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
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-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
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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
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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
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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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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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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
I I
FIGURE 10
Therma l Behavior in Experiment 1 0 - 13A Conducted with P red r i ed Ai r
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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.
-41 - HW - 67 79 2
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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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