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Activation analysis of tritium breeder materials in the FDS-II fusion power reactor
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Fusion Engineering and Design 82 (2007) 2641–2646
Activation analysis of tritium breeder materials in the FDS-II fusion power reactor M. Chen ∗ , Q. Huang, S.L. Zheng Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, China Received 31 July 2006; received in revised form 5 May 2007; accepted 6 May 2007 Available online 2 July 2007
Abstract Lithium lead (Li17 Pb83 ) is presently considered as a primary candidate tritium breeder for fusion power reactors because of its attractive properties. Its induced radioactivity by D–T fusion neutrons was calculated with the fusion power reactor (FDS-II) as the reference reactor. The results were compared with the activation levels of other tritium breeders, such as Li4 SiO4 , Li2 TiO3 , Li2 O, Li, etc., under the same irradiation conditions. Furthermore, the dominant nuclides to dose rate of Li17 Pb83 and the effects of irradiation time on the activation characteristics of Li17 Pb83 were analyzed. © 2007 Elsevier B.V. All rights reserved. Keywords: Activation; Tritium breeders; Lithium lead
1. Introduction R&D of fusion materials, especially their activation characteristics, is one of the key issues for fusion research in the world [1–7]. The activation of the materials would substantially influence not only the safety of the fusion reactors but also the recycling of used reactor materials and waste management. The selection of suitable kinds of materials including structural material and tritium breeder is a very important and effective method to control the neutron-induced activation in fusion reactors and to ensure the attractiveness of fusion nuclear power regarding safety and envi∗
Corresponding author. Fax: +86 551 559 3326. E-mail address: [email protected] (M. Chen).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.05.033
ronmental aspects [8–12]. Research on low activation materials, such as ferritic steels and vanadium alloys, is being done throughout the world. Tritium breeder is another important material in fusion power reactors, which also has an important impact on the neutroninduced activation in the fusion system. Li17 Pb83 , Li4 SiO4 , Li2 TiO3 , Li2 O and Li etc. are considered to be the candidate tritium breeders. Li17 Pb83 is selected as the tritium breeder in many fusion reactors designs for its attractive properties, such as in FDS-II [13–14], ARIES-RS [15] and ARIES-ST [16], etc. It can not only breed tritium but also multiply neutrons and be a coolant in the designs. In this contribution, the activation levels of Li17 Pb83 with the FDS-II as the reference reactor were calculated and compared with the activation levels of other tritium breeders under the same
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conditions. In addition, the dominant nuclides for the dose rate of Li17 Pb83 and the effects of irradiation time on the activation levels of Li17 Pb83 were analyzed.
2. Calculation procedure The compositions of the tritium breeders (expressed in mass%) are given in Table 1. The enrichments of 6 Li in the tritium breeders are all assumed to be natural in this paper. All the elements except Li, Pb, Ti, Si and O are the impurities in the tritium breeders. The impurities compositions and their levels in the tritium breeders depend on different fabrication conditions. The effects of the impurities on the activation characteristic were analyzed except for Li2 TiO3 and Li2 O. The geometrical and material configurations of dual functional lithium lead (DLL) blanket concept design of FDS-II were selected as the model for neutron transport calculation with home-developed multifunctional (transport/burnup/optimization/activation) code system VisualBUS and the multi-group (175 neutron groups–42 gamma groups coupled) data library HENDL1.0/MG (Hybrid Evaluation Nuclear Data Library) [17,18]. The average neutron wall loading on the first wall is 2.72 MW/m2 . Li17 Pb83 and other tritium breeder materials mentioned above are considered to be the tritium breeders in the blanket and are assumed to be replaced after 30 years (the life time of FDS-II). The details of the DLL blanket model of the FDS-II are in Refs. [13,14,19]. The neutron spectrum was used as an input of the code system VisualBUS to calculate the activation levels of the tritium breeders. The activation data library file EAF-99 [20] was used in the activation calculation. Table 1 The compositions of tritium breeders (wt%) Components
Composition
Li17 Pb83
Li: 0.7; Na: 1.8e−4; Ca: 1.8e−4; Ag: 1.0e−3; Bi: 4.1e−3; Pb: balance; K: 1.2e−4; Cu: 2.0e−4; Sb: 3.1e−4 Li: 3.2; Si: 23.4; O: balance; Fe: 8.3e−3; Mn: 3.3e−4 Li: 12.0; Ti: 44.0; O: 44.0 Li: 46.5; O: balance; Si: 1.8e−2; Fe: 1.0e−3; Al: 3.6e−2; Ni: 1.0e−4 Li: 100
Li4 SiO4 Li2 TiO3 Li2 O Li
Fig. 1. Neutron spectrum for the tritium breeder in FDS-II.
3. Results and analysis The activation characteristics and the factors affecting them have been studied for the tritium breeders in the DLL blanket of FDS-II in the following sections. The tritium generated in the tritium breeders has to be extracted as fusion fuel and the activation level of tritium depends on the tritium extraction efficiency in the system. The effects of the tritium on the activation characteristics of tritium breeders were not included here. 3.1. Neutron spectra Neutron spectrum is one of the main factors leading to different activation characteristics. The neutron spectrum in the tritium breeder zone is shown in Fig. 1. The energies for most of the neutrons are between 100 eV and 2 MeV. The total neutron flux was 1.42 × l015 n/(cm2 s). 3.2. Activation levels of tritium breeders After an irradiation for 30 years, the afterheats, activities and contact dose rates for the above tritium breeders vary with cooling time (CT) as shown in Figs. 2–4, respectively (note that tritium has been ignored). The activation levels of the tritium breeders are mainly due to the other elements rather than Li in them. The activation levels of Li are almost zero. After a cooling time of ∼100 years, Li17 Pb83 has the
M. Chen et al. / Fusion Engineering and Design 82 (2007) 2641–2646
Fig. 2. Specific afterheats of tritium breeders as a function of cooling time.
lowest level of activity and afterheat but the highest level of dose rate compared with Li4 SiO4 , Li2 TiO3 and Li2 O. The afterheats for all the tritium breeders are not relevant for the waste management and will be lower than simple recycle material limit of 1 W/m3 [21] when decay periods of more than 100 years are assumed. The clearance index Ic [22] of the activated tritium breeders are assessed, taking into account the contribution of each radionuclide’s activity contained. All the clearance index of tritium breeders except for Li are high above 1.0 even after more than 103 years decay and cannot be cleared. If we consider hands-on and remote recycling limits of dose rate to be 10 Sv/h and 10 mSv/h [23,24],
Fig. 3. Specific activities of tritium breeders as a function of cooling time.
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Fig. 4. Dose rates of tritium breeders as a function of cooling time.
respectively, the contact dose rates for Li17 Pb83 , Li4 SiO4 and Li2 O cannot reduce to the hands-on recycling limit during the cooling time less than 104 years, while the cooling time is ∼300 years for Li2 TiO3 to reduce to the hands-on recycling limit. Cooling times of ∼300 years, ∼4 years, ∼6 years and ∼2 years would be needed for Li17 Pb83 , Li4 SiO4 , Li2 TiO3 and Li2 O to reduce to the remote recycling limit, respectively. 3.3. Dominant nuclides for dose rate of lithium lead After 30 years of irradiation, the dominant nuclides for dose rate of Li17 Pb83 as a function of cooling time are shown in Fig. 5 (note that tritium
Fig. 5. Dominant nuclides to dose rate of Li17 Pb83 .
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has been ignored). At shutdown time, the dose rate level of Li17 Pb83 is dominated by the nuclide Pb207m (T1/2 = 0.8 s), which is mainly produced via the reactions Pb-208(n,2n)Pb-207m and Pb-207(n,n )Pb207m, and Pb-207m decays very rapidly. A few minutes later, the dose rate level of Li17 Pb83 is dominated by the nuclide Pb-204m (T1/2 = 66.9 min) up to ∼2 h after shutdown. The nuclide Pb-204m is mainly from the reactions Pb-204(n,n )Pb-204m and Pb-206(n,2n)Pb-205(n,2n)Pb-204m. During the cooling time from ∼2 h to ∼60 days after shutdown, the nuclides Pb-203 (T1/2 = 52.1 h) and T1-202 (T1/2 = 12 days) dominate the total dose rate of Li17 Pb83 . The nuclides Pb-203 and T1-202 are produced mainly from the reactions Pb-204(n,2n)Pb-203m(IT)Pb-203, Pb-204(n,2n)Pb-203 and Pb-204(n,2n)Pb-203(+ )T1203(n,2n)T1-202, respectively. Then the total dose rate of Li17 Pb83 is mainly due to the nuclide Bi-207 (T1/2 = 31.8 years) up to ∼250 years after shutdown. The nuclide Bi-207 is produced mainly via the reactions Pb-208(n,␥)Pb-209(− )Bi-209(n,2n)Bi208(n,2n)Bi-207 and Bi-209(n,2n)Bi-208(n,2n)Bi207. After that time, the total dose rate of Li17 Pb83 is dominated by the nuclide Ag-108m (T1/2 = 438 years) up to ∼1000 years after shutdown, which is generated by the impurity mainly through the reactions Ag-109(n,2n)Ag-108m and Ag-107(n,␥)Ag108m. The nuclide Bi-208 (T1/2 = 3.7 × l05 years) becomes the dominant nuclide for the total dose rate of Li17 Pb83 thereafter. The nuclide Bi-208 is formed mainly from the reactions Pb-208(n,␥)Pb209(− )Bi209(n,2n)Bi-208 and Bi-209(n,2n)Bi-208. The Bi impurity contributes significantly to the generation of the nuclides Bi-207 and Bi-208. So the impurities levels of Ag and Bi must be very low to control the long-term dose rate. In addition, Bi can not only be introduced during the fabrication process, but also be generated via the reaction Pb-208(n,␥)Pb-209(− )Bi-209 during the irradiation process. Although the radio-active nuclides Po-210 (T1/2 = 138.4 days) and Hg-203 (T1/2 = 46.6 days) make no contribution to dose rate, they are seriously radio-toxic and are a safety concern for fusion power reactors due to their high volatility and mobilization. They are generated mainly through the reactions Pb-208(n,␥)Pb-209(− )Bi209(n,␥)Bi-210(− )Po-210, Pb-206(n,␣)Hg-203 and Pb-207(n,␣)Hg-204(n,2n)Hg-203, respectively.
Fig. 6. Contact dose rates of Li17 Pb83 for different irradiation time.
At shutdown time, the activities of the Po-210 and Hg-203 are 8.8 × 109 Bq (4.8 × 10−3 % of total) and 3.5 × 1010 Bq (1.9 × 10−2 % of total), respectively. 3.4. Effects of irradiation time The irradiation time is an important factor to influence the activation level of the reactor materials, especially for the long-term activation. The dose rate levels of the tritium breeder Li17 Pb83 for different irradiation times and the same neutron spectrum in FDS-II are shown in Fig. 6. The difference of dose rate for Li17 Pb83 under different irradiation times is relatively small for cooling times less than 1 day. Thereafter, the difference becomes bigger. At a cooling time of 100 years, its dose rate for an irradiation time of 1 year is 4.1 × 10−4 Sv/h. But its dose rate for an irradiation time of 30 years, 10 years and 5 years are about ∼200, ∼18.7, and ∼6 times larger than that for an irradiation time of 1 year, respectively. None of the dose rates of Li17 Pb83 for an irradiation time of 1 year, 5 years, 10 years or 30 years can reduce to the hands-on recycling limit 10 Sv/h during a cooling time less than 104 years. This is mainly because the impurities Ag and Bi generate long half-life nuclides Bi-207, Bi-208 and Ag-108m.
4. Conclusions The activation levels of Li17 Pb83 were calculated with the home-developed multi-functional code sys-
M. Chen et al. / Fusion Engineering and Design 82 (2007) 2641–2646
tem VisualBUS and the data library HENDL1.0/MG and European Activation File EAF-99 for the fusion power reactor FDS-II. The results were compared with other tritium breeders, such as Li4 SiO4 , Li2 TiO3 , Li2 O and Li. After an irradiation for 30 years, if the generated tritium contribution to the activation characteristics of tritium breeders is not considered, the activation levels of the tritium breeders are mainly due to the other elements rather than Li in them. After a cooling of ∼100 years, Li17 Pb83 has the lowest level of activity and afterheat but highest level of dose rate compared with Li4 SiO4 , Li2 TiO3 and Li2 O. The afterheats for all the tritium breeders are not relevant for the waste management and will be lower than simple recycle material limit of 1 W/m3 when decay periods of more than 100 years are assumed. All the clearance Index of tritium breeders except for Li are high above 1.0 even after more than 103 years’ decay and cannot be cleared. The contact dose rate for Li17 Pb83 , Li4 SiO4 and Li2 O can not reduce to the hands-on recycling limit 10 Sv/h during a cooling time less than 104 years. A cooling time of ∼300 years is required to reduce to the hands-on recycling limit for Li2 TiO3 . Cooling times of ∼300 years, ∼4 years, ∼6 years and ∼2 years would be needed for Li17 Pb83 , Li4 SiO4 , Li2 TiO3 and Li2 O to reduce to the remote recycling limit 10 mSv/h, respectively. The dominant nuclides for dose rate of Li17 Pb83 are analyzed and the dose rate of Li17 Pb83 is dominated by nuclides Pb-207m, Pb-204m, T1-202, Bi-207, Ag108m and Bi-208 at cooling times of ∼2 min, ∼1 h, ∼60 days, ∼200 years, ∼103 years and 104 years, respectively. The impurities Ag and Bi have an important effect on the long-term contact dose rate because they can generate Bi-207, Ag-108m and Bi-208 during the irradiation process. In addition, Bi will not only be introduced during the fabrication process but also be generated via the reaction Pb-208(n,␥)Pb-209(− )Bi209 during the irradiation process. The irradiation time is an important factor of the activation level of Li17 Pb83 having an important influence on the long-term activation and a small influence on the short-term activation.
Acknowledgements This work is supported by the National Natural Science Foundation with grant no. 10375067, National
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Natural Science Foundation with grant no. 10675123 and the Knowledge Innovation Program of Chinese Academy of Sciences.
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Fusion Engineering and Design 82 (2007) 2641–2646
Activation analysis of tritium breeder materials in the FDS-II fusion power reactor M. Chen ∗ , Q. Huang, S.L. Zheng Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, China Received 31 July 2006; received in revised form 5 May 2007; accepted 6 May 2007 Available online 2 July 2007
Abstract Lithium lead (Li17 Pb83 ) is presently considered as a primary candidate tritium breeder for fusion power reactors because of its attractive properties. Its induced radioactivity by D–T fusion neutrons was calculated with the fusion power reactor (FDS-II) as the reference reactor. The results were compared with the activation levels of other tritium breeders, such as Li4 SiO4 , Li2 TiO3 , Li2 O, Li, etc., under the same irradiation conditions. Furthermore, the dominant nuclides to dose rate of Li17 Pb83 and the effects of irradiation time on the activation characteristics of Li17 Pb83 were analyzed. © 2007 Elsevier B.V. All rights reserved. Keywords: Activation; Tritium breeders; Lithium lead
1. Introduction R&D of fusion materials, especially their activation characteristics, is one of the key issues for fusion research in the world [1–7]. The activation of the materials would substantially influence not only the safety of the fusion reactors but also the recycling of used reactor materials and waste management. The selection of suitable kinds of materials including structural material and tritium breeder is a very important and effective method to control the neutron-induced activation in fusion reactors and to ensure the attractiveness of fusion nuclear power regarding safety and envi∗
Corresponding author. Fax: +86 551 559 3326. E-mail address: [email protected] (M. Chen).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.05.033
ronmental aspects [8–12]. Research on low activation materials, such as ferritic steels and vanadium alloys, is being done throughout the world. Tritium breeder is another important material in fusion power reactors, which also has an important impact on the neutroninduced activation in the fusion system. Li17 Pb83 , Li4 SiO4 , Li2 TiO3 , Li2 O and Li etc. are considered to be the candidate tritium breeders. Li17 Pb83 is selected as the tritium breeder in many fusion reactors designs for its attractive properties, such as in FDS-II [13–14], ARIES-RS [15] and ARIES-ST [16], etc. It can not only breed tritium but also multiply neutrons and be a coolant in the designs. In this contribution, the activation levels of Li17 Pb83 with the FDS-II as the reference reactor were calculated and compared with the activation levels of other tritium breeders under the same
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conditions. In addition, the dominant nuclides for the dose rate of Li17 Pb83 and the effects of irradiation time on the activation levels of Li17 Pb83 were analyzed.
2. Calculation procedure The compositions of the tritium breeders (expressed in mass%) are given in Table 1. The enrichments of 6 Li in the tritium breeders are all assumed to be natural in this paper. All the elements except Li, Pb, Ti, Si and O are the impurities in the tritium breeders. The impurities compositions and their levels in the tritium breeders depend on different fabrication conditions. The effects of the impurities on the activation characteristic were analyzed except for Li2 TiO3 and Li2 O. The geometrical and material configurations of dual functional lithium lead (DLL) blanket concept design of FDS-II were selected as the model for neutron transport calculation with home-developed multifunctional (transport/burnup/optimization/activation) code system VisualBUS and the multi-group (175 neutron groups–42 gamma groups coupled) data library HENDL1.0/MG (Hybrid Evaluation Nuclear Data Library) [17,18]. The average neutron wall loading on the first wall is 2.72 MW/m2 . Li17 Pb83 and other tritium breeder materials mentioned above are considered to be the tritium breeders in the blanket and are assumed to be replaced after 30 years (the life time of FDS-II). The details of the DLL blanket model of the FDS-II are in Refs. [13,14,19]. The neutron spectrum was used as an input of the code system VisualBUS to calculate the activation levels of the tritium breeders. The activation data library file EAF-99 [20] was used in the activation calculation. Table 1 The compositions of tritium breeders (wt%) Components
Composition
Li17 Pb83
Li: 0.7; Na: 1.8e−4; Ca: 1.8e−4; Ag: 1.0e−3; Bi: 4.1e−3; Pb: balance; K: 1.2e−4; Cu: 2.0e−4; Sb: 3.1e−4 Li: 3.2; Si: 23.4; O: balance; Fe: 8.3e−3; Mn: 3.3e−4 Li: 12.0; Ti: 44.0; O: 44.0 Li: 46.5; O: balance; Si: 1.8e−2; Fe: 1.0e−3; Al: 3.6e−2; Ni: 1.0e−4 Li: 100
Li4 SiO4 Li2 TiO3 Li2 O Li
Fig. 1. Neutron spectrum for the tritium breeder in FDS-II.
3. Results and analysis The activation characteristics and the factors affecting them have been studied for the tritium breeders in the DLL blanket of FDS-II in the following sections. The tritium generated in the tritium breeders has to be extracted as fusion fuel and the activation level of tritium depends on the tritium extraction efficiency in the system. The effects of the tritium on the activation characteristics of tritium breeders were not included here. 3.1. Neutron spectra Neutron spectrum is one of the main factors leading to different activation characteristics. The neutron spectrum in the tritium breeder zone is shown in Fig. 1. The energies for most of the neutrons are between 100 eV and 2 MeV. The total neutron flux was 1.42 × l015 n/(cm2 s). 3.2. Activation levels of tritium breeders After an irradiation for 30 years, the afterheats, activities and contact dose rates for the above tritium breeders vary with cooling time (CT) as shown in Figs. 2–4, respectively (note that tritium has been ignored). The activation levels of the tritium breeders are mainly due to the other elements rather than Li in them. The activation levels of Li are almost zero. After a cooling time of ∼100 years, Li17 Pb83 has the
M. Chen et al. / Fusion Engineering and Design 82 (2007) 2641–2646
Fig. 2. Specific afterheats of tritium breeders as a function of cooling time.
lowest level of activity and afterheat but the highest level of dose rate compared with Li4 SiO4 , Li2 TiO3 and Li2 O. The afterheats for all the tritium breeders are not relevant for the waste management and will be lower than simple recycle material limit of 1 W/m3 [21] when decay periods of more than 100 years are assumed. The clearance index Ic [22] of the activated tritium breeders are assessed, taking into account the contribution of each radionuclide’s activity contained. All the clearance index of tritium breeders except for Li are high above 1.0 even after more than 103 years decay and cannot be cleared. If we consider hands-on and remote recycling limits of dose rate to be 10 Sv/h and 10 mSv/h [23,24],
Fig. 3. Specific activities of tritium breeders as a function of cooling time.
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Fig. 4. Dose rates of tritium breeders as a function of cooling time.
respectively, the contact dose rates for Li17 Pb83 , Li4 SiO4 and Li2 O cannot reduce to the hands-on recycling limit during the cooling time less than 104 years, while the cooling time is ∼300 years for Li2 TiO3 to reduce to the hands-on recycling limit. Cooling times of ∼300 years, ∼4 years, ∼6 years and ∼2 years would be needed for Li17 Pb83 , Li4 SiO4 , Li2 TiO3 and Li2 O to reduce to the remote recycling limit, respectively. 3.3. Dominant nuclides for dose rate of lithium lead After 30 years of irradiation, the dominant nuclides for dose rate of Li17 Pb83 as a function of cooling time are shown in Fig. 5 (note that tritium
Fig. 5. Dominant nuclides to dose rate of Li17 Pb83 .
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has been ignored). At shutdown time, the dose rate level of Li17 Pb83 is dominated by the nuclide Pb207m (T1/2 = 0.8 s), which is mainly produced via the reactions Pb-208(n,2n)Pb-207m and Pb-207(n,n )Pb207m, and Pb-207m decays very rapidly. A few minutes later, the dose rate level of Li17 Pb83 is dominated by the nuclide Pb-204m (T1/2 = 66.9 min) up to ∼2 h after shutdown. The nuclide Pb-204m is mainly from the reactions Pb-204(n,n )Pb-204m and Pb-206(n,2n)Pb-205(n,2n)Pb-204m. During the cooling time from ∼2 h to ∼60 days after shutdown, the nuclides Pb-203 (T1/2 = 52.1 h) and T1-202 (T1/2 = 12 days) dominate the total dose rate of Li17 Pb83 . The nuclides Pb-203 and T1-202 are produced mainly from the reactions Pb-204(n,2n)Pb-203m(IT)Pb-203, Pb-204(n,2n)Pb-203 and Pb-204(n,2n)Pb-203(+ )T1203(n,2n)T1-202, respectively. Then the total dose rate of Li17 Pb83 is mainly due to the nuclide Bi-207 (T1/2 = 31.8 years) up to ∼250 years after shutdown. The nuclide Bi-207 is produced mainly via the reactions Pb-208(n,␥)Pb-209(− )Bi-209(n,2n)Bi208(n,2n)Bi-207 and Bi-209(n,2n)Bi-208(n,2n)Bi207. After that time, the total dose rate of Li17 Pb83 is dominated by the nuclide Ag-108m (T1/2 = 438 years) up to ∼1000 years after shutdown, which is generated by the impurity mainly through the reactions Ag-109(n,2n)Ag-108m and Ag-107(n,␥)Ag108m. The nuclide Bi-208 (T1/2 = 3.7 × l05 years) becomes the dominant nuclide for the total dose rate of Li17 Pb83 thereafter. The nuclide Bi-208 is formed mainly from the reactions Pb-208(n,␥)Pb209(− )Bi209(n,2n)Bi-208 and Bi-209(n,2n)Bi-208. The Bi impurity contributes significantly to the generation of the nuclides Bi-207 and Bi-208. So the impurities levels of Ag and Bi must be very low to control the long-term dose rate. In addition, Bi can not only be introduced during the fabrication process, but also be generated via the reaction Pb-208(n,␥)Pb-209(− )Bi-209 during the irradiation process. Although the radio-active nuclides Po-210 (T1/2 = 138.4 days) and Hg-203 (T1/2 = 46.6 days) make no contribution to dose rate, they are seriously radio-toxic and are a safety concern for fusion power reactors due to their high volatility and mobilization. They are generated mainly through the reactions Pb-208(n,␥)Pb-209(− )Bi209(n,␥)Bi-210(− )Po-210, Pb-206(n,␣)Hg-203 and Pb-207(n,␣)Hg-204(n,2n)Hg-203, respectively.
Fig. 6. Contact dose rates of Li17 Pb83 for different irradiation time.
At shutdown time, the activities of the Po-210 and Hg-203 are 8.8 × 109 Bq (4.8 × 10−3 % of total) and 3.5 × 1010 Bq (1.9 × 10−2 % of total), respectively. 3.4. Effects of irradiation time The irradiation time is an important factor to influence the activation level of the reactor materials, especially for the long-term activation. The dose rate levels of the tritium breeder Li17 Pb83 for different irradiation times and the same neutron spectrum in FDS-II are shown in Fig. 6. The difference of dose rate for Li17 Pb83 under different irradiation times is relatively small for cooling times less than 1 day. Thereafter, the difference becomes bigger. At a cooling time of 100 years, its dose rate for an irradiation time of 1 year is 4.1 × 10−4 Sv/h. But its dose rate for an irradiation time of 30 years, 10 years and 5 years are about ∼200, ∼18.7, and ∼6 times larger than that for an irradiation time of 1 year, respectively. None of the dose rates of Li17 Pb83 for an irradiation time of 1 year, 5 years, 10 years or 30 years can reduce to the hands-on recycling limit 10 Sv/h during a cooling time less than 104 years. This is mainly because the impurities Ag and Bi generate long half-life nuclides Bi-207, Bi-208 and Ag-108m.
4. Conclusions The activation levels of Li17 Pb83 were calculated with the home-developed multi-functional code sys-
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tem VisualBUS and the data library HENDL1.0/MG and European Activation File EAF-99 for the fusion power reactor FDS-II. The results were compared with other tritium breeders, such as Li4 SiO4 , Li2 TiO3 , Li2 O and Li. After an irradiation for 30 years, if the generated tritium contribution to the activation characteristics of tritium breeders is not considered, the activation levels of the tritium breeders are mainly due to the other elements rather than Li in them. After a cooling of ∼100 years, Li17 Pb83 has the lowest level of activity and afterheat but highest level of dose rate compared with Li4 SiO4 , Li2 TiO3 and Li2 O. The afterheats for all the tritium breeders are not relevant for the waste management and will be lower than simple recycle material limit of 1 W/m3 when decay periods of more than 100 years are assumed. All the clearance Index of tritium breeders except for Li are high above 1.0 even after more than 103 years’ decay and cannot be cleared. The contact dose rate for Li17 Pb83 , Li4 SiO4 and Li2 O can not reduce to the hands-on recycling limit 10 Sv/h during a cooling time less than 104 years. A cooling time of ∼300 years is required to reduce to the hands-on recycling limit for Li2 TiO3 . Cooling times of ∼300 years, ∼4 years, ∼6 years and ∼2 years would be needed for Li17 Pb83 , Li4 SiO4 , Li2 TiO3 and Li2 O to reduce to the remote recycling limit 10 mSv/h, respectively. The dominant nuclides for dose rate of Li17 Pb83 are analyzed and the dose rate of Li17 Pb83 is dominated by nuclides Pb-207m, Pb-204m, T1-202, Bi-207, Ag108m and Bi-208 at cooling times of ∼2 min, ∼1 h, ∼60 days, ∼200 years, ∼103 years and 104 years, respectively. The impurities Ag and Bi have an important effect on the long-term contact dose rate because they can generate Bi-207, Ag-108m and Bi-208 during the irradiation process. In addition, Bi will not only be introduced during the fabrication process but also be generated via the reaction Pb-208(n,␥)Pb-209(− )Bi209 during the irradiation process. The irradiation time is an important factor of the activation level of Li17 Pb83 having an important influence on the long-term activation and a small influence on the short-term activation.
Acknowledgements This work is supported by the National Natural Science Foundation with grant no. 10375067, National
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Natural Science Foundation with grant no. 10675123 and the Knowledge Innovation Program of Chinese Academy of Sciences.
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