- Email: [email protected]
Fission-gas release in a spinel-based fuel used for actinide transmutation
Progress in Nuclenr Energy, Vol. 38, No. 3-4, pp. 313-316, 2001 Q 2001 Published by Elsevier Science Ltd. All rights reserved
Pergamon
Printed in Great Britain 0149-1970/01/$ - see front matter
www.elsevier.com/locate/pnucene
PII: SO149.1970(00)00124-4
Fission-gas release in a spinel-based fuel used for actinide transmutation
K. Bakker*‘, R. Belvroy’ , F.A. van den Berg’, S. Casalta*, R. Conrad* , E.A.C. Neeft’ , R.P.C. Schram’, W. Tams’
2 European
i NRG, Westerduinweg 3, 1755 ZG, Petten, the Netherlands Commission, Institute for Advanced Materials, Petten, The Netherlands
Abstract The present paper discusses the fission-gas release as measured from three fuels considered for the transmutation process. The releases were measured in spinel-based fuels from the EFTTRA-T3 irradiation [1,2]. Gas puncturing and EPMA data are discussed for three different inert-matrix concepts. The differences in the fission-gas release for the three spinel-based fuel concepts are most likely connected with the concentrations of fission-gas atoms in the fissile phase and in the spine1 phase. The present data suggest that homogeneously distributing the fission-gas atoms over the various phases, as would arise from micro-dispersed fuel, gives a lower release than implanting the fission-gas only in the fissile phase. 0 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords:
fission-gas
release, inert-matrix
fuel, post-irradiation
examination,
modelling
1. Introduction Several fuel concepts for transmutation are envisaged at present: 1. A solid solution of the inert matrix material and the fissile material (e.g. (Y,Pu,Zr)Oz.,. 2. A micro-dispersed type of fuel, which are very small fissile inclusions (~1 pm) in an inert matrix. 3. A macro-dispersed type of fuel, which are large fissile inclusions in an inert matrix phase. concepts 1 and 3. An example of this 4. Hybrid fuel, which is a combination of the above-mentioned is a mixture of (Y,Pu,Zr)Oz, solid-solution inclusions (- 20-200 urn) mixed in spinel. The macro- and micro-dispersed types of fuels are made of materials that do not form a solid solution, such as PUOZ and MgA1204. This paper focuses on the fission-gas release in concept 2, 3, and 4 with spine1 as an inert matrix. Data from a recent HFR-Petten irradiation (EFTTRA-T3 “) are used [1,2]. 2. Irradiation
experiments
on MgA1204-based
targets in the EFTTRA-T3
irradiation
Table 1 gives an overview of the EFTTRA-T3 data on spine1 targets, which are a macro- and a microdispersion of UOZ in spine1 and a macro dispersion of Y5.7sUO,-inclusions in spine1 [l]. Irradiation . * Corresponding
author: tel: +31 224564386;
fax: +31 224563608; 313
e-mail: [email protected]
K. Bakker et al.
314
conditions are given in [2]. The Fission Gas (F.G.) release was measured after the experiment using puncturing and analysis of the gas volume and gas composition [l]. The fuel temperature was about 673-973 K and the initial uranium concentration in the three samples is about 6 w%. The bumup was 17-20 % of the uranium. With a penetration depth of 8.6-12 micrometer it is computed, using a simple geometrical model, that about 10% of the fission products are implanted in the inert matrix (Table 1).
Table 1. Collection of data on spinel-based fuel from EFITRA-T3 Diameter fissile inclusions (pm)
Experiment
UOz-macro,
pin 11
U02-micro,
pin 9
F.G. Release (%)
F.G.-density (m”) Inert matrix Fissile phase
=: 150
>40
1.1x1o27
#
90
o-2
2.5 ~10~~
2.5 ~10~~ #
90
= 150 5-10 Ys.7sUOx-macro, pin 13 1.5 x1o26 # The xenon density in the inert matrix is inhomogeneous for these samples.
3. Fission-gas behaviour
Fraction of F.G. amount implanted in the fissile phase (%) 3
in the three types of fuel
The fission-gas release from the UOz-macro is large (>40 %), which can be attributed to the high concentration of fission-gas atoms implanted in the UOZ inclusions. Studies on high-bumup standard U02 fuel [3] have shown that when the pellet average bumup is 6 % FIMA, the local bumup at the pellet edge is about 12% FIMA. In this so-called rim region, re-crystallisation of the UOz grains and extensive pore formation are observed. The rim region has a similar temperature (673-873 K) and a slightly lower bumup than the UOz-macro fuel. In the UOz inclusions of the UOz-macro fuel a similar porous structure [l] is observed, as in the rim of UO2 fuel. The exact fission-gas release from the rim region is not completely clear, especially for the high U-bumups in the present UO2-macro fuel, but it is likely to be high, which is in good agreement with the present high fission-gas release. The UOz-macro fuel has a strongly fractured structure which causes that fission-gas released from the inclusions can directly leave the fuel. EPMA results have been obtained on the UO2-macro fuel, which shows that a relatively high concentration of xenon is observed in the spine1 that is O-15 urn from the UO2 inclusions (Fig. 1). At the positions in the spine1 where the EPMA is performed in Fig. 1 no porosity is visible while the UOz is highly porous. It is difficult to analyse the Xe-concentration as measured in the UOZ matrix due to this porosity. The measured Xe-profile in the spine1 and the fission-product Zr-profile in both phases are in agreement with that expected from fission-product implantation from the UOz phase. It can be concluded that spine1 has a good retention of fission gas under the EFTTRA-T3 conditions. In the Y5.7sUOx-macro fuel the fission-gas release and the implanted concentration of fission gas in the fissile phase are intermediate (5-10%) between those for the UO2 macro and micro fuels. The Xe-gas release from the micro-dispersed fuel is very small (O-2 %). Table 1 shows for the micro-U02 fuel that both in the spine1 and the UOZ the concentration of implanted Xe-atoms is rather low, since nearly all fission-products are implanted from the UO2 inclusions into the spine1 matrix. An increase of the local fission-gas density in standard UOZ fuel increases the fraction of fission-gas released. This relation probably also holds for other materials,
315
Fission-gas release
txe +Zr -A-Al
*Mg
-0-U
20
30
Position (micron)
Fig. 1. The distribution of Mg, Al, Xe, Zr and U (in arbitrary units) in the UOz-macro fuel, along a path through the spinel/UOz interface. since a higher fission-gas concentration enhances pore formation and pore movement and thereby fission-gas release. The much lower fission-gas density in the micro-dispersed UOZ fuels presently studied apparently causes the better fission-gas retention compared with the macro-dispersed UOz fuels presently studied. Another parameter, which has a strong influence on the fission-gas retention, is temperature. Since the EFTTRA-T3 fuels presently studied have similar temperatures, the temperature is probably not the parameter that causes the large difference between the fission-gas behaviour in the micro- and macro-U02 fuels. For UO2 the combined influence of fission-gas concentration and temperature on the fission-gas release is shown by the Vitanza limit [4]. This empirical limit, which has been derived from test in the Halden reactor (Fig. 2) describes the bum-up and central-fuel temperature dependence of the I-% fission-gas release threshold of standard UO2 fuel. For the design and operation of commercial UOz-fuel the Vitanza limit is an important criterion since the fuel in a LWR is operated in such a way that the fuel central temperature stays close to the Vitanza limit. When the fuel central temperature is considerably below the Vitanza limit insufficient power is generated, which has a commercial disadvantage. On the other hand when the central temperature is much higher than the Vitanza limit excessive fission-gas release is caused which might have safety related consequences. In homogenous standard UOZ fuel there is a direct relation between the bum-up and the concentration of implanted fission-atoms. In the micro-U02 nearly all fission gas atoms generated during fission are injected in the spine1 matrix, while in the UOz-macro and the Y=JJOx-macro most of the fission products are injected in the fissile inclusions. This suggests that the fission-gas release for these three fuels is determined by the properties of the spine1 (UOz-micro) or of the fissile phases (both macro-dispersed samples). Therefore the three concentrations can be used in a similar way as the Vitanza limit and are inserted in Figure 2. In the unlikely case that the fission-gas release is due to the UOz-phase in the micro-dispersed sample or from the spine1 phase in both macro-dispersed samples the values inserted in Fig. 2 would be too high and therefore the values in Fig. 2 are upper limits. For spine1 no relation is yet known which is similar to the Vitanza limit for UO2, but would help designing spinel-based fuel.
K. Bakker et al.
316
1% release threshold in UO,
$
750
Macro UY,,,O,
Micro UOp
-z = lOOO8
l
-
Macro UOp l
l
O-2 % release
5-10 % release
>40 % release
500
lmpanted xenon concentration
(atoms mm3)
Fig. 2. The Vitanza limit that describes the temperature and burn-up threshold above which more than 1% of the fission gas is released in UOz fuel (after [4]). The three points included represent the release from spine1 and the fissile phases, as discussed in the text.
4. Discussion and conclusions The EFITRA-T3 data suggest that the fission-gas release is strongly connected to the distribution of the fission gas over the various phases in the fuel. The micro-U02 fuel has a better, fission-gas retention than both macro-dispersed fuels. This good fission-gas retention and the EPMA results on the macro UOz show that spine1 has rather good fission-gas retention properties, although in the present report no comparison is made with other inert matrices. In order to extrapolate the present results on spinel-based fuel, to large-scale commercial transmutation of actinides, various aspects, which are atypical in the present experiment, should be taken into account: l In the present study UO;? is used, instead of a Pu- or Am-containing fissile phase. The fissiongas retention in these fissile phases should be studied for high bum up. l The temperatures and fission-gas concentrations in the present experiment are probably nontypical for large-scale transmutation.
References [I] E.A.C. Neeft, K. Bakker, H.A. Buurveld, J. Minkema, A. Paardekooper, R.P.C. Schram, C. Sciolla, 0. Zwaagstra, B. Beemsterboer, J.R.W. Woittiez, P. van Vlaanderen, W.J. Tams, H. Hein, R. Conrad and A. van Veen, Proc. 6’h IMF, Prog. Nucl. Energy 38 (2001). 121 R.P.C. Schram, K. Bakker, J.G. Boshoven, E.A.C. Neeft, G. Dassel, H. Hein, R.R. van der Laan, R.J.M. Konings, R. Conrad, Proceeding Global ‘99, Wyoming, Aug 29- Sept 3 (1999). S. Kashibe, K. Une, K. Nogita, J. Nucl. Mater 206 (1993) 22. 131 W. Wiesenack, M. McGrath, “Performance of MOX fuel: An overview of the experimental [41 programme of the OECD Halden Reactor Project and review of selected results” IAEA report IAEA-SM-358/18 (2000).
Pergamon
Printed in Great Britain 0149-1970/01/$ - see front matter
www.elsevier.com/locate/pnucene
PII: SO149.1970(00)00124-4
Fission-gas release in a spinel-based fuel used for actinide transmutation
K. Bakker*‘, R. Belvroy’ , F.A. van den Berg’, S. Casalta*, R. Conrad* , E.A.C. Neeft’ , R.P.C. Schram’, W. Tams’
2 European
i NRG, Westerduinweg 3, 1755 ZG, Petten, the Netherlands Commission, Institute for Advanced Materials, Petten, The Netherlands
Abstract The present paper discusses the fission-gas release as measured from three fuels considered for the transmutation process. The releases were measured in spinel-based fuels from the EFTTRA-T3 irradiation [1,2]. Gas puncturing and EPMA data are discussed for three different inert-matrix concepts. The differences in the fission-gas release for the three spinel-based fuel concepts are most likely connected with the concentrations of fission-gas atoms in the fissile phase and in the spine1 phase. The present data suggest that homogeneously distributing the fission-gas atoms over the various phases, as would arise from micro-dispersed fuel, gives a lower release than implanting the fission-gas only in the fissile phase. 0 2001 Published by Elsevier Science Ltd. All rights reserved. Keywords:
fission-gas
release, inert-matrix
fuel, post-irradiation
examination,
modelling
1. Introduction Several fuel concepts for transmutation are envisaged at present: 1. A solid solution of the inert matrix material and the fissile material (e.g. (Y,Pu,Zr)Oz.,. 2. A micro-dispersed type of fuel, which are very small fissile inclusions (~1 pm) in an inert matrix. 3. A macro-dispersed type of fuel, which are large fissile inclusions in an inert matrix phase. concepts 1 and 3. An example of this 4. Hybrid fuel, which is a combination of the above-mentioned is a mixture of (Y,Pu,Zr)Oz, solid-solution inclusions (- 20-200 urn) mixed in spinel. The macro- and micro-dispersed types of fuels are made of materials that do not form a solid solution, such as PUOZ and MgA1204. This paper focuses on the fission-gas release in concept 2, 3, and 4 with spine1 as an inert matrix. Data from a recent HFR-Petten irradiation (EFTTRA-T3 “) are used [1,2]. 2. Irradiation
experiments
on MgA1204-based
targets in the EFTTRA-T3
irradiation
Table 1 gives an overview of the EFTTRA-T3 data on spine1 targets, which are a macro- and a microdispersion of UOZ in spine1 and a macro dispersion of Y5.7sUO,-inclusions in spine1 [l]. Irradiation . * Corresponding
author: tel: +31 224564386;
fax: +31 224563608; 313
e-mail: [email protected]
K. Bakker et al.
314
conditions are given in [2]. The Fission Gas (F.G.) release was measured after the experiment using puncturing and analysis of the gas volume and gas composition [l]. The fuel temperature was about 673-973 K and the initial uranium concentration in the three samples is about 6 w%. The bumup was 17-20 % of the uranium. With a penetration depth of 8.6-12 micrometer it is computed, using a simple geometrical model, that about 10% of the fission products are implanted in the inert matrix (Table 1).
Table 1. Collection of data on spinel-based fuel from EFITRA-T3 Diameter fissile inclusions (pm)
Experiment
UOz-macro,
pin 11
U02-micro,
pin 9
F.G. Release (%)
F.G.-density (m”) Inert matrix Fissile phase
=: 150
>40
1.1x1o27
#
90
o-2
2.5 ~10~~
2.5 ~10~~ #
90
= 150 5-10 Ys.7sUOx-macro, pin 13 1.5 x1o26 # The xenon density in the inert matrix is inhomogeneous for these samples.
3. Fission-gas behaviour
Fraction of F.G. amount implanted in the fissile phase (%) 3
in the three types of fuel
The fission-gas release from the UOz-macro is large (>40 %), which can be attributed to the high concentration of fission-gas atoms implanted in the UOZ inclusions. Studies on high-bumup standard U02 fuel [3] have shown that when the pellet average bumup is 6 % FIMA, the local bumup at the pellet edge is about 12% FIMA. In this so-called rim region, re-crystallisation of the UOz grains and extensive pore formation are observed. The rim region has a similar temperature (673-873 K) and a slightly lower bumup than the UOz-macro fuel. In the UOz inclusions of the UOz-macro fuel a similar porous structure [l] is observed, as in the rim of UO2 fuel. The exact fission-gas release from the rim region is not completely clear, especially for the high U-bumups in the present UO2-macro fuel, but it is likely to be high, which is in good agreement with the present high fission-gas release. The UOz-macro fuel has a strongly fractured structure which causes that fission-gas released from the inclusions can directly leave the fuel. EPMA results have been obtained on the UO2-macro fuel, which shows that a relatively high concentration of xenon is observed in the spine1 that is O-15 urn from the UO2 inclusions (Fig. 1). At the positions in the spine1 where the EPMA is performed in Fig. 1 no porosity is visible while the UOz is highly porous. It is difficult to analyse the Xe-concentration as measured in the UOZ matrix due to this porosity. The measured Xe-profile in the spine1 and the fission-product Zr-profile in both phases are in agreement with that expected from fission-product implantation from the UOz phase. It can be concluded that spine1 has a good retention of fission gas under the EFTTRA-T3 conditions. In the Y5.7sUOx-macro fuel the fission-gas release and the implanted concentration of fission gas in the fissile phase are intermediate (5-10%) between those for the UO2 macro and micro fuels. The Xe-gas release from the micro-dispersed fuel is very small (O-2 %). Table 1 shows for the micro-U02 fuel that both in the spine1 and the UOZ the concentration of implanted Xe-atoms is rather low, since nearly all fission-products are implanted from the UO2 inclusions into the spine1 matrix. An increase of the local fission-gas density in standard UOZ fuel increases the fraction of fission-gas released. This relation probably also holds for other materials,
315
Fission-gas release
txe +Zr -A-Al
*Mg
-0-U
20
30
Position (micron)
Fig. 1. The distribution of Mg, Al, Xe, Zr and U (in arbitrary units) in the UOz-macro fuel, along a path through the spinel/UOz interface. since a higher fission-gas concentration enhances pore formation and pore movement and thereby fission-gas release. The much lower fission-gas density in the micro-dispersed UOZ fuels presently studied apparently causes the better fission-gas retention compared with the macro-dispersed UOz fuels presently studied. Another parameter, which has a strong influence on the fission-gas retention, is temperature. Since the EFTTRA-T3 fuels presently studied have similar temperatures, the temperature is probably not the parameter that causes the large difference between the fission-gas behaviour in the micro- and macro-U02 fuels. For UO2 the combined influence of fission-gas concentration and temperature on the fission-gas release is shown by the Vitanza limit [4]. This empirical limit, which has been derived from test in the Halden reactor (Fig. 2) describes the bum-up and central-fuel temperature dependence of the I-% fission-gas release threshold of standard UO2 fuel. For the design and operation of commercial UOz-fuel the Vitanza limit is an important criterion since the fuel in a LWR is operated in such a way that the fuel central temperature stays close to the Vitanza limit. When the fuel central temperature is considerably below the Vitanza limit insufficient power is generated, which has a commercial disadvantage. On the other hand when the central temperature is much higher than the Vitanza limit excessive fission-gas release is caused which might have safety related consequences. In homogenous standard UOZ fuel there is a direct relation between the bum-up and the concentration of implanted fission-atoms. In the micro-U02 nearly all fission gas atoms generated during fission are injected in the spine1 matrix, while in the UOz-macro and the Y=JJOx-macro most of the fission products are injected in the fissile inclusions. This suggests that the fission-gas release for these three fuels is determined by the properties of the spine1 (UOz-micro) or of the fissile phases (both macro-dispersed samples). Therefore the three concentrations can be used in a similar way as the Vitanza limit and are inserted in Figure 2. In the unlikely case that the fission-gas release is due to the UOz-phase in the micro-dispersed sample or from the spine1 phase in both macro-dispersed samples the values inserted in Fig. 2 would be too high and therefore the values in Fig. 2 are upper limits. For spine1 no relation is yet known which is similar to the Vitanza limit for UO2, but would help designing spinel-based fuel.
K. Bakker et al.
316
1% release threshold in UO,
$
750
Macro UY,,,O,
Micro UOp
-z = lOOO8
l
-
Macro UOp l
l
O-2 % release
5-10 % release
>40 % release
500
lmpanted xenon concentration
(atoms mm3)
Fig. 2. The Vitanza limit that describes the temperature and burn-up threshold above which more than 1% of the fission gas is released in UOz fuel (after [4]). The three points included represent the release from spine1 and the fissile phases, as discussed in the text.
4. Discussion and conclusions The EFITRA-T3 data suggest that the fission-gas release is strongly connected to the distribution of the fission gas over the various phases in the fuel. The micro-U02 fuel has a better, fission-gas retention than both macro-dispersed fuels. This good fission-gas retention and the EPMA results on the macro UOz show that spine1 has rather good fission-gas retention properties, although in the present report no comparison is made with other inert matrices. In order to extrapolate the present results on spinel-based fuel, to large-scale commercial transmutation of actinides, various aspects, which are atypical in the present experiment, should be taken into account: l In the present study UO;? is used, instead of a Pu- or Am-containing fissile phase. The fissiongas retention in these fissile phases should be studied for high bum up. l The temperatures and fission-gas concentrations in the present experiment are probably nontypical for large-scale transmutation.
References [I] E.A.C. Neeft, K. Bakker, H.A. Buurveld, J. Minkema, A. Paardekooper, R.P.C. Schram, C. Sciolla, 0. Zwaagstra, B. Beemsterboer, J.R.W. Woittiez, P. van Vlaanderen, W.J. Tams, H. Hein, R. Conrad and A. van Veen, Proc. 6’h IMF, Prog. Nucl. Energy 38 (2001). 121 R.P.C. Schram, K. Bakker, J.G. Boshoven, E.A.C. Neeft, G. Dassel, H. Hein, R.R. van der Laan, R.J.M. Konings, R. Conrad, Proceeding Global ‘99, Wyoming, Aug 29- Sept 3 (1999). S. Kashibe, K. Une, K. Nogita, J. Nucl. Mater 206 (1993) 22. 131 W. Wiesenack, M. McGrath, “Performance of MOX fuel: An overview of the experimental [41 programme of the OECD Halden Reactor Project and review of selected results” IAEA report IAEA-SM-358/18 (2000).