- Email: [email protected]
Thermomechanical behaviour of ceramic breeder pebble stacks for HICU
Fusion Engineering and Design 69 (2003) 425 /429 www.elsevier.com/locate/fusengdes
Thermomechanical behaviour of ceramic breeder pebble stacks for HICU J.B.J. Hegeman a,*, E.D.L. van Essen a, M. Jong a, J.G. van der Laan a, J. Reimann b a
NRG Petten */Materials, Monitoring and Inspection, P.O. Box 25, 1755 ZG Petten, The Netherlands b Forschungszentrum Karlsruhe, Karlsruhe, Germany
Abstract This paper reports on the thermomechanical behaviour of small pebble beds or pebble stacks in order to lay out irradiation test specimens. Uniaxial compression tests (UCTs) of the pebble stacks with relatively small diameters are performed at 27 and 800 8C i.e. the maximum irradiation temperature. The H/D ratio of the specimens influences the thermomechanical behaviour because of the friction between the pebbles and the steel tube. The results are compared with the thermomechanical data obtained from standard UCTs, which apply for larger pebble-beds. The outcome of the comparison will be used for the detailed design of the constrained pebble-beds in the high fluence irradiation. In addition, the compatibility between the ceramic breeder materials and the containment is studied. It has been shown that platinum has the best compatibility at 800 8C for both the lithium meta-titanate and lithium ortho-silicate ceramics. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Blanket materials; Breeding materials; Pebble beds
1. Introduction As a part of the European programme for the development of the Helium Cooled Pebble Bed blanket concept a high fluence irradiation, HICU, in the High Flux Reactor is under development [1]. The HICU project concerns the investigation of the impact of neutron spectrum and the influence * Corresponding author. Tel.: /31-224-56-4246; fax: /31224-56-8883. E-mail address: [email protected] (J.B.J. Hegeman).
of constraint conditions on the thermomechanical behaviour of ceramic breeder pebble beds. The size of the irradiation specimens is limited due to the Cd-shielding that is used to tailor the neutron spectrum. The preliminary designs of the pebble stacks imply that they might be too small in order to apply the presently available tools for thermomechanical analysis of pebble beds. In particular, the ratio of pebble size to stack diameter is so large that the pebble bed can be no longer considered as a continuum. This prevents a straightforward analysis of stresses during irradiation and the
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00086-3
426
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
determination of pebble swelling and the effects on heat transfer. Therefore, thermomechanical pretesting of the specimens is required. In addition, the compatibility between the ceramic breeder materials and the containment is studied.
2. Materials The breeder materials used for the compatibility tests as well as for the uniaxial compression tests (UCTs) of the pebbles stacks are lithium orthosilicate produced by the melt-spraying process and lithium meta-titanate produced by the extrusion / spheronization /sintering process. The Li4SiO4 has a pebble size distribution from 0.25 to 0.63 mm and a pebble density of 2.35 g/cm3, which is 98% of the theoretical density. The size distribution of Li2TiO3-pebbles is 0.9 /1.2 mm with a density of 3.11 g/cm3.
3. Compatibility The pre-design thermal analyses of pebble stacks under neutron irradiation for DEMO relevant conditions showed that during irradiation the cladding will have temperatures over 550 8C, even up to 800 8C. At the highest temperatures, the interaction between the cladding and the breeder material may lead to the formation of a reaction layer on the cladding or may cause degradation of the cladding. Moreover, those interactions may influence the thermomechanical properties, swelling and heat transfer i.e. key properties to be studied during the high fluence irradiation. Furthermore, since the central temperature needs to be measured, the thermocouple cladding has to be compatible with the breeder material at 800 8C. Therefore, the compatibility of various structural materials with ceramic breeder materials has been studied. Solid breeder pebbles have been crushed and subsequently poured in tubes of different cladding materials. The specimens were heated up to 800 8C and were purged with helium, 0.1% hydrogen for times varying from 200 to 4000 h. The tests have been performed with crushed
pebbles ortho-silicate and meta-titanate breeder materials in contact with Eurofer-97, AISI 321 (pre-oxidised), Inconel 718 (with Ni coating), 15 / 15 Ti steel and platinum foil. After the heat treatment the tubes were impregnated, cut and polished. SEM and EDS analyses were used to study interaction, i.e. the reaction layers on the cladding and on the breeder material. A summary of the compatibility tests is given in Table 1. In Figs. 1 and 2, SEM micrographs of the cross-section of the compatibility tests are shown. In Fig. 1, a clear interaction between the orthosilicate and the AISI 312 stainless steel is demonstrated. Various oxide reaction layers are observed on the stainless steel, like iron-chromiumoxide and chromiumoxide. Also, on the breeder ceramic, reaction layers have been observed. For the AISI 321 cladding with a platinum protection foil no interaction is observed after 4000 h at 800 8C (Fig. 2). However, EDS analyses revealed that there is little chromium diffusion from the steel and silicon diffusion (may be from the steel or from the orthosilicate) into the Pt foil but there is no chromium diffusion into the ortho-silicate breeder after 4000 h. Table 1 Results of the compatibility tests at 800 8C purged with He/ 0.1% Ne for ortho-silicate and meta-titanate pebbles Materials
Time (h) MTi pebbles OSi pebbles
AISI 321
500 1000
/// ///
/// /
Inconel 718
500 1000
/// /
/// /
Eurofer97
500
///
/
Inconel 718 Ni-coated
500
///
/
15 /15 Ti steel
200 500
/ /
/ /
Pre-oxidised AISI 321
200 500
/// /
/ /
/ / / /
/ / / /
Platinum
200 1000 2000 4000
‘/’ are serious interactions between cladding and pebbles, ‘///’ some reactions and ‘/’ means negligible interactions.
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
Fig. 1. SEM micrographs of Pre-oxidised AISI 321 (4 h) 500 h in contact with OSi at 800 8C.
Fig. 2. SEM micrograph of the reaction between 321SS steel with Pt foil */4000 h in contact with OSi at 800 8C.
From Table 1, it can be concluded that orthosilicate is more susceptible to interaction with the cladding material than meta-titanate. Both breeder materials can sufficiently be protected from interaction with the structural material at 800 8C by a platinum foil of approx. 100 mm.
4. Thermomechanical behaviour UCTs have been performed with small pebble stacks with a large height to diameter ratio H/D. It had been shown previously that this ratio has a significant influence on the thermomechanical behaviour of ceramic pebble beds [2,3]. When the
427
H/D ratio is large, friction forces between the wall and the pebbles become important. Besides, the small ratio of stack diameter to pebble size affects the packing factor and packing density. Although the pebble stack is not representative for the blanket breeder bed, the thermomechanical behaviour needs to be studied to analyse the stresses, swelling and heat transfer during high fluence neutron irradiation. Nimonic steel tubes with a platinum foil preventing direct contact between the breeder materials have been designed with inner diameters of 4, 8 and 18 mm for the UCTs. The design allows compression from both sides of the cylinder in order to reduce friction effects with the wall, which will result in a non-homogeneous stress distribution in the pebble bed. For the UCTs an INSTRON tensile test-machine is used. After filling, the specimens were tapped and vibrated to densify the pebble beds. Subsequently, the bed height, filling weight and the packing factor were measured at room temperature. Filling procedures have been repeated 10 times to obtain accurate results for the filling factors. Then, the specimens were isothermally heated up to the test-temperature and, after stabilising in order to allow the granular material to expand freely, the compression test was performed with a loading/unloading speed of 0.8 MPa/min and a holding time of 5 min at 5 MPa. The specimens of ¥ 8/12 mm at 800 8C were compressed 3 times using the same (un)loading speed and holding time. The strain was calculated using the displacement of the actuator and the measured initial length after vibrating at room temperature. The test matrix, listed in Table 2 includes the values of the packing factors. It can be observed that for stack diameters smaller than approx. 10 / the average pebble diameter, the packing factor decreases with decreasing stack diameter. For the present stack diameters, this effect is more expressed for meta-titanate compared to orthosilicate due to the larger pebble size. The results of the UCTs are plotted in Figs. 3 and 4. At ambient temperature, the beds are much stiffer due to wall friction effects compared to standard UCTs performed with a H/D of 0.16, compare [2,3]. At 800 8C, the pebble beds show a
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
428
Table 2 Test matrix of the UCT of breeder ceramics for various H/D ratios Material
Stack dimension (mm /mm)
H/D
Bed density (g/cm3)
Packing factor (%)
Test temperature (8C)
Li4SiO4
¥4.14 /15.8 ¥4.18 /40.9 ¥8.10 /40.4 ¥8.10 /40.3 ¥12.13/39.7
3.8 9.8 5.0 5.0 3.3
1.45 1.48 1.53 1.53 1.50
61 62 64 64 64
803 795 794 22 791
Li2TiO3
¥4.18 /16.3 ¥4.18 /40.7 ¥8.12 /39.4 ¥8.12 /39.6 ¥12.16/40.1
3.9 9.7 4.8 4.9 3.3
1.68 1.74 1.90 1.90 1.93
54 55 61 61 62
815 790 795 22 789
Fig. 3. UCT for various stack heights and diameters of orthosilicate at 800 8C and room temperature.
Fig. 4. UCT for various stack heights and diameters of metatitanate at 800 8C and room temperature.
significant amount of creep. Thermal creep for the type of meta-titanate used in the present experiments is larger than for ortho-silicate, which is consistent with previous experiments [4]. The quantitative comparison of the present data with the previous creep results is difficult due to the small creep period in the present experiments. The creep strain is larger for the smaller beds. This indicates an additional influence of the ratio of pebble diameter to stack diameter. During the first unloading, still some creep can be noticed from the slight increase in strain in the graphs, which is not the case during unloading at room temperature. No significant fragmentation of pebbles was observed after loading up to 5 MPa during the UCT.
5. Conclusions A dedicated irradiation experiment, HICU, is being designed to assess the impact of neutron spectrum and neutron fluence on individual ceramic breeder pebbles and */if possible */the influence of constraint conditions on the thermomechanical behaviour of the pebbles in the irradiation capsules considered as pebble beds. From the pre-irradiation tests of representative stack geometries the following results were obtained: . Pt has been found as the most suitable material that is compatible with breeder ceramics at 800 8C in blanket typical purge gas condition
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
(He/0.1% H2). Pt-foil enhances the number of structural materials that can be used for the design. . Pebble stack size and constrains are being selected carefully to be reasonably representative for a pebble bed based breeding blanket! . After the performance of UCTs, no significant fragmentation was observed after loading up to 5 MPa for OSi and MTi. . UCTs with irradiation capsule relevant dimensions have shown distinct differences to standard UCTs. This is due to wall friction effects that influence strongly the present results (influence of stack height to diameter ratio) and fact that the pebble diameter is no longer always negligibly small compared to the stack diameter (most expressed for the meta-titanate pebbles and the smallest stack diameter). Therefore, considering the pebble bed assemblies in the irradiation capsules as pebble beds and using the corresponding thermomechanical modelling tools might be justified for ortho-
429
silicate and stack diameters larger than 8 mm but not for other conditions.
References [1] J.G. van der Laan et al., Key issues for the ‘HICU’ project */a high fluence irradiation of ceramic breeder pebble beds, CBBI-10, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 101 /106. [2] J. Reimann, D. Ericher, G. Wo¨rner, Influence of pebble bed dimensions and filling factor on mechnical pebble bed properties, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 161 /174. [3] J. Reimann, L. Boccaccini, M. Enoeda, A. Ying, Thermomechanics of solid breeder and Be pebble bed materials, 6th Int. Symp. Fusion Nucl. Techn., San Diego, USA, April 7 / 12, 2002. [4] J. Reimann, J.D. Lulewicz, N. Roux, G. Wo¨rner, Thermal creep of metatitanate pebble beds, CBBI-10, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 175 /184.
Thermomechanical behaviour of ceramic breeder pebble stacks for HICU J.B.J. Hegeman a,*, E.D.L. van Essen a, M. Jong a, J.G. van der Laan a, J. Reimann b a
NRG Petten */Materials, Monitoring and Inspection, P.O. Box 25, 1755 ZG Petten, The Netherlands b Forschungszentrum Karlsruhe, Karlsruhe, Germany
Abstract This paper reports on the thermomechanical behaviour of small pebble beds or pebble stacks in order to lay out irradiation test specimens. Uniaxial compression tests (UCTs) of the pebble stacks with relatively small diameters are performed at 27 and 800 8C i.e. the maximum irradiation temperature. The H/D ratio of the specimens influences the thermomechanical behaviour because of the friction between the pebbles and the steel tube. The results are compared with the thermomechanical data obtained from standard UCTs, which apply for larger pebble-beds. The outcome of the comparison will be used for the detailed design of the constrained pebble-beds in the high fluence irradiation. In addition, the compatibility between the ceramic breeder materials and the containment is studied. It has been shown that platinum has the best compatibility at 800 8C for both the lithium meta-titanate and lithium ortho-silicate ceramics. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Blanket materials; Breeding materials; Pebble beds
1. Introduction As a part of the European programme for the development of the Helium Cooled Pebble Bed blanket concept a high fluence irradiation, HICU, in the High Flux Reactor is under development [1]. The HICU project concerns the investigation of the impact of neutron spectrum and the influence * Corresponding author. Tel.: /31-224-56-4246; fax: /31224-56-8883. E-mail address: [email protected] (J.B.J. Hegeman).
of constraint conditions on the thermomechanical behaviour of ceramic breeder pebble beds. The size of the irradiation specimens is limited due to the Cd-shielding that is used to tailor the neutron spectrum. The preliminary designs of the pebble stacks imply that they might be too small in order to apply the presently available tools for thermomechanical analysis of pebble beds. In particular, the ratio of pebble size to stack diameter is so large that the pebble bed can be no longer considered as a continuum. This prevents a straightforward analysis of stresses during irradiation and the
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00086-3
426
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
determination of pebble swelling and the effects on heat transfer. Therefore, thermomechanical pretesting of the specimens is required. In addition, the compatibility between the ceramic breeder materials and the containment is studied.
2. Materials The breeder materials used for the compatibility tests as well as for the uniaxial compression tests (UCTs) of the pebbles stacks are lithium orthosilicate produced by the melt-spraying process and lithium meta-titanate produced by the extrusion / spheronization /sintering process. The Li4SiO4 has a pebble size distribution from 0.25 to 0.63 mm and a pebble density of 2.35 g/cm3, which is 98% of the theoretical density. The size distribution of Li2TiO3-pebbles is 0.9 /1.2 mm with a density of 3.11 g/cm3.
3. Compatibility The pre-design thermal analyses of pebble stacks under neutron irradiation for DEMO relevant conditions showed that during irradiation the cladding will have temperatures over 550 8C, even up to 800 8C. At the highest temperatures, the interaction between the cladding and the breeder material may lead to the formation of a reaction layer on the cladding or may cause degradation of the cladding. Moreover, those interactions may influence the thermomechanical properties, swelling and heat transfer i.e. key properties to be studied during the high fluence irradiation. Furthermore, since the central temperature needs to be measured, the thermocouple cladding has to be compatible with the breeder material at 800 8C. Therefore, the compatibility of various structural materials with ceramic breeder materials has been studied. Solid breeder pebbles have been crushed and subsequently poured in tubes of different cladding materials. The specimens were heated up to 800 8C and were purged with helium, 0.1% hydrogen for times varying from 200 to 4000 h. The tests have been performed with crushed
pebbles ortho-silicate and meta-titanate breeder materials in contact with Eurofer-97, AISI 321 (pre-oxidised), Inconel 718 (with Ni coating), 15 / 15 Ti steel and platinum foil. After the heat treatment the tubes were impregnated, cut and polished. SEM and EDS analyses were used to study interaction, i.e. the reaction layers on the cladding and on the breeder material. A summary of the compatibility tests is given in Table 1. In Figs. 1 and 2, SEM micrographs of the cross-section of the compatibility tests are shown. In Fig. 1, a clear interaction between the orthosilicate and the AISI 312 stainless steel is demonstrated. Various oxide reaction layers are observed on the stainless steel, like iron-chromiumoxide and chromiumoxide. Also, on the breeder ceramic, reaction layers have been observed. For the AISI 321 cladding with a platinum protection foil no interaction is observed after 4000 h at 800 8C (Fig. 2). However, EDS analyses revealed that there is little chromium diffusion from the steel and silicon diffusion (may be from the steel or from the orthosilicate) into the Pt foil but there is no chromium diffusion into the ortho-silicate breeder after 4000 h. Table 1 Results of the compatibility tests at 800 8C purged with He/ 0.1% Ne for ortho-silicate and meta-titanate pebbles Materials
Time (h) MTi pebbles OSi pebbles
AISI 321
500 1000
/// ///
/// /
Inconel 718
500 1000
/// /
/// /
Eurofer97
500
///
/
Inconel 718 Ni-coated
500
///
/
15 /15 Ti steel
200 500
/ /
/ /
Pre-oxidised AISI 321
200 500
/// /
/ /
/ / / /
/ / / /
Platinum
200 1000 2000 4000
‘/’ are serious interactions between cladding and pebbles, ‘///’ some reactions and ‘/’ means negligible interactions.
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
Fig. 1. SEM micrographs of Pre-oxidised AISI 321 (4 h) 500 h in contact with OSi at 800 8C.
Fig. 2. SEM micrograph of the reaction between 321SS steel with Pt foil */4000 h in contact with OSi at 800 8C.
From Table 1, it can be concluded that orthosilicate is more susceptible to interaction with the cladding material than meta-titanate. Both breeder materials can sufficiently be protected from interaction with the structural material at 800 8C by a platinum foil of approx. 100 mm.
4. Thermomechanical behaviour UCTs have been performed with small pebble stacks with a large height to diameter ratio H/D. It had been shown previously that this ratio has a significant influence on the thermomechanical behaviour of ceramic pebble beds [2,3]. When the
427
H/D ratio is large, friction forces between the wall and the pebbles become important. Besides, the small ratio of stack diameter to pebble size affects the packing factor and packing density. Although the pebble stack is not representative for the blanket breeder bed, the thermomechanical behaviour needs to be studied to analyse the stresses, swelling and heat transfer during high fluence neutron irradiation. Nimonic steel tubes with a platinum foil preventing direct contact between the breeder materials have been designed with inner diameters of 4, 8 and 18 mm for the UCTs. The design allows compression from both sides of the cylinder in order to reduce friction effects with the wall, which will result in a non-homogeneous stress distribution in the pebble bed. For the UCTs an INSTRON tensile test-machine is used. After filling, the specimens were tapped and vibrated to densify the pebble beds. Subsequently, the bed height, filling weight and the packing factor were measured at room temperature. Filling procedures have been repeated 10 times to obtain accurate results for the filling factors. Then, the specimens were isothermally heated up to the test-temperature and, after stabilising in order to allow the granular material to expand freely, the compression test was performed with a loading/unloading speed of 0.8 MPa/min and a holding time of 5 min at 5 MPa. The specimens of ¥ 8/12 mm at 800 8C were compressed 3 times using the same (un)loading speed and holding time. The strain was calculated using the displacement of the actuator and the measured initial length after vibrating at room temperature. The test matrix, listed in Table 2 includes the values of the packing factors. It can be observed that for stack diameters smaller than approx. 10 / the average pebble diameter, the packing factor decreases with decreasing stack diameter. For the present stack diameters, this effect is more expressed for meta-titanate compared to orthosilicate due to the larger pebble size. The results of the UCTs are plotted in Figs. 3 and 4. At ambient temperature, the beds are much stiffer due to wall friction effects compared to standard UCTs performed with a H/D of 0.16, compare [2,3]. At 800 8C, the pebble beds show a
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
428
Table 2 Test matrix of the UCT of breeder ceramics for various H/D ratios Material
Stack dimension (mm /mm)
H/D
Bed density (g/cm3)
Packing factor (%)
Test temperature (8C)
Li4SiO4
¥4.14 /15.8 ¥4.18 /40.9 ¥8.10 /40.4 ¥8.10 /40.3 ¥12.13/39.7
3.8 9.8 5.0 5.0 3.3
1.45 1.48 1.53 1.53 1.50
61 62 64 64 64
803 795 794 22 791
Li2TiO3
¥4.18 /16.3 ¥4.18 /40.7 ¥8.12 /39.4 ¥8.12 /39.6 ¥12.16/40.1
3.9 9.7 4.8 4.9 3.3
1.68 1.74 1.90 1.90 1.93
54 55 61 61 62
815 790 795 22 789
Fig. 3. UCT for various stack heights and diameters of orthosilicate at 800 8C and room temperature.
Fig. 4. UCT for various stack heights and diameters of metatitanate at 800 8C and room temperature.
significant amount of creep. Thermal creep for the type of meta-titanate used in the present experiments is larger than for ortho-silicate, which is consistent with previous experiments [4]. The quantitative comparison of the present data with the previous creep results is difficult due to the small creep period in the present experiments. The creep strain is larger for the smaller beds. This indicates an additional influence of the ratio of pebble diameter to stack diameter. During the first unloading, still some creep can be noticed from the slight increase in strain in the graphs, which is not the case during unloading at room temperature. No significant fragmentation of pebbles was observed after loading up to 5 MPa during the UCT.
5. Conclusions A dedicated irradiation experiment, HICU, is being designed to assess the impact of neutron spectrum and neutron fluence on individual ceramic breeder pebbles and */if possible */the influence of constraint conditions on the thermomechanical behaviour of the pebbles in the irradiation capsules considered as pebble beds. From the pre-irradiation tests of representative stack geometries the following results were obtained: . Pt has been found as the most suitable material that is compatible with breeder ceramics at 800 8C in blanket typical purge gas condition
J.B.J. Hegeman et al. / Fusion Engineering and Design 69 (2003) 425 /429
(He/0.1% H2). Pt-foil enhances the number of structural materials that can be used for the design. . Pebble stack size and constrains are being selected carefully to be reasonably representative for a pebble bed based breeding blanket! . After the performance of UCTs, no significant fragmentation was observed after loading up to 5 MPa for OSi and MTi. . UCTs with irradiation capsule relevant dimensions have shown distinct differences to standard UCTs. This is due to wall friction effects that influence strongly the present results (influence of stack height to diameter ratio) and fact that the pebble diameter is no longer always negligibly small compared to the stack diameter (most expressed for the meta-titanate pebbles and the smallest stack diameter). Therefore, considering the pebble bed assemblies in the irradiation capsules as pebble beds and using the corresponding thermomechanical modelling tools might be justified for ortho-
429
silicate and stack diameters larger than 8 mm but not for other conditions.
References [1] J.G. van der Laan et al., Key issues for the ‘HICU’ project */a high fluence irradiation of ceramic breeder pebble beds, CBBI-10, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 101 /106. [2] J. Reimann, D. Ericher, G. Wo¨rner, Influence of pebble bed dimensions and filling factor on mechnical pebble bed properties, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 161 /174. [3] J. Reimann, L. Boccaccini, M. Enoeda, A. Ying, Thermomechanics of solid breeder and Be pebble bed materials, 6th Int. Symp. Fusion Nucl. Techn., San Diego, USA, April 7 / 12, 2002. [4] J. Reimann, J.D. Lulewicz, N. Roux, G. Wo¨rner, Thermal creep of metatitanate pebble beds, CBBI-10, Proceedings of the 10th International Workshop on Ceramic Breeder Blanket Interactions, Karlsruhe, Germany, FZKA 6720, June 2002, pp. 175 /184.