- Email: [email protected]
Cl in Japanese surface soil
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 486–490 www.elsevier.com/locate/nimb
Isotopic ratios of a,*
R. Seki
b
36
Cl/Cl in Japanese surface soil
, T. Matsuhiro a, Y. Nagashima a, T. Takahashi a, K. Sasa a, K. Sueki a, Y. Tosaki a, K. Bessho b, H. Matsumura b, T. Miura b
a AMS Group, Tandem Accelerator Complex, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan Radiation Science Center, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
Available online 8 February 2007
Abstract We have measured the 36Cl/Cl ratio of uncultivated surface soil samples collected from 11 areas distributed throughout Japan to determine the undisturbed value of the ratio. The ratio was found to be on the order of 10 13 except for the Tokai-mura area, where four research reactors, two commercial nuclear power plants and a nuclear fuel reprocessing plant have been operated. The observed ratio in the Tokai-mura area was higher than 10 12. Notably, soil samples collected from a site of commercial BWR nuclear power plants in Fukushima prefecture showed no significant increase in 36Cl/Cl ratio. The 36Cl/Cl ratio depth profiles of soil samples collected at both of Makabe-town and Tokai-mura were also measured. Since Makabe-town is located about 50 km apart from Tokai-mura, we do not expect it to be affected by the nuclear facilities. No large variations were observed in the Makabe depth profile; the measured ratios ranged from 3 to 5 · 10 13. The result obtained for Tokai-mura is significantly different in that from the surface to about 80 cm depth, the measured ratios, 10 12, are much higher than any at Makabe. At depth below 80 cm, the Tokai-mura ratios are lower and become indistinguishable from those at Makabe. The 36Cl/Cl ratio in unaffected areas of Japan is estimated to be 3–4 · 10 13. 2007 Elsevier B.V. All rights reserved. PACS: 82.80.Kp; 89.60. k Keywords: AMS;
36
Cl; Isotopic ratio
36
Cl/Cl; Neutron; Surface soil
1. Introduction Chlorine is a common element distributed widely in the environment. A very long-lived radionuclide, 36Cl, is produced in nature by cosmic-rays interacting with Ar in the atmosphere. It is also produced through the thermal neutron capture reaction on 35Cl, which has a large cross section outside the resonance region. 36Cl has been effectively used as a monitoring tool of thermal neutron dose in the environment. The 36Cl produced by nuclear tests on barges or small islands between 1952 and 1958 have been extensively studied after the first measurements by Elmore et al. [1]. The 36 Cl bomb pulse has been used as a good tracer in shallow *
Corresponding author. Tel.: +81 851 0638; fax: +81 846 3977. E-mail address: [email protected] (R. Seki).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.227
groundwaters [2]. The measurements of 36Cl have provided an estimate of thermal neutron flux at the Hiroshima Atomic bomb site [3]. Recently nuclear power industries, which possibly enhance the 36Cl level in the environment, have been steadily developing. A criticality accident occurred on September 30, 1990 at the uranium conversion facility of JCO Company Ltd. in Tokai-mura. The 36Cl/Cl ratio of surface soil samples 2.5 m from the criticality accident site is 2.8 · 10 11. The ratios of reagents (CsCl, BaCl2 and NiCl2) collected in the factory 56 m apart from the accident site are around 1.2 · 10 14, almost same as that measured for table salt sampled at 60 km away from JCO [4]. Milton et al. reported that concentrations of 36Cl around the Chalk River Laboratories (CRL), which were slightly in excess of natural production rates, were a byproduct of reactor operations and radioisotope production at the site
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
487
[5]. They discussed two probable modes of 36Cl production, neutron activation of 36Ar or 35Cl in the air passing through the reactor or chlorinated water used for cooling some support structures. The former production mode is accompanied with 41Ar. Large amounts of 41Ar emissions were reported in the exhaust gas of the Gas Cooled Reactor (GCR) and the Heavy Water Reactor (HWR) [6]. Concentrations of 36Cl in water samples from near the Savannah River Site (SRS) and Idaho National Engineering Laboratory (INEL) nuclear fuel reprocessing plants were higher than natural levels [7,8]. There are four research reactors, two commercial nuclear power plants and a nuclear fuel reprocessing plant in Tokai-mura. One of the nuclear power plants in Tokaimura is a GCR and was in operation 1966–1998. It has been reported in ‘Quarterly Report of Ibaraki Environmental Radiation Monitoring’ that the research reactors in Tokai-mura have emitted 41Ar [9]. On the basis of the above, measurements of 36Cl/Cl ratios can provide a useful tool for monitoring neutron dose. For this purpose, the background level of 36Cl/Cl should be known. We report the background level of 36 Cl/Cl in surface soils in areas unaffected as well as affected by nuclear facilities. We measured 36Cl/Cl ratios of soil samples collected from areas falling into the following three categories: 1. Tokai-mura, where there are many types of nuclear facilities. 2. Fukushima prefecture, where there are 10 BWR nuclear power plants. 3. Several areas unaffected by nuclear installations. 36
Cl/Cl depth profiles were also obtained for a site in Tokai-mura and a site in an unaffected area.
2. Experimental procedure 2.1. Soil samples and preparation Surface soil samples spanning 0–5 cm depth were collected from the surface at uncultivated locations such as courtyards in small shrines. The analytical method was described previously [10] and is shown briefly in Fig. 1. 200 g of air dried soil samples were taken for analysis. After adding the 200 ml of 0.8 M HNO3 solution, the soil samples were heated and stirred to homogenize. The chlorine and 36Cl were extracted from the soil samples by centrifugation. Then about 100 ml of pure water from a Milli-Q Water Purification System was added to the separated residue and chlorine isotopes were extracted by centrifugation. Impurities such as iron were eliminated as a precipitate by the addition of NH4OH. The combined solution was concentrated to 10 ml by volume. For AMS measurement, chlorine isotopes were changed to AgCl by adding AgNO3 solution. The potential 36S
Fig. 1. Sample preparation method of soil samples for ment by AMS.
36
Cl/Cl measure-
interference in 36Cl AMS measurement was eliminated by adding Ba(NO3)2 solution. 2.2. Measurement of
36
Cl
The isotopic ratios 36Cl/Cl of soil samples were determined by accelerator mass spectrometry at the Tandem Accelerator Complex, University of Tsukuba. The method was described previously [10]. A ‘tri-molecular pilot beam’ method is used to stabilize the terminal voltage. A small amount of fullerene (C60), MTR Ltd., saturated in benzene was added to AgCl sample as a pilot beam for 36Cl/Cl measurement. The well-mixed target material was put in a target holder and compressed to a routine pressure. The measured ratios were normalized to standards prepared by diluting the reference material NIST 4943. 3. Results and discussion Fig. 2 shows the collecting locations for unaffected area samples in Japan. Fig. 3 is a map of Ibaraki prefecture indicating Tokai-mura, Makabe-town and Tsukuba. (Makabetown became Sakuragawa city by consolidation.)
488
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Fig. 2. Sampling locations of surface soil samples in undisturbed area in Japan.
The peaceful use of atomic energy in Japan started at Tokai-mura in the 1960s. Four research reactors of JAERI started from 1960. The first commercial nuclear power plant (GCR) operated from 1966 to 1998. The nuclear fuel reprocessing plant has been operating since 1977. Among many nuclear facilities in Tokai-mura, the facilities which may release 36Cl to the environment are JCO, JNC and JAPC. JNC (Japan Nuclear Cycle Development) has a nuclear fuel reprocessing plant in Tokai-mura. [JNC has turned to JAEA, Japan Atomic Energy Agency by consolidation with JAERI (Japan Atomic Energy Research Institute).] Beasley et al. (1992, 1993) reported that two nuclear fuel reprocessing plants (SRS and INEL) released 36Cl to the environment [7,8]. Certainly 36Cl is produced by neutron activation of chloride impurities in the fuel elements during reactor operations. It is not clear that 36Cl is emitted to the environment. Iodine–129 which belongs to same halogen family has been reported to be released from JNC to the environment occasionally [9,11]. Beasley et al. mentioned that the amount of 36Cl released through the same pathway from the fuel reprocessing plant was less than 2% of 129I. JAPC (Japan Atomic Power Company Co., Ltd.) has two nuclear power plants in Tokai-mura. The first one is a gas cooled reactor operated from 1966 to 1998. Argon41 has been reported as having been released from the reactor [9]. Argon-41 is produced by neutron activation of Ar in the air. Probably 36Cl can be produced by fast neutron reaction (n,p) of Ar in the plant. The second power plant is a BWR type. It was reported that 41Ar has not been released from the plant [9]. 41Ar releases by research reactors in JAERI were less than 30 % of GCR of JAPC [9]. Though the released activities from reactors have gradually decreased recently, the 36Cl/Cl ratios in soils has proved to be significantly higher in Tokai-mura than in unaffected regions. 3.2. Near the nuclear power plants
Fig. 3. Sampling locations in Ibaraki prefecture.
3.1. Surface soil in Tokai-mura Sampling places and their measured surface soil 36Cl/Cl ratios in Tokai-mura are indicated in Fig. 4. The highest ratio, (279 ± 5) · 10 13, was obtained from the sample taken 2.5 m from the accident site in JCO. The second highest, (202 ± 4) · 10 13, was from near the JAPC and JAERI. It is obvious that 36Cl/Cl in surface soils in Tokai-mura is higher than in other areas in Japan. The mean ratio of 36 Cl/Cl in Tokai-mura area is (72 ± 57) · 10 13.
Two commercial nuclear power stations, one having 6 BWRs and a second one having 4 BWRs, have been installed in Fukushima prefecture (Fig. 5). Each power plant generates about one million kilowatts. The first power station has been operating since 1971 and the second has been operating since 1982. Around the power stations surface soil samples were collected at 21 places in the courtyards of small shrines. The 36Cl/Cl ratios are indicated in Fig. 3. The mean value of 36Cl/Cl ratios of all samples collected in this area is (4.6 ± 2.7) · 10 13. This indicates that there has been no significant release of 36Cl from the stations, because this ratio is essentially identical to ratios in unaffected area of Japan. 3.3. Unaffected area of Japan We have collected 24 surface soil samples from 10 areas of Japan. The measured isotopic ratios of these samples are
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Fig. 4. Sampling locations in and around Tokai-mura and
489
36
Cl/Cl ratios in surface soil.
Table 1 36 C/Cl ratios in unaffected areas in Japan 36
Sampling location
Fig. 5. Sampling locations around nuclear power stations and ratios in surface soil.
Sapporo
6.4 ± 0.3
Aomori prefecture
Aomori Ajigaura
1.6 ± 0.8 1.4 ± 0.3
Tochigi prefecture
Batou Batou
13 ± 1b 13 ± 1b
Ibaraki prefecture
Nakaminato Tsukuba Tsukuba Tsukuba Daigo
14 ± 1b 1.7 ± 0.4 1.6 ± 0.2 3.2 ± 0.6 4.5 ± 0.4
Fukui prefecture
Katsuyama Katsuyama Katsuyama Katsuyama
3.4 ± 0.5 4.5 ± 0.9 4.2 ± 1.0 11 ± 1b
Osaka prefecture
Kumatori Kumatori
8.0 ± 0.5 2.8 ± 0.3
Hiroshima prefecture
Kurose
1.5 ± 0.2
Kouchi prefecture
Tosa Tosa
1.4 ± 0.5 1.0 ± 0.3
Cl/Cl
(1) Batou, which is 50 km NW of Tokai, is possibly influenced by Tokai. (2) Nakaminato, 14 km S of Tokai, is also possibly influenced by Tokai. (3) The sample from Katsuyama showing higher value of 36 Cl/Cl ratio was collected in 1994. All other samples were collected in 2001–2002. Possibly the fall-out level has changed over the intervening years. Excepting these four samples, the mean value of the Cl/Cl ratio is 3.4 · 10 13. This ratio is taken to be the typical 36Cl/Cl ratio for undisturbed areas in Japan. 36
Hokkaido
36
shown in Table 1. The mean value of all samples is (4.6 ± 3.5) · 10 13. Four samples (marked with ‘b’ in Table 1) had slightly higher ratios than the others and these may be explained by the following reasons;
Cl/Cl (10
Fukuoka prefecture
Fukuoka
4.2 ± 2.2
Nagasaki prefecture
Shimabara Aino Tarami Tarami
5.6 ± 0.3 6.8 ± 0.3 1.2 ± 0.1 2.8 ± 0.2
a b
13 a
)
Errors represent 1r confidence intervals. Each values are slightly larger than others.
3.4. Depth profiles at Tokai and Makabe Surface soil 36Cl/Cl depth profiles measured at Tokai (near nuclear facilities) and at Makabe (50 km southeast away from Tokai) are shown in Fig. 6. The Makabe depth profile is essentially flat at 3 · 10 13. The Tokai depth profile shows very different behavior. From the surface to
490
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Based on our measurements, we estimate that the isotopic ratio of 36Cl/Cl in surface soil in unaffected areas of Japan is 3–4 · 10 13. Knowledge of this ‘unaffected’ value will be useful in attempts to detect any unusual emissions or production of 36Cl in the environment. Acknowledgements
Fig. 6. Depth profiles of
36
Cl/Cl in Tokai-mura and Makabe.
80 cm depth, the Tokai 36Cl/Cl ratios are significantly higher than those at Makabe and below 80 cm the ratios drop to become essentially identical to the Makabe ratios. This suggests that Tokai soils at depth <80 cm have been affected by some anthropogenic input of 36Cl. The unaffected 36Cl/Cl ratio obtained from the Makabe depth profile is 3.8 · 10 13. 4. Summary We have obtained the following results from AMS measurements of 36Cl/Cl ratios in surface soils from 4 different areas: (1) The JCO accident site (2) The Tokai-mura area (3) Fukushima (near BWR plants) (4) Unaffected areas in Japan
279 · 10 13 (Max) 13.5 to 202 · 10 13 4.6 · 10 13 4.6 · 10 13
The 0–100 cm 36Cl/Cl depth profile from a Tokai-mura soil showed different behavior from that observed in a soil from a Makabe site, 50 km southeast from Tokai-mura. The Makabe depth profile was essentially flat at 3.8 · 10 13 while the Tokai depth profile was significantly higher over its upper 80 cm.
We would like to thank Dr. K. Nishiizumi (Space Sciences Laboratory, University of California) for helpful discussions and encouragement. The authors are grateful to the technical staff of the Tandem Accelerator Complex, University of Tsukuba. This work is partially supported by the Grants-in-Aid for the Developmental Scientific Research Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan No. 13680619. References [1] D. Elmore, B.R. Fulton, M.R. Clover, J.R. Marsden, H.G. Gove, H. Naylor, K.H. Purser, L.R. Kilius, R.P. Beukens, A.E. Litherland, Nature 277 (1979) 22. [2] H.W. Benley, F.M. Phillips, S.N. Davis, S. Gifford, D. Elmore, L.E. Tubbs, H.E. Gove, Nature 300 (1982) 737. [3] Y. Nagashima, R. Seki, T. Matsuhiro, T. Takahashi, K. Sasa, K. Sueki, M. Hoshi, S. Fujita, K. Shizuma, H. Hasai, Nucl. Instr. and Meth. B 223–224 (2004) 782. [4] R. Seki, D. Arai, Y. Nagashima, T. Imanaka, T. Takahashi, T. Matsuhiro, J. Radioanal. Nucl. Chem. 225 (2003) 245. [5] G.M. Milton, H.R. Andrews, S.E. Causey, L.A. Chant, R.J. Cornett, W.G. Davis, B.F. Greiner, V.T. Koslowsky, Y. Imahori, S.J. Kramer, J.W. McKay, J.C.D. Milton, Nucl. Instr. and Meth. B 92 (1994) 376. [6] Sources and effects of ionizing radiation UNSCEAR 2000 Report. [7] T.M. Beasley, D. Elmore, P.W. Kubik, P. Sharma, Ground Water 30 (1992) 539. [8] T.M. Beasley, L.D. Cecil, P. Sharma, P.W. Kubik, U. Fehn, L.J. Mann, H.E. Gove, Ground Water 31 (1993) 302. [9] Quarterly Report of Ibaraki Environmental Radiation Monitoring, 1970–2003. (Committee of Ibaraki Environmental Radiation Monitoring) (in Japanese). [10] Y. Nagashima, R. Seki, T. Takahashi, D. Arai, Nucl. Instr. and Meth. B 172 (2000) 129. [11] R. Seki, E. Kimura, T. Takahashi, N. Ikeda, J. Radioanal. Nucl. Chem. 138 (1990) 17.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 486–490 www.elsevier.com/locate/nimb
Isotopic ratios of a,*
R. Seki
b
36
Cl/Cl in Japanese surface soil
, T. Matsuhiro a, Y. Nagashima a, T. Takahashi a, K. Sasa a, K. Sueki a, Y. Tosaki a, K. Bessho b, H. Matsumura b, T. Miura b
a AMS Group, Tandem Accelerator Complex, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan Radiation Science Center, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
Available online 8 February 2007
Abstract We have measured the 36Cl/Cl ratio of uncultivated surface soil samples collected from 11 areas distributed throughout Japan to determine the undisturbed value of the ratio. The ratio was found to be on the order of 10 13 except for the Tokai-mura area, where four research reactors, two commercial nuclear power plants and a nuclear fuel reprocessing plant have been operated. The observed ratio in the Tokai-mura area was higher than 10 12. Notably, soil samples collected from a site of commercial BWR nuclear power plants in Fukushima prefecture showed no significant increase in 36Cl/Cl ratio. The 36Cl/Cl ratio depth profiles of soil samples collected at both of Makabe-town and Tokai-mura were also measured. Since Makabe-town is located about 50 km apart from Tokai-mura, we do not expect it to be affected by the nuclear facilities. No large variations were observed in the Makabe depth profile; the measured ratios ranged from 3 to 5 · 10 13. The result obtained for Tokai-mura is significantly different in that from the surface to about 80 cm depth, the measured ratios, 10 12, are much higher than any at Makabe. At depth below 80 cm, the Tokai-mura ratios are lower and become indistinguishable from those at Makabe. The 36Cl/Cl ratio in unaffected areas of Japan is estimated to be 3–4 · 10 13. 2007 Elsevier B.V. All rights reserved. PACS: 82.80.Kp; 89.60. k Keywords: AMS;
36
Cl; Isotopic ratio
36
Cl/Cl; Neutron; Surface soil
1. Introduction Chlorine is a common element distributed widely in the environment. A very long-lived radionuclide, 36Cl, is produced in nature by cosmic-rays interacting with Ar in the atmosphere. It is also produced through the thermal neutron capture reaction on 35Cl, which has a large cross section outside the resonance region. 36Cl has been effectively used as a monitoring tool of thermal neutron dose in the environment. The 36Cl produced by nuclear tests on barges or small islands between 1952 and 1958 have been extensively studied after the first measurements by Elmore et al. [1]. The 36 Cl bomb pulse has been used as a good tracer in shallow *
Corresponding author. Tel.: +81 851 0638; fax: +81 846 3977. E-mail address: [email protected] (R. Seki).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.227
groundwaters [2]. The measurements of 36Cl have provided an estimate of thermal neutron flux at the Hiroshima Atomic bomb site [3]. Recently nuclear power industries, which possibly enhance the 36Cl level in the environment, have been steadily developing. A criticality accident occurred on September 30, 1990 at the uranium conversion facility of JCO Company Ltd. in Tokai-mura. The 36Cl/Cl ratio of surface soil samples 2.5 m from the criticality accident site is 2.8 · 10 11. The ratios of reagents (CsCl, BaCl2 and NiCl2) collected in the factory 56 m apart from the accident site are around 1.2 · 10 14, almost same as that measured for table salt sampled at 60 km away from JCO [4]. Milton et al. reported that concentrations of 36Cl around the Chalk River Laboratories (CRL), which were slightly in excess of natural production rates, were a byproduct of reactor operations and radioisotope production at the site
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
487
[5]. They discussed two probable modes of 36Cl production, neutron activation of 36Ar or 35Cl in the air passing through the reactor or chlorinated water used for cooling some support structures. The former production mode is accompanied with 41Ar. Large amounts of 41Ar emissions were reported in the exhaust gas of the Gas Cooled Reactor (GCR) and the Heavy Water Reactor (HWR) [6]. Concentrations of 36Cl in water samples from near the Savannah River Site (SRS) and Idaho National Engineering Laboratory (INEL) nuclear fuel reprocessing plants were higher than natural levels [7,8]. There are four research reactors, two commercial nuclear power plants and a nuclear fuel reprocessing plant in Tokai-mura. One of the nuclear power plants in Tokaimura is a GCR and was in operation 1966–1998. It has been reported in ‘Quarterly Report of Ibaraki Environmental Radiation Monitoring’ that the research reactors in Tokai-mura have emitted 41Ar [9]. On the basis of the above, measurements of 36Cl/Cl ratios can provide a useful tool for monitoring neutron dose. For this purpose, the background level of 36Cl/Cl should be known. We report the background level of 36 Cl/Cl in surface soils in areas unaffected as well as affected by nuclear facilities. We measured 36Cl/Cl ratios of soil samples collected from areas falling into the following three categories: 1. Tokai-mura, where there are many types of nuclear facilities. 2. Fukushima prefecture, where there are 10 BWR nuclear power plants. 3. Several areas unaffected by nuclear installations. 36
Cl/Cl depth profiles were also obtained for a site in Tokai-mura and a site in an unaffected area.
2. Experimental procedure 2.1. Soil samples and preparation Surface soil samples spanning 0–5 cm depth were collected from the surface at uncultivated locations such as courtyards in small shrines. The analytical method was described previously [10] and is shown briefly in Fig. 1. 200 g of air dried soil samples were taken for analysis. After adding the 200 ml of 0.8 M HNO3 solution, the soil samples were heated and stirred to homogenize. The chlorine and 36Cl were extracted from the soil samples by centrifugation. Then about 100 ml of pure water from a Milli-Q Water Purification System was added to the separated residue and chlorine isotopes were extracted by centrifugation. Impurities such as iron were eliminated as a precipitate by the addition of NH4OH. The combined solution was concentrated to 10 ml by volume. For AMS measurement, chlorine isotopes were changed to AgCl by adding AgNO3 solution. The potential 36S
Fig. 1. Sample preparation method of soil samples for ment by AMS.
36
Cl/Cl measure-
interference in 36Cl AMS measurement was eliminated by adding Ba(NO3)2 solution. 2.2. Measurement of
36
Cl
The isotopic ratios 36Cl/Cl of soil samples were determined by accelerator mass spectrometry at the Tandem Accelerator Complex, University of Tsukuba. The method was described previously [10]. A ‘tri-molecular pilot beam’ method is used to stabilize the terminal voltage. A small amount of fullerene (C60), MTR Ltd., saturated in benzene was added to AgCl sample as a pilot beam for 36Cl/Cl measurement. The well-mixed target material was put in a target holder and compressed to a routine pressure. The measured ratios were normalized to standards prepared by diluting the reference material NIST 4943. 3. Results and discussion Fig. 2 shows the collecting locations for unaffected area samples in Japan. Fig. 3 is a map of Ibaraki prefecture indicating Tokai-mura, Makabe-town and Tsukuba. (Makabetown became Sakuragawa city by consolidation.)
488
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Fig. 2. Sampling locations of surface soil samples in undisturbed area in Japan.
The peaceful use of atomic energy in Japan started at Tokai-mura in the 1960s. Four research reactors of JAERI started from 1960. The first commercial nuclear power plant (GCR) operated from 1966 to 1998. The nuclear fuel reprocessing plant has been operating since 1977. Among many nuclear facilities in Tokai-mura, the facilities which may release 36Cl to the environment are JCO, JNC and JAPC. JNC (Japan Nuclear Cycle Development) has a nuclear fuel reprocessing plant in Tokai-mura. [JNC has turned to JAEA, Japan Atomic Energy Agency by consolidation with JAERI (Japan Atomic Energy Research Institute).] Beasley et al. (1992, 1993) reported that two nuclear fuel reprocessing plants (SRS and INEL) released 36Cl to the environment [7,8]. Certainly 36Cl is produced by neutron activation of chloride impurities in the fuel elements during reactor operations. It is not clear that 36Cl is emitted to the environment. Iodine–129 which belongs to same halogen family has been reported to be released from JNC to the environment occasionally [9,11]. Beasley et al. mentioned that the amount of 36Cl released through the same pathway from the fuel reprocessing plant was less than 2% of 129I. JAPC (Japan Atomic Power Company Co., Ltd.) has two nuclear power plants in Tokai-mura. The first one is a gas cooled reactor operated from 1966 to 1998. Argon41 has been reported as having been released from the reactor [9]. Argon-41 is produced by neutron activation of Ar in the air. Probably 36Cl can be produced by fast neutron reaction (n,p) of Ar in the plant. The second power plant is a BWR type. It was reported that 41Ar has not been released from the plant [9]. 41Ar releases by research reactors in JAERI were less than 30 % of GCR of JAPC [9]. Though the released activities from reactors have gradually decreased recently, the 36Cl/Cl ratios in soils has proved to be significantly higher in Tokai-mura than in unaffected regions. 3.2. Near the nuclear power plants
Fig. 3. Sampling locations in Ibaraki prefecture.
3.1. Surface soil in Tokai-mura Sampling places and their measured surface soil 36Cl/Cl ratios in Tokai-mura are indicated in Fig. 4. The highest ratio, (279 ± 5) · 10 13, was obtained from the sample taken 2.5 m from the accident site in JCO. The second highest, (202 ± 4) · 10 13, was from near the JAPC and JAERI. It is obvious that 36Cl/Cl in surface soils in Tokai-mura is higher than in other areas in Japan. The mean ratio of 36 Cl/Cl in Tokai-mura area is (72 ± 57) · 10 13.
Two commercial nuclear power stations, one having 6 BWRs and a second one having 4 BWRs, have been installed in Fukushima prefecture (Fig. 5). Each power plant generates about one million kilowatts. The first power station has been operating since 1971 and the second has been operating since 1982. Around the power stations surface soil samples were collected at 21 places in the courtyards of small shrines. The 36Cl/Cl ratios are indicated in Fig. 3. The mean value of 36Cl/Cl ratios of all samples collected in this area is (4.6 ± 2.7) · 10 13. This indicates that there has been no significant release of 36Cl from the stations, because this ratio is essentially identical to ratios in unaffected area of Japan. 3.3. Unaffected area of Japan We have collected 24 surface soil samples from 10 areas of Japan. The measured isotopic ratios of these samples are
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Fig. 4. Sampling locations in and around Tokai-mura and
489
36
Cl/Cl ratios in surface soil.
Table 1 36 C/Cl ratios in unaffected areas in Japan 36
Sampling location
Fig. 5. Sampling locations around nuclear power stations and ratios in surface soil.
Sapporo
6.4 ± 0.3
Aomori prefecture
Aomori Ajigaura
1.6 ± 0.8 1.4 ± 0.3
Tochigi prefecture
Batou Batou
13 ± 1b 13 ± 1b
Ibaraki prefecture
Nakaminato Tsukuba Tsukuba Tsukuba Daigo
14 ± 1b 1.7 ± 0.4 1.6 ± 0.2 3.2 ± 0.6 4.5 ± 0.4
Fukui prefecture
Katsuyama Katsuyama Katsuyama Katsuyama
3.4 ± 0.5 4.5 ± 0.9 4.2 ± 1.0 11 ± 1b
Osaka prefecture
Kumatori Kumatori
8.0 ± 0.5 2.8 ± 0.3
Hiroshima prefecture
Kurose
1.5 ± 0.2
Kouchi prefecture
Tosa Tosa
1.4 ± 0.5 1.0 ± 0.3
Cl/Cl
(1) Batou, which is 50 km NW of Tokai, is possibly influenced by Tokai. (2) Nakaminato, 14 km S of Tokai, is also possibly influenced by Tokai. (3) The sample from Katsuyama showing higher value of 36 Cl/Cl ratio was collected in 1994. All other samples were collected in 2001–2002. Possibly the fall-out level has changed over the intervening years. Excepting these four samples, the mean value of the Cl/Cl ratio is 3.4 · 10 13. This ratio is taken to be the typical 36Cl/Cl ratio for undisturbed areas in Japan. 36
Hokkaido
36
shown in Table 1. The mean value of all samples is (4.6 ± 3.5) · 10 13. Four samples (marked with ‘b’ in Table 1) had slightly higher ratios than the others and these may be explained by the following reasons;
Cl/Cl (10
Fukuoka prefecture
Fukuoka
4.2 ± 2.2
Nagasaki prefecture
Shimabara Aino Tarami Tarami
5.6 ± 0.3 6.8 ± 0.3 1.2 ± 0.1 2.8 ± 0.2
a b
13 a
)
Errors represent 1r confidence intervals. Each values are slightly larger than others.
3.4. Depth profiles at Tokai and Makabe Surface soil 36Cl/Cl depth profiles measured at Tokai (near nuclear facilities) and at Makabe (50 km southeast away from Tokai) are shown in Fig. 6. The Makabe depth profile is essentially flat at 3 · 10 13. The Tokai depth profile shows very different behavior. From the surface to
490
R. Seki et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 486–490
Based on our measurements, we estimate that the isotopic ratio of 36Cl/Cl in surface soil in unaffected areas of Japan is 3–4 · 10 13. Knowledge of this ‘unaffected’ value will be useful in attempts to detect any unusual emissions or production of 36Cl in the environment. Acknowledgements
Fig. 6. Depth profiles of
36
Cl/Cl in Tokai-mura and Makabe.
80 cm depth, the Tokai 36Cl/Cl ratios are significantly higher than those at Makabe and below 80 cm the ratios drop to become essentially identical to the Makabe ratios. This suggests that Tokai soils at depth <80 cm have been affected by some anthropogenic input of 36Cl. The unaffected 36Cl/Cl ratio obtained from the Makabe depth profile is 3.8 · 10 13. 4. Summary We have obtained the following results from AMS measurements of 36Cl/Cl ratios in surface soils from 4 different areas: (1) The JCO accident site (2) The Tokai-mura area (3) Fukushima (near BWR plants) (4) Unaffected areas in Japan
279 · 10 13 (Max) 13.5 to 202 · 10 13 4.6 · 10 13 4.6 · 10 13
The 0–100 cm 36Cl/Cl depth profile from a Tokai-mura soil showed different behavior from that observed in a soil from a Makabe site, 50 km southeast from Tokai-mura. The Makabe depth profile was essentially flat at 3.8 · 10 13 while the Tokai depth profile was significantly higher over its upper 80 cm.
We would like to thank Dr. K. Nishiizumi (Space Sciences Laboratory, University of California) for helpful discussions and encouragement. The authors are grateful to the technical staff of the Tandem Accelerator Complex, University of Tsukuba. This work is partially supported by the Grants-in-Aid for the Developmental Scientific Research Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan No. 13680619. References [1] D. Elmore, B.R. Fulton, M.R. Clover, J.R. Marsden, H.G. Gove, H. Naylor, K.H. Purser, L.R. Kilius, R.P. Beukens, A.E. Litherland, Nature 277 (1979) 22. [2] H.W. Benley, F.M. Phillips, S.N. Davis, S. Gifford, D. Elmore, L.E. Tubbs, H.E. Gove, Nature 300 (1982) 737. [3] Y. Nagashima, R. Seki, T. Matsuhiro, T. Takahashi, K. Sasa, K. Sueki, M. Hoshi, S. Fujita, K. Shizuma, H. Hasai, Nucl. Instr. and Meth. B 223–224 (2004) 782. [4] R. Seki, D. Arai, Y. Nagashima, T. Imanaka, T. Takahashi, T. Matsuhiro, J. Radioanal. Nucl. Chem. 225 (2003) 245. [5] G.M. Milton, H.R. Andrews, S.E. Causey, L.A. Chant, R.J. Cornett, W.G. Davis, B.F. Greiner, V.T. Koslowsky, Y. Imahori, S.J. Kramer, J.W. McKay, J.C.D. Milton, Nucl. Instr. and Meth. B 92 (1994) 376. [6] Sources and effects of ionizing radiation UNSCEAR 2000 Report. [7] T.M. Beasley, D. Elmore, P.W. Kubik, P. Sharma, Ground Water 30 (1992) 539. [8] T.M. Beasley, L.D. Cecil, P. Sharma, P.W. Kubik, U. Fehn, L.J. Mann, H.E. Gove, Ground Water 31 (1993) 302. [9] Quarterly Report of Ibaraki Environmental Radiation Monitoring, 1970–2003. (Committee of Ibaraki Environmental Radiation Monitoring) (in Japanese). [10] Y. Nagashima, R. Seki, T. Takahashi, D. Arai, Nucl. Instr. and Meth. B 172 (2000) 129. [11] R. Seki, E. Kimura, T. Takahashi, N. Ikeda, J. Radioanal. Nucl. Chem. 138 (1990) 17.