EXPERIMENTS

DISCLAIMER

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HW-67792

UC-80, Reac tor Technology (TID-4500, 17th Ed. )

EXPERIMENTAL EVALUATION O F THE COMBUSTION

HAZARD TO THE EXPERIMENTAL GAS-COOLED REACTOR-

PRELIMINARY BURNING RIG EXPERIMENTS

R. E. Dah1

Mate r i a l s Development Reac to r and F u e l s R e s e a r c h and Development Operation

Hanfor d Labor at o r i e s

November, 1961

HANFORD ATOMIC PRODUCTS OPERATION RICH LAND, WASHINGTON

Work pe r fo rmed under Contract No. A T ( 45-1)-1350 between the Atomic Energy Commiss ion and Genera l E lec t r i c Company

P r in t ed by / fo r the U. S. Atomic Ene rgy Commiss ion

P r in t ed in USA. Price $1. 00 Available from the Office of Technical Serv ices Department of Commerce Washington 25, D . C .

- 2 - HW-67792

ABSTRACT

An as sembly was constructed which simulated the modera tor coolant

annulus in the Experimental Gas-Cooled Reac tor .

heated to var ious t e m p e r a t u r e s and air was passed through the coolant

annulus. Under ce r t a in conditions it was demonstrated that self -sustained

combustion of the graphi te could occur .

general ly less than 1 C p e r minute until the graphi te t empera tu re exceeded

700 C and then r i s e r a t e s became ve ry rapid.

Th i s a s sembly was pre-

Ra te s of t empera tu re rise were

In these exper iments , the assembly was not opera ted in such a

manner as to give ignition t empera tu res charac te r i s t ic of the EGCR. These

t e s t s w e r e designed only to investigate the e f f ec t s of changing such param- e t e r s a s the r a t e of coolant flow, a i r humidity, p r i o r oxidation on the

graphi te , and air inlet t empera ture .

closely duplicate the EGCR operat ing conditions h a s been completed and will be repor ted in a second r epor t , HW-71182.

A l a t e r s e r i e s of exper iments to m o r e

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TABLE OF CONTENTS Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . 4

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . 4

EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . . . . 5 Descr ipt ion of Burning Rig . . . . . . . . . . . . . . 5 Graphi te . . . . . . . . . . . . . . . . . . . . . . 12 P r o c e d u r e . . . . . . . . . . . . . . . . . . . . . 13

DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . 13

Data . . . . . . . . . . . . . . . . . . . . . . . . 13

Theory . . . . . . . . . . . . . . . . . . . . . . . 23

ACKNOWLEDGEMENT . . . . . . . . . . . . . . . . . . . . 32

APPENDIX I 33

SUMMARY OF BURNING RIG TESTS AND OBSERVATIONS

APPENDIX11 . . . . . . . . . . . . . . . . . . . . . . . . 35

CALCULATION O F IGNITION TEMPERATURES IN THE

EGCR BURNING RIG

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 41

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EXPERIMENTAL EVALUATION O F THE COMBUSTION

HAZARD TO THE EXPERIMENTAL GAS-COOLED REACTOR -

PRELIMINARY BURNING RIG EXPERIMENTS

INTRODUCTION

The self-sustained combustion of graphite in air is a potential

hazard to high-temperature , gas -cooled, graphi te-moderated r eac to r s and

could conceivably resu l t f r o m an accident in which the p r i m a r y coolant

sys tem was ruptured. If such a rupture occurred , the r eac to r would depres -

su r i ze and a i r would be passed over the hot graphite, causing seve re oxida- tion. Since the oxidation of graphite i n a i r is exothermic, i t is possible

that heat could be generated m o r e rapidly than it could be removed,

such a case , t empera tu res would r i s e , causing a m o r e rapid react ion and

g r e a t e r heat generation thereby completing the cycle for self - sustained

combustion.

In

Th i s experimental investigation was undertaken to a s s i s t in eval-

uating the potential hazard of runaway oxidation in the Experimental Gas -

Cooled Reac tor (EGCR).

in the combustion of a r eac to r channel, i t w a s considered necessa ry to

conduct experimental t e s t s on a prototypical scale .

Because of the l a rge number of var iab les involved

The r e su l t s a r e published in two r epor t s . P re l imina ry tes t data, derivation of the model, and description of the apparatus a r e included in

th i s repor t .

a r e m o r e closely simulated a r e included in the second repor t .

Data f rom experiments in which r eac to r accident conditions (1)

SUMMARY

An experimental p rog ram w a s conducted to study combusion cha rac -

t e r i s t i c s of the EGCR moderator . which the fuel assembly w a s replaced with a solid co re , was tes ted under

the flow and t empera tu re conditions expected in the modera tor coolant

annulus region of an EGCR fuel channel.

A mock-up of an EGCR fuel channel, in

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Runaway oxidation was experimentally demonstrated at t empera tu res

as low a s 400 C under cer ta in conditions.

low ( < 0. 5 C / m i n ) at graphite t empera tu res l e s s than 600 C and increased

exponentially with t ime above about 700 C.

T e m p e r a t u r e - r i s e r a t e s were

The influence of variation in a i r flow r a t e s , inlet air tempera ture ,

par t ia l p r e s s u r e of water vapor, and p r io r graphite oxidation on combustion

w e r e studied in the burning r i g to permit extrapolation of data f r o m sma l l

s amples to mass ive specimens.

Variat ions in flow had a s t rong influence on combustion. Higher

a i r flows increased the ignition t empera tu res , but they acce lera ted runaway

oxidation once it had been established, EGCR depressur iza t ion accident conditions, and therefore flow in all t e s t s

was l amina r .

cant influence on the the rma l behavior of the r ig , presumably because of the low flow r a t e s .

The investigation w a s l imited to

Variat ions in the inlet a i r t empera tu re did not exer t a signifi-

Th i s observation was substantiated analytically.

Water vapor in concentrations up to 0. 5 weight p e r cent did not

detectably influence runaway oxidation in the burning r i g experiments .

Oxidation of the graphite p r i o r to the combustion t e s t s acce lera ted

Ignition t empera tu res were reduced and t empera tu res runaway oxidation.

r o s e much m o r e rapidly when combustion had been established.

A s e r i e s of exper iments were conducted with a s teel-clad center

column to study the protection which would be afforded to the modera tor

by a fuel s leeve sur face which did not r eac t with oxygen.

ignition t empera tu res were inc reased about 100 C, and runaway oxidation

was l e s s s e v e r e once initiated.

In th i s ca se ,

EXPERIMENTAL

Description of Burning Rig

Exper iments were conducted in the EGCR burning r i g in which a mock-up of an EGCR fuel channel was subjected to t empera tu res and a i r

- 6 - HW - 67 7 9 2

flows which would follow credible EGCR depressurizat ion accidents.

r ig included a t e s t section and auxiliary equipment necessa ry to provide the

des i r ed conditions.

The

A photograph of the assembly is shown in F igu re 1.

An EGCR modera tor coolant channel was simulated full sca le radially

and approximately one-third sca le axially in the tes t section. section was contained in an insulated pipe 78 inches in length. T e s t s in th i s

phase of the study were made on a simplified geometry in o r d e r to facil i tate

evaluation of such var iab les as graphite tempera ture , water vapor content,

and p r i o r oxidation. The EGCR modera tor was simulated with a column of

nested s leeves 1 2 inches in length by 5. 2 5 inches ID by 8. 16 inches OD, maintaining the radial dimensions of a la t t ice unit and the modera tor coolant

annulus.

of the p rope r outer diameter .

in t h i s assembly s ince the cent ra l channel containing the fuel e lements w a s

eliminated when the graphi te column w a s substituted fo r the fuel assembly . Radial space r plugs extended f r o m the outer surface of both columns to in su re

c o r r e c t spacing and to prevent skewing.

The tes t

The EGCR fuel a s sembl i e s were replaced with a graphi te column

Thus , the annulus w a s the only coolant channel

The graphite w a s heated with eight Calrod hea te r s , four of which

w e r e positioned a t 90-degree in te rva ls in the modera tor on a 6. 69-inch

c i r c l e and four in the cen te r column on a 3. 00-inch c i rc le .

view of the tes t section i s presented in F igu re 2. h e a t e r s was to off-set rad ia l heat l o s s e s f r o m the assembly simulating a fuel

channel in an infinite la t t ice a r r a y in which no heat would flow a c r o s s cell

boundaries. During init i tal t e s t s , the power to these hea te r s w a s se t a t a

level w'hich would maintain the conditions a t the s t a r t of the run.

l a t e r t e s t s , these hea te r s were controlled by a differential thermocouple so that heat generation caused by oxidation w a s matched by an inc rease in hea ter power to maintain adiabatic conditions. Hea te r s were positioned in the center

column principally to a s s i s t in attaining des i r ed init ial t empera tu res in a

reasonable t ime , These hea te r s were maintained a t a constant power setting

throughout the cour se of the first exper iments a d were off during the exper i -

ments in which the outer hea te r s were controlled by the differential thermocouple.

A c r o s s sectional

The purpose of the modera tor

During

6 I 4

I m

-3

-8 -

Moderator Cooling Annulus

H W - 6 7 7 9 2

Heaters

P o s i t i o n

FIGURE 2

Radial C r o s s Sectional View of Test Assembly Showing Heater and Thermocouple P lacements

A L C - G L R I C H L L I I O W A S H

- 9 - HW-67792

Gas-flow paths a r e shown on the flow diagram in F igu re 3 . Air was

taken f r o m the room and passed through a removable d r y e r and a prehea ter

F r o m th is point, the a i r could be passed through the t e s t chamber or diverted

through a bypass l ine to the vent.

flow and t empera tu re p r i o r to commencement of the t e s t . Outlet g a s could

be passed direct ly to vent or through a cooler and scrub column which was

used to remove noxious g a s e s tes ted a s oxidation inhibitors.

was a packed column with 1 /2- inch plast ic Raschig r ings and employed a

water sc rub .

The bypass allowed stabil ization of gas

The scrubber

Flow was metered by a ro t ame te r and was controlled with a throt t le

valve ups t ream f r o m the tes t chamber . bypass l ine during init ial t e s t s , and i t was necessa ry to calculate flow through

the t e s t a s sembly by comparing p r e s s u r e drops through the two l ines . The

flow m e t e r was moved to an inline position with the t e s t assembly f o r l a t e r

experiments . Flow r a t e s used in the t e s t s ranged f r o m 4 to 55 l b / h r . The

corresponding Reynold 's number in the annular region ranged between 68

and 930; thus the t e s t s were conducted with l amina r flow.

e r a t u r e s up to 450 C were provided by the prehea ter .

The flow m e t e r was located in the

Inlet a i r t emp-

T e s t s w e r e run with and without the s i l ica gel d r y e r in the l ine. P r e -

dr ied a i r had a maximum water vapor content of l e s s than 0. 05 weight p e r

cent. weight p e r cent.

When the d r y e r was removed, the water vapor content was 0. 1 to 0. 5

T h e r m a l reac t ions within the t e s t chamber were monitored with

thermocouples located within 1 / 3 2 inch of the oxidizing graphi te sur faces .

T e m p e r a t u r e s w e r e measu red every 90 seconds a t 22 points along the length

of the column.

to-experiment and is shown in F igu re 4.

faces , the air s t r e a m and the sur face of the cen te r column were measu red

in each 12-inch zone. In addition, the sur face t empera tu re of the modera tor

and of the cent ra l column w e r e measu red at the s a m e elevation in the center

t h r e e feet of the assembly.

a lumel .

in the graphi te had isolated hot junctions.

The axial thermocouple placement var ied l i t t le f r o m experiment -

The t empera tu re of modera tor s u r -

All of the thermocouples were sheathed ch romel -

Those monitoring a i r t empera tu re had exposed t ips; those embedded

- 1 0 - HW-67792

Gas Scrubber

Heat Exchanger

Blower

Burning Rig

FIGURE 3 G a s F l o w Diagram for EGCR Burning Rig, Exper iments 1 Through 11

A E C - C F R I C H L I H O . W I S ”

-11- H W - 6 7 7 9 2

F o a m G l a s s Graph i t e

Insulation (Column) Graph i t e 6-111. Th ick Graph i t e , (h.Ioderator)

Inches

T h e r n Elevat (Inche

- 7 2

- 69

-66

- 60

- 57

- 54

- 1 2

9 -

- 6

loco .ion s fr'

muple

om Inlet)

Axial C r o s s Sectional View of Tes t Assembly Showing Thermocouple Elevations (Not to Scale)

-12- HW-67792

Heater power and graphite t empera tu re were controlled e i ther manually o r automatically. Automatic control was achieved through a c i r -

cuit composed of a saturable r eac to r , magnetic amplif ier , and three-mode

control ler . With th i s a r rangement , s tep less control could be achieved with

an accuracy of f 2 C in the control zone.

h e a t e r s a t the end of the column to offset end lo s ses , the t empera tu re differ-

ence between the center and the ends of the column at the beginning of the

t e s t s seldom exceeded 50 C.

Although t h e r e were no boos ter

G r aphit e

Graphi te react ivi ty is an important fac tor in determining graphi te

oxidation r a t e s because the oxidation r a t e s es tabl ish heat generat ion and

possible runaway oxidation conditions. In th i s type of investigation, i t

becomes essent ia l that the react ivi ty of the graphi te to be used in the r eac to r

is duplicated o r approximated a s near ly a s possible.

not available for these t e s t s ; however, graphi te produced under nea r ly

identical specifications was obtained, and the react ivi ty r a t io between these

similar graphi tes was determined.

t hese graphi tes towards oxidation by a i r under identical conditions a r e

l i s ted in Table I.

EGCR graphi te was

Comparative chemical reac t iv i t ies of

TABLE I

REACTIVITY OF TYPES OF NUCLEAR GRAPHITE USED

Graphit e Exper iments Used Relative Chemical Reactivity

Speer Nuclear Grade I1 1 - 9

NC - 9 (Hanford designation - National Carbon Co. graphi te) 1 0 , 11

EGCR graphi te - - -

0. 9

1 . 0

1 . 0

n -13- HW-67792

P rocedure

Although experiments were run with var ious init ial conditions, the

genera l procedure was near ly the same.

begun, the graphite w a s brought t o the des i r ed init ial t empera tu res with

s ta t ic air or a ve ry slow flow of argon in the annulus, was adjusted to maintain the proper tempera ture ,

heated, flow was s ta r ted through the prehea ter and diverted through the

bypass l ine, When equilibrium had been reached, air was introduced a t the

requi red t empera tu re and flow, and the tempera ture of the exposed graphite

su r faces was monitored.

Before the oxidation t e s t s were

The hea ter power

If the air was to be p r e -

DISCUSSION

Data

Exper iments in th i s phase of the study fall into two genera l categories:

Experi.ments 1 through 9 were designed to t e s t the combustion cha rac t e r i s t i c s

of the a s sembly under approximate EGCR tempera tu re and flow conditions. In addition, the effects of flow and inlet air t empera tu re on combustion were

a l so studied. The second se t , Exper iments 1 0 and 11, was designed to t e s t

m o r e specific EGCR accident c i rcumstances and the effects of p r i o r oxida-

tion and water vapor upon combustion.

summar ized in Appendix I.

Data f r o m a l l the exper iments a r e

Exper iments 1 through 9, consisting of 29 sepa ra t e t e s t s , demon-

s t r a t ed that under cer ta in c i rcumstances runaway oxidation can occur a t

t empera tu res as low a s 400 C. four oxidation t e s t s which were conducted adiabatically with init ial graphite

t empera tu res varying between 400 and 600 C and with air flow r a t e s of 4 . 4 ,

19, and 55 l b / h r a t 20 C inlet air tempera ture . Tempera tu res r o s e in each case , although the r a t e of r i s e below 600 C w a s low ( l e s s than 1 C /min ) .

However, each c a s e must be considered runaway oxidation and would r equ i r e

remedia l action.

substantially the s a m e as those of Experiment 2 except that the inlet a i r

was preheated, again demonstrat ing that runaway oxidation can occur in the

t empera tu re range of in te res t to the EGCR.

Experiment 2 ( s e e F igu re 5) consis ted of

Exper iments 4 through 9 consisted of 20 t e s t s which were

700

a, h 7 cd h a,

c

? ; 2 600 2 a cd

u E z .- X (d z

500

400

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1 9 A i r F l o w ( I b / h r )

E x p e r i m e n t H e a t e r P o w e r

W a t e r V a p o r I n l e t A i r

T e m p e r a t u r e

2 ( S o l i d Core) A d i a b a t i c

C o n d i t i o n s c0.05 W / O H20

20 c

4. 4 A i r F l o w ( I b / h r )

/ /

- - -_ 1 9 A i r F l o w ( l b / h r ) _ _ - - - - --- 3 4 A i r F l o w ( I b / h r )

0

-15- HW - 6 7 7 9 2

A ve ry significant observation was made in Experiment 2 on the

effect of.flow r a t e on combustion. Below 500 C an inc rease in the a i r flow

inhibited the t empera tu re r i s e , but above 600 C, the effect was r e v e r s e d and an inc rease in the a i r flow caused m o r e rapid runaway. This observa-

tion is quite important because it had been considered that one possible way

of controll ing runaway oxidation would be to inc rease a i r flow.

Experiment 3-1 ( s e e F igu re 6 ) was run to investigate fur ther the

effect of flow r a t e .

t empera tu re of 607 C, 4 l b / h r a i r flow, an inlet a i r t empera tu re of 150 C

and sufficient power to the guard hea te r s to offset rad ia l heat l o s ses .

t e s t proceeded for 70 minutes with constant a i r flow and with those power

adjustments necessa ry to cause the react ion to proceed adiabatically.

Graphite t empera tu res r o s e near ly l inear ly a t approximately 0. 6 C /min

during th i s period. The a i r flow was then increased fifteen-fold, and an

exponential t empera tu re rise occur red which continued unchecked when the

guard h e a t e r s were turned off. Af te r

a shor t t ime the a i r flow was stopped, and the en t i re assembly began to

cool. t u r e was 700 C and, then the a i r flow was resumed at 55 l b / h r with the guard

h e a t e r s off. Combustion occur red immediately and t empera tu res again r o s e

exponentially up to a r a t e of 40 C /min . marked effect of a i r f low on runaway oxidation and the inherent danger in

attempting to blow out an established runaway.

laminar ; however, a t 55 l b / h r the flow exceeded the maximum m a s s flow

r a t e which could be provided to the annulus in a n EGCR maximum credible

accident.

of 4 l b / h r , and when the a i r flow was increased , heat generation was

acce lera ted to a much g r e a t e r degree than heat removal .

Initial conditions f o r th i s run were a maximum graphi te

The

The r a t e of r i s e re.ached 20 C /min .

Cooling was allowed to continue until the maximum graphi te t e m p e r a -

Th i s experiment demonstrated the

A i r flow in each c a s e was

Apparently oxygen s tarvat ion was re ta rd ing oxidation a t a flow

Experiment 1 0 consisted of 21 sepa ra t e t e s t s in which init ial graphi te

t e m p e r a t u r e s ranged f r o m 470 C t o 730 C and with flow r a t e s which could be expected in the EGCR following cer ta in depressur iza t ion accidents . All of

the t e s t s in th i s s e r i e s were made on the same graphi te assembly , thereby

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W-67792

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-17- HW-67792

permitt ing observation of the effects of p r io r oxidation.

t e s t s a r e presented a s t ime-versus- tempera ture plots in F igu res 7 through

11. demonstrate the complexity of the sys tem.

generally exist in the channel, a zone which is cooling, one which is in

the rma l equilibrium and one which is heating. t r a t e s the effect of oxygen depletion which occurred in a lmost all t e s t s

because of the low flow r a t e s used.

oxygen supply to such an extent that oxidation could not be supported in the

downstream sections, and the zone of maximum tempera tu re moved toward

the inlet.

conduction, o r , in some instances, cooled. This effect w a s demonstrated

in Experiment 10-9 (F igure 7 ) in which the point of maximum tempera tu re

moved f r o m the mid-plane to a point approximately 9 inches f r o m the inlet.

Data f r o m these

Graphite t empera tu res a t a number of axial elevations a r e plotted to It may be seen that t h ree regions

Th i s type of plot a l so i l lus-

Combustion in many c a s e s depleted the

The r e s t of the column in these c a s e s e i ther heated f r o m axial

P r i o r oxidation of graphite has been observed to inc rease the oxida-

tion r a t e of laboratory samples and could affect the course of runaway

oxidation. Rates general ly increased through approximately the f i r s t 5 pe r cent burnoff and then stabil ize through a wide range of burnoff, Th i s effect

could cause a reduction in the c r i t i ca l t empera tu re if the graphite had p r e -

viously been oxidized s e v e r a l pe r cent. Experiment 1 0 to s tudy th i s effect.

10-7 were repeated a s near ly a s possible in Experiment 10- 11 (F igu res

8 and 9) but runaway w a s much less pronounced in the f i r s t t es t than the la t te r ; thus i t i s c l ea r ly demonstrated that p r i o r oxidation had a significant

influence on runaway oxidation.

o r d e r to confirm th is conclusion.

Several t e s t s were repeated during The init ial conditions of Experiment

Several other t e s t s were a l so repeated in

The influence of low water vapor concentrations on combustion was

studied in Experiments 13A and 15 by tes t ing with and without the drying

column.

cent water and for a tmospheric conditions a t the t ime of the t e s t (0. 5 weight

The r e su l t s f o r predr ied air containing less than 0. 05 weight p e r

I I I I I I I I I I I I I I I I I I

-18

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W-67792

-19-

HW-67792

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Experiment 10- 13A (Solid Core) Heater Power 2 .16 kw Air Flow 4. 00 l b / h r

Inlet A i r

G r a phi t e 1’ e m pc rat u r c + - Water Vapor <O. 05 w/o H 2 0 b - Inc. h E I evat 1 on

- . - - . - A

Temperature 20 C

800

720

2 640 . . al 3 c

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.- /./ - I 12-Inch Elevation

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FIGURE 10

Therma l Behavior in Experiment 1 0 - 13A Conducted with P red r i ed Ai r

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- 2 3 - H W - 6 7 7 9 2

p e r cent wa te r ) a r e i l lustrated in F igu res 1 0 and 11.

ence was noted in the the rma l behavior between t e s t s .

that the magnitude of the effect was not enough to influence the cour se of

runaway oxidat ion.

No significant differ-

One may thus conclude

The potential hazards of self -sustained combustion made it des i rab le

that a safeguard procedure o r device be developed.

make the sur face of the fuel s leeve oxidation res i s tan t thus reducing the heat

generation.

impermeable , r e f r ac to ry coating to the sleeve. However, coated s leeves

w e r e not available for these t e s t s .

a s ta in less s t ee l pipe was fitted over the center column. The outside d iam- e t e r of th i s pipe was 4. 50 inches causing an inc rease in the hydraulic d iam-

e t e r by a fac tor of t h r e e and an inc rease in the annular flow r a t e by a fac tor

of 2. 65. The Reynold's numbers , however, were only inc reased by 13 p e r cent and the flow w a s s t i l l l aminar .

One approach was to

In a r eac to r core , t h i s may be accomplished by applying an

In o r d e r to s imulate th i s protect ive effect,

Experiment 11, consisting of seven t e s t s , was conducted with the

nonreact ive c o r e assembly; the maximum initial graphi te t e m p e r a t u r e s

ranged f r o m 490 C to 680 C.

the init ial graphi te t empera tu re was below 600 C.

l e s s than 0. 5 C/min , were observed with init ial graphi te t empera tu res

between 600 C and 700 C. observed, but instead s teady- s ta te t he rma l conditions appeared to be

established at high t empera tu res .

th i s s e r i e s . elevated 150 degrees by an iner t s leeve and that runaway oxidation, once

init iated, was m o r e eas i ly controlled.

Cooling of the assembly was observed when

Slow t empera tu re r i s e s ,

In no t e s t s was an exponential t empera tu re r i s e

F i g u r e s 1 2 and 13 a r e i l lustrat ive of

Experiment 11 demonstrated that the ignition t empera tu re was

Theory

Runaway oxidation o c c u r s when the r a t e of heat generat ion exceeds

the r a t e of heat removal . The c r i t i ca l condition can then be determined by

establishing a balance between these r a t e s according to the genera l method

of Robinson and Taylor . ( 2 )

HW

-67792 -2

4-

.

I i

i i I

I I I I I I

i i .

I i i i

I I

I

d 3, *am

jeJadu

iaL

0

N

0

d

m

.-- v

0

0

In

v

x3

n

0

3

0

0

0

0

x3 W

m

W

W

v

N

P-

Q

960

880 t 800

720

y 640

s c

f 560 c 480

400

Graphite Temperature + &Inch Elevation

-. / 24-Inch Elevatlon

-.- - I -/-I- /- /- /- /-L-/-

/-/ / -_,,,-, ,, -7-d.'. -...-...- ...-... -

/-. // /k -_.--. // -1

-... 40-Inch Elevation - .. . - ... / -'

/-'

/ -'

72-Inch Elevation _. --. //-I/-

Experiment Heater Pqwer 4 . 4 6 kw A i r Flow 4 . 00 lb /hr Water Vapor Inlet Air

I 1 - 5 (Solid Core Enclosed in Sta inless Steel Tube)

<O. 05 w / o H 2 0

Temperature 20 C

320 r

FIGURE 13

Therma l Behavior i n Experiment 11 - 1 Conducted with Steel-Clad Fuel Sleeve at Tempera tures Exceeding Those Expected in the EGCR

-26- HW-67792

Heat generation, QHJ depends upon the r a t e of oxidation, the amount

of reac t ing ma te r i a l and the heat of combustion according to

QH = y p V A H c a l / h r

where

y = f ract ional weight l o s s of graphite p e r hour ( for uniform

oxidation)

p = density ( g / c m )

V = react ing volume ( c m )

3

3

AH = heat of combustion of graphite ( = 7900 c a l / g )

The t empera tu re dependence of the r a t e of react ion follows the Ar rhen ius equation

y = a b exp ( - C / T ) ( h r - l ) (2)

where -1 a = constant ( h r )

b = a fac tor which can be included to account f o r any

react ivi ty change which does not affect the activation

energy

E = activation energy ( = 50 kca l /mole) R = gas constant ( = 1 .987 cal /mole/deg)

T = absolute t empera tu re (OK)

The react ing volume fo r mass ive spec imens under i so thermal conditions

in which the oxidation is controlled by diffusion in the graphi te is:

v = P L P c m

where

P = exposed p e r i m e t e r (cm)

L = length of graphi te column (cm)

,f! = depth of oxidation ( c m )

(3)

- 2 7 - HW-67792

The depth of oxidation ( f ) i s not well established; however, according to a

study on Br i t i sh P i le Grade A Graphite:.. .I.( 3 )

The t e r m ( 273 lo . 38 ranges between 1 . 4 1 at 400 C and 1. 62 a t 700 C; thus,

a constant value of 1. 5 can be used with reasonable accuracy.

The r a t e of heat generation p e r unit length of graphi te column is then

QH = L - a c a l / h r (5) where

g-ca l a = combustion constant = 0. 050 exp ( -C /2T)p P AH ___ - h r cm

Heat is removed according to the equation

Qc = hAx (T - ta) c a l / h r

where 2 h = heat t r ans fe r coefficient ( c a l / h r , c m , " C )

2 A = PL = cooled sur face a r e a ( c m ) X

T = graphite t empera tu re ("C)

ta = a i r t empera tu re ("c)

in which all t e r m s a re known o r eas i ly determined except the a i r t empera ture .

The air t empera tu re will r i s e along the channel according to

- - dta - A (T - ta) dx ( 7 )

where

W = mass flow r a t e ( g / h r )

Cp = heat capacity of air = 0. 255 ( ca l /g , C )

If the graphi te t empera tu re is considered constant through the increment of length used, the solution to Equation (7) is:

'::Note added in proof: Very- recent work by the author h a s indicated that the constant t e r m , 0. 033, in Equation ( 4 ) i s too high for EGCR graphite, and fu r the rmore , v a r i e s with the amount of oxidation which h a s occurred . Thus, the t rea tment in th i s text tends to overest imate the heat generation.

- 2 8 -

(T - t a ) = (T - to ) exp (-Ax)

where to = inlet a i r t empera ture

fo r the boundary conditions

HW-67792

( 8 )

A sys t em in which the graphi te t empera tu re is not constant can be consid-

e r e d by inclusion of an appropr ia te temperature-dis t r ibut ion function in

Equation (7) .

the cooling r a t e is:

However, i f Equation (8) is substituted into Equation (61,

= h P L (T - to ) exp (-Ax) ( 9 ) QC

and the c r i t i ca l t empera ture , i. e . , the t empera tu re a t which heat genera-

t ion exactly equals heat removal , can now be determined by equating the

r a t e s of heat generation and removal a s given by Equations (5 ) and ( 9 ) :

This model gives a sat isfactory approximation to the c r i t i ca l t empera tu re

if the length (x) i s re la t ively s m a l l a s it would be e i ther in a sma l l - sca l e test rig, or in a f ini te difference solution fo r a l a rge sca le assembly . However, a more exact solution can be obtained by considering the v a r i a -

tion of T with x, as demonstrated in Appendix 11.

Application of Equation (10) to the configuration used in the EGCR

burning r i g with the range of flow r a t e of i n t e re s t ( s e e Appendix 11) yields

the r e s u l t s given in F i g u r e s 1 4 and 1 5 and in Table 11.

100 C variat ion in inlet a i r t empera tu re can be seen by comparing the two

f igures .

The effects of a

0 0

e

-29

- H

W-67792

-30

- H

W-67792

0 0

4

ALC CL R

ICH

LIIIO

. WlS

H

-31 -

TABLE I1

HW-67792

CRITICAL CONDITIONS IN EGCR BURNING RIG

Mass Flow Rate , l b / h r

6

1 2

1 9

55 100

Cri t ica l Tempera tu re , C t a = 27 C t a = 1 2 7 C

_ _ 109

221 203

3 62 3 48

47 7 465

534 525

A bas i c assumption made in these calculations is that t he re is no

mechanism fo r heat l o s s other than removal of heat by the coolant gas .

The calculations must therefore be considered a s a f i r s t approximation

f o r experimental behavior s ince axial and rad ia l heat l o s s e s do occur to

some extent in the t e s t assembly. Because of the axial heat l o s s e s , the

column is not init ially under i so thermal conditions.

combustion to be expected under the axial t he rma l gradient in the burning

r i g is given in Appendix 11. Th i s analysis shows that the i so the rma l t r e a t

ment tends to es t imate the c r i t i ca l t empera tu re too low.

An ana lys is of the

Thus , even

though agreement between the i so thermal ana lyses and experimental o b s e r -

vat ions were good in seve ra l ins tances , th i s may have been due in pa r t to two compensating effects.

that the Equation (4 ) may overes t imate the depth of oxidation.

t h i s inaccuracy would have been par t ia l ly compensated by considering the

graphi te to be i so thermal .

A s stated e a r l i e r t h e r e is recent evidence:::

However,

The i so thermal calculation shows that runaway oxidation is possible

a t low t empera tu res (on the o r d e r of 400 C) and i s a lmost unaffected by inlet air t empera tu re in th i s system. These effects were confirmed experiment-

ally, but the calculation demonst ra tes that the insensi t iveness i s due to the

l amina r flow and the relat ively long channel; i. e . , the a i r t empera tu re

::See footnote on page 27.

- 3 2 - H W - 6 7 7 9 2

approaches the graphite t empera tu re short ly a f te r the a i r e n t e r s the channel.

Thus, the main importance of the analytical t rea tment in i t s p resent f o r m

is that i t explains the general t rends observed in the Burning r ig . Before

accu ra t e values can be calculated for the c r i t i ca l conditions the effective depth of oxidation must be co r rec t ly accounted for. :% Work is continuing to

es t imate the depth of oxidationunder conditions of in te res t in the EGCR. When t h i s work is completed, i t should be possible to m o r e closely predict

the conditions under which the graphite wi l l ignite.

ACKNOWLEDGEMENT

The author is grateful to S. E . Nichols for his ass i s tance in design-

ing the appara tus and conducting the experiments .

:%See footnote on page 27.

-33- HW-67792

APPENDIX I

SUMMARY O F BURNING RIG TESTS AND OBSERVATIONS

Initial Maximum Graphi te Total Inlet A i r Tempera tu re , C Heating

Rate , OClrnin

Exper i - A i r Flow, Tempera tu re , O C Sleeve ment lb/hr:i: Annulus Center Modera tor o r Core

1 - Solid cent ra l core ; Speer Nuclear Grade I1 graphite; p redr ied a i r ; h e a t e r s off during runs

635 650 7 1 5 735

- - 1-1 4 20 1 - 2 55 220 1 - 3 55 220 1 -4 55 20

_ _ _ - - -

635 -0 . 50 650 -0 . 50 715 +5. 4 735 +7 .3

2, 3, and 4 - Same, except h e a t e r s controlled t o give adiabatic conditions during exper iments

41 0 2 - 1 4 . 4 20 2 -2 4 . 4 20 - - 440

47 0 2-3 4 . 4 20 4 60 2 -4 19 .0 20 640 2-5 1 9 . 0 20 680 2-6 54. 5 20

_ -

- - - - - - _ -

410 + O . 08 440 +o. 33 470 + O . 25 460 + O . 1 7 640 + l . 3 68 0 + 3 . 3

3-1 4 to 60 220 _ _ 61 0 61 0 rise

425 4-1 4 to 60 220 4-2 55 220 - - 700

_ - 425 rise 7 0 0 +9. 8

5 - New graphi te assembly; solid cen t r a l - co re ; Speer Nuclear Grade 11; p red r i ed air; h e a t e r s off during r u n s

47 5 5-1 4 180 530 5 -2 4 170 57 0 5-3 4 180

5 -4 4 185 - - 600 5-5 4 200 _ - 630

- - - - _ -

4 7 5 - 0 , 1 6 530 - 0 . 1 6 57 0 -0.16 600 -0 .16 63 0 -0. 08

6, 7, and 8 - Same, except h e a t e r s controlled to give adiabatic conditions during r u n s

6-1 4 130 - - 480 53 5 6-2 4 140

4 50 7 -1 4 120 - - 4 60 7-2 20 110 - - 590 7-3 20 145 640 7 -4 20 150 - - 69 5 7-5 55 110

- -

- -

_ -

48 0 +O. 25 535 +O. 25 450 +o. 33 4 60 + O . 08 590 + O . 27 64 0 + l . 5 69 5 +4.7

-34- HW-67792

SUMMARY OF BURNING RIG TESTS AND OBSERVATIONS (Contd.

Initial Maximum Graphite Total Inlet A i r Tempera ture , C Heating

Exper i - A i r Flow, Tempera tu re , OC Sleeve Rate, ment lb / h r Annulus Center Moderator or Core OC/min

8-1 4 120 - - 41 0 410 0. 00 8 - 2 4 120 - - 445 445 + O . 16

9 - Same, except hea te r s off during run 9-1 4 600 120 - - 600 - 1 . 7

1 0 - New graphi te assembly; National Carbon Co. Graphite; p redr ied a i r except a f te r Experiment 10-1 3; hea te rs controlled to give adiabatic conditions during r u n s

- - 10-1 4 20 10-2 4 20 10-3 4 20 10-4 4 20 10-5 4 120 10-6 20 20 10-7 20 120 10-7A 20 120 - - 10-8 4 20 1 0 - 9 4 20 10-10 12 20 10-11 1 5 20 10-12 4 20 10-13 4 20 10-13A 4 20

- - - - - - _ - - - - -

- - - - - - - - _ - - - _ -

A i r predr ie r removed - - 10-14 4 20

10-15 4 20 10-16 1 5 20 10-17 1 5 20 10-17A 4 20 10-18 35 20

- - - - - - - - - -

450 58 5 550 57 5 52 5 530 555 630 700 700 580 550 625 5 50 625

540 61 5 67 0 58 5 665 600

450 58 5 550 57 5 525 530 555 630 700 700 58 0 550 62 5 550 625

540 61 5 68 0 58 5 67 0 600

+ O . 16

+ O . 1 6 + O . 16

+ O . 13 + O . 08 + O . 03 + O . 42 + l . 1 + O . 42 + O . 50 + l . 5 + l . 4 + O . 42 + l . 0 + O . 92

+l. 3 + O . 8 +5. 0 + l . 3 + O . 67 +2. 0

11 - New graphi te assembly; solid c o r e enclosed in s ta in less s tee l tube; National Carbon Co. graphite; hea t e r s controlled to give adiabatic conditions during runs

11-1 4 20 - - 600 520 11-2 4 20 565 11-3 4 20 650 11-4 4 20 7 00 11-5 4 20 500 11-6 4 20 560 11 -7 4 20

- - - - - - - - - - - -

600 -0. 58 520 +o. 20 565 + O . 08 650 +o. 33 700 + O . 42 505 + O . 25 565 +O. 67

-35-

APPENDIX I1

HW - 677 9 2

CALCULATION O F IGNITION TEMPERATURES IN THE EGCR BURNING RIG

Data D1 = 1 2 . 7 c m

D2 13 .3 c m

L 1 . 9 8 c m

Equivalent d iameter

De = D2 - D1 = 1 3 . 3 - 12.7 = 0. 63 c m

C r o s s sectional area of annulus

2 2 ( D i - D ) 13 .1 c m n A = m X 1

A i r p rope r t i e s evaluated at 540 C

= 0. 255 c a l / g C

p = 0.036 g / c m sec cP

Npr = 0. 690

k = 1. 41 x g - c a l / s e c c m 2 C / c m

p = l . 7 0 g / c m 3

The Reynold 's number for the g rea t e s t mass flow r a t e (55 l b / h r o r 24. 9 k g / h r ) is:

- (1 -3) = 930 - - - DG = DW = (2. 08 x lo- ' ) (55)

(8.82 x 1 0 - 2 ) ( l . 4 x l o m 2 ) NRe c!

Flow in all c a s e s may b e considered laminar ; hence, the heat t r a n s f e r coefficient will b e : (1 -4)

0 .4 0 . 8 0 .14 D 0. 05 0. 5 pa h = (NRe) o ' 4 5 D e (1. 02 Npr) (G) [e) NGr

(See Reference 4 )

HW - 67 792 -36-

The Grashof number i s :

An average t empera tu re difference of 1 0 0 C between the graphite and

the air may be reasonably assumed

t . = 540 C air = 43.9 NGr

Heat t r a n s f e r coefficients calculated on the b a s i s of the preceeding

data and Equation (1 -4 ) a r e presented in Table HI).

TABLE I11

REYNOLD'S NUMBERS AND HEAT TRANSFER COEFFICIENTS FOR

EGCR BURNING RIG EXPERIMENTS

M a s s Flow ( l b / h r ) A i r

4

8

10

1 2

1 9 55

Reynold ' s Number

68

135

169

203

321 930

Heat T r a n s f e r Coefficient g-ca l / sec-cm' - ' c x 104

1. 69

2 . 30

2. 53

2 . 7 5

3. 08 5 . 4 5

Cr i t ica l graphi te t empera tu res in the EGCR Burning Rig a r e calcu-

la ted and shown in Table I1 in the text.

Sample calculation is given f o r the c a s e of 55 l b / a i r flow

Data W = 55 l b / h r

y = 6. 97 x l o l o exp ( - 2 5 , 000/T) ( h r - l ) (EGCR graphi te react ing with air in the absence of radiation, s e e Reference 5)

-37 - HW-67792 n

h = 5.45 x g - c a l / s e c - c m 2 = OC

= 300 K = 3.27 x c a l l h r - c m 2 - O C ta

Assuming constant graphite tempera ture , Equation (1 2) may be applied

where: C = 2 5 , 0 0 0 K

P h / a - = 1. 91 x exp (-12: 500/T)

A = - Ph - - 0. 0252 c m - I WCP

L = 1. 98

then r 1

exp (12, 5 0 0 / T ) ( T - 300) exp (-4. 99) J 25J O o 0 = - I n ~ 1 . 91 x T

l 2 > 5 0 0 = 22.78 - I n (T - 300) T

T = 750 K = 477 C

Similar t r ea tmen t yielded the values for o ther a i r flows l is ted in Table 11.

A m o r e exact determination can be made by consider ing the actual

modera to r t empera tu re distribution which follows a chopped sinusoidal function in most ca ses , Hence, Equation ( 7 ) would be: 1

(1 -6) 1 dt

dx - a = A(T - t a ) = A [Tmax s in (bx + a) - ta

The solution is:

(1 - 7 ) - AT [ : A s in . (bx + a) - b COS (bx + a) 1 + c’~-AX

ta - A2 + b2

C’ can b e evaluated by considering the boundary condition

a t x = 0 ta = to

.@

t

-38-

Hence,

H W - 677 92

A T [A sin a - b cos a 1 C = t o - A 2 + b2

(1 -8)

Because the tempera ture . distribution can be represented a s shown schemat ic ally below

1 in i n - -a L

x- 0 L / 2 L+a

the t r igonment r ic p a r a m e t e r s can most easi ly be evaluated by consider ing

the f o r m of the function, and let

so

and

rr(x + a ) ( b x + a ) L + 2a

TT

L + 2a b =

rra L + 2a a =

A s an example, the c r i t i ca l t empera tu re for the EGCR Burning Rig

under the conditions of the preceding ana lys i s is recalculated with the actual

graphi te t empera tu re distribution considered. In the r i g the minimum temp-

e r a t u r e s which were at the ends of the column, were approximately 90 p e r

cent of the maximum which w a s a t the midplane, under s ta t ic conditions.

- 3 9 - HW - 67 7 9 2

There fo re :

a t x = O = sin (bx + a) = 0. 9 T’Tmax

s in a = 0. 9

a = 0 . 3 6 rr

and because rra

L + 2a a =

0. 36 L L = 1.98m 1 - 0 . 7 2 , a =

= 2. 5 5 m

then

= 4 .43

Values f o r A, - Ph and C a r e unchanged. a -

The c r i t i ca l t empera tu re can now be calculate( by sms t i tu t ing

Equation (1 -9 ) into equation (1 3) to obtain the t ranscendental relationship:

ATlA s in (bx+ a ) - b c o s (bx +a))+ ,-Axt AT(A s i n a - b cosa) 0 A2 + b 2 - 2T = - l n [ F [ T - [ A 2 + b 2

L

(1-10)

substi tuting values into 1-10 and solving fo r the case

x = L, = 300

12, 500 = 1 7 . 77 - I n (0 . lOOT - 2. 01) T

T = 7 8 0 K = 507 c The est imat ion of c r i t i ca l t empera tu re is thus r a i sed 30 C by consider ing the t empera tu re gradient existing in the graphite.

A -40- HW - 67792

REFERENCES

1.

2.

3 .

4.

5.

Dahl, R. E. F ina l Report - Experimental Evaluation of the Combustion

Hazard to the ExDerimental Gas Cooled Reac to r , HW-71 182. ~~~~ ~

To b e published.

Robinson, P. J. and J. C. Taylor . The rma l Instabil i ty Due t o Oxidation

of a Graphite Channel Carrying an Ai r Flow, TID-7597 BK-2: 453-503.

March, 1961.

Robinson, P. J. The Effects of Diffusional Control of Oxidation on the

Highest Safe Tempera tu re in A i r , TID-7597, BK-1: 414-451. March, 1961.

Jakob, M. Heat T r a n s f e r , Vol. 1: 551. New York: John Wiley & Sons.

1949.

Dahl, R. E. Oxidation of Reactor Graphite Under High Tempera tu re

Reac tor Conditions, HW-68493. July, 1961.

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Number of Copies 1 RAND Corporation 1 Rensse laer -Polytechnic Institute 1 Republic Aviation Corporation 1 1 Sandia Corporation, Albuquerque 1 1 Space Technology Labora tor ies (PRL) 1 States Marine Lines, Inc. 1 Stevens Institute of Technology 1 Surgeon General 1 Sylvania Elec t r ic Products , Inc. 1 Technical Research Group 1 Tennessee Valley Authority 1 Texas Nuclear Corporation 2 8 1 United Nuclear Corporation (NDA) 1 U. S. Geological Survey, Denver 1 U. S. Patent Office 1 University of California, Berkeley 2 University of California, L ivermore 1 University of Puer to Rico 1 University of Rochester 2 University of Rochester (Marshak) 1 Walter Reed A r m y Medical Center 1 Watertown Arsena l 4 Westinghouse Bet t i s Atomic Power Laboratory 2 Westinghouse Elec t r ic Corporation 1 Yankee Atomic Elec t r ic Company

R e s e a r ch Analysis Corporation

Schenectady Naval Reac tors Operations Off ice

Union Carbide Nuclear Company (ORGDP) Union Carbide Nuclear Company (ORNL)

3 2 5 Division of Technical Information Extension 7 5 Office of Technical Services , Washington

UC-80, 17th Ed. J

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