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Stored energy in the Oak Ridge graphite reactor—1960
Reactor
Science and Technology
(Journal
01 Nuclt~r
STORED ENERGY
Energy Parts A/B)
1961. Vol. 15. pp. 185 to 191.
IN THE OAK RIDGE
Pergamon
Press Ltd.
GRAPHITE
Printed
ia Nonhem
Ireland
REACTOR-1960
M. C. wIlTELF Solid State Division, Oak Ridge National Laboratory,’
Oak Ridge, Tennessee, U.S.A.
(&caked 20 January 1961) Ah&act-The stored-energy status of the Oak Ridge Graphite Reactor, which went into operation in November, 1943, has heen analysed following a low temperature anneal in which fission heat and a reverse air flow system were employed. From more than 100 post-annealing core samples, calorimetric studies revealed that the success of the annealing operation closely conforms to the maximum temperature piofiles reached during the anneal. Between 3+45 MWh of stored energy in a 15 ft diameter region was released during the pile operation, and the stored energy in that area was reduced to a level where a spontaneous release is now not possible. This was achieved with a maximum fuel element temperature of 275°C and a maximum graphite temperature of 236°C emphasizing the significance of the low temperature method. The peripheral regions of the fuel zone, including the outer three rows, did not reach sufficiently high temperatures for annealing primarily hecause of sharply dropping thermal gradients at the edges of the moderator stack. The future stored-energy status of the reactor is discussed.
INTRODUCTION
A CONTROLLED stored-energy release was conducted in September, 1960, in the world’s oldest operating reactor, the Oak Ridge Graphite Reactor. As a preliminary to this operation, graphite sample cores were removed from the moderator stack for storedenergy measurements. The purpose of these measurements was threefold: (1) to check on previously determined stored-energy growth rates, (2) to conduct low temperature annealing experiments as an aid in the upcoming annealing operation, and (3) to provide a direct analysis of the success of the annealing operation by means of a later comparison with postannealing measurements in these same areas. The thermal characteristics of the reactor (STANFORD, 1961) made it apparent that the reactor could be imagined to consist of two regions: (1) a core region approximately 15 ft in diameter that could be easily annealed by the previously proposed method (STANFORD, 1960) and (2) peripheral region encompassing the outer three fuel channel rows, in which it was recognized that significantly high annealing temperatures might not be easily reached. A typical channel in each of these regions was carefully scanned before and after the annealing operation and will, as shown later, depict the annealing behaviour of the entire reactor fairly accurately. More than 100 cores were taken from the moderator stack, post-anneal, to completely
analyse the stored-energy whole. METHOD
status in the reactor as a
OF STORED-ENERGY
MEASUREMENT
All of the stored-energy measurements described in this report were made by the radiation-calorimetry method (WECHSLER, 1959). A higher oil-bath temperature (24O’C) was employed than before, however, since this was a more suitable temperature for producing a quasi-adiabatic rise and, therefore, was more informative. The maximum temperature reached during the measurements ranged between 238°C and 300°C so that in no case did the quasi-adiabatic rise exceed the oil-bath temperature by more than 60°C. The integrated stored-energy release was obtained from calculations on the IBM 704 computer for each of the measurements.
* Oak Ridge National Laboratory is operated by Union Carbide Corporation for the United States Atomic Energy Commission. 185
PRE-ANNEALING
DATA
Cores that contained the largest amounts of stored energy were subsectioned and annealed at low temperatures slowly and subsequently measured for stored energy. The results confirmed previous observations that a low temperature soak (140”-165°C) greatly reduces the temperature rise in a subsequent spontaneous release of stored energy. In Fig. 1 the temperature-time curves for a typical experiment of this kind are shown. The annealed samples were heated at O*S”C/minute to the stated annealing temperatures and were then measured for stored energy in the usual manner. It should be pointed out that after the 161°C
186
M. c. WIll’ELS
-
.?a
250
230
” 0 ;
240
(90
420 y
z aog
f,
B
‘I
80 P
p :: 4s %” ato
5 4’ (70
g
604
9
40
5
20
z s ?
0
150
0
2
4
6
*
MSTAKE
I30
90
70
50
IL
0
2
4
6
8
FIG. I.-Time-temperature
(0 TIME.
(2
curve release.
(4 m,n
during
16
(8
20
0 12
14
16
18
ma.
22
stored-energy
anneal only 22.4 Cal/g remained up to 250°C and, therefore, a spontaneous release in this graphite would no longer be possible. From the time-temperature data shown in Fig. I, similar curves can be calculated showing the rate of energy release for the same three cases (Fig. 2). This analysis shows that at a heating rate of 45”C/minute the energy release rate, at peak, is 0.55 watts/g and persists for approximately 30 seconds. Stated in other terms, if the entire mass of damaged graphite in the reactor were heated at 45”C/minute in a quasi-adiabatic environment, a peak power release of approximately 50 MW could be sustained for about 30 seconds. This
20
22
24
n
FIG. 3.--Stored-energy distribution before annealing in channel 2167.
-
tt0
to
FRW EC3 Gv.P”liE
and
after
point is entirely academic, however, since these environmental conditions are not possible in this massive graphite structure. The energy-release curves for the low-temperature annealed samples only further emphasize the significance of a slow, low temperature anneal in reducing the dangers of a spontaneous release of stored energy. A typical profile of the stored-energy distribution before annealing for the core region channel, 2167, is shown in Fig. 3, together with a graphite temperature traverse. In this case the peak damage position and peak reversed-temperature position coincide and therefore present an ideal annealing configuration. A similar profile for the peripheral channel, 3068, is shown in Fig. 4 where it is evident that the damagetemperature protiles are not ideally suited for an annealing operation and, in addition, the peak temperatures are too low for adequate annealing. It should be emphasized that the graphite temperature profiles in these two channels were for conditions of reactor power on: air coolant flowing, and maximum fuel element temperatures, while the graphite temperatures were steady-state. 160
40
0
(50
3cm
450
600 TU,,E,
FIG. 2.-Rate of stored-energy graphite on immersion
750
9cxl
(0%
tzca
1350
0
0
sei
release in channel in 240-C oil bath.
2167
pile
FIG.
2
4
6 8 10 $2 14 O,S-dNCE f-=-w EAST GRAPF
0 LICE,
IS
4.--Stored-energy distribution before annealing in channel 3068.
PO
22
24
0
1,
and
after
pile
Stored energy in the Oak Ridge Graphite Reactor-1960
187
5t 52 53 54 55 56 57 58 59 60 6f 62 63 64 65 66 67 60 69 70 7i 72 73 74 75 76 77 78 79 80 81 82 83 04 65 86
?? CHANNEL SCANNED FOR STORED
ENERGY
06 R Looding Face FIG. 5.-Graphite
channels scanned for post-annealing
POST-ANNEALING DATA Following the annealing operation the stored-energy distributions in the two above-mentioned channels were again determined and are shown in Figs. 3 and 4. The reversed air-flow temperature profiles were taken during the annealing operation, but after the energy release, when steady-state conditions prevailed. As indicated in the pre-annealing discussion the operation was very successful in channel 2167, and in Channel 3068 it was apparent that only a minor degree of annealing had occurred. From 36 other channels in the stack (Fig. 5) stored energy distributions along the channels were determined and are shown graphically in Figs. 6 and 7 for
stored energy.
the two halves of the reactor. The stored energies shown are for the total energy released during the quasi-adiabatic run and are therefore not necessarily related to the identical temperature span. Several facts are evident in this scan: (1) annealing was successful in the 15 ft core region of the graphite moderator (Fig. 8), (2) annealing was unsuccessful in the peripheral region of the moderator, (3) stored energy in the west half of the moderator is nil except for small stringers that extend a few feet west of centre in the peripheral region only, (4) the peak damage position in the south peripheral area is slightly east of-that for the north peripheral region and is obviously due to the normal colder temperaturedistribution in the north half
M. C. WI~~ELS SO-
40 --
_.. P 5 30 __ >= B E tl (L 20 co m
lO----
to0
I-_
0
2
4
DISTANCE
Graphite
6
8
IO
FROM EAST GRAPHITE
Stored Energy Distribution Moderator Channels
12
14
FACE, in North.Half
16 ft of
FIG. 6.-Post-annealing stored-energy distribution in south half of graphite moderator measured at temperatures up to 300°C.
of the reactor, and (5) the stored-energy distribution in the north peripheral region is also somewhat broader for the same reason. The unusual curves for channels 2965 and 3068 are due to the proximity of control rod positions. The spatial distribution of stored energy capable of spontaneous release is shown in Fig. 9. This region presently contains approximately 3.5 MWh of releasable energy up to 250°C. Analysis of the annealed core region indicates that 3.54.5 MWh were released during the annealing operation. DISCUSSION
X-ray lattice parameter analysis (WUTH_S, 1957) had previously revealed that the stored-energy accumulation had extended into the outermost row of fuel channels on the north edge of the reactor (3268). It is therefore,apparent that the other three lateral graphite edges of the fuel region also have suffered similar damage since the fast flux and temperatures in all four of these peripheral areas are comparable. The discovery of stored-energy accumulations in the comers of the graphite moderator is not so well understood, however, and suggests that the fast flux distribution in the Oak Ridge Graphite Reactor might be somewhat different from the well-known thermal flux
2
4
DISTANCE
Graphite
6
8
IO
FROM EAST GRAPHITE
Stored Energy Distribution Moderotor Channels
(2 FACE, in Swth
14
16
ft Half of
FIG. ‘I.-Post-annealing stored-energy distribution in north half of graphite moderator3gOured at temperatures up to
1955) distribution (cosine) at the corners of the reactor. This is the case at the core-reflector interface but is of too low intensity to affect the fast flux by more than 15 per cent. It therefore seems more probable that the stored-energy growth rate at temperatures below 30°C may be as much as three times as high as the damage rates at 3O”C-100°C (DAVIDSON, 1958; WOODS, 1955). It further suggests that controlled radiation damage experiments with graphite at temperatures between 4°K and 300°K may provide interesting results. The overall effect of the annealing operation was a complete success from the standpoint of a low temperature procedure confined to narrow operational limits. For example, the maximum permissible fuel element temperature (285°C) was closely adhered to, and in fact only reached a maximum of 275°C. Of even greater significance were the heat transfer properties of the reactor during that period when stored energy was being released. Continuous recording from more than 300 thermocouple positions revealed that at no time during the stored-energy release, with reactor power on, was energy transferred from graphite to uranium; and, therefore, the reactor was under rigid control during the most critical annealing phase. Further, the
(RAMSEY,
Stored energy in
the Oak Ridge Graphite Reactor-1960
Front Face View; FIG. S.-Outline
of peripheral
Unonneoled
unannealed
rate of spontaneous release of stored energy was reduced to the extent that the maximum graphite temperature with reactor power on was only 203"C, and with the reactor scrammed at high temperature, and air flow blocked, this maximum recorded graphite temperature only reached 236°C. The reduction of intensity of a spontaneous release coupled wjth the subsequent use of fission heat as a power source for higher annealing temperatures is in sharp contrast to other annealing methods (DICKSON, 1958; CO~~RELL, 1958; RIMMER, 1959) where the kinetics of the stored-energy release itself are utilized as a major source of power. The final two stages of the annealing operation itself offer conclusive evidence of the continuing safe operation of the reactor with air flow in either direction. In the next-to-last stage, with fuel elements near maximum temperature, the reactor
189
Region
region on reactor loading face.
ran for more than twenty hours with a thermally steady-state condition. This means that a large mass of graphite and fuel elements was maintained at temperatures over 100°C higher than was maintained at any time during the reactor’s previous seventeen years’ history. During the final annealing operation with the reactor scrammed from a maximum temperature condition, and the air flow blocked, there was simulated what might be considered, academically, a maximum credible accident environment. As a result of this operation, as well as of the previous annealing steps, not a single fuel element was ruptured, and the maximum temperatures previously described were not exceeded. FUTURE STORED-ENERGY STATUS The evidence for the presently safe stored-energy
190
M.
C.
WI-ITELS
Stored energy in the Oak Ridge Graphite Reactor-1968
191
status in the Oak Ridge Graphite Reactor has already exceeded. If the reactor were reloaded with fuel been discussed. As for the future stored-energy status, elements capable of safe operation at 35O”C,then there it will be assumed that the reactor is to be operated seems to be very little doubt that sufficiently high continuously and indefinitely. It is evident that the annealing temperatures could be attained rather easily. 15 ft core region offers no great difficulties because As a routine operating measure it is believed that the it is within a region of easily attainable annealing reactor should be placed in a bake-out condition from temperature (Fig. 3) now that the reactor has a the reverse flow operation, perhaps annually, to attenupermanently installed reverse air-flow system. The ate the accumulation of stored energy throughout the unannealed peripheral regions (Figs. 4, 8 and 9) will reactor and thereby reduce the higher temperature continue to accumulate stored energy at the rate of stored-energy growth as well. This should be a approximately 2 cal/g/yr up to 250°C. That this does routine experience now that the stored-energy content not constitute a great hazard can be concluded for has been largely eliminated in the central regions of several reasons. The already cited evidence for the the reactor core. initiation of annealing (Fig. 4) in a peripheral channel indicates that a longer bake-out period following a Acknowle&ments-It is a pleasure to thank F. A. SH~RRILLfor his assistance during the stored-energy measurements and M. T. scram condition, say 16-20 hours, might produce ROBINSONfor programming the data for IBM-7W computation. sufficiently high annealing temperatures in the periphMany personnel of the Operations Division contributed to eral area. This probability can be further increased the extensive remote coring programme and the author is by reducing the thermal losses to air flowing in the especially indebted to L. E. STANFORD,who directed the annealTheir unstinting efforts reflector channels by restricting the air flow in these ing operation, and to C. B. Gm. made this report possible. channels. Also, the shifting of higher temperatures to the peripheral region could also be accomplished by REFERENCES orificing designated fuel channels. If these efforts do COVERALLA. H., BELL J. C., GRJZENOUGH G. B., LoW. M. not succeed in annealing the outermost fuel channels, and SIMMONSJ. H. W. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic no great hazard presents itself in any event, since Energy, Geneva, P/2485,7, p. 315. United Nations, N.Y. release temperatures would therefore be virtually DAVIDSON J. M. (1958) Stored Energy in Irradiated Graphite. impossible to achieve under any conditions. It might HW-55736. be noted at this point that concern of a ‘thermal wave’ DICKMN J. L., KMCHIN G. H., JACKSONR. F., LOMERW. M. occurring through an accidental stored-energy release and SIMMONSJ. H. W. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic during normal reactor operations is unrealistic if an Energy, Geneva, P/1805,7, p. 250. United Nations, N.Y. adequate air coolant flow is available. During the RA~.BEYM. E. and CAGLE C. D. (1955) Proceedings of the First annealing operation it was strongly apparent that with Znternational Conference on the Peaceful Uses of Atomic coolant air flowing at 60 per cent of normal capacity Energy, Geneva, P/486,2, p. 281. United Nations, N.Y. and stored energy being released, the thermal resist- RIMMERD. E. (1959) The Validity of the Constant Activation Energy Model for the Release of Stored Energy in Graphite. ance at the boundaries of individual graphite blocks AERE-R 3061. was so great as to prevent any large-scale heat transfer from graphite block to graphite block. Instead, the STANFORD L. E. (1960) Safeguard Report on the Proposed Method of Annealing Graphite in the X-10 Reactor. ORNLheat transfer due to the stored-energy release was 2725. mainly to the coolant air stream and along the STANFORDL. E. (1961) To be published. WECHSLER M. S., C~LTMAN R. R., KJXNOHAN R. H. and unbroken graphite stringers themselves and therefore Wtrrsrs M. C. (1959) J. appl. Phys. 30 (l), 42. is narrowly confined in space. In addition, the effecW~LS M. C. (1957) Evaluation of the ORNL Graphite Reactor. tively dispersed nature of the peripheral area is an TID-7565 (Pt. I), 64-81. isolating feature in itself. WEEDS W. K., BURP L. P. and FLETCHER J. F. (1955)ProceeaYngs This discussion has assumed that the present maxiof the First International Conference on the Peaceful uses of mum fuel element temoeratures (285°C) would not be Atomic Enerpv. Geneva, Pl746. United Nations, N.Y.
Science and Technology
(Journal
01 Nuclt~r
STORED ENERGY
Energy Parts A/B)
1961. Vol. 15. pp. 185 to 191.
IN THE OAK RIDGE
Pergamon
Press Ltd.
GRAPHITE
Printed
ia Nonhem
Ireland
REACTOR-1960
M. C. wIlTELF Solid State Division, Oak Ridge National Laboratory,’
Oak Ridge, Tennessee, U.S.A.
(&caked 20 January 1961) Ah&act-The stored-energy status of the Oak Ridge Graphite Reactor, which went into operation in November, 1943, has heen analysed following a low temperature anneal in which fission heat and a reverse air flow system were employed. From more than 100 post-annealing core samples, calorimetric studies revealed that the success of the annealing operation closely conforms to the maximum temperature piofiles reached during the anneal. Between 3+45 MWh of stored energy in a 15 ft diameter region was released during the pile operation, and the stored energy in that area was reduced to a level where a spontaneous release is now not possible. This was achieved with a maximum fuel element temperature of 275°C and a maximum graphite temperature of 236°C emphasizing the significance of the low temperature method. The peripheral regions of the fuel zone, including the outer three rows, did not reach sufficiently high temperatures for annealing primarily hecause of sharply dropping thermal gradients at the edges of the moderator stack. The future stored-energy status of the reactor is discussed.
INTRODUCTION
A CONTROLLED stored-energy release was conducted in September, 1960, in the world’s oldest operating reactor, the Oak Ridge Graphite Reactor. As a preliminary to this operation, graphite sample cores were removed from the moderator stack for storedenergy measurements. The purpose of these measurements was threefold: (1) to check on previously determined stored-energy growth rates, (2) to conduct low temperature annealing experiments as an aid in the upcoming annealing operation, and (3) to provide a direct analysis of the success of the annealing operation by means of a later comparison with postannealing measurements in these same areas. The thermal characteristics of the reactor (STANFORD, 1961) made it apparent that the reactor could be imagined to consist of two regions: (1) a core region approximately 15 ft in diameter that could be easily annealed by the previously proposed method (STANFORD, 1960) and (2) peripheral region encompassing the outer three fuel channel rows, in which it was recognized that significantly high annealing temperatures might not be easily reached. A typical channel in each of these regions was carefully scanned before and after the annealing operation and will, as shown later, depict the annealing behaviour of the entire reactor fairly accurately. More than 100 cores were taken from the moderator stack, post-anneal, to completely
analyse the stored-energy whole. METHOD
status in the reactor as a
OF STORED-ENERGY
MEASUREMENT
All of the stored-energy measurements described in this report were made by the radiation-calorimetry method (WECHSLER, 1959). A higher oil-bath temperature (24O’C) was employed than before, however, since this was a more suitable temperature for producing a quasi-adiabatic rise and, therefore, was more informative. The maximum temperature reached during the measurements ranged between 238°C and 300°C so that in no case did the quasi-adiabatic rise exceed the oil-bath temperature by more than 60°C. The integrated stored-energy release was obtained from calculations on the IBM 704 computer for each of the measurements.
* Oak Ridge National Laboratory is operated by Union Carbide Corporation for the United States Atomic Energy Commission. 185
PRE-ANNEALING
DATA
Cores that contained the largest amounts of stored energy were subsectioned and annealed at low temperatures slowly and subsequently measured for stored energy. The results confirmed previous observations that a low temperature soak (140”-165°C) greatly reduces the temperature rise in a subsequent spontaneous release of stored energy. In Fig. 1 the temperature-time curves for a typical experiment of this kind are shown. The annealed samples were heated at O*S”C/minute to the stated annealing temperatures and were then measured for stored energy in the usual manner. It should be pointed out that after the 161°C
186
M. c. WIll’ELS
-
.?a
250
230
” 0 ;
240
(90
420 y
z aog
f,
B
‘I
80 P
p :: 4s %” ato
5 4’ (70
g
604
9
40
5
20
z s ?
0
150
0
2
4
6
*
MSTAKE
I30
90
70
50
IL
0
2
4
6
8
FIG. I.-Time-temperature
(0 TIME.
(2
curve release.
(4 m,n
during
16
(8
20
0 12
14
16
18
ma.
22
stored-energy
anneal only 22.4 Cal/g remained up to 250°C and, therefore, a spontaneous release in this graphite would no longer be possible. From the time-temperature data shown in Fig. I, similar curves can be calculated showing the rate of energy release for the same three cases (Fig. 2). This analysis shows that at a heating rate of 45”C/minute the energy release rate, at peak, is 0.55 watts/g and persists for approximately 30 seconds. Stated in other terms, if the entire mass of damaged graphite in the reactor were heated at 45”C/minute in a quasi-adiabatic environment, a peak power release of approximately 50 MW could be sustained for about 30 seconds. This
20
22
24
n
FIG. 3.--Stored-energy distribution before annealing in channel 2167.
-
tt0
to
FRW EC3 Gv.P”liE
and
after
point is entirely academic, however, since these environmental conditions are not possible in this massive graphite structure. The energy-release curves for the low-temperature annealed samples only further emphasize the significance of a slow, low temperature anneal in reducing the dangers of a spontaneous release of stored energy. A typical profile of the stored-energy distribution before annealing for the core region channel, 2167, is shown in Fig. 3, together with a graphite temperature traverse. In this case the peak damage position and peak reversed-temperature position coincide and therefore present an ideal annealing configuration. A similar profile for the peripheral channel, 3068, is shown in Fig. 4 where it is evident that the damagetemperature protiles are not ideally suited for an annealing operation and, in addition, the peak temperatures are too low for adequate annealing. It should be emphasized that the graphite temperature profiles in these two channels were for conditions of reactor power on: air coolant flowing, and maximum fuel element temperatures, while the graphite temperatures were steady-state. 160
40
0
(50
3cm
450
600 TU,,E,
FIG. 2.-Rate of stored-energy graphite on immersion
750
9cxl
(0%
tzca
1350
0
0
sei
release in channel in 240-C oil bath.
2167
pile
FIG.
2
4
6 8 10 $2 14 O,S-dNCE f-=-w EAST GRAPF
0 LICE,
IS
4.--Stored-energy distribution before annealing in channel 3068.
PO
22
24
0
1,
and
after
pile
Stored energy in the Oak Ridge Graphite Reactor-1960
187
5t 52 53 54 55 56 57 58 59 60 6f 62 63 64 65 66 67 60 69 70 7i 72 73 74 75 76 77 78 79 80 81 82 83 04 65 86
?? CHANNEL SCANNED FOR STORED
ENERGY
06 R Looding Face FIG. 5.-Graphite
channels scanned for post-annealing
POST-ANNEALING DATA Following the annealing operation the stored-energy distributions in the two above-mentioned channels were again determined and are shown in Figs. 3 and 4. The reversed air-flow temperature profiles were taken during the annealing operation, but after the energy release, when steady-state conditions prevailed. As indicated in the pre-annealing discussion the operation was very successful in channel 2167, and in Channel 3068 it was apparent that only a minor degree of annealing had occurred. From 36 other channels in the stack (Fig. 5) stored energy distributions along the channels were determined and are shown graphically in Figs. 6 and 7 for
stored energy.
the two halves of the reactor. The stored energies shown are for the total energy released during the quasi-adiabatic run and are therefore not necessarily related to the identical temperature span. Several facts are evident in this scan: (1) annealing was successful in the 15 ft core region of the graphite moderator (Fig. 8), (2) annealing was unsuccessful in the peripheral region of the moderator, (3) stored energy in the west half of the moderator is nil except for small stringers that extend a few feet west of centre in the peripheral region only, (4) the peak damage position in the south peripheral area is slightly east of-that for the north peripheral region and is obviously due to the normal colder temperaturedistribution in the north half
M. C. WI~~ELS SO-
40 --
_.. P 5 30 __ >= B E tl (L 20 co m
lO----
to0
I-_
0
2
4
DISTANCE
Graphite
6
8
IO
FROM EAST GRAPHITE
Stored Energy Distribution Moderator Channels
12
14
FACE, in North.Half
16 ft of
FIG. 6.-Post-annealing stored-energy distribution in south half of graphite moderator measured at temperatures up to 300°C.
of the reactor, and (5) the stored-energy distribution in the north peripheral region is also somewhat broader for the same reason. The unusual curves for channels 2965 and 3068 are due to the proximity of control rod positions. The spatial distribution of stored energy capable of spontaneous release is shown in Fig. 9. This region presently contains approximately 3.5 MWh of releasable energy up to 250°C. Analysis of the annealed core region indicates that 3.54.5 MWh were released during the annealing operation. DISCUSSION
X-ray lattice parameter analysis (WUTH_S, 1957) had previously revealed that the stored-energy accumulation had extended into the outermost row of fuel channels on the north edge of the reactor (3268). It is therefore,apparent that the other three lateral graphite edges of the fuel region also have suffered similar damage since the fast flux and temperatures in all four of these peripheral areas are comparable. The discovery of stored-energy accumulations in the comers of the graphite moderator is not so well understood, however, and suggests that the fast flux distribution in the Oak Ridge Graphite Reactor might be somewhat different from the well-known thermal flux
2
4
DISTANCE
Graphite
6
8
IO
FROM EAST GRAPHITE
Stored Energy Distribution Moderotor Channels
(2 FACE, in Swth
14
16
ft Half of
FIG. ‘I.-Post-annealing stored-energy distribution in north half of graphite moderator3gOured at temperatures up to
1955) distribution (cosine) at the corners of the reactor. This is the case at the core-reflector interface but is of too low intensity to affect the fast flux by more than 15 per cent. It therefore seems more probable that the stored-energy growth rate at temperatures below 30°C may be as much as three times as high as the damage rates at 3O”C-100°C (DAVIDSON, 1958; WOODS, 1955). It further suggests that controlled radiation damage experiments with graphite at temperatures between 4°K and 300°K may provide interesting results. The overall effect of the annealing operation was a complete success from the standpoint of a low temperature procedure confined to narrow operational limits. For example, the maximum permissible fuel element temperature (285°C) was closely adhered to, and in fact only reached a maximum of 275°C. Of even greater significance were the heat transfer properties of the reactor during that period when stored energy was being released. Continuous recording from more than 300 thermocouple positions revealed that at no time during the stored-energy release, with reactor power on, was energy transferred from graphite to uranium; and, therefore, the reactor was under rigid control during the most critical annealing phase. Further, the
(RAMSEY,
Stored energy in
the Oak Ridge Graphite Reactor-1960
Front Face View; FIG. S.-Outline
of peripheral
Unonneoled
unannealed
rate of spontaneous release of stored energy was reduced to the extent that the maximum graphite temperature with reactor power on was only 203"C, and with the reactor scrammed at high temperature, and air flow blocked, this maximum recorded graphite temperature only reached 236°C. The reduction of intensity of a spontaneous release coupled wjth the subsequent use of fission heat as a power source for higher annealing temperatures is in sharp contrast to other annealing methods (DICKSON, 1958; CO~~RELL, 1958; RIMMER, 1959) where the kinetics of the stored-energy release itself are utilized as a major source of power. The final two stages of the annealing operation itself offer conclusive evidence of the continuing safe operation of the reactor with air flow in either direction. In the next-to-last stage, with fuel elements near maximum temperature, the reactor
189
Region
region on reactor loading face.
ran for more than twenty hours with a thermally steady-state condition. This means that a large mass of graphite and fuel elements was maintained at temperatures over 100°C higher than was maintained at any time during the reactor’s previous seventeen years’ history. During the final annealing operation with the reactor scrammed from a maximum temperature condition, and the air flow blocked, there was simulated what might be considered, academically, a maximum credible accident environment. As a result of this operation, as well as of the previous annealing steps, not a single fuel element was ruptured, and the maximum temperatures previously described were not exceeded. FUTURE STORED-ENERGY STATUS The evidence for the presently safe stored-energy
190
M.
C.
WI-ITELS
Stored energy in the Oak Ridge Graphite Reactor-1968
191
status in the Oak Ridge Graphite Reactor has already exceeded. If the reactor were reloaded with fuel been discussed. As for the future stored-energy status, elements capable of safe operation at 35O”C,then there it will be assumed that the reactor is to be operated seems to be very little doubt that sufficiently high continuously and indefinitely. It is evident that the annealing temperatures could be attained rather easily. 15 ft core region offers no great difficulties because As a routine operating measure it is believed that the it is within a region of easily attainable annealing reactor should be placed in a bake-out condition from temperature (Fig. 3) now that the reactor has a the reverse flow operation, perhaps annually, to attenupermanently installed reverse air-flow system. The ate the accumulation of stored energy throughout the unannealed peripheral regions (Figs. 4, 8 and 9) will reactor and thereby reduce the higher temperature continue to accumulate stored energy at the rate of stored-energy growth as well. This should be a approximately 2 cal/g/yr up to 250°C. That this does routine experience now that the stored-energy content not constitute a great hazard can be concluded for has been largely eliminated in the central regions of several reasons. The already cited evidence for the the reactor core. initiation of annealing (Fig. 4) in a peripheral channel indicates that a longer bake-out period following a Acknowle&ments-It is a pleasure to thank F. A. SH~RRILLfor his assistance during the stored-energy measurements and M. T. scram condition, say 16-20 hours, might produce ROBINSONfor programming the data for IBM-7W computation. sufficiently high annealing temperatures in the periphMany personnel of the Operations Division contributed to eral area. This probability can be further increased the extensive remote coring programme and the author is by reducing the thermal losses to air flowing in the especially indebted to L. E. STANFORD,who directed the annealTheir unstinting efforts reflector channels by restricting the air flow in these ing operation, and to C. B. Gm. made this report possible. channels. Also, the shifting of higher temperatures to the peripheral region could also be accomplished by REFERENCES orificing designated fuel channels. If these efforts do COVERALLA. H., BELL J. C., GRJZENOUGH G. B., LoW. M. not succeed in annealing the outermost fuel channels, and SIMMONSJ. H. W. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic no great hazard presents itself in any event, since Energy, Geneva, P/2485,7, p. 315. United Nations, N.Y. release temperatures would therefore be virtually DAVIDSON J. M. (1958) Stored Energy in Irradiated Graphite. impossible to achieve under any conditions. It might HW-55736. be noted at this point that concern of a ‘thermal wave’ DICKMN J. L., KMCHIN G. H., JACKSONR. F., LOMERW. M. occurring through an accidental stored-energy release and SIMMONSJ. H. W. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic during normal reactor operations is unrealistic if an Energy, Geneva, P/1805,7, p. 250. United Nations, N.Y. adequate air coolant flow is available. During the RA~.BEYM. E. and CAGLE C. D. (1955) Proceedings of the First annealing operation it was strongly apparent that with Znternational Conference on the Peaceful Uses of Atomic coolant air flowing at 60 per cent of normal capacity Energy, Geneva, P/486,2, p. 281. United Nations, N.Y. and stored energy being released, the thermal resist- RIMMERD. E. (1959) The Validity of the Constant Activation Energy Model for the Release of Stored Energy in Graphite. ance at the boundaries of individual graphite blocks AERE-R 3061. was so great as to prevent any large-scale heat transfer from graphite block to graphite block. Instead, the STANFORD L. E. (1960) Safeguard Report on the Proposed Method of Annealing Graphite in the X-10 Reactor. ORNLheat transfer due to the stored-energy release was 2725. mainly to the coolant air stream and along the STANFORDL. E. (1961) To be published. WECHSLER M. S., C~LTMAN R. R., KJXNOHAN R. H. and unbroken graphite stringers themselves and therefore Wtrrsrs M. C. (1959) J. appl. Phys. 30 (l), 42. is narrowly confined in space. In addition, the effecW~LS M. C. (1957) Evaluation of the ORNL Graphite Reactor. tively dispersed nature of the peripheral area is an TID-7565 (Pt. I), 64-81. isolating feature in itself. WEEDS W. K., BURP L. P. and FLETCHER J. F. (1955)ProceeaYngs This discussion has assumed that the present maxiof the First International Conference on the Peaceful uses of mum fuel element temoeratures (285°C) would not be Atomic Enerpv. Geneva, Pl746. United Nations, N.Y.