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Journal of Environmental Radioactivity 111 (2012) 13e17

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Journal of Environmental Radioactivity

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

2011 Fukushima Dai-ichi nuclear power plant accident: summary of regionalradioactive deposition monitoring results

Katsumi Hirose a,b,*

aDepartment of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, JapanbGeosphere Research Institute, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan

a r t i c l e i n f o

Article history:Received 16 July 2011Received in revised form23 August 2011Accepted 5 September 2011Available online 26 November 2011

Keywords:Fukushima Dai-ichi NPPNuclear accident131I137CsDepositionSpatial distribution

* Former address: Geochemical Research DepartmInstitute, Nagamine 1-1, Tsukuba, Ibaraki 305-0052, J

E-mail address: hirose45037@mail2.accsnet.ne.jp.

0265-931X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jenvrad.2011.09.003

a b s t r a c t

After the Great East Japan Earthquake and resulting Tsunami on March 11, 2011, serious accident of theFukushima Dai-ichi Nuclear Power Plant has been occurred. Huge amounts of radionuclides werereleased in atmosphere and ocean. Japanese prefectural governments have carried out environmentalradioactivity monitoring; external dose rate, radioactivity measurements in environmental samples andothers. Since March 18, 2011, daily and monthly deposition samples were collected in 45 stationscovering Japanese Islands and radionuclides in the deposition samples were determined. We summarizeradioactive deposition data reported by Japanese Government and study the depositional behaviors ofthe Fukushima-derived radionuclides. The results revealed that Fukushima-derived radioactive clouddominantly affected in the central and eastern part of Honshu-Island, although it affected all of Japaneseland area and also western North Pacific. The temporal change of the Fukushima-derived 137Cs revealedthat the apparent atmospheric residence time of the Fukushima-derived 137Cs in sites within 300 kmfrom the Fukushima Dai-ichi NPPis about 10 d.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

On March 11, 2011, a big earthquake (The Great East JapanEarthquake), which was M 9.0 with a hypocentral region of 500 kmlong and 200 km width, attacked the northeast Honshu-island,Japan. After the big earthquake, gigantic tsunami hit east coast ofthe northeast Honshu-Island, whose wave height was more than10 m. Just after the earthquake, fission reaction in all of reactorsconstructed in the northeast Honshu Island was safely stopped.However, gigantic tsunami seriously damaged the electric systemof the Fukushima Dai-ichi Nuclear Power Plant (NPP) (37.42 �N,140.97 �E). As a result, cooling system of nuclear reactors in theFukushima Dai-ichi NPP was break down. This was a cause ofhydrogen explosions of No.1 Reactor and No. 3 Reactor buildings inthe Fukushima Dai-ichi NPP, which happened on March 12 and 14,respectively. In addition to explosions of nuclear reactors, anexplosion occurred in the No. 4 Reactor building on March 15, inwhich sources of flammable gas is unknown because spent fuel inthe No. 4 reactor in spent fuel pool appeared to have been coveredwith water. Explosion occurred in the No. 2 Reactor on March 15,

ent, Meteorological Researchapan.

All rights reserved.

which suffered to lower suppression area (RJG, 2011). On March 12early morning, atmospheric emission of radionuclides from theFukushima Dai-ichi NPP was started. Major radioactivity emissionoccurred during the period of March 15e16, 2011 (TEPCO, 2011).Especially, explosions of the Fukushima Dai-ichi NPP lead to seriousradioactivity release in the atmosphere. However, reconstruction ofexact emission history of the Fukushima-derived radionuclides inatmosphere requires further study.

Japanese Government and also prefectural governments startedemergency radioactivity monitoring to assess the environmentaleffect of radioactivity due to the Fukushima Dai-ichi NPP accident(MEXT, 2011). Results of external dose rate observed in IbarakiPrefecture revealed that radioactive plume first arrived in the KantoPlain on March 15, 2011 (IPG, 2011). As a result of local fallout bysnowfall, high radioactive contamination area spread from thereactor site to about 60 km northwest region in the FukushimaPrefecture. The effect of local fallout spread over the Kanto Plain,which include Tokyo and others. Japanese Government estimatedto be 160 PBq of 131I and 15 PBq of 137Cs as total atmosphericreleases of radioactivity from the Fukushima Dai-ichi NPP (RJG,2011), which are about one order of magnitude less than theChernobyl accident (IAEA, 1986; UNSCEAR, 2000). Measurementsof radionuclides in daily deposition samples, which include wetand dry depositions, started in 18 March (MEXT, 2011). Highradioactive deposition derived from the Fukushima Dai-ichi NPP

K. Hirose / Journal of Environmental Radioactivity 111 (2012) 13e1714

accident started in wide area of the Kanto Plain and south Tohokuarea from March 21e23, 2011.

In this report, we summarize regional distributions andtemporal variations of Fukushima-derived radionuclides in depo-sition samples, which were mainly determined by prefecturalgovernments and discuss depositional behaviors of the Fukushima-derived radioactivity.

2. Sampling and method

Daily and monthly deposition samples (rainwater and fallingdust) were collected by rainwater samplers with surface areas of0.05 m2 and 0.5 m2, respectively, which were usually installed onthe roof of main monitoring building in each monitoring site. Dailydeposition sampling in 45 sites covering Japanese Island was star-ted on March 18, 2011. Daily rainwater and/or falling dust sampleswas collected in every 9 o’clock. The falling dust was gathered bypure water. The water sample was transferred into a measuringbottle (100 ml or 2 L) and sealed by plastic film. The bottle wassubjected by gamma spectrometry. Counting timewas generally 6 hdue to emergency measurements. Detection limits were rangedfrom 0.5 Bq to 10 Bq per bottle. Higher detection limit in the sitesaffected by radioactive plume was caused by contamination ofdetectors. The counting error was not described in the MEXTdocument. Therefore, the data is given as an order estimation ofradioactive fallout at emergency. We cited Fukushima-derived

Fig. 1. Temporal changes of daily 131I and 137Cs deposition observed in the Kanto Plain. A: Hit139.70 �E). Dotted line shows daily precipitation. The value described as ND (not detected)

radionuclide data in several monthly deposition samplescollected in the other NPP sites (Satsuma-Sendai (KPG, 2011),Higashi-dori (APG, 2011) and Iwanai (HG, 2011)), and JapanRadioactivity Survey sites (MEXT, 2011) including Fukui (FERC,2011) and Fukuoka (FPG, 2011). Unfortunately we have less dataof radioactive deposition at Fukushima Prefecture because ofserious damages of monitoring stations by the earthquake andresulting power failure.

3. Results and discussion

Iodine-131 (8.02 d half-life) and 137Cs (30 y half-life) weredocumented at the early stage as dominant radionuclides from theFukushima Dai-ichi NPP accident in deposition samples by MEXT(MEXT, 2011). The temporal changes of daily 131I and 137Cs deposi-tion rates during the period of March to April together withprecipitation amount are shown in Fig. 1. On March 21, the level of131I, a dominant radionuclide, at Hitachinaka (36.40 �N, 140.54 �E),which is located about 120 km south from the Fukushima Dai-ichiNPP, was 91 kBqm�2 d�1 as a maximumvalue, and 137Cs depositionat Hitachinaka was 13 kBq m�2 d�1. High radionuclide depositionrates (131I, 58 kBqm�2 d�1; 137Cs, 4.3 kBqm�2 d�1) were observed inYamagata (38.26 �N, 140.34 �E), which is about 110 km northwestapart from the Fukushima Dai-ichi NPP. On March 21, high deposi-tion rates were observed in all of the monitoring sites of the KantoPlain (Fig. 1). High radioactive deposition rates accompanied with

achinaka (36.40 �N, 140.54 �E), B: Utsunomiya (36.55 �N, 139.88 �E), C: Tokyo (35.59 �N,is tentatively shown at 1 Bq m�2 in this figure.

Fig. 2. Spatial distributions of monthly 137Cs deposition observed in Japan. (Unit: Bq m�2) A: March, B: April, C: May. Location is as follows: AK: Akita, NI: Niigata, TY: Toyama, FK:Fukui, TT: Tottori, AM: Aomori, YM: Yamagata, MO: Morioka, UM: Utsunomiya, HN: Hitacihnaka, ST: Saitama, TK: Tokyo, KF: Kofu, IK: Ichihara, CS: Chigasaki, SO: Shizuoka, NN:Nagano, FO: Fukuoka, KC: Kochi, OS: Osaka, TM: Iwanai, HT: Higashi-Dori, SS: Satsuma-Sendai, SP: Sapporo.

K. Hirose / Journal of Environmental Radioactivity 111 (2012) 13e17 15

rainfall continued until March 23. On March 24, the radioactivedeposition rates decreased dramatically due to fine weather withcoverage of a high pressure system. Most of the Fukushima-derivedradionuclides,whichwere transported from the FukushimaDai-ichiNPP by northeast wind, were deposited on land surface by rainfallduring March 21e23. The radioactive deposition derived from theFukushima reactor accidentwas observed inApril 2011. InMay 2011,the daily deposition rate of the Fukushima-derived radionuclides inmost of the Japanese monitoring sites decreased below detectionlimit except Fukushima. The 131I/137Cs activity ratio in daily depo-sition samples, ranged from 0.3 to 230, varied temporally andspatially. The 131I/137Cs activity ratio in total atmospheric releasewas estimated to be 11 (NSC, 2011). To compare with the 131I/137Csactivity ratio at the initial release of radionuclides from the

Fukushima Dai-ichi NPP, we corrected radioactive decay of 131I atMarch 12 when the first emission of radionuclides occurred. Thedecay-corrected 131I/137Cs activity ratios ranged from 3.5 to 550,with a median value of 15. High decay-corrected 131I/137Cs activityratios occurred during the period from March 20 to 23, which cor-responded to time observed maximum deposition rates. Higher131I/137Cs activity ratio appeared at downstream inland sites ofradioactive plume, which means that the depositional behaviorbetween 131I and 137Cs largely differed from each other. This findingsuggests that the Fukushima-derived 137Cs was preferentiallyremoved from atmosphere comparing with 131I.

Local governments determined the Fukushima-derived radio-nuclides in monthly deposition samples. Iodine-131, 134Cs (2.06 yhalf-life) and 137Cs were measured as dominant radionuclides in

Fig. 3. Temporal changes of monthly 137Cs deposition observed at several sites of eastHonshu Island. The apparent residence times of atmospheric 137Cs at Yamagata,Hitachinaka, Tokyo, Shizuoka (34.63 �N, 138.14 �E) and Morioka were 8.8, 11.3, 8.8, 10.9and 14.4 d, respectively.

K. Hirose / Journal of Environmental Radioactivity 111 (2012) 13e1716

monthly deposition samples in Japan, whereas 129mTe (33.6 d half-life), 129Te (69.6m half-life), 136Cs (35 d half-life), 110mAg (250 d half-life), 95Zr (64 d half-life), 95Nb (35 d half-life) 140Ba (12.7 d half-life)and 140La (1.68 d half-life) were detected as minor radionuclides inthe sites within 300 km apart from the Fukushima Dai-ichi NPP. Wefocused long-lived radionuclides (134Cs, 137Cs and 110mAg) in thispaper because monthly deposition rates of the short-lived radio-nuclides are not well defined. Activity ratios of 134Cs/137Cs observedon March at the sites, ranged 1.0e1.1 as an average of 1.03, werealmost constant except Ichihara, in case when the monthly 137Csdeposition exceeded 100 Bq m�2. The 134Cs/137Cs activity ratios onApril andMay in corresponding sites were from 0.99 to 1.09 with anaverage of 1.03 and from 0.93 to 1.04 with an average of 0.99,respectively. The systematically lower 134Cs/137Cs activity ratios(0.84e0.90) occurred at Ichihara (35.50 �N, 140.11 �E) near Tokyo(35.59 �N, 139.70 �E), whose cause may be problems on analysis ofg-spectrometry. The result suggests that there were no spatial andtemporal variations of the 134Cs/137Cs activity ratios in the monthlydeposition samples during the period from March to May 2011. Itmust be noted that the almost same level of 134Cs as 137Cs depositedin Japan because the 134Cs/137Cs activity ratio in the Fukushima-derived radionuclides was nearly 1.0, whose value is higher thanthat in the Chernobyl fallout (134Cs/137Cs ratio: 0.5) (IAEA, 1986;UNSCEAR, 2000). Activity ratios of 110mAg/137Cs observed onMarch ranged 0.0009e0.006 as an average of 0.0019.

The spatial distributions of monthly 137Cs deposition in March,April and May are depicted in Fig. 2AeC, respectively. The monthly137Cs depositions in the sites within 300 km apart from theFukushima Dai-ichi NPP except Kofu (35.65 �N, 138.57 �E; inlandsite) and the Japan Sea side sites (Niigata: 37.91 �N, 139.04 �E andAkita: 39.72 �N, 140.10 �E) were in the range from 1.1 kBq m�2 to17 kBq m�2, which are higher than the maximum monthly 137Csdeposition (0.55 kBq m�2) originating from the 1961e1962 large-scale atmospheric nuclear testing observed at Koenji (Tokyo) in1963 (Hirose et al., 2008; Katsuragi, 1983). The results reveal thatareas with the high 137Cs deposited areas, comparable to thecumulative amount of the 137Cs deposition at Tokyo until mid-1960(about 7 kBq m�2), appeared within a region band from 100 to300 km from the Fukushima Dai-ichi NPP. The spatial distributionof monthly 137Cs deposition in March revealed that the majordeposition of the Fukushima-derived radionuclides occurred in theNorth Pacific coast and inland area of the east Honshu Islandwhereas there was less contribution of the Fukushima-derivedradionuclides in the Japan Sea side sites of the east HonshuIsland. These findings suggest that transport of the radioactiveplume is affected by land topography and that most of theFukushima-derived radionuclides might be injected in theboundary layer (about 1000m). Higher 137Cs deposition, which wasone order of magnitude higher than pre-Fukushima levels (Igarashiet al., 2003), was observed in March at Fukuoka (33.59 �N,130.40 �E), where is located more than 1000 km southwest apartfrom the Fukushima Dai-ichi NPP. Observation that 134Cs wasdetected in the same sample revealed that the Fukushima-derivedradionuclides was transported to Fukuoka. Model simulation(Takemura et al., 2011) suggested that the Fukushima-derivedradioactivity cloud was transported in Europe on March 24 andspread far eastern Siberia. It is likely that the Fukushima-derivedradionuclides in Fukuoka were transported via the east Siberiarather than rounded the globe. In April, higher monthly 137Csdepositions were observed in the North Pacific side sites and EastJapan inland sites, although the levels decreased markedly. Thissuggests that the atmospheric emission of radionuclides from theFukushima Dai-ichi NPP at least continued within April 2011,although release rate dramatically decreased. On the other hand,the monthly 137Cs deposition increased at southwest sites in Japan

and at the Japan Sea side sites comparing with that in March,suggesting that the Fukushima-derived radioactive cloud affectednorth part of the Northern Hemisphere atmosphere. In May,monthly 137Cs depositions decreased in all of sites of Japan (Fig. 2C),although higher 137Cs depositions occurredwithin 300 km from theFukushima Dai-ichi NPP.

In order to elucidate depositional behaviors of the Fukushima-derived radionuclides, it is important to examine the temporalchange of the monthly 137Cs deposition. The monthly 137Cs deposi-tion at the sites in 300 km apart from the Fukushima Dai-ichi NPPexhibited exponential decreases during the period ofMarcheMayasshown in Fig. 3. We calculated apparent residence times of theFukushima-derived atmospheric 137Cs; these are 8.8 d for Tokyo to14 d for Morioka (39.70 �N, 141.16 �E). The apparent atmosphericresidence time of the Fukushima-derived 137Cs is similar to that (8 d)estimated for 210Pb over the west central United States, based onradioactive equilibrium of 210Pb with its short-lived progenies(Moore et al., 1973). Lambert et al. (1982) estimated a global meanaerosol residence time of 6.5 d by using atmospheric inventories of222Rn and 210Pb extrapolated from observations and computing the210Pb deposition sink to balance 222Rn decay. The residence times oftropospheric aerosols inferred from a global three-dimensionalsimulation of 210Pb were 5e10 d, which depends on season andlatitude (Balkanski et al., 1993). On the other hand, longer tropo-sphere residence time of aerosols (about 30 d) was obtained fromradioactive debris of atmospheric nuclear explosions (Katsuragi,1983). A similar long residence time was estimated for the Cher-nobyl 137Cs (25 d) (Aoyama, 1988), which was emitted as sub-micrometer particles (Hirose, 1995). The residence times of aero-sols in troposphere, which are in the range of 5e30 d, have beendetermined by natural and anthropogenic radionuclides, whichdepend on particles size and altitude (Ehhalt, 1973). The shorterresidence time of the Fukushima-derived radionuclides suggeststhatmostof theFukushima-derived radionuclideswere injected intothe lower layer of the troposphere and/or emitted as larger particlesin the atmosphere. As another possible process, the apparent resi-dence timemay partly reflect a history of the atmospheric emissionbecause the radioactive emission from the Fukushima Dai-ichi NPPcontinued still now (Aug. 2011) although the emission rate ofradioactivity dramatically decreased (TEPCO, 2011).

K. Hirose / Journal of Environmental Radioactivity 111 (2012) 13e17 17

4. Conclusion

Radioactivity originating from the Fukushima Dai-ichi NPPaccident dominantly spread in Fukushima and the Kanto Plain bysouthwest and northeast wind, respectively. As a result of snowfalland southwest wind during March 15e16, serious radioactivecontaminated area appeared in Fukushima Prefecture (MEXT,2011). The Kanto Plain was mainly contaminated by theFukushima-derived radionuclides due to movement of radioactivecloud by northeast wind and following rainfall during March21e23. The radioactivity monitoring result suggests that theFukushima-derived radionuclides spread in the north part of theNorthern Hemisphere atmosphere. The apparent residence time ofthe Fukushima-derived radionuclides observed in the Kanto Plainand northeast Honshu Island is about 10 d, which suggests thatmajor radioactive materials emitted from the Fukushima Dai-ichiNPP accident were injected into the lower layer of the troposphere.

The Fukushima Dai-ichi NPP accident is still ongoing (TEPCO,2011), although major atmospheric emission of radionuclides hasbeen ceased apparently. We have only limited information on theenvironmental impact of the Fukushima Dai-ichi NPP-derivedradioactivity. In order to do adequate response to public concern,further investigation is required.

Acknowledgement

Author would appreciate stuff members of the prefecturalgovernments in Japan andMEXT for great effort onmeasurements ofradioactivity in deposition samples. Author thanks Dr S. Hisamatsuandanonymous reviewer forconstructive commentsandsuggestion.

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