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Role of energetic tritium chemistry on developing thermonuclear fusion reactors
Fusion Engineering and Design 86 (2011) 2358–2361
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Role of energetic tritium chemistry on developing thermonuclear fusion reactors Kenji Okuno ∗ , Makoto Kobayashi, Rie Kurata, Yasuhisa Oya Radio-Science Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
a r t i c l e
i n f o
Article history: Available online 1 June 2011 Keywords: Tritium Irradiation defects Migration process Lithium oxides Thermal annealing
a b s t r a c t The reaction kinetics of the two conversion processes for the chemical state of tritium in neutron irradiated Li2 O during the thermal annealing was investigated. The chemical states of tritium were T− state and T+ state. The abundance of T− state was increased in the temperature region of 400–500 K (Conversion I) and it was decreased above 500 K (Conversion II). These kinetic constants were determined. The kinetics of Conversion I and Conversion II was determined to be 7.79 exp(0.45 ± 0.04 eV/kT) s−1 and 8.53 × 104 exp(0.90 ± 0.08 eV/kT) s−1 , respectively. The annealing kinetics of irradiation defects of F-centers (oxygen vacancy occupied by one electron), which was a major defect in neutron-irradiated Li2 O, was also studied. Based on those experimental results, a comprehensive migration model for energetic tritium produced in lithium oxides including the dynamics of irradiation defects was proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Knowledge about chemical behavior of energetic hydrogen isotopes, especially tritium, in solid states has increasingly required for development of thermonuclear fusion reactors. In D-T fusion reactors, energetic tritium will be produced by the nuclear reactions of 6 Li(n, ␣)T and 7 Li(n, n ␣)T in lithium-bearing materials, and also will escape from the D-T plasma, and then implanted into the plasma facing materials, such as graphite, beryllium, tungsten and so on. Both of them could behave hot-atomically in the lithium ceramics and plasma-facing materials. Especially, the chemical behavior of energetic tritium produced in the tritium breeding materials is interested from the viewpoint of radiochemistry. In the present paper, we will review hot atom chemical behavior of energetic tritium produced in the tritium breeding materials of fusion reactors. In our previous work, it was reported that the chemical state of energetic tritium in thermal neutron irradiated Li2 O was mainly two types, namely T+ state and T− state [1]. Tritium as T+ state was retained by the interaction with oxygen in Li2 O. On the other hand, tritium as T− state was considered to be interacted with lithium. Therefore, the tritium would be trapped in oxygen vacancy, indicating that the irradiation defects would be a trapping site of tritium in tritium breeder materials. It was found that these abundances for chemical states of tritium were changed by thermal annealing process: the T+ to T− conversion was occurred at 400–500 K and the T− species were
∗ Corresponding author. E-mail address: [email protected] (K. Okuno). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.04.014
eventually conversed to the T+ species at the temperature region of 500–600 K. It was also found that the intensity of F+ -center, which is oxygen vacancy occupied by one electron was annihilated in the same temperature region. In addition, almost tritium in Li2 O was not released below 600 K [2]. These results would indicate that these conversions for chemical states of tritium were associated with the dynamics of irradiation defects like F+ -center in thermal annealing process. Moreover, it have been clarified that the tritium release processes were related to the annihilation of irradiation defects in thermal neutron irradiated ternary lithium oxides [3,4]. In addition, the detail studies on the annihilation kinetics of the irradiation defects induced by the nuclear reactions, 14 MeV neutrons, and gamma-ray in the ternary lithium oxides have been carried out by means of the Electron Spin Resonance (ESR) method [5]. As the results of ESR measurement for gamma-ray irradiated Li2 O, two types of the irradiation defects were existed in Li2 O [6]. One was F+ -center, and the other was O− -center that is the oxygen atom in interstitial site of lattice. These defects were referred as Frenkel pair and these defects will be annihilated by the recombination with each other. It was considered that tritium would interact with F+ -center and O− -center, leading the tritium retention with the chemical state of T− and T+ , respectively. In the thermal annealing process, F+ -center and O− -center was quickly annihilated in the temperature range of 300–400 K. In this temperature region, it was observed that F-centers were diffused in the Li2 O and formed the F-center aggregation (FA). FA was stable in the temperature below 500 K and showed the decomposition above that temperature. These dynamic annihilation–aggregation behaviors of irradiation defects would be correlated with the chemical states of tritium.
K. Okuno et al. / Fusion Engineering and Design 86 (2011) 2358–2361
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In this study, the reaction kinetic rates of two conversion processes for the chemical states of tritium in the thermal annealing processes were determined. These were compared and discussed with the dynamics of irradiation defects which would be trapping sites of tritium. Finally, a comprehensive migration model for energetic tritium produced in the lithium oxides including the dynamics of irradiation defects was proposed. 2. Experiment 2.1. Materials Crystalline powder of Li2 O (99% pure) was the product of Cerac/Pure Inc. Before usage, the powder was heated at 930 K for 50 h in an ultra high vacuum (UHV) system to decompose LiOH and Li2 CO3 impurities. The LiOH content in the degassed material was 0.1 wt% or less. The particle size was about 10 m on the average and the isotopic abundance of 6 Li was 7.4 ± 0.1%. Deuterium oxide (D2 O) with 99.8% of isotopic purity purchased from Merck Sharp & Dohm Canada Ltd. was used without further purification. 2.2. Neutron irradiation The degassed materials (10–20 mg) were sealed in quartz ampoules under vacuum and irradiated in the T-pipe of the JRR4 reactor of the Japan Atomic Energy Agency. The thermal and fast neutron fluxes at the irradiation position, monitored with Co and Ni foils, were 4 × 1013 and 3 × 1012 cm−2 s−1 , respectively. The temperature at the target surface was about 320 K. The irradiated materials were stored in a container cooled with liquid nitrogen. 2.3. Analytical procedures The tritium species (T+ , T− and T0 ) existing in Li2 O were analyzed with a radiometric method based on the following reactions [7]. When the irradiated material is dissolved in heavy water, the labile tritium (T+ ) in the solid is converted to tritiated water through the immediate exchange reaction. T+ + D2 O → D+ + DTO
(1a)
or OT− + D2 O → OD− + DTO (T− ),
which would be bonded to Tritide anion gaseous DT on hydrolysis, Li+ T− + D2 O → Li+ (OD)− + DT
(1b) Li+ ,
gives rise to (2)
The exchange between tritium (T0 ), which would be in the form of molecular hydrogen (HT, T2 ) as well as hydrocarbons, and deuterium of heavy water is very slow in absence of catalysts, so that the T0 species is evolved in the form of HT, T2 and hydrocarbon-T. Consequently the tritium remaining in the aqueous phase corresponds to the T+ species. The T− species is observed as gaseous DT and can be separated from the T0 species which is evolved as HT, T2 and hydrocarbon-T. Details of the apparatus and the analytical procedures were described in the previous paper [1]. 3. Results and discussion The abundances of T− state in isothermal annealing experiments at the temperature of 449, 499, 511, 549 K are shown in Fig. 1. It was thought that tritium in T− state is corresponding to tritium trapped in defects like F+ -center and that in T+ state is tritium
Fig. 1. The abundance of T− state at various temperatures as a function of annealing time.
interacting with oxygen in Li2 O. The abundance of tritium in T0 state was few percent. During the isothermal annealing, a few percent of tritium was released [2]. It was found that the abundance of T− was increased at first, and eventually decreased. These behaviors in thermal annealing processes suggested that there were two conversion regimes for the chemical states of tritium before release; first regime is the conversion of T+ state to T− state (Conversion I), and the second is conversion of T− state to T+ state (Conversion II). These results showed that some tritium in T+ state which is retained by the interaction with oxygen was formerly detrapped and re-trapped by the defects. Thereafter, the tritium in the defects was detrapped from defects and trapped by oxygen again. As mentioned above, two conversion processes for the chemical states of tritium were considered in thermal annealing process. To understand the tritium migration inside tritium breeder materials, evaluation of the kinetics for these conversions should be important. It was thought that these conversions were occurred by the interaction of tritium with irradiation defects. Here, it can be considered that the amount of irradiation defects would be excess compared to that of bred tritium by its high energy, deducing that the kinetics for conversions of chemical states of tritium would only be determined by the concentration of tritium in the sample. Therefore, it can be assumed that each conversion would be proceeded as the first order reaction. Based on this assumption, the reaction rate constants of each conversion process in different annealing temperatures are obtained from Fig. 1. Fig. 2 presents the Arrhenius type plot of reaction rate constant for Conversion I (Fig. 2a) and Conversion II (Fig. 2b). The kinetic rates of Conversion I and Conversion II were determined to be 7.79 exp(0.45 ± 0.04 eV/kT) s−1 and 8.53 × 104 exp(0.90 ± 0.08 eV/kT) s−1 , respectively. By using this result, the change of the chemical states of tritium reported in Ref. [1] was analyzed again. Fig. 3a is the change of chemical states of tritium in Li2 O, namely T+ state and T− state with increasing of annealing temperature, reported in Ref. [1]. Fig. 3b shows the simulation results of the chemical states of tritium with the elevated temperature using reaction kinetics for each conversion process obtained from Fig. 2. The simulation results from the
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Fig. 4. The model of tritium migration processes in lithium oxides with the annihilation and aggregation dynamics of irradiation defects.
Fig. 2. Arrhenius type plot for (a) Conversion I and (b) Conversion II.
analysis showed a good agreement with the results obtained in experiment. For understanding of the conversion for the chemical state of tritium in thermal annealing processes, the dynamics of irradiation defects should be the key issues. In temperature range of 300–400 K, the Frenkel pair, those are F+ -center and O− -center would be recombined. In this process, the tritium trapped in F+ -center and O− -center as T− state and T+ state, respectively was detrapped. Simultaneously, the F-centers were aggregated in this temperature by thermal diffusion [8]. The aggregated F-centers would form the F-center aggregation (FA). It was considered that tritium trapped in FA should behave as T− state. The FA is stable in the temperature range of 400–500 K. In this temperature, the abundance of T− state was increased. It was interpreted that tritium was trapped by FA in this temperature region. This tritium was expected to be from Frenkel pair which was disappeared by
recombination below 400 K. Then, the FA will be decomposed above 500 K. The abundance of T− state was decreased and T+ state was increased above 500 K. Above 500 K, the tritium trapped in FA as T− state was detraped by the decomposition of FA. Hayashi et al. previously reported that detrapped tritium from irradiation defects would be diffused via Li vacancy and at that time, tritium interacts with oxygen, indicating that the chemical state of tritium will be converted from T− state to T+ state above 500 K [9]. Finally, diffused tritium will reach on the surface of Li2 O and released by the recombination reaction with other tritium or the isotope exchange reaction with purge gas. From these ideas, the model of tritium migration in Li2 O was established and shown in Fig. 4. In this model, the interaction of tritium with irradiation defects was focused. After generation of energetic tritium by the nuclear reaction of neutron with lithium, the tritium will be thermalized by producing irradiation defects. Thermal tritium will be trapped in irradiation defects like F+ -center and O− -center as T− state and T+ state, respectively. In thermal annealing process, some F+ -center and O− -center are recombined and disappeared. Simultaneously, FA is formed by thermal diffusion of F-centers. Tritium detrapped from Frenkel pair is re-trapped in
Fig. 3. The abundance of tritium with each chemical state (a) in neutron irradiated Li2 O in Ref. [1] and (b) simulated by the reaction kinetics of Conversion I and Conversion II.
K. Okuno et al. / Fusion Engineering and Design 86 (2011) 2358–2361
FA and behaves as T− state. The FA will be decomposed in higher temperature and trapped tritium in FA is detrapped in this time. Through the diffusion processes of tritium by interacting with oxygen, the tritium will be released. For establishment of this model, Li2 O was used as specimen. It was considered that this model would be capable to apply to tritium migration in the other tritium breeder candidates as the Li2 O is the simplest lithium oxides. The kinetics of tritium interacting with irradiation defects was revealed in this model. However, for the establishment of comprehensive tritium migration model, the diffusion kinetics of tritium and the surface reaction have not been clarified. In the future work, the diffusion kinetics of tritium and the effects of purge gas on the release behavior of tritium should be investigated.
with the dynamics of irradiation defects which would be trapping sites of tritium. F-centers were aggregated in the temperature region of 300–400 K and formed F-center aggregation (FA). FA was stable in 400–500 K and decomposed above 500 K. It was considered that tritium trapped in Frenkel pair was de-trapped and re-trapped in FA in 400–500 K, leading that the abundance of T− state of tritium was increased. In the decomposition process of FA above 500 K, the tritium was detraped and diffused to the surface and finally released. At last, a comprehensive migration model for energetic tritium produced in the lithium oxides including the dynamics of irradiation defects was proposed. This model would be capable to apply to tritium migration in other tritium breeder candidates as well as Li2 O, one of the simplest lithium oxides.
4. Conclusion References In this study, it was found that the major chemical states of tritium produced in neutron-irradiated Li2 O were T− state and T+ state. The abundance of T− state was increased in the temperature region of 400–500 K and it was decreased above 500 K. This suggested there were two conversion processes of the tritium existing states in Li2 O. The reaction kinetic rates of the two conversion processes in the thermal annealing processes were determined. These kinetics data were compared and discussed
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[1] [2] [3] [4] [5] [6] [7] [8] [9]
K. Okuno, H. Kudo, J. Nucl. Mater. 138 (1986) 31. H. Kudo, K. Okuno, S. O’hira, J. Nucl. Mater. 155 (1988) 524. S. Akahori, et al., J. Radiochem. Nucl. Chem. 255 (2003) 257. M. Oyaidzu, et al., Phys. Scr. T 108 (2004) 42. Y. Morimoto, et al., Fusion Technol. 39 (2001) 634. Y. Ueda, et al., J. Appl. Phys 16 (1977) 1743. P. Van Urk, Thesis, University of Amsterdam (1970). H. Moriyama, A. Okada, Y. Asaoka, Y. Ito, J. Nucl. Mater. 179 (1991) 839. T. Hayashi, S. Konishi, K. Okuno, J. Nucl. Mater. 170 (1990) 60.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Role of energetic tritium chemistry on developing thermonuclear fusion reactors Kenji Okuno ∗ , Makoto Kobayashi, Rie Kurata, Yasuhisa Oya Radio-Science Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
a r t i c l e
i n f o
Article history: Available online 1 June 2011 Keywords: Tritium Irradiation defects Migration process Lithium oxides Thermal annealing
a b s t r a c t The reaction kinetics of the two conversion processes for the chemical state of tritium in neutron irradiated Li2 O during the thermal annealing was investigated. The chemical states of tritium were T− state and T+ state. The abundance of T− state was increased in the temperature region of 400–500 K (Conversion I) and it was decreased above 500 K (Conversion II). These kinetic constants were determined. The kinetics of Conversion I and Conversion II was determined to be 7.79 exp(0.45 ± 0.04 eV/kT) s−1 and 8.53 × 104 exp(0.90 ± 0.08 eV/kT) s−1 , respectively. The annealing kinetics of irradiation defects of F-centers (oxygen vacancy occupied by one electron), which was a major defect in neutron-irradiated Li2 O, was also studied. Based on those experimental results, a comprehensive migration model for energetic tritium produced in lithium oxides including the dynamics of irradiation defects was proposed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Knowledge about chemical behavior of energetic hydrogen isotopes, especially tritium, in solid states has increasingly required for development of thermonuclear fusion reactors. In D-T fusion reactors, energetic tritium will be produced by the nuclear reactions of 6 Li(n, ␣)T and 7 Li(n, n ␣)T in lithium-bearing materials, and also will escape from the D-T plasma, and then implanted into the plasma facing materials, such as graphite, beryllium, tungsten and so on. Both of them could behave hot-atomically in the lithium ceramics and plasma-facing materials. Especially, the chemical behavior of energetic tritium produced in the tritium breeding materials is interested from the viewpoint of radiochemistry. In the present paper, we will review hot atom chemical behavior of energetic tritium produced in the tritium breeding materials of fusion reactors. In our previous work, it was reported that the chemical state of energetic tritium in thermal neutron irradiated Li2 O was mainly two types, namely T+ state and T− state [1]. Tritium as T+ state was retained by the interaction with oxygen in Li2 O. On the other hand, tritium as T− state was considered to be interacted with lithium. Therefore, the tritium would be trapped in oxygen vacancy, indicating that the irradiation defects would be a trapping site of tritium in tritium breeder materials. It was found that these abundances for chemical states of tritium were changed by thermal annealing process: the T+ to T− conversion was occurred at 400–500 K and the T− species were
∗ Corresponding author. E-mail address: [email protected] (K. Okuno). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.04.014
eventually conversed to the T+ species at the temperature region of 500–600 K. It was also found that the intensity of F+ -center, which is oxygen vacancy occupied by one electron was annihilated in the same temperature region. In addition, almost tritium in Li2 O was not released below 600 K [2]. These results would indicate that these conversions for chemical states of tritium were associated with the dynamics of irradiation defects like F+ -center in thermal annealing process. Moreover, it have been clarified that the tritium release processes were related to the annihilation of irradiation defects in thermal neutron irradiated ternary lithium oxides [3,4]. In addition, the detail studies on the annihilation kinetics of the irradiation defects induced by the nuclear reactions, 14 MeV neutrons, and gamma-ray in the ternary lithium oxides have been carried out by means of the Electron Spin Resonance (ESR) method [5]. As the results of ESR measurement for gamma-ray irradiated Li2 O, two types of the irradiation defects were existed in Li2 O [6]. One was F+ -center, and the other was O− -center that is the oxygen atom in interstitial site of lattice. These defects were referred as Frenkel pair and these defects will be annihilated by the recombination with each other. It was considered that tritium would interact with F+ -center and O− -center, leading the tritium retention with the chemical state of T− and T+ , respectively. In the thermal annealing process, F+ -center and O− -center was quickly annihilated in the temperature range of 300–400 K. In this temperature region, it was observed that F-centers were diffused in the Li2 O and formed the F-center aggregation (FA). FA was stable in the temperature below 500 K and showed the decomposition above that temperature. These dynamic annihilation–aggregation behaviors of irradiation defects would be correlated with the chemical states of tritium.
K. Okuno et al. / Fusion Engineering and Design 86 (2011) 2358–2361
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In this study, the reaction kinetic rates of two conversion processes for the chemical states of tritium in the thermal annealing processes were determined. These were compared and discussed with the dynamics of irradiation defects which would be trapping sites of tritium. Finally, a comprehensive migration model for energetic tritium produced in the lithium oxides including the dynamics of irradiation defects was proposed. 2. Experiment 2.1. Materials Crystalline powder of Li2 O (99% pure) was the product of Cerac/Pure Inc. Before usage, the powder was heated at 930 K for 50 h in an ultra high vacuum (UHV) system to decompose LiOH and Li2 CO3 impurities. The LiOH content in the degassed material was 0.1 wt% or less. The particle size was about 10 m on the average and the isotopic abundance of 6 Li was 7.4 ± 0.1%. Deuterium oxide (D2 O) with 99.8% of isotopic purity purchased from Merck Sharp & Dohm Canada Ltd. was used without further purification. 2.2. Neutron irradiation The degassed materials (10–20 mg) were sealed in quartz ampoules under vacuum and irradiated in the T-pipe of the JRR4 reactor of the Japan Atomic Energy Agency. The thermal and fast neutron fluxes at the irradiation position, monitored with Co and Ni foils, were 4 × 1013 and 3 × 1012 cm−2 s−1 , respectively. The temperature at the target surface was about 320 K. The irradiated materials were stored in a container cooled with liquid nitrogen. 2.3. Analytical procedures The tritium species (T+ , T− and T0 ) existing in Li2 O were analyzed with a radiometric method based on the following reactions [7]. When the irradiated material is dissolved in heavy water, the labile tritium (T+ ) in the solid is converted to tritiated water through the immediate exchange reaction. T+ + D2 O → D+ + DTO
(1a)
or OT− + D2 O → OD− + DTO (T− ),
which would be bonded to Tritide anion gaseous DT on hydrolysis, Li+ T− + D2 O → Li+ (OD)− + DT
(1b) Li+ ,
gives rise to (2)
The exchange between tritium (T0 ), which would be in the form of molecular hydrogen (HT, T2 ) as well as hydrocarbons, and deuterium of heavy water is very slow in absence of catalysts, so that the T0 species is evolved in the form of HT, T2 and hydrocarbon-T. Consequently the tritium remaining in the aqueous phase corresponds to the T+ species. The T− species is observed as gaseous DT and can be separated from the T0 species which is evolved as HT, T2 and hydrocarbon-T. Details of the apparatus and the analytical procedures were described in the previous paper [1]. 3. Results and discussion The abundances of T− state in isothermal annealing experiments at the temperature of 449, 499, 511, 549 K are shown in Fig. 1. It was thought that tritium in T− state is corresponding to tritium trapped in defects like F+ -center and that in T+ state is tritium
Fig. 1. The abundance of T− state at various temperatures as a function of annealing time.
interacting with oxygen in Li2 O. The abundance of tritium in T0 state was few percent. During the isothermal annealing, a few percent of tritium was released [2]. It was found that the abundance of T− was increased at first, and eventually decreased. These behaviors in thermal annealing processes suggested that there were two conversion regimes for the chemical states of tritium before release; first regime is the conversion of T+ state to T− state (Conversion I), and the second is conversion of T− state to T+ state (Conversion II). These results showed that some tritium in T+ state which is retained by the interaction with oxygen was formerly detrapped and re-trapped by the defects. Thereafter, the tritium in the defects was detrapped from defects and trapped by oxygen again. As mentioned above, two conversion processes for the chemical states of tritium were considered in thermal annealing process. To understand the tritium migration inside tritium breeder materials, evaluation of the kinetics for these conversions should be important. It was thought that these conversions were occurred by the interaction of tritium with irradiation defects. Here, it can be considered that the amount of irradiation defects would be excess compared to that of bred tritium by its high energy, deducing that the kinetics for conversions of chemical states of tritium would only be determined by the concentration of tritium in the sample. Therefore, it can be assumed that each conversion would be proceeded as the first order reaction. Based on this assumption, the reaction rate constants of each conversion process in different annealing temperatures are obtained from Fig. 1. Fig. 2 presents the Arrhenius type plot of reaction rate constant for Conversion I (Fig. 2a) and Conversion II (Fig. 2b). The kinetic rates of Conversion I and Conversion II were determined to be 7.79 exp(0.45 ± 0.04 eV/kT) s−1 and 8.53 × 104 exp(0.90 ± 0.08 eV/kT) s−1 , respectively. By using this result, the change of the chemical states of tritium reported in Ref. [1] was analyzed again. Fig. 3a is the change of chemical states of tritium in Li2 O, namely T+ state and T− state with increasing of annealing temperature, reported in Ref. [1]. Fig. 3b shows the simulation results of the chemical states of tritium with the elevated temperature using reaction kinetics for each conversion process obtained from Fig. 2. The simulation results from the
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K. Okuno et al. / Fusion Engineering and Design 86 (2011) 2358–2361
Fig. 4. The model of tritium migration processes in lithium oxides with the annihilation and aggregation dynamics of irradiation defects.
Fig. 2. Arrhenius type plot for (a) Conversion I and (b) Conversion II.
analysis showed a good agreement with the results obtained in experiment. For understanding of the conversion for the chemical state of tritium in thermal annealing processes, the dynamics of irradiation defects should be the key issues. In temperature range of 300–400 K, the Frenkel pair, those are F+ -center and O− -center would be recombined. In this process, the tritium trapped in F+ -center and O− -center as T− state and T+ state, respectively was detrapped. Simultaneously, the F-centers were aggregated in this temperature by thermal diffusion [8]. The aggregated F-centers would form the F-center aggregation (FA). It was considered that tritium trapped in FA should behave as T− state. The FA is stable in the temperature range of 400–500 K. In this temperature, the abundance of T− state was increased. It was interpreted that tritium was trapped by FA in this temperature region. This tritium was expected to be from Frenkel pair which was disappeared by
recombination below 400 K. Then, the FA will be decomposed above 500 K. The abundance of T− state was decreased and T+ state was increased above 500 K. Above 500 K, the tritium trapped in FA as T− state was detraped by the decomposition of FA. Hayashi et al. previously reported that detrapped tritium from irradiation defects would be diffused via Li vacancy and at that time, tritium interacts with oxygen, indicating that the chemical state of tritium will be converted from T− state to T+ state above 500 K [9]. Finally, diffused tritium will reach on the surface of Li2 O and released by the recombination reaction with other tritium or the isotope exchange reaction with purge gas. From these ideas, the model of tritium migration in Li2 O was established and shown in Fig. 4. In this model, the interaction of tritium with irradiation defects was focused. After generation of energetic tritium by the nuclear reaction of neutron with lithium, the tritium will be thermalized by producing irradiation defects. Thermal tritium will be trapped in irradiation defects like F+ -center and O− -center as T− state and T+ state, respectively. In thermal annealing process, some F+ -center and O− -center are recombined and disappeared. Simultaneously, FA is formed by thermal diffusion of F-centers. Tritium detrapped from Frenkel pair is re-trapped in
Fig. 3. The abundance of tritium with each chemical state (a) in neutron irradiated Li2 O in Ref. [1] and (b) simulated by the reaction kinetics of Conversion I and Conversion II.
K. Okuno et al. / Fusion Engineering and Design 86 (2011) 2358–2361
FA and behaves as T− state. The FA will be decomposed in higher temperature and trapped tritium in FA is detrapped in this time. Through the diffusion processes of tritium by interacting with oxygen, the tritium will be released. For establishment of this model, Li2 O was used as specimen. It was considered that this model would be capable to apply to tritium migration in the other tritium breeder candidates as the Li2 O is the simplest lithium oxides. The kinetics of tritium interacting with irradiation defects was revealed in this model. However, for the establishment of comprehensive tritium migration model, the diffusion kinetics of tritium and the surface reaction have not been clarified. In the future work, the diffusion kinetics of tritium and the effects of purge gas on the release behavior of tritium should be investigated.
with the dynamics of irradiation defects which would be trapping sites of tritium. F-centers were aggregated in the temperature region of 300–400 K and formed F-center aggregation (FA). FA was stable in 400–500 K and decomposed above 500 K. It was considered that tritium trapped in Frenkel pair was de-trapped and re-trapped in FA in 400–500 K, leading that the abundance of T− state of tritium was increased. In the decomposition process of FA above 500 K, the tritium was detraped and diffused to the surface and finally released. At last, a comprehensive migration model for energetic tritium produced in the lithium oxides including the dynamics of irradiation defects was proposed. This model would be capable to apply to tritium migration in other tritium breeder candidates as well as Li2 O, one of the simplest lithium oxides.
4. Conclusion References In this study, it was found that the major chemical states of tritium produced in neutron-irradiated Li2 O were T− state and T+ state. The abundance of T− state was increased in the temperature region of 400–500 K and it was decreased above 500 K. This suggested there were two conversion processes of the tritium existing states in Li2 O. The reaction kinetic rates of the two conversion processes in the thermal annealing processes were determined. These kinetics data were compared and discussed
2361
[1] [2] [3] [4] [5] [6] [7] [8] [9]
K. Okuno, H. Kudo, J. Nucl. Mater. 138 (1986) 31. H. Kudo, K. Okuno, S. O’hira, J. Nucl. Mater. 155 (1988) 524. S. Akahori, et al., J. Radiochem. Nucl. Chem. 255 (2003) 257. M. Oyaidzu, et al., Phys. Scr. T 108 (2004) 42. Y. Morimoto, et al., Fusion Technol. 39 (2001) 634. Y. Ueda, et al., J. Appl. Phys 16 (1977) 1743. P. Van Urk, Thesis, University of Amsterdam (1970). H. Moriyama, A. Okada, Y. Asaoka, Y. Ito, J. Nucl. Mater. 179 (1991) 839. T. Hayashi, S. Konishi, K. Okuno, J. Nucl. Mater. 170 (1990) 60.