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Compatibility of stainless steels and lithium based ceramics with beryllium
joumalof nuclear materials
Journal of Nuclear Materials 191-194 (1992) 163-167 North-Holland
Compatibility of stainless steels and lithium based ceramics with beryllium T. Flament, D. Herpin, L. Feve and J. Sannier DTM/ SCECF / SECNAU, CEN Fonlenay, BP6, 92265 Fontenay aux Roses Cedex, France 1ntroduction of beryllium as neutron multiplier in ceramic blankets of thermonuclear fusion reactors may give rise to the following compatibility problems: (a) oxidation of beryllium by ceramics or by water vapour, due to the high stability of beryllium oxide, (b) interaction of belyllium and structural materials. The present report completes previous results and constitutes a synthesis of our work carried out in the compatibility field: (j) interaction beryllium-steels: a mathematical expression giving the thickness of the diffusion zone as a function of exposure duration and temperature for 3J 6L and 1.4914 steels is proposed; (jj) oxidation of beryllium in contact with Li 2 0 is noticeable, this phenomenon being much less pronounced with ternary lithium based ceramics; (iii) the oxidation rate of beryllium in the presence of high water vapour content helium is high above 700°C.
1. Introduction
The introduction of beryllium as a neutron multiplier in ceramics blankets of thermonuclear fusion reactors may give rise to the following compatibility problems: (a) oxidation of beryllium by ceramics or by water vapour, due to the high stability of beryllium oxide; (b) interaction of beryllium and structural materials (austenitic and martensitic steels). In the framework of the European Programme on Fusion Technology, these problems are studied by using three types of devices: - type 1: capsules where couples of the different materials (e.g. steel and beryllium or ceramics and beryllium) are maintained under dynamic vacuum; - type 2: a loop where beryllium and ceramics arc swept by wet helium simulating tritium purge gas (10 to 1000 vppm of water vapour); - type 3: a loop where beryllium is swept by helium with high water vapour content (several %, corresponding to accidental conditions). Previous results obtained in the type 1 and 2 devices [1-3J, indicated a negligible interaction of beryllium with t~rnary ceramics below 650°C and the diffusion of beryllium in austenitic and martensitic steels from 600°C. The present report completes these results and constitutes a synthesis of our work.
2. Materials Three types of beryllium have been tested: - high oxygen-content beryllium (° 2 : 1000-5000 w1. ppm, Al: 500-650 w1. ppm, Fe: 300-500 wt. ppm), - low oxygen-content beryllium (02 : 50-100 wt. ppm), - Be-O.4%Ca alloy (Oz < 200 wt. ppm).
The two European reference structural materials, namely 316L austenitic steel (Cr: 17.44; Ni: 12.33; Mo: 2.3; Mn: 1.82 wt.%) and 1.4914 martensitic steel (Cr: 10.6; C: 0.13; Ni: 0.87; Mo: 0.8 wt. %) have been investigated. The LizO specimens were supplied by Argonne National LaboratOlY and dehydrated 3 h at 600 c C before testing.
3. Beryllium-steel interaction 3.1. Experimental
The experimental conditions of the hew series of tests are summarized in table 1. Contrary to the previous tests which included simultaneously beryllium, steels and ceramics, this series has been carried out only with steels and beryllium specimens in order to avoid a possible competition between metal oxidation and intermetallic formation. The major part of the tests has been performed under dynamic vacuum (type 1 test); however, for comparison, some tests have been done in the presence of dry sweeping gas (He or
He + Hz). 3.2. Corrosion morphology
The interaction of beryllium and steels leads to the formation of a discontinuous brittle superficial layer adherent to the steel and to the concomitant generation of holes in beryllium (fig. 1). Moreover, observations carried out in this layer by using backscattered electron microscopy and energy dispersive spectroscopy have revealed an important (up to 40 wt.%)
0022-3115j92j$05.00 © 1992 - Elsevier Science Publishers B.V. Ali rights reserved
164
T. Flament et al. / Compatibility of steels and ceramics with Be
Be
Interaction · la er
316L 100 JLm Fig. 1. Cross-section micrography of a Be-316L diffusion couple after 3000 h at 600°C.
and not homogeneous beryllium content (fig. 2). Three zones can be distinguished: ~ a dark zone with the highest Be-content and approximate concentrations of: 30% Fe, 30% Ni, 8% Cr, a gray zone containing 52% Fe, 7% Ni, 14% Cr, an internal zone with the lowest Be-content and containing white particles. These particles are too small to permit their analysis but seem to be chromium rich.
Values or diffusion-zone thickness are grouped on table 1. 3.3. Influence of exposure duration
As it can be deduced from these values, the thickness of the diffusion zone seems to depend linearly on the square root of the exposure duration for 316L and 1.49l4 steels. This parabolic law is in accordance with
Be
Int raction la er
10j.Lm
16L
Fig. 2. Back-scattered electrons image of a Be-316L diffusion couple after 3000 h at 600°C.
165
T. Flament et al. / Compatibility of steels and ceramics with Be Table 1 Experimental conditions and results of belyJlium--steels interaction tests Type of steel
Temperature [DC]
1.4914
700
1.4914
Type of beryllium
Type of test
500
ca
uv
12
600 600 650 700 700
500 3000 1500 500 500
ho ho ho ho ho
uv uv uv uv uv
30 20 20 30
1.4914
700
500
10
uv
25
316L
600 700 700
3000 500 500
ca
He uv uv
150 40 65
550
3000
ho ho ho ho ho
uv uv uv uv uv He uy uy uv uv
28 40 40 100 60 100 65 100 65
uy He uv uv
50 160 90 50
316L
316L
Exposure duration [h]
580 600 600 600 600 650 700 700 750
500 500 1500 3000 3000 1500 500 500
600
3000
600 700 750
500
3000 500 500
ca ca
ho ho ho ho ho
10 10 10
10
Maximal thickness of the layer 111m)
12
10
Abbreviations: ho, high oxygen-content beryllium; 10, low oxygen-content beryllium; ca, Be-O.4%Ca alloy; under swee\)ing He.
what can be expected when a diffusion process is occurring and with the results reported by Hofmann [4]. However, we have to point out that even if it is taken care to avoid any oxidation of the metallic surface, the steel-beryllium interaction appears to be irregular and not reproducible; for example, in one test with 316L and in two tests with 1.4914 steel no interaction was observed even after 3000 h at 60Q°C. This irregular behaviour is probably related to the more or less protective properties of the thin oxide layer always existing at the beryllium surface.
UY,
3.5. Type of steel
Our results show that 1.4914 steel exhibits a better behaviour than 316L steel probably because of its much lower nickel content, the diffusion of beryllium
Interaction of Be with structural steels
10000
0
0
• 316 L !CEA) 0316 \KfK\ • 1.4914 (CEAI
• •
i
-"'" , ~
3.4. Influence of temperature
As shown by fig. 3, the thickness of the diffusion zone is very temperature dependent. A linear regression, derived from this Arrhenius plot, leads to the following dependence of the thickness of the diffusion zone as a function of temperature and exposure duration:
e316 dJ..l.m] "" 2.3 X 10 6 exp( -105000/1.98T)t°.5[hJ, e1. 4914 [lJ..m] = 425 exp( -49000/1.98T)t. 0 .5[h].
under vacuum; He,
'""'
>,
E
1000
.::!-
'" '"c t5 Vl
::c
~
100
• 10 '=-----:c':----,J:----::'-:-----:-'::-----~ 0.8 0.9 1.0 1.1 lOOO/T
Fig. 3. Influence of temperature on thickness of the beryllium-steel interaction layer.
T. Flament et al. / Compatibility of steels and ceramics with Be
166
Table 2 Oxidation of beryllium in contact with LizO Temperature rOC]
Exposure duration [h)
Type of test
Thickness of the layer [f1m)
550 650 700
1700 500 500
He+H z
40 20 40
uv uv
He + Hz: under sweeping He + H z mixture; uv: under vacuum.
being faster in nickel than in iron [5]. Analysis of intermetallic compounds formed with austenitic steels indicates that beryllium rich zones exhibit a high nickel content. As shown in fig. 3, the difference between the two steels becomes smaller when the temperature is close to that of the steel-beryllium interface in the solid blanket design currently studied in Europe. 3.6. Type of beryllium Taking into account these results and those already published, the thickness of the diffusion zones measured with low oxygen-content beryllium or Be-O.4% Ca alloy remains in the scatter band of that obtained with high oxygen-content beryllium.
4. Beryllium-Li 2 0 interaction Whatever the temperature (550, 650, 700°C) and the environment (vacuum or He + 0.1 % Hz), the morphol-
Table 3 Oxidation of beryllium in the presence of helium containing 1.5 vol.% HzO Temperature rOC)
Exposure duration [h)
Weight loss
700 700 750 750 750
100 500 100 100 500
1.8± 0.6 8.6± 0.8 29 ± 9 49 ±14 152 ± 7
[mgcm-Z]
ogy of the interaction is similar. Beryllium is covered by a white oxide layer and its interface with LizO is indented (fig. 4). X-rays analysis has shown the presence of three crystallized phases: - BeD, - Li zC0 3 corresponding certainly to a carbonatation of LizO in the atmosphere after the compatibility test, - a phase whose X-ray diagram is close to Li 4 Be0 3 . Whatever the temperature, the interaction amplitude is relatively high. For example, it reaches 40 !Lm after 1700 h at 550°C or 500 h at 70Q°C. If we compare the thickness of the oxide layer measured in these tests (table 2) to those obtained with Li Z Zr0 3 or LiAlO z [1-3], it appears that LizO is much more reactive with beryllium than these ternary-based ceramics. As for Li 4 Si0 4 , a difference in behaviour is detectable at 550°C only [3,4]. This higher reactivity of LizO is in agreement with thermodynamic expectations deduced from the stability of the various ceramics. However, the influence of irradiation has to be taken
Fig. 4. Cross-section micrography of beryllium after 500 h at 70QoC in contact with LizO.
T. Flament et at. / Compatibility of steels and ceramics with Be
into account before concluding definitively on the comparison between the various lithium based ceramics.
5. Oxidation of beryllium by water
In order to assess the oxidation rate of beryllium in accidental conditions (leak of water used as coolant of some parts of the tokamak), a set of compatibility tests has been carried out in the type 3 test device. The experimental conditions are summarized in table 3. After exposition to sweeping helium containing about 1.5 vol.% water vapuur, the beryllium specimens (high oxygen-content) appear to be covered by a white oxide layer which is not adherent. Moreover cross-section micrographies of beryllium specimens show an irregular interface and some intergranular attack especially in zones where the cold worked layer due to machining (central hole and short faces of the specimen) has not been eliminated by polishing. As visible in table 3, the weight losses vary linearly, at 700 and 750°C, with respect to the exposure duration. This observation is consistent with the poor adherence of the oxide layer. Moreover, the oxidation rate is strongly temperature dependent since an increase of 50°C leads to an oxidation rate 20 times higher. Consequently, in the temperature range studied, the oxidation rate of beryllium can be expressed by the following expression:
L1m[ mg em -2]
=
10 24 exp( - 480 OOO/1.98T)t[h].
These values are close to those obtained in similar conditions by Almose et al. [6].
167
6. Conclusion From this compatibility study performed in inactive conditions, it appears that the critical issues relating to a ceramics blanket are the following ones: - interaction of beryllium and austenitic or martensitic steel when in contact; mathematical expression giving the thickness of the diffusion zone as a function of exposure duration and temperature for 316L and 1.4914 steel are proposed in this paper; - noticeable oxidation of beryllium in contact of Li 2 0, this phenomenon being much less pronounced with ternary lithium based ceramics; - high oxidation rate of beryllium in the presence of high water vapour content helium. Irradiation effect has also to be taken into account and results obtained in that field by means of the in-pile compatibility experiment, named SlBELIUS, are presented in these Proceedings [7].
References [1] T. Flament, P. Fauvet and J. Sannier, J. Nuc\. Mater. 155-157 (1988) 496. [2] A. Terlain, D. Herpin, P. Perodeaud and T. Flament, Fusion Techn. 2 (1988) 1175. [3] M. Broc, T. Flament, A. Terlain and J. Sannier J. Nuc1. Mater. 179 (1991) 820. [4] P. Hofman and W. Dienst, Proc. 16th Symp. on Fusion Technology, vo!. J, eds. E.E. Keen et a1. (North-Holland, 1991) p. 777. [5] A.G. Knapton and K.B.C. West, J. Nuc!. Mater. 3 (196]) 239. [6] D.W. Almose, J.S. Gregg and W.E. Jepson, J. Nuc!. Mater. 3 (1961) J90. [7] N. Roux et a!., in these Proceedings (ICFRM-S), J. Nuc!. Mater. J91-194 (1992) 168.
Journal of Nuclear Materials 191-194 (1992) 163-167 North-Holland
Compatibility of stainless steels and lithium based ceramics with beryllium T. Flament, D. Herpin, L. Feve and J. Sannier DTM/ SCECF / SECNAU, CEN Fonlenay, BP6, 92265 Fontenay aux Roses Cedex, France 1ntroduction of beryllium as neutron multiplier in ceramic blankets of thermonuclear fusion reactors may give rise to the following compatibility problems: (a) oxidation of beryllium by ceramics or by water vapour, due to the high stability of beryllium oxide, (b) interaction of belyllium and structural materials. The present report completes previous results and constitutes a synthesis of our work carried out in the compatibility field: (j) interaction beryllium-steels: a mathematical expression giving the thickness of the diffusion zone as a function of exposure duration and temperature for 3J 6L and 1.4914 steels is proposed; (jj) oxidation of beryllium in contact with Li 2 0 is noticeable, this phenomenon being much less pronounced with ternary lithium based ceramics; (iii) the oxidation rate of beryllium in the presence of high water vapour content helium is high above 700°C.
1. Introduction
The introduction of beryllium as a neutron multiplier in ceramics blankets of thermonuclear fusion reactors may give rise to the following compatibility problems: (a) oxidation of beryllium by ceramics or by water vapour, due to the high stability of beryllium oxide; (b) interaction of beryllium and structural materials (austenitic and martensitic steels). In the framework of the European Programme on Fusion Technology, these problems are studied by using three types of devices: - type 1: capsules where couples of the different materials (e.g. steel and beryllium or ceramics and beryllium) are maintained under dynamic vacuum; - type 2: a loop where beryllium and ceramics arc swept by wet helium simulating tritium purge gas (10 to 1000 vppm of water vapour); - type 3: a loop where beryllium is swept by helium with high water vapour content (several %, corresponding to accidental conditions). Previous results obtained in the type 1 and 2 devices [1-3J, indicated a negligible interaction of beryllium with t~rnary ceramics below 650°C and the diffusion of beryllium in austenitic and martensitic steels from 600°C. The present report completes these results and constitutes a synthesis of our work.
2. Materials Three types of beryllium have been tested: - high oxygen-content beryllium (° 2 : 1000-5000 w1. ppm, Al: 500-650 w1. ppm, Fe: 300-500 wt. ppm), - low oxygen-content beryllium (02 : 50-100 wt. ppm), - Be-O.4%Ca alloy (Oz < 200 wt. ppm).
The two European reference structural materials, namely 316L austenitic steel (Cr: 17.44; Ni: 12.33; Mo: 2.3; Mn: 1.82 wt.%) and 1.4914 martensitic steel (Cr: 10.6; C: 0.13; Ni: 0.87; Mo: 0.8 wt. %) have been investigated. The LizO specimens were supplied by Argonne National LaboratOlY and dehydrated 3 h at 600 c C before testing.
3. Beryllium-steel interaction 3.1. Experimental
The experimental conditions of the hew series of tests are summarized in table 1. Contrary to the previous tests which included simultaneously beryllium, steels and ceramics, this series has been carried out only with steels and beryllium specimens in order to avoid a possible competition between metal oxidation and intermetallic formation. The major part of the tests has been performed under dynamic vacuum (type 1 test); however, for comparison, some tests have been done in the presence of dry sweeping gas (He or
He + Hz). 3.2. Corrosion morphology
The interaction of beryllium and steels leads to the formation of a discontinuous brittle superficial layer adherent to the steel and to the concomitant generation of holes in beryllium (fig. 1). Moreover, observations carried out in this layer by using backscattered electron microscopy and energy dispersive spectroscopy have revealed an important (up to 40 wt.%)
0022-3115j92j$05.00 © 1992 - Elsevier Science Publishers B.V. Ali rights reserved
164
T. Flament et al. / Compatibility of steels and ceramics with Be
Be
Interaction · la er
316L 100 JLm Fig. 1. Cross-section micrography of a Be-316L diffusion couple after 3000 h at 600°C.
and not homogeneous beryllium content (fig. 2). Three zones can be distinguished: ~ a dark zone with the highest Be-content and approximate concentrations of: 30% Fe, 30% Ni, 8% Cr, a gray zone containing 52% Fe, 7% Ni, 14% Cr, an internal zone with the lowest Be-content and containing white particles. These particles are too small to permit their analysis but seem to be chromium rich.
Values or diffusion-zone thickness are grouped on table 1. 3.3. Influence of exposure duration
As it can be deduced from these values, the thickness of the diffusion zone seems to depend linearly on the square root of the exposure duration for 316L and 1.49l4 steels. This parabolic law is in accordance with
Be
Int raction la er
10j.Lm
16L
Fig. 2. Back-scattered electrons image of a Be-316L diffusion couple after 3000 h at 600°C.
165
T. Flament et al. / Compatibility of steels and ceramics with Be Table 1 Experimental conditions and results of belyJlium--steels interaction tests Type of steel
Temperature [DC]
1.4914
700
1.4914
Type of beryllium
Type of test
500
ca
uv
12
600 600 650 700 700
500 3000 1500 500 500
ho ho ho ho ho
uv uv uv uv uv
30 20 20 30
1.4914
700
500
10
uv
25
316L
600 700 700
3000 500 500
ca
He uv uv
150 40 65
550
3000
ho ho ho ho ho
uv uv uv uv uv He uy uy uv uv
28 40 40 100 60 100 65 100 65
uy He uv uv
50 160 90 50
316L
316L
Exposure duration [h]
580 600 600 600 600 650 700 700 750
500 500 1500 3000 3000 1500 500 500
600
3000
600 700 750
500
3000 500 500
ca ca
ho ho ho ho ho
10 10 10
10
Maximal thickness of the layer 111m)
12
10
Abbreviations: ho, high oxygen-content beryllium; 10, low oxygen-content beryllium; ca, Be-O.4%Ca alloy; under swee\)ing He.
what can be expected when a diffusion process is occurring and with the results reported by Hofmann [4]. However, we have to point out that even if it is taken care to avoid any oxidation of the metallic surface, the steel-beryllium interaction appears to be irregular and not reproducible; for example, in one test with 316L and in two tests with 1.4914 steel no interaction was observed even after 3000 h at 60Q°C. This irregular behaviour is probably related to the more or less protective properties of the thin oxide layer always existing at the beryllium surface.
UY,
3.5. Type of steel
Our results show that 1.4914 steel exhibits a better behaviour than 316L steel probably because of its much lower nickel content, the diffusion of beryllium
Interaction of Be with structural steels
10000
0
0
• 316 L !CEA) 0316 \KfK\ • 1.4914 (CEAI
• •
i
-"'" , ~
3.4. Influence of temperature
As shown by fig. 3, the thickness of the diffusion zone is very temperature dependent. A linear regression, derived from this Arrhenius plot, leads to the following dependence of the thickness of the diffusion zone as a function of temperature and exposure duration:
e316 dJ..l.m] "" 2.3 X 10 6 exp( -105000/1.98T)t°.5[hJ, e1. 4914 [lJ..m] = 425 exp( -49000/1.98T)t. 0 .5[h].
under vacuum; He,
'""'
>,
E
1000
.::!-
'" '"c t5 Vl
::c
~
100
• 10 '=-----:c':----,J:----::'-:-----:-'::-----~ 0.8 0.9 1.0 1.1 lOOO/T
Fig. 3. Influence of temperature on thickness of the beryllium-steel interaction layer.
T. Flament et al. / Compatibility of steels and ceramics with Be
166
Table 2 Oxidation of beryllium in contact with LizO Temperature rOC]
Exposure duration [h)
Type of test
Thickness of the layer [f1m)
550 650 700
1700 500 500
He+H z
40 20 40
uv uv
He + Hz: under sweeping He + H z mixture; uv: under vacuum.
being faster in nickel than in iron [5]. Analysis of intermetallic compounds formed with austenitic steels indicates that beryllium rich zones exhibit a high nickel content. As shown in fig. 3, the difference between the two steels becomes smaller when the temperature is close to that of the steel-beryllium interface in the solid blanket design currently studied in Europe. 3.6. Type of beryllium Taking into account these results and those already published, the thickness of the diffusion zones measured with low oxygen-content beryllium or Be-O.4% Ca alloy remains in the scatter band of that obtained with high oxygen-content beryllium.
4. Beryllium-Li 2 0 interaction Whatever the temperature (550, 650, 700°C) and the environment (vacuum or He + 0.1 % Hz), the morphol-
Table 3 Oxidation of beryllium in the presence of helium containing 1.5 vol.% HzO Temperature rOC)
Exposure duration [h)
Weight loss
700 700 750 750 750
100 500 100 100 500
1.8± 0.6 8.6± 0.8 29 ± 9 49 ±14 152 ± 7
[mgcm-Z]
ogy of the interaction is similar. Beryllium is covered by a white oxide layer and its interface with LizO is indented (fig. 4). X-rays analysis has shown the presence of three crystallized phases: - BeD, - Li zC0 3 corresponding certainly to a carbonatation of LizO in the atmosphere after the compatibility test, - a phase whose X-ray diagram is close to Li 4 Be0 3 . Whatever the temperature, the interaction amplitude is relatively high. For example, it reaches 40 !Lm after 1700 h at 550°C or 500 h at 70Q°C. If we compare the thickness of the oxide layer measured in these tests (table 2) to those obtained with Li Z Zr0 3 or LiAlO z [1-3], it appears that LizO is much more reactive with beryllium than these ternary-based ceramics. As for Li 4 Si0 4 , a difference in behaviour is detectable at 550°C only [3,4]. This higher reactivity of LizO is in agreement with thermodynamic expectations deduced from the stability of the various ceramics. However, the influence of irradiation has to be taken
Fig. 4. Cross-section micrography of beryllium after 500 h at 70QoC in contact with LizO.
T. Flament et at. / Compatibility of steels and ceramics with Be
into account before concluding definitively on the comparison between the various lithium based ceramics.
5. Oxidation of beryllium by water
In order to assess the oxidation rate of beryllium in accidental conditions (leak of water used as coolant of some parts of the tokamak), a set of compatibility tests has been carried out in the type 3 test device. The experimental conditions are summarized in table 3. After exposition to sweeping helium containing about 1.5 vol.% water vapuur, the beryllium specimens (high oxygen-content) appear to be covered by a white oxide layer which is not adherent. Moreover cross-section micrographies of beryllium specimens show an irregular interface and some intergranular attack especially in zones where the cold worked layer due to machining (central hole and short faces of the specimen) has not been eliminated by polishing. As visible in table 3, the weight losses vary linearly, at 700 and 750°C, with respect to the exposure duration. This observation is consistent with the poor adherence of the oxide layer. Moreover, the oxidation rate is strongly temperature dependent since an increase of 50°C leads to an oxidation rate 20 times higher. Consequently, in the temperature range studied, the oxidation rate of beryllium can be expressed by the following expression:
L1m[ mg em -2]
=
10 24 exp( - 480 OOO/1.98T)t[h].
These values are close to those obtained in similar conditions by Almose et al. [6].
167
6. Conclusion From this compatibility study performed in inactive conditions, it appears that the critical issues relating to a ceramics blanket are the following ones: - interaction of beryllium and austenitic or martensitic steel when in contact; mathematical expression giving the thickness of the diffusion zone as a function of exposure duration and temperature for 316L and 1.4914 steel are proposed in this paper; - noticeable oxidation of beryllium in contact of Li 2 0, this phenomenon being much less pronounced with ternary lithium based ceramics; - high oxidation rate of beryllium in the presence of high water vapour content helium. Irradiation effect has also to be taken into account and results obtained in that field by means of the in-pile compatibility experiment, named SlBELIUS, are presented in these Proceedings [7].
References [1] T. Flament, P. Fauvet and J. Sannier, J. Nuc\. Mater. 155-157 (1988) 496. [2] A. Terlain, D. Herpin, P. Perodeaud and T. Flament, Fusion Techn. 2 (1988) 1175. [3] M. Broc, T. Flament, A. Terlain and J. Sannier J. Nuc1. Mater. 179 (1991) 820. [4] P. Hofman and W. Dienst, Proc. 16th Symp. on Fusion Technology, vo!. J, eds. E.E. Keen et a1. (North-Holland, 1991) p. 777. [5] A.G. Knapton and K.B.C. West, J. Nuc!. Mater. 3 (196]) 239. [6] D.W. Almose, J.S. Gregg and W.E. Jepson, J. Nuc!. Mater. 3 (1961) J90. [7] N. Roux et a!., in these Proceedings (ICFRM-S), J. Nuc!. Mater. J91-194 (1992) 168.