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Distributions of 134Cs and 137Cs in the axial UO2 blankets of irradiated (U,Pu)O2 fuel pins
J. inorg, nucl. Chem., 1974,Vol. 36, pp. 17 23. PergamonPress.Printedin Great Britain.
DISTRIBUTIONS OF 134Cs AND 137Cs IN THE AXIAL U O 2 BLANKETS OF IRRADIATED (U,Pu)O 2 FUEL PINS J. R. PHILLIPS, G. R. WATERBURY and N. E. VANDERBORGH* Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (First received 4 October 1972; in revised form 16 March 1973)
Abstract--The axial distributions of 134Cs and t 37Cs in the axial blanket column of uranium-plutonium oxide fuel pins showed that the migration properties of the precursors of these two Cs isotopes were significantly different. The isotopic distributions were directly related to the half-lives of the two principal precursors of each cesium isotope, t33I and 13aXe for 134Cs, and 1371 and t37Xe for 137Cs. Both cesium isotopes preferentially collected on natural UO 2 blanket pellets having high oxygen content.
INTRODUCTION
to 90.45 per cent. The materials used in the pins designated as A, B, C, D and E are summarized in Table 1. Gross gamma scans were obtained on each of the five fuel pins to determine anomalous regions and relative gross gamma activities as a function of discrete position. In this mode of data collection, the fuel pin was moved axially in pre-determined increments and the gamma rays with energies greater than 100 keV were counted from an area precisely defined by a lead collimating slit. A detailed description of the equipment and methods of data processing used in this investigation have been presented before[l]. As the gross gamma scans of fuel pins A and B (Figs. 1 and 2) showed high gross gamma activity over the axial UO2 blanket columns, the two blanket columns were removed from the pins for further examination. Each column was sealed in an inert atmosphere. The two UO 2 blanket columns and the three blanket columns of pins C, D and E were also scanned in the multispectral mode of data collection to identify the principal isotopes contributing to the high gross gamma activities in these regions. The multispectral scanning consisted of collecting complete gammaray spectra between 0 and 1500 keV from precisely defined areas of the axial blankets[l]. The spectra were unfolded using a summation function of a Gaussian and three exponential functions to determine the net areas of the full-energy gamma-ray peaks[2] which are directly proportional to the amount of the specific isotope present in the defined volume segment of the sample. The net areas were plotted as a function of position, resulting in isotopic distributions of the specific fission products. Selected UO2 pellets from pins A and B were examined to determine their total oxygen contents. To assure adequate sample for repeated determinations, two or three pellets from an area having relatively large amounts of cesium (pellets 2, 4 and 5 from pin A) were combined as one sample, as were pellets 3 and 7 which had smaller amounts of cesium. These two samples were analyzed repeatedly for oxygen by a highly reliable method in which weighed portions were reacted at 2000-2200°C with carbon to convert the oxygen to carbon monoxide and carbon dioxide which were
IN STUDYING cesium distributions by precision gamma scanning in fast reactor fuel pins, differences in relative concentrations of 134Cs and 137Cs were found as a function of axial position. This was especially true for measurements of the two cesium isotopes in the natural UO2 axial blankets of five u r a n i u m - p l u t o n i u m oxide fuel pins irradiated in EBR-II to about 6 a t m burnup with a peak heat generation rate of 15 kW/ft. Evidence was obtained that cesium was concentrating selectively on specific UO2 pellets and, also that the 134Cs and 137Cs distributions were significantly different. To determine the effect of pellet oxygen content on cesium distributions, selected UO2 pellets were destructively analyzed for their total oxygen contents. The relative cesium concentrations of individual pellets were correlated to the measured total oxygen contents. EXPERIMENTAL Each of the five fuel pins contained a short UO 2 insulator pellet, a 14.2-in mixed (U0.sPu0.2)O 2 fuel column and a 6.25-in natural UO2 upper axial blanket column. The upper end of the pellet column was held in place by means of an extensiometer spring and spacer tube. A gas plenum, about 14.0-in long, was located above the axial blanket column. There were three types of fuel pellets in the five pins: master mix, mechanically blended and coprecipitated material. Each fuel column contained two of these types with one type in the top half and another type in the bottom half. The initial oxygen-to-metal atom ratios (O/M) varied from 1-94 to 2.00 and the theoretical densities varied from 82'38 * The University of New Mexico, Albuquerque, N.M. This work was sponsored by the Fuels and Materials Branch of the Division of Reactor Development and Technology of the U.S. Atomic Energy Commission. 17
J. R. PHILLIPS,G. R. WATERBURYand N. E. VANDERBORGH
18
Table 1 Pin
Section
O/M
~o Theo. density
Feed material
A
Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom
1'94 1.96 2-00 1-99 1.97 1.96 1.97 .1.99 2-00 2.00
82.38 82"85 83.55 82.40 90.31 90.13 89.90 90.45 89"59 88'02
Coprecipitated Mechanically blended Mechanically blended Coprecipitated Master mix Mechanically blended Mechanically blended Master mix Mechanically blended Master mix
B C D E
measured gravimetrically[3]. As the measured total oxygen content of pellets 2, 4 and 5 was slightly higher than that of pellets 3 and 7, the results indicated that the amount of cesium absorbed on a UO2 pellet might be related to the amount of oxygen present. To check this observation, three samples of two pellets each from pin A and three samples from pin B were analyzed for total oxygen content. Conditions to avoid oxygen contamination of these pellets were carefully maintained, and the method was calibrated using certified standards. The results for the latter samples are given in Table 2. RESULTS AND DISCUSSION T h e complete gamma-ray spectra indicated that the two cesium isotopes were the dominant gammaemitting isotopes in the UO2 blanket regions in pins A and B. The 134Cs activity generally increased as the distance from the enriched fuel column increased. The dashed lines have been drawn to accentuate this general trend. In pin A (Fig. 3) the 137Cs activity decreased as the distance from the fuel column increased, in a manner opposite to that for the x34Cs isotope. The migration of 137Cs to the blanket in pin B (Fig. 4) was much less. The slight activity around 16"0in showed that 137Cs had migrated only to the first two blanket pellets. This lower migration of x37Cs to the blanket in pin B as compared to pin A correlates well
with the lower O/M atom ratio of the fuel in pin A (Table 1) which would result in more movement of the volatile metallic cesium. It was obvious that the two isotopes collected preferentially on specific UO2 pellets. From the cesium isotopic distributions and the results of oxygen measurements, Table 2, it was concluded that the amount of 134Cs adsorbed on the natural UO2 blanket pellets was directly related to the oxygen contents of the pellets. The 137Cs distribution in pin A, Fig. 3, was also directly related to the oxygen content of the UO2 blanket pellets. The 134Cs and aaVCs distributions for fuel pins C, D and E are shown in Figs. 5, 6 and 7, respectively. The top section of the natural UO2 blanket region is shown in each figure. The two isotopes exhibited distinctively different distributions with the 137Cs activity being related to the positions of the UO2 blanket pellets and the 134Cs isotope concentrating at specific locations in the blanket region. As 134Cs was produced from the neutron capture reaction of 133Cs, the 134Cs distribution was dependent upon the variations in the thermal neutron flux profile. A monitor of the thermal flux profile is the relative 6°C0 activity, and the fast flux profile is shown by the distribution of the threshold monitor, 54Mn. As 6°C0 and S4Mn are activation products of the stainless steel
I
Fuel colurn~
103
i
i0 z
Gos plenum
:
i.i~ir.; !
-
j~
~ r.~._r
I°~ o
......,., ,~.;~,~-"~ ~ ~
.r:..~:.1
!.
"
8"0
160 24"0 AxioI posit"ion, ill
320
40'0
Fig. 1. The gross gamma activity as a function along the principal axis of fuel pin A. This scan shows unusually high activity in the region of the axial UO2 blanket (17.0-22.0 in).
Distributions of 134Cs and 137Cs in fuel pins
19
io4
column
Axial blanket
#
;!:~i:, ]
Gas Henum
"~
~,'r~.~_;
,jj
~Oz
~! ; , ~ ,
io' ,o:'~ :::" :'
8"0
16 o
24"0
Axi a l
position,
520
40"0
in
Fig. 2. The gross gamma activity as a function of axial position of fuel pin B. This scan shows unusually high activity in the axial UO 2 blanket region (17.0-22.0 in). cladding; the relative flux profiles were determined by plotting the two isotopic distributions. The distributions were relatively smooth over the scanned regions. The higher 6°Co activity in pin E around 17'0in (Fig. 8), than at other locations was due to a short stainless steel spacer located between the top pellet and second pellet. The effect of the spacer can be seen in the 1arCs distributions for pins C, D and E (Figs. 5, 6 and 7, respectively). The thermal neutron flux profile, illustrated by the 6°Co isotopic distribution in Fig. 8, corresponded with the gradient observed 6000'
I
I
I
in the 134Cs distributions for pins A and B (Figs. 3 and 4). Similar gradients were evident in pins C, D and E. The precursors of 134Cs and 137Cs were then considered in seeking an explanation for their different distributions. The two cesium isotopes result from tie beta decay of precursor fission products in the following ways: 134Cs ~ #~331
#-
Cs 134(796keVJ.
~4 ~ - Gas plenum 5000
#-
--~ ~33Xe _-> 133Cs(n,~ ) ~34Cs --~ ~34Ba 21h 5-27d 2-1y
T
1
Fuelcc~ulnn
#
u 5 ~
8
I
--
"'
IE
4000
I
5000
15
2000
I000
~ ,20,000~
Gas plenum
~
z 90,000
/
60,000 30,000
2 22
0
9
I1,0
12.0
Axial
13-0
posifion,
14.c
150
16.0
17+0
in
Fig. 3. The isotopic distributions of 134Cs and 1arCs over the axial blanket region of the fuel pin A. Note the significantly different distributions of the two isotopes. (The error bars for each point are drawn in the plots). The UO 2 pellets are numbered consecutively from the spacer towards the fuel.
J. R.
20
PHILLIPS,
G. R. WATERBURYand N. E. VANDERBORGH Table 2
Fuel pin
Sample
A
Pellet numbers
Oxygen (wt. %)
~34Csactivity
1
16, 17
2 3
21, 22 23, 25
11.87 + 0-03 11.64 + 0.03 11.81 + 0.03
high low high
4 5 6
1,2 4,7 5,6
11.72 _ 0.03 11.89 + 0.03 11.73 _ 0.03
low high low
entially adsorbed on the natural UO2 blanket pellets with high oxygen contents. The average oxygen-tometal atom ratio of the enriched fuel material in fuel pin A was 1.95. The O/M atom ratio for the fuel column in fuel pin B was significantly higher, 1.99. The oxygen content of the original fuel material in pin B may have been sufficiently high to permit the laTCs to form an oxide and remain in the enriched fuel column as was indicated by the very low activity of 137Cs in the UO2 blanket shown in Fig. 4. The O/M atom ratios in fuel pins C, D and E (1-97, 1'98 and 2.00, respectively) also were relatively higher than the 1-95 O/M ratio of fuel pin A. It was concluded that in the higher O/M fuels most of the 137Cs remained in the fuel region probably as an oxide. The respective ~37Cs isotopic distributions (Figs. 5, 6 and 7) of the blanket regions showed relatively
137Cs: ##.a1371 ~ 137Xe ._+ 137Cs ~ 137rnBa ~ 24s
4.2m
30.Oy
137Ba '
2-6m
For the la4Cs isotopic distributions the net areas of three gamma-ray peaks (566, 604.6 and 795"8 keV) were calculated and plotted. Only the gamma-ray peak at 661"6 keV was plotted for ~aVCs. Consideration of the short half-lives of the precursors of ~37Cs and the ~a7Cs isotopic distribution of pin A (Fig. 3) led to the conclusion that either cesium or a cesium complex was the principal migrating species foi" 137Cs. The la7Cs apparently did not migrate as one of the two principal precursors. Once the cesium had moved from the enriched fuel column, it prefer-
5ooo
i
134Cs (796 keY) r
'
<--Gas plenum
I
[
Fuel column
I
--~
°°I" I .ooor
l[
zoooI
7-1~1 &
100o
c
t)
0 50,000
I
I
'~'Cs (662 keY)
< - Gas plenum
Fuel column
I
40,000
"-)
I
30,000
20,000
I0,000 /
1 2 5 0 9"0
I0"0
o5o 1
I1'0
12'0 Axial
F!g. 4. The
134
Cs and
137
_ j,,I t3"O position,
14"0
15"0
16"0
17'0
in
Cs isotopic distributions over the axial blanket region of the fuel pin B. Note the
absence of la7Cs except for the region next to the enriched fuel column. (The error bars for each point are drawn on the plots). The UO2 pellets are numbered consecutively from the spacer towards the fuel.
Distributions of 13¢Cs and 137Cs in fuel pins I~Cs( 796 keV ) I I
21
75001
Gosplenum
I
Fuel column "~
6000
4500
:/
3000
1500
0 5000
~ '37Cs (662 keY ) Gas plenum
Fuel column -->
1
4000
3000
2000
I000
J 5"5
6.0
6'5 Axial
70
7'5
position,
in
8'0
8'5
Fig. 5. The 134Cs and laTCs isotopic distributions of pin C showing the differences in the distributions. The 137Cs activity appears to be the result of fissions within the UO2 and the 134Cs activity from the deposition of isoto ,es that have migrated from the fuel matrix. J
I0,000
~- Gas
B4Cs (796keY)
ptenum
Fuel column "->
8000 i 6000
I
4000
!.
2000
~ 0
0
f"
2000
~cs IbbE keV
I
~- Gas plenum 1600
Fuel column
"~
1200
800
400
o
16"0
Lu
170
18'0 Axial
position,
190
20"0
2bO
in
Fig. 6. The 134Cs and t3VCs isotopic distributions of pin D showing the differences in the distributions. The 137Cs activity appears to be the result of fissions within the UO2 and the 13'*Cs activity from the deposition of isotopes that have migrated from the fuel matrix.
22
J.R. PHILLIPS,G. R. WATERBURYand N. E. VANDERBORGH Io,ooo
3~s 796keV
r
1
Gas ple~m
Fuel column
8 000
i
I
6 000
4 000
2
O01
4000
0
I ~ Gasplenum
'S~s(662 keY)
I Fuel column --~
3200
2400
16"0
17"0
18"0 Axial
19-0
position,
20"0
21 "0
in
Fig. 7. The 134Cs and '37Cs isotopic distributions of pin E showing the differences in the distributions. The '37Cs activity appears to be the result of fissions within the UO2 and the ~34Cs activity from the deposition of isotopes that have migrated from the fuel matrix. smooth distributions, indicating that the 137Cs activity present was primarily due to the fissions in the natural UO2. The lS*Cs isotope arrives at its distribution in a more complicated manner. The two ~a4Cs precursors, t33I and 13SXe have half-lives of 21h and 5-27d, respectively. The long half-lives of 133I and ~33Xe, relative to the ~STCs precursors, would permit their diffusion from the enriched fuel matrix and movement into the inert gas in the plenum. The 133Xedecays by
beta emission to 133Cs which is adsorbed on the surface of the UO2 pellets; the 133Cs is then neutron activated to 134Cs which is measured by gamma scanning. The 134Cs concentrations should be higher at the gas plenum end of the axial UO2 blanket and decrease toward the enriched fuel column as a result of the thermal neutron flux profiles, the adsorption at available unsaturated sites, and the relatively large gas volume above the blanket. The general decreasing trend was observed for fuel pins A and B (Figs. 3 and 4)
L~,O00
16,000
~
~-~_
12.000
,~
c O
8000
4000
016"0
17"0
180 Axial position,
I~0
20"0
210
in
Fig. 8. The isotopic distribution of 6°Co of pin E shows the thermal neutron flux profile over the axial blanket region shown in Fig. 7. The thermal flux gradient increases as the distance from the reactor core increases. This would cause a slight increase in the t34Cs activity from the higher neutron flux for the (n, ?) reaction of 133Cs.
23
Distributions of 134Cs and 137Cs in fuel pins and to a lesser degree in pins C, D and E (Figs. 5, 6 and 7). Variations in the total oxygen contents of the individual UO2 blanket pellets would significantly perturb the general trend. The perturbations in the 134Cs isotopic distributions observed in pins C, D and E (Figs. 5, 6 and 7) may be the result of ~34Cs concentrating in cracks which would expose more unsaturated sites, as well as, concentrating on specific pellets. As shown by the 6°Co distribution (Fig. 8), the thermal flux profile varied by about a factor or two over the axial blanket region, but the 134Cs varied much more. It is obvious that several factors combine to create the observed ~34Cs distributions. The theoretical densities of fuel pins C, D and E were higher than the densities of pins A and B (Table 1) resulting in less 133I and 13axe migrating from the enriched fuel matrix. The xa4Cs distributions of fuel pins C, D and E in Figs. 5, 6 and 7, respectively, are significantly different from the corresponding 137Cs distributions. The localized concentration of the a34Cs isotope is evident in all three plots. General trends in the aa~Cs activity, similar to those of pins A and B, were found in fuel pins D and E (Figs. 6 and 7). Only a short region of the axial blanket of pin C was scanned, eliminating the possibility of determining any trend in the 134Cs activity. CONCLUSIONS The isotopic distributions of the two cesium isotopes, a34Cs and 137Cs, indicate that their distributions are related to the half-lives of their I and Xe precursors.
The 137Cs isotope migrates as the metal or as a compound, whereas the final distribution of the 134Cs is a result of migration of either a331 or a3axe precursor which deposits as aa3Cs on UO2 blanket pellets with higher oxygen contents. The examination of six samples from the blanket regions of the two fuel pins indicate that both cesium isotopes preferentially concentrate on the pellets having oxygen contents above 11-81 per cent as compared to pellets with 11-73 per cent oxygen content or less. The movement of 137Cs from the enriched fuel region is retarded by high oxygen-to-metal ratios in the fuel. Fuel matrices with higher densities reduce the probability that the relative long-lived precursors of 134Cs will escape and migrate to the axial U O 2 blanket regions. Acknowledgements--The authors acknowledge the encouragement and suggestions offered by John O. Barner. The oxygen determinations were performed by Carolyn S. MacDougall. The assistance of Jennie Netuschil, Glenn H. Mottaz and Johnny N. Quintana in obtaining and processing the data is gratefully acknowledged. REFERENCES
1. J. R. Phillips, G. R. Waterbury, G. H. Mottaz and J. N. Quintana, In Proc. 20th Conf. Remote Syst. Technol., p. 115. Amer. Nuclear Society, Hinsdale, Ill. (1972). 2. W. M. Sanders and D. M. Holm, Los Alamos Scientific Laboratory report LA-4030, March 1969. 3. C. S. MacDougall, M. E. Smith and G. R. Waterbury, Proe. 20th Conf. Remote Syst. Teehnol., p. 137. Amer. Nuclear Society, Hinsdale, Ill. (1972).
DISTRIBUTIONS OF 134Cs AND 137Cs IN THE AXIAL U O 2 BLANKETS OF IRRADIATED (U,Pu)O 2 FUEL PINS J. R. PHILLIPS, G. R. WATERBURY and N. E. VANDERBORGH* Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (First received 4 October 1972; in revised form 16 March 1973)
Abstract--The axial distributions of 134Cs and t 37Cs in the axial blanket column of uranium-plutonium oxide fuel pins showed that the migration properties of the precursors of these two Cs isotopes were significantly different. The isotopic distributions were directly related to the half-lives of the two principal precursors of each cesium isotope, t33I and 13aXe for 134Cs, and 1371 and t37Xe for 137Cs. Both cesium isotopes preferentially collected on natural UO 2 blanket pellets having high oxygen content.
INTRODUCTION
to 90.45 per cent. The materials used in the pins designated as A, B, C, D and E are summarized in Table 1. Gross gamma scans were obtained on each of the five fuel pins to determine anomalous regions and relative gross gamma activities as a function of discrete position. In this mode of data collection, the fuel pin was moved axially in pre-determined increments and the gamma rays with energies greater than 100 keV were counted from an area precisely defined by a lead collimating slit. A detailed description of the equipment and methods of data processing used in this investigation have been presented before[l]. As the gross gamma scans of fuel pins A and B (Figs. 1 and 2) showed high gross gamma activity over the axial UO2 blanket columns, the two blanket columns were removed from the pins for further examination. Each column was sealed in an inert atmosphere. The two UO 2 blanket columns and the three blanket columns of pins C, D and E were also scanned in the multispectral mode of data collection to identify the principal isotopes contributing to the high gross gamma activities in these regions. The multispectral scanning consisted of collecting complete gammaray spectra between 0 and 1500 keV from precisely defined areas of the axial blankets[l]. The spectra were unfolded using a summation function of a Gaussian and three exponential functions to determine the net areas of the full-energy gamma-ray peaks[2] which are directly proportional to the amount of the specific isotope present in the defined volume segment of the sample. The net areas were plotted as a function of position, resulting in isotopic distributions of the specific fission products. Selected UO2 pellets from pins A and B were examined to determine their total oxygen contents. To assure adequate sample for repeated determinations, two or three pellets from an area having relatively large amounts of cesium (pellets 2, 4 and 5 from pin A) were combined as one sample, as were pellets 3 and 7 which had smaller amounts of cesium. These two samples were analyzed repeatedly for oxygen by a highly reliable method in which weighed portions were reacted at 2000-2200°C with carbon to convert the oxygen to carbon monoxide and carbon dioxide which were
IN STUDYING cesium distributions by precision gamma scanning in fast reactor fuel pins, differences in relative concentrations of 134Cs and 137Cs were found as a function of axial position. This was especially true for measurements of the two cesium isotopes in the natural UO2 axial blankets of five u r a n i u m - p l u t o n i u m oxide fuel pins irradiated in EBR-II to about 6 a t m burnup with a peak heat generation rate of 15 kW/ft. Evidence was obtained that cesium was concentrating selectively on specific UO2 pellets and, also that the 134Cs and 137Cs distributions were significantly different. To determine the effect of pellet oxygen content on cesium distributions, selected UO2 pellets were destructively analyzed for their total oxygen contents. The relative cesium concentrations of individual pellets were correlated to the measured total oxygen contents. EXPERIMENTAL Each of the five fuel pins contained a short UO 2 insulator pellet, a 14.2-in mixed (U0.sPu0.2)O 2 fuel column and a 6.25-in natural UO2 upper axial blanket column. The upper end of the pellet column was held in place by means of an extensiometer spring and spacer tube. A gas plenum, about 14.0-in long, was located above the axial blanket column. There were three types of fuel pellets in the five pins: master mix, mechanically blended and coprecipitated material. Each fuel column contained two of these types with one type in the top half and another type in the bottom half. The initial oxygen-to-metal atom ratios (O/M) varied from 1-94 to 2.00 and the theoretical densities varied from 82'38 * The University of New Mexico, Albuquerque, N.M. This work was sponsored by the Fuels and Materials Branch of the Division of Reactor Development and Technology of the U.S. Atomic Energy Commission. 17
J. R. PHILLIPS,G. R. WATERBURYand N. E. VANDERBORGH
18
Table 1 Pin
Section
O/M
~o Theo. density
Feed material
A
Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom
1'94 1.96 2-00 1-99 1.97 1.96 1.97 .1.99 2-00 2.00
82.38 82"85 83.55 82.40 90.31 90.13 89.90 90.45 89"59 88'02
Coprecipitated Mechanically blended Mechanically blended Coprecipitated Master mix Mechanically blended Mechanically blended Master mix Mechanically blended Master mix
B C D E
measured gravimetrically[3]. As the measured total oxygen content of pellets 2, 4 and 5 was slightly higher than that of pellets 3 and 7, the results indicated that the amount of cesium absorbed on a UO2 pellet might be related to the amount of oxygen present. To check this observation, three samples of two pellets each from pin A and three samples from pin B were analyzed for total oxygen content. Conditions to avoid oxygen contamination of these pellets were carefully maintained, and the method was calibrated using certified standards. The results for the latter samples are given in Table 2. RESULTS AND DISCUSSION T h e complete gamma-ray spectra indicated that the two cesium isotopes were the dominant gammaemitting isotopes in the UO2 blanket regions in pins A and B. The 134Cs activity generally increased as the distance from the enriched fuel column increased. The dashed lines have been drawn to accentuate this general trend. In pin A (Fig. 3) the 137Cs activity decreased as the distance from the fuel column increased, in a manner opposite to that for the x34Cs isotope. The migration of 137Cs to the blanket in pin B (Fig. 4) was much less. The slight activity around 16"0in showed that 137Cs had migrated only to the first two blanket pellets. This lower migration of x37Cs to the blanket in pin B as compared to pin A correlates well
with the lower O/M atom ratio of the fuel in pin A (Table 1) which would result in more movement of the volatile metallic cesium. It was obvious that the two isotopes collected preferentially on specific UO2 pellets. From the cesium isotopic distributions and the results of oxygen measurements, Table 2, it was concluded that the amount of 134Cs adsorbed on the natural UO2 blanket pellets was directly related to the oxygen contents of the pellets. The 137Cs distribution in pin A, Fig. 3, was also directly related to the oxygen content of the UO2 blanket pellets. The 134Cs and aaVCs distributions for fuel pins C, D and E are shown in Figs. 5, 6 and 7, respectively. The top section of the natural UO2 blanket region is shown in each figure. The two isotopes exhibited distinctively different distributions with the 137Cs activity being related to the positions of the UO2 blanket pellets and the 134Cs isotope concentrating at specific locations in the blanket region. As 134Cs was produced from the neutron capture reaction of 133Cs, the 134Cs distribution was dependent upon the variations in the thermal neutron flux profile. A monitor of the thermal flux profile is the relative 6°C0 activity, and the fast flux profile is shown by the distribution of the threshold monitor, 54Mn. As 6°C0 and S4Mn are activation products of the stainless steel
I
Fuel colurn~
103
i
i0 z
Gos plenum
:
i.i~ir.; !
-
j~
~ r.~._r
I°~ o
......,., ,~.;~,~-"~ ~ ~
.r:..~:.1
!.
"
8"0
160 24"0 AxioI posit"ion, ill
320
40'0
Fig. 1. The gross gamma activity as a function along the principal axis of fuel pin A. This scan shows unusually high activity in the region of the axial UO2 blanket (17.0-22.0 in).
Distributions of 134Cs and 137Cs in fuel pins
19
io4
column
Axial blanket
#
;!:~i:, ]
Gas Henum
"~
~,'r~.~_;
,jj
~Oz
~! ; , ~ ,
io' ,o:'~ :::" :'
8"0
16 o
24"0
Axi a l
position,
520
40"0
in
Fig. 2. The gross gamma activity as a function of axial position of fuel pin B. This scan shows unusually high activity in the axial UO 2 blanket region (17.0-22.0 in). cladding; the relative flux profiles were determined by plotting the two isotopic distributions. The distributions were relatively smooth over the scanned regions. The higher 6°Co activity in pin E around 17'0in (Fig. 8), than at other locations was due to a short stainless steel spacer located between the top pellet and second pellet. The effect of the spacer can be seen in the 1arCs distributions for pins C, D and E (Figs. 5, 6 and 7, respectively). The thermal neutron flux profile, illustrated by the 6°Co isotopic distribution in Fig. 8, corresponded with the gradient observed 6000'
I
I
I
in the 134Cs distributions for pins A and B (Figs. 3 and 4). Similar gradients were evident in pins C, D and E. The precursors of 134Cs and 137Cs were then considered in seeking an explanation for their different distributions. The two cesium isotopes result from tie beta decay of precursor fission products in the following ways: 134Cs ~ #~331
#-
Cs 134(796keVJ.
~4 ~ - Gas plenum 5000
#-
--~ ~33Xe _-> 133Cs(n,~ ) ~34Cs --~ ~34Ba 21h 5-27d 2-1y
T
1
Fuelcc~ulnn
#
u 5 ~
8
I
--
"'
IE
4000
I
5000
15
2000
I000
~ ,20,000~
Gas plenum
~
z 90,000
/
60,000 30,000
2 22
0
9
I1,0
12.0
Axial
13-0
posifion,
14.c
150
16.0
17+0
in
Fig. 3. The isotopic distributions of 134Cs and 1arCs over the axial blanket region of the fuel pin A. Note the significantly different distributions of the two isotopes. (The error bars for each point are drawn in the plots). The UO 2 pellets are numbered consecutively from the spacer towards the fuel.
J. R.
20
PHILLIPS,
G. R. WATERBURYand N. E. VANDERBORGH Table 2
Fuel pin
Sample
A
Pellet numbers
Oxygen (wt. %)
~34Csactivity
1
16, 17
2 3
21, 22 23, 25
11.87 + 0-03 11.64 + 0.03 11.81 + 0.03
high low high
4 5 6
1,2 4,7 5,6
11.72 _ 0.03 11.89 + 0.03 11.73 _ 0.03
low high low
entially adsorbed on the natural UO2 blanket pellets with high oxygen contents. The average oxygen-tometal atom ratio of the enriched fuel material in fuel pin A was 1.95. The O/M atom ratio for the fuel column in fuel pin B was significantly higher, 1.99. The oxygen content of the original fuel material in pin B may have been sufficiently high to permit the laTCs to form an oxide and remain in the enriched fuel column as was indicated by the very low activity of 137Cs in the UO2 blanket shown in Fig. 4. The O/M atom ratios in fuel pins C, D and E (1-97, 1'98 and 2.00, respectively) also were relatively higher than the 1-95 O/M ratio of fuel pin A. It was concluded that in the higher O/M fuels most of the 137Cs remained in the fuel region probably as an oxide. The respective ~37Cs isotopic distributions (Figs. 5, 6 and 7) of the blanket regions showed relatively
137Cs: ##.a1371 ~ 137Xe ._+ 137Cs ~ 137rnBa ~ 24s
4.2m
30.Oy
137Ba '
2-6m
For the la4Cs isotopic distributions the net areas of three gamma-ray peaks (566, 604.6 and 795"8 keV) were calculated and plotted. Only the gamma-ray peak at 661"6 keV was plotted for ~aVCs. Consideration of the short half-lives of the precursors of ~37Cs and the ~a7Cs isotopic distribution of pin A (Fig. 3) led to the conclusion that either cesium or a cesium complex was the principal migrating species foi" 137Cs. The la7Cs apparently did not migrate as one of the two principal precursors. Once the cesium had moved from the enriched fuel column, it prefer-
5ooo
i
134Cs (796 keY) r
'
<--Gas plenum
I
[
Fuel column
I
--~
°°I" I .ooor
l[
zoooI
7-1~1 &
100o
c
t)
0 50,000
I
I
'~'Cs (662 keY)
< - Gas plenum
Fuel column
I
40,000
"-)
I
30,000
20,000
I0,000 /
1 2 5 0 9"0
I0"0
o5o 1
I1'0
12'0 Axial
F!g. 4. The
134
Cs and
137
_ j,,I t3"O position,
14"0
15"0
16"0
17'0
in
Cs isotopic distributions over the axial blanket region of the fuel pin B. Note the
absence of la7Cs except for the region next to the enriched fuel column. (The error bars for each point are drawn on the plots). The UO2 pellets are numbered consecutively from the spacer towards the fuel.
Distributions of 13¢Cs and 137Cs in fuel pins I~Cs( 796 keV ) I I
21
75001
Gosplenum
I
Fuel column "~
6000
4500
:/
3000
1500
0 5000
~ '37Cs (662 keY ) Gas plenum
Fuel column -->
1
4000
3000
2000
I000
J 5"5
6.0
6'5 Axial
70
7'5
position,
in
8'0
8'5
Fig. 5. The 134Cs and laTCs isotopic distributions of pin C showing the differences in the distributions. The 137Cs activity appears to be the result of fissions within the UO2 and the 134Cs activity from the deposition of isoto ,es that have migrated from the fuel matrix. J
I0,000
~- Gas
B4Cs (796keY)
ptenum
Fuel column "->
8000 i 6000
I
4000
!.
2000
~ 0
0
f"
2000
~cs IbbE keV
I
~- Gas plenum 1600
Fuel column
"~
1200
800
400
o
16"0
Lu
170
18'0 Axial
position,
190
20"0
2bO
in
Fig. 6. The 134Cs and t3VCs isotopic distributions of pin D showing the differences in the distributions. The 137Cs activity appears to be the result of fissions within the UO2 and the 13'*Cs activity from the deposition of isotopes that have migrated from the fuel matrix.
22
J.R. PHILLIPS,G. R. WATERBURYand N. E. VANDERBORGH Io,ooo
3~s 796keV
r
1
Gas ple~m
Fuel column
8 000
i
I
6 000
4 000
2
O01
4000
0
I ~ Gasplenum
'S~s(662 keY)
I Fuel column --~
3200
2400
16"0
17"0
18"0 Axial
19-0
position,
20"0
21 "0
in
Fig. 7. The 134Cs and '37Cs isotopic distributions of pin E showing the differences in the distributions. The '37Cs activity appears to be the result of fissions within the UO2 and the ~34Cs activity from the deposition of isotopes that have migrated from the fuel matrix. smooth distributions, indicating that the 137Cs activity present was primarily due to the fissions in the natural UO2. The lS*Cs isotope arrives at its distribution in a more complicated manner. The two ~a4Cs precursors, t33I and 13SXe have half-lives of 21h and 5-27d, respectively. The long half-lives of 133I and ~33Xe, relative to the ~STCs precursors, would permit their diffusion from the enriched fuel matrix and movement into the inert gas in the plenum. The 133Xedecays by
beta emission to 133Cs which is adsorbed on the surface of the UO2 pellets; the 133Cs is then neutron activated to 134Cs which is measured by gamma scanning. The 134Cs concentrations should be higher at the gas plenum end of the axial UO2 blanket and decrease toward the enriched fuel column as a result of the thermal neutron flux profiles, the adsorption at available unsaturated sites, and the relatively large gas volume above the blanket. The general decreasing trend was observed for fuel pins A and B (Figs. 3 and 4)
L~,O00
16,000
~
~-~_
12.000
,~
c O
8000
4000
016"0
17"0
180 Axial position,
I~0
20"0
210
in
Fig. 8. The isotopic distribution of 6°Co of pin E shows the thermal neutron flux profile over the axial blanket region shown in Fig. 7. The thermal flux gradient increases as the distance from the reactor core increases. This would cause a slight increase in the t34Cs activity from the higher neutron flux for the (n, ?) reaction of 133Cs.
23
Distributions of 134Cs and 137Cs in fuel pins and to a lesser degree in pins C, D and E (Figs. 5, 6 and 7). Variations in the total oxygen contents of the individual UO2 blanket pellets would significantly perturb the general trend. The perturbations in the 134Cs isotopic distributions observed in pins C, D and E (Figs. 5, 6 and 7) may be the result of ~34Cs concentrating in cracks which would expose more unsaturated sites, as well as, concentrating on specific pellets. As shown by the 6°Co distribution (Fig. 8), the thermal flux profile varied by about a factor or two over the axial blanket region, but the 134Cs varied much more. It is obvious that several factors combine to create the observed ~34Cs distributions. The theoretical densities of fuel pins C, D and E were higher than the densities of pins A and B (Table 1) resulting in less 133I and 13axe migrating from the enriched fuel matrix. The xa4Cs distributions of fuel pins C, D and E in Figs. 5, 6 and 7, respectively, are significantly different from the corresponding 137Cs distributions. The localized concentration of the a34Cs isotope is evident in all three plots. General trends in the aa~Cs activity, similar to those of pins A and B, were found in fuel pins D and E (Figs. 6 and 7). Only a short region of the axial blanket of pin C was scanned, eliminating the possibility of determining any trend in the 134Cs activity. CONCLUSIONS The isotopic distributions of the two cesium isotopes, a34Cs and 137Cs, indicate that their distributions are related to the half-lives of their I and Xe precursors.
The 137Cs isotope migrates as the metal or as a compound, whereas the final distribution of the 134Cs is a result of migration of either a331 or a3axe precursor which deposits as aa3Cs on UO2 blanket pellets with higher oxygen contents. The examination of six samples from the blanket regions of the two fuel pins indicate that both cesium isotopes preferentially concentrate on the pellets having oxygen contents above 11-81 per cent as compared to pellets with 11-73 per cent oxygen content or less. The movement of 137Cs from the enriched fuel region is retarded by high oxygen-to-metal ratios in the fuel. Fuel matrices with higher densities reduce the probability that the relative long-lived precursors of 134Cs will escape and migrate to the axial U O 2 blanket regions. Acknowledgements--The authors acknowledge the encouragement and suggestions offered by John O. Barner. The oxygen determinations were performed by Carolyn S. MacDougall. The assistance of Jennie Netuschil, Glenn H. Mottaz and Johnny N. Quintana in obtaining and processing the data is gratefully acknowledged. REFERENCES
1. J. R. Phillips, G. R. Waterbury, G. H. Mottaz and J. N. Quintana, In Proc. 20th Conf. Remote Syst. Technol., p. 115. Amer. Nuclear Society, Hinsdale, Ill. (1972). 2. W. M. Sanders and D. M. Holm, Los Alamos Scientific Laboratory report LA-4030, March 1969. 3. C. S. MacDougall, M. E. Smith and G. R. Waterbury, Proe. 20th Conf. Remote Syst. Teehnol., p. 137. Amer. Nuclear Society, Hinsdale, Ill. (1972).