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Effects of oxygen potential on tritium release from Li2O
journal of
nuclear materials
Journal of Nuclear Materials 191-194 (1992) 204-208 North-Holland
Effects of oxygen potential on tritium release from Li 2 0 Daiju Yamaki, Satam Tanaka and Michia Yamawaki Nuclear Engineering Research Laboratory, University of Tokyo, Japan
Effects of oxygen potential on tritium recovery from LizO were Quantitatively studied by in-situ tritIUm release experiments named TTTEx at the University of Tokyo using sweep gases of He+H z and He+H z +H 2 0. The oxygen potential in the experimental system was calculated from the chemical composition of the sweep gas. It was found to be influenced by the oxygen concentration at the surface of samples which was strongly affected by the method of pre-treatment. The tritium release rate was found to be affected both by the oxygen potential and by the swamping effect with Hz and/or HzO in the sweep gas. Chemical form of the recovered tritium also had a strong relationship with the oxygen potential in the sweep gases.
1. Introduction A precise estimation of the tritium inventory in the fusion reactor blanket is essential for the design of the fusion reactor fuel cycle. In order to calculate the tritium inventory in the blanket system, thorough understanding of the tritium release behavior from breeding material is required. In sintered solid breeding material, tritium release is composed of a series of migration steps such as diffusion within the grain, diffusion and percolation through interconnected open pores, and reaction at the surface. Among them, surface reaction sometimes becomes the rate-determining process. The surface reaction is affected by sweep gas chemical composition, sample pre-treatment and container materials. However, the mechanisms involved remain to be elucidated. Tritium release behavior from solid breeding materials has been studied by combining in-situ tritium release experiments, out-of-pile experiments such as desorption and exchange reactions, and modeling studies [1]. A sweep gas of He + Hz has been proposed to enhance tritium release. A small amount of H zO is inevitable in the sweep gas. The effects of Hz and HzO in the sweep gas on tritium release have been studied by many authors [2-8]. Suggested mechanism for enhancing tritium release includes surface swamping with hydrogen, exchange reaction, and larger desorption rate by lower oxygen potential. Chemical form of the released tritium was found to be strongly affected by oxygen activity from thermodynamic considerations [9], tritium release experiments, and modeling studies [10-12]. In recent work, the present authors have shown that the behavior of surface reaction is significantly affected by the oxygen potential which is determined by the chemical composition of the sweep gas, purification of
the sample and the surface nature of the container material [5]. In the present paper, the oxygen potential in the system is quantitatively evaluated from the water vapor and the hydrogen partial pressures in the sweep gas. Then, correlations of the tritium residence time and the released chemical form with the evaluated oxygen potential are discussed.
2. Experiments Polyerystalline particles of LizO were used in the experiments. The LizO sample used was supplied by Rare Metallic Inc. It was crushed and sieved into particle sizes of 7-9 mesh. About 3-5 g of the sample were loaded as a packed column in the type-316 stainless steel (SS) sample cell, and 316 SS wool was used to fix the sample in the cell. The sample was dehumidified by heat-treating in dry N z gas or dry He + 3%H z gas at 500a C for one to two weeks. The experimental apparatus and procedure were explained before [4,5,13]. Tritium residence time and the recovered chemical form were evaluated in these experiments. Two important factors can be considered which decide the oxygen potential in the experimental system. One is oxygen or OH concentration at the surface of the sample that is determined by the method of sample pre-treatment such as purification, preservation and so on. The other is the sweep gas chemical composition which affects the oxygen potential in the sweep gas near the sample. In order to study the effects of oxygen concentration at the surface, two kinds of LizO sample were prepared: sample 1 and sample 2. Sample 1 had been previously dehumidified by heat-treatment before irradiation by dry N z gas, and was used in He + HzO
0022-3115/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
D. Fmnaki ct al. / E(f('cts of 0 potential on Ti relcase [rom U 2()
sweep gas experiments. Sample 2 had been dehumidified hy dry He + 3%H 2 gas, and W,lS not used in the sweep gas of He + H 2 0 or He + H 2 + H 2 0 before the cxpcrimcnts using He + H 2 sweep gas. Therefore, the surface oxygen concentration of sample 2 was considered to be remarkably lower than that of sample 1. To study the effcct of oxygen potential on tritium release, it is essential to estimate precisely the oxygen potential in the sweep gas ncar the sample. However, it was difficult in our experimental system to measurc the sweep gas composition at the sample. Thus, we estimated it from the chemical composition 1.5 m downstream from the sample. Water vapor pressure was measured by a SHAW hygrometer which had been calibrated before usc. Detection limit was - BOoe or 0.53 ppm. H 2 partial pressure at the outlet was consid· ered to he equal to that in the charged sweep gas when equilibrium was reached. From the H 2 0 and H 2 partial pressures, oxygen partial pressures were calculated using the equilibrium constant K for the reaction H 2 + 0.50 z = H 2 0. The value of K at 50Q a e is 7.08 X 10 n
H 2, reaction rate for sample 1. was found to be much smaller than that for 8amplc 2. Dependences on H 2 concentration were also different between the two samples. This means that the oxygen concentration on the surface strongly affects the tritium release mechanism [5]. To verify this, sample I was dehumidified by dry He + 3%H 2 gas at 500D e for about two weeks, and the experiments with He + H 2 sweep gas were repeated on sample 1. Fig. I also compares the surface tritium residence times for sample 1 dehumidified by He + 3%H 2' for the same sample before He + H 2 drying and for sample 2. Dehumidification by He + 3%H z increased the tritium release rate for sample 1. Hence the oxygen concentration at the surface was considered to be decreased by He + Hz drying. However the tritium residence time at the surface and its hydrogen pressure dependency for the He + Hz dried sample 1 did not agree with those for sample 2. The reason for this disagreement could be attributable to irreversible change of the surface nature which was caused by frequent adsorption and desorption of H 2 0. It was considered that the oxygen concentration at the surface strongly affected the H zO partial pressure in the sweep gas downstream of the sample. Fig. 2 shows the relatil)nship between the oxygen pressure and the hydrogen partial pressure of the sweep gas downstream. The oxygen pressure for sample 2 was found to be the lowest. For sample 1 before He + H 2 drying, the oxygen pressure was higher by several orders of magnitude than that for sample 2. By He + H 2
atm- 1/2 .
3. Results and discussion 3. I. Effects of sample pre-treatment Fig. 1 compares the surface tritium residence time between sample 1 and sample 2. Unless dried by He +
~,
",
8c:
(U
"'0
....
(U
•
" ,,
Sample 2 --",, ,, ,,
'u;
..
•
..
• ,
Sample 1
.... before He+H2 Drying £::, 0 after He + H2 Drying
E ~
....-
....
...
£::,
•
0
205
t
t
•
Start up Shut down
10 f L..---..l._ _-'-----l_--l..._---'_--l..._.....L-.._--'-_-'-_...L..-_-'-----J 5 1016 4 10 3 102 10 1
XH2/Fig. 1. Hydrogen pressure dependence of tritium surface residence time for sample 1 (previously dried by N2 and used for the experiments of He+H 2 0 sweep gas) and sample 2 (dried by He+3%H 2 ). For sample 1, change by He+H 2 drying was also examined.
206
D. Yamaki et al. / Effects of 0 potential on Ti release from Li 20
potentials. However, as will bc explained in the next section, tritium release behavior was found not to be determined only by oxygen potential.
•• • o
o ...
•
0
o
-25
eX: , 0 -
Q
.......
°
• •
o
N
o'?
3.2. Effects of s}l'eep gas composition
... Sample 2 ... ~ Sample 1 o after He t H2 Drying ... -30
10
10
• 0
belore He t H2 Drying
10
I
10 2
10 3
10 4
10
5
PH2/ Pa Fig. 2. Hydrogen pressure dependence of estimated oxygen pressure for sample 1 and sample 2.
drying, the oxygen pressure for sample 1 decreased to a level close to that of sample 2, but still slightly higher. Hence, the surface oxygen concentration for sample 1 was considered to be decreased by drying. This difference in oxygen pressure provides an explanation for the difference in tritium surface residence times: larger tritium residence times correspond to higher oxygen
Fig. 3 shows the relationship between the oxygen pressure in the sweep gas and the tritium residence time. Constant tritium residence time at low oxygen pressure was considered to result from diffusional resistance. Generally, the tritium residence time was found to increase with the higher oxygen pressure. However, the tritium residence times for the three samples (sample 1 and sample 2 (Hc + Hz, He + Hz + HzO)) were found not to be identical even at the samc values of oxygen pressure. This implies that tritium residence time is not determined only by oxygen pressure. Other reasons such as the swamping effect should be considered. For sample 2 with He + Hz + HzO sweep gas, short residence times were observed at P02 above 10 - 27. Pa (fig. 3). This could be also cxplained by the swamping effect caused by high HzO partial pressure. In order to study the isotopic swamping, effects of H 2 and HzO partial pressures on the residence time were examined for a series of runs with nearly the same values of oxygen pressures. Oxygen pressures of about 5 X 10- 26 Pa and 3 X 10- 24 Pa were selected. Results are shown in table 1. It was found that, in spite of the same oxygen pressure, the tritium residence time increased with decreasing Hz and/or H 2 0 pressures. It can be concluded that the tritium release rate is affected by both the swamping effect caused by Hz
•
-
• .t..t.
.t.. •
Q) (.)
cQ)
I:>
"0
.iii
2
~ 10 E
..
• Sample I, He +H2 ... Sample 2,He+H2 LJ Sample 2,He+H2tH2D-
;j
-...
10
•
I . . - - _ - ' - _ - - ' -_ _I . . - - _ - l . -_ _-'---:-I_-L.-_~
_ _L _ _ . . . l . __
'W30
_J
W- 25
P0 2 / Po Fig. 3. Oxygen pressure dependence of tritium residence time for samples 1 and 2.
1020
D. Yamaki et al. I Effects 01" 0 potential on Ti release from Li"O
10
2
P02 / Po 1625
30
10 I
207
16 20 I
I
6/
10 '
/'
Li 20 500t
......
Do
::I:
I::I:
t: . . D.
... ... 10'4
b6 , /
I}
...
,/ 10- 3
t:.
\./
10
/
2
/
6./
~
10-2
6
t:.
0 ........ 10 0
16'
/
/
t:.
./'
• Sample 1, He+ H2 2, He +H2 .6. Sample 2, He+ H2i'H20
.6 Sample
I
10- 1
10
I
0
10 '
10
2
PHzO/ PHz Fig. 4. Correlation between the recovered tritium chemical form HTO IHT and partial pressures' ratio or the estimated oxygen pressure.
and/or H 2 0 in the sweep gas and the oxygen potential at the surface of the sample. Another important index is the chemical form of the released tritium and its interrelation with the sweep gas chemical composition. Fig. 4 shows the dependence of the tritium chemical form on the ratio PH,O/P H , and on the oxygen partial pressure in the sweep gas. There is a strong relationship between them regardless of sample difference (sample 1, sample 2) and sweep gas composition (He + H 2 , He + H 2 + H 2 0), This shows that the chemical form of recovered tritium is mainly determined by PH,O/P H, ratio or Po,. As fig. 4 shows, the relation of [HTOV(HT] = 1 was observed at the oxygen pressure of Po, == 10- 23 Pa = 10- 28 atm. Table 1 Effects of H 2 and H 2 0 partial pressures on tritium residence time. Experimental runs with nearly same oxygen partial pressures were compared Run no. 6106 6602 6610
Po, [Pal 5.16 X 10- 26
6203 6108 6603 6609
6.07XI0- 26 3.85xlO- 26 4.60xlO- 26 4.20x 1O- z6 3.08XlO- 24 2.68 X 10- 24 2.81 X 10- 24
6611 6213
2.54 X 10- 24 2.33 X 10- 24
6614
6613
3.67x 10- 24
(lIS) [s]
1.01 X 10 4 3.64x 10 2 3.64XI0 2 3.64 X 10 2 1.01 X 10 1 1.01 X 10 3 3.07 x 10 1 3.07X 10 1 3.07XIO I 7.65XlO° 3.04X lOU
5.12XI0 z 2.00 X 10 1 1.59 X 10 1 1.74 x 10 1
5.29X 1O~1 3.97X 10 2 U2xlO i 1.14 X 10 1 1.31 x10 1 2.71 x 10° 1.03 X 10°
]40.6 165.4 286.9 210.9
702.7 327.9 691.8 520.9 566.4 794.0 2347.0
Pll20
I PH,
in the sweep gas
-
This pressure is lower than the value (Po, = 10- 24 atm) where P T, is equal to PT,o at 900 K as thermochemically evaluated by Fischer and Johnson (9). However, the surface effects of the container and tubing materials are also involved in our experiments. Therefore the result calculated by them can not be verified by this comparison. When PH ,O/P H2 was higher than 0.1, [HTO]/[HT] was found nearly cqual to PH,O/PH1 · It was not clear whether this relationship was established by the tritium release from the surface or whether the sample container and tubing changed the chemical form by an exchange reaction with H 2 or H 2 0 in the sweep gas. The trend of [HTO]j[HT] > PH,O/P H2 was observed in the pressure region PH20/PHl < 0.1. From this divergence, the effect of re-exchange reaction at the tube surface could be evaluated. However there was doubt about the reliability of the data in this region because of a relatively large experimental error. Further investigation is necessary to establish the relationship between the chemical form of the recovered tritium and the sweep gas composition, especially at low PH 2 Effects of sweep gas flow rate and tubing material would also be important.
4. Conclusion Effects of oxygen potential on tritium release behavior from Li 2 0 were investigated by in situ experiments using He + H 2 and He + Hz + H 2 0 sweep gases. The tritium release rate from Li 2 0 was affected both by the
208
D. Yamaki et al. / Effects of 0 potential on Ti release from Li 2 0
oxygen potential at thc sample and by the Hz or HzO swamping effect. Chemical form of the recovered tritium showed a strong relationship with the oxygen potential in the sweep gas.
References
c.E. Johnson, T. Kondo, N. Roux, S. Tanaka and D. VoJlath, Fusion Eng. and Des. 16 (1991) 127. [2] S. Tanaka, T. Terai, H. Mohri and Y. Takahashi, J. Nuel. Mater. 155-157 (1988) 533. [3] S. Tanaka, A Kawamoto, M. Yamawaki, T. Terai, Y. Takahashi, H. Kawamura and M. Saito, Fusion Eng. and Des. 8 (1989) ISS. [4] S. Tanaka, A. Kawamoto, D. Yamaki, K. Yamaguchi and M. Yamawaki, J. Nucl. Mater. 179-18J (1991) 867.
[1]
[5] D. Yamaki, S. Tanaka and M. Yamawaki, Fusion Eng. and Des. 17 (1991) 37. [6] T. Kurasawa and H. Watanabe, J. Nucl. Mater. 179-181 (]991) 851. [7] S. Tanaka, K. Uozumi and M. Yamawaki, presented at 3rd Int. Symp. on the Fabrication and Properties of Lithium Ceramics, Cincinnati, 1991. [8] S. Tanaka, T. Usami and M. Yamawaki, Proc. 16th Symp. on Fusion Technology, London, England, 1990 (Elsevier, Amsterdam, 1991) PP. 782-786. (9) AX Fischer and c.E. Johnson, J. Nucl. Mater. 133&134 (1985) 186. [10] G. Federici, A.R. Raffray and M.A. Abdou, J. Nuel Mater. 173 (1990) 185. [11] G. Federici, A.R. Raffray and MA Abdou, J. Nuel Mater. 173 (]990) 214. [I2J S. Tanaka, D. Yamaki and M. Yamawaki, Fusion Technol. 19 (1991) 1018. [13] T. Terai, Y. Takahashi and S. Tanaka, Fusion Eng. and Des. 7 (989) 345.
nuclear materials
Journal of Nuclear Materials 191-194 (1992) 204-208 North-Holland
Effects of oxygen potential on tritium release from Li 2 0 Daiju Yamaki, Satam Tanaka and Michia Yamawaki Nuclear Engineering Research Laboratory, University of Tokyo, Japan
Effects of oxygen potential on tritium recovery from LizO were Quantitatively studied by in-situ tritIUm release experiments named TTTEx at the University of Tokyo using sweep gases of He+H z and He+H z +H 2 0. The oxygen potential in the experimental system was calculated from the chemical composition of the sweep gas. It was found to be influenced by the oxygen concentration at the surface of samples which was strongly affected by the method of pre-treatment. The tritium release rate was found to be affected both by the oxygen potential and by the swamping effect with Hz and/or HzO in the sweep gas. Chemical form of the recovered tritium also had a strong relationship with the oxygen potential in the sweep gases.
1. Introduction A precise estimation of the tritium inventory in the fusion reactor blanket is essential for the design of the fusion reactor fuel cycle. In order to calculate the tritium inventory in the blanket system, thorough understanding of the tritium release behavior from breeding material is required. In sintered solid breeding material, tritium release is composed of a series of migration steps such as diffusion within the grain, diffusion and percolation through interconnected open pores, and reaction at the surface. Among them, surface reaction sometimes becomes the rate-determining process. The surface reaction is affected by sweep gas chemical composition, sample pre-treatment and container materials. However, the mechanisms involved remain to be elucidated. Tritium release behavior from solid breeding materials has been studied by combining in-situ tritium release experiments, out-of-pile experiments such as desorption and exchange reactions, and modeling studies [1]. A sweep gas of He + Hz has been proposed to enhance tritium release. A small amount of H zO is inevitable in the sweep gas. The effects of Hz and HzO in the sweep gas on tritium release have been studied by many authors [2-8]. Suggested mechanism for enhancing tritium release includes surface swamping with hydrogen, exchange reaction, and larger desorption rate by lower oxygen potential. Chemical form of the released tritium was found to be strongly affected by oxygen activity from thermodynamic considerations [9], tritium release experiments, and modeling studies [10-12]. In recent work, the present authors have shown that the behavior of surface reaction is significantly affected by the oxygen potential which is determined by the chemical composition of the sweep gas, purification of
the sample and the surface nature of the container material [5]. In the present paper, the oxygen potential in the system is quantitatively evaluated from the water vapor and the hydrogen partial pressures in the sweep gas. Then, correlations of the tritium residence time and the released chemical form with the evaluated oxygen potential are discussed.
2. Experiments Polyerystalline particles of LizO were used in the experiments. The LizO sample used was supplied by Rare Metallic Inc. It was crushed and sieved into particle sizes of 7-9 mesh. About 3-5 g of the sample were loaded as a packed column in the type-316 stainless steel (SS) sample cell, and 316 SS wool was used to fix the sample in the cell. The sample was dehumidified by heat-treating in dry N z gas or dry He + 3%H z gas at 500a C for one to two weeks. The experimental apparatus and procedure were explained before [4,5,13]. Tritium residence time and the recovered chemical form were evaluated in these experiments. Two important factors can be considered which decide the oxygen potential in the experimental system. One is oxygen or OH concentration at the surface of the sample that is determined by the method of sample pre-treatment such as purification, preservation and so on. The other is the sweep gas chemical composition which affects the oxygen potential in the sweep gas near the sample. In order to study the effects of oxygen concentration at the surface, two kinds of LizO sample were prepared: sample 1 and sample 2. Sample 1 had been previously dehumidified by heat-treatment before irradiation by dry N z gas, and was used in He + HzO
0022-3115/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
D. Fmnaki ct al. / E(f('cts of 0 potential on Ti relcase [rom U 2()
sweep gas experiments. Sample 2 had been dehumidified hy dry He + 3%H 2 gas, and W,lS not used in the sweep gas of He + H 2 0 or He + H 2 + H 2 0 before the cxpcrimcnts using He + H 2 sweep gas. Therefore, the surface oxygen concentration of sample 2 was considered to be remarkably lower than that of sample 1. To study the effcct of oxygen potential on tritium release, it is essential to estimate precisely the oxygen potential in the sweep gas ncar the sample. However, it was difficult in our experimental system to measurc the sweep gas composition at the sample. Thus, we estimated it from the chemical composition 1.5 m downstream from the sample. Water vapor pressure was measured by a SHAW hygrometer which had been calibrated before usc. Detection limit was - BOoe or 0.53 ppm. H 2 partial pressure at the outlet was consid· ered to he equal to that in the charged sweep gas when equilibrium was reached. From the H 2 0 and H 2 partial pressures, oxygen partial pressures were calculated using the equilibrium constant K for the reaction H 2 + 0.50 z = H 2 0. The value of K at 50Q a e is 7.08 X 10 n
H 2, reaction rate for sample 1. was found to be much smaller than that for 8amplc 2. Dependences on H 2 concentration were also different between the two samples. This means that the oxygen concentration on the surface strongly affects the tritium release mechanism [5]. To verify this, sample I was dehumidified by dry He + 3%H 2 gas at 500D e for about two weeks, and the experiments with He + H 2 sweep gas were repeated on sample 1. Fig. I also compares the surface tritium residence times for sample 1 dehumidified by He + 3%H 2' for the same sample before He + H 2 drying and for sample 2. Dehumidification by He + 3%H z increased the tritium release rate for sample 1. Hence the oxygen concentration at the surface was considered to be decreased by He + Hz drying. However the tritium residence time at the surface and its hydrogen pressure dependency for the He + Hz dried sample 1 did not agree with those for sample 2. The reason for this disagreement could be attributable to irreversible change of the surface nature which was caused by frequent adsorption and desorption of H 2 0. It was considered that the oxygen concentration at the surface strongly affected the H zO partial pressure in the sweep gas downstream of the sample. Fig. 2 shows the relatil)nship between the oxygen pressure and the hydrogen partial pressure of the sweep gas downstream. The oxygen pressure for sample 2 was found to be the lowest. For sample 1 before He + H 2 drying, the oxygen pressure was higher by several orders of magnitude than that for sample 2. By He + H 2
atm- 1/2 .
3. Results and discussion 3. I. Effects of sample pre-treatment Fig. 1 compares the surface tritium residence time between sample 1 and sample 2. Unless dried by He +
~,
",
8c:
(U
"'0
....
(U
•
" ,,
Sample 2 --",, ,, ,,
'u;
..
•
..
• ,
Sample 1
.... before He+H2 Drying £::, 0 after He + H2 Drying
E ~
....-
....
...
£::,
•
0
205
t
t
•
Start up Shut down
10 f L..---..l._ _-'-----l_--l..._---'_--l..._.....L-.._--'-_-'-_...L..-_-'-----J 5 1016 4 10 3 102 10 1
XH2/Fig. 1. Hydrogen pressure dependence of tritium surface residence time for sample 1 (previously dried by N2 and used for the experiments of He+H 2 0 sweep gas) and sample 2 (dried by He+3%H 2 ). For sample 1, change by He+H 2 drying was also examined.
206
D. Yamaki et al. / Effects of 0 potential on Ti release from Li 20
potentials. However, as will bc explained in the next section, tritium release behavior was found not to be determined only by oxygen potential.
•• • o
o ...
•
0
o
-25
eX: , 0 -
Q
.......
°
• •
o
N
o'?
3.2. Effects of s}l'eep gas composition
... Sample 2 ... ~ Sample 1 o after He t H2 Drying ... -30
10
10
• 0
belore He t H2 Drying
10
I
10 2
10 3
10 4
10
5
PH2/ Pa Fig. 2. Hydrogen pressure dependence of estimated oxygen pressure for sample 1 and sample 2.
drying, the oxygen pressure for sample 1 decreased to a level close to that of sample 2, but still slightly higher. Hence, the surface oxygen concentration for sample 1 was considered to be decreased by drying. This difference in oxygen pressure provides an explanation for the difference in tritium surface residence times: larger tritium residence times correspond to higher oxygen
Fig. 3 shows the relationship between the oxygen pressure in the sweep gas and the tritium residence time. Constant tritium residence time at low oxygen pressure was considered to result from diffusional resistance. Generally, the tritium residence time was found to increase with the higher oxygen pressure. However, the tritium residence times for the three samples (sample 1 and sample 2 (Hc + Hz, He + Hz + HzO)) were found not to be identical even at the samc values of oxygen pressure. This implies that tritium residence time is not determined only by oxygen pressure. Other reasons such as the swamping effect should be considered. For sample 2 with He + Hz + HzO sweep gas, short residence times were observed at P02 above 10 - 27. Pa (fig. 3). This could be also cxplained by the swamping effect caused by high HzO partial pressure. In order to study the isotopic swamping, effects of H 2 and HzO partial pressures on the residence time were examined for a series of runs with nearly the same values of oxygen pressures. Oxygen pressures of about 5 X 10- 26 Pa and 3 X 10- 24 Pa were selected. Results are shown in table 1. It was found that, in spite of the same oxygen pressure, the tritium residence time increased with decreasing Hz and/or H 2 0 pressures. It can be concluded that the tritium release rate is affected by both the swamping effect caused by Hz
•
-
• .t..t.
.t.. •
Q) (.)
cQ)
I:>
"0
.iii
2
~ 10 E
..
• Sample I, He +H2 ... Sample 2,He+H2 LJ Sample 2,He+H2tH2D-
;j
-...
10
•
I . . - - _ - ' - _ - - ' -_ _I . . - - _ - l . -_ _-'---:-I_-L.-_~
_ _L _ _ . . . l . __
'W30
_J
W- 25
P0 2 / Po Fig. 3. Oxygen pressure dependence of tritium residence time for samples 1 and 2.
1020
D. Yamaki et al. I Effects 01" 0 potential on Ti release from Li"O
10
2
P02 / Po 1625
30
10 I
207
16 20 I
I
6/
10 '
/'
Li 20 500t
......
Do
::I:
I::I:
t: . . D.
... ... 10'4
b6 , /
I}
...
,/ 10- 3
t:.
\./
10
/
2
/
6./
~
10-2
6
t:.
0 ........ 10 0
16'
/
/
t:.
./'
• Sample 1, He+ H2 2, He +H2 .6. Sample 2, He+ H2i'H20
.6 Sample
I
10- 1
10
I
0
10 '
10
2
PHzO/ PHz Fig. 4. Correlation between the recovered tritium chemical form HTO IHT and partial pressures' ratio or the estimated oxygen pressure.
and/or H 2 0 in the sweep gas and the oxygen potential at the surface of the sample. Another important index is the chemical form of the released tritium and its interrelation with the sweep gas chemical composition. Fig. 4 shows the dependence of the tritium chemical form on the ratio PH,O/P H , and on the oxygen partial pressure in the sweep gas. There is a strong relationship between them regardless of sample difference (sample 1, sample 2) and sweep gas composition (He + H 2 , He + H 2 + H 2 0), This shows that the chemical form of recovered tritium is mainly determined by PH,O/P H, ratio or Po,. As fig. 4 shows, the relation of [HTOV(HT] = 1 was observed at the oxygen pressure of Po, == 10- 23 Pa = 10- 28 atm. Table 1 Effects of H 2 and H 2 0 partial pressures on tritium residence time. Experimental runs with nearly same oxygen partial pressures were compared Run no. 6106 6602 6610
Po, [Pal 5.16 X 10- 26
6203 6108 6603 6609
6.07XI0- 26 3.85xlO- 26 4.60xlO- 26 4.20x 1O- z6 3.08XlO- 24 2.68 X 10- 24 2.81 X 10- 24
6611 6213
2.54 X 10- 24 2.33 X 10- 24
6614
6613
3.67x 10- 24
(lIS) [s]
1.01 X 10 4 3.64x 10 2 3.64XI0 2 3.64 X 10 2 1.01 X 10 1 1.01 X 10 3 3.07 x 10 1 3.07X 10 1 3.07XIO I 7.65XlO° 3.04X lOU
5.12XI0 z 2.00 X 10 1 1.59 X 10 1 1.74 x 10 1
5.29X 1O~1 3.97X 10 2 U2xlO i 1.14 X 10 1 1.31 x10 1 2.71 x 10° 1.03 X 10°
]40.6 165.4 286.9 210.9
702.7 327.9 691.8 520.9 566.4 794.0 2347.0
Pll20
I PH,
in the sweep gas
-
This pressure is lower than the value (Po, = 10- 24 atm) where P T, is equal to PT,o at 900 K as thermochemically evaluated by Fischer and Johnson (9). However, the surface effects of the container and tubing materials are also involved in our experiments. Therefore the result calculated by them can not be verified by this comparison. When PH ,O/P H2 was higher than 0.1, [HTO]/[HT] was found nearly cqual to PH,O/PH1 · It was not clear whether this relationship was established by the tritium release from the surface or whether the sample container and tubing changed the chemical form by an exchange reaction with H 2 or H 2 0 in the sweep gas. The trend of [HTO]j[HT] > PH,O/P H2 was observed in the pressure region PH20/PHl < 0.1. From this divergence, the effect of re-exchange reaction at the tube surface could be evaluated. However there was doubt about the reliability of the data in this region because of a relatively large experimental error. Further investigation is necessary to establish the relationship between the chemical form of the recovered tritium and the sweep gas composition, especially at low PH 2 Effects of sweep gas flow rate and tubing material would also be important.
4. Conclusion Effects of oxygen potential on tritium release behavior from Li 2 0 were investigated by in situ experiments using He + H 2 and He + Hz + H 2 0 sweep gases. The tritium release rate from Li 2 0 was affected both by the
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D. Yamaki et al. / Effects of 0 potential on Ti release from Li 2 0
oxygen potential at thc sample and by the Hz or HzO swamping effect. Chemical form of the recovered tritium showed a strong relationship with the oxygen potential in the sweep gas.
References
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