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Effect of surface water on tritium release behavior from Li2TiO3
Journal of Nuclear Materials 417 (2011) 735–738
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of surface water on tritium release behavior from Li2TiO3 T. Hanada, M. Nishikawa ⇑, T. Kanazawa, H. Yamasaki, N. Yamashita, S. Fukada Interdisciplinary Graduate School of Engineering Science, Kyushu University, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 25 December 2010
a b s t r a c t The model to describe tritium release behavior from solid breeder materials has been developed by the blanket group of Kyushu University. It has been found in the course of this experiment that water is released from the solid breeder materials to the purge gas and that this water affects the tritium release behavior. The amount of adsorbed water and the capacity of water formation reaction of Li2TiO3 pebbles are quantified in this study, where Li2TiO3 pebbles considered to use in the test blanket module at ITER by JAEA is experimented. The tritium release behavior from Li2TiO3 blanket is also estimated for the commercial reactor condition or the ITER burning condition. Moreover, the effects of grain size and temperature on the tritium inventory in Li2TiO3 blanket under the steady state condition are evaluated and the optimal grain size is discussed. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The tritium release model to represent the release behavior of bred tritium from solid breeder materials (Li2TiO3, Li4SiO4, LiAlO2, and Li2ZrO3) has been developed by the present authors [1–3], and the model constructed is shown by Fig. 1. This model takes into account tritium diffusion in solid breeder grain, tritium transfer from bulk of grain to surface water at the interfacial layer of grain, and release of tritium from surface water to purge gas through such surface reactions as water adsorption/desorption, isotope exchange reaction between hydrogen in purge gas and tritium in surface water (isotope exchange reaction (1)), isotope exchange reaction with water vapor in purge gas (isotope exchange reaction (2)), and water generation reaction using hydrogen in purge gas. The interfacial layer is hypothetically assumed to adjust the overall tritium transfer rate and details of the tritium release model are explained in the previous paper by the present authors [1]. Besides desorption of physical and chemical adsorbed water, water is also released to the purge from solid breeder materials through water generation reaction when hydrogen is added to the purge gas at high temperature and it is recognized that existence of water in the blanket changes the tritium release behavior rather largely. Accordingly, it is necessary to understand the chemical form of bred tritium and the amount of water released to the purge gas for designing of the tritium recovery system because recovery of almost all bred tritium is expected to keep the tritium balance in a D–T fusion reactor.
⇑ Corresponding author. Address: Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812-8581, Japan. Tel.: +81 92 642 3783; fax: +81 92 642 3784. E-mail address: [email protected] (M. Nishikawa). 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.130
The amount of water released from Li2TiO3 was experimentally quantified in this study using pebbles considered to use in the ITER test blanket module and the effect of tritium release behavior in the ITER test blanket module was also discussed. 2. Experimental The Li2TiO3 pebbles supplied from Nuclear Fuel Industry (NFI) was used in this study. It was confirmed from SEM pictures that the average grain diameter is 5.0 lm. After desorption of physical adsorbed water by the purge with dry N2 gas at the room temperature, the sample bed packed with Li2TiO3 pebbles in a quartz tube was heated up to 900 °C with a ramp rate of 5 °C/min and change of the water vapor concentration at outlet of the sample bed was measured using a hygrometer. In the case when the release rate was evaluated, the temperature of the sample bed was changed in stepwise also from room temperature to 900 °C. The capacity of chemical adsorbed water was quantified from the experiments using the dry N2 gas, and the water generation capacity was quantified from the experiments using the N2 gas mixed with hydrogen. 3. Results and discussion Fig. 2 shows an example of the water release curve obtained when dry the N2 gas mixed with 10,000 ppm hydrogen is used as the purge gas. As can be seen from peak (1) in this figure, a certain amount of water is released to the purge gas at the room temperature and this water is called the physical adsorbed water in this study. It has been also observed in the previous paper that the capacity of this physical adsorbed water has the vapor pressure
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T. Hanada et al. / Journal of Nuclear Materials 417 (2011) 735–738
400
400 300
200 200 100
100
0
1
2
Fig. 1. The tritium release model from solid breeder materials.
(2)
(1) 0
0
200
(3)
2
4
0
Time [hour]
ð2Þ
The above observation indicates that the chemical adsorbed water of about 1.3 ton is released rapidly to the purge gas when blanket temperature is raised above 300 °C if Li2TiO3 of 550 ton is used in the blanket of a commercial D–T fusion reactor. The water generation capacity for the Li2TiO3 pebbles used in this study, Qwg [mol water/mol Li2TiO3], and the rate constant of the water generation reaction, Kwg [m3/mol s] are also obtained using the similar way as,
Q wg ¼ f1:4 108 expð168000=RTÞg=f1 þ 1011 expð168000=RTÞg
Fig. 2. An example of the water release curve when temperature is raised continuously.
dependency [1]. The temperature of the sample bed was raised to 900 °C with a ramp rate of 5 °C/min after the water vapor in the purge gas became lower than the detectable limit. Then a concentration peak of the desorbed water at around 250 °C was observed in the case of Li2TiO3. The overall capacity of physical or chemical adsorbed water was estimated from the curves obtained in this manner. After release of chemical adsorbed water, further release of water was observed in this figure at the higher temperature than 500 °C. This phenomenon was found by the present authors for ceramic breeder materials and named as the water generation reaction [4,5]. The third peak shown in Fig. 2 is not observed when dry nitrogen is used as the purge gas though similar peaks for physical adsorbed water and chemical adsorbed water are observed. Fig. 3 shows an example of the water release curve of chemical adsorbed water where dry N2 gas is introduced to the Li2TiO3 bed and the temperature is changed in stepwise. The capacity of chemical adsorbed water for the Li2TiO3 pebbles used in this study, Qc [mol water/mol Li2TiO3], was evaluated from observed release curves at each temperature as
Q c ¼ f6:9 1011 expð71500=RTÞg=f1 þ 4:9 109 expð71500=RTÞg:
0
K c ¼ 2:2 109 expð115000=RTÞ þ 0:00139:
ð1Þ
This equation has no vapor pressure dependency because the sample bed is kept in the atmosphere of dry nitrogen gas after release of physical adsorbed water. It is also observed by the present authors that recovery of chemical adsorbed water proceeds slowly when dried Li2TiO3 sample is placed in the humid atmosphere.
þ f2:5 102 expð130000Þ=RTg=f1 þ 1:3 expð130000=RTÞg þ 2 104 ;
ð3Þ
K wg ¼ 3 103 expð4000=RTÞ:
ð4Þ
The above equation indicates that water of 0.15 ton is released due to the water generation reaction when the blanket temperature exceeds 500 °C if the blanket is packed with 550 ton of Li2TiO3 pebbles and is purged by helium gas mixed with hydrogen.The water generation capacity per surface area is compared in Fig. 4
Temperature [°C] 1000 800
600
400
200 0.1
0.1
Amount of generated water/ABET [mol H2O/m2]
10,000ppm H2/N2 purge gas Li2TiO3 (NFI) 1.0g
200
Temperature [°C]
Water vapor concentration [ppm]
400
5
The rate constant in desorption of the chemical adsorbed water, Kc [g/m2 s] is obtained using the curve fitting method to the release curve at each temperature step as,
600
400
4
Fig. 3. An example of the water release curve when temperature is raised in stepwise.
800
600
3
Time [hour]
1000
Temperature
500
300
0
800
600
Temperature
dry N2 purge gas Li2TiO3 (NFI) 1.0g Estimation
Temperature [°C]
Water vapor concentration [ppm]
500
Li2TiO3 (NFI) Li2TiO3 (CEA) LiAlO2 (JAERI) Li4SiO4 (FzK) Li2ZrO3 (MAPI)
0.01
0.01
1E-3
1E-3
1E-4
1E-4
1E-5
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1000/T [K-1] Fig. 4. Water generation capacity per surface area.
1E-5 2.2
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T. Hanada et al. / Journal of Nuclear Materials 417 (2011) 735–738
the blanket is kept at 900 °C after increase from the room temperature to 900 °C with a ramp rate of 5 °C/min. It is also assumed in this estimation that only physical adsorbed water is eliminated before operation.Fig. 6 shows the release curves for tritium and water to the purge gas, where the curves (1–3) represent tritium released as HT, tritium released as HTO + HT, and H2O, respectively. This figure shows that the chemical adsorbed water is rapidly released at the initial stage of operation. However, tritium release is delayed because the diffusion rate of tritium in grain is rather slow at the lower temperature than 300 °C. With increase of tritium in the surface water, release of HTO to the purge gas increases through the water adsorption/desorption and isotope exchange reaction (2). It is shown that release of tritium in HT form increases when the blanket temperature becomes higher than 500 °C because contribution of the isotope exchange reaction (1) is promoted. HTO is formed because of existence of water from water generation reaction at the higher temperature than 500 °C so far as the water generation capacity remains. Fig. 7 shows the comparison of tritium inventory distribution in the blanket where curves (1–4) in this figure represent tritium inventory in the bulk of grain, inventory in the interfacial layer, inventory in the surface water, and the total tritium inventory, respectively. The tritium inventory in the bulk of grain increases rapidly soon after beginning of neutron irradiation and then decreases rapidly when the temperature is raised. This occurs because tritium diffusion from the bulk to the interfacial layer is promoted with temperature rise. With decrease of the tritium inventory in the bulk, tritium inventory in the interfacial layer or the surface water increases. Then, the tritium inventory
800
40
600
20
400
0
200
(3) Temperature
30
1
2
(2)
103 HT HT + HTO 2 H2O in purge gas 10
(1) (2) (3)
20
101
10
(1) 0
0
104
40
0
2
4
6
8
100
800 600 400 200 0
Time [hour]
Time [hour]
Fig. 6. A simulation result of tritium and water release.
Fig. 5-1. A simulation result of water release curve for initial 4 shots.
10-2
30
Tritium inventory [mol]
Li2TiO3(NFI) 19.9kg Flow rate: 27.3 L/min (STP) Purge gas: He with 100Pa H2 20
1000
H2O in purge gas
10
1000
800
Total Tritium inventory [mol] 600
10-4
Tritium inventory in the surface water [mol]
400
Tritium inventory in the interfacial layer [mol] 200
Tritium inventory in the bulk [mol] 10-6
0 0
100
200
Time [hour] Fig. 5-2. A simulation result of water release curves.
300
0
2
4
6
8
0
Time [hour] Fig. 7. Change of tritium inventory distribution with time in blanket.
Temperature [°C]
60
Tritium concentration [µCi/cc]
1000
105
50
1200
Temperature
H2O in purge gas
80
Water concentration [Pa]
Water concentration [Pa]
100
1400
Temperature [°C]
Li2TiO3(NFI) 19.9kg Flow rate: 27.3 L/min (STP) Purge gas: He with 100Pa H2
Temperature [°C]
120
Partial pressure of H2O in gas phase [Pa]
with the generation capacity for other blanket materials reported elsewhere [6]. It is known from this figure that the water generation capacity per surface area observed for Li2TiO3 is almost same as the capacity for Li4SiO4 and smaller than the capacity observed for other blanket materials when the blanket temperature is higher than 700 °C. The water release behavior from an ITER test blanket module is estimated using observed values in this study where repetition of burning for 400 s and dwelling for 1400 s is considered following the ITER burning plan. The blanket module considered in this study is a simplified one from the design of ITER-TBM suggested by Japan Atomic Energy Agency (JAEA) [7] and details of the simplified blanket module are explained by the present authors elsewhere [1]. It is assumed in this estimation that the pebbles are preparatory dried to eliminate the physical and chemical adsorbed water. Fig. 5–1 shows the water release curve estimated for the initial four shots where 100 Pa hydrogen gas mixed with helium is used as the purge gas under the total pressure of 100 kPa. The flow rate is 27.3 L/min in the STP condition. Fig. 5–1 implies that about 20% of hydrogen in the purge gas will change to water through the water generation reaction at the initial stage of blanket operation. The water release behavior until the whole water generation capacity is used is shown in Fig. 5–2. It is known from this figure that effect of water generation on tritium release behavior will not be ignored even after operation of 400 shots.The tritium release behavior under the condition of commercial reactor operation from the blanket module packed with Li2TiO3 is estimated assuming that the similar blanket module stated above is used and that the temperature of
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900
0 0.10 0.50 1.0 3.0 5.0 7.0 10 20
Temperature [°C]
800
700
T2mol
600
500
10-6
10-5
10-4
10-3
Grain size [m]
4. Conclusion The capacities of chemical adsorbed water and that of water generation reaction of Li2TiO3 pebbles are quantified using pebbles considered to use in the ITER-TBM by JAEA. It is observed that the water generation capacity of Li2TiO3 is almost same as the capacity observed for Li4SiO4 and that Li2TiO3 has the smaller water formation capacity than the capacity observed for Li2ZrO3 or LiAlO2. The estimation in this study implies that the generation of water from the ITER-TBM continues for about 400 shots. It is confirmed from simulation of water release behavior or tritium release behavior in this study that water generation can give the profound effect on the tritium release behavior from the blanket. The optimal grain size of Li2TiO3 pebble is about 10 lm from the viewpoint of the tritium inventory in blanket. Acknowledgment
Fig. 8. Effect of grain size and temperature on total tritium inventory.
The authors acknowledge Dr. M. Enoeda in Japan Atomic Energy Agency for useful advice in our study. in the interfacial layer decreases and gradually approaches to the steady-state value. With decrease of inventory in the interfacial layer, the tritium inventory in the surface water also decreases as shown by the curve (3) in Fig. 7. This figure also shows that most of the tritium inventory in the blanket packed with Li2TiO3 pebbles of which grain size is 5.0 lm is occupied by the inventory in the surface water. Change of the total tritium inventory in Li2TiO3 blanket with grain size and blanket temperature is estimated in Fig. 8. The total tritium inventory increases when the grain size is smaller than 10 lm because the tritium inventory in the surface water increases due to increase of total surface area of grains. When the grain size is expanded, inventory in the bulk of grain increases while inventory in the surface water decreases. It is known from this figure that the optimal grain size of Li2TiO3 is about 10 lm from the viewpoint of the tritium inventory.
References [1] T. Kinjyo, M. Nishikawa, M. Enoeda, J. Nucl. Mater. 367–370 (2007) 1361–1365. [2] T. Kinjyo, M. Nishikawa, M. Enoeda, S. Fukada, Fusion Eng. Des. 83 (2008) 580– 587. [3] K. Suematsu, M. Nishikawa, S. Fukada, T. Kinjyo, T. Koyama, N. Yamashita, Fusion Sci. Technol. 54 (2008) 561–564. [4] M. Nishikawa, Y. Kawamura, K. Munakata, H. Matsumoto, J. Nucl. Mater. 174 (1990) 121–123. [5] M. Nishikawa, T. Kinjyo, Y. Nishida, J. Nucl. Mater. 325 (2004) 87–93. 2004. [6] Y. Kawamura, M. Nishikawa, T. Shiraishi, K. Okuno, J. Nucl. Mater. 230 (1996) 287–294. [7] D. Tsuru, et al., Achievements of the Water Cooled Solid Breeder Test Blanket Module of Japan to the Milestones for Installation in ITER, 22nd IAEA Fusion Energy Conference, Geneva, 2008.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Effect of surface water on tritium release behavior from Li2TiO3 T. Hanada, M. Nishikawa ⇑, T. Kanazawa, H. Yamasaki, N. Yamashita, S. Fukada Interdisciplinary Graduate School of Engineering Science, Kyushu University, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 25 December 2010
a b s t r a c t The model to describe tritium release behavior from solid breeder materials has been developed by the blanket group of Kyushu University. It has been found in the course of this experiment that water is released from the solid breeder materials to the purge gas and that this water affects the tritium release behavior. The amount of adsorbed water and the capacity of water formation reaction of Li2TiO3 pebbles are quantified in this study, where Li2TiO3 pebbles considered to use in the test blanket module at ITER by JAEA is experimented. The tritium release behavior from Li2TiO3 blanket is also estimated for the commercial reactor condition or the ITER burning condition. Moreover, the effects of grain size and temperature on the tritium inventory in Li2TiO3 blanket under the steady state condition are evaluated and the optimal grain size is discussed. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The tritium release model to represent the release behavior of bred tritium from solid breeder materials (Li2TiO3, Li4SiO4, LiAlO2, and Li2ZrO3) has been developed by the present authors [1–3], and the model constructed is shown by Fig. 1. This model takes into account tritium diffusion in solid breeder grain, tritium transfer from bulk of grain to surface water at the interfacial layer of grain, and release of tritium from surface water to purge gas through such surface reactions as water adsorption/desorption, isotope exchange reaction between hydrogen in purge gas and tritium in surface water (isotope exchange reaction (1)), isotope exchange reaction with water vapor in purge gas (isotope exchange reaction (2)), and water generation reaction using hydrogen in purge gas. The interfacial layer is hypothetically assumed to adjust the overall tritium transfer rate and details of the tritium release model are explained in the previous paper by the present authors [1]. Besides desorption of physical and chemical adsorbed water, water is also released to the purge from solid breeder materials through water generation reaction when hydrogen is added to the purge gas at high temperature and it is recognized that existence of water in the blanket changes the tritium release behavior rather largely. Accordingly, it is necessary to understand the chemical form of bred tritium and the amount of water released to the purge gas for designing of the tritium recovery system because recovery of almost all bred tritium is expected to keep the tritium balance in a D–T fusion reactor.
⇑ Corresponding author. Address: Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812-8581, Japan. Tel.: +81 92 642 3783; fax: +81 92 642 3784. E-mail address: [email protected] (M. Nishikawa). 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.130
The amount of water released from Li2TiO3 was experimentally quantified in this study using pebbles considered to use in the ITER test blanket module and the effect of tritium release behavior in the ITER test blanket module was also discussed. 2. Experimental The Li2TiO3 pebbles supplied from Nuclear Fuel Industry (NFI) was used in this study. It was confirmed from SEM pictures that the average grain diameter is 5.0 lm. After desorption of physical adsorbed water by the purge with dry N2 gas at the room temperature, the sample bed packed with Li2TiO3 pebbles in a quartz tube was heated up to 900 °C with a ramp rate of 5 °C/min and change of the water vapor concentration at outlet of the sample bed was measured using a hygrometer. In the case when the release rate was evaluated, the temperature of the sample bed was changed in stepwise also from room temperature to 900 °C. The capacity of chemical adsorbed water was quantified from the experiments using the dry N2 gas, and the water generation capacity was quantified from the experiments using the N2 gas mixed with hydrogen. 3. Results and discussion Fig. 2 shows an example of the water release curve obtained when dry the N2 gas mixed with 10,000 ppm hydrogen is used as the purge gas. As can be seen from peak (1) in this figure, a certain amount of water is released to the purge gas at the room temperature and this water is called the physical adsorbed water in this study. It has been also observed in the previous paper that the capacity of this physical adsorbed water has the vapor pressure
736
T. Hanada et al. / Journal of Nuclear Materials 417 (2011) 735–738
400
400 300
200 200 100
100
0
1
2
Fig. 1. The tritium release model from solid breeder materials.
(2)
(1) 0
0
200
(3)
2
4
0
Time [hour]
ð2Þ
The above observation indicates that the chemical adsorbed water of about 1.3 ton is released rapidly to the purge gas when blanket temperature is raised above 300 °C if Li2TiO3 of 550 ton is used in the blanket of a commercial D–T fusion reactor. The water generation capacity for the Li2TiO3 pebbles used in this study, Qwg [mol water/mol Li2TiO3], and the rate constant of the water generation reaction, Kwg [m3/mol s] are also obtained using the similar way as,
Q wg ¼ f1:4 108 expð168000=RTÞg=f1 þ 1011 expð168000=RTÞg
Fig. 2. An example of the water release curve when temperature is raised continuously.
dependency [1]. The temperature of the sample bed was raised to 900 °C with a ramp rate of 5 °C/min after the water vapor in the purge gas became lower than the detectable limit. Then a concentration peak of the desorbed water at around 250 °C was observed in the case of Li2TiO3. The overall capacity of physical or chemical adsorbed water was estimated from the curves obtained in this manner. After release of chemical adsorbed water, further release of water was observed in this figure at the higher temperature than 500 °C. This phenomenon was found by the present authors for ceramic breeder materials and named as the water generation reaction [4,5]. The third peak shown in Fig. 2 is not observed when dry nitrogen is used as the purge gas though similar peaks for physical adsorbed water and chemical adsorbed water are observed. Fig. 3 shows an example of the water release curve of chemical adsorbed water where dry N2 gas is introduced to the Li2TiO3 bed and the temperature is changed in stepwise. The capacity of chemical adsorbed water for the Li2TiO3 pebbles used in this study, Qc [mol water/mol Li2TiO3], was evaluated from observed release curves at each temperature as
Q c ¼ f6:9 1011 expð71500=RTÞg=f1 þ 4:9 109 expð71500=RTÞg:
0
K c ¼ 2:2 109 expð115000=RTÞ þ 0:00139:
ð1Þ
This equation has no vapor pressure dependency because the sample bed is kept in the atmosphere of dry nitrogen gas after release of physical adsorbed water. It is also observed by the present authors that recovery of chemical adsorbed water proceeds slowly when dried Li2TiO3 sample is placed in the humid atmosphere.
þ f2:5 102 expð130000Þ=RTg=f1 þ 1:3 expð130000=RTÞg þ 2 104 ;
ð3Þ
K wg ¼ 3 103 expð4000=RTÞ:
ð4Þ
The above equation indicates that water of 0.15 ton is released due to the water generation reaction when the blanket temperature exceeds 500 °C if the blanket is packed with 550 ton of Li2TiO3 pebbles and is purged by helium gas mixed with hydrogen.The water generation capacity per surface area is compared in Fig. 4
Temperature [°C] 1000 800
600
400
200 0.1
0.1
Amount of generated water/ABET [mol H2O/m2]
10,000ppm H2/N2 purge gas Li2TiO3 (NFI) 1.0g
200
Temperature [°C]
Water vapor concentration [ppm]
400
5
The rate constant in desorption of the chemical adsorbed water, Kc [g/m2 s] is obtained using the curve fitting method to the release curve at each temperature step as,
600
400
4
Fig. 3. An example of the water release curve when temperature is raised in stepwise.
800
600
3
Time [hour]
1000
Temperature
500
300
0
800
600
Temperature
dry N2 purge gas Li2TiO3 (NFI) 1.0g Estimation
Temperature [°C]
Water vapor concentration [ppm]
500
Li2TiO3 (NFI) Li2TiO3 (CEA) LiAlO2 (JAERI) Li4SiO4 (FzK) Li2ZrO3 (MAPI)
0.01
0.01
1E-3
1E-3
1E-4
1E-4
1E-5
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1000/T [K-1] Fig. 4. Water generation capacity per surface area.
1E-5 2.2
737
T. Hanada et al. / Journal of Nuclear Materials 417 (2011) 735–738
the blanket is kept at 900 °C after increase from the room temperature to 900 °C with a ramp rate of 5 °C/min. It is also assumed in this estimation that only physical adsorbed water is eliminated before operation.Fig. 6 shows the release curves for tritium and water to the purge gas, where the curves (1–3) represent tritium released as HT, tritium released as HTO + HT, and H2O, respectively. This figure shows that the chemical adsorbed water is rapidly released at the initial stage of operation. However, tritium release is delayed because the diffusion rate of tritium in grain is rather slow at the lower temperature than 300 °C. With increase of tritium in the surface water, release of HTO to the purge gas increases through the water adsorption/desorption and isotope exchange reaction (2). It is shown that release of tritium in HT form increases when the blanket temperature becomes higher than 500 °C because contribution of the isotope exchange reaction (1) is promoted. HTO is formed because of existence of water from water generation reaction at the higher temperature than 500 °C so far as the water generation capacity remains. Fig. 7 shows the comparison of tritium inventory distribution in the blanket where curves (1–4) in this figure represent tritium inventory in the bulk of grain, inventory in the interfacial layer, inventory in the surface water, and the total tritium inventory, respectively. The tritium inventory in the bulk of grain increases rapidly soon after beginning of neutron irradiation and then decreases rapidly when the temperature is raised. This occurs because tritium diffusion from the bulk to the interfacial layer is promoted with temperature rise. With decrease of the tritium inventory in the bulk, tritium inventory in the interfacial layer or the surface water increases. Then, the tritium inventory
800
40
600
20
400
0
200
(3) Temperature
30
1
2
(2)
103 HT HT + HTO 2 H2O in purge gas 10
(1) (2) (3)
20
101
10
(1) 0
0
104
40
0
2
4
6
8
100
800 600 400 200 0
Time [hour]
Time [hour]
Fig. 6. A simulation result of tritium and water release.
Fig. 5-1. A simulation result of water release curve for initial 4 shots.
10-2
30
Tritium inventory [mol]
Li2TiO3(NFI) 19.9kg Flow rate: 27.3 L/min (STP) Purge gas: He with 100Pa H2 20
1000
H2O in purge gas
10
1000
800
Total Tritium inventory [mol] 600
10-4
Tritium inventory in the surface water [mol]
400
Tritium inventory in the interfacial layer [mol] 200
Tritium inventory in the bulk [mol] 10-6
0 0
100
200
Time [hour] Fig. 5-2. A simulation result of water release curves.
300
0
2
4
6
8
0
Time [hour] Fig. 7. Change of tritium inventory distribution with time in blanket.
Temperature [°C]
60
Tritium concentration [µCi/cc]
1000
105
50
1200
Temperature
H2O in purge gas
80
Water concentration [Pa]
Water concentration [Pa]
100
1400
Temperature [°C]
Li2TiO3(NFI) 19.9kg Flow rate: 27.3 L/min (STP) Purge gas: He with 100Pa H2
Temperature [°C]
120
Partial pressure of H2O in gas phase [Pa]
with the generation capacity for other blanket materials reported elsewhere [6]. It is known from this figure that the water generation capacity per surface area observed for Li2TiO3 is almost same as the capacity for Li4SiO4 and smaller than the capacity observed for other blanket materials when the blanket temperature is higher than 700 °C. The water release behavior from an ITER test blanket module is estimated using observed values in this study where repetition of burning for 400 s and dwelling for 1400 s is considered following the ITER burning plan. The blanket module considered in this study is a simplified one from the design of ITER-TBM suggested by Japan Atomic Energy Agency (JAEA) [7] and details of the simplified blanket module are explained by the present authors elsewhere [1]. It is assumed in this estimation that the pebbles are preparatory dried to eliminate the physical and chemical adsorbed water. Fig. 5–1 shows the water release curve estimated for the initial four shots where 100 Pa hydrogen gas mixed with helium is used as the purge gas under the total pressure of 100 kPa. The flow rate is 27.3 L/min in the STP condition. Fig. 5–1 implies that about 20% of hydrogen in the purge gas will change to water through the water generation reaction at the initial stage of blanket operation. The water release behavior until the whole water generation capacity is used is shown in Fig. 5–2. It is known from this figure that effect of water generation on tritium release behavior will not be ignored even after operation of 400 shots.The tritium release behavior under the condition of commercial reactor operation from the blanket module packed with Li2TiO3 is estimated assuming that the similar blanket module stated above is used and that the temperature of
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900
0 0.10 0.50 1.0 3.0 5.0 7.0 10 20
Temperature [°C]
800
700
T2mol
600
500
10-6
10-5
10-4
10-3
Grain size [m]
4. Conclusion The capacities of chemical adsorbed water and that of water generation reaction of Li2TiO3 pebbles are quantified using pebbles considered to use in the ITER-TBM by JAEA. It is observed that the water generation capacity of Li2TiO3 is almost same as the capacity observed for Li4SiO4 and that Li2TiO3 has the smaller water formation capacity than the capacity observed for Li2ZrO3 or LiAlO2. The estimation in this study implies that the generation of water from the ITER-TBM continues for about 400 shots. It is confirmed from simulation of water release behavior or tritium release behavior in this study that water generation can give the profound effect on the tritium release behavior from the blanket. The optimal grain size of Li2TiO3 pebble is about 10 lm from the viewpoint of the tritium inventory in blanket. Acknowledgment
Fig. 8. Effect of grain size and temperature on total tritium inventory.
The authors acknowledge Dr. M. Enoeda in Japan Atomic Energy Agency for useful advice in our study. in the interfacial layer decreases and gradually approaches to the steady-state value. With decrease of inventory in the interfacial layer, the tritium inventory in the surface water also decreases as shown by the curve (3) in Fig. 7. This figure also shows that most of the tritium inventory in the blanket packed with Li2TiO3 pebbles of which grain size is 5.0 lm is occupied by the inventory in the surface water. Change of the total tritium inventory in Li2TiO3 blanket with grain size and blanket temperature is estimated in Fig. 8. The total tritium inventory increases when the grain size is smaller than 10 lm because the tritium inventory in the surface water increases due to increase of total surface area of grains. When the grain size is expanded, inventory in the bulk of grain increases while inventory in the surface water decreases. It is known from this figure that the optimal grain size of Li2TiO3 is about 10 lm from the viewpoint of the tritium inventory.
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