Journal of Nuclear and Radiochemical Sciences, Vol. 15, No.1, pp. 1-6, 2015

ArticlesPu Distribution in Seawater in the Near Coastal Area off Fukushima after the Fukushima
Daiichi Nuclear Power Plant Accident
W. T. Bu,a,b J. Zheng,*,a T. Aono,a J. W. Wu,a,c K. Tagami,a S. Uchida,a Q. J. Guo,b and M.
YamadadaResearch Center for Radiation Protection, National Institute of Radiological Sciences, Anagawa 4-9-1, Inageku, Chiba 263-8555, Japanb
State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871,
P. R. ChinacState Key Laboratory of Marine Environment Science, Xiamen University, Xiamen, Fujian 361005, P. R. Chinad
Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1,
Hon-Cho, Hirosaki, Aomori 036-8564, Japan
Received February 16, 2015; Received in revised form March 24, 2015; Accepted March 25, 2015
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released large amount of radionuclides into themarine environment. Compared with the fission products, data on the distributions of Pu in the marine
environment of the western North Pacific after the accident is limited. To better understand the Pu contaminationin the marine environment after the accident, for the first time, we determined Pu isotope ratio (240Pu/ 239Pu) in
addition to 239+240Pu activity in seawater collected in the near coastal area (mostly within the 30 km zone) off the
FDNPP site. The 239+240Pu activities were 4.16-5.52 mBq/m3 and the 240Pu/ 239Pu atom ratios varied from 0.221 to
0.295. These values were compared with the baseline data for Pu distribution in the near coast seawaters before
the FDNPP accident (2008-2010). The results suggested that there is no significant Pu contamination in seawaterin the near coastal area off the FDNPP site from the accident two years after the accident.

1. Introduction
Plutonium isotopes are normally produced in nuclear reactor
fuel by neutron capture reactions. The predominant Pu isotope
is 239Pu produced by neutron capture of 238U. When a fuel element containing 239Pu is left in a reactor for any length of timefurther multiple neutron capture reactions can occur to yieldhigher Pu isotopes, such as 240Pu, 241Pu and 242Pu. In addition,
small quantities of 236Pu and 238Pu can be produced during theirradiation [1]. In Fukushima Daiichi Nuclear Power Plant(FDNPP) reactors, most of fuel used was UO2 , but in Unit 3
reactor, ca. 4% of the core loading was mixed-oxide fuelassemblies containing ~6% Pu [2]. Thus, after the FDNPP
accident in March 2011, the release of Pu isotopes has attracted
great public attention because they present a high risk for internal radiation exposure via ingestion of contaminated agricultural crops and marine products [3].
The FDNPP accident caused radioactive contamination in
the environment through the deposition of the radionuclides
released into the atmosphere as well as through the direct discharge of large amount of radioactive liquids [4,5]. In the terrestrial environment around the FDNPP site, the Pu isotopes
released from the accident were characterized by high atom
ratios of 240 Pu/ 239 Pu (0.30 – 0.33) and 241 Pu/ 239 Pu (0.103 –
0.135) [6], and were detected in various environmental samples, such as, soil, litter, dust (black substances) and aerosol
samples and the total released amounts of Pu isotopes were
estimated to be 1.0 – 3.5 × 109, 1.1 – 2.6 × 1011 Bq, and 2.9 –6.9 × 109 Bq for 239+240Pu, 241Pu and 238Pu, respectively [3,6-10].
However, the amount of Pu isotopes directly released into themarine environment remains unknown. In the high level radioactive accumulated water collected at the FDNPP after theaccident, high level radioactivities of Pu isotopes (ca. 10 -3 Bq/*Cor responding author. E-mail: jzheng@nirs.go.jp T EL :+81-043-206-4634; FAX: +81-043-255-0721mL) were detected [11]. These values were 6 to 7 orders ofmagnitudes higher than that of the seawater in the western
North Pacific. In addition, a new study on Pu isotopes inrecently deposited sediment found along rivers draining theregions of most contaminated by the inland radioactive plumes
that were dispersed over Fukushima Prefecture suggested there
was a potential sediment-borne Pu supply from Fukushimacoastal rivers to the Pacific Ocean [12]. Thus more attention
should be paid to the contamination situation of Pu isotopes in
the marine environment off Fukushima since the FDNPP accident.
After the FDNPP accident, several studies reported theactivity levels and isotopic compositions of Pu in the marine
sediments of the western North Pacific [13-17]. Bu et al. alsostudied Pu distribution in seawater samples collected 30 km off
the FDNPP site after the accident [18]. These studies suggested that the release of Pu isotopes from the FDNPP accident
into the wider marine environment could be ignored. However,no data about Pu characterization in seawater within the 30 km
zone around the FDNPP site was reported after the accident.
Numerical modeling showed that the impact of Pu from theaccident on the marine environment would mainly remain
within the 30 km zone around FDNPP site considering the low
mobility of Pu [19]. Therefore, Pu isotopes in seawater off the
FDNPP site, especially within the 30 km zone, needs to be routinely investigated. In this study we analyzed the 239+240 Pu
activities and 240Pu/239Pu atom ratios in seawater samples in thenear coastal area off the FDNPP site (mostly within the 30 kmzone) collected in May 2013, two years after the accident. A
comparison was made between the obtained results and thebaseline data of Pu information in seawater in the western
North Pacific and its marginal seas, especially the near coast
seawater before the accident (2008-2010) to identify the sourcesof Pu in the studied area and assess the possible Pu contamination to the marine environment due to the FDNPP accident.

© 2015 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 05/13/2015

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J. Nucl. Radiochem. Sci., Vol. 15, No. 1, 2015

TABLE 1: Summary of distributions of 239+240Pu activities and 240Pu/ 239Pu atom ratios in surface seawater in the western
North Pacific and its marginal seasa

Study area

GPS location

Coastal Water off Aomori Prefecture
Coastal Water off Aomori Prefecture
East China Sea
East Sea/Japan Sea
East Sea/Japan Sea
East Sea/Japan Sea
East Sea/Japan Sea
Japan Sea
Japan Sea
Sagami Bay
South China Sea
Western North PacificWestern North Pacific/Japan Sea
Western North Pacific/Japan Sea
Yellow Sea/Korea Strait/East Sea

40-42ºN, 141-142ºE
40-42ºN, 141-143ºE
30-35ºN, 120-130ºE
35-43ºN, 130-138ºE
38-42ºN, 133-135ºE
35-41ºN, 129-137ºE
35-41ºN, 129-137ºE
38-39ºN, 132-136ºE
38-45ºN, 132-141ºE
35º’N, 139º’E
1-8ºN, 102-106ºE
14-32ºN, 128-138ºE
37-41ºN, 137-142ºE
38-46ºN, 135-145ºE
33-39ºN, 124-132ºE

Sampling
date
1999-2000
2001-2005
1987
1993
1995
1993-1996
1993
1984, 1993
1986-1997
1992
2008
1996
1991-1993
1998
1999-2000

239+240

Pu activity
range (mBq/m3)
4.6-8.8
2.8-103.2-10a
6-10
8-25
3.5-10.0
3.4-20.8
6.7-9.5
3.0-14.0
13.4
2.3-7.9
1.52-1.87
4.9-7.8
ND
3.1-22.3

240

Pu/239Pu atom
Reference
ratio range
ND
280.243-0.245
29
0.199-0.246
32
ND
33
ND
34
ND
35
ND
36
0.233-0.235
37
ND
38
0.224
39
ND
40
0.199-0.224
41
0.221-0.235
42
0.204-0.225
43
0.18-0.33
22

Data that are apparently influenced by Changjiang River is not adopted there. “ND”, stands for not detected.

2. Spatial and temporal distribution of Pu in seawater inthe western North Pacific before the FDNPP accident
Before the FDNPP accident, Pu was deposited over thewestern North Pacific mainly as a result of global fallout
following the atmospheric nuclear weapons tests about 50 years
ago. Besides, the Pacific Proving Ground (PPG) close-in
fallout is another important source other than global fallout,which made the 239+240 Pu inventories in the seawater of thewestern North Pacific more than two times higher than thoseof the terrestrial environment at the same latitude [20,21]. The
PPG sourced Pu was transported by ocean current to thewestern North Pacific and made the 240Pu/ 239Pu atom ratio in
this sea area higher than that of global fallout [21-24]. Table 1
summarized the related studies on the distribution of Pu isotopes (239+249Pu activity and 240Pu/ 239Pu atom ratio) in seawaterof the western North Pacific conducted from 1984 to 2008.
Since the early 1970s, continuous investigation on the distribution of Pu activities in surface seawater showed a latitudinal
distribution, with higher concentration in the mid-latitude
region of the western North Pacific and relatively lower concentration in the equatorial region because the amount ofradioactive deposition is controlled by the precipitation amountand stratosphere-troposphere air change [25-27]. For thatreason, Pu activity in seawater of the western North Pacifictook on a large variation (i.e. 1.52-22.3 mBq/m 3 as Table 1
shows) [22, 28-43].
Besides spatial distribution, temporal distribution of Pu in
the North Pacific needs to be considered when we discuss thebaseline data of Pu in seawater because 239+240 Pu activities
slowly decreased over time especially in surface seawater.
Bowen et al. reported the 239+240Pu activities in central North
Pacific seawater collected in 1974 and 1978 [20]. The results
showed that Pu activities in the surface seawater decreased in1978 compared with 1974. Hirose et al. estimated the 239+240Pu
activities in the surface water of the Pacific ocean based on
HAM database and obtained the temporal variation of 239+240Pu
activities in the western North Pacific (25°N-40°N) from 1971
to 1998 and a clearly decrease of 239+240Pu activities with time
was observed [26]. The 239+240Pu activities in the surface seawater of the western North Pacific decreased exponentiallyover recent decades. In the early 1970s, the 239+240Pu activities
ranged from 8.1 to 35 mBq/m3, while in 2000, they have been
estimated to be 0.3-2.7 mBq/m3 in the open ocean seawaters.
Oikawa et al. determined Pu isotopes in seawater collected

off Rokkasho in northern Japan during 1991-2005 [29]. The
239+240
Pu activities in surface seawater of this region varied
from 2.8 to 10 mBq/m3 with an average of 5.3 mBq/m3. The
estimated apparent half-life of Pu in the surface seawater was
about 17.8 y at the end of the study period and it tended togradually increase because the decrease rate of 239+240 Pu in
surface seawater would probably lessen with time. Recently,
more comprehensive baseline data on 239+240Pu activities in nearcoastal seawaters collected off the sites of commercial nuclear
power plants (including FDNPP and Fukushima Daini NPP)
around the Japanese islands at 37 locations were summarized[30,31]. Based on this investigation, the 239+240Pu activity concentration in the surface water was independent of seawater
properties, such as salinity and temperature, and the 239+240Pu
activity ranged from 3.1 to 12 mBq/m3 with the average of 5.2± 1.5 mBq/m3 for the surface water samples collected during
the 3 years (2008-2010) just before the FDNPP accident.
Although 239 +240 Pu activities in surface seawater slowly
decrease with time, the 240 Pu/ 239 Pu atom ratio in seawater
seems to be invariant with time. Yamada and Zheng determined the 240Pu/239Pu atom ratios in the water column of JapanSea collected during 1984-1993, and no temporal variation was
found [24]. After adequately mixing of seawater, 240Pu/ 239Pu
atom ratio in seawater can be regarded as a consistent value if
there isn’t any other Pu source injection. As shown in Table 1,
recent studies within 10 years showed a centralized 240Pu/239Pu
atom ratio in seawater in the western North Pacific from 0.199
to 0.246 [32-43], with higher ratios (0.18-0.33) in the coastal
waters of the Korean Peninsula [22]. These Korean Peninsula
values are apparently higher than the characteristic 240Pu/239Pu
atom ratio of 0.18 for global fallout and lower than the PPG
derived Pu with a 240 Pu/ 239Pu atom ratio of 0.36 [21,44,45],
indicating a mix of Pu from these two sources in the western
North Pacific. The baseline data on 240 Pu/ 239Pu atom ratios
compiled by Oikawa et al.[30], based on a ministry report onenvironmental radioactivity investigations that found the240
Pu/ 239Pu atom ratios ranged from 0.172 to 0.304, with theaverage of 0.234 ± 0.025 for the surface water samples collected during the 3 years (2008-2010) just before the FDNPP
accident [31].
3. Materials and methods
3.1. Seawater sampling. Surface seawater (0-60 m)
samples (20 L) were collected at the six locations described inPu Distribution in Seawater in the Near Coastal Area off Fukushima

J. Nucl. Radiochem. Sci., Vol. 15, No. 1, 2015

3

TABLE 2: Pu isotopes in surface seawater in the near coastal area off Fukushima after the FDNPP accident
239+240

Pu activity(mBq/m3)

240

Pu/239Pu atom
ratio

Sample location ID

Sampling time

Water sampling depth(m)

GPS location

M01

2013/5/17

1037°33’N, 141°13’E

4.36±0.57

0.221±0.040

M012013/5/17

50

37°33’N, 141°13’E4.73±0.50

0.251±0.039

N012013/5/15

037°30’N, 141°30’E

4.23±0.48

0.295±0.048

N012013/5/15

20

37°30’N, 141°30’E

4.16±0.36

0.235±0.030

I022013/5/20

037°14’N, 141°14’E

4.23±0.34

0.242±0.037

NP1

2013/5/17

40

37°25’N, 141°11’E5.30±0.42

0.249±0.034

NP12013/5/17

60

37°25’N, 141°11’E5.52±0.43

0.293±0.038

AN62013/5/15

037°17’N, 141°05’E

4.56±0.44

0.286±0.056

AN6
NP2

2013/5/15
2013/5/17

20
20

37°17’N, 141°05’E
37°25’N, 141°06’E

4.55±0.74
4.73±0.45

0.227±0.041
0.278±0.040

Uncertainties are expressed as the expanded standard uncertainty with a coverage factor of 2.

140°40'140°50'141°00'141°10'141°20'141°30'141°40'a

37°50'

139°
40°

140°

141°

200 km39°

37°40'AN6

M01

100 kmN01
37°30'

FDNPP
37°20'

37°30'

NP2 NP1

10 km

38°

37°20'

I02

37°

37°10'

37°00'

37°00'
141°00'141°10'

P11

K06

TR23
37°

P8
F1

36°

30 km

140°50'

38°

30 km

FDNPP

20 km

37°10'

140°40'

144°
40°

b

37°50'
39°

37°40'

143°

142°

141°20'141°30'

35°
139°

TR26

36°

Tokyo Bay

140°

141°

142°

143°

35°
144°

141°40'Figure 1. Map showing the locations for (a) seawater collected in the near coastal areas around the FDNPP site and (b) seawater collected outside
the 30 km zone around the FDNPP site (redrawn from ref. 18). (The map is generated using Surfer 8.0)

Table 2 in the western North Pacific during the UM 13-05
cruise (T/S Umitaka-maru, Tokyo University of Marine
Science and Technology) in May 2013. All the sampling
locations were within the 30 km zone around the FDNPP site
except N01, which was about 38 km away. Fig. 1 shows theseawater sampling locations in this study together withlocations outside the 30 km zone off the FDNPP site whichwere investigated previously. The specific sampling date andwater depth are given in Table 2.
3.2. Seawater sample analytical procedures. An analytical method based on anion-exchange chromatography using
Dowex 1X8 resin and SF-ICP-MS for the determination of plutonium isotopes at ultra-trace level in seawater was establishedand validated in our previous work [18], and it was employedin this study to determine the 239+240Pu activities and 240Pu/239Pu
atom ratios in the collected seawater samples. This method
was characterized by a sufficiently low detection limit at subfg/mL level for 239Pu and 240Pu isotopes. About 0.57 pg 242Pu
was added to the seawater sample (pre-filtrated with an
Advantec cartridge, 0.45 µm) as a yield tracer. Fe(OH)3 wasused for initial concentration to collect Pu by co-precipitation.

After allowing the precipitate to settle, the sample supernatant
was carefully siphoning away once and then again after a centrifuging step. The Fe precipitates together with Pu were dissolved in 8 M HNO3. Then NaNO2 was added to the sample
solution and this was heated at 40°C to convert Pu to the tetravalent. Pretreated resin was packed in a 2.5 mL PTFE column
and then preconditioned with 20 mL 8 M HNO3–0.2 M NaNO2
solution. The seawater sample (which now included the tetravalent Pu) was loaded onto the column and then the column
was rinsed first with 50 mL 8 M HNO3 and then with 30 mL
10 M HCl. Then 30 mL 0.1 M NH4I–8.5 M HCl was loadedfor Pu elution. The eluent was heated to dryness after adding 5
mL conc. HNO3. Then 4 M CH3COOH was added to dissolve
the residue and the obtained solution was loaded to the second
preconditioned resin column. Solution flowing through theresin was later mixed with the eluent of 20 mL conc. HBr.
This solution was heated to dryness and 1 mL conc. HNO3 wasadded and evaporated to dryness. After that, the final residue
was dissolved in about 0.7 ml 4% HNO3 for the SF-ICP-MS
analysis. The information about the SF-ICP-MS analyticalsystem and the detailed operation setup can be seen in anotherwork [46].

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J. Nucl. Radiochem. Sci., Vol. 15, No. 1, 2015240

Pu/239Pu atom ratio in seawater

0.400
Average 240Pu/239Pu ratio:
0.254 ± 0.026

0.350
0.300
0.250
0.200

Global fallout 0.176 ± 0.014

0.150
0.100
0.050
0.000

1.0

2.0

3.0
239+240

4.0

5.0

6.0

7.0

Pu activity (mBq/m3) in seawater

Figure 2. Relationship between 239+240Pu activity (mBq/m3) and 240Pu/ 239Pu atom ratio found in the surface waters collected within the 30 km
zone (blue circles) and in the zone of 30 km – 200 km (red circles). Data for 30 km – 200 km surface water were cited from Bu et al. [18].

4. Results and Discussion
The analytical results of Pu isotopes in the near coastal
seawater samples off the FDNPP site were shown in Table 2.
The 239+240Pu activities showed small variation with range from
4.16 to 5.52 mBq/m3 in the surface seawaters collected within
30 km zone. The lower value of 4.16 mBq/m3 was detected insampling Station N01, located outside 30 km zone (ca. 38 km
from the FDNPP). The 240 Pu/ 239Pu atom ratios ranged from0.221 to 0.295, with an average of 0.254 ± 0.026. Relationshipbetween 239+240Pu activity (mBq/m3) and 240Pu/239Pu atom ratio
found in the surface waters collected within the 30 km zone
and in the zone of 30 km – 200 km was plotted in Fig. 2. Data
for 30 km – 200 km surface water were cited from Bu et al[18]. The 240Pu/ 239Pu atom ratios in the surface waters showedno significant difference and they were independent of sampling locations, and there was no clear relationship between the239+240
Pu activities and the 240Pu/239Pu atom ratios.
As summarized Table 1, there was a large variation of
239+240
Pu activities in seawater in the western North Pacific dueto the latitudinal dispersion, the influence of oceanographic
processes, and the temporal variation [27]. The 239 +240 Pu
activities in 1990s varied from 3.1 mBq/m3 to 22.3 mBq/m3. If
we suppose that the apparent half-residence time of Pu still
remained to at 17.8 y during the past two decades, from thatviewpoint, 239+240Pu activities in the seawater in 2011 could beless than 10 mBq/m3.
Compared with the 239+240Pu activities in the western North
Pacific in the 1990s, 239+240Pu after 2000 varies in a relatively
smaller range (i.e. 2.8 mBq/m 3 -10 mBq/m 3). As mentioned
before, 239 +2 4 0 Pu activities in the surface seawater slowly
decrease with time and the decrease rate is getting slower.
During 2008 to 2010, just before the FDNPP accident, 239+240Pu
activities in near coastal seawaters collected off the sites ofcommercial nuclear power plants around the Japanese islands
ranged from 3.1 to 12 mBq/m 3 with the average of 5.2 ± 1.5
mBq/m 3. These data provided reliable baseline of 239+240 Pu
activities before the accident.
The comparison of the 239+240 Pu activities and 240 Pu/ 239Pu
atom ratios in seawater before and after the accident in thewestern North Pacific and the near coastal waters was
illustrated in Fig. 3. The 239+240Pu activities of 4.16 mBq/m3 to
5.52 mBq/m3 observed in surface seawaters within the 30 km
zone off the FDNPP site are within the range of baseline data

even when we take account of the temporal distribution of
baseline data. Furthermore, these results are also comparable
to the results of Oikawa et al. (3.1-12 mBq/m 3 ) for the nearcoastal seawaters off the sites of commercial nuclear power
plants during 2008 to 2010 [30]. In a previous study [18], we
analyzed Pu isotopes in seawater samples collected in thewestern North Pacific 30 - 200 km off the FDNPP site after theaccident and found that the 239+240Pu activities were 0.43-5.59
mBq/m3. The 239+240Pu activities in the near coastal seawater
showed a smaller variation compared with that in seawater 30 200 km off the FDNPP site. This is because in the near coastal
area, the strong coastal current made the efficient mixing of Pu
of different water masses, and also the vertical mixing with Pu
originated from bottom sediment taking into account the shallow water depth of several tens meters. All the observed
239+240
Pu activities in the Fukushima seawater samples wereconsistent with the background data before the accident,
suggesting no significant extra injection of Pu into the marine
environment after the accident. In the Nuclear Regulation
Authority (NRA) report [47], there are two sampling sites close
to the FDNPP: T-1 was located 30 m north from the mouth ofthe discharge canal for reactors 5 and 6, and T-2-1 was 1.3 km
south from the discharge canal for reactors 1 to 4. In thesesampling sites, 239 + 2 4 0 Pu concentrations in seawater werereported in 2012-2014 and the range was from detection limit
to 14 mBq/m3 except 31 mBq/m3 observed at T-2-1 site on 10
April 2014. Thus most data were within the range of 239+240Pu
activity data before the accident as reported above. However,in the NRA report [47], there was no additional informationsuch as 240Pu/ 239Pu atom ratio, therefore, further discussion is
not available.
The particle scavenging and upwelling processes may haveless influence on the 240Pu/239Pu atom ratio than on the 239+240Pu
activity in seawater samples because the 240Pu/ 239Pu atom ratio
in seawater distributed homogenously with water depth and thisratio in seawater column also doesn’t vary with time [20,22,37].
Thus, compared with 239+240Pu activity, 240Pu/ 239Pu atom ratio
could provide further information for source identification.
The 240Pu/ 239Pu atom ratio in seawater in the western North
Pacific before the FDNPP accident ranged widely from 0.18 to
0.33 as presented in Table 1, suggesting a mixing of global
fallout Pu and the PPG derived Pu. More recently, the240
Pu/ 239Pu atom ratios in seawater collected off the coastal
area of Japan from 2008 to 2010 ranged from 0.173 to 0.304

Pu Distribution in Seawater in the Near Coastal Area off Fukushima

J. Nucl. Radiochem. Sci., Vol. 15, No. 1, 2015

30 km zone off FDNPP

30 km zone off FDNPPThis study

This study

Near coast of Japan (2008-2010)

Near coast of Japan (2008-2010)

WN Pacific and its marginal seas

WN Pacific and its marginal seas

1984 - 2008

0

7

14

5

1984 - 200821

28

239+240

Pu activity (mBq/m3)

0.1

0.2
240

0.3

0.4

Pu/239 Pu atom ratio

Figure 3. Comparison of 239+240Pu activities and 240Pu/ 239Pu atom ratios in seawater samples within the 30 km zone off the FDNPP site with thebackground level in near coast of Japan [30,31] and in the western North Pacific and its marginal seas (Summarized from data in Table 1).

[30,31]. Our determined 240Pu/ 239Pu atom ratios in the surface
seawater within the 30 km zone off the FDNPP after theaccident were 0.221-0.295, typically in the range of the
background data in the western North Pacific before theaccident. Oikawa et al. reported the 240Pu/ 239Pu atom ratios of
surface seawater collected off the FDNPP and Fukushima
Daini NPP (about 10 km south of the FDNPP) ranged from0.195 to 0.259 during the period of 2008 to 2010 [30]. In addition, among the studies listed in Table 1, the sample locationsin the work of Yamada and Zheng [42] and of Oikawa et al.
[29] were relatively closer to the FDNPP site than others. The
240
Pu/ 239Pu atom ratios in these two studies were 0.221-0.235,
and 0.243-0.245, respectively, similar to the data obtained inour study. These results suggested that the sources of Pu in themarine environment in the near coastal area off the FDNPP
site were the same before and after the nuclear accident. In ourprevious work [16], we determined the Pu distribution in
sediment samples collected within the 30 km zone around the
FDNPP site. The obtained isotopic ratios of Pu (240Pu/ 239Pu,
2 41
Pu/ 239 Pu) were found to be different from those of the
FDNPP accident released Pu [6,16], and showed that global
fallout and the PPG close-in fallout were still the two main
sources for Pu contamination in the sediments. Based on theseawater results obtained in the present study, we can reach thesame conclusion that there was no significant Pu contamination der ived from the F DN PP accident in the ma r ine
environment two years after the its occurrence.
5. Conclusions
In this study we analyzed the 239+240Pu activities and 240Pu/
Pu atom ratios in seawater samples collected in the nearcoastal area off the FDNPP site (mostly within the 30 km zone)
two years after the nuclear accident. The 239+240Pu activities inthe seawater samples ranged from 4.16-5.52 mBq/m3, while the240
Pu/ 239Pu atom ratios were 0.221-0.295. Our results agreed
well with the baseline data before the accident, revealing thatno significant Pu was released into the marine environment
from the FDNPP accident. Global fallout and the PPG close-in
fallout are still the two main sources for Pu contamination in239

the western North Pacific. Considering that the seawater
samples in this study were collected 2 years after the FDNPP
accident, we couldn’t conclude whether Pu was discharged into
the marine environment during the accident. This remains to
be clarified by analyzing the seawater samples collected in
April or May of 2011, the period of heaviest radioactive contamination in the ocean after the accident.
Acknowledgements. We are grateful to colleagues and thecrew aboard the T/S Umitaka-maru. In particular, our samples
were made possible by Prof. T. Ishimaru and Prof. J. Kanda.
The authors wish to acknowledge Mr. Y.Y. Ni for useful
discussion. This work was supported by the Kakenhi Grant-inAid for Scientific Research on Innovative Areas (24110004),
and partly supported by the Agency for Natural Resources andEnergy (METI), Japan.
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