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Study of the microstructure of neutron irradiated beryllium for the validation of the ANFIBE code
Fusion Engineering and Design 61 /62 (2002) 769 /773 www.elsevier.com/locate/fusengdes
Study of the microstructure of neutron irradiated beryllium for the validation of the ANFIBE code E. Rabaglino a,b, , C. Ferrero c, J. Reimann a, C. Ronchi b, T. Schulenberg a a
b
Forschungszentrum Karlsruhe, Institute of Nuclear and Energy Technologies, P.O. Box 3640, D-76021, Karlsruhe, Germany European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany c European Synchrotron Radiation Facility, P.O. Box 220, F-38043 Grenoble, France
Abstract The behaviour of beryllium under fast neutron irradiation is a key issue of the helium cooled pebble bed tritium breeding blanket, due to the production of large quantities of helium and of a non-negligible amount of tritium. To optimise the design, a reliable prediction of swelling due to helium bubbles and of tritium inventory during normal and off-normal operation of a fusion power reactor is needed. The ANFIBE code (ANalysis of Fusion Irradiated BEryllium) is being developed to meet this need. The code has to be applied in a range of irradiation conditions where no experimental data are available, therefore a detailed gas kinetics model, and a specific and particularly careful validation strategy are needed. The validation procedure of the first version of the code was based on macroscopic data of swelling and tritium release. This approach is, however, incomplete, since a verification of the microscopic behaviour of the gas in the metal is necessary to obtain a reliable description of swelling. This paper discusses a general strategy for a thorough validation of the gas kinetics models in ANFIBE. The microstructure characterisation of weakly irradiated beryllium pebbles, with different visual examination techniques, is then presented as an example of the application of this strategy. In particular, the advantage of developing 3D techniques, such as X-ray microtomography, is demonstrated. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas-in-solid; Gas retention; Irradiated beryllium
1. Introduction In the European concept of a solid breeder blanket for fusion reactors, the helium cooled pebble bed (HCPB) blanket, the neutron multiplier is metallic beryllium. Fast neutrons produce in
Corresponding author. Tel.: /49-7247-82-3476; fax: /497247-82-4837 E-mail address: [email protected] (E. Rabaglino).
beryllium large quantities of helium and a nonnegligible amount of tritium. Swelling due to helium bubbles and tritium retention are considered to be key issues of the concept, see Malang [1]. For design and safety assessment purposes, it is essential to give a reliable prediction of these phenomena, in the full range of operating and accidental conditions of a fusion power reactor. The code ANalysis of Fusion Irradiated BEryllium (ANFIBE) is being upgraded to meet this need. The gas kinetics model in ANFIBE, described by
0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 1 4 9 - 7
770
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
Scaffidi-Argentina [2], is a so-called ‘3-stage’ diffusion model. It takes into account: (1) gas atomic diffusion across the grain and to grain boundaries, (2) nucleation of bubbles inside the grain (intra granular bubbles) and their coalescence and diffusion to grain boundaries, (3) growth and coalescence of bubbles at grain boundaries (intergranular bubbles) and related formation of open porosity networks, through which gas escapes to the outside. In the scenario of the existing gas-insolid models, ANFIBE accounts for the highest number of structural details and gas migration mechanisms. This substantially higher level of detail is necessary to make the extrapolation of the models possible, provided that, in the range of irradiation conditions where experimental evidence is available, such detailed solid-state models are correctly validated.
2. The validation procedure required for ANFIBE The first version of the code was validated on the basis of macroscopic swelling data of different kinds of beryllium samples, measured at the end of irradiation in fission reactors or during subsequent out-of-pile heating, see Scaffidi-Argentina [3]. The tritium retention model was validated on the basis of out-of-pile tritium release measurements. Relevant data on the microstructure of irradiated beryllium were at that time not available. Such a procedure is not fully satisfactory, since geometrical swelling and gas release are integral quantities, which cannot be univocally related to single microscopic quantities described by the gas kinetics model. In particular, the following phenomena have to be correctly predicted by the model at a microscopic level: (a) The nucleation of intragranular bubbles . In beryllium, fast neutrons produce large quantities of gas and a strong lattice damage at the same time. Dislocations are obstacles for the free diffusion of gas atoms and tend to act as traps. As a consequence, there is a strong interaction between dislocations kinetics and bubble nucleation. (b) The characteristics of bubbles. Bubbles are subdivided into classes: spherical bubbles inside
the grains and, at grain boundaries, significantly larger bubbles of a lenticular shape. As a consequence of growth, diffusion and coalescence phenomena, each class evolves in size, shape and concentration. (c) The formation of open porosity networks . The growth and coalescence of bubbles at grain boundaries lead to the formation of a complex 3D open porosity network. According to the model, this is the main escape way for the gas; gas atomic diffusion to free surfaces plays a negligible role. Furthermore, a sharp gas release peak has been observed during fast out-of-pile heating by some authors, e.g. Scaffidi-Argentina [4] and Rabaglino [5], and this phenomenon is not correctly predicted by ANFIBE, see Scaffidi-Argentina [3]. This behaviour has not been satisfactorily explained yet: it might be due to high temperature embrittlement of beryllium, see Pokrovsky [6], which may lead to extended microcracking, as suggested by Scaffidi-Argentina [7]. A careful microstructure characterisation of irradiated beryllium is necessary to obtain evidence of the above mentioned phenomena and to verify whether their description in the model is adequate or additional models should be implemented. In this case, the experimental results suggest a strategy for further theoretical developments. The relevant microscopic quantities that should be correctly predicted are, for each class of bubbles, their average size and volume/surface concentration, as a function of in-pile history and out-of-pile thermal treatments. The microstructure analyses should be coupled to gas release measurements, in order to correlate release stages to microscopic diffusion steps, see Rabaglino [5]. Since gas kinetics is influenced by the original microstructure and impurity content, the investigated material should be as similar as possible to the reference pebbles for the HCPB blanket, i.e. 1mm diameter pebbles, produced by NGK by rotating electrode process, impurity content specifications according to Piazza [8]. The relevance of the samples is then related to distinct model aspects which are to be validated, e.g. in order to study bubble nucleation, a weakly irradiated material should be used, since in a highly irra-
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
diated material this stage is not visible any more. On the other hand, in order to provide evidence of open porosity formation, highly irradiated material should be studied, or, as an alternative, also weakly irradiated samples, provided that they are heated to high temperatures. Also the investigation technique should be chosen on the basis of the investigated object: transmission electron microscopy (TEM) is suitable to investigate dislocations and about nanometer size bubbles at low neutron fluence and/or low temperature; optical microscopy is needed to study about micrometer size bubbles and gas release pathways at high fluence and/or high temperature. In order to prove the formation of a 3D open porosity network, the classical 2D techniques have obvious limitations: the following step is to develop a 3D microtomography method, based on synchrotron X-ray light.
3. The microstructure characterisation of pebbles from the BERYLLIUM irradiation experiment In order to provide data for a better validation of ANFIBE, weakly irradiated 2-mm diameter beryllium pebbles, fabricated by Brush and Wellman by fluoride reduction process, were characterised. The samples were irradiated in 1994 in the BERYLLIUM experiment in the high flux reactor (Petten). The essential irradiation data are as follows: irradiation temperature 780 K; fast neutron fluence (E /0.1 MeV) 1.24 /1025 nm 2, 480 appm 4He and 12 appm 3H production, according to Conrad [9]. Compared to the reference material, these pebbles are of lower metallurgical quality, their shape is irregular and they have a higher impurity content. Nevertheless, as far as grain size and microstructure are concerned, these samples are similar to the reference material, since they have large grains (40 /200 mm) without dislocations induced by cold-working, see Rabaglino [10]. Furthermore, their irradiation history is very well known. These characteristics make them suitable to study gas diffusion phenomena at low dose, with the aim to provide significant validation data for ANFIBE. By thermal treatments, the effect of temperature on bubble formation and develop-
771
ment was studied. The TEM micrograph in Fig. 1a shows the conditions of the inside of a grain at the end of the irradiation, which was carried out at about 780 K: in spite of the low dose and relatively high temperature, a high density of dislocations were formed during irradiation, but no visible gas bubbles. The gas is probably trapped in dislocations and impurities. This proves that dislocations have a relevant effect on gas diffusion also at high temperature and, vice versa, dislocations might be hindered in their movement and healing by the gas trapped in them. The TEM micrograph in Fig. 1b shows the conditions of the inside of a grain after a post-irradiation heating to about 1000 K. Small elliptic bubbles of some nanometres size have appeared on dislocations: this gives further confirmation to the above mentioned hypothesis that, at the end of the irradiation, a relevant part of the gas is trapped in dislocations. The optical microscopy micrograph in Fig. 1c shows the conditions of a grain after post-irradiation heating to about 1300 K. The intragranular bubbles have dramatically grown up to about micron sizes, and at grain boundaries large and elongated bubbles are visible: the formation of a porosity network appears to be in progress. In order to investigate gas release mechanisms, which are related to the development of porosity networks until they reach the outer surface of the pebble, a 2D characterisation technique is not sufficient, because of the inherent 3D geometry of these channels. To meet this need, a 3D microtomography investigation technique, based on X-ray synchrotron light, appears to be promising. The basic principle of this technique is that, starting from a series of 2D slices of the pebble along a certain axis, a 3D reconstruction of the sample and of the detail of its internal features can be attempted. Fig. 1d shows a high resolution 2D slice of a pebble after post-irradiation heating up to about 1450 K, obtained by microtomography: extended porosities are visible. These patterns have to be analysed with topological models to validate the percolation model in ANFIBE. The straight channel crossing the bottom left corner might be the result of a grain detachment. If confirmed by the 3D reconstruction, this could help to explain the occurrence of a sharp gas release peak at this temperature, see Rabaglino [5].
772
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
Fig. 1. Microstructure of beryllium pebbles from the BERYLLIUM irradiation experiment. Irradiation data: temperature 780 K; fast neutron fluence (E /0.1 MeV) 1.24 /1025 nm 2, 480 appm 4He and 12 appm 3H production. (a) After irradiation at 780 K, dislocations and Mg impurities (black dots) are visible, but no gas bubbles (TEM). (b) After post-irradiation heating to about 1000 K. Small elliptic bubbles appear (TEM). (c) After post-irradiation heating up to about 1300 K (optical microscopy). Elongated bubbles at grain boundaries are developing. (d) After post-irradiation heating up to about 1450 K (microtomography). Porosity networks and grain detachment are visible.
4. Conclusions In order to validate the code ANFIBE with the particular effectiveness required for far extrapolating predictions, single gas diffusion phenomena have to be studied in detail, at a microscopic, mechanistic level. A microstructure characterisation of relevant irradiated beryllium samples is essential. As an example, some results of the study of beryllium pebbles from the BERYLLIUM irradiation experiment have been presented. The applicability and attractiveness of the X-ray microtomography for the analysis of gas release
patterns at high dose or high temperature has also been proved.
Acknowledgements The authors would like to express a special thank to S. Malang and L. V. Boccaccini, for promoting and supporting the definition of the programme of activities for the validation and development of the ANFIBE code. The authors are then grateful to the EFDA-CSU Garching for financing the programme.
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
References [1] S. Malang, Limitations on blanket performance, Fus. Eng. Des. 46 (1999) 193 /206. [2] F. Scaffidi-Argentina, M. Dalle Donne, C. Ronchi, C. Ferrero, ANFIBE: a comprehensive model for swelling and tritium release from neutron irradiated beryllium-I: theory and model capabilities, Fus. Techn. 32 (1997) 179 / 195. [3] F. Scaffidi-Argentina, M. Dalle Donne, C. Ronchi, C. Ferrero, ANFIBE: a comprehensive model for swelling and tritium release from neutron irradiated beryllium-II: comparison of model predictions with experimental results, Fus. Techn. 33 (1998) 146 /163. [4] F. Scaffidi-Argentina, Tritium and helium release from neutron irradiated beryllium pebbles from the EXOTIC-8 irradiation, Fus. Eng. Des. 58 /59 (2001) 641 /645. [5] E. Rabaglino, J.P. Hiernaut, C. Ronchi, F. ScaffidiArgentina, Helium and tritium kinetics in irradiated
[6]
[7]
[8]
[9] [10]
773
beryllium pebbles, Proc. 10th Int. Conf. on Fusion Reactor Materials, J. Nucl. Mat., in press. A.S. Pokrovsky, S.A. Fabritsiev, R.M. Bagautdinov, Yu. Goncharenko, High temperature beryllium embrittlement, J. Nucl. Mat. 233 /237 (1996) 841 /846. F. Scaffidi-Argentina, C. Sand, C.H. Wu, Tritium and helium retention in neutron irradiated beryllium, Proc. 5th Int. Workshop on Hydrogen in Solids, Stockholm, 2000. G. Piazza, in W. Bahm (ed.), Nuclear Fusion Programme Annual Report of the Association Forschungszentrum Karlsruhe/EURATOM, October 2000 /September 2001, Report FZKA 6650/EUR 20162 EN, Forschungszentrum Karlsruhe, 2002. R. Conrad, R. May, BERYLLIUM Final irradiation report, EC-JRC-IAM Report P/F1/96/15, 1996. E. Rabaglino, C. Ronchi, F. Scaffidi-Argentina, T. Wiss, Helium and tritium release and sub-microscopic restructuring in irradiated beryllium pebbles, Proceeding of the fifth IEA International Workshop on Beryllium Technology for Fusion, Moscow, 2001, in press.
Study of the microstructure of neutron irradiated beryllium for the validation of the ANFIBE code E. Rabaglino a,b, , C. Ferrero c, J. Reimann a, C. Ronchi b, T. Schulenberg a a
b
Forschungszentrum Karlsruhe, Institute of Nuclear and Energy Technologies, P.O. Box 3640, D-76021, Karlsruhe, Germany European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany c European Synchrotron Radiation Facility, P.O. Box 220, F-38043 Grenoble, France
Abstract The behaviour of beryllium under fast neutron irradiation is a key issue of the helium cooled pebble bed tritium breeding blanket, due to the production of large quantities of helium and of a non-negligible amount of tritium. To optimise the design, a reliable prediction of swelling due to helium bubbles and of tritium inventory during normal and off-normal operation of a fusion power reactor is needed. The ANFIBE code (ANalysis of Fusion Irradiated BEryllium) is being developed to meet this need. The code has to be applied in a range of irradiation conditions where no experimental data are available, therefore a detailed gas kinetics model, and a specific and particularly careful validation strategy are needed. The validation procedure of the first version of the code was based on macroscopic data of swelling and tritium release. This approach is, however, incomplete, since a verification of the microscopic behaviour of the gas in the metal is necessary to obtain a reliable description of swelling. This paper discusses a general strategy for a thorough validation of the gas kinetics models in ANFIBE. The microstructure characterisation of weakly irradiated beryllium pebbles, with different visual examination techniques, is then presented as an example of the application of this strategy. In particular, the advantage of developing 3D techniques, such as X-ray microtomography, is demonstrated. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas-in-solid; Gas retention; Irradiated beryllium
1. Introduction In the European concept of a solid breeder blanket for fusion reactors, the helium cooled pebble bed (HCPB) blanket, the neutron multiplier is metallic beryllium. Fast neutrons produce in
Corresponding author. Tel.: /49-7247-82-3476; fax: /497247-82-4837 E-mail address: [email protected] (E. Rabaglino).
beryllium large quantities of helium and a nonnegligible amount of tritium. Swelling due to helium bubbles and tritium retention are considered to be key issues of the concept, see Malang [1]. For design and safety assessment purposes, it is essential to give a reliable prediction of these phenomena, in the full range of operating and accidental conditions of a fusion power reactor. The code ANalysis of Fusion Irradiated BEryllium (ANFIBE) is being upgraded to meet this need. The gas kinetics model in ANFIBE, described by
0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 1 4 9 - 7
770
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
Scaffidi-Argentina [2], is a so-called ‘3-stage’ diffusion model. It takes into account: (1) gas atomic diffusion across the grain and to grain boundaries, (2) nucleation of bubbles inside the grain (intra granular bubbles) and their coalescence and diffusion to grain boundaries, (3) growth and coalescence of bubbles at grain boundaries (intergranular bubbles) and related formation of open porosity networks, through which gas escapes to the outside. In the scenario of the existing gas-insolid models, ANFIBE accounts for the highest number of structural details and gas migration mechanisms. This substantially higher level of detail is necessary to make the extrapolation of the models possible, provided that, in the range of irradiation conditions where experimental evidence is available, such detailed solid-state models are correctly validated.
2. The validation procedure required for ANFIBE The first version of the code was validated on the basis of macroscopic swelling data of different kinds of beryllium samples, measured at the end of irradiation in fission reactors or during subsequent out-of-pile heating, see Scaffidi-Argentina [3]. The tritium retention model was validated on the basis of out-of-pile tritium release measurements. Relevant data on the microstructure of irradiated beryllium were at that time not available. Such a procedure is not fully satisfactory, since geometrical swelling and gas release are integral quantities, which cannot be univocally related to single microscopic quantities described by the gas kinetics model. In particular, the following phenomena have to be correctly predicted by the model at a microscopic level: (a) The nucleation of intragranular bubbles . In beryllium, fast neutrons produce large quantities of gas and a strong lattice damage at the same time. Dislocations are obstacles for the free diffusion of gas atoms and tend to act as traps. As a consequence, there is a strong interaction between dislocations kinetics and bubble nucleation. (b) The characteristics of bubbles. Bubbles are subdivided into classes: spherical bubbles inside
the grains and, at grain boundaries, significantly larger bubbles of a lenticular shape. As a consequence of growth, diffusion and coalescence phenomena, each class evolves in size, shape and concentration. (c) The formation of open porosity networks . The growth and coalescence of bubbles at grain boundaries lead to the formation of a complex 3D open porosity network. According to the model, this is the main escape way for the gas; gas atomic diffusion to free surfaces plays a negligible role. Furthermore, a sharp gas release peak has been observed during fast out-of-pile heating by some authors, e.g. Scaffidi-Argentina [4] and Rabaglino [5], and this phenomenon is not correctly predicted by ANFIBE, see Scaffidi-Argentina [3]. This behaviour has not been satisfactorily explained yet: it might be due to high temperature embrittlement of beryllium, see Pokrovsky [6], which may lead to extended microcracking, as suggested by Scaffidi-Argentina [7]. A careful microstructure characterisation of irradiated beryllium is necessary to obtain evidence of the above mentioned phenomena and to verify whether their description in the model is adequate or additional models should be implemented. In this case, the experimental results suggest a strategy for further theoretical developments. The relevant microscopic quantities that should be correctly predicted are, for each class of bubbles, their average size and volume/surface concentration, as a function of in-pile history and out-of-pile thermal treatments. The microstructure analyses should be coupled to gas release measurements, in order to correlate release stages to microscopic diffusion steps, see Rabaglino [5]. Since gas kinetics is influenced by the original microstructure and impurity content, the investigated material should be as similar as possible to the reference pebbles for the HCPB blanket, i.e. 1mm diameter pebbles, produced by NGK by rotating electrode process, impurity content specifications according to Piazza [8]. The relevance of the samples is then related to distinct model aspects which are to be validated, e.g. in order to study bubble nucleation, a weakly irradiated material should be used, since in a highly irra-
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
diated material this stage is not visible any more. On the other hand, in order to provide evidence of open porosity formation, highly irradiated material should be studied, or, as an alternative, also weakly irradiated samples, provided that they are heated to high temperatures. Also the investigation technique should be chosen on the basis of the investigated object: transmission electron microscopy (TEM) is suitable to investigate dislocations and about nanometer size bubbles at low neutron fluence and/or low temperature; optical microscopy is needed to study about micrometer size bubbles and gas release pathways at high fluence and/or high temperature. In order to prove the formation of a 3D open porosity network, the classical 2D techniques have obvious limitations: the following step is to develop a 3D microtomography method, based on synchrotron X-ray light.
3. The microstructure characterisation of pebbles from the BERYLLIUM irradiation experiment In order to provide data for a better validation of ANFIBE, weakly irradiated 2-mm diameter beryllium pebbles, fabricated by Brush and Wellman by fluoride reduction process, were characterised. The samples were irradiated in 1994 in the BERYLLIUM experiment in the high flux reactor (Petten). The essential irradiation data are as follows: irradiation temperature 780 K; fast neutron fluence (E /0.1 MeV) 1.24 /1025 nm 2, 480 appm 4He and 12 appm 3H production, according to Conrad [9]. Compared to the reference material, these pebbles are of lower metallurgical quality, their shape is irregular and they have a higher impurity content. Nevertheless, as far as grain size and microstructure are concerned, these samples are similar to the reference material, since they have large grains (40 /200 mm) without dislocations induced by cold-working, see Rabaglino [10]. Furthermore, their irradiation history is very well known. These characteristics make them suitable to study gas diffusion phenomena at low dose, with the aim to provide significant validation data for ANFIBE. By thermal treatments, the effect of temperature on bubble formation and develop-
771
ment was studied. The TEM micrograph in Fig. 1a shows the conditions of the inside of a grain at the end of the irradiation, which was carried out at about 780 K: in spite of the low dose and relatively high temperature, a high density of dislocations were formed during irradiation, but no visible gas bubbles. The gas is probably trapped in dislocations and impurities. This proves that dislocations have a relevant effect on gas diffusion also at high temperature and, vice versa, dislocations might be hindered in their movement and healing by the gas trapped in them. The TEM micrograph in Fig. 1b shows the conditions of the inside of a grain after a post-irradiation heating to about 1000 K. Small elliptic bubbles of some nanometres size have appeared on dislocations: this gives further confirmation to the above mentioned hypothesis that, at the end of the irradiation, a relevant part of the gas is trapped in dislocations. The optical microscopy micrograph in Fig. 1c shows the conditions of a grain after post-irradiation heating to about 1300 K. The intragranular bubbles have dramatically grown up to about micron sizes, and at grain boundaries large and elongated bubbles are visible: the formation of a porosity network appears to be in progress. In order to investigate gas release mechanisms, which are related to the development of porosity networks until they reach the outer surface of the pebble, a 2D characterisation technique is not sufficient, because of the inherent 3D geometry of these channels. To meet this need, a 3D microtomography investigation technique, based on X-ray synchrotron light, appears to be promising. The basic principle of this technique is that, starting from a series of 2D slices of the pebble along a certain axis, a 3D reconstruction of the sample and of the detail of its internal features can be attempted. Fig. 1d shows a high resolution 2D slice of a pebble after post-irradiation heating up to about 1450 K, obtained by microtomography: extended porosities are visible. These patterns have to be analysed with topological models to validate the percolation model in ANFIBE. The straight channel crossing the bottom left corner might be the result of a grain detachment. If confirmed by the 3D reconstruction, this could help to explain the occurrence of a sharp gas release peak at this temperature, see Rabaglino [5].
772
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
Fig. 1. Microstructure of beryllium pebbles from the BERYLLIUM irradiation experiment. Irradiation data: temperature 780 K; fast neutron fluence (E /0.1 MeV) 1.24 /1025 nm 2, 480 appm 4He and 12 appm 3H production. (a) After irradiation at 780 K, dislocations and Mg impurities (black dots) are visible, but no gas bubbles (TEM). (b) After post-irradiation heating to about 1000 K. Small elliptic bubbles appear (TEM). (c) After post-irradiation heating up to about 1300 K (optical microscopy). Elongated bubbles at grain boundaries are developing. (d) After post-irradiation heating up to about 1450 K (microtomography). Porosity networks and grain detachment are visible.
4. Conclusions In order to validate the code ANFIBE with the particular effectiveness required for far extrapolating predictions, single gas diffusion phenomena have to be studied in detail, at a microscopic, mechanistic level. A microstructure characterisation of relevant irradiated beryllium samples is essential. As an example, some results of the study of beryllium pebbles from the BERYLLIUM irradiation experiment have been presented. The applicability and attractiveness of the X-ray microtomography for the analysis of gas release
patterns at high dose or high temperature has also been proved.
Acknowledgements The authors would like to express a special thank to S. Malang and L. V. Boccaccini, for promoting and supporting the definition of the programme of activities for the validation and development of the ANFIBE code. The authors are then grateful to the EFDA-CSU Garching for financing the programme.
E. Rabaglino et al. / Fusion Engineering and Design 61 /62 (2002) 769 /773
References [1] S. Malang, Limitations on blanket performance, Fus. Eng. Des. 46 (1999) 193 /206. [2] F. Scaffidi-Argentina, M. Dalle Donne, C. Ronchi, C. Ferrero, ANFIBE: a comprehensive model for swelling and tritium release from neutron irradiated beryllium-I: theory and model capabilities, Fus. Techn. 32 (1997) 179 / 195. [3] F. Scaffidi-Argentina, M. Dalle Donne, C. Ronchi, C. Ferrero, ANFIBE: a comprehensive model for swelling and tritium release from neutron irradiated beryllium-II: comparison of model predictions with experimental results, Fus. Techn. 33 (1998) 146 /163. [4] F. Scaffidi-Argentina, Tritium and helium release from neutron irradiated beryllium pebbles from the EXOTIC-8 irradiation, Fus. Eng. Des. 58 /59 (2001) 641 /645. [5] E. Rabaglino, J.P. Hiernaut, C. Ronchi, F. ScaffidiArgentina, Helium and tritium kinetics in irradiated
[6]
[7]
[8]
[9] [10]
773
beryllium pebbles, Proc. 10th Int. Conf. on Fusion Reactor Materials, J. Nucl. Mat., in press. A.S. Pokrovsky, S.A. Fabritsiev, R.M. Bagautdinov, Yu. Goncharenko, High temperature beryllium embrittlement, J. Nucl. Mat. 233 /237 (1996) 841 /846. F. Scaffidi-Argentina, C. Sand, C.H. Wu, Tritium and helium retention in neutron irradiated beryllium, Proc. 5th Int. Workshop on Hydrogen in Solids, Stockholm, 2000. G. Piazza, in W. Bahm (ed.), Nuclear Fusion Programme Annual Report of the Association Forschungszentrum Karlsruhe/EURATOM, October 2000 /September 2001, Report FZKA 6650/EUR 20162 EN, Forschungszentrum Karlsruhe, 2002. R. Conrad, R. May, BERYLLIUM Final irradiation report, EC-JRC-IAM Report P/F1/96/15, 1996. E. Rabaglino, C. Ronchi, F. Scaffidi-Argentina, T. Wiss, Helium and tritium release and sub-microscopic restructuring in irradiated beryllium pebbles, Proceeding of the fifth IEA International Workshop on Beryllium Technology for Fusion, Moscow, 2001, in press.