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Tritium release from neutron-irradiated li2o; constant rate heating measurements
Journal of Nuclear Materials 95 (1980) 108-118 0 North-Holland Publishing Company
TRITIUM RELEASE FROM NEUTRON-IRRADIATED L&O; CONSTANT RATE HEATING MEASUREMENTS
Takaaki TANIFUJI, Kenji NODA, Shoichi NASU and Katsuya UCHIDA * Japan Atomic Energy Research Institute, Tokai-mura,Naka-gun, Ibarakiken, 319-11 Japan Received 20 March 1980; in revised form 27 May 1980
Based on the constant rate heating method, a,study of tritium release from neutron irradiated Liz0 has beenperformed for sintered pellets and single crystals. About 8% of the tritiated species’were released in gaseous forms and the other was released in condensible forms, both from sintered pellets (86% TD) and from single crystals. The rate limiting process of the release of condensible species from the sintered pellets was inferred to be due to the desorption mechanism. The apparent activation energies of the tritium release were determined from the maximum release temperatures of the release curves. The neutron fluence and flux effects on the tritium release were also examined.
1. Introduction
dies of tritium release from Liz0 due to the diffusion process have been reported by many workers [S-lo]. The isothermal annealing method was applied for studies of tritium release from commercially available Li20 powder [5-71. The diffusion constants at1873 and 923 K and the activation energies were determined on the basis of the equivalent sphere model [5], by the model described by Katz et al. [ll] and by assuming thermal decomposition of tritiated hydroxide formed in Li20. On the other hand, the isochronal annealing method was applied for studies of tritium release from Liz0 powder [8,9]. These studies were focused on the determination of chemical forms of release, tritiated species and their release temperatures. Unfortunately, these diffusion measurements mentioned above have been made using powder, and appropriate corrections such as surface effects must be made. In the sintered pellets, the tritium release rate is controlled by a number of factors, for instance, pellet density, gram size, pore distribution, temperature, neutron fluence, neutron flux and sweep gas effects. However, up to the present, the effect of sintered pellet density on the release temperature was preliminarily examined on the basis of the isochronal annealing method [9,10]. The important features of these previous works are summarized in table 1. In the present study, the constant rate of heating
In the conceptional designs of a tokamak experimental fusion reactor, JXFR, in JAERI [l] and a laser fusion reactor, SOLASE, in University of Wisconsin [2], lithium oxide (Li,O) has been selected as a solid state blanket breeding material because of its high lithium density (8.2 X 1O28Li atoms/m’) and high melting point (1700 K). Recovery of tritium from Liz0 produced by the 6Li(n,cr)3H and the ‘Li(n,n ‘o) 3H reactions is one of the essential problems for the design of fusion reactors. Tritium atoms are released from Liz0 by several processes, that is, diffusion, recoil and knockout processes. In the case of powder or sintered fine particles, such as the design of SOLASE, the tritium release due to the recoil and knockout processes are more dominant. The behavior of the recoil process has been studied using sintered Liz0 pellets [3] and Li20 single crystals [4] in our previous works. On the other hand, the tritium release due to the diffusion process is more dominant, in case of considerably large compacts such as the design of JXFR in which tritium will be extracted by helium gas coolant. Stu-
* On leave of absence from the postgraduated course of Nagoya University as a JAERI Research Student, 1977-1980. 108
T. Tanifuji etal. / Tritium release from neutron-irwdiated Liz0
109
Table 1 Summary of the previous works on tritium release from Liz0 Researchers
Sample form a)
Specifications powder diameter
Exper. method b,
Atmosphere
Neutron fluence G)
Neutron flux (m-? s-t)
Parameters determined
Wiswall and Wirsing ]Sl
P
-18 Bmand -16 brn
T
He stream
3.6 x 102’
10”
Diffusion constant
Gugii et al. (61
P
-1Opm
T
He stream
2
Kudo et al. 171
P
T and C
He and vacuum
4.8 x 1o19 3.8 x 10ZO
4 x 1o16 2.0-3.2 X 10”
Activation energy, release temperature
Kudo and Tanaka
P
C
Vacuum
4.8 x 1o19 3.6 x 10”
4 3
x 1o16 x 10”
Release temperature
Tanaka et al. ]91
PandS
C
Vacuum
3.6 X 10”
3
x 10”
Release temperature, density dependence
Hasu et al. [lo]
S
C
Vacuum
3
1o15
181
Activation energy
x 1o19
x 10ZO
Release temperature, density dependence
a) P: powder, S: sintered pellets. b, T: isothermal annealing method, C: isochronal annealing method.
method is applied for the first time for study of tritium release from Liz0 in the stream of helium gas using sintered pellets and single crystals irradiated from 2 X 1019 to 2 X 10” thermal neutrons/m’ in order to collect information about the activation energy without corrections for effective sample surface areas, release mechanism and fluence and flux effects of tritium release from Li?O.
2. Experimental 2.1. Specimens
Sintered pellets and single crystals of Li20 were used as specimens in this tritium release experiment. The sintered pellets were prepared by the usual presssintering method. Since the preparation method is described in detail elsewhere [ 121, the most important features are briefly presented here. The Liz0 powder (99% in purity, Cerac/Pure Co. Ltd.) was
pressed into a pellet of about 12 mm in diameter and 10 mm in height, and was sintered for 4 h at 1570 K under vacuum. Li20 pellets of 86% theoretical density (TD) (grain size was about 30 W) were cut into cubes (1.6 X 1.6 X 1.6 mm, -8 mg) with a diamond cutter. The specimens were annealed for 2 h at 1070 K under vacuum to decompose LiOH and/or Li2COs which may be formed on the surfaces of specimens. Liz0 single crystals were prepared by the floating zone growth method reported in this journal [13]. The powder of Liz0 was hydrostatically pressed into a rod of about 10 mm in diameter and 100 mm in length and sintered for 2 h at 1370 K under vacuum. This rod was grown to be a single crystal by the floating zone growth method using an infrared imaging furnace with a pair of halogen lamps in an argon atmosphere. The crystal was crushed into small pieces and those with reasonable size (-2 mm in one dimension) and weight (5-12 mg) were manually selected. Typical impurities in Liz0 were analyzed by
T. Tanifuji et al. / Tritium release porn neutron-irradiatedLiz0
110
Dl
HELIUM GAS -
’ RECORDER DIGITAL FRI NTER
T
AMPLIFIER SCALER
FLOW CONTROLLER
Z
vv STACK *
I
I
IONIZATION TUBE
METHANE GAS Fig. 1. Apparatus used for measurement of release of total tritiated species.
extraction-photometry; Fe(61 ppm), Al(55 ppm), Co(<2 ppm), Cr(
2.3. Apparatus and procedure The apparatus for measuring the total tritiated species released from the specimens is schematically shown in fig. 1. The total tritiated species released from Liz0 were introduced into a proportional counter with pure helium carrier passing through a
trap cooled down to liquid nitrogen temperature to remove traces of water. The specimens were heated in a bent platinum plate in a resistance furnace to avoid contact with quartz walls, because Liz0 reacts with quartz above 670 K [ 141. Since it is known that most tritium is released from Liz0 as TzO and HTO [9,10], zinc powder cleaned with dilute nitric acid, of which the temperature was kept at about 670 K, was used to reduce T20 and HTO to tritium. The flow rate of helium was kept 60 ml/mm which was mixed with methane gas in the ratio 5 : 1 before the mixture of the gas was fed into a proportional counter. The apparatus for measuring gaseous trititum is schematically shown in fig. 2. The gaseous fraction of tritiated species through a cold trap (dry ice-alcohol) at 201 K is directly fed to the proportional counter. Sample temperatures were raised linearly with heating rates of 1, 5 and 10 K/min up to 1270 K and were maintained at constant temperature until the measured tritium activity became negligibly small, when it is compared with back ground. After the above procedure, the specimens were dissolved into water and the residual tritium in the specimens were determined to be neglisibly small by liquid scintillation counting.
111
T. Tanifuii et al. / Tritium release from neutron-irmdiated Liz0
HELIUM
TEMPERATURE CONTROLLER
GAS
I I
RECORDER DIGITA_L _-INTER
&THERMOCOUPLE
I
I
FURNACE
LIQUID NITROGEN
FI-_nw. . CONTROLLER
~1 IA--) I
DRY IC ALCOHOL
,
I
1
I
I
IU
YIALK
, GAS
Fig. 2. Apparatus used for measurement of release of gaseous tritiated species.
3. Results and discussion
10
3.1. Wtium releasefrom sintered pellets
6
3.1.1. Total t-Matedspecies The release behavior of total tritiated species from the pellets having 86% TD is shown in fig. 3 as a function of temperature for neutron fluences of 2 X 1019, 2 X 102’ and 2 X 102’ neutrons/m’. The percentage of the released tritiated species at heating rates of 1, 5 and 10 K/mm, the data of which showed some scattering for each run, was about 95%, 83% and 77%, respectively after heating up to 870 K, and about lOO%, 98% and 96%, respectively after heating up to 1270 K for every fluence. Observed maximum release temperatures are shown in table 2. For analysis of the release behavior, the first order case is assumed, in which the release rate has the Arrhenius form,
6
66%TD PELLET TOTAL TRITIUM 1 K / min 5K /min -
-o---a----b-
W/dt =A(1 -F)
exp(-E/RT),
10K /min
(1)
where dF/dt is the release rate, F the fraction released A the constant term and E the apparent activation energy. The temperature is raised linearly with time as T = To + Bt, where B is the linear heating rate and
Fig. 3. Fractional release of total tritiated species from sintered pellets. TEMPERATURE
, K
T. Tan@@ et al. / Tritium release from neutron-irmdiated Liz0
112
dT = B dt. This leads to dF/dT = @/B)(l - F) exp(-E/RT)
.
(2)
The maximum release temperature in the release curve is defined by the condition of d2F/dT2 = 0, which yields ln(TE/B)
= E/RT, + constant ,
(3)
where TP is the maximum peak temperature. Plots of ln(Ti/B) versus l/T, yields activation energies for each fluence as shown in fig. 4. Table 2 also represents the activation energies calculated by the least squares method for each fluence. This result shows that the activation energies are independent of neutron fluence and flux. The average activation energy is obtained to be 148 + 18 kJ/mol taking three activation energies of each fluence as shown in table 5, which is higher than that reported by Kudo et al. [7] (78.6 kJ/mol) for condensed tritiated species released from Liz0 powder under vacuum. This lack of agreement may be due to differences of sample form. It is well known that the release rates for powders may be much greater than the Values predicted from single crystal or sintered pellet data by several
orders of magnitude and also may be characterized by different activation energies. Lehovec [ 151 has discussed that surface charge effects would influence gas release rates. This effect could be particularly important in powders. 3.1.2. Gaseous fraction Fig. 5 shows the release behavior of gaseous fraction of tritiated species from the sintered pellets having 86% TD. The amount of gaseous tritiated species was determined by comparing each specimen weight used for the experiments of total and gaseous tritiated species, on the assumption that the amount of tritiated species produced in the specimens irradiated to the same fluence was proportional to the specimen weight. The percentage of gaseous tritiated species released from the specimens and the maximum release temperatures are summarized in table 3. The release behavior for gaseous tritiated species is analyzed by the same procedure given in the previous section. The Arrhenius plots for each fluence and their activation energies are shown in fig. 6 and in table 5, respectively. The release behavior of gaseous
Table 2 Maximum release temperatures and activation energies for the release of total tritiated species and chemisorbed from sintered pellets (2-4 X 10” n/m2 . s*; 2-4 X 1015 n/m2 . s) X
Neutron fluence
2
1019 n/m2
Heating rate (K/min)
Maximum release temperature W)
X
2 X 102t n/m2
Desorption from unirradiated peilet surface
Maximum release temperature W)
Maximum release temperature W)
Maximum release temperature W
2
10” n/m2
1 1 1 1
669 678 678 679
671 *678 680
663 *673
671 679
5 5 5
696 703 *705
*699 702 703
709 *719
688 703 707
10 10 10 10
733 736 738 738
*721 723 735
720 *736
718 720 723 738
170 * 20
135 f 19
148 f 26
Activation energy (kJ/mol) -
140 i 16
tritiated water
113
T. Tanifui et.al. / Tritium release from neutron-irmdiated Liz0 I
I
6.0 -
tritiated species is independent of the neutron fluence and flux as it is the case of the release behavior of total tritiated species. From the above results, it is found that the condensible fraction, which has been identified as tritiated water [lo], was about 92% of the total tritiated species released from the sintered pellets.
I
I
66% TD PELLET TOTAL TRITIUM
140
E=
kJ/mol
3.1.3. Desorption of chemisorbed sintered pellets
1170
E
6.0 t
2x102’
tritium on the
Desorption of chemisorbed tritium on the sintered pellets was investigated based on the temperature programmed desorption (TPD) technique to clarify the mechanism of tritiated water release from Li20 pellets. The unirradiated and irradiated specimens were set in an entrance of the trap cooled down to dry icealcohol temperature (201 K) and in the specimen holder in the apparatus shown in fig. 2, respectively.
kJ/mol
n/n+
^m 7.5 . “i -
fz
7.0 t
Fig. 4.ln(Ti/B) versus l/Tp for total tritiated species released from sintered pellets, where Tp is the maximum release temperature of the fractional release, B the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ 5 n/m2 . s, circles: 2-4 X 10” n/m2 -
kJ/mol
6.5 -
I I
I
I
I
I.3
1.4
1.5
lo3
Table 3 Maximum release temperatures 2-4 X 1015 n/m2 . s) X
/
I
,
1.6
9.
Tp , K-’
and percentage of gaseous tritiated species released from sintered pellets (2-4
Neutron fluence
2
10” n/m2
Heating rate Wmin)
Maximum release temperature (K)
2 Gaseous fraction (%I
X
10” n/m2
Maximum release temperature W
2 Gaseous fraction (%I
X
x
IOr’ n/m2 . s*;
102’ n/m2
Maximum release temperature (0
Gaseous fraction (%I
1 1 1
661 669 679
2.5 7.4 6.1
683 686 699
2.0 4.2 1.8
678 *688
2.2 3.8
5 5 5
*718 741 748
2.9 7.3 3.9
721 746 *763
13.8 10.0 8.2
763 *783
8.7 4.2
10 10 10
795 *808 813
6.7 8.8 4.8
*773 776 788
13.3 6.5 6.6
817 *830
10.6 16.4
Average
5.6 f 2.2
7.4 f 4.4
7.6 f 5.3
114
T. Tanififi et al. / Tn’tium release from neutron-irradkted Liz0 1
0.3 0.4
GASEOUS
!WWTlON
-o-
1 K /min
---*----o-iOK
5K /min
8.0
I
,
86% TD
PELLET GASEOUS FRACTION
/min
0.3
8.0
1’
2 xiOM n/n-?
I
1.2
I
1.3
1.4
1.5
lo3 / Tp , K-’
TEMPERATURE
,
K
Fig. 6.ln(Ti/B) versus l/Tp for gaseous tritiated species released from sintered pellets, Tp is the maximum release temperature, E the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ s n/m* - s, circles: 2-4 X 10’ ’ n/m* . s).
Fig. 5. Fractional release of gaseous tritiated species from sintered pellets.
Tritiated water was then adsorbed on the unirradiated specimen. After this procedure, the irradiated specimen was taken from the specimen holder, the chemisorbed specimen was set in the specimen holder, and desorbed tritium was measured by the same manner described in the previous sections. Typical results are shown in fig. 7. The maximum release temperatures were observed at similar temperatures to those in case of total tritiated species released from the pellets. According to Lord and Kittelberger [ 161, the method traditionally used for determining the activa-
tion energy in first order cases can be extended to second and probably higher order cases regardless of the kinetic order in case of thermal desorption experiments by employing the constant rate heating method. Therefore, the activation energy of thermal desorption was determined to be 148 f 26 kJ/mol by plots of ln(Z$/B) versus l/T, assuming a first order case as shown in fig. 8. This value is in good agreement with the average activation energy for the release of total tritiated species from the sintered pellets. These facts suggest that the release of tritiated water from the sintered Liz0 pellets irradiated by
115
T. Tanifuji et al, / Tritium release from neutron-hradiated Liz0
---m---
--A-
5 K
/min
10K /min
kJ/
I
1.3
1.4
mol
1.5
103/Tp,K-’ Fig. 8.ln(Ti/B) versus l/Tp for desorption of tritiated water chemisorbed on sintered pellets, where Tp is the maximum release temperature, B the linear heating rate. The slope of its line gives the activation energy. Fig. 7. Fractional desorption on sintered pellets.
of tritiated water chemisorbed
neutrons is controlled by the desorption mechanism. Recently, Cathcart et al. [17] and Scott et al. [ 181 concluded that during diffusion of tritium in TiOz (rutile) and BeO, tritium ions were associated with the oxygen ions of the host material, and they suggested that diffusion occurred either by movement of tritium ions from one oxygen to another or by migration of the tritiated hydroxide ions. It may be possible that tritiated hydroxide ions migrate to the surface and stayed in situ due to chemisorption or due to formation of tritiated hydroxide before tritiated water is released from the pellet surface. Kudo et al. [7] suggested that the recoiled tritium was first stabilized as tritiated hydroxide in the LizO, and tritiated water was released by the decomposition of tritiated hydroxide. Kudo [ 191 also studies the decomposition reaction of LiOH under vacuum and determined the activation energy to be 123 ?r 5 kJ/ mol, which is in good agreement with that obtained in this experiment (148 + 18 kJ/mol). However, there is no evidence that the release rate of tritiated water is controlled by decomposion of tritiated hydroxide, since, in the present experiment, the behavior of a micro amount of tritiated species was studied under dynamic helium atmosphere, while in the study of LiOH docomposition a reasonable amount of specimens was used under vacuum.
3.2. Tritium release from single crystals 3.2.1. Total Wiated species The typical release curves of the total tritiated species from single crystals are shown in fig. 9. The percentage of released tritiated species, which depends slightly on neutron fluence, was about 35%, 15% and 10% after heating up to 870 K and was about 98%, 85% and 70% after heating up to 1270 K at heating rates of 1,s and 10 K/min, respectively. In comparison with the sintered pellets, the release rates for the single crystals are considerably lower and the release peaks shift to the higher temperature side. Most of the tritiated species was tritiated water as it is the case for sintered pellets (this identification wilI be described in the following section). These results suggest that tritiated hydroxide ions were removed very fast through the grain boundaries in the pellets, while in the single crystals, tritiated hydroxide ions were removed very slowly through the grain, that is, the grain boundaries in the pellets fill the role of a high-diffusivity path in the migration of tritiated hydroxide ions. It is interesting to note that the peak shape becomes sharper with increasing neutron fluence especially at the heating rate of 1 K/mm. At a fluence of 2 X 102’ neutrons/m2, the peak shape was the sharpest and the maximum release temperature was about 920 K. The activation energy was not evaluated owing to a broad and unresolved peak shape.
T. Tamfiji et al. / Tritium release from neutron-irradiatedLiz0
116
&NGLE CRY&IL 5
TOTAL
4
o.5
TRITIUM 0
1 K /min
l
SKImin
2 x i0”
n /m2
SINGLE
CRYSTAL
1’
A lOK/min
OI
ae
I
_ 0.5
2 X~020n/m2
h k -
F
:
0.4
2 =
0.3
0
a 0.2 < g
0.1
5 2
0.5
0
IL
I
’
I
2x ic?‘” nff,o”o,
0.3
0.
A 0*
0.2
TEMPERATURE TEMPERATURE,
K
Fig. 9. Fractional release of total tritiated species from single crystals.
3.2.2. Gaseous fraction
Fig. 10 illustrates the gaseous fraction release features from the single crystals. The maximum peak temperatures and the percentage of gaseous tritiated species are represented in table 4. The release behavior of gaseous fraction from single crystals is independent of neutron fluence and flux and the amount of gaseous tritiated species released from single crystals is comparable (8%) to that from the sintered pellets. The maximum release temperatures
, K
Fig. 10. Fractional release of gaseous tritiated species from single crystals.
are higher than those for sintered pellets. The activation energies are shown in table 5, the Arrhenius plots are shown in fig. 11. The values of the activation energies depend on neutron fluence and are higher than those for the sintered pellets. In the case of high fluences (2 X 10zo and 2 X 10” neutrons/m2), the activation energies are considerably lower than those for the specimens irradiated to 2 X 1Ol9 neutrons/m2. This suggests that irradiation induced defects and clusters play an important role in the gaseous tritium release from single crystals.
Table 4 Maximum release temperatures 2-4 x 10” n/m2 * s)
and percentage of gaseous tritiated species released from single crystals (2-4
Neutron fluence
2 X lOI
n/m*
Heating rate Wnin)
Maximum reIease temperature WI
2 Gaseous fraction (%I
X
lo*’ n/m2
2
Maximum release temperature WI
X
X
Gaseous fraction (%I
1 1 1
835 842 843
14.0 8.6 9.4
771 778 808
6.8 3.7 4.6
796 *803
5.8 8.8
5 5 5
*876 888 898
2.7 8.2 3.7
*833 *848 851
13.1 6.4 4.7
*831 858 *888
13.1 9.2 12.6
10 10 10
911 *911 913
5.0 9.9 6.9
865 *873 876
11.2 5.4 11.0
893 *896 903
5.3 8.6 16.1
Average
7.6 i 3.5
. s*;
10” n/m’
Maximum release temperature (K)
Gaseous fraction (%I
10” n/m2
9.9 * 3.7
7.4 f 3.4
SINGLE CRYSTAL GASEOUS FRACTION
&!
Table 5 Activation energies for the release of total and gaseous tritiated species from sintered pellets and single crystals (kJ/mol) Neutron fluence (n/m2)
E- 180 kJ/moi
-
kJ/mol
~
Single crystals
Sintered pellets
Total tritiated species
Gaseous fraction
Gaseous fraction
2 x io*r
140 f 16 170 f 20 135 f 19
62 f 8 85 f 10 64 f 5
184 f 14 125 * 16 110 f 19
Average
148 f 18
70 f 8
8.0
E = Ii0 kJ/mol Fig. 11. ~(T~/~) versus l/Z’n for gaseous tritiated species released from single crystals, where Tp is the m~imum release temperature, B the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ s n/ m* . s, circles: 2-4 X 10” n/m* 0s).
6.5
I
1
L
I
I
1.0
1.1
1.2
1.3
IO3 / Tp , K-’
1
118
T. Tonifiji et al. / Tritium release from neutron-irradiated Liz0
4. summary (1) About 8% of the tritiated species were released in the gaseous form, and the rest was released in the condesible form, which was collected in the cold trap cooled down to 201 K, both from the sintered pellets and from the single crystals. (2) The release features of the total tritiated species and the gaseous species from the sintered pellets (86% TD) are independent of neutron fluence and flux. (3) In the sintered pellets (86% TD), the release features of the total tritiated species such as the maximum release temperature and the apparent activation energy are similar to those of desorption of tritiated water chemisorbed on the sintered pellets. (4) In the single crystals, the release features of the total tritiated species depend on neutron fluence but do not depend on neutron flux.
Acknowledgements The authors wish to express their thanks to Drs. S. Mori, S. Nomura, J. Shimokawa, Y. Obata, R. Nagasaki, T. Kikuchi, K. Iwamoto and K. Sako for their interest in this work.
References [l) K. Sako, M. Ohta, Y. Seki, H. Yamato, T. Hiraoka, T. Tanaka, N. Asami and S. Mori, JAERI-M 5502 (1973).
[21 E.M. Larsen, S.I. Abdel-Khahk and MS. Ortman, Nucl. Technol. 41 (1978) 12. [3] M. Akabori, K. Uchida, K. Noda, T. Tanifuji and S. Nasu, J. Nucl. Mater. 83 (1979) 330. [4] K. Uchida, M. Akabori, K. Noda, T. Tanifuji and S. Nasu, J. Nucl. Mater. 89 (1980) 92. [S] R.H. WiswaB and E. Wlrsing, BNL-50748 (1977). [ 61 D. Cuggi, H.R. Ihel and U. Kurz, in: Proc. 9th Symp. on Fusion Technology (1976) p. 337. [7] H. Kudo, K. Tanaka and H. Amano, J. Inorg. Nucl. Chem. 40 (1978) 363. [S] H. Kudo and K. Tanaka, Radiochem. Radional Letters 23 (1975) 57. [9] K. Tanaka, H. Kudo and H. Amano, in: Proc. Intern. Conf. on Radiation Effects and Tritium Technology for Fusion Reactors, GatIinburg, TN, 1975. [lo] S. Nasu, H. Kudo, K. Shiozawa, T. Takahashi, T. Kurasawa, M. Tachiki and K. Tanaka, J. Nucl. Mater. 68 (1977) 261. [ 111 U. Katz, M. Guinan and R.J. Borg, Phys. Rev. B4 (1971) 330. [12] T. Takahashi and T. Kikuchi, JAERI-M 7518 (1978). [ 131 I. Shindo, S. Kimura, K. Noda, T. Kurasawa and S. Nasu, J. Nucl. Mater. 79 (1979) 375. [ 141 T. Tanifuji and S. Nasu, to be published. [15] K. Lehovec, J. Chem. Phys. 21 (1953) 1123. [ 161 F.M. Lord and J.S. Kittelberger, Surface Sci. 43 (1974) 173. [17] J.V. Cathcart, R.A. Parkins, J.B. Bate and L.C. Manley, J. Appl. Phys. 50 (1979) 4110. [18] K.T. Scott and L.L. Was&, Proc. Brit. Ceram. Sot. 7 (1967) 375. 119) H. Kudo, J. Nucl. Mater. 87 (1979) 185.
TRITIUM RELEASE FROM NEUTRON-IRRADIATED L&O; CONSTANT RATE HEATING MEASUREMENTS
Takaaki TANIFUJI, Kenji NODA, Shoichi NASU and Katsuya UCHIDA * Japan Atomic Energy Research Institute, Tokai-mura,Naka-gun, Ibarakiken, 319-11 Japan Received 20 March 1980; in revised form 27 May 1980
Based on the constant rate heating method, a,study of tritium release from neutron irradiated Liz0 has beenperformed for sintered pellets and single crystals. About 8% of the tritiated species’were released in gaseous forms and the other was released in condensible forms, both from sintered pellets (86% TD) and from single crystals. The rate limiting process of the release of condensible species from the sintered pellets was inferred to be due to the desorption mechanism. The apparent activation energies of the tritium release were determined from the maximum release temperatures of the release curves. The neutron fluence and flux effects on the tritium release were also examined.
1. Introduction
dies of tritium release from Liz0 due to the diffusion process have been reported by many workers [S-lo]. The isothermal annealing method was applied for studies of tritium release from commercially available Li20 powder [5-71. The diffusion constants at1873 and 923 K and the activation energies were determined on the basis of the equivalent sphere model [5], by the model described by Katz et al. [ll] and by assuming thermal decomposition of tritiated hydroxide formed in Li20. On the other hand, the isochronal annealing method was applied for studies of tritium release from Liz0 powder [8,9]. These studies were focused on the determination of chemical forms of release, tritiated species and their release temperatures. Unfortunately, these diffusion measurements mentioned above have been made using powder, and appropriate corrections such as surface effects must be made. In the sintered pellets, the tritium release rate is controlled by a number of factors, for instance, pellet density, gram size, pore distribution, temperature, neutron fluence, neutron flux and sweep gas effects. However, up to the present, the effect of sintered pellet density on the release temperature was preliminarily examined on the basis of the isochronal annealing method [9,10]. The important features of these previous works are summarized in table 1. In the present study, the constant rate of heating
In the conceptional designs of a tokamak experimental fusion reactor, JXFR, in JAERI [l] and a laser fusion reactor, SOLASE, in University of Wisconsin [2], lithium oxide (Li,O) has been selected as a solid state blanket breeding material because of its high lithium density (8.2 X 1O28Li atoms/m’) and high melting point (1700 K). Recovery of tritium from Liz0 produced by the 6Li(n,cr)3H and the ‘Li(n,n ‘o) 3H reactions is one of the essential problems for the design of fusion reactors. Tritium atoms are released from Liz0 by several processes, that is, diffusion, recoil and knockout processes. In the case of powder or sintered fine particles, such as the design of SOLASE, the tritium release due to the recoil and knockout processes are more dominant. The behavior of the recoil process has been studied using sintered Liz0 pellets [3] and Li20 single crystals [4] in our previous works. On the other hand, the tritium release due to the diffusion process is more dominant, in case of considerably large compacts such as the design of JXFR in which tritium will be extracted by helium gas coolant. Stu-
* On leave of absence from the postgraduated course of Nagoya University as a JAERI Research Student, 1977-1980. 108
T. Tanifuji etal. / Tritium release from neutron-irwdiated Liz0
109
Table 1 Summary of the previous works on tritium release from Liz0 Researchers
Sample form a)
Specifications powder diameter
Exper. method b,
Atmosphere
Neutron fluence G)
Neutron flux (m-? s-t)
Parameters determined
Wiswall and Wirsing ]Sl
P
-18 Bmand -16 brn
T
He stream
3.6 x 102’
10”
Diffusion constant
Gugii et al. (61
P
-1Opm
T
He stream
2
Kudo et al. 171
P
T and C
He and vacuum
4.8 x 1o19 3.8 x 10ZO
4 x 1o16 2.0-3.2 X 10”
Activation energy, release temperature
Kudo and Tanaka
P
C
Vacuum
4.8 x 1o19 3.6 x 10”
4 3
x 1o16 x 10”
Release temperature
Tanaka et al. ]91
PandS
C
Vacuum
3.6 X 10”
3
x 10”
Release temperature, density dependence
Hasu et al. [lo]
S
C
Vacuum
3
1o15
181
Activation energy
x 1o19
x 10ZO
Release temperature, density dependence
a) P: powder, S: sintered pellets. b, T: isothermal annealing method, C: isochronal annealing method.
method is applied for the first time for study of tritium release from Liz0 in the stream of helium gas using sintered pellets and single crystals irradiated from 2 X 1019 to 2 X 10” thermal neutrons/m’ in order to collect information about the activation energy without corrections for effective sample surface areas, release mechanism and fluence and flux effects of tritium release from Li?O.
2. Experimental 2.1. Specimens
Sintered pellets and single crystals of Li20 were used as specimens in this tritium release experiment. The sintered pellets were prepared by the usual presssintering method. Since the preparation method is described in detail elsewhere [ 121, the most important features are briefly presented here. The Liz0 powder (99% in purity, Cerac/Pure Co. Ltd.) was
pressed into a pellet of about 12 mm in diameter and 10 mm in height, and was sintered for 4 h at 1570 K under vacuum. Li20 pellets of 86% theoretical density (TD) (grain size was about 30 W) were cut into cubes (1.6 X 1.6 X 1.6 mm, -8 mg) with a diamond cutter. The specimens were annealed for 2 h at 1070 K under vacuum to decompose LiOH and/or Li2COs which may be formed on the surfaces of specimens. Liz0 single crystals were prepared by the floating zone growth method reported in this journal [13]. The powder of Liz0 was hydrostatically pressed into a rod of about 10 mm in diameter and 100 mm in length and sintered for 2 h at 1370 K under vacuum. This rod was grown to be a single crystal by the floating zone growth method using an infrared imaging furnace with a pair of halogen lamps in an argon atmosphere. The crystal was crushed into small pieces and those with reasonable size (-2 mm in one dimension) and weight (5-12 mg) were manually selected. Typical impurities in Liz0 were analyzed by
T. Tanifuji et al. / Tritium release porn neutron-irradiatedLiz0
110
Dl
HELIUM GAS -
’ RECORDER DIGITAL FRI NTER
T
AMPLIFIER SCALER
FLOW CONTROLLER
Z
vv STACK *
I
I
IONIZATION TUBE
METHANE GAS Fig. 1. Apparatus used for measurement of release of total tritiated species.
extraction-photometry; Fe(61 ppm), Al(55 ppm), Co(<2 ppm), Cr(
2.3. Apparatus and procedure The apparatus for measuring the total tritiated species released from the specimens is schematically shown in fig. 1. The total tritiated species released from Liz0 were introduced into a proportional counter with pure helium carrier passing through a
trap cooled down to liquid nitrogen temperature to remove traces of water. The specimens were heated in a bent platinum plate in a resistance furnace to avoid contact with quartz walls, because Liz0 reacts with quartz above 670 K [ 141. Since it is known that most tritium is released from Liz0 as TzO and HTO [9,10], zinc powder cleaned with dilute nitric acid, of which the temperature was kept at about 670 K, was used to reduce T20 and HTO to tritium. The flow rate of helium was kept 60 ml/mm which was mixed with methane gas in the ratio 5 : 1 before the mixture of the gas was fed into a proportional counter. The apparatus for measuring gaseous trititum is schematically shown in fig. 2. The gaseous fraction of tritiated species through a cold trap (dry ice-alcohol) at 201 K is directly fed to the proportional counter. Sample temperatures were raised linearly with heating rates of 1, 5 and 10 K/min up to 1270 K and were maintained at constant temperature until the measured tritium activity became negligibly small, when it is compared with back ground. After the above procedure, the specimens were dissolved into water and the residual tritium in the specimens were determined to be neglisibly small by liquid scintillation counting.
111
T. Tanifuii et al. / Tritium release from neutron-irmdiated Liz0
HELIUM
TEMPERATURE CONTROLLER
GAS
I I
RECORDER DIGITA_L _-INTER
&THERMOCOUPLE
I
I
FURNACE
LIQUID NITROGEN
FI-_nw. . CONTROLLER
~1 IA--) I
DRY IC ALCOHOL
,
I
1
I
I
IU
YIALK
, GAS
Fig. 2. Apparatus used for measurement of release of gaseous tritiated species.
3. Results and discussion
10
3.1. Wtium releasefrom sintered pellets
6
3.1.1. Total t-Matedspecies The release behavior of total tritiated species from the pellets having 86% TD is shown in fig. 3 as a function of temperature for neutron fluences of 2 X 1019, 2 X 102’ and 2 X 102’ neutrons/m’. The percentage of the released tritiated species at heating rates of 1, 5 and 10 K/mm, the data of which showed some scattering for each run, was about 95%, 83% and 77%, respectively after heating up to 870 K, and about lOO%, 98% and 96%, respectively after heating up to 1270 K for every fluence. Observed maximum release temperatures are shown in table 2. For analysis of the release behavior, the first order case is assumed, in which the release rate has the Arrhenius form,
6
66%TD PELLET TOTAL TRITIUM 1 K / min 5K /min -
-o---a----b-
W/dt =A(1 -F)
exp(-E/RT),
10K /min
(1)
where dF/dt is the release rate, F the fraction released A the constant term and E the apparent activation energy. The temperature is raised linearly with time as T = To + Bt, where B is the linear heating rate and
Fig. 3. Fractional release of total tritiated species from sintered pellets. TEMPERATURE
, K
T. Tan@@ et al. / Tritium release from neutron-irmdiated Liz0
112
dT = B dt. This leads to dF/dT = @/B)(l - F) exp(-E/RT)
.
(2)
The maximum release temperature in the release curve is defined by the condition of d2F/dT2 = 0, which yields ln(TE/B)
= E/RT, + constant ,
(3)
where TP is the maximum peak temperature. Plots of ln(Ti/B) versus l/T, yields activation energies for each fluence as shown in fig. 4. Table 2 also represents the activation energies calculated by the least squares method for each fluence. This result shows that the activation energies are independent of neutron fluence and flux. The average activation energy is obtained to be 148 + 18 kJ/mol taking three activation energies of each fluence as shown in table 5, which is higher than that reported by Kudo et al. [7] (78.6 kJ/mol) for condensed tritiated species released from Liz0 powder under vacuum. This lack of agreement may be due to differences of sample form. It is well known that the release rates for powders may be much greater than the Values predicted from single crystal or sintered pellet data by several
orders of magnitude and also may be characterized by different activation energies. Lehovec [ 151 has discussed that surface charge effects would influence gas release rates. This effect could be particularly important in powders. 3.1.2. Gaseous fraction Fig. 5 shows the release behavior of gaseous fraction of tritiated species from the sintered pellets having 86% TD. The amount of gaseous tritiated species was determined by comparing each specimen weight used for the experiments of total and gaseous tritiated species, on the assumption that the amount of tritiated species produced in the specimens irradiated to the same fluence was proportional to the specimen weight. The percentage of gaseous tritiated species released from the specimens and the maximum release temperatures are summarized in table 3. The release behavior for gaseous tritiated species is analyzed by the same procedure given in the previous section. The Arrhenius plots for each fluence and their activation energies are shown in fig. 6 and in table 5, respectively. The release behavior of gaseous
Table 2 Maximum release temperatures and activation energies for the release of total tritiated species and chemisorbed from sintered pellets (2-4 X 10” n/m2 . s*; 2-4 X 1015 n/m2 . s) X
Neutron fluence
2
1019 n/m2
Heating rate (K/min)
Maximum release temperature W)
X
2 X 102t n/m2
Desorption from unirradiated peilet surface
Maximum release temperature W)
Maximum release temperature W)
Maximum release temperature W
2
10” n/m2
1 1 1 1
669 678 678 679
671 *678 680
663 *673
671 679
5 5 5
696 703 *705
*699 702 703
709 *719
688 703 707
10 10 10 10
733 736 738 738
*721 723 735
720 *736
718 720 723 738
170 * 20
135 f 19
148 f 26
Activation energy (kJ/mol) -
140 i 16
tritiated water
113
T. Tanifui et.al. / Tritium release from neutron-irmdiated Liz0 I
I
6.0 -
tritiated species is independent of the neutron fluence and flux as it is the case of the release behavior of total tritiated species. From the above results, it is found that the condensible fraction, which has been identified as tritiated water [lo], was about 92% of the total tritiated species released from the sintered pellets.
I
I
66% TD PELLET TOTAL TRITIUM
140
E=
kJ/mol
3.1.3. Desorption of chemisorbed sintered pellets
1170
E
6.0 t
2x102’
tritium on the
Desorption of chemisorbed tritium on the sintered pellets was investigated based on the temperature programmed desorption (TPD) technique to clarify the mechanism of tritiated water release from Li20 pellets. The unirradiated and irradiated specimens were set in an entrance of the trap cooled down to dry icealcohol temperature (201 K) and in the specimen holder in the apparatus shown in fig. 2, respectively.
kJ/mol
n/n+
^m 7.5 . “i -
fz
7.0 t
Fig. 4.ln(Ti/B) versus l/Tp for total tritiated species released from sintered pellets, where Tp is the maximum release temperature of the fractional release, B the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ 5 n/m2 . s, circles: 2-4 X 10” n/m2 -
kJ/mol
6.5 -
I I
I
I
I
I.3
1.4
1.5
lo3
Table 3 Maximum release temperatures 2-4 X 1015 n/m2 . s) X
/
I
,
1.6
9.
Tp , K-’
and percentage of gaseous tritiated species released from sintered pellets (2-4
Neutron fluence
2
10” n/m2
Heating rate Wmin)
Maximum release temperature (K)
2 Gaseous fraction (%I
X
10” n/m2
Maximum release temperature W
2 Gaseous fraction (%I
X
x
IOr’ n/m2 . s*;
102’ n/m2
Maximum release temperature (0
Gaseous fraction (%I
1 1 1
661 669 679
2.5 7.4 6.1
683 686 699
2.0 4.2 1.8
678 *688
2.2 3.8
5 5 5
*718 741 748
2.9 7.3 3.9
721 746 *763
13.8 10.0 8.2
763 *783
8.7 4.2
10 10 10
795 *808 813
6.7 8.8 4.8
*773 776 788
13.3 6.5 6.6
817 *830
10.6 16.4
Average
5.6 f 2.2
7.4 f 4.4
7.6 f 5.3
114
T. Tanififi et al. / Tn’tium release from neutron-irradkted Liz0 1
0.3 0.4
GASEOUS
!WWTlON
-o-
1 K /min
---*----o-iOK
5K /min
8.0
I
,
86% TD
PELLET GASEOUS FRACTION
/min
0.3
8.0
1’
2 xiOM n/n-?
I
1.2
I
1.3
1.4
1.5
lo3 / Tp , K-’
TEMPERATURE
,
K
Fig. 6.ln(Ti/B) versus l/Tp for gaseous tritiated species released from sintered pellets, Tp is the maximum release temperature, E the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ s n/m* - s, circles: 2-4 X 10’ ’ n/m* . s).
Fig. 5. Fractional release of gaseous tritiated species from sintered pellets.
Tritiated water was then adsorbed on the unirradiated specimen. After this procedure, the irradiated specimen was taken from the specimen holder, the chemisorbed specimen was set in the specimen holder, and desorbed tritium was measured by the same manner described in the previous sections. Typical results are shown in fig. 7. The maximum release temperatures were observed at similar temperatures to those in case of total tritiated species released from the pellets. According to Lord and Kittelberger [ 161, the method traditionally used for determining the activa-
tion energy in first order cases can be extended to second and probably higher order cases regardless of the kinetic order in case of thermal desorption experiments by employing the constant rate heating method. Therefore, the activation energy of thermal desorption was determined to be 148 f 26 kJ/mol by plots of ln(Z$/B) versus l/T, assuming a first order case as shown in fig. 8. This value is in good agreement with the average activation energy for the release of total tritiated species from the sintered pellets. These facts suggest that the release of tritiated water from the sintered Liz0 pellets irradiated by
115
T. Tanifuji et al, / Tritium release from neutron-hradiated Liz0
---m---
--A-
5 K
/min
10K /min
kJ/
I
1.3
1.4
mol
1.5
103/Tp,K-’ Fig. 8.ln(Ti/B) versus l/Tp for desorption of tritiated water chemisorbed on sintered pellets, where Tp is the maximum release temperature, B the linear heating rate. The slope of its line gives the activation energy. Fig. 7. Fractional desorption on sintered pellets.
of tritiated water chemisorbed
neutrons is controlled by the desorption mechanism. Recently, Cathcart et al. [17] and Scott et al. [ 181 concluded that during diffusion of tritium in TiOz (rutile) and BeO, tritium ions were associated with the oxygen ions of the host material, and they suggested that diffusion occurred either by movement of tritium ions from one oxygen to another or by migration of the tritiated hydroxide ions. It may be possible that tritiated hydroxide ions migrate to the surface and stayed in situ due to chemisorption or due to formation of tritiated hydroxide before tritiated water is released from the pellet surface. Kudo et al. [7] suggested that the recoiled tritium was first stabilized as tritiated hydroxide in the LizO, and tritiated water was released by the decomposition of tritiated hydroxide. Kudo [ 191 also studies the decomposition reaction of LiOH under vacuum and determined the activation energy to be 123 ?r 5 kJ/ mol, which is in good agreement with that obtained in this experiment (148 + 18 kJ/mol). However, there is no evidence that the release rate of tritiated water is controlled by decomposion of tritiated hydroxide, since, in the present experiment, the behavior of a micro amount of tritiated species was studied under dynamic helium atmosphere, while in the study of LiOH docomposition a reasonable amount of specimens was used under vacuum.
3.2. Tritium release from single crystals 3.2.1. Total Wiated species The typical release curves of the total tritiated species from single crystals are shown in fig. 9. The percentage of released tritiated species, which depends slightly on neutron fluence, was about 35%, 15% and 10% after heating up to 870 K and was about 98%, 85% and 70% after heating up to 1270 K at heating rates of 1,s and 10 K/min, respectively. In comparison with the sintered pellets, the release rates for the single crystals are considerably lower and the release peaks shift to the higher temperature side. Most of the tritiated species was tritiated water as it is the case for sintered pellets (this identification wilI be described in the following section). These results suggest that tritiated hydroxide ions were removed very fast through the grain boundaries in the pellets, while in the single crystals, tritiated hydroxide ions were removed very slowly through the grain, that is, the grain boundaries in the pellets fill the role of a high-diffusivity path in the migration of tritiated hydroxide ions. It is interesting to note that the peak shape becomes sharper with increasing neutron fluence especially at the heating rate of 1 K/mm. At a fluence of 2 X 102’ neutrons/m2, the peak shape was the sharpest and the maximum release temperature was about 920 K. The activation energy was not evaluated owing to a broad and unresolved peak shape.
T. Tamfiji et al. / Tritium release from neutron-irradiatedLiz0
116
&NGLE CRY&IL 5
TOTAL
4
o.5
TRITIUM 0
1 K /min
l
SKImin
2 x i0”
n /m2
SINGLE
CRYSTAL
1’
A lOK/min
OI
ae
I
_ 0.5
2 X~020n/m2
h k -
F
:
0.4
2 =
0.3
0
a 0.2 < g
0.1
5 2
0.5
0
IL
I
’
I
2x ic?‘” nff,o”o,
0.3
0.
A 0*
0.2
TEMPERATURE TEMPERATURE,
K
Fig. 9. Fractional release of total tritiated species from single crystals.
3.2.2. Gaseous fraction
Fig. 10 illustrates the gaseous fraction release features from the single crystals. The maximum peak temperatures and the percentage of gaseous tritiated species are represented in table 4. The release behavior of gaseous fraction from single crystals is independent of neutron fluence and flux and the amount of gaseous tritiated species released from single crystals is comparable (8%) to that from the sintered pellets. The maximum release temperatures
, K
Fig. 10. Fractional release of gaseous tritiated species from single crystals.
are higher than those for sintered pellets. The activation energies are shown in table 5, the Arrhenius plots are shown in fig. 11. The values of the activation energies depend on neutron fluence and are higher than those for the sintered pellets. In the case of high fluences (2 X 10zo and 2 X 10” neutrons/m2), the activation energies are considerably lower than those for the specimens irradiated to 2 X 1Ol9 neutrons/m2. This suggests that irradiation induced defects and clusters play an important role in the gaseous tritium release from single crystals.
Table 4 Maximum release temperatures 2-4 x 10” n/m2 * s)
and percentage of gaseous tritiated species released from single crystals (2-4
Neutron fluence
2 X lOI
n/m*
Heating rate Wnin)
Maximum reIease temperature WI
2 Gaseous fraction (%I
X
lo*’ n/m2
2
Maximum release temperature WI
X
X
Gaseous fraction (%I
1 1 1
835 842 843
14.0 8.6 9.4
771 778 808
6.8 3.7 4.6
796 *803
5.8 8.8
5 5 5
*876 888 898
2.7 8.2 3.7
*833 *848 851
13.1 6.4 4.7
*831 858 *888
13.1 9.2 12.6
10 10 10
911 *911 913
5.0 9.9 6.9
865 *873 876
11.2 5.4 11.0
893 *896 903
5.3 8.6 16.1
Average
7.6 i 3.5
. s*;
10” n/m’
Maximum release temperature (K)
Gaseous fraction (%I
10” n/m2
9.9 * 3.7
7.4 f 3.4
SINGLE CRYSTAL GASEOUS FRACTION
&!
Table 5 Activation energies for the release of total and gaseous tritiated species from sintered pellets and single crystals (kJ/mol) Neutron fluence (n/m2)
E- 180 kJ/moi
-
kJ/mol
~
Single crystals
Sintered pellets
Total tritiated species
Gaseous fraction
Gaseous fraction
2 x io*r
140 f 16 170 f 20 135 f 19
62 f 8 85 f 10 64 f 5
184 f 14 125 * 16 110 f 19
Average
148 f 18
70 f 8
8.0
E = Ii0 kJ/mol Fig. 11. ~(T~/~) versus l/Z’n for gaseous tritiated species released from single crystals, where Tp is the m~imum release temperature, B the linear heating rate. The slope of its line gives the activation energy (triangles: 2-4 X 10’ s n/ m* . s, circles: 2-4 X 10” n/m* 0s).
6.5
I
1
L
I
I
1.0
1.1
1.2
1.3
IO3 / Tp , K-’
1
118
T. Tonifiji et al. / Tritium release from neutron-irradiated Liz0
4. summary (1) About 8% of the tritiated species were released in the gaseous form, and the rest was released in the condesible form, which was collected in the cold trap cooled down to 201 K, both from the sintered pellets and from the single crystals. (2) The release features of the total tritiated species and the gaseous species from the sintered pellets (86% TD) are independent of neutron fluence and flux. (3) In the sintered pellets (86% TD), the release features of the total tritiated species such as the maximum release temperature and the apparent activation energy are similar to those of desorption of tritiated water chemisorbed on the sintered pellets. (4) In the single crystals, the release features of the total tritiated species depend on neutron fluence but do not depend on neutron flux.
Acknowledgements The authors wish to express their thanks to Drs. S. Mori, S. Nomura, J. Shimokawa, Y. Obata, R. Nagasaki, T. Kikuchi, K. Iwamoto and K. Sako for their interest in this work.
References [l) K. Sako, M. Ohta, Y. Seki, H. Yamato, T. Hiraoka, T. Tanaka, N. Asami and S. Mori, JAERI-M 5502 (1973).
[21 E.M. Larsen, S.I. Abdel-Khahk and MS. Ortman, Nucl. Technol. 41 (1978) 12. [3] M. Akabori, K. Uchida, K. Noda, T. Tanifuji and S. Nasu, J. Nucl. Mater. 83 (1979) 330. [4] K. Uchida, M. Akabori, K. Noda, T. Tanifuji and S. Nasu, J. Nucl. Mater. 89 (1980) 92. [S] R.H. WiswaB and E. Wlrsing, BNL-50748 (1977). [ 61 D. Cuggi, H.R. Ihel and U. Kurz, in: Proc. 9th Symp. on Fusion Technology (1976) p. 337. [7] H. Kudo, K. Tanaka and H. Amano, J. Inorg. Nucl. Chem. 40 (1978) 363. [S] H. Kudo and K. Tanaka, Radiochem. Radional Letters 23 (1975) 57. [9] K. Tanaka, H. Kudo and H. Amano, in: Proc. Intern. Conf. on Radiation Effects and Tritium Technology for Fusion Reactors, GatIinburg, TN, 1975. [lo] S. Nasu, H. Kudo, K. Shiozawa, T. Takahashi, T. Kurasawa, M. Tachiki and K. Tanaka, J. Nucl. Mater. 68 (1977) 261. [ 111 U. Katz, M. Guinan and R.J. Borg, Phys. Rev. B4 (1971) 330. [12] T. Takahashi and T. Kikuchi, JAERI-M 7518 (1978). [ 131 I. Shindo, S. Kimura, K. Noda, T. Kurasawa and S. Nasu, J. Nucl. Mater. 79 (1979) 375. [ 141 T. Tanifuji and S. Nasu, to be published. [15] K. Lehovec, J. Chem. Phys. 21 (1953) 1123. [ 161 F.M. Lord and J.S. Kittelberger, Surface Sci. 43 (1974) 173. [17] J.V. Cathcart, R.A. Parkins, J.B. Bate and L.C. Manley, J. Appl. Phys. 50 (1979) 4110. [18] K.T. Scott and L.L. Was&, Proc. Brit. Ceram. Sot. 7 (1967) 375. 119) H. Kudo, J. Nucl. Mater. 87 (1979) 185.