0 

A

0 .0 , 0\

A03

0

0 L

I I I I I I I I I I I

0 0 10 20 30 40 50 60

Steam Concentration (%)

Figure 6.2 Ignition limits for Whiteshell tests with the GM AC 7G glow plug

igniter (fans off). Dashed line represents flammability limit

correlation from FITS tests (Equation 2.1).

49
80

70

60

0

4-1

0

L._

C-Q) U

C) 0

Q) On

0

-1

50

40

30

20

10

0

0 10 20 30 40 50 60

Steam Concentration (%)

Figure 6.3 Ignition limits for Whiteshell tests with the GM AC 7G glow plug

igniter (fans on). Dashed line represents flammability limit

correlation from FITS tests (Equation 2.1).

50
20 I I I I I I I I I I

o Ignition

" Marginal

o No Ignition

x Fans On

0

01)

0

()

15

10

5

0

03 130

69

U'l 0

0

E

U

A

0

&

H a

I I I , I I I I I II 0 I

0 10 20 30 40 50 60

Steam Concentration (%)

Figure 6.4 Ignition limits for Whiteshell tests with the Tayco helical igniter.
6.3 Combustion Completeness

Combustion completeness was determined for each of the tests by mass

spectrometry analyses and static pressure measurements. Figure 6.5 shows the

combustion completeness for the dry (hydrogen:air) tests. In these tests,

complete combustion occurred for all burns with initial hydrogen concentrations

greater than 9-10%. Figure 6.6 shows the combustion completeness for the steam

tests, based on the initial hydrogen concentration in hydrogen:air only. The

presence of steam lessened the combustion completeness, especially for mixtures

with initial steam concentration greater than 30%. Both figures demonstrate

that turbulent burns tended to be more complete than equivalent quiescent

tests.

6.4 Combustion Pressure Rises

The normalized peak pressure (ratio of peak-to-initial pressure) for each of

the Whiteshell tests is shown in Figures 6.7 and 6.8, with the steam tests

shown as a function of initial hydrogen concentration in hydrogen:air only.

Tests with the fans on resulted in higher peak pressures due to the increase in

combustion completeness and burn rate. Steam acted as a diluent in reducing

the peak combustion pressure of the hydrogen:air:steam burns when compared to

equivalent hydrogen:air burns (i.e., burns with equivalent hydrogen-to-air

ratio).

6.5 Condensation Experiments

Experiments were also performed at Whiteshell with steam condensing conditions

where hydrogen:air:steam mixtures, initially steam inerted, were cooled to

bring the concentration into the flammable regime. The results of these tests

are plotted in Figure 6.9. The initial concentration and concentration at

initiation of combustion is plotted for each experiment, with a connecting line

to indicate its history. The figure also shows the ignition limits presented

in Figure 6.2. The results of these experiments match closely with those

obtained without condensation.

52
U'

U)

W

0~

E

0

U110

100

90

80

70

60

50

40

30

20

10

0

I~~1

- 00

I I

o0m

0

I

- 0

x Fans On

--

I I I

0 20 40 60

Concentration

80 100

Hydrogen (%)

Figure 6.5 Combustion completeness for Whiteshell dry tests.
CL

C:

0

4a, Ea

E

0

110

100

90

80

70

60

50

40

30

20

I I i i i

INlok10A

+*

*0

0

a

*

0o

+

41'

0

*a

0

0 a

+

*÷ 0

x Fans On

o 10% Steam

0 20% Steam

a 30% Steam

0 40% Steam

* 50% Steam

10 - + >50% Steam +- I + I I I

0

0 20 40 60 80 100

Hydrogen Concentration in Hydrogen:Air Only (%)

Figure 6.6 Combustion completeness for Whiteshell steam tests.
5

0

0

(D

(D 0

0

0 C

C)

-o

E

0 Fans On z

0 ,

.. 020 40 ':60 80 1)00

.. Hydrogen Concentrotion

Figure 6.7 Normalized peak combustion pressure for Whiteshell dry tests.
5

0

(I)

:3)

0~

0

E

0

Z)

n

A

4

.0

A

9

0

01

3

a

* 00 0

0 A

0

0

[]

k

0

2

1

+ **

* **

0 S+ + +

x Fans On

o 10% Steam

0 205 Steam

A 30% Steam

0 40% Steam

* 50% Steam

+ >50% Steam

-W

0 IIII I I I II

0 20 40 60 80 100

Hydrogen Concentration in Hydrogen:Air Only (%)

Figure 6.8 Normalized peak combustion pressure for Whiteshell steam tests.
70

60

50

c

0

0

L

4- 40

C

4-.'

a)U Initial c

30

",4 0 C 30

0

>' 2020 Final

10

0-

0 20 40 60

Steam Concentration (%)

Figure 6.9 Ignition limits for Whiteshell tests in steam condensing atmosphere.
58
7. ACUREX TESTS

The Acurex Corporation conducted a series of intermediate-scale hydrogen

combustion experiments as a part of EPRI's Hydrogen Combustion and Control

Program. The Acurex program, conducted in a 17.8 10 cylindrical vessel,

studied the combustion behavior of hydrogen:air:steam mixtures in both premixed

and continuous injection tests [17,18]. The purpose of the program was to

investigate the effects of hydrogen and steam flow rates, igniter location, and

water sprays and fogs on the deliberate ignition of flammable containment

atmospheres resulting from postulated degraded core accidents.

7.1 Facility Description

A schematic of the Acurex facility is shown in Figure 7.1. The vessel is 5.2 m

tall with a 2.1 m inside diameter and has penetrations for injecting hydrogen

or hydrOgen:steam and water spray or microfog. The ignition sources are 14 Vac

GM AC 7G glow plugs mounted with spray shields. An air-powered fan is used to

mix the vessel atmosphere before the premixed tests and to investigate the

effects of turbulence and mixing on burning during the continuous injection

tests. Instrumentation is present to measure vessel wall and gas temperature,

dynamic and static pressure, flame front location, and pre- and post-test gas

concentrations.

7.2 Burn Characteristics in Premixed Tests

A limited number of premixed combustion tests were performed and are summarized

in Table 7.1. Nominal hydrogen concentrations of 5, 7.5, and 10.7% were

examined in dry hydrogen:air tests; hydrogen concentrations of 7.5 and 10.7%

were.tested with the addition of microfogs of different droplet sizes.

Table 7.1 Results of Acurex Premixed Tests

Nominal H2 Measured H2 Combustion Burn

Concentration Concentration Completeness Overpressure

(vol%) Spray (vol%) (%) (kPa)

5 4.7 30 55

7.5 7.8 90 235

138 kPa microfog 7.2 >99 275

207 kPa microfog 7.6 >99 270

10.7 138 kPa microfog 9.7 >99 327

207 kPa microfog 10.2 >99 337

10.2 >99 332

59
• sampling Gas

Exhaust

Air

Figure 7.1 Schematic of Acurex test facility (taken from Reference 19).
A fog nozzle with a pressure drop of 138 kPa produced droplet diameters of 11j,

while a 207 kPa nozzle produced 8 p diameter droplets. The combustion pressure

rise, as well the completeness of combustion, was observed to increase with

increasing initial hydrogen concentration. The presence of microfog had the

effect of increasing the combustion completeness and pressure rise, due to

induced turbulence created by the fog flow and microturbulence caused by the

presence of the water drops themselves.

7.3 Effect of Igniter Location

The effect of igniter location on burn characteristics can be seen in

Figure 7.2 for three continuous injection tests without steam or sprays. For

the case of bottom igniter location, the position of the injector caused

hydrogen to bypass. the igniter, with a subsequent buildup of hydrogen

throughout the vessel until a global burn occurred at about 450 sec. With the

igniter in the center position, earlier and frequent intermittent local

combustion took place, resulting in lower pressure rises and more complete

combustion. The test with the igniter located near the top showed a partial

burn occurring at about 300 sec as the ignition limit for lateral flame

propagation was reached. A global burn eventually occurred at about 500 sec as

the limit for downward propagation was reached.

7.4 Effect of Steam, Spray. and Fog

Figure 7.3 shows the effect of steam injection and water sprays on the

combustion characteristics for continuous injection tests. The three tests

were conducted with the igniter in the center position. When large quantities

of steam were added to the hydrogen flow, the combustion pressure rise was

lowered slightly and the combustion was more erratic, taking place over a

longer time period. When a water spray was introduced to the conditions of the

previous test, multiple combustion events occurred much more uniformly, and the

combustion pressure rise was considerably less than that for dry hydrogen

injection.

The effects of water fog in the continuous injection tests is shown in

Figure 7.4. The water fog aided mixing in the vessel, contributing to local

combustion near the igniter (rather than global burns) and very little pressure

rise. Steam injection with the fog produced several discrete burns spread out

over the experiment, and slightly greater combustion pressure rises than the

hydrogen-only tests.

Varying the hydrogen flow rate in the continuous injection tests from 0.016-

0.048 kg/min (0.035-0.105 1b./min) had little effect on the general combustion

characteristics and pressure rises, even in the presence of steam. In all

tests of this type, increasing the hydrogen injection rate caused burning to

start earlier.

61
300

200

100

0

300

200 0)

f 100

0

300

200

100

0

Top igniter

18 m3 Vessel

Center igniter

40

30

20

10

0

40

30

20

10

0

40

30

20

10

0

0.

0)

U)

C,) 0)

0.

0 200 400 600 800 1000 1200

Time (sec)

Figure 7.2 Effect of igniter location in Acurex continuous injection tests

(taken from Reference 19).

62
I.,fl..

Hyroe ijcto

20

Hydrogen injection.

100 -

75.-

50-

15

-10

I0

I I I -0

100 200 3;0 400 500 600 700 800 9010 1000 110O 120o 300.o

TIME (SEC)

U.

I

1%11

20

L125

S75

~50

CL. 25

0

.150

125

Hydrogen + steam injection

15 W

10

0

0. 1

0 too 2;0 30o 400 50 600 7;0 80o 9;0 1000 1100 12o 1 3O

TIME (SEC)

2Hydrogen + steam injection with water spray

-20

-15

-10

-5

50-

25-

n . A- A

0 1o0 200 300 400 500 600 700 B0O 900 W00 uo0 120o 130

TIME (SEC)

Figure 7.3 Effects of steam injection and water sprays in Acurex continuous

injection tests (taken from Reference 19).

63
150-

i- ?0

V)

12511 Hydrogen Injection i with Water Fog Spray

100-"

75 -

50 J

crj

-10

rlll .... V.-

0 100 200 300 400

TIME

50O

(SEC)

600o 760 800 900

AI~u

-- I

0J

125- Hydrogen + Steam Injection

with Water Fog Spray

50 •

25

10 J

so-

-20

-15 V

-10 e

-5

I A

0 100 200 300 400 500 600 700 80 90 1000 1100 1200

TIME (SEC)

1300

Figure 7.4 Effects of steam injection and water fog in Acurex continuous

injection tests (taken from Reference 19).

64
8. FENWAL TESTS

Fenwal Incorporated conducted an experimental program for Westinghouse Electric

Corporation and several utilities to determine the effectiveness of glow plug

igniters in deliberate ignition systems [19]. Both premixed and continuous

injection combustion tests were conducted with various mixtures of hydrogen,

air, and steam to test the performance and durability of the glow plug igniter.

The effect of fans and water sprays on the combustion process was also

investigated here.

8.1 Facility Description

A schematic of the Fenwal facility is shown in Figure 8.1. The Fenwal tests

were conducted in a 3.8 m3 (1000 gal) spherical carbon steel vessel with a

stainless steel liner and a pressure rating of 3.5 MPa (500 psig). The outside

surface is insulated with 8 cm thick fiberglass insulation and there are

penetrations for hydrogen injection, steam and water spray supply, and vessel

drainage. Mixing of the gases in the vessel is accomplished with a 10 cm

diameter fan. The igniter type used throughout the experiments is the GM AC 7G

glow plug, installed with a spray shield to prevent direct water spray

impingement.

8.2 Combustion Pressure Rises

Figure 8.2 presents the normalized peak pressure (ratio of peak-to-initial

pressure) as a function of hydrogen concentration in hydrogen:air for each of

the Fenwal premixed tests. Hydrogen concentrations tested ranged from 5 to 12%

by volume, with steam fractions of up to 40%. The results show a sharp rise to

nearly complete combustion at -8% hydrogen. The presence of steam tended to

decrease the peak pressure when compared to equivalent dry burns (i.e., burns

with equivalent hydrogen-to-air ratio). The presence of water sprays tended to

increase the peak pressure by inducing turbulence in the vessel mixture and

introducing microturbulence due to the presence of water drops. Fans had a

similar effect on the combustion peak pressure. The effect of sprays and fans

was less at higher hydrogen concentrations.

8.3 Burn Characteristics in Premixed Tests

The Fenwal premixed tests confirmed the presence of a flammability threshold

for hydrogen combustion. Figure 8.3 illustrates this effect for combustion in

hydrogen:air (dry) mixtures. The figure shows that a burn can propagate upward

at -4% hydrogen concentration. A burn can propagate sideways at -6%

concentration, and at -8.5% a burn can proceed in all three directions. For

concentrations of 8.5% or greater, a substantial fraction of the hydrogen will

burn and a significant pressure rise will occur. The figure-also shows the

theoretical deflagration/detonation ranges for hydrogen:air combustion at

ambient conditions [20].

65
HYC~DROGEN 7 EPRTR YRECORDER COOLING/CONDENSING

FLOWMETER CHAMBER

VESSEL DRAIN

Figure 8.1 Schematic of the Fenwal test facility.

66
5.0

x xFans On

0)

4.

0~

N

E

0

0

4.0

3.0

2.0

1.0

o <10% Steam

o 10%-20% Steam 8 0

o80

I

A A

0

20%-30% Steam

>30% Steam

0

0

0 1

@9

0 0

R

H

A

0

i a a a I a I I I I a I I 0.0

0 5 10 15 20

Hydrogen Concentration in Hydrogen:Air Only (s)

Figure 8.2 Normalized peak combustion pressure for the hydrogen:air:steam burns in the Fenwal vessel as

a-function of the hydrogen concentration in hydrogen:air only.
FLMMABLE SNOT

-- DEFLAGRATION DETONATION

HYDROGEN

PERCENT

HYDROGEN

BURNS

4% 6% 8.5% 12%

I I I

7 5%4

1 1 1

I I

UP UP UP

SIDEWAYS SIDEWAYS

DOWN

COMPLETELY

F BENIGN BURN'-

-- SIGNIFICANT PRESSURE RISE-]

Figure 8.3 Combustion characteristics of hydrogen:air (dry) burns.

68
8.4 Burn Characteristics in Continuous Injection Tests

The dynamic combustion tests involved the continuous injection of hydrogen and

steam (when specified) into the vessel with the glow plug energized. The

hydrogen flow rate was -0.01 kg/min (4 ft 3/min) for all cases, with a steam flow rate of -1.4 kg/min (3 lb/min) for the hydrogen:steam injection test. An

additional test was performed with a water spray turned on before hydrogen

injection.

All cases show an initial pressure increase due to the hydrogen (and steam)

injection before ignition. The calculated hydrogen concentration in the vessel

mixture was about 5% at the first ignition for all three cases. The first

ignition in the dry hydrogen injection test occurred at 100 sec, compared to 84

sec for the hydrogen:steam injection test. The pressure response for the

hydrogen:steam case indicated multiple burns with generally increasing

frequency, as opposed to the dry hydrogen case, which showed only two ignitions

throughout the test. The ratio of the experimental pressure rise to the

adiabatic pressure rise for the first ignition was similar (-28%) in both the

dry hydrogen and hydrogen:steam cases. The presence of a water spray in a dry

hydrogen injection test reduced the combustion pressure rise to only 12% of the

adiabatic value, but had little influence on the time of ignition.
70
9. 1/4-SCALE MARK III CONTAINMENT HYDROGEN COMBUSTION PROGRAM

The Hydrogen Control Owners Group (HCOG) sponsored the 1/4-Scale Mark III

Containment Hydrogen Combustion Program [21] to determine the thermal

environment to which critical plant equipment in a Mark III containment may be

subjected as a result of hydrogen combustion during a postulated degraded core

accident. Results from this program provide an extensive data base for input

to the evaluation of equipment thermal response and survivability under

hydrogen burn conditions. The 1/4-Scale Program also provides a significant

increase in the quality of hydrogen burn data applicable to Mark III

containment facilities. This is due to improved modeling of internal

structures and an increase in overall scale in comparison to earlier

experiments, such as those conducted in a 1/20-scale model of a Mark III

containment [22].

9.1 Facility Description

The test facility is a 1/4 linear scale model of a Mark III containment

designed and constructed by Factory Mutual Research Corporation and located in

West Glouster, Rhode Island. The facility was designed so that it could be

easily modified to simulate the internal geometry of the various existing

Mark III containments. The four plants studied were Perry Nuclear Power Plant,

Grand Gulf Nuclear Station, River Bend Station, and Clinton Power Station. The

facility was designed using Froude modeling [23] and provides a reasonably

accurate simulation of the details of the containment systems which are

expected to have an important effect on the modeling of the combustion

phenomenon. These include a simulation of the distributed ignition system

installed in Mark III containments, modeling of SRV sparger geometry and

suppression pool characteristics, containment sprays, and fan coolers. The

facility is capable of simulating the programmed simultaneous release of

hydrogen and steam to the suppression pool.

A schematic of the 1/4-Scale Test Facility is shown in Figure 9.1. The test

vessel is 15.0 m (49.4 ft) high and 9.6 m (31.5 ft) in diameter, containing a

smaller tank (7.0 m (22.9 ft) high and 6.3 m (20.8 ft) in diameter) to simulate

the drywell. The gap between the two tanks is fitted with modules to simulate

the floors and large enclosed volumes present in actual containments. A body

of water at the bottom of the gap simulates the suppression pool. The entire

test facility is heavily instrumented to measure gas and surface temperatures,

velocities, gas concentrations, heat fluxes, and pressure.

9.2 Approach of the 1/4-Scale Tests

Tests were performed in the 1/4-scale facility for four different facility

configurations, simulating the internal geometries of Perry Nuclear Power

Plant, Grand Gulf Nuclear Station, River Bend Station, and Clinton Power

Station. Hydrogen was released either through the spargers of the automatic

depressurization system (ADS) and through another sparger unit simulating a

stuck-open relief valve (SORV) or through the simulated LOCA vents. Two

hydrogen release histories were studied, using stepwise simulations obtained by

hand fitting predictions from the BWRCHUC computer model [24], which calculates

71
31.0 1 D

49.4'

41.5'

29'

22.9'

17.2'

10.6'

S0•

-- it'

r/7~/?77 4 y

Figure 9.1 Schematic of the 1/4-scale test facility (taken from Reference 22).
core response to loss of coolant. The release histories presented here are for

Grand Gulf; the rate of hydrogen release for the other plants was adjusted in

proportion to core size. The first release history corresponds to a

0.57 m3/min (150 gpm) reflood of the core at 3100 seconds in the accident

sequence. The hydrogen release rate in this case reaches a peak of -0.45 kg/s

(1.0 lb/s) at -3500 seconds, dropping during the following 1000 seconds to less

than 0.07 kg/s (0.15 Ib/s). The total release up to this portion of the

transient corresponds to 23.8% MWR. The 1/4-Scale stepwise simulation extends

the release by appending a tail of constant 0.07 kg/s up to a total release

corresponding to 75% MWR. The second release history corresponds to a

19 m3/min (5000 gpm) core reflood at 3870 seconds. In this case, the hydrogen

production rate reaches a peak of 4.3 kg/s (9.5 lb/s) for a very short period,

then decreases to essentially zero in the next 160 seconds. This peak value

exceeded the capability of the hydrogen metering system. Since the peak value

could not be simulated, the total hydrogen released in it was matched by

providing full flow (-1.1 kg/s (2.4 lb/s) full scale) for the time needed to

simulate the total hydrogen release, in this case corresponding to 17.3% MWR.

9.3 Results of the 1/4-Scale Tests

Results from the 1/4-Scale Test Program were presented for experiments

conducted in a geometry applicable to the Perry Nuclear Power Plant. Results

of additional tests for the other plant geometries were still being analyzed by

HCOG at the time of this writing. The results presented so far demonstrate

that, over the conditions tested, the distributed glow plug igniter system

provides an effective means for limiting the accumulation of hydrogen in

Mark III containments. Hydrogen concentrations throughout the test facility

were maintained below 4-5% by volume (dry basis) even when the oxygen was

depleted to a level of 7.2%, regardless of the magnitude of the hydrogen

release rate.

For the first release history (peak of 0.45 kg/s full scale followed by drop to

0.07 kg/s), the global hydrogen concentration typically increased to about 2%

by volume before ignition occurred in the wetwell. Ignition transients were

modest, with no pressure excursions over 10 kPa (1.4 psi). Following ignition,

diffusion flames became established on the surface of the suppression pool.

This was the dominant mode of combustion, and occurred for oxygen

concentrations above 8% and hydrogen injection rates greater than 0.07 kg/s.

Flames were fairly steady during this phase and reached a maximum height of

about 2.4 m (8 ft) full scale for a hydrogen injection rate of 0.45 kg/s

(1,Ib/s). Combustion was essentially complete during this high-release portion

of the transient, indicated by a fairly constant background hydrogen

concentration. Gas temperatures over the bulk of the test volume were

generally low (below 90"C (200"F)) for this type of burning, with selected

areas (above the hydraulic control unit (HCU) floors) over 1800C (3500F).

Actual absolute maximum temperatures (i.e. flame temperatures) were not

detected by the instrumentation, and would be higher than those of the bulk

gas.

As the hydrogen flow decreased to 0.07 kg/s (0.15 lb/s), combustion became less

complete and the flames less stable. Flame extinction on the suppression pool

surface would occur for a period until the hydrogen concentration increased.

The pool surface would be reignited and the background hydrogen concentration

in the wetwell stabilized at about 4.5%. As the oxygen concentration

73
decreased, the flames became weaker and the flame height increased slightly.

Under certain conditions in 'which the oxygen concentration fell below 8%, the

diffusion flames on the surface of the pool were extinguished and

re-established at the HCU floor level above the SORV. This phenomenon, which

was also observed in 1/20-scale testing, is referred to as secondary burning,

or lifted diffusion flames. Conclusions regarding lifted diffusion flames

cannot be made with certainty from the 1/4-Scale tests due to lack of available

data.

At very low hydrogen injection rates of 0.03-0.01 kg/s full sca~le (0.07-

0.022 lb/s), flames were extinguished on the pool surface and localized

combustion. was measured above the HCU floor, concentrated mostly in chimney

areas. This type of burning appeared to be relatively benign, but there was

insufficient instrumentation to fully assess the effects. Localized combustion

was more widespread and more intense at low oxygen conditions, and was

accompanied by slightly higher background hydrogen concentrations. The

activation of sprays at the low hydrogen injection rates had the effect of

concentrating the burning in one chimney and reducing it in the others.

Scoping tests demonstrated that varying the injection rate of steam by a factor

of 2.0-3.5 with the same hydrogen release had no measurable effect on flame

behavior and peak gas temperatures. Therefore, this parameter was not varied

during production tests, and a constant steam injection rate of 2.5 m3/min

(go ft 3/min) was used. The presence of a grating near the suppression pool

tended to increase the heat losses from the hydrogen flames, resulting in a

cooler environment at the HCU floor. This was indicated by a 2 kPa (0.3 psi)

or -10% difference between the average combustion pressure rise for tests with

and without the grating. The major effect of containment sprays was to lower

the background gas temperature to the spray water temperature. Though improved

gas mixing is a likely result of spray operation, there was no evidence of

change in the overall flow circulation patterns as a result of containment

sprays. Results do indicate, however, that spray operation may cause a slight

decrease in the flame extinguishment limit.

74
10. SUMMARY

This report summarizes the results of eight experimental research programs

regarding the performance of thermal igniters for deliberate ignition systems

in LWR's. The results of the research programs provide a broad data base

covering nearly all aspects of hydrogen combustion related to the performance

of deliberate ignition systems (See Table 1.1, p. 5). Though clear conclusions

regarding certain combustion characteristics can be difficult to draw due to

uncertainties in the scaling of experimental data and differences in testing

methods (test apparatus, initial conditions, instrumentation, data reporting),

general trends regarding most aspects of the combustion processes can clearly

be defined.

10.1 Flammability Limits

Data regarding the flammability limits of hydrogen:air:steam mixtures were

reported for the FITS, LLNL, and Whiteshell tests. Figures 2.2, 2.3, and

6.2-6.4 present the flammability data for the FITS and Whiteshell tests. A

correlation of the FITS data was developed to describe the three-component

flammability limit (Equation 2.1). The Whiteshell data includes a comparison

of the lean flammability limits for the GM AC 7G glow plug and the Tayco

helical igniter, and matches the FITS correlation for the lean and rich

flammability limits and steam-inert concentration. Data from LLNL and other

test programs provides useful information on hydrogen:air:steam flammability

over a smaller range of hydrogen and steam concentrations.

For mixtures of hydrogen and air only, the lower flammability limit for glow

plug ignition is approximately 5% hydrogen (by volume) for quiescent mixtures

and between 4% and 5% hydrogen for turbulent mixtures. The upper flammability

limit for hydrogen:air mixtures is approximately 75% hydrogen for both

quiescent and turbulent mixtures. Turbulence has less of an effect on the

flammability limits for hydrogen-rich mixtures than for lean mixtures.

In the presence of steam, mixtures of hydrogen and air can be inerted if the

steam concentration is greater than -55% by volume. The VGES tests

demonstrated that CO2 has a comparable effect, and that -54% CO2 by volume will

inert a hydrogen:air mixture.

10.2 Combustion Pressure Rises

Combustion pressure rises were measured and presented for all of the test

programs, usually in terms of normalized peak pressure (ratio of peak-toinitial pressure) as a function of hydrogen concentration in hydrogen:air only.

When presented in this manner, various trends can be observed. Peak combustion

pressure increases rapidly with hydrogen concentration from 5% to 8%, and

becomes a substantial fraction of the adiabatic isochoric complete combustion

peak pressure for hydrogen concentrations above 8%. The normalized peak

pressure increases with increasing hydrogen concentrations up to -30%, at which

point it decreases with further increases in hydrogen concentration.

Pre-combustion gas motion is very important for hydrogen concentrations below

10%, as demonstrated in the VGES tests (Figure 3.3). Burns in turbulent

mixtures produce higher peak pressures than burns in quiescent mixtures, due to

an increase in combustion completeness and energy release rate. Water sprays

75
tend to induce more turbulence than fans for both pre-mixed combustion and

burns involving continuous injection of hydrogen. This is illustrated in

Figures 4.4 and 4.5 for the NTS experiments. Sprays also introduce

microturbulence due to the mere presence of water droplets. The effect of

sprays and fans is diminished at higher hydrogen concentrations.

The presence of steam in mixtures of hydrogen and air tends to lessen the

combustion severity when compared to burns with equivalent hydrogen:air volume

ratios in the absence of steam. This is shown in all test programs involving

steam. Steam acts as a heat sink and inert diluent in reducing combustion

pressure excursions and peak gas temperatures. Pre-combustion gas pressure can

influence the combustion characteristics of hydrogen:air and hydrogen:air:steam

burns. Reducing the pre-combustion gas pressure causes a slight decrease in

the normalized peak pressure due to a reduction in the number of moles

available for combustion.

10.3 Combustion ComDleteness

Data on combustion completeness, or the fraction of original hydrogen consumed,

were presented for all the test programs investigating premixed combustion.

Figures 4.2, 5.2-5.3, and 6.5-6.6 present combustion completeness data from the

NTS, LLNL, and Whiteshell test programs, respectively. Generally, there is a

direct relationship between the initial concentration of hydrogen and the

fraction of hydrogeni burned. Complete combustion occurs in hydrogen:air

mixtures if the hydrogen concentration is greater than -8% by volume. The

activation of fans or water sprays will induce turbulence and increase the

completeness (compared to equivalent quiescent burns) in mixtures with less

than 8% hydrogen. The presence of steam tends to lessen the combustion

completeness, especially for mixtures with initial steam concentrations greater

than 30%.

10.4 Effect of Igniter Type and Location

The two types of igniters currently installed in deliberate ignition systems

are the GM AC 7G glow plug and the Tayco helical igniter, shown in Figures 1.1

and 1.2. The VGES and Whiteshell programs compare the effectiveness of the two

igniter types. Both igniters prove reliable in igniting hydrogen:air mixtures

at the lower flammability limit. Table 3.1 shows some performance parameters

for the GM AC 7G glow plug and the Tayco helical igniter. The surface

temperature required for ignition is about IO0°C higher for the glow plug than

for the helical igniter. The helical igniter, however, is more strongly

affected by impinging water sprays than is the glow plug. The VGES tests

demonstrate that the Tayco igniter does not operate properly in unshielded

spray fluxes above 24 L/m2 .min, while the GM glow plug is able to operate in

spray fluxes of more than twice that value. In addition, these threshold spray

fluxes are reduced if the igniters are not allowed to heat up before the sprays

are activated. VGES also demonstrates that gas flows between 10 and 20 m/s are

sufficient to cool both igniter types to temperatures below which they will

satisfactorily ignite lean (6%) hydrogen:air mixtures.

The effect of igniter location was investigated in most test programs. Igniter

location is only important in the combustion of lean (<8%) hydrogen mixtures,

since downward flame propagation is limited in these cases. The combustion

completeness is lowered if an igniter is located high in a volume because the

76
flame propagates only upward. Igniters distributed throughout a volume, as

they are in containments employing deliberate ignition systems, are more

effective in igniting and preventing the accumulation of combustible mixtures

of hydrogen. The distributed glow plug system utilized in the 1/4-Scale tests

maintained global hydrogen concentrations below 4-5% by volume (dry basis) over

the range of hydrogen injection rates investigated.

10.5 Flame Speeds

Data on combustion flame speeds were reported for nearly all the test programs.

Figures 3.4, 3.5, and 4.7 present data on flame speed or burn duration for the

VGES and NTS tests. The results show that the average flame speed increases with initial hydrogen concentration. Higher flame speeds are attained in burns

with fans on (turbulent) as opposed to burns with fans off (quiescent). This

is especially true for hydrogen concentrations below -10%. Figures 3.4 and 3.5

show that upward flame propagation is generally faster than downward

propagation. A comparison of data between test programs suggests that upward

flame speeds at lean hydrogen concentrations increase with vessel size,

presumably due to buoyancy effects.

The NTS tests demonstrated that flame spherical propagation is not meaningful

for representing large volumetric burns with low hydrogen concentration (5%-8%)

due to variations in geometry. A more accurate representation of large scale

combustion phenomena is burn duration, defined as the time interval from

combustion initiation to the time of peak pressure. Figure 4.7 shows that burn

duration decreases with increasing hydrogen concentration due to increased

turbulence in the flames, and that sprays are more effective than fans in

inducing turbulent mixing.

10.6 Heat Transfer Characteristics

Measurements of heat transfer or energy deposition to vessel walls were carried

out for the FITS, VGES, Fenwal, and 1/4-Scale test programs. Figures 2.6 and

2.7 present ratios of radiative to total energy deposition in the FITS vessel

for both hydrogen:air and hydrogen:air:steam burns. Figures 4.8 and 4.9 show

the peak radiative and total heat flux results from the NTS program for 5%

steam and >10% steam tests.

Convection is the dominant energy transfer mechanism for lean (<10%)

hydrogen:air and hydrogen:air:steam burns. Radiation is slightly more

prevalent for dry burns of near stoichiometric mixtures. Radiation is the

dominant cooling mechanism for hydrogen:air:steam burns with moderate

quantities of steam (<20%) due to the increase in bulk gas emittance.

Convection and radiation are equally important mechanisms for very rich steam

concentrations due to the reduced combustion severity and lower postcombustion gas temperature.

10.7 Combustion Characteristics of 1/4-Scale Tests

The 1/4-Scale tests are unique when compared to the other test programs

reviewed here in that these are the only tests carried out in a scale model of

a containment. Due to the increased overall scale and modeling of internal

containment structures, the 1/4-Scale program provides a significant increase

77
in the quality of hydrogen diffusion flame data applicable to Mark III

containment facilities.

The 1/4-Scale experiments demonstrated that, over the conditions tested, the

distributed glow plug igniter system provides an effective means for

maintaining the hydrogen concentration below 4-5% by volume. Ignition

transients during the tests were modest, with no pressure excursions over

1.4 psi, and ignition usually occurred at average hydrogen levels of 1-2%.

The dominant mode of combustion for oxygen concentrations above 8% and hydrogen

injection rates greater than 0.15 lb/s (full scale) was diffusion flames

anchored to the surface of the suppression pool. The injection rate for flame

stability, however, was dependent on background hydrogen concentration, and

could decrease to -0.03 lb/s (full scale) as hydrogen concentrations approached

4.5%. Gas temperatures over the bulk of the test volume were generally low

(below 200°F) for diffusion flame burning, with selected areas (above the

hydraulic control unit floors) over 350°F. At low oxygen concentrations (below

8%), lifted diffusion flames were observed as flames were extinguished on the

pool surface and re-established at the HCU floor level. In experiments

involving very low hydrogen injection rates (0.02-0.07 lb/s'full scale),

relatively benign localized combustion was measured above the HCU floor, but

there was insufficient instrumentation to fully assess the effects.

10.8 Scaling Considerations

The experimental research p,'ograms reviewed here provide deflagration and

diffusion flame data in test vessels varying in volume from 0.02 to 2000 i3, in

addition to data obtained from the 1/4-Scale experiments. Although scaling

relationships are available for diffusion flame lengths and temperature and

velocity distributions (See Ref. 25), direct extrapolation of the test data to

full-scale containment volumes (30,000 to 40,000 n?) is not feasible due to

geometric dissimilarities and the lack of direct scaling laws for the turbulent

combustion and heat transfer phenomena associated with lean hydrogen:air:steam

mixture burns. The subscale test data presented here are useful in identifying

scaling trends and providing insights into relevant combustion phenomena. The

test data also provides validation and input for containment system computer

codes, which are well-equipped to incorporate geometry and heat transfer

effects in modeling actual containment combustion scenarios.

78
11. REFERENCES

1. Rasmussen, N.C., H.J.C. Kouts, E.K. Dabora, G.M. Faeth, A.K. Oppenheim,

D.J. Seery, and R.G. Zalosh, "Technical Aspects of Hydrogen Control and

Combustion in Severe Light-Water Reactor Accidents," National Research

Council Committee on Hydrogen Combustion, 1986.

2. Orrin, J.E., I.M. Vince, and F.J. Weinberg, "A Study of Plasma Jet

Ignition Mechanisms," The Combustion Institute, Eighteenth Symposium

(International) on Combustion, Pittsburgh, Pa., 1981.

3. Mittinti, D.N.R., and E.K. Dabora, "Plasma Jet Ignition Studies," The

Combustion Institute, Twentieth Symposium (International) on Combustion,

Pittsburgh, Pa., 1984.

4. Thorne, L.R., J.V. Volponi, and W.J. McLean, "Platinum Catalytic Igniters:

for Lean Hydrogen/Air Mixtures," NUREG/CR-5086, SAND88-8201, Sandia

National Laboratories, Livermore, CA, September 1988.

5. Renfro, D., L. Smith, L. Thompson, and R. Clever, "Development and Testing

of Hydrogen Ignition Devices," Proceedings of the Second International

Conference on the Impact of Hydrogen on Water Reactor Safety, Albuquerque,

NM, October 3-7, 1982.

6. Marshall, B.W., '"Hydrogen:Air:Steam Flammability Limits and Combustion

Characteristics in the FITS Vessel," NUREG/CR-3468, SAND84-0383, Sandia

National Laboratories, Albuquerque, NM, December 1986.

7. Ratzel, A.C., S.N. Kempka, J.E. Shephard, and A.W. Reed, "SMOKE: A Data

Reduction Package for Analysis of Combustion Experiments," NUREG/CR-4136,

SAND83-2657, Sandia National Laboratories, Albuquerque, NM, September

1985.

8. Benedick, W.B., J.C. Cummings, and P.G. Prassinos, "Combustion of

Hydrogen: Air Mixtures in the VGES Cylindrical Tank," NUREG/CR-3273,

SAND83-1022, Sandia National Laboratories, Albuquerque, NM, May 1984.

9. Thompson, R.T., R.C. Torok, D.S. Randall, J.S. Sullivan, L.B. Thompson,

and J.J. Haugh, "Large-Scale Hydrogen Combustion Experiments,"

EPRI NP-3878, Vols. I and 2, 1985.

10. Hacking, D.B., "Nevada Test Site Data Evaluation," Hydrogen Control Owners

Group Program Plan Task 14, HGN-100, July 1986.

11. Ratzel, A.C., "Data Analyses for Nevada Test Site Premixed Combustion

Tests," NUREG/CR-4138, SAND85-0135, Sandia National Laboratories,

Albuquerque, NM, May 1985.

12. Shepard, J.E., "Analysis of Diffusion Flame Tests," NUREG/CR-4534,

SAND86-0419, Sandia National Laboratories, Albuquerque, NM, August 1987.

13. Lowry, W.E., B.R. Bowman, and B.W. Davis, "Final Results of the Hydrogen

Igniter Experimental Program," NUREG/CR-2486, UCRL-53036, February 1982.

79
14. Altenbach, T.J., B.W. Davis, R.W. Martin, and T.A. Reitter, "Data

Analysis of the LLNL Hydrogen Igniter Experiments," NUREG/CR-3225,

UCRL-53049, -May 1983.

15. Tamm, H., R. McFarlane, and D.D.S. Liu, "Effectiveness of Thermal Ignition

Devices in Lean Hydrogen-Air-Steam Mixtures," EPRI NP-2956, March 1985.

16. Tamm, H., M. Ungurian, and R.K. Kumar, "Ignition Effectiveness of the

GM AC 7G Glow Plug in Rich Hydrogen-Air-Steam Mixtures," Hydrogen Control

Owners Group Final Whiteshell Ignition Test Report, HGN-017, October 1983.

17. Torok, R.C., K. Siefert, and W. Wachtler, "Hydrogen Combustion and Control

Studies in Intermediate Scale," EPRI NP-2953, June 1983.

18. Torok, R.C., K. Siefert, and L.B. Thompson, "Hydrogen Control Tests in

Intermediate Scale,!' Proceedings of the Second International Conference on

the Impact of Hydrogen on Water Reactor Safety, Albuquerque, NM,

October 3-7, 1982.

19. Liparulo, N.J., J.E. Olhoeft, and D.F. Paddleford,

Tests in H2 Mixtures," Proceedings of the Workshop

Hydrogen on Water Reactor Safety, Albuquerque, NM,

"Glow Plug Ignitor

on the Impact of

January 26-28, 1981.

20. Tieszen, S.R., M.P. Sherman, W.B. Benedick, and M. Berman, "Detonability

of H2-Air-Diluent Mixtures," NUREG/CR-4905, SAND85-1263, Sandia National

Laboratories, Albuquerque, NM, June 1987.

21. Tamanini, F., E.A. Ural, J.L. Chaffee, "Hydrogen Combustion Experiments in

a 1/4-Scale model of a Mark III Nuclear Reactor Containment," Research

Project Y101-1 Interim Report, Factory Mutual Research Corporation, May

1987.

22. Brown, R., S. Suttmann, and K. Wolfe, "Hydrogen Combustion Testing in a

One-Twentieth-Scale Model of a BWR6/Mark III Containment. Volume I:

Facility Description and Results Summary," Technical Report on EPRI

Research Project 1932-32/YI01-3, Acurex Corporation, February 1984.

23. Tamanini,

(The Case

Technical

F., "Reduced-Scale Modeling of Diffusion Flames in

of Hydrogen Burning in the Mark III Containment),"

Report J.I. OH5R8.RK, February 1983.

Enclosures

FMRC

24. Letter from J.R. Langley (HCOG) to R. Bernero (NRC), "BWR Core Heatup Code

Report," HGN-096, dated June 30, 1986.

25. Shepard, J.E., "Hydrogen-Steam Jet-flame Facility and Experiments,"

NUREG/CR-3638, SAND84-0060, Sandia National Laboratories,

Albuquerque, NM, 1985.

80
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NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION I. REPORT NUMBER

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2. TITLE AND SUBTITLE NUREG/CR-5079

SAND87-3139

Experimental Results Pertaining to the Performance of 3. DATE REPORT PUBLISHED

Thermal Igniters MONTH Y YEAR

October 1989

4. FIN OR GRANT NUMBER

FIN AM301

5. AUTHOR(S) 6. TYPE OF REPORT

Michael K. Carmel Topical

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10. SUPPLEMENTARY NOTES

11. ABSTRACT (200 words or less)

This report summarizes the results of various experimental programs regarding the

performance of thermal igniters for the deliberate ignition of hydrogen in light water

reactors. Experiments involving both premixed combustion and combustion with continuous hydrogen injection are reviewed. Combustion characteristics examined include

flammability limits of hydrogen: air and hydrogen: air: steam mixtures, combustion

pressure rises, combustion completeness, flame speeds, and heat transfer aspects.

Comparisons of igniter type and igniter reliability under simulated reactor accident

conditions are included. The results of the research programs provide a broad data

base covering nearly all aspects of hydrogen combustion related to the performance

of deliberate ignition systems.

12. KEY WORDS/DESCR! PTORS (List words or phrases that will assist researchers in locating the report.) 13. AVAILABILITY STATEMENT

Unlimited

hydrogen combustion, igniters, deliberate ignition 14. SECURITY CLASSIFICATION

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Unclassified

(This Report)

Tinrl nS~c fg-t.S 15. NUMBER OF PAGES

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