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Applied Radiation and Isotopes 66 (2008) 103–107

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Determination of 240Pu/239Pu atom ratio in coastal surface seawatersfrom the western North Pacific Ocean and Japan Sea

Masatoshi Yamada�, Jian Zheng

Nakaminato Laboratory for Marine Radioecology, Environmental Radiation Effects Research Group, National Institute of Radiological Sciences,

Isozaki 3609, Hitachinaka, Ibaraki 311-1202, Japan

Received 21 May 2007; accepted 4 August 2007

Abstract

Surface seawater samples were collected from a site in the vicinity of the nuclear fuel reprocessing facility at Rokkasho, Japan and sites

along the Japan Sea coast. 239+240Pu activities and 240Pu/239Pu atom ratios were determined by a-spectrometry and isotope-dilution

sector-field ICP-MS. The 240Pu/239Pu atom ratio with the mean value of 0.22770.006 was significantly higher than the mean global

fallout ratio of 0.18. The contribution of the Pacific Proving Grounds close-in fallout was estimated to be 33% of the 239+240Pu.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: 240Pu/239Pu atom ratio; ID-SF-ICP-MS; Nuclear fuel reprocessing facility; Close-in fallout; Western North Pacific Ocean; Japan Sea

1. Introduction

Significant quantities of anthropogenic radionuclidessuch as 239Pu (half-life of 2.41� 104 yr) and 240Pu (half-life of 6.56� 103 yr) have been released into the environ-ment as the result of atmospheric nuclear weapons testing,nuclear weapons production activities, nuclear fuel-cyclereprocessing operations, disposal of nuclear waste, andaccidents involving nuclear materials, etc. (e.g. UN-SCEAR, 2000; Hamilton, 2004). Large quantities ofradioactive waste have been discharged from the Sellafieldnuclear complex in the United Kingdom and from the LaHague reprocessing facility in France. For example,239+240Pu of�0.6 PBq was discharged into the Irish Seafrom the Sellafield nuclear facility up to 1992 (Kershaw etal., 1995a). The nuclear fuel reprocessing facility atRokkasho, Aomori Prefecture, Japan began active testingat the end of March 2006 and its full commercial operationwill begin in November 2007. Liquid wastes will be releasedfrom an outlet pipeline into coastal waters at a distance of3 km from shore. The waste discharge could cause radio-active contamination in the coastal waters.

e front matter r 2007 Elsevier Ltd. All rights reserved.

radiso.2007.08.003

ing author. Tel.: +8129 265 7130; fax: +81 29 265 9883.

ess: m_yamada@nirs.go.jp (M. Yamada).

The atom ratio of 240Pu/239Pu is known to be a usefultracer to identify the sources of plutonium in the ocean(Bertine et al., 1986; Buesseler, 1997; Chiappini et al., 1999;Kim et al., 2003; Lee et al., 2004; Zheng and Yamada,2004; Wang and Yamada, 2005). Reported atom ratios of240Pu/239Pu from global stratospheric fallout are0.17670.014 (Krey et al., 1976) and 0.18070.014 (Kelleyet al., 1999), whereas those from close-in troposphericfallout from the Pacific Proving Grounds (PPG) in theMarshall Islands are 0.33–0.36 (Diamond et al., 1960;Komura et al., 1984; Koide et al., 1985, Buesseler, 1997). Afew measurements of 240Pu/239Pu atom ratios in surfacewaters were carried out in the western North Pacific Oceanand its adjacent seas (Kim et al., 2004; Norisuye et al.,2006; Yamada et al., 2006). Investigations of 240Pu/239Puatom ratios in seawaters have not been done in the coastalareas of the nuclear fuel reprocessing facility at Rokkasho.The objectives of this study are (1) to measure the

239+240Pu activities and 240Pu/239Pu atom ratios in surfaceseawater samples collected in the vicinity of the nuclear fuelreprocessing facility at Rokkasho and along the Japan Seacoast; (2) to provide background data on 239+240Puactivities and 240Pu/239Pu atom ratios before the start-upof the nuclear fuel reprocessing facility; (3) to estimate therelative contributions of the global fallout Pu and the PPG

ARTICLE IN PRESSM. Yamada, J. Zheng / Applied Radiation and Isotopes 66 (2008) 103–107104

fallout Pu in seawater in the studied areas; and (4) topropose pathways for delivery of the PPG fallout Pu to thestudied areas.

TomariAjigasawa

Sado

Pacific Ocean

Japan Sea

Japan

Russia

Reprocessing

plant

140°°E135°E130°E45°N

40°N

35°N

145°E

Aomori

Fig. 1. Map of the western North Pacific Ocean and Japan Sea showing

the sampling locations. The open circles are the sampling locations for this

study and the open square represents the site for the nuclear fuel

reprocessing plant and other nuclear fuel facilities in Rokkasho, Aomori

Prefecture, Japan.

2. Materials and methods

2.1. Sample collection and analytical procedure

Seawater samples (�200L each) for Pu determinationwere obtained at Tomari (sampling dates: September 1991and June 1993) on the western North Pacific coast, atAjigasawa (sampling date: June 1993) on the northeasternJapan Sea coast and at Sado Island (sampling date: June1993) in the Japan Sea (Table 1). The Tomari samplinglocation was in the vicinity of the then-future nuclear fuelreprocessing plant and other nuclear fuel facilities inRokkasho (Fig. 1). The samples were collected from theend of a pier by using an acid-washed plastic bucket. Theunfiltered water samples were transferred into 20-L plasticcontainers and then acidified to pH of about 2 withhydrochloric acid. The containers were transferred to aland-based laboratory for further treatment. Afteradding 3 g of Fe3+ carrier and a known amount of242Pu, and leaving the samples to stand overnight, thesolution was neutralized with ammonia solution to formiron hydroxide precipitate (Nakanishi et al., 1995). Theiron hydroxide precipitate was allowed to settle and then itwas separated from most of the supernatant water bydecantation.

The analytical procedure for Pu isotopes in seawater wasessentially the same as that described previously (Yamadaet al., 1996, 2007). Briefly, after extracting most of the ironcarrier into isopropyl ether, Pu was purified three times byanion-exchange chromatography (Bio-Rad, AG 1-� 8,100–200 mesh). The purified Pu was electrodeposited ontoa stainless-steel disc (25mm in diameter). The activities ofPu isotopes were measured with a spectrometers equippedwith passivated ion implanted silicon detectors and amultichannel analyzer (Yamada and Aono, 2002). Afterdetermining 239+240Pu activities by a-spectrometry, Pu onthe stainless-steel disc was extracted with HNO3 and HF.Pu isotopes in the extracted solution were further purifiedby anion-exchange chromatography and finally dissolvedin 4% nitric acid (Yamada et al., 2006, 2007).

Table 1239+240Pu activities and 240Pu/239Pu atom ratios in surface waters of the weste

Sample code Sampling location Sampling date (YYMM) Sam

TO-91 Tomari 9109 132

TO-93 Tomari 9306 190

SA-93 Sado 9306 200

AJ-93 Ajigasawa 9306 200

aThe uncertainties quoted for 239+240Pu activities are 1s values derived frombThe uncertainties quoted for Pu atom ratios are 1 standard deviation (tota

2.2. Determination of Pu isotopes by SF-ICP-MS

Pu isotopes were measured with a double-focusing SF-ICP-MS (Element 2, Thermo Electron), which wasequipped with a guard electrode to eliminate secondarydischarge in the plasma and to enhance overall sensitivity.The SF-ICP-MS was used in the low-resolution mode inorder to get the maximum instrument sensitivity. AnAPEX-Q high-efficiency sample introduction system (Ele-mental Scientific Inc.) with a membrane desolvation unitwas used for sample introduction. Additionally, the normalskimmer cone was replaced by a high-efficiency cone (X-cone, Thermo Electron), further increasing sensitivity ofSF-ICP-MS. All the measurements were made in the self-aspirating mode to reduce the risk of contamination by theperistaltic pump tubing. Details about optimization foroperation conditions were described elsewhere (Zheng andYamada, 2006a, 2007). The Pu isotope standard solution(NBS-947) with a known 240Pu/239Pu atom ratio was usedto correct the mass bias of the instrument. The detectionlimit of 0.14 fgmL�1 Pu was obtained (Zheng and

rn North Pacific and Japan Sea

ple volume (L) 239+240Pua (mBqm�3) 240Pu/239Pub (atom ratio)

7.670.7 0.23170.022

7.870.3 0.23570.023

4.970.3 0.22170.029

5.970.3 0.22170.019

a counting statistics.

l scans of 51; 17 runs� 3 passes) from the ICP-MS measurements.

ARTICLE IN PRESS

130°°E 140°E 150°E120°E

40°N

30°N

50°N

0.204±0.034

0.225±0.037

0.225±0.030

0.220±0.042

Japan

Russia

China

Korea

0.231±0.0220.235±0.023

0.221±0.019

0.221±0.029

0.247(0.19-0.33)

0.260(0.21-0.31)

0.243(0.18-0.33)

0.224±0.014

Pacific OceanEast China

Sea

Japan Sea

Sea of

Okhotsk

JSYS

TS

Fig. 2. Map showing the 240Pu/239Pu atom ratios in surface waters. The

black squares are the sampling locations of Norisuye et al. (2006) and the

open square is the sampling location of Yamada et al. (2007). The grey

shadings indicate the areas studied by Kim et al. (2004) in the

southwestern Japan Sea (JS), Tsushima Strait (TS) and Yellow Sea

(YS). Numbers in parentheses given under a mean 240Pu/239Pu atom ratio

represent the range of the 240Pu/239Pu atom ratio in the JS, TS and YS

(Kim et al., 2004).

M. Yamada, J. Zheng / Applied Radiation and Isotopes 66 (2008) 103–107 105

Yamada, 2006a). The concentrations of 239Pu and 240Puwere calculated by the isotope dilution method using a242Pu yield tracer. The relative standard deviations for thedetermination of 240Pu/239Pu atom ratios in surfaceseawater samples ranged from 8.4% to 12.4%(10.271.6% on average).

3. Results and discussion

3.1. 239+240Pu activities

Analytical results for 239+240Pu activities and240Pu/239Pu atom ratios are given in Table 1. The239+240Pu activities in the surface water obtained for 1991and 1993 at Tomari were 7.6–7.8mBqm�3. Tomari islocated in Rokkasho, on the Pacific coast of AomoriPrefecture, Japan (Fig. 1). A nuclear fuel reprocessingplant, a uranium enrichment plant and an undergrounddisposal facility for low-level radioactive waste are locatedin Rokkasho. Kondo et al. (2003) reported that thedetected 239+240Pu activities obtained for 1999–2000 incoastal waters near the future release site for the dischargewater of the reprocessing plant ranged from 4.6 to8.8mBqm�3. Misonoh et al. (2007) reported that therange in surface water obtained for 1991–2004 off thePacific coast of Aomori Prefecture had altered little, with amean value of 573mBqm�3. However, Shima et al. (2006)suggested that three water masses, the subarctic OyashioCurrent (OC), the subtropical Kuroshio Current (KC) andthe Tsugaru Warm Current (TWC), coexisted in thesurface layer off the Pacific coast of Aomori Prefectureand the mean 239+240Pu activity was clearly different foreach water mass; this was especially apparent whencomparing the TWC and OC.

The 239+240Pu activities in the surface water obtained for1993 at Sado Island and Ajigasawa on the Japan Sea coastwere 4.9–5.9mBqm�3. Hirose et al. (2002) reported thatthe mean value of 239+240Pu activity in surface water of theJapan Sea obtained for 1986–1997 was 5.6mBqm�3 andactivities ranged between 2.1 and 14.0mBqm�3. They alsosuggested that the 239+240Pu activities in surface waters ofthe Japan Sea, including historical data, showed notemporal variation from 1977 to 1997. The 239+240Puactivities in coastal surface waters from the western NorthPacific Ocean and Japan Sea in the present study were onthe same level as the previously reported values by Hiroseet al. (2002), Kondo et al. (2003), Shima et al. (2006) andMisonoh et al. (2007).

3.2. 240Pu/239Pu atom ratios

The atom ratio of 240Pu/239Pu at Tomari on the Pacificcoast was 0.23170.022 for 1991 and 0.23570.023 for 1993and showed no notable variation between 1991 and 1993(Table 1). Okubo et al. (2007) also observed a range of240Pu/239Pu atom ratios between 0.196 and 0.239 insuspended particles collected off the Pacific coast of

Aomori Prefecture. The atom ratio of 0.221 for the JapanSea coast was not significantly different from that for thePacific coast (Table 1). The atom ratios in the internalorgans of Surume-squid (Todarodes Pacificus) collected inthe Japan Sea were observed to range between0.1970.025–0.2470.028 (Kishimoto et al., 2002) and0.23070.003–0.23770.002 (Oikawa and Yamamoto,2007). The reported atom ratios of 240Pu/239Pu in surfacewater from the western North Pacific Ocean and itsmarginal seas are compiled in Fig. 2. The atom ratios forthe Pacific coast, near the Rokkasho nuclear fuel reproces-sing plant, were approximately the same as the0.22570.037 ratio obtained from the western North Pacific(Norisuye et al., 2006) and the 0.22470.014 ratio obtainedfrom Sagami Bay, western North Pacific margin (Yamadaet al., 2007). The atom ratios from the Japan Sea coast inthe present study were also similar to that from the centralJapan Sea (Norisuye et al., 2006). Kim et al. (2004)reported a range of 240Pu/239Pu atom ratios between 0.19and 0.33 with a mean value of 0.247 from the southwesternJapan Sea (JS), between 0.21 and 0.31 with a mean value of0.260 from the Tsushima Straight (TS) and between 0.18and 0.33 with a mean value of 0.243 from the Yellow Sea(YS). The atom ratios from the Japan Sea in the presentstudy and Norisuye et al. (2006) were lower than the meanvalues from JS, TS and YS by Kim et al. The atom ratiofrom the Sea of Okhotsk was the lowest among them.The atom ratio of 240Pu/239Pu is useful to identify the

sources of plutonium and to understand the transportprocess in the ocean. The atom ratio depends on thespecific weapons design and test yield (Warneke et al.,

ARTICLE IN PRESSM. Yamada, J. Zheng / Applied Radiation and Isotopes 66 (2008) 103–107106

2002). The atom ratios of 240Pu/239Pu from westernEuropean nuclear fuel reprocessing facilities were observedto be 0.3470.03 in the effluent from the Cap la Hague(Oughton et al., 1999) and in the range of 0.05–0.25 insediment cores collected from a site close to the Sellafieldoutfall (Kershaw et al., 1995b). Two high-energy thermo-nuclear detonations were conducted at the PPG in theMarshall Islands: at Enewetak Atoll in 1952 and at BikiniAtoll in 1954. The 240Pu/239Pu atom ratios were reported tobe 0.33870.051 in Bikini Island soil (Komura et al., 1984)and 0.06570.004 in Runit Island (Enewetak Atoll) soil(Muramatsu et al., 2001). The mean atom ratios of240Pu/239Pu from global stratospheric fallout were0.17670.014 (Krey et al., 1976) or 0.18070.014 (Kelleyet al., 1999) based on soil samples data. Muramatsu et al.(2003) also reported 240Pu/239Pu atom ratios between 0.155and 0.194 with a mean value of 0.18070.011 in Japanesesoil samples. Sediment core records of 240Pu/239Pu atomratios were well investigated recently in the western NorthPacific Ocean and its marginal seas (Kim et al., 2003; Lee etal., 2004, 2005; Zheng and Yamada, 2004, 2005, 2006b, c;Wang and Yamada, 2005) and these studies have clarifiedthe existence of close-in fallout plutonium originating fromthe PPG.

The atom ratios of 240Pu/239Pu in coastal surfaceseawaters from the western North Pacific and Japan Seain the present study were significantly higher than the meanglobal fallout ratio of 0.176 7 0.014 (Krey et al., 1976) or0.18070.014 (Kelley et al., 1999). Komura et al. (1984)reported the 240Pu/239Pu atom ratio of 0.31870.033 inhemp-palm leaves of fishing floats which were exposed tofallout from the Bravo shot in 1954. Airborne debris fromthe Mike shot in 1952 had the 240Pu/239Pu atom ratio of0.36370.004 (Diamond et al., 1960). These high atomratios in the present study may prove the existence of close-in fallout plutonium originating from the PPG. Kim et al.(2004) observed higher atom ratios with a mean value of0.2570.03 in seawaters around the Korean Peninsula andsuggested that the Chernobyl accident fallout, the dis-charges from nuclear reprocessing plants and radioactivewaste dumping by the former Soviet Union and RussianFederation were not the potential sources of Pu in the seaareas adjacent to the Korean Peninsula. The NorthEquatorial Current and Kuroshio Current were proposedas pathways for transporting PPG Pu to the western NorthPacific and its adjacent seas (Kim et al., 2004; Lee et al.,2004; Zheng and Yamada, 2004, 2005; Wang and Yamada,2005; Yamada et al., 2006). The Kuroshio Currentbranches off south of Kyushu Island and the TsushimaCurrent flows northeastward passing the Tsushima Strait.The Tsushima Current may continuously deliver close-infallout Pu from the PPG to the northern Japan Sea (Zhengand Yamada, 2005). The Tsushima Current splits off theJapan Sea coast of Aomori Prefecture into two flows andone flow, called the Tsugaru Warm Current, continues offthe Pacific coast of Aomori Prefecture after passingthrough the Tsugaru Strait. The two end-member mixing

model proposed by Krey et al. (1976) could be used toevaluate the relative contributions of the global strato-spheric fallout and the PPG close-in fallout. The240Pu/239Pu atom ratio of 0.18 from the global strato-spheric fallout (Krey et al., 1976; Kelley et al., 1999), thatof 0.363 from the PPG close-in fallout (Diamond et al.,1960) and the mean atom ratio of 0.22770.006 in thepresent study were used for the calculation. The contribu-tion of the PPG close-in fallout was calculated to be 33%on average. The remaining 67% represented the contribu-tion from the global stratospheric fallout.

4. Conclusions

The atom ratios of 240Pu/239Pu in coastal surfaceseawater, ranging from 0.22170.019 to 0.23570.023, weresignificantly higher than the mean global fallout ratio of0.18. The contribution of the PPG close-in fallout Pu wasestimated to be 33% using a two end-member model. Itwas proposed that the oceanic currents accounted fordelivery of close-in Pu from the PPG to the studied areas.239Pu and 240Pu derived from the two sources of globalfallout and close-in fallout were homogenized in thesurface water of the Pacific coast and Japan Sea coast.Data on 240Pu/239Pu atom ratios in seawater samplescollected in the vicinity of the Rokkasho nuclear fuelreprocessing plant will provide useful keys for under-standing the process controlling plutonium transport andfor distinguishing potential sources of Pu.

Acknowledgements

The authors wish to express their gratitude to T. Aono,K. Nakamura, S. Hirano, R. Nakamura and T. Nakaharafor their help in sample collection. T. Nakanishi is thankedfor preparing the sampling location map.

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