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Evaluation of radiation shielding, nuclear heating and dose rate for JT-60 superconducting modification
Fusion Engineering and Design 63 /64 (2002) 115 /120 www.elsevier.com/locate/fusengdes
Evaluation of radiation shielding, nuclear heating and dose rate for JT-60 superconducting modification A. Morioka , A. Sakasai, K. Masaki, S. Ishida, N. Miya, M. Matsukawa, A. Kaminaga, A. Oikawa Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, 801-1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193, Japan
Abstract The radiation shielding, nuclear heating and dose rate have been evaluated for JT-60 superconducting modification. The double-walled structure of a vacuum vessel was adopted so that the maximum nuclear heating at the inboard side of the superconducting TF coil could be suppressed to be lower than 2.5 mW/cm3. The 316 stainless steel (SS316L) boards outside the vacuum vessel are installed for effective gamma-ray shielding. The dose rate with a reduced activation ferritic steel pedestal for the first wall in the vacuum vessel was estimated. This pedestal reduced the in-vessel dose rate by 40% compared to SS316L pedestal. The dose rate outside the boron carbide doped concrete cryostat was estimated at shutdown after 10 years operation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: JT-60SC; Radiation shielding; Vacuum vessel structure; Nuclear heating; Dose rate
1. Introduction The JT-60 superconducting modification (JT60SC) will pursue demonstration of high-performance steady state operation and demonstration of applicability of ferritic steel (F82H) with plasmas [1 /3]. The JT-60SC is planned to be utilized in the existing JT-60 experimental building and operate for 10 years with DD discharges. Corresponding to the high performance discharges of 100 s, the annual neutron emission from the JT60SC plasma should increase by about 6 times the Corresponding author. Tel.: /81-29-270-7328; fax: /8129-270-7419 E-mail address: [email protected] (A. Morioka).
present amount of neutron fluence in the JT-60U. It is important to evaluate the nuclear responses such as the radiation shielding, nuclear heating, induced activity and dose rate in various locations within the building during DD operation and after a shutdown. The nuclear responses in tokamak designs using the superconducting coil such as International Thermonuclear Experimental Reactor had been reported [4,5]. Important issues for the nuclear responses can be resolved into the following three points: (1) the safety operation of the superconducting coil during the operation; (2) estimation of biological shielding during the operation; (3) estimation of the dose rate in areas required to access after the operation. Estimation of the neutron and gamma-
0920-3796/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 2 ) 0 0 1 4 2 - 4
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ray fluxes on the structure of vacuum vessel and the nuclear heating of TF coil are assessed for JT60SC. For safety operation of the superconducting TF coils, the vacuum vessel is required to suppress the nuclear heating at the TF coil. The basic structures of the vacuum vessel should be considered in the calculation results of the radioactivity in the vacuum vessel and the JT-60 building. For maintenance in areas required to access after the operation, the material selection of the first wall pedestal in the vacuum vessel are presented. The dose rate in the area varies with the calculated results of the activity of the candidate materials F82H and SS316L contaminated with cobalt. For the biological shielding during the operation, shielding effects of the cryostat are discussed. It is necessary to install the cryostat for radiation shielding in addition to the thermal shielding of the TF coil in JT-60SC. The dose rate outside the cryostat was calculated after shutdown of operations.
used for the calculations. The radiation shielding and the nuclear heating are estimated by these fluxes. The neutron emission rate for the calculation is shown in Table 1. The poloidal view of the JT-60SC and JT-60U are shown in Fig. 2. The neutron flux is used to calculate the induced activities and gamma-ray source intensity by the ACT-4 code at various times after the shutdown. On the basis of the gamma-ray source intensity, the gamma-ray flux distribution is calculated with the transport codes and the group constant of gamma-ray transport of GROUPIN composed of 54 groups, which is included in the THIDA-2 code system [8]. The dose rate distribution with 10 years operation is converted from the gamma-ray flux distribution at 1 day, 1 month and 3 months after a shutdown.
3. Results and discussion 3.1. Vacuum vessel design on the radiation shielding
2. Calculation procedure The neutron and gamma-ray fluxes during an operation are calculated with the ANISN code [6] and the DOT3.5 code [7] as shown in Fig. 1. A transport group constant set of FUSION-40, which consists of 42 neutron groups and 21 gamma-ray groups based on JENDL3.1, was
The nuclear heating of the superconducting TF coil has been calculated. The nuclear heating was compared for the following structures of the vacuum vessel. In Fig. 3(a), the vacuum vessel made of SS316L is designed to be a double-walled structure. Multilayer shielding structure, which is the double walls filled with multi layers of water and high-Mn steel boards for cooling and radiation shielding, had been designed in the previous work on JT-60SU [9]. As shown in Fig. 3(b), single-layer shielding structure consisting of double walls filled with Table 1 Neutron emission rate of JT-60SC Emission rate JT-60U
JT-60SC For nuclear heating For dose rate and Argon
(n/s) (n/week) (n/3 months) (n/year) Fig. 1. Calculation flow chart for
THIDA-2
system.
2 /1017 3.1 /1018 2.1 /1019 3.1 /1019
4 /1017 4 /1019 1.5 /1020 2 /1020
7 /1018 6.3 /1019
A. Morioka et al. / Fusion Engineering and Design 63 /64 (2002) 115 /120
Fig. 2. The poloidal views of JT-60U and JT-60SC.
water. The shielding material such as high-Mn steel (VC9) or SS316L outside the double wall is installed to surpress additionally the nuclear heating of the TF coil. The nuclear heating is taken account as follows:
Wtotal
n X 42 X i 0
fn (E)Kn (E)
117
For the calculation of the radiation shielding and the nuclear heating, the maximum neutron emission rate was assumed with DD neutrons of 4 /1017 n/s and DT neutrons of 1.2 /1016 n/s: 3% of DD neutrons. From the viewpoint of the operation of the TF coil, the neutron emission rate is limited for 10 s. In the case of DD neutrons, the calculated neutron flux and gamma-ray flux dependence through the vacuum vessel on the ratio of VC9 thickness to the total shielding thickness of 130 mm (VC9/(VC9/H2O)) water of the vacuum vessel are shown in Fig. 4. The neutron flux for both structures increases with the ratio of VC9 thickness to the total thickness of the vacuum vessel. The neutron flux through the multi-layer structure is smaller than that through the singlelayer structure. The gamma-ray flux generated by the multi-layer structure is slightly larger than that by single-layer structure. The performance of the
k 0
n X 21 X i 0
fg (E)Kg (E)
k 0
where Wtotal is a total nuclear heating, fn (E ) is neutron flux, Kn (E ) is neutron kerma factor, fg (E ) is the gamma-ray flux and Kg (E) is gamma-ray kerma factor are the function of the energy, i is component of the material and k is the group of energy.
Fig. 4. Neuron and gamma-ray flux through the vacuum vessel of MLT and SLT.
Fig. 3. Calculational 1D-model of vacuum vessel for JT-60SC: (a) MLT, multi layer type. (b) SLT, single layer type.
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neutron shielding in the multi-layer structure is superior to the single-layer structure. In contrast, in respect of the gamma-ray flux, the shielding effect by the single-layer structure is rather better than the multi-layer structure. The JT-60SU had been planned to carry out DD and DT discharges [9]. On the other hand, the operation of JT-60SC has been planned for only DD discharges. From the view of the neutron energy, the structure of the vacuum vessel is possible to be simplified. Fig. 5 shows the nuclear heating of the TF coil in the case of (a) neutron and gamma-ray irradiation, (b) neutron irradiation and (c) gamma-ray irradiation through the vacuum vessel with the ratio of VC9 to water. In the case of DD and DT neutron emission, the same neutron emission rate was assumed. In Fig. 5(a), for the ratio of 0.5, the maximum nuclear heating at inboard of the TF coil was
estimated to be 1.9 mW/cm3 for DD neutrons. In the case of DT neutrons, for the ratio of 0.8, the maximum nuclear heating is minimized. The nuclear heating of the TF coil with the single-layer structure reduced by about 20% for DD neutrons and about 10% for DT neutrons in comparison with the multi-layer structure, respectively. In Fig. 5(b), with increasing the ratio from 0.1 to 0.5, the nuclear heating by neutron irradiation are almost 0.1 mW/cm3 in the case of DD neutrons. The nuclear heating in the TF coil due to gammaray irradiation created by the shielding structure decreases for DT neutrons with increasing ratio. In Fig. 5(c), the nuclear heating by gamma-ray irradiation is minimized for the ratio of 0.5. As a result, the minimum nuclear heating is estimated to be about 1.8 mW/cm3 for DD neutrons. The nuclear heating is minimized for the ratio of 0.8 with single-layer structure for DT neutrons.
Fig. 5. (a) Maximum nuclear heating in the TF coil of the vacuum vessel of MLT and SLT. (b) Nuclear heating in the TF coil by neutron irradiation through the vacuum vessel of MLT and SLT. (c) Nuclear heating in the TF coil by gamma-ray irradiation created by the vacuum vessel of MLT and SLT.
A. Morioka et al. / Fusion Engineering and Design 63 /64 (2002) 115 /120
It was found that gamma-ray irradiation causes almost all nuclear heating of the TF coil. This is a result of the radiation shielding performance of the single-layer structure being better than that of the multi-layer structure. From the viewpoint of the nuclear heating of the superconducting coil, it is possible to thin the shielding material for the vacuum vessel structure. Therefore, from the result of Fig. 5(a), the total weight of the vacuum vessel can be reduced when the single-layer structure is adopted. The single-layer structure for vacuum vessel was decided so that the maximum nuclear heating at inboard side of the TF coil could be 2.45 mW/cm3 as specified from the refrigerator design for the superconducting coil. The ratio of SS316L to the total shielding thickness 0.21 was obtained in the single-layer structure under the above specification. The structure of vacuum vessel is shown in Fig. 6. In VC9 and SS316L, from the calculation result of the nuclear heating of the coil, the value of nuclear heating was almost the same. As the shielding material, SS316 with the reliability had been adopted. The weight of the vacuum vessel for the single-layer structure reduced by about 80 tons compared with the multi-layer structure. 3.2. Dose rate in vacuum vessel The dose rate at the first wall in the vacuum vessel depends on the results from the activity of the pedestal materials, F82H (0.005 wt.% cobalt) [10] and SS316L (0.05 wt.% cobalt). For the calculation of the dose rate, the neutron emission rate was assumed with DD neutrons of 7 /1018 n/ week and 6.3 /1018 n/3 months. When the F82H
Fig. 6. Structure of vacuum vessel for JT-60SC.
119
material is used for the pedestal in the vacuum vessel, the dose rate at the F82H pedestal in the first wall was 109 mSv/h. It is found that the dose rate in the vacuum vessel is reduced by 40% compared to the SS316L material. In Fig. 7, the induced activities of F82H were calculated at 1 s, 1 day, 1 week, 1 month, 1 year, 10 and 50 years after the shutdown under the neutron yield of 10 years operation. The principal nuclides contributing to the activity after shutdown are 54Mn, 55Fe(non gamma-ray) and 58Co at 1 year after the shutdown. 3.3. Cryostat design The role of cryostat is the thermal shielding of the superconducting coil and radiation shielding. The concrete added boron carbide (B4C), which is corresponding to 1% of natural boron, is adopted to improve the shielding efficiency of the thermal neutrons through the vacuum vessel. The cryostat consists of the B4C concrete of 200-mm thickness and SS316L of 34 mm inside and 6-mm thickness outside. By using the B4C concrete, the activation of argon in the air could be reduced in the JT-60 torus hall. The specific 41Ar-activity (40Ar (n, g)) in the JT60 torus hall was estimated to be 7.6 /10 5 Bq/ cm3 with the B4C concrete. In the case of the normal concrete, the 41Ar-activity was 5.9 /10 4
Fig. 7. Radioactivities at F82H pedestal at different coding time and corresponding decay curves calculated with ACT-4 code.
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Bq/cm3. Therefore, it was found that the 41Aractivity with the B4C concrete was reduced by about 90% in comparison with the normal concrete. In the case of the B4C concrete for the shielding material of cryostat, the calculated dose rate outside the cryostat was 1.8 mSv/h after shutdown of 10 years operation. The dose rate of JT-60SC site boundary is also satisfied without any additional shielding performance for the JT-60 building.
4. Conclusion The radiation shielding, nuclear heating and dose rate for the JT-60SC has been evaluated. From the view of the nuclear heating of the superconducting coil, the structure of the vacuum vessel of the single-layer structure is superior to the multi-layer structure. The single-layer structure for vacuum vessel was chosen so that the maximum nuclear heating at inboard side of the TF coil could be 2.45 mW/cm3. The weight of the vacuum vessel for the single-layer structure reduced by about 80 ton in compared to the multi-layer structure. The dose rate at the F82H pedestal in the vacuum vessel is 40% improved over the SS316L pedestal. The pedestal in the vacuum vessel is planned to be F82H. The dose rate outside the cryostat was estimated to be 1.8 mSv/h after shutdown of 10 years operation. By using the B4C concrete, it was found that the 41Ar-activity with the B4C concrete was reduced by about 90% in comparison with the normal concrete. The use of the B4C concrete shows that the dose rate of JT60SC site boundary is also satisfied without any
additional shielding performance for the JT-60 building.
Acknowledgements The authors would like to acknowledge Dr K. Ushigusa for their kind suggestions. They thank Mr H. Kawasaki of CRC solutions Inc. for preparing the two-dimensional code for the work.
References [1] S. Ishida et al., Recent results and future plan of JT-60U, in: Proceedings of 19th SOFE, Atlantic City, NJ, in press. [2] A. Sakasai et al., Engineering Design Study of JT-60 Superconducting Modification, ibid. [3] M. Matsukawa et al., Design and R&D issues for the JT60 modification to a full superconducting tokamak, this conference. [4] K. Maki, et al., Evaluation of dose rate and reactor core and nuclear properties for superconductive magnets in ITER, Fus. Eng. Des. 42 (1998) 173 /185. [5] Plant design specification and plant description document of ITER (2001). [6] Engle, W.A. Jr., A User’s Manual for ANISN, A onedimensional discrete ordinates transport code with anisotropic scattering, K-1693 (1967). [7] Rhoades, W.A. et al., DOT3.5 two-dimensional discrete ordinates radiation transport code, CCC-276 (1977). [8] Y. Seki et al., THIDA-2: An advanced code system for calculation of transmutation, activation, decay heat and dose rate, JAERI 1301, Japan Atomic Energy Research Institute (1986). [9] M. Kikuchi et al., Design progress of JT-60SU, IAEA-CN64/G2-3 (1996) 451 /466. [10] N. Yamanouchi, et al., Accumulation of engineering data for practical use of reduced activation ferric steel: 8%Cr2%W-0.2%V-0.04%Ta /Fe, J. Nucl. Mater. 191 /194 (1992) 822 /826.
Evaluation of radiation shielding, nuclear heating and dose rate for JT-60 superconducting modification A. Morioka , A. Sakasai, K. Masaki, S. Ishida, N. Miya, M. Matsukawa, A. Kaminaga, A. Oikawa Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, 801-1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193, Japan
Abstract The radiation shielding, nuclear heating and dose rate have been evaluated for JT-60 superconducting modification. The double-walled structure of a vacuum vessel was adopted so that the maximum nuclear heating at the inboard side of the superconducting TF coil could be suppressed to be lower than 2.5 mW/cm3. The 316 stainless steel (SS316L) boards outside the vacuum vessel are installed for effective gamma-ray shielding. The dose rate with a reduced activation ferritic steel pedestal for the first wall in the vacuum vessel was estimated. This pedestal reduced the in-vessel dose rate by 40% compared to SS316L pedestal. The dose rate outside the boron carbide doped concrete cryostat was estimated at shutdown after 10 years operation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: JT-60SC; Radiation shielding; Vacuum vessel structure; Nuclear heating; Dose rate
1. Introduction The JT-60 superconducting modification (JT60SC) will pursue demonstration of high-performance steady state operation and demonstration of applicability of ferritic steel (F82H) with plasmas [1 /3]. The JT-60SC is planned to be utilized in the existing JT-60 experimental building and operate for 10 years with DD discharges. Corresponding to the high performance discharges of 100 s, the annual neutron emission from the JT60SC plasma should increase by about 6 times the Corresponding author. Tel.: /81-29-270-7328; fax: /8129-270-7419 E-mail address: [email protected] (A. Morioka).
present amount of neutron fluence in the JT-60U. It is important to evaluate the nuclear responses such as the radiation shielding, nuclear heating, induced activity and dose rate in various locations within the building during DD operation and after a shutdown. The nuclear responses in tokamak designs using the superconducting coil such as International Thermonuclear Experimental Reactor had been reported [4,5]. Important issues for the nuclear responses can be resolved into the following three points: (1) the safety operation of the superconducting coil during the operation; (2) estimation of biological shielding during the operation; (3) estimation of the dose rate in areas required to access after the operation. Estimation of the neutron and gamma-
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ray fluxes on the structure of vacuum vessel and the nuclear heating of TF coil are assessed for JT60SC. For safety operation of the superconducting TF coils, the vacuum vessel is required to suppress the nuclear heating at the TF coil. The basic structures of the vacuum vessel should be considered in the calculation results of the radioactivity in the vacuum vessel and the JT-60 building. For maintenance in areas required to access after the operation, the material selection of the first wall pedestal in the vacuum vessel are presented. The dose rate in the area varies with the calculated results of the activity of the candidate materials F82H and SS316L contaminated with cobalt. For the biological shielding during the operation, shielding effects of the cryostat are discussed. It is necessary to install the cryostat for radiation shielding in addition to the thermal shielding of the TF coil in JT-60SC. The dose rate outside the cryostat was calculated after shutdown of operations.
used for the calculations. The radiation shielding and the nuclear heating are estimated by these fluxes. The neutron emission rate for the calculation is shown in Table 1. The poloidal view of the JT-60SC and JT-60U are shown in Fig. 2. The neutron flux is used to calculate the induced activities and gamma-ray source intensity by the ACT-4 code at various times after the shutdown. On the basis of the gamma-ray source intensity, the gamma-ray flux distribution is calculated with the transport codes and the group constant of gamma-ray transport of GROUPIN composed of 54 groups, which is included in the THIDA-2 code system [8]. The dose rate distribution with 10 years operation is converted from the gamma-ray flux distribution at 1 day, 1 month and 3 months after a shutdown.
3. Results and discussion 3.1. Vacuum vessel design on the radiation shielding
2. Calculation procedure The neutron and gamma-ray fluxes during an operation are calculated with the ANISN code [6] and the DOT3.5 code [7] as shown in Fig. 1. A transport group constant set of FUSION-40, which consists of 42 neutron groups and 21 gamma-ray groups based on JENDL3.1, was
The nuclear heating of the superconducting TF coil has been calculated. The nuclear heating was compared for the following structures of the vacuum vessel. In Fig. 3(a), the vacuum vessel made of SS316L is designed to be a double-walled structure. Multilayer shielding structure, which is the double walls filled with multi layers of water and high-Mn steel boards for cooling and radiation shielding, had been designed in the previous work on JT-60SU [9]. As shown in Fig. 3(b), single-layer shielding structure consisting of double walls filled with Table 1 Neutron emission rate of JT-60SC Emission rate JT-60U
JT-60SC For nuclear heating For dose rate and Argon
(n/s) (n/week) (n/3 months) (n/year) Fig. 1. Calculation flow chart for
THIDA-2
system.
2 /1017 3.1 /1018 2.1 /1019 3.1 /1019
4 /1017 4 /1019 1.5 /1020 2 /1020
7 /1018 6.3 /1019
A. Morioka et al. / Fusion Engineering and Design 63 /64 (2002) 115 /120
Fig. 2. The poloidal views of JT-60U and JT-60SC.
water. The shielding material such as high-Mn steel (VC9) or SS316L outside the double wall is installed to surpress additionally the nuclear heating of the TF coil. The nuclear heating is taken account as follows:
Wtotal
n X 42 X i 0
fn (E)Kn (E)
117
For the calculation of the radiation shielding and the nuclear heating, the maximum neutron emission rate was assumed with DD neutrons of 4 /1017 n/s and DT neutrons of 1.2 /1016 n/s: 3% of DD neutrons. From the viewpoint of the operation of the TF coil, the neutron emission rate is limited for 10 s. In the case of DD neutrons, the calculated neutron flux and gamma-ray flux dependence through the vacuum vessel on the ratio of VC9 thickness to the total shielding thickness of 130 mm (VC9/(VC9/H2O)) water of the vacuum vessel are shown in Fig. 4. The neutron flux for both structures increases with the ratio of VC9 thickness to the total thickness of the vacuum vessel. The neutron flux through the multi-layer structure is smaller than that through the singlelayer structure. The gamma-ray flux generated by the multi-layer structure is slightly larger than that by single-layer structure. The performance of the
k 0
n X 21 X i 0
fg (E)Kg (E)
k 0
where Wtotal is a total nuclear heating, fn (E ) is neutron flux, Kn (E ) is neutron kerma factor, fg (E ) is the gamma-ray flux and Kg (E) is gamma-ray kerma factor are the function of the energy, i is component of the material and k is the group of energy.
Fig. 4. Neuron and gamma-ray flux through the vacuum vessel of MLT and SLT.
Fig. 3. Calculational 1D-model of vacuum vessel for JT-60SC: (a) MLT, multi layer type. (b) SLT, single layer type.
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neutron shielding in the multi-layer structure is superior to the single-layer structure. In contrast, in respect of the gamma-ray flux, the shielding effect by the single-layer structure is rather better than the multi-layer structure. The JT-60SU had been planned to carry out DD and DT discharges [9]. On the other hand, the operation of JT-60SC has been planned for only DD discharges. From the view of the neutron energy, the structure of the vacuum vessel is possible to be simplified. Fig. 5 shows the nuclear heating of the TF coil in the case of (a) neutron and gamma-ray irradiation, (b) neutron irradiation and (c) gamma-ray irradiation through the vacuum vessel with the ratio of VC9 to water. In the case of DD and DT neutron emission, the same neutron emission rate was assumed. In Fig. 5(a), for the ratio of 0.5, the maximum nuclear heating at inboard of the TF coil was
estimated to be 1.9 mW/cm3 for DD neutrons. In the case of DT neutrons, for the ratio of 0.8, the maximum nuclear heating is minimized. The nuclear heating of the TF coil with the single-layer structure reduced by about 20% for DD neutrons and about 10% for DT neutrons in comparison with the multi-layer structure, respectively. In Fig. 5(b), with increasing the ratio from 0.1 to 0.5, the nuclear heating by neutron irradiation are almost 0.1 mW/cm3 in the case of DD neutrons. The nuclear heating in the TF coil due to gammaray irradiation created by the shielding structure decreases for DT neutrons with increasing ratio. In Fig. 5(c), the nuclear heating by gamma-ray irradiation is minimized for the ratio of 0.5. As a result, the minimum nuclear heating is estimated to be about 1.8 mW/cm3 for DD neutrons. The nuclear heating is minimized for the ratio of 0.8 with single-layer structure for DT neutrons.
Fig. 5. (a) Maximum nuclear heating in the TF coil of the vacuum vessel of MLT and SLT. (b) Nuclear heating in the TF coil by neutron irradiation through the vacuum vessel of MLT and SLT. (c) Nuclear heating in the TF coil by gamma-ray irradiation created by the vacuum vessel of MLT and SLT.
A. Morioka et al. / Fusion Engineering and Design 63 /64 (2002) 115 /120
It was found that gamma-ray irradiation causes almost all nuclear heating of the TF coil. This is a result of the radiation shielding performance of the single-layer structure being better than that of the multi-layer structure. From the viewpoint of the nuclear heating of the superconducting coil, it is possible to thin the shielding material for the vacuum vessel structure. Therefore, from the result of Fig. 5(a), the total weight of the vacuum vessel can be reduced when the single-layer structure is adopted. The single-layer structure for vacuum vessel was decided so that the maximum nuclear heating at inboard side of the TF coil could be 2.45 mW/cm3 as specified from the refrigerator design for the superconducting coil. The ratio of SS316L to the total shielding thickness 0.21 was obtained in the single-layer structure under the above specification. The structure of vacuum vessel is shown in Fig. 6. In VC9 and SS316L, from the calculation result of the nuclear heating of the coil, the value of nuclear heating was almost the same. As the shielding material, SS316 with the reliability had been adopted. The weight of the vacuum vessel for the single-layer structure reduced by about 80 tons compared with the multi-layer structure. 3.2. Dose rate in vacuum vessel The dose rate at the first wall in the vacuum vessel depends on the results from the activity of the pedestal materials, F82H (0.005 wt.% cobalt) [10] and SS316L (0.05 wt.% cobalt). For the calculation of the dose rate, the neutron emission rate was assumed with DD neutrons of 7 /1018 n/ week and 6.3 /1018 n/3 months. When the F82H
Fig. 6. Structure of vacuum vessel for JT-60SC.
119
material is used for the pedestal in the vacuum vessel, the dose rate at the F82H pedestal in the first wall was 109 mSv/h. It is found that the dose rate in the vacuum vessel is reduced by 40% compared to the SS316L material. In Fig. 7, the induced activities of F82H were calculated at 1 s, 1 day, 1 week, 1 month, 1 year, 10 and 50 years after the shutdown under the neutron yield of 10 years operation. The principal nuclides contributing to the activity after shutdown are 54Mn, 55Fe(non gamma-ray) and 58Co at 1 year after the shutdown. 3.3. Cryostat design The role of cryostat is the thermal shielding of the superconducting coil and radiation shielding. The concrete added boron carbide (B4C), which is corresponding to 1% of natural boron, is adopted to improve the shielding efficiency of the thermal neutrons through the vacuum vessel. The cryostat consists of the B4C concrete of 200-mm thickness and SS316L of 34 mm inside and 6-mm thickness outside. By using the B4C concrete, the activation of argon in the air could be reduced in the JT-60 torus hall. The specific 41Ar-activity (40Ar (n, g)) in the JT60 torus hall was estimated to be 7.6 /10 5 Bq/ cm3 with the B4C concrete. In the case of the normal concrete, the 41Ar-activity was 5.9 /10 4
Fig. 7. Radioactivities at F82H pedestal at different coding time and corresponding decay curves calculated with ACT-4 code.
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Bq/cm3. Therefore, it was found that the 41Aractivity with the B4C concrete was reduced by about 90% in comparison with the normal concrete. In the case of the B4C concrete for the shielding material of cryostat, the calculated dose rate outside the cryostat was 1.8 mSv/h after shutdown of 10 years operation. The dose rate of JT-60SC site boundary is also satisfied without any additional shielding performance for the JT-60 building.
4. Conclusion The radiation shielding, nuclear heating and dose rate for the JT-60SC has been evaluated. From the view of the nuclear heating of the superconducting coil, the structure of the vacuum vessel of the single-layer structure is superior to the multi-layer structure. The single-layer structure for vacuum vessel was chosen so that the maximum nuclear heating at inboard side of the TF coil could be 2.45 mW/cm3. The weight of the vacuum vessel for the single-layer structure reduced by about 80 ton in compared to the multi-layer structure. The dose rate at the F82H pedestal in the vacuum vessel is 40% improved over the SS316L pedestal. The pedestal in the vacuum vessel is planned to be F82H. The dose rate outside the cryostat was estimated to be 1.8 mSv/h after shutdown of 10 years operation. By using the B4C concrete, it was found that the 41Ar-activity with the B4C concrete was reduced by about 90% in comparison with the normal concrete. The use of the B4C concrete shows that the dose rate of JT60SC site boundary is also satisfied without any
additional shielding performance for the JT-60 building.
Acknowledgements The authors would like to acknowledge Dr K. Ushigusa for their kind suggestions. They thank Mr H. Kawasaki of CRC solutions Inc. for preparing the two-dimensional code for the work.
References [1] S. Ishida et al., Recent results and future plan of JT-60U, in: Proceedings of 19th SOFE, Atlantic City, NJ, in press. [2] A. Sakasai et al., Engineering Design Study of JT-60 Superconducting Modification, ibid. [3] M. Matsukawa et al., Design and R&D issues for the JT60 modification to a full superconducting tokamak, this conference. [4] K. Maki, et al., Evaluation of dose rate and reactor core and nuclear properties for superconductive magnets in ITER, Fus. Eng. Des. 42 (1998) 173 /185. [5] Plant design specification and plant description document of ITER (2001). [6] Engle, W.A. Jr., A User’s Manual for ANISN, A onedimensional discrete ordinates transport code with anisotropic scattering, K-1693 (1967). [7] Rhoades, W.A. et al., DOT3.5 two-dimensional discrete ordinates radiation transport code, CCC-276 (1977). [8] Y. Seki et al., THIDA-2: An advanced code system for calculation of transmutation, activation, decay heat and dose rate, JAERI 1301, Japan Atomic Energy Research Institute (1986). [9] M. Kikuchi et al., Design progress of JT-60SU, IAEA-CN64/G2-3 (1996) 451 /466. [10] N. Yamanouchi, et al., Accumulation of engineering data for practical use of reduced activation ferric steel: 8%Cr2%W-0.2%V-0.04%Ta /Fe, J. Nucl. Mater. 191 /194 (1992) 822 /826.