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Journal of Environmental Radioactivity 111 (2012) 100e108

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate / jenvrad

Distribution of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plantsimulated numerically by a regional ocean model

Daisuke Tsumune a,*, Takaki Tsubono a, Michio Aoyama b, Katsumi Hirose c

a Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba-ken 270-1194, JapanbGeochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba-shi, Ibaraki-ken 305-0052, Japanc Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 101-8554, Japan

a r t i c l e i n f o

Article history:Received 31 July 2011Received in revised form1 October 2011Accepted 11 October 2011Available online 8 November 2011

Keywords:Fukushima reactor accidentRegional ocean modelRelease rate137Cs131I/137Cs activity ratio

* Corresponding author. Tel.: þ81 4 7182 1181; fax:E-mail address: tsumune@criepi.denken.or.jp (D. T

0265-931X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jenvrad.2011.10.007

a b s t r a c t

Radioactive materials were released to the environment from the Fukushima Dai-ichi Nuclear PowerPlant as a result of the reactor accident after the Tohoku earthquake and tsunami of 11 March 2011. Themeasured 137Cs concentration in a seawater sample near the Fukushima Dai-ichi Nuclear Power Plant sitereached 68 kBq L�1 (6.8� 104 Bq L�1) on 6 April. The two major likely pathways from the accident site tothe ocean existed: direct release of high radioactive liquid wastes to the ocean and the deposition ofairborne radioactivity to the ocean surface. By analysis of the 131I/137Cs activity ratio, we determined thatdirect release from the site contributed more to the measured 137Cs concentration than atmosphericdeposition did.

We then used a regional ocean model to simulate the 137Cs concentrations resulting from the directrelease to the ocean off Fukushima and found that from March 26 to the end of May the total amount of137Cs directly released was 3.5� 0.7 PBq ((3.5� 0.7)� 1015 Bq). The simulated temporal change in 137Csconcentrations near the Fukushima Daini Nuclear Power Plant site agreed well with observations. Oursimulation results showed that (1) the released 137Cs advected southward along the coast during thesimulation period; (2) the eastward-flowing Kuroshio and its extension transported 137C during May2011; and (3) 137Cs concentrations decreased to less than 10 Bq L�1 by the end of May 2011 in the wholesimulation domain as a result of oceanic advection and diffusion.

We compared the total amount and concentration of 137Cs released from the Fukushima Dai-ichireactors to the ocean with the 137Cs released to the ocean by global fallout. Even though the measured137Cs concentration from the Fukushima accident was the highest recorded, the total released amount of137Cs was not very large. Therefore, the effect of 137Cs released from the Fukushima Dai-ichi reactors onconcentration in the whole North Pacific was smaller than that of past release events such as globalfallout, and the amount of 137Cs expected to reach other oceanic basins is negligible comparing with thepast radioactive input.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Radioactive materials were released to the environment fromthe Fukushima Dai-ichi Nuclear Power Plant (1F NPP), operated bythe Tokyo Electric Power Company (TEPCO), as a result of thereactor accident after the Tohoku earthquake and tsunami on 11March 2011. The total estimated release amounts of 131I and 137Csfrom the 1F NPP reactors (No. 1, No. 2 and No. 3) to the atmospherewere 160 PBq (1.6�1017 Bq) and 15 PBq (1.5�1016 Bq), respec-tively (Nuclear Emergency Response Headquarters, Government of

þ81 4 7183 2966.sumune).

All rights reserved.

Japan, NERH, 2011). There are several potential pathways by whichthese materials might reach the ocean (Table 1). Some of theradioactive materials released to the atmosphere were introducedinto the ocean by wet and dry atmospheric deposition. The totalestimated amount of 131I, 134Cs, and 137Cs released directly to theocean during 1e6 April, observed near the water intake of the 1FNPP No. 2 reactor, was 4.7 PBq (4.7�1015 Bq), with 137Csaccounting for 0.94 PBq (9.4�1014 Bq) (NERH, 2011). In addition,an estimated 20 TBq (2.0�1013 Bq) of radioactive materials werereleased near the water intake of No. 3 reactor from 2:00 on 10Mayto 19:00 on 11 May. The radioactivity in a planned release of low-level waste water was 42 GBq (4.2�1010 Bq). Thus, the directrelease from 1F NPP No .2 reactor was the largest among thesethree direct release pathways from the 1F NPP reactors to the

Table 1Potential release routes from the 1F NPP reactors to the ocean.

Potential release route This study Simulated by other studies

Atmospheric deposition Ignored (Discussed) Toulouse University,2011; JAEA, 2011

Direct release Simulated Toulouse University,2011; JAEA, 2011;JAMSTEC, 2011

Groundwater discharge IgnoredFreshwater runoff from

the 1F NPP siteIgnored

River runoff IgnoredPlanned release of

low-level waste waterIgnored NERH, 2011; JAEA, 2011

Fig. 1. Observed points off Fukushima coast (by MEXT and TEPCO). 5e6 dischargecanal and south discharge canal are at the north and south of 1F NPP site, respectively.Circle shows the 30 km distance from 1F NPP.

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108 101

ocean. The estimates of the release rate from the 1F NPP No. 2reactor to the ocean, however, have considerable uncertaintybecause they are not taken measured concentrations of radioactivematerials in the ocean into account.

To distinguish the contributions of atmospheric deposition anddirect release to the oceanic radioactive contamination, we exam-ined 131I/137Cs observed data. Although freshwater runoff(including both groundwater discharge and river runoff) of radio-active materials deposited at the 1F NPP site and subsequentlywashed out to sea by precipitation and drainage is another possiblerelease pathway to the ocean, we ignored it in this study under theassumption that its contribution was small. In fact, 137Cs depositedon land surface is difficult to be leached out by river runoff (Hiroseet al., 1990). More detailed estimates may need to consider otherpotential release routes. We also ignored the planned release oflow-level radioactive wastewater in this study because the amountof radioactivity was far smaller than the estimated direct releasefrom the 1F NPP No. 2 reactor.

Because it is more difficult tomake observations from ships thanto make similar observations on land, the available observations ofthe released radioactive materials in the ocean are too few to char-acterize the behavior of thosematerials in the ocean. Oceanicmodelsimulations can interpolate andextrapolate sparse observations andthus can help us understand the oceanic behavior of radioactivematerials (i.e., Tsumune et al., 2003a,b, 2011). In addition, oceancirculation model simulations can provide useful information thatallows efficient planning of future observations. Several institutionsand agencies have performed simulations of the behavior of radio-active materials in the ocean, including the Japan Agency forMarine-Earth Science and Technology (JAMSTEC, 2011) and theJapan Atomic Energy Agency (JAEA, 2011) , and Toulouse University,France (Toulouse University, 2011). Intercomparison betweensimulation results obtained by multiple models will be useful tohave better understanding of the behavior of oceanic tracer andimprove ocean modeling. Moreover, by comparing observedconcentrations of a radioactive tracer material with simulatedresults it is possible to assess the skill of an ocean circulationmodel.An accurate estimation of the release rate of 137Cs from the 1F NPPreactors will be useful for intercomparison of models used forsimulating the behavior of radioactive materials in the ocean.

In this paper, we describe the observation data and the regionalocean model in Section 2. In Section 3 (Results and Discussion), weanalyze the observed data for 131I and 137Cs to determine theamounts released directly and by atmospheric deposition. We alsoestimate the amount of 137Cs released directly from the 1F NPP No.2 reactor by comparing the observed data with simulated results.In addition, we simulate the behavior of 137Cs released from the 1FNPP No. 2 reactor off Fukushima to understand the distributionof oceanic contamination. We verify the simulated result bycomparison with the observed data. Finally, we compare the

amount of oceanic contamination with 137Cs between theFukushima accident and past radioactive contamination events dueto atmospheric weapons tests since 1945, the Chernobyl accident in1986, and releases from the British Nuclear Fuels, Ltd (BNFL)reprocessing site at Sellafield, UK since 1952.

2. Material and methods

2.1. Observed data

TEPCO has observed radioactivity in seawater near shore atseveral sites, the discharge canal for 5 and 6 reactors (5e6discharge canal) at the north of 1F NPP site and the discharge canalfor 1, 2, 3 and 4 reactors (south discharge canal) at the south of the1F NPP site, the north discharge canal at the Fukushima DainiNuclear Power Plant (2F NPP) site (10 km south of the 1F NPP site),and off Iwasawa (16 km south of the 1F NPP site), since 21 March2011 (TEPCO, 2011a). MEXT has observed the concentrations of 131I,134Cs, and 137Cs at eight points 30 km off Fukushima Prefecturefrom 23 March to 8 May (MEXT, 2011).

The number of sampling points observed by MEXT and TEPCOhas increased since mid-April, in addition Fukushima Prefecturesince themid of May. However it must be noted that detection limitof 137Cs in seawater is high (9 Bq L�1) and uncertainties of radioac-tivity measurements are not reported. Here we analyzed the MEXTand TEPCO observed data for 131I and 137Cs for the period from lateMarch to the end ofMay 2011. Fig.1 shows the observed points usedin this study. TEPCO andMEXT have been observing radioactivity inseawater at these sites since long before the Fukushima accident justbefore a start of operation of the 1F NPP in 1970.

2.2. Model

We employed the Regional Ocean Modeling System (ROMS;Shchepetkin and McWilliams, 2005) to simulate the behavior of137Cs released from the 1F NPP reactors off Fukushima. The ROMS isa three-dimensional Boussinesq free-surface ocean circulationmodel formulated using terrain-following coordinates. The modeldomain in this study covered the oceanic area off Fukushima(35�200Ne38�400N, 140�200Ee142�400E). The horizontal resolutionwas 1 km in both zonal and meridional directions with 289 meshes

Fig. 2. 137Cs concentration at the 5e6 and south discharge canals near 1F NPP site.

Fig. 3. 137Cs concentrations near the 2F NPP site: at the 2F north discharge canal(10 km south of the 1F NPP site) and off Iwasawa (16 km south of the 1F NPP site).

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108102

in the zonal direction and 397 in the meridional direction.The vertical resolution of the s coordinate was 20 layers. The oceanbottom was set at 250 m depth to reduce the computer resourcesneeded for the simulation. The actual ocean depth reaches morethan 1500 m in this region.

We used a third-order upwind difference for the advectionscheme for both momentum and tracers and a fourth-ordercentered difference scheme for viscosity and diffusivity in themodel. The horizontal viscosity and diffusion coefficient were5.0 m2 s�1. The vertical viscosity and diffusion coefficient wereobtained by K-profile parameterization (Large et al., 1994). Thebackground value of the vertical viscosity and diffusion coefficientwas 10�5 m2 s�1.

The model was forced at the sea surface by wind stress and heatand freshwater fluxes, whose values were acquired by a real-timenested simulation system (NuWFAS, Hashimoto et al., 2010) ofthe Weather Research and Forecasting Model (Skamarock et al.,2008), a global spectral model used for numerical weatherprediction modeling by the Japan Meteorological Agency (JMA).The horizontal resolution of the systemwas 5 km in both zonal andmeridional directions. The time step of output from the real-timesimulation system was 1 h.

Horizontal current, temperature, salinity and sea surface heightwere restored to the Real-time 1/12� Global HYCOM (HYbridCoordinate Ocean Model) Nowcast/Forecast System results(Chassignet et al., 2006) along the open boundary during simula-tion period. Temperature and salinity were nudged to the HYCOMreanalysis results to represent the mesoscale eddy during simula-tion period. The nudging parameter was 1 day�1. Tidal effects wereset by the Global Inverse Tide Model (TPX, 2011). The simulationperiod was from 1 March to 31 May 2011. The initial conditions oftemperature, salinity, horizontal current velocity, and sea surfaceheight were set by the HYCOM reanalysis output.

137Cs is modeled as a passive tracer that advects and diffusesinto the ocean interior, and the 137Cs concentration in seawaterdecreases by radioactive decay with a half-life of 30 years. Thedecay effect was negligible during the simulation period of a fewmonths used in this study.

3. Results and discussion

3.1. Observed data

3.1.1. Temporal changes in 137Cs concentrationsWe estimated the behavior of 137Cs in the ocean by examining

temporal changes in 137Cs concentrations in the 5e6 and southdischarge canals at the 1F NPP site (Fig. 2), although the directrelease did not occur at these discharge canals. The 137Cs concen-trations in the discharge canals increased to more than 10 kBq L�1

(1.0�104 Bq L�1) from 21 to 31 March 2011. The 137Cs concentra-tion peaks occurred on 30 March (47 kBq L�1) and 6 April(68 kBqoL�1). Then, The 137Cs concentrations decreased from 6April to late April, because TEPCO stopped the visible direct leakfrom reactor 2 with an injection of sodium silicate on 6 April 2011.The 137Cs concentrations in near shore waters have remainedapproximately constant at 100 Bq L�1 since late April to the end ofMay 2011, which might be supported by continuous leak of smalleramount of radioactive waste water.

At the 2F NPP north discharge canal (10 km south of the 1F NPPsite) and off Iwasawa (16 km south of the 1F NPP site) near the 2FNPP site, 137Cs concentrations ranged from 10 to 100 Bq L�1 during21e26 March 2011 (Fig. 3). On 27 March, they increased rapidly to570 Bq L�1, and then abruptly decreased again. They then increasedrapidly again to 1.4 kBq L�1 on 5 April, after which they decreased,reaching concentrations on the order of 10 Bq L�1 by the end of

April 2011. The 137Cs concentrations ranged from 10 to 40 Bq L�1

during May 2011.Surface-water 137Cs concentrations at the eight points 30 km

offshore of Fukushima (Fig. 1) ranged from 1 to 30 Bq L�1 (Fig. 4).They were less than 10 Bq L�1 before 5 April 2011, increased to190 Bq L�1 by mid-April, and then decreased.

3.1.2. 131I/137Cs activity ratioWe analyzed the 131I/137Cs activity ratio to differentiate the

release pathways of radioactive materials for 137Cs observed databecause the 131I/137Cs activity ratio should not change during theoceanic transport due to weak interaction of both 131I and 137Cswith biogenic particles. On the other hand, the ratio might changedue to difference of depositional behaviors between 131I and 137Csduring atmospheric transport. Both 131I and 137Cs exist in water indissolved forms and their behavior follows that of the water mass.Therefore, during oceanic transport, the 131I/137Cs activity ratioshould decrease in accordance with the decay of 131I, which hasa half-life of 8 days. The decay of 137Cs, with a half-life of 30 years, isnegligible on the timescale of this study of several months. On theother hand, 131I and 137Cs were dominantly released to the atmo-sphere from the 1F NPP reactors at very high temperature ingaseous and particle forms. During transport in the atmosphere atnormal temperature, the released 131I existed as a gas and as smallparticles less than 1 mm in size, whereas the released 137Cs existedas larger particles of 1e2 mm (Masumoto et al., 2011). Because thewet deposition rate depends on the size of particles (Aoyama,1999;Hirose et al., 1993), the 131I/137Cs activity ratio increases ordecreases during atmospheric transport (Igarashi et al., 2011). Asa result, the temporal changes in 131I/137Cs activity ratios do notfollow the expected decay curve for 131I. Therefore, the 131I/137Csactivity ratio is a useful tool for determining whether 137Cs

Fig. 6. 131I/137Cs activity ratios near the 2F NPP site: at the 2F north discharge canal(10 km south of 1F NPP) and off Iwasawa (16 km south of 1F NPP) compared with thedecay curve for a half-life for a 131I/137Cs activity ratio of 5.7 on 26 March 2011.

Fig. 4. Surface 137Cs concentrations 30 km offshore from Fukushima coast. Pointnumbers (1e8) show the observed points in Fig. 1.

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108 103

concentrations in the ocean result from direct release into theocean or atmospheric deposition.

We compared the temporal changes in the 131I/137Cs activityratios at the 5e6 and south discharge canals at the 1F NPP site(Fig. 5) with the radioactive decay curve of 131I for an 131I/137Csactivity ratio of 5.7 on 26March 2011 (black line), which is the ratiomeasured in a puddle of water in the basement of the 1F NPPreactor 2 turbine building on 26March (TEPCO, 2011b). This puddlewas estimated to be a direct release source. From 26 March to 6April, the ratios agreed with the radioactive decay curve of 131I,which suggests that the observed 137Cs concentrations during thatperiod originated as direct releases from the 1F NPP reactors. It alsosuggests that the scavenging behavior of 137Cs in seawater wassimilar to that of 131I. Scavenging rate of 131I is very small becauseinteraction between 131I and oceanic particles is generally weak(IAEA, 2004). Therefore the effects of scavenging on 131I and 137Csconcentrations were negligible during this period.

The 131I/137Cs activity ratios before 25 March were scattered anddo not fall on the decay curve; therefore, before that date, theobserved 137Cs concentrations originated from atmospheric depo-sition. The contribution to the 137Cs concentration from atmo-spheric deposition was less than that from direct release, however,until mid-April. The 131I/137Cs activity ratios were apparentlyconstant at 0.1 from mid-May, but ratios of less than 0.05 were notreported because the 131I concentration decreased to below thedetection limit in mid-May.

We also compared temporal changes in the 131I/137Cs activityratio at the 2F NPP north discharge canal and off the Iwasawa coastnear the 2F NPP site with the radioactive decay curve of 131I fora 131I/137Cs activity ratio of 5.7 on 26 March 2011 (Fig. 6). Theactivity ratios before 27 March did not agree with the decay curve,

Fig. 5. 131I/137Cs activity ratios at the 5e6 and south discharge canals near FukushimaDai-ichi compared with the decay curve for a half-life for a 131I/137Cs ratio of 5.7 on 26March 2011.

suggesting that the surface-water 137Cs concentrations observedbefore that date originated from atmospheric deposition. It alsosuggests that the 137Cs concentration peak of 500 Bq L�1 observedon 27 March 2011 (Fig. 3) originated from 137Cs directly released tothe ocean and transported from the 1F NPP site, because the131I/137Cs activity ratio on that date was plotted on the radioactivedecay curve of 131I. Here as well, the contribution of atmosphericdeposition to the oceanic 137Cs concentration was lower than thedirect release contribution until mid-April 2011.

We then compared the temporal changes in surface 131I/137Csactivity ratios 30 km offshore of Fukushima at sampling sites 1e8(locations shown in Fig. 1) with the 8-day half-life decay curve fora 131I/137Cs activity ratio of 5.7 on 26March 2011 (Fig. 7).We inferredthat the 137Cs concentrations observed before 7 April originatedfrom atmospheric deposition because the 131I/137Cs activity ratioswere scattered and did not plot on the decay curve. In contrast, weinferred that the 137Cs concentrations in surface water observedduringmid-April originated from direct release because the activityratios were plotted on the decay curve. Directly released 137Cs wasthus transported by ocean currents and diffusion to 30 km offshoreby 9 April. The highest observed 137Cs concentrations in seawater(Fig. 4), recorded in mid-April, thus correspond to 131I/137Cs activityratios that are plotted on the decay curve of 131I. Aftermid-April, the137Cs concentrations in 30 km offshore water decreased.

3.1.3. Summary of the observed data analysis resultsThe results of our analysis of 137Cs concentrations and 131I/137Cs

activity ratios suggest that direct release of 137Cs from the 1F NPPreactors occurred for 12 d, from 26 March to 6 April 2011. On the

Fig. 7. Surface-water 131I/137Cs activity ratios 30 km offshore of Fukushima comparedwith the decay curve for a half-life for a 131I/137Cs activity ratio of 5.7 on 26 March 2011.Points 1e8 represent the observed points shown in Fig. 1.

Fig. 10. Comparison of simulated 137Cs concentrations and daily mean observedconcentrations at the 5e6 (north) and south discharge canals near 1F NPP.

Fig. 8. Estimated release rates of 137Cs calculated by using observed and simulationdata.

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108104

other hand, direct release was apparently observed to occur onlyfrom 1 April, because the radioactive dose near the screen of reactor2 increased from 1.5 mSv h�1 on 1 April to over 1000 mSv h�1 on 2April (NERH, 2011). The results of our analysis of the observed datasuggest that there was another, non-visible pathway of directrelease from the 1F NPP reactor. Moreover, we inferred that the137Cs directly released from the 1F NPP reactor on 26 March wastransported to the coast near the 2F NPP site by 27 March and to30 km offshore by 9 April.

The maximum oceanic 137Cs concentration originating fromdirect release was 68 kBq L�1 near the 1F NPP site on 6 April,1.4 kBq L�1 near the 2F NPP site on 5 April, and 186 Bq L�1 30 kmoffshore on 15 April.

Surface-water 137Cs concentrations originating from atmo-spheric depositionwere detected near the 1F NPP site by 25 March,near the 2F NPP site by 26 March, and 30 km offshore by 7 April.The estimated 137Cs concentrations originating from atmosphericdeposition were less than 2 kBq L�1 near the 1F NPP site, less than100 Bq L�1 near the 2F NPP site, and less than 30 Bq L�1 30 kmoffshore, based on our analysis of observed 131I/137Cs activity ratios.

The 137Cs concentrations estimated to have originated fromdirect release were thus larger than those originating from atmo-spheric deposition.

3.2. Estimation of the release rate of 137Cs

The results of our analysis of 137Cs concentrations and 131I/137Csactivity ratios suggest that direct release of 137Cs from the 1F NPP

Fig. 9. Observed 137Cs concentrations at the 5e6 (north) and south discharge canalsnear 1F NPP and simulated 137Cs concentrations in a release mesh in front of the 1FNPP site.

reactor occurred for 12 d from 26March to 6 April 2011. The average137Cs concentration detected near the 1F NPP site was 11 kBq L�1

(1.1�104 Bq L�1). We applied a simple release scenario to estimatethe direct release rate over the entire 12 d. In this scenario, therelease of 137Cs into an area covered by amesh in front of the 1F NPPsite, to all 20 vertical layers, was simulated at a constant rate of1 Bq s�1 during the12 d from26March to 6April.We then calculateda scaling factor between the observed and simulated concentrationswith a constant release rate of 1 Bq s�1 to result in the simulatedaverage 137Cs concentrations in the mesh from 26 March to 6 Aprilbeing equal to the 137Cs concentrations observed at the 5e6 andsouth discharge canals near the 1F NPP site. The resulting scalingfactor, 2.55�109, was used to estimate the direct release rate of 137Csfrom 26 March to 6 April to be 220 TBq d�1 (2.2�1014 Bq d�1).The observed 137Cs concentrations decreased exponentially after 6April and then remained constant (Fig. 2).We therefore set the directrelease rate in the simulation to also decrease exponentially after 6April and then remain constant. Fig. 8 shows the estimated dailyrelease rate of 137Cs from 26 March to 31 May.

The simulated 137Cs concentrations in the release mesh were ingood agreement with the observed concentrations in the 5e6 andsouth discharge canal waters at the 1F NPP site (Fig. 9). The peaks in

Fig. 11. Observed and simulated 137Cs concentrations at the 2F north discharge canal(10 km south of the 1F NPP site) and off Iwasawa (16 km south of the 1F NPP site) near2F NPP.

Fig. 12. Simulated distributions of surface-water 137Cs concentrations on (a) 8 April, (b) 13 April, (c) 1 May, and (d) 24 May.

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108 105

the observed 137Cs concentrations on 30 March and 6 April werereproduced by the simulation with a constant release rate. Theconcentration simulated under a constant release rate increasedwhen the current velocity was low in the release mesh, whereas itdecreased when the current velocity was high in the release mesh.We then compared the daily mean observed concentrations in the5e6 and south discharge canal waters with the simulatedconcentrations near the 1F NPP site. The error was estimated fromthe sum of the squares of the residual errors between daily meanobserved and simulated concentrations near the 1F NPP site shownin Fig. 10. The total amount of 137Cs directly released from 26Marchto the end of May was estimated to be 3.5� 0.7 PBq.

The estimated total amount of 137Cs released from the 1F NPPreactor to the atmosphere was 15 PBq (NERH, 2011). Some portion

of that 15 PBq was deposited in the ocean. A numerical simulationhas shown that the radioactive materials released from the 1F NPPreactor to the atmosphere mainly advected eastward and spreadacross the North Pacific (Takemura et al., 2011). The estimated totalamount of atmospheric deposition in the simulation area offFukushima was therefore not very much larger than the totalamount of direct release.

3.3. Simulated temporal changes in 137Cs concentrations near the2F NPP site

We compared the temporal changes of the observed andsimulated 137Cs concentrations in the 2F NPP north discharge canaland off Iwasawa water near the 2F NPP site and found that the

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108106

simulated results were in good agreement with observations untilmid-April 2011 (Fig. 11). The observed 137Cs concentrations weredue to atmospheric deposition from 21 to 26 March because the131I/137Cs activity ratios did not fall on the decay curve of 131I(Fig. 6). Therefore, the 137Cs concentrations before 26 March werenot simulated. The 137Cs concentration attributable to directrelease was over 1000 Bq L�1 on 27March, decreased to 100 Bq L�1

by 4 April, and then increased quickly to over 1000 Bq L�1 on 5April. The current velocity in this coastal zone depends on thewind velocity. The model, which is forced by the realistic windstress by the mesoscale prediction system (NuWFAS), showed thatthe quick increase in 137Cs on 5 April was due to a change in thecoastal current velocity. Cesium-137 originating from directrelease from the 1F NPP reactor was advected southward by thecoastal current in late March. The 137Cs concentrations near the 2FNPP site decreased when the southward coastal current weakenedfrom 28 March to 4 April. The 137Cs concentration of over1000 Bq L�1 at the 2F NPP site on 5 April reflected an enhancedsouthward coastal current from 3 to 5 April. The 137Cs concentra-tions decreased after 5 April owing to reduction of the directrelease rate (Fig. 9). Simulated 137Cs concentrations were under-estimated by one order of magnitude after mid-April (Fig. 11).Simulation using a model with a resolution of 1 km� 1 km mightnot correctly represent the temporal changes in the 137Csconcentrations in seawater at observed points very close to thecoast. We plan to consider other possible release routes (Table 1) infuture simulations with a finer resolution model to determine thereason for the underestimation.

VideoS1.

3.4. Surface-water distribution of 137Cs concentrations

Fig. 12 shows the distribution of surface 137Cs concentrations on8 and 13 April and on 1 and 24 May 2011 (an animation of thechanges is available in the Supplementary material). Cesium-137released directly from the 1F NPP reactor was advected along thecoast and transported to the open ocean by a mesoscale eddy on 8April. The behavior of the released 137Cs in the open ocean wascomplex because of the mesoscale eddy. The released 137Cs wasinitially advected southward (Fig. 12a). Cesium-137 with over100 Bq L�1 was transported to the Ibakaki coast by the coastalcurrent and to the open ocean 30 km offshore by the mesoscaleeddy by 13 April (Fig. 12b). The released 137Cs was advected to theChiba and Miyagi coasts and then transported eastward by theKuroshio Current during May (Fig. 12c, d). This simulation suggeststhat the 137Cs concentration was reduced to less than 10 Bq L�1 inMay by advection and diffusion in the open ocean.

Mesoscale eddies play an important role in offshore transport inthis area, causing the behavior of the tracer to be very complex. Wecompared the observed surface 137Cs concentrations at points 1e8(Fig. 1) 30 km off Fukushima with the simulated results at point 8(Fig. 13, line). The highest simulated concentration (point 8) is ingood agreement with the highest observed 137Cs concentration(point 4) in mid-April, which suggests that the model was able tosimulate both the concentration of the transported 137Cs and thetiming of the transport to the open ocean. On the other hand,because of the complexity of the offshore transport process, theexact distribution of the tracer was not correctly simulated.The simulated results at other points (1e7) underestimated. Thus,there is room for improvement of the model.

Fig. 13. Surface-water 137Cs concentrations observed 30 km offshore at points 1e8(Fig. 1) and simulated results at point 8 (line).

3.5. Comparison with past 137Cs release events

We compared the amounts of 137Cs released to the ocean by theFukushima accident and past events (summarized in Table 2).

Cesium-137 was deposited in the ocean by global fallout due toatmospheric nuclear weapons testing after 1945. Aoyama et al.(2006) reconstructed the precise geographical distribution ofglobal 137Cs deposition as 10� �10� grid data, primarily by usingworldwide measurements of 137Cs in rain, seawater, and soil. Thedecay-corrected cumulative deposition in 1970 was 765�79 PBq((765�79)� 1015 Bq) in the Northern Hemisphere (land andocean) and 290 PBq in the North Pacific (Aoyama et al., 2006). In theNorth Pacific, the decay-corrected cumulative deposition in 2003was 150 PBq, whereas the simulated total inventory was 86 PBq(Tsumune et al., 2011). The difference between the cumulativedeposition and inventory in the North Pacific was advected to theIndian Ocean, the South Pacific, and the Arctic Ocean (Tsumuneet al., 2011). Thus, over a period of several decades, the NorthPacific can be a source region of radioactive contaminants trans-ported to other basins.

Aoyama and Hirose (2004), and update, compiled observedoceanic 137Cs concentrations in the HAM database, which contains

Table 2Comparison of input amounts to the ocean and maximum concentrations betweenpast release events and the Fukushima accident.

Event Domain Input to theocean (PBq)

Maximumconcentration(Bq L�1)

Global fallout North Pacific 290 (at 1970)a,b 0.08f

Global fallout North Pacific 150 (at 2003)a,c 0.08f

Chernobyl accident Baltic Sea e 1.0g

Sellafield release Irish Sea 41d 200h

Fukushimadirect release

3.5� 0.7 68,000i

Fukushima fallout Some portion of 15e e

Fukushima plannedrelease

0.000042e e

a Decay-corrected value for the target year.b Aoyama et al., 2006.c Tsumune et al., 2011.d Smith et al., 2003.e NERH, 2011.f Aoyama and Hirose, 2004 and updates.g Buesseler and Livingston, 1996.h Inomata et al., 2009.i TEPCO, 2011a.

D. Tsumune et al. / Journal of Environmental Radioactivity 111 (2012) 100e108 107

anthropogenic radionuclide concentration data for the Pacific,Indian, and Atlantic Oceans from 1958 to the present. Themaximum 137Cs concentration due to global fallout is 0.08 Bq L�1 inthis database.

A total amount oceanic input of 137Cs released by the Chernobylaccident was not estimated because the Chernobyl site is far fromthe ocean. The observed 137Cs concentrations in seawater increasedslightly in the North Pacific and markedly in the Baltic Sea and theBlack Sea after the Chernobyl accident. In the Baltic Sea, a high 137Csconcentration of over 1.0 Bq L�1 was observed (Buesseler andLivingston, 1996).

A total of 41 PBq 137Cs was released from the BNFL nuclear fuelreprocessing plant at Sellafield, UK to the Irish Sea from 1952 to1998 (Smith et al., 2003). The maximum amount of annual releasewas 5.2 PBq in 1975. Themaximum observed 137Cs concentration inthe Irish Sea was 200 Bq L�1 in 1974 (Inomata et al., 2009).

From the 1F NPP reactor, a total of 3.5� 0.7 PBq of 137Cs wasdirectly released to the ocean, and some portion of 15 PBq reachedthe ocean by atmospheric deposition. Thus, the total amount of137Cs released from the 1F NPP reactor was smaller than thatentering the ocean from global fallout. On 6 April 2011, a 137Csconcentration of 68 kBq L�1 (6.8� 104 Bq L�1) was detected nearthe 1F NPP site, the highest value among all of the release events(Table 2). Our simulation results suggest that the 137Cs concentra-tion was reduced to less than 10 Bq L�1 in the open ocean by theend of May. If no additional release occurs, the effect of the 137Csrelease might thus be smaller than that of global fallout in theNorth Pacific. The timescale of transport to other basins is severaldecades. The effect of the 137Cs release on other oceanic basinsmight be negligible.

4. Conclusion

We differentiated two major release pathways by analyzingobserved 131I/137Cs activity ratios, direct release and atmosphericdeposition. The direct release of radioactivity-contaminated waterstarted from the 1F NPP reactor from 26 March 2011. The largedirect release of 137Cs continued until 6 April and then decreased,but the direct release did not stop altogether until the end of May.The contribution of direct release to the observed 137Cs concen-trations was larger than that of atmospheric deposition.

The total amount of 137Cs originating from direct release wasestimated to be 3.5� 0.7 PBq by calculation of a scaling factor from

a comparison of observed 137Cs concentrations near the 1F NPPsite and simulated concentrations in a mesh in front of the 1F NPPsite. The 137Cs concentrations simulated by using the estimateddirect release rate were in good agreement with observations. Themodel showed that the rapid changes in the observed 137Csconcentration near the 2F NPP site were due to changes in thecoastal current velocity. The model also simulated the transport tothe open ocean taking into account a mesoscale eddy indicated byHYCOM reanalysis data. The simulation result indicated that the137Cs concentration in the open ocean decreased to less than10 Bq L�1 by May 2011 as a result of oceanic advection anddiffusion.

We compared the total amount and concentration of 137Csreleased from the 1F NPP reactor to the ocean with those of 137Csreleased to the ocean by past events such as global fallout.The observed 137Cs concentration from the Fukushima accidentshowed highest level within the past observations but the totalreleased amount of 137Cs was comparatively not very large. Thiscomparison suggested that the effect of 137Cs released from the 1FNPP reactor to the North Pacific was smaller than that of past eventsand is likely to be negligible in other oceanic basins. However, in thefuture, the oceanic assessment of this accident should continue tobe observed in the North Pacific even though we estimate that theeffect will be small.

Acknowledgments

We thank Motoyoshi Ikeda, Mitso Uematsu and members of theEarthquake Disaster Response Working Group of the Oceano-graphic Society of Japan for their helpful discussion. We also thankHiromaru Hirakuchi and Atsushi Hashimoto for providing theNuWFAS results. We also thank Fukiko Taguchi and Ryosuke Niwafor technical support.

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