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Formation of lithium-tritide by hot atom reactions of tritium produced in Pb-16Li
Fusion Engineering and Design 88 (2013) 2328–2331
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Formation of lithium-tritide by hot atom reactions of tritium produced in Pb-16Li Kenji Okuno a,∗ , Makoto Kobayashi a , Toshihiko Yamanishi b , Yasuhisa Oya a a b
Radioscience Research Laboratory, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan Japan Atomic Energy Agency, Ibaraki, Japan
h i g h l i g h t s • • • •
The solubility of hydrogen isotope was evaluated to be S = 6.56 × 10−7 exp(−0.11 [eV]/kT) [at. fr, Pa0.5 ]. About 5% of tritium was released at single release stage around 600 K for neutron irradiated Li17 Pb83 . No tritium release was found at around 600 K for the hydrogen isotope-doped Li17 Pb83 . LiT was formed in Li17 Pb83 by thermal neutron irradiation from the kinetic analysis.
a r t i c l e
i n f o
Article history: Received 14 September 2012 Received in revised form 21 January 2013 Accepted 19 March 2013 Available online 20 April 2013 Keywords: Tritium Pb-16Li LiT Solubility TDS
a b s t r a c t The release behaviors of hydrogen isotopes in Pb-16Li introduced by thermal gas exposure or produced by thermal neutron irradiation were compared to investigate hot atom reactions of tritium. The solubility of hydrogen isotope was evaluated to be S = 6.56 × 10−7 exp(−0.11 [eV]/kT) [at. fr, Pa0.5 ], which is consistent with the literature values. The tritium TDS spectra for Pb-16Li with thermal neutron irradiation showed that about 5% of tritium was released at single release stage around 600 K, which was higher than the melting point of Pb-16Li, although no release peak was found at around 600 K for the hydrogen isotope-doped Pb-16Li. The kinetic analysis indicated that the tritium release would be associated with the dissociation of Li-T bond, indicating that LiT was formed in Pb-16Li by thermal neutron irradiation, suggesting that the formation of Li-T bonds should be considered to estimate tritium retention in Pb-16Li eutectic blanket systems in fusion reactors. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In blanket system of fusion reactors, lithium-containing compounds will be introduced to produce tritium artificially by the reaction between lithium and neutron such as 6 Li(n, ␣T. A blanket concept using lithium (Li)-lead (Pb) eutectic alloy is considered as one of the promising liquid blanket systems for fusion reactors due to large tritium-breeding ratio (TBR), neutron multiplying ratio by lead, quick tritium recovery by low tritium solubility, high thermal conductivity, and so on [1,2]. Pb-16Li eutectic alloy is the most promising material as liquid tritium breeder because of its lowest melting point compared to that of other lithium-lead eutectic alloys. For the assessments of tritium safety, evaluation of tritium inventory in Pb-16Li blanket system is important. The solubility of tritium has been mainly investigated by hydrogen isotope gas
∗ Corresponding author. Tel.: +81 54 238 4802; fax: +81 54 238 3989. E-mail address: [email protected] (K. Okuno). 0920-3796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2013.03.064
charging for the liquid state of Pb-16Li under thermal equilibrium [3–7]. It is found that tritium solubility in Pb-16Li is generally low, however the data of solubility is largely scattered. For actual fusion reactor environment, the reactions of tritium with lithium should be also considered because lithium has high affinity to tritium. It is well known that tritium can solute exothermically into lithium with the formation of LiT, which may enhance tritium solubility in lithium [8]. The solution mechanisms of hydrogen isotopes in Li-Pb alloys have been systematically studied with changing the atomic fraction of lithium and lead in alloys [9–11]. Wu has observed that the heat of solution for deuterium decreased as lead content increased [10]. The heat of solution for deuterium into LiPb alloys was clearly dropped at the lithium fractional content of 0.17, where the solution process was changed from the exothermally to the endothermally. These facts represent that the affinity of lithium to tritium was sufficiently low in Pb-16Li. It can be seen that tritium inventory in Pb-16Li can be estimated with regardless of LiT formation according to these experimental results obtained in thermal equilibrium condition. However, Moriyama has reported that the fraction of tritium anion (T− ) in Li-Pb alloys depends on
QMS
TMP Reactor tube Furnace
2329
1
D exposure
10
Neutron irradiation (JRR-3) Neutron irradiation (KUR)
14
Capacitance manometer
H isotope release rate / 10 s g
D2 bottle
-1 -1
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
SP
0
10
10
-1
10
-2
Mo crucible
400 Fig. 1. Schematic drawing of the apparatus for deuterium gas exposure and TDS experiments. QMS, quadropole mass spectrometer; TMP, turbo molecular pump; SP, scroll pump.
lithium fractional content in alloys, and about 30% of tritium was retained as T− state (tritium bound to lithium) in Pb-16Li [11], indicating that this report has showed the possibility of the LiT formation in Pb-16Li although the formation mechanism of LiT was not expected from solubility data under thermal equilibrium. It was seen that the hot atom reactions which are non-equilibrium reactions of energetic atoms, would contribute to the formation of LiT in Pb-16Li during neutron irradiation. The formation of LiT during neutron irradiation will result in the increase of tritium hazard potential in fusion reactor. These results motivate us to investigate the possibility of LiT formation in Pb-16Li under neutron irradiation circumstance and to discuss about the interaction of ‘hot’ (energetic) tritium with Pb-16Li. Two types of hydrogen isotope introduction methods were adapted to elucidate the effect of hydrogen isotope energy on the formation of LiT in Pb-16Li in this study. The first was thermal neutron irradiation for the Pb-16Li eutectic alloy at ambient temperature to produce hot tritium into the alloy. The other was deuterium gas exposure to introduce deuterium under thermal equilibrium. The release behaviors for these samples were compared by thermal desorption spectroscopy (TDS) and the mechanism of hot atom reactions of tritium produced in the Pb-16Li eutectic alloy was discussed. 2. Experimental 2.1. Deuterium thermal doping and deuterium TDS experiments Pb-16Li eutectic alloy fabricated from pure lithium rod and lead granular was used. The fabrication method and the characteristics of Pb-16Li eutectic alloy are found in elsewhere [12]. The alloy was set into Mo crucible with the size of 10 mmt × 10 mmt , then Mo crucible was introduced into quartz tube under vacuum as found in Fig. 1. The pressure of this system was around 10−5 Pa. The quartz tube can be heated by ceramic furnace up to the temperature of 1173 K. Pb-16Li eutectic alloy was heated under vacuum to be the liquid state. Thereafter, the exposure of deuterium (D2 ) gas with the isotopic purity of 99.4% was performed for 5 h to introduce deuterium uniformly in the alloy. The D2 gas pressure and exposure temperature were changed in 7.7–200 kPa and 603–873 K, respectively. After the exposure, the quartz tube with Mo crucible was cooled down quickly by water and liquid nitrogen and the residual D2 gas was evacuated by TMP (turbo molecular pump). The TDS experiments were carried out using QMS (quadrupole mass spectrometer) from room temperature to 973 K with the heating rate of 5 K/min. Mass number of 4 corresponding to D2 was monitored by QMS to estimate the deuterium retention in the alloy.
500
600
700
Temperature/ K Fig. 2. TDS spectra of hydrogen isotopes released from Pb-16Li exposed to D2 gas or irradiated with thermal neutron.
2.2. Thermal neutron irradiation and out-of-pile tritium release experiments 0.3 g of the alloy was encapsulated in a quartz tube under vacuum. Thermal neutron irradiation was carried out in Japan Research Reactor-3 (JRR-3) and Kyoto University Research Reactor (KUR). Thermal neutron flux and fluence at JRR-3 were 5.2 × 1013 n cm−2 s−1 and 3.1 × 1015 n cm−2 , respectively. Those at KUR were 5.2 × 1012 n cm−2 s−1 and 3.3 × 1016 n cm−2 , respectively. Thermal neutron fluence in KUR was about ten times higher than that in JRR-3. The maximum sample temperature during the neutron irradiation was estimated to be less than 373 K, indicating that the Pb-16Li eutectic alloy was solid state during the neutron irradiation. The TDS experiments were carried out in the temperature range of 300–1173 K by tritium TDS system at Shizuoka University, whose detail was described in ref [13]. The radioactivity of released tritium was measured by a proportional counter. The different chemical forms as water (HTO) and molecule (HT) were trapped separately by water bubblers. Total amount of released tritium was determined by using a liquid scintillation counter. 3. Results and discussion Fig. 2 shows TDS spectra for D2 gas exposed Pb-16Li and thermal neutron irradiated one. The D2 gas exposure was done with the pressure of 105 kPa and the temperature of 673 K. The heating rates for all spectra were 5 K/min. In the both cases of D2 gas exposure and thermal neutron irradiation, a sharp release peak was observed at around 520 K, which was slightly higher than the melting point of Pb-16Li eutectic alloy. It was reported in our previous studies that the major hydrogen isotope release is governed by diffusion process in the liquid phase of Pb-16Li [12]. The major hydrogen isotope release was triggered by the phase transition of Pb-16Li from the solid phase to the liquid phase. For the alloy with thermal neutron irradiation, the additional release peak was appeared at around 600 K, which was only observed for the thermal neutron irradiated Pb-16Li. The deuterium concentration in the alloy with D2 gas exposure as a function of D2 gas pressure under various exposure temperatures are shown in Fig. 3. The linear relationship between the deuterium concentration in the alloy and the square root of D2 gas pressure was found even if the exposure temperature was changed, indicating that the dissociative absorption of D2 gas into the liquid state of Pb-16Li would be proceeded. Fig. 4 summarizes the solubility of deuterium in Pb-16Li. The literature data of hydrogen isotope solubility in Pb-16Li is also added in this figure [4–7]. The deuterium solubility obtained in this study has a slight temperature dependency while some of the researchers have reported that hydrogen
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
Tritium release rate / kBq g s
D2 gas exposure temp. 873 K 773 K 673 K 603 K
4.0
-5
D concentration / 10 at. fr.
5.0
-1 -1
2330
3.0 2.0 1.0
0
10
-1
10
450
0.0 0
100
200
300
400
2 K / min. 5 K / min. 15 K / min. 30 K / min.
1
10
500
550
600
650
700
750
800
Temperature / K
500
0.5
Square root of pressure / Pa
Fig. 5. Heating rate dependence on tritium TDS spectra for Pb-16Li irradiated with thermal neutron at JRR-3.
Fig. 3. Deuterium concentration in Pb-16Li at various temperatures as a function of square root of D2 gas exposure pressure.
isotope solubility in Pb-16Li is independent on temperature. The deuterium solubility was almost consistent with the literature data by Maeda, Chan and Veleckis [5,7], and can be expressed as follows: S = 6.56 × 10−7 exp
−0.11 [eV] kT
[at.fr, Pa0.5 ]
(1)
where the heat of solution of deuterium in Pb-16Li was 0.11 eV, indicating that deuterium solution proceeds endothermally. The additional peak at 600 K was found for thermal neutron irradiated Pb-16Li as the thermal neutron fluence increased, whose tritium retention was estimated to be about 5% to total tritium retention. Engbæk et al. have been reported that the releases of hydrogen and lithium were observed at around 600 K during TDS experiment for LiH [8]. The TDS experiments with different heating rates were performed to determine the tritium release process at 600 K. Fig. 5 shows the TDS spectra with various heating rates for thermal neutron irradiated Pb-16Li. It was found that the peak temperature was shifted toward higher temperature side as the heating rate increased. The tritium release peak in higher temperature region was analyzed by Kissinger-Akahira-Sunose (KAS) model-free-kinetics method which has widely applied for the analysis of TDS spectra [14]. In this method, ln(ˇ/TP2 ) are plotted against 1/TP to determine the activation energy as shown in Fig. 6, where ˇ is the heating rate and TP is the peak top temperature, based on following equation.
= ln
RA Ea g(˛)
−
Ea RTP
(2)
-13.5
ln (β / T )
-14.0 2
ln
ˇ T2
where R is gas constant, A is pre-exponential factor, Ea is activation energy of reaction. The term of g(˛) is a model-function determined by late limiting process of reaction. The linear relation between ln(ˇ/TP2 ) and 1/TP was deduced as Eq. (2), which slope is representing −Ea /R. The activation energy was determined to be 1.4 ± 0.1 eV, which was consistent with that for decomposition of LiH, indicating that tritium generated by the nuclear reaction can be bound to lithium to form LiT in Pb-16Li [8]. The tritium retention for the sample irradiated in JRR-3 was lower than solubility limit estimated by Eq. (1). Therefore, the formation of LiT would not result from oversaturation of tritium solution site in the alloy but hot tritium reaction with the alloy. The formation of LiT in Pb-16Li was only observed for the sample with thermal neutron irradiation, not formed by hydrogen isotope gas exposure. This fact indicates that the energetic tritium would induce the LiT formation. Almost all of deuterium introduced by gas exposure was released by diffusion process, indicating that most of deuterium was trapped weakly in the solid state of Pb-16Li alloy, where the deuterium trapping site was the interstitial site of Pb-16Li lattice. On the other hand, the bred tritium in Pb-16Li by neutron irradiation has recoil energy of 2.78 MeV and is thermalized by the knock-on processes of constituent atoms of lithium and lead. During these processes, this ‘hot’ tritium scatters the lead atoms surrounding lithium atom, then disordered structure can be formed locally, leading to the formation of LiT. The fraction of tritium retained as LiT was about 5% in the present study. However, this fraction should be increased after the tritium retention reached solubility limit which can be deduced by Eq. (1), at any operation temperature. Then, LiT would be the major source of tritium retention in Li-Pb system and tritium inventory would be underestimated with regardless of the formation of LiT. The formation equilibrium of LiT in Pb-16Li was not clarified in this study.
Results Fitting
-14.5 -15.0 -15.5 -16.0 -16.5
Fig. 4. Summary of deuterium solubility in Pb-16Li with the literature data.
1.55 1.60 1.65 -3 -1 Reciprocal temp. / 10 K
1.70
Fig. 6. Analytical results of tritium release peak at higher temperature region.
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
Therefore, the formation behavior of LiT in Pb-16Li irradiated with high neutron fluence where the tritium solution is limited (saturated) should be necessary to estimate actual tritium inventory in Li-Pb blanket system for long term reactor operation. 4. Conclusion The release behaviors of hydrogen isotopes in Pb-16Li introduced by thermal gas exposure or produced by thermal neutron irradiation were compared to investigate hot atom reactions of tritium. The solubility of hydrogen isotope was also determined in this study, and was consistent with the literature values. The tritium TDS spectra for Pb-16Li with thermal neutron irradiation showed that about 5% of tritium was release at single release stage around 600 K at the heating rate of 5 K/min, which was higher than the melting point of Pb-16Li, although no release peak was found at around 600 K for the hydrogen isotope-doped Pb-16Li. This fact indicates that recoil tritium can be trapped at the stable trapping site in Pb-16Li due to its recoil energy. The kinetic analysis indicated that the tritium release at around 600 K would be associated with the dissociation of Li-T bond, indicating that LiT was formed in Pb-16Li by thermal neutron irradiation by scattering lead atom surrounding lithium atom. The formation of LiT should be considered to estimate tritium retention in Pb-16Li eutectic blanket systems in fusion reactors because the amount of LiT should be increased after the tritium retention reached solubility limit.
2331
Acknowledgements This work was supported by the Japan Atomic Energy Agency under the Joint Work contract # 23K146, as a part of Broader Approach activities. This work was also performed under the Shared Facility Use Program of JAEA. This work was also performed by using facilities of the Research Reactor Institute, Kyoto University. References [1] W. Farabolini, A. Ciampichetti, F. Dabbene, M.A. Fütterer, L. Giancarli, G. Laffont, et al., Fusion Engineering and Design 81 (2006) 753. [2] A. Aiello, A. Ciampichetti, G. Benamati, Fusion Engineering and Design 81 (2006) 639. [3] H. Katsuta, H. Iwamoto, H. Ohno, Journal of Nuclear Materials 133 (1985) 167. [4] C.H. Wu, H.R. Ihle, Journal of Nuclear Materials 130 (1985) 454. [5] Y. Maeda, Y. Edao, S. Yamaguchi, S. Fukada, Fusion Science and Technology 54 (2008) 131. [6] F. Reiter, Fusion Engineering and Design 14 (1991) 207. [7] Y.C. Chan, E. Veleckis, Journal of Nuclear Materials 122 (1984) 935. [8] J. Engbæk, G. Nielsen, J.H. Nielsen, Ib. Chorkendorff, Surface Science 600 (2006) 1468. [9] C.H. Wu, Journal of Nuclear Materials 114 (1983) 30. [10] C.H. Wu, Journal of Nuclear Materials 122 (1984) 941. [11] H. Moriyama, J. Oishi, K. Kawamura, Journal of Nuclear Materials 158 (1988) 137. [12] M. Kobayashi, A. Hamada, K. Matsuoka, M. Suzuki, J. Osuo, Y. Edao, et al., Fusion Science and Technology 62 (2012) 56. [13] M. Kobayashi, K. Kawasaki, T. Fujishima, Y. Miyahara, Y. Oya, K. Okuno, Fusion Engineering and Design 87 (2012) 471. [14] T. Ozawa, Netsu Sokutei 31 (3) (2004) 133.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Formation of lithium-tritide by hot atom reactions of tritium produced in Pb-16Li Kenji Okuno a,∗ , Makoto Kobayashi a , Toshihiko Yamanishi b , Yasuhisa Oya a a b
Radioscience Research Laboratory, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan Japan Atomic Energy Agency, Ibaraki, Japan
h i g h l i g h t s • • • •
The solubility of hydrogen isotope was evaluated to be S = 6.56 × 10−7 exp(−0.11 [eV]/kT) [at. fr, Pa0.5 ]. About 5% of tritium was released at single release stage around 600 K for neutron irradiated Li17 Pb83 . No tritium release was found at around 600 K for the hydrogen isotope-doped Li17 Pb83 . LiT was formed in Li17 Pb83 by thermal neutron irradiation from the kinetic analysis.
a r t i c l e
i n f o
Article history: Received 14 September 2012 Received in revised form 21 January 2013 Accepted 19 March 2013 Available online 20 April 2013 Keywords: Tritium Pb-16Li LiT Solubility TDS
a b s t r a c t The release behaviors of hydrogen isotopes in Pb-16Li introduced by thermal gas exposure or produced by thermal neutron irradiation were compared to investigate hot atom reactions of tritium. The solubility of hydrogen isotope was evaluated to be S = 6.56 × 10−7 exp(−0.11 [eV]/kT) [at. fr, Pa0.5 ], which is consistent with the literature values. The tritium TDS spectra for Pb-16Li with thermal neutron irradiation showed that about 5% of tritium was released at single release stage around 600 K, which was higher than the melting point of Pb-16Li, although no release peak was found at around 600 K for the hydrogen isotope-doped Pb-16Li. The kinetic analysis indicated that the tritium release would be associated with the dissociation of Li-T bond, indicating that LiT was formed in Pb-16Li by thermal neutron irradiation, suggesting that the formation of Li-T bonds should be considered to estimate tritium retention in Pb-16Li eutectic blanket systems in fusion reactors. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In blanket system of fusion reactors, lithium-containing compounds will be introduced to produce tritium artificially by the reaction between lithium and neutron such as 6 Li(n, ␣T. A blanket concept using lithium (Li)-lead (Pb) eutectic alloy is considered as one of the promising liquid blanket systems for fusion reactors due to large tritium-breeding ratio (TBR), neutron multiplying ratio by lead, quick tritium recovery by low tritium solubility, high thermal conductivity, and so on [1,2]. Pb-16Li eutectic alloy is the most promising material as liquid tritium breeder because of its lowest melting point compared to that of other lithium-lead eutectic alloys. For the assessments of tritium safety, evaluation of tritium inventory in Pb-16Li blanket system is important. The solubility of tritium has been mainly investigated by hydrogen isotope gas
∗ Corresponding author. Tel.: +81 54 238 4802; fax: +81 54 238 3989. E-mail address: [email protected] (K. Okuno). 0920-3796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2013.03.064
charging for the liquid state of Pb-16Li under thermal equilibrium [3–7]. It is found that tritium solubility in Pb-16Li is generally low, however the data of solubility is largely scattered. For actual fusion reactor environment, the reactions of tritium with lithium should be also considered because lithium has high affinity to tritium. It is well known that tritium can solute exothermically into lithium with the formation of LiT, which may enhance tritium solubility in lithium [8]. The solution mechanisms of hydrogen isotopes in Li-Pb alloys have been systematically studied with changing the atomic fraction of lithium and lead in alloys [9–11]. Wu has observed that the heat of solution for deuterium decreased as lead content increased [10]. The heat of solution for deuterium into LiPb alloys was clearly dropped at the lithium fractional content of 0.17, where the solution process was changed from the exothermally to the endothermally. These facts represent that the affinity of lithium to tritium was sufficiently low in Pb-16Li. It can be seen that tritium inventory in Pb-16Li can be estimated with regardless of LiT formation according to these experimental results obtained in thermal equilibrium condition. However, Moriyama has reported that the fraction of tritium anion (T− ) in Li-Pb alloys depends on
QMS
TMP Reactor tube Furnace
2329
1
D exposure
10
Neutron irradiation (JRR-3) Neutron irradiation (KUR)
14
Capacitance manometer
H isotope release rate / 10 s g
D2 bottle
-1 -1
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
SP
0
10
10
-1
10
-2
Mo crucible
400 Fig. 1. Schematic drawing of the apparatus for deuterium gas exposure and TDS experiments. QMS, quadropole mass spectrometer; TMP, turbo molecular pump; SP, scroll pump.
lithium fractional content in alloys, and about 30% of tritium was retained as T− state (tritium bound to lithium) in Pb-16Li [11], indicating that this report has showed the possibility of the LiT formation in Pb-16Li although the formation mechanism of LiT was not expected from solubility data under thermal equilibrium. It was seen that the hot atom reactions which are non-equilibrium reactions of energetic atoms, would contribute to the formation of LiT in Pb-16Li during neutron irradiation. The formation of LiT during neutron irradiation will result in the increase of tritium hazard potential in fusion reactor. These results motivate us to investigate the possibility of LiT formation in Pb-16Li under neutron irradiation circumstance and to discuss about the interaction of ‘hot’ (energetic) tritium with Pb-16Li. Two types of hydrogen isotope introduction methods were adapted to elucidate the effect of hydrogen isotope energy on the formation of LiT in Pb-16Li in this study. The first was thermal neutron irradiation for the Pb-16Li eutectic alloy at ambient temperature to produce hot tritium into the alloy. The other was deuterium gas exposure to introduce deuterium under thermal equilibrium. The release behaviors for these samples were compared by thermal desorption spectroscopy (TDS) and the mechanism of hot atom reactions of tritium produced in the Pb-16Li eutectic alloy was discussed. 2. Experimental 2.1. Deuterium thermal doping and deuterium TDS experiments Pb-16Li eutectic alloy fabricated from pure lithium rod and lead granular was used. The fabrication method and the characteristics of Pb-16Li eutectic alloy are found in elsewhere [12]. The alloy was set into Mo crucible with the size of 10 mmt × 10 mmt , then Mo crucible was introduced into quartz tube under vacuum as found in Fig. 1. The pressure of this system was around 10−5 Pa. The quartz tube can be heated by ceramic furnace up to the temperature of 1173 K. Pb-16Li eutectic alloy was heated under vacuum to be the liquid state. Thereafter, the exposure of deuterium (D2 ) gas with the isotopic purity of 99.4% was performed for 5 h to introduce deuterium uniformly in the alloy. The D2 gas pressure and exposure temperature were changed in 7.7–200 kPa and 603–873 K, respectively. After the exposure, the quartz tube with Mo crucible was cooled down quickly by water and liquid nitrogen and the residual D2 gas was evacuated by TMP (turbo molecular pump). The TDS experiments were carried out using QMS (quadrupole mass spectrometer) from room temperature to 973 K with the heating rate of 5 K/min. Mass number of 4 corresponding to D2 was monitored by QMS to estimate the deuterium retention in the alloy.
500
600
700
Temperature/ K Fig. 2. TDS spectra of hydrogen isotopes released from Pb-16Li exposed to D2 gas or irradiated with thermal neutron.
2.2. Thermal neutron irradiation and out-of-pile tritium release experiments 0.3 g of the alloy was encapsulated in a quartz tube under vacuum. Thermal neutron irradiation was carried out in Japan Research Reactor-3 (JRR-3) and Kyoto University Research Reactor (KUR). Thermal neutron flux and fluence at JRR-3 were 5.2 × 1013 n cm−2 s−1 and 3.1 × 1015 n cm−2 , respectively. Those at KUR were 5.2 × 1012 n cm−2 s−1 and 3.3 × 1016 n cm−2 , respectively. Thermal neutron fluence in KUR was about ten times higher than that in JRR-3. The maximum sample temperature during the neutron irradiation was estimated to be less than 373 K, indicating that the Pb-16Li eutectic alloy was solid state during the neutron irradiation. The TDS experiments were carried out in the temperature range of 300–1173 K by tritium TDS system at Shizuoka University, whose detail was described in ref [13]. The radioactivity of released tritium was measured by a proportional counter. The different chemical forms as water (HTO) and molecule (HT) were trapped separately by water bubblers. Total amount of released tritium was determined by using a liquid scintillation counter. 3. Results and discussion Fig. 2 shows TDS spectra for D2 gas exposed Pb-16Li and thermal neutron irradiated one. The D2 gas exposure was done with the pressure of 105 kPa and the temperature of 673 K. The heating rates for all spectra were 5 K/min. In the both cases of D2 gas exposure and thermal neutron irradiation, a sharp release peak was observed at around 520 K, which was slightly higher than the melting point of Pb-16Li eutectic alloy. It was reported in our previous studies that the major hydrogen isotope release is governed by diffusion process in the liquid phase of Pb-16Li [12]. The major hydrogen isotope release was triggered by the phase transition of Pb-16Li from the solid phase to the liquid phase. For the alloy with thermal neutron irradiation, the additional release peak was appeared at around 600 K, which was only observed for the thermal neutron irradiated Pb-16Li. The deuterium concentration in the alloy with D2 gas exposure as a function of D2 gas pressure under various exposure temperatures are shown in Fig. 3. The linear relationship between the deuterium concentration in the alloy and the square root of D2 gas pressure was found even if the exposure temperature was changed, indicating that the dissociative absorption of D2 gas into the liquid state of Pb-16Li would be proceeded. Fig. 4 summarizes the solubility of deuterium in Pb-16Li. The literature data of hydrogen isotope solubility in Pb-16Li is also added in this figure [4–7]. The deuterium solubility obtained in this study has a slight temperature dependency while some of the researchers have reported that hydrogen
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
Tritium release rate / kBq g s
D2 gas exposure temp. 873 K 773 K 673 K 603 K
4.0
-5
D concentration / 10 at. fr.
5.0
-1 -1
2330
3.0 2.0 1.0
0
10
-1
10
450
0.0 0
100
200
300
400
2 K / min. 5 K / min. 15 K / min. 30 K / min.
1
10
500
550
600
650
700
750
800
Temperature / K
500
0.5
Square root of pressure / Pa
Fig. 5. Heating rate dependence on tritium TDS spectra for Pb-16Li irradiated with thermal neutron at JRR-3.
Fig. 3. Deuterium concentration in Pb-16Li at various temperatures as a function of square root of D2 gas exposure pressure.
isotope solubility in Pb-16Li is independent on temperature. The deuterium solubility was almost consistent with the literature data by Maeda, Chan and Veleckis [5,7], and can be expressed as follows: S = 6.56 × 10−7 exp
−0.11 [eV] kT
[at.fr, Pa0.5 ]
(1)
where the heat of solution of deuterium in Pb-16Li was 0.11 eV, indicating that deuterium solution proceeds endothermally. The additional peak at 600 K was found for thermal neutron irradiated Pb-16Li as the thermal neutron fluence increased, whose tritium retention was estimated to be about 5% to total tritium retention. Engbæk et al. have been reported that the releases of hydrogen and lithium were observed at around 600 K during TDS experiment for LiH [8]. The TDS experiments with different heating rates were performed to determine the tritium release process at 600 K. Fig. 5 shows the TDS spectra with various heating rates for thermal neutron irradiated Pb-16Li. It was found that the peak temperature was shifted toward higher temperature side as the heating rate increased. The tritium release peak in higher temperature region was analyzed by Kissinger-Akahira-Sunose (KAS) model-free-kinetics method which has widely applied for the analysis of TDS spectra [14]. In this method, ln(ˇ/TP2 ) are plotted against 1/TP to determine the activation energy as shown in Fig. 6, where ˇ is the heating rate and TP is the peak top temperature, based on following equation.
= ln
RA Ea g(˛)
−
Ea RTP
(2)
-13.5
ln (β / T )
-14.0 2
ln
ˇ T2
where R is gas constant, A is pre-exponential factor, Ea is activation energy of reaction. The term of g(˛) is a model-function determined by late limiting process of reaction. The linear relation between ln(ˇ/TP2 ) and 1/TP was deduced as Eq. (2), which slope is representing −Ea /R. The activation energy was determined to be 1.4 ± 0.1 eV, which was consistent with that for decomposition of LiH, indicating that tritium generated by the nuclear reaction can be bound to lithium to form LiT in Pb-16Li [8]. The tritium retention for the sample irradiated in JRR-3 was lower than solubility limit estimated by Eq. (1). Therefore, the formation of LiT would not result from oversaturation of tritium solution site in the alloy but hot tritium reaction with the alloy. The formation of LiT in Pb-16Li was only observed for the sample with thermal neutron irradiation, not formed by hydrogen isotope gas exposure. This fact indicates that the energetic tritium would induce the LiT formation. Almost all of deuterium introduced by gas exposure was released by diffusion process, indicating that most of deuterium was trapped weakly in the solid state of Pb-16Li alloy, where the deuterium trapping site was the interstitial site of Pb-16Li lattice. On the other hand, the bred tritium in Pb-16Li by neutron irradiation has recoil energy of 2.78 MeV and is thermalized by the knock-on processes of constituent atoms of lithium and lead. During these processes, this ‘hot’ tritium scatters the lead atoms surrounding lithium atom, then disordered structure can be formed locally, leading to the formation of LiT. The fraction of tritium retained as LiT was about 5% in the present study. However, this fraction should be increased after the tritium retention reached solubility limit which can be deduced by Eq. (1), at any operation temperature. Then, LiT would be the major source of tritium retention in Li-Pb system and tritium inventory would be underestimated with regardless of the formation of LiT. The formation equilibrium of LiT in Pb-16Li was not clarified in this study.
Results Fitting
-14.5 -15.0 -15.5 -16.0 -16.5
Fig. 4. Summary of deuterium solubility in Pb-16Li with the literature data.
1.55 1.60 1.65 -3 -1 Reciprocal temp. / 10 K
1.70
Fig. 6. Analytical results of tritium release peak at higher temperature region.
K. Okuno et al. / Fusion Engineering and Design 88 (2013) 2328–2331
Therefore, the formation behavior of LiT in Pb-16Li irradiated with high neutron fluence where the tritium solution is limited (saturated) should be necessary to estimate actual tritium inventory in Li-Pb blanket system for long term reactor operation. 4. Conclusion The release behaviors of hydrogen isotopes in Pb-16Li introduced by thermal gas exposure or produced by thermal neutron irradiation were compared to investigate hot atom reactions of tritium. The solubility of hydrogen isotope was also determined in this study, and was consistent with the literature values. The tritium TDS spectra for Pb-16Li with thermal neutron irradiation showed that about 5% of tritium was release at single release stage around 600 K at the heating rate of 5 K/min, which was higher than the melting point of Pb-16Li, although no release peak was found at around 600 K for the hydrogen isotope-doped Pb-16Li. This fact indicates that recoil tritium can be trapped at the stable trapping site in Pb-16Li due to its recoil energy. The kinetic analysis indicated that the tritium release at around 600 K would be associated with the dissociation of Li-T bond, indicating that LiT was formed in Pb-16Li by thermal neutron irradiation by scattering lead atom surrounding lithium atom. The formation of LiT should be considered to estimate tritium retention in Pb-16Li eutectic blanket systems in fusion reactors because the amount of LiT should be increased after the tritium retention reached solubility limit.
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Acknowledgements This work was supported by the Japan Atomic Energy Agency under the Joint Work contract # 23K146, as a part of Broader Approach activities. This work was also performed under the Shared Facility Use Program of JAEA. This work was also performed by using facilities of the Research Reactor Institute, Kyoto University. References [1] W. Farabolini, A. Ciampichetti, F. Dabbene, M.A. Fütterer, L. Giancarli, G. Laffont, et al., Fusion Engineering and Design 81 (2006) 753. [2] A. Aiello, A. Ciampichetti, G. Benamati, Fusion Engineering and Design 81 (2006) 639. [3] H. Katsuta, H. Iwamoto, H. Ohno, Journal of Nuclear Materials 133 (1985) 167. [4] C.H. Wu, H.R. Ihle, Journal of Nuclear Materials 130 (1985) 454. [5] Y. Maeda, Y. Edao, S. Yamaguchi, S. Fukada, Fusion Science and Technology 54 (2008) 131. [6] F. Reiter, Fusion Engineering and Design 14 (1991) 207. [7] Y.C. Chan, E. Veleckis, Journal of Nuclear Materials 122 (1984) 935. [8] J. Engbæk, G. Nielsen, J.H. Nielsen, Ib. Chorkendorff, Surface Science 600 (2006) 1468. [9] C.H. Wu, Journal of Nuclear Materials 114 (1983) 30. [10] C.H. Wu, Journal of Nuclear Materials 122 (1984) 941. [11] H. Moriyama, J. Oishi, K. Kawamura, Journal of Nuclear Materials 158 (1988) 137. [12] M. Kobayashi, A. Hamada, K. Matsuoka, M. Suzuki, J. Osuo, Y. Edao, et al., Fusion Science and Technology 62 (2012) 56. [13] M. Kobayashi, K. Kawasaki, T. Fujishima, Y. Miyahara, Y. Oya, K. Okuno, Fusion Engineering and Design 87 (2012) 471. [14] T. Ozawa, Netsu Sokutei 31 (3) (2004) 133.