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Radial plutonium and fission product isotope profiles in mixed oxide fuel pins evaluated by secondary ion mass spectrometry
Journal of Nuclear Materials 202 (1993) 65-69 North-Holland
m Radial plutonium and fission product isotope profiles in mixed oxide fuel pins evaluated by secondary ion mass spectrometry H.U. Zwicky 1, E.T. Aerne, A. Hermann and H.A. Thomi Paul Scherrer Institut Wiirenlingen und Villigen, CH-5232 Villigen PSI, Switzerland
M. Lippens Belgonucldaire, Avenue Ariane 4, B-1200 Brussels, Belgium
Received 30 September 1992; accepted 29 January 1993
In the frame of the PRIMO programme the irradiation behaviour of uranium-plutonium mixed oxide fuel in pressurized water reactors is investigated. Secondary ion mass spectrometry was applied for the determination of radial profdes of burnup and of plutonium and fission product isotopes, providing data for the analysis of mixed oxide neutronic behaviour.
1. Introduction
Mixed oxide (MOX) nuclear fuel has been used in light water reactors for many years. The experience gained is thus significant but still incomplete. In order to extend the data base for MOX fuel behaviour to be used for design and licensing purposes, Belgonucl6aire and CEN/SCK * Mol co-organized the PRIMO ( P W R Reference Irradiation of M O X fuel) international programme. One of the subjects of the PRIMO phase 1 programme was to analyze the neutronic behaviour of MOX fuel rods by determining the radial profiles of burnup and of uranium and plutonium isotopes [1]. A classical method to determine burnup and isotopic compositions used in destructive postirradiation examinations is the dissolution of fuel samples followed by radiochemical separation and mass spectrometric isotopic dilution analysis. To evaluate radial burnup and isotopic composition profiles, the same method could be applied to samples produced by
Present address: Kernkraftwerk Leibstadt AG, CH-4353 Leibstadt, Switzerland. * Centre d'Etude de l'Energie Nucl6aire, Studiecentrum voor Kernenergie, Pare S6ny, B-1160 Brussels, Belgium.
mechanical or ultrasonic microdrilling. This technique is time-consuming and the lateral resolution is limited by the sampling procedure. A powerful technique for the characterization of irradiated fuel is electron probe microanalysis (EPMA). Uranium, plutonium and fission product distributions can be analyzed quantitatively. A complement, providing isotopic information with a lateral resoh:tion comparable to EPMA, is secondary ion mass spectrometry (SIMS). This paper describes the successful use of the SIMS technique on MOX fuel samples from the PRIMO-1 programme. The technique has ~al~ been successfully applied for the evaluation of the radial distribution of gadolinium isotopes in irradiated nuclear fuel pins doped with gadolinium oxide [2].
2. Materials and methods
A secondary ion mass spectrometer (SIMS) modified for the analysis of highly radioactive samples is installed in the Hot-Laboratory of the Paul Scherrer Institute [3]. The instrument is equipped with a quadrupole mass analyzer. Spectrometer settings, sample positioning and data collection are controlled by a HP 9836 desk computer and a HP 6942 multiprogammer.
0022-3115/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
H.U. ZwicAw et aL / Radial plutonium and fission product isotope profiles
66
Table 1 Count rate combinations used for the elimination of geometrical and instrumental effects
For the analysis of polished cross sections of irradiated fuel, an O~ primary ion beam of 12 keV energy was applied. The beam current used was about 200 nA with a spot size of about 50 lxm. The primary beam was scanned over an area of about 0.1 × 0.1 mm 2 in order to average out local inhomogeneities. Point by point measurements were performed along two perpendicular diameters as sketched in fig. 1 by moving the sample in x or y-direction between the counting cycles. Uranium, plutonium and some selected fission product isotopes were measured as M +, MO + or MO~+ ions. The burnup distribution in the analyzed samples was asymmetric. Therefore, a 13/~ autoradiograph was used to fix the orientation in the SIMS instrument in ~,_,ch a way that the region with the highest burnup was located at the bottom of the ),-axis. .M_OX fuel samples with average burnup values between 16 and 55 G W d / t M have been characterized. As an example, results from the analysis of a 35 G W d / t M sample are presented. A photomacrograph of the cut and the polished fuel cross section is shown in fig. 1. Absolute secondary ion count rates can vary considerably [2]. In order to eliminate these effects as far as
Quantity to be determined Plutonium isotopic composition Chemical plutonium content Relative fission product distribution
Count rates used E
Pu
x-14SNd+ E U+ y ' E Pu ~' x" laSNd+ Y'.U+ y ' E Pu a)
~) x: Factor for the conversion of n4SNd count rate into approximate number of fissions estimated by comparing the count rate ratio of n48Nd and 2asU with the ratio of n4SNd and 23SU atoms determined by radiochemistry/mass spectrometry. y: Ratio of secondary ion counting yields of U and Pu estimated by comparing the ratio of the Pu and U count rates with the ratio of Pu and U atoms determined by radiochemistry/mass spectrometry.
possible, all count rates were divided by a combination of count rates depending on the quantity to be determined according to table 1. Average values from the five measurements representing about the same radius
I
•
•
O
I
0
Q
•
•
O
nm f 5 .Points
,
)
~
0.1 m m
o o o e e
Y o o l e l
Fig. 1. Photomacrograph of a ~ypicai cut and polished Nel cross section and pnnciple of SIMS measurements along two pe~endicular diameters.
H.U. Zwicky et al. / Radial plutonium and fission product isotope profiles were then calculated. As the signal of 241pu is disturbed by 241Am, average values determined by radiochemical separation and mass spectrometric isotopic dilution analysis on an adjacent sample were used for the transformation of SIMS count rate ratios into local isotopic Pu abundances. Due to the asymmetric burnup, it was not possible to use a central symmetry for the calculation. It was assumed that the relative count rate c~ measured at the distance r~ from the pellet center is representative for a half annulus limite~ by the radii (r i - 0.Sd) and (r i + 0.5d) and a line rectangular to the measuring direction through the center. An average value for the whole sample was calculated according to eq. (1): d R cmj = R--~ ~ , c i j l r i l ,
¢mj
so
!
t
L~ Pu-241
~" 2o --
1
Pu-240 [] Pu-239 0
-t.
where Cmj is the average relative count rate of isotope j, d the step between two measured points, R the pellet radius, c~i the relative count rate of isotope j at position i, r~ the distance of position i from the pellet center. Local isotopic compositions were then calculated according to eq. (2): amj
60-
(1)
-R
aij = - - c i j ,
67
(2)
where a,j is the isotopic fraction of isotope j at position i, and a m / t h e average isotopic fraction of isotope j determined by radiochemistry/mass spectrometry. The same procedure was used for the transformation of the sum of plutonium count rates into local Putot/(U + Pu)to t values, neglecting ~SPu, which cainnot be measured in a sample containing ~SU. To compare the behaviour of different fission products, fission product data were normalized in the same way assuming an average value for the cross-section area of I. Counting times were generally set long enough to accumulate at least 500 counts for every individual mass. The statistical error of a single count rate is therefore lower than 5%. According to error propagation rules the error of an average value calculated from five adjacent points is lower than 2%, the error of a count rate ratio c o lower than 3%. The average relative count rate cmj (eq. (1)) is calculated from about 80 individual c u values, leading to an error of less than 0.5%. Errors for average isotopic fractions amj determined by isotopic dilution analysis are in the order of 1 - 2 % . This means that the random error of an isotopic fraction aij of an individual isotope j at position i (eq.
-3
-2
-1
0
1
2
3
Disumce f ~ m pellet center [ m m ]
Fig. 2. Radial distribution of plutonium isotopes in y-direction on the sample shown in fig. 1.
(2)) is lower than 4%. Not included in this value is the influence of cracks and pores on the sample surface, which may lead to locally limited systematic deviations from measured isotopic ratios.
3. R e s u l t s a n d d i s c u s s i o n
Local isotopic plutonium compositions, local total plutonium contents+ Putot/(U + Pu)to t, and some relative fission product distributions measured along the vertical diameter (y-direction, fig. 1) are shown in figs. 2-5. The radial distribution of plutonium isotopes shown in fig. 2 is the result of different nuclear reactions occuring during the irradiation. The main reactions are: - fission of z~'~Pu and 241pu by thermal neutrons, - production of ~-~9Pu from ~-~SU by thermal neutron capture and consecutive I~-decays, - formation of 24°pu, 241pu and 242pu by thermal neutron capture from 239pu, 24°pu and 241pu, respectively, - decay of 2+: Pu (half life: 14.4 a). Spectrum and flux of the neutrons vary as a function of pin radius and of irradiation time. As reaction cross sections are dependent on the neutron spectrum, reaction rate ratios vary across a fuel pellet and with time. This leads to isotopic plutonium distributions
68
H.U. Zwicio" et al. / Radial ph~tonium and fission product isotope profiles 2.5-
m
Ii
2.0 ~,~0 ~ "~ g 1.s ~1_ •=.~ . z ~
#6 ~g
...~
~~.vI,,
x Nd-148 -t- Nd-143 A Pr-141 0 Ce-140 La-139
a_ 4-
~L0-
I.
"N
~o.s
-~,
-3
-2
-1
0
I
2
0.0
3
Distance from pellet center [mm ]
Distance from pellet center [ mm ]
Fig. 3. Radial distribution of total plutonium content in y-direction on the sample shown in fig. 1.
characteristic for the neutronic conditions during irradiation. The Puto t radial distribution presented in fig. 3 shows a more pronounced consumption of plutonium at the pellet periphery, as expected from the thermal neutron flux depression effect. Irregularities in the radial distribution originate from some local inhomo2.S-
g-. II
~= 2.0 ~ O . d ~ 1.5
4X A
C.s-137 Ba-138 Cs-133
Q
Sr-88
[] Rb-85
2 ~.o.
._~ vl
~ 0.5,
0.0
-~
-"
-~ -'I o ~ 2 Distance from pellet center [ mm ]
3
4
Fig. 4. Relative radial distribution of SSRb, 88Sr, ta3cs, 137Cs and 13SBa in y-direction on the sample shown in fig. 1 normalized to an average value of 1.0.
Fig. 5. Relative radial distribution of 139La, t4°Ce, 14tpr, t43Nd and 14SNd in y-direction on the sample shown in fig. 1, normalized to an average value of 1.0.
geneities in the plutonium distribution before irradiation. More than 90% of fissions in the investigated samples were plutonium fissions. Therefore, the Putot/(U + Pu)to t pattern of fig. 3 can also be recognized in the fission product distributions (figs. 4 and 5). If, e.g., the 14aNd distribution is normalized to the average burnup, the relative distribution can directly be converted into a local burnup dis',ribution. This burnup distribution and the fission product distributions are radially very smooth and perturbated to a minor extent only by the presence of some heterogeneities in the initial plutonium distribution. The comparison of the relative 8SRb, ]a3Cs and ~37Cs distributions with the 8SSr and ~a8Ba distribution in fig. 4 shows, that no significant rubidium and cesium migration occurred during the irradiation. "l]le fission product distributions in figs. 4 and 5 and the isotopic plutonium distribution in fig. 2 confirm the burnup asymmetry. At the pellet rim at the bottom of the y-axis (fig. 1) the burnup is almost twice as high as at the opposite rim.
4. C o n c l u s i o n s
Radial profiles of burnup and of uranium and plutonium isotopes are needed for the analysis of MOX fuel neutronic behaviour. It could be demonstrated
H.U. Zwicky et al. / Radial plutonimn and fission product isotope profiles
69
that SIMS is a powerful method not only for the measurement of isotopic distributions, but also for the evaluation of the distribution of many fission products.
in particular A. Biichli and A. Erne. D. Orciuolo made the mass spectrometric measurements for the isotopic dilution analysis.
Aclmowledgemtats
References
PRIMO programme management and committee members, especially D. Boulanger of Beigonucl6aire and R.W. Stratton of PSI, contributed to the succes of this work with their advice and encouragement. Helpful discussions with G. Bart are appreciated. Sample preparation was performed by the PSI Hot-Cell Group,
[1] D. Haas, Proc. ANS/ENS Int. Topical Meeting on LWR Fuel Performance (ANS/ENS, Avignon, 1991) p. 948. [2] H.U. Zwicky, E.T. Aeme, G. Bart, F. Petrik and H.A. Thomi, Radiochim. Acta 47 (1989) 9. [3] G. Bart, E.T. Aerne, U. Fliickiger and E. Sprunger, Nucl. Instr. and Meth. 180 (1981) 109.
m Radial plutonium and fission product isotope profiles in mixed oxide fuel pins evaluated by secondary ion mass spectrometry H.U. Zwicky 1, E.T. Aerne, A. Hermann and H.A. Thomi Paul Scherrer Institut Wiirenlingen und Villigen, CH-5232 Villigen PSI, Switzerland
M. Lippens Belgonucldaire, Avenue Ariane 4, B-1200 Brussels, Belgium
Received 30 September 1992; accepted 29 January 1993
In the frame of the PRIMO programme the irradiation behaviour of uranium-plutonium mixed oxide fuel in pressurized water reactors is investigated. Secondary ion mass spectrometry was applied for the determination of radial profdes of burnup and of plutonium and fission product isotopes, providing data for the analysis of mixed oxide neutronic behaviour.
1. Introduction
Mixed oxide (MOX) nuclear fuel has been used in light water reactors for many years. The experience gained is thus significant but still incomplete. In order to extend the data base for MOX fuel behaviour to be used for design and licensing purposes, Belgonucl6aire and CEN/SCK * Mol co-organized the PRIMO ( P W R Reference Irradiation of M O X fuel) international programme. One of the subjects of the PRIMO phase 1 programme was to analyze the neutronic behaviour of MOX fuel rods by determining the radial profiles of burnup and of uranium and plutonium isotopes [1]. A classical method to determine burnup and isotopic compositions used in destructive postirradiation examinations is the dissolution of fuel samples followed by radiochemical separation and mass spectrometric isotopic dilution analysis. To evaluate radial burnup and isotopic composition profiles, the same method could be applied to samples produced by
Present address: Kernkraftwerk Leibstadt AG, CH-4353 Leibstadt, Switzerland. * Centre d'Etude de l'Energie Nucl6aire, Studiecentrum voor Kernenergie, Pare S6ny, B-1160 Brussels, Belgium.
mechanical or ultrasonic microdrilling. This technique is time-consuming and the lateral resolution is limited by the sampling procedure. A powerful technique for the characterization of irradiated fuel is electron probe microanalysis (EPMA). Uranium, plutonium and fission product distributions can be analyzed quantitatively. A complement, providing isotopic information with a lateral resoh:tion comparable to EPMA, is secondary ion mass spectrometry (SIMS). This paper describes the successful use of the SIMS technique on MOX fuel samples from the PRIMO-1 programme. The technique has ~al~ been successfully applied for the evaluation of the radial distribution of gadolinium isotopes in irradiated nuclear fuel pins doped with gadolinium oxide [2].
2. Materials and methods
A secondary ion mass spectrometer (SIMS) modified for the analysis of highly radioactive samples is installed in the Hot-Laboratory of the Paul Scherrer Institute [3]. The instrument is equipped with a quadrupole mass analyzer. Spectrometer settings, sample positioning and data collection are controlled by a HP 9836 desk computer and a HP 6942 multiprogammer.
0022-3115/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
H.U. ZwicAw et aL / Radial plutonium and fission product isotope profiles
66
Table 1 Count rate combinations used for the elimination of geometrical and instrumental effects
For the analysis of polished cross sections of irradiated fuel, an O~ primary ion beam of 12 keV energy was applied. The beam current used was about 200 nA with a spot size of about 50 lxm. The primary beam was scanned over an area of about 0.1 × 0.1 mm 2 in order to average out local inhomogeneities. Point by point measurements were performed along two perpendicular diameters as sketched in fig. 1 by moving the sample in x or y-direction between the counting cycles. Uranium, plutonium and some selected fission product isotopes were measured as M +, MO + or MO~+ ions. The burnup distribution in the analyzed samples was asymmetric. Therefore, a 13/~ autoradiograph was used to fix the orientation in the SIMS instrument in ~,_,ch a way that the region with the highest burnup was located at the bottom of the ),-axis. .M_OX fuel samples with average burnup values between 16 and 55 G W d / t M have been characterized. As an example, results from the analysis of a 35 G W d / t M sample are presented. A photomacrograph of the cut and the polished fuel cross section is shown in fig. 1. Absolute secondary ion count rates can vary considerably [2]. In order to eliminate these effects as far as
Quantity to be determined Plutonium isotopic composition Chemical plutonium content Relative fission product distribution
Count rates used E
Pu
x-14SNd+ E U+ y ' E Pu ~' x" laSNd+ Y'.U+ y ' E Pu a)
~) x: Factor for the conversion of n4SNd count rate into approximate number of fissions estimated by comparing the count rate ratio of n48Nd and 2asU with the ratio of n4SNd and 23SU atoms determined by radiochemistry/mass spectrometry. y: Ratio of secondary ion counting yields of U and Pu estimated by comparing the ratio of the Pu and U count rates with the ratio of Pu and U atoms determined by radiochemistry/mass spectrometry.
possible, all count rates were divided by a combination of count rates depending on the quantity to be determined according to table 1. Average values from the five measurements representing about the same radius
I
•
•
O
I
0
Q
•
•
O
nm f 5 .Points
,
)
~
0.1 m m
o o o e e
Y o o l e l
Fig. 1. Photomacrograph of a ~ypicai cut and polished Nel cross section and pnnciple of SIMS measurements along two pe~endicular diameters.
H.U. Zwicky et al. / Radial plutonium and fission product isotope profiles were then calculated. As the signal of 241pu is disturbed by 241Am, average values determined by radiochemical separation and mass spectrometric isotopic dilution analysis on an adjacent sample were used for the transformation of SIMS count rate ratios into local isotopic Pu abundances. Due to the asymmetric burnup, it was not possible to use a central symmetry for the calculation. It was assumed that the relative count rate c~ measured at the distance r~ from the pellet center is representative for a half annulus limite~ by the radii (r i - 0.Sd) and (r i + 0.5d) and a line rectangular to the measuring direction through the center. An average value for the whole sample was calculated according to eq. (1): d R cmj = R--~ ~ , c i j l r i l ,
¢mj
so
!
t
L~ Pu-241
~" 2o --
1
Pu-240 [] Pu-239 0
-t.
where Cmj is the average relative count rate of isotope j, d the step between two measured points, R the pellet radius, c~i the relative count rate of isotope j at position i, r~ the distance of position i from the pellet center. Local isotopic compositions were then calculated according to eq. (2): amj
60-
(1)
-R
aij = - - c i j ,
67
(2)
where a,j is the isotopic fraction of isotope j at position i, and a m / t h e average isotopic fraction of isotope j determined by radiochemistry/mass spectrometry. The same procedure was used for the transformation of the sum of plutonium count rates into local Putot/(U + Pu)to t values, neglecting ~SPu, which cainnot be measured in a sample containing ~SU. To compare the behaviour of different fission products, fission product data were normalized in the same way assuming an average value for the cross-section area of I. Counting times were generally set long enough to accumulate at least 500 counts for every individual mass. The statistical error of a single count rate is therefore lower than 5%. According to error propagation rules the error of an average value calculated from five adjacent points is lower than 2%, the error of a count rate ratio c o lower than 3%. The average relative count rate cmj (eq. (1)) is calculated from about 80 individual c u values, leading to an error of less than 0.5%. Errors for average isotopic fractions amj determined by isotopic dilution analysis are in the order of 1 - 2 % . This means that the random error of an isotopic fraction aij of an individual isotope j at position i (eq.
-3
-2
-1
0
1
2
3
Disumce f ~ m pellet center [ m m ]
Fig. 2. Radial distribution of plutonium isotopes in y-direction on the sample shown in fig. 1.
(2)) is lower than 4%. Not included in this value is the influence of cracks and pores on the sample surface, which may lead to locally limited systematic deviations from measured isotopic ratios.
3. R e s u l t s a n d d i s c u s s i o n
Local isotopic plutonium compositions, local total plutonium contents+ Putot/(U + Pu)to t, and some relative fission product distributions measured along the vertical diameter (y-direction, fig. 1) are shown in figs. 2-5. The radial distribution of plutonium isotopes shown in fig. 2 is the result of different nuclear reactions occuring during the irradiation. The main reactions are: - fission of z~'~Pu and 241pu by thermal neutrons, - production of ~-~9Pu from ~-~SU by thermal neutron capture and consecutive I~-decays, - formation of 24°pu, 241pu and 242pu by thermal neutron capture from 239pu, 24°pu and 241pu, respectively, - decay of 2+: Pu (half life: 14.4 a). Spectrum and flux of the neutrons vary as a function of pin radius and of irradiation time. As reaction cross sections are dependent on the neutron spectrum, reaction rate ratios vary across a fuel pellet and with time. This leads to isotopic plutonium distributions
68
H.U. Zwicio" et al. / Radial ph~tonium and fission product isotope profiles 2.5-
m
Ii
2.0 ~,~0 ~ "~ g 1.s ~1_ •=.~ . z ~
#6 ~g
...~
~~.vI,,
x Nd-148 -t- Nd-143 A Pr-141 0 Ce-140 La-139
a_ 4-
~L0-
I.
"N
~o.s
-~,
-3
-2
-1
0
I
2
0.0
3
Distance from pellet center [mm ]
Distance from pellet center [ mm ]
Fig. 3. Radial distribution of total plutonium content in y-direction on the sample shown in fig. 1.
characteristic for the neutronic conditions during irradiation. The Puto t radial distribution presented in fig. 3 shows a more pronounced consumption of plutonium at the pellet periphery, as expected from the thermal neutron flux depression effect. Irregularities in the radial distribution originate from some local inhomo2.S-
g-. II
~= 2.0 ~ O . d ~ 1.5
4X A
C.s-137 Ba-138 Cs-133
Q
Sr-88
[] Rb-85
2 ~.o.
._~ vl
~ 0.5,
0.0
-~
-"
-~ -'I o ~ 2 Distance from pellet center [ mm ]
3
4
Fig. 4. Relative radial distribution of SSRb, 88Sr, ta3cs, 137Cs and 13SBa in y-direction on the sample shown in fig. 1 normalized to an average value of 1.0.
Fig. 5. Relative radial distribution of 139La, t4°Ce, 14tpr, t43Nd and 14SNd in y-direction on the sample shown in fig. 1, normalized to an average value of 1.0.
geneities in the plutonium distribution before irradiation. More than 90% of fissions in the investigated samples were plutonium fissions. Therefore, the Putot/(U + Pu)to t pattern of fig. 3 can also be recognized in the fission product distributions (figs. 4 and 5). If, e.g., the 14aNd distribution is normalized to the average burnup, the relative distribution can directly be converted into a local burnup dis',ribution. This burnup distribution and the fission product distributions are radially very smooth and perturbated to a minor extent only by the presence of some heterogeneities in the initial plutonium distribution. The comparison of the relative 8SRb, ]a3Cs and ~37Cs distributions with the 8SSr and ~a8Ba distribution in fig. 4 shows, that no significant rubidium and cesium migration occurred during the irradiation. "l]le fission product distributions in figs. 4 and 5 and the isotopic plutonium distribution in fig. 2 confirm the burnup asymmetry. At the pellet rim at the bottom of the y-axis (fig. 1) the burnup is almost twice as high as at the opposite rim.
4. C o n c l u s i o n s
Radial profiles of burnup and of uranium and plutonium isotopes are needed for the analysis of MOX fuel neutronic behaviour. It could be demonstrated
H.U. Zwicky et al. / Radial plutonimn and fission product isotope profiles
69
that SIMS is a powerful method not only for the measurement of isotopic distributions, but also for the evaluation of the distribution of many fission products.
in particular A. Biichli and A. Erne. D. Orciuolo made the mass spectrometric measurements for the isotopic dilution analysis.
Aclmowledgemtats
References
PRIMO programme management and committee members, especially D. Boulanger of Beigonucl6aire and R.W. Stratton of PSI, contributed to the succes of this work with their advice and encouragement. Helpful discussions with G. Bart are appreciated. Sample preparation was performed by the PSI Hot-Cell Group,
[1] D. Haas, Proc. ANS/ENS Int. Topical Meeting on LWR Fuel Performance (ANS/ENS, Avignon, 1991) p. 948. [2] H.U. Zwicky, E.T. Aeme, G. Bart, F. Petrik and H.A. Thomi, Radiochim. Acta 47 (1989) 9. [3] G. Bart, E.T. Aerne, U. Fliickiger and E. Sprunger, Nucl. Instr. and Meth. 180 (1981) 109.