Radioactive pollution from Fukushima Daiichi nuclear power plant in the terrestrial environment
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RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHINUCLEAR POWER PLANT IN THE TERRESTRIAL ENVIRONMENTH. Tazoe1,*, M. Hosoda2, A. Sorimachi1, A. Nakata1, M. A. Yoshida1, S. Tokonami1 and M Yamada11Hirosaki University, Institute of Radiation Emergency Medicine, 66-1 Hon-cho, Hirosaki,Aomori 036-8564, Japan2Hirosaki University, Graduate School of Health Sciences, 66-1 Hon-cho, Hirosaki, Aomori 036-8564,Japan
*Corresponding author: firstname.lastname@example.org
Major contaminants from venting and hydrogen explosions at the Fukushima Daiichi nuclear reactors between 12 and 15March 2011 were transported northwestward and deposited on soil and plants via precipitation. Surface soils and plant leaveswere sampled at 64 sites in the Fukushima Prefecture. The highest concentrations of 134Cs (84.4 kBq kg21) and 137Cs (82.0kBq kg21) in surface soils were observed at Nagadoro in Iidate village located 32 km northwest from the Fukushima Daiichinuclear power plant. Furthermore, 131I, 129Te, 129mTe, 110mAg and 140La were detected in the same samples. Outer surface ofplant leaves, such as bamboo, cabbage and grasses were highly contaminated at the high-dose rate areas of Tsushima andMinami-Tsushima in Namie town. Mugwort leaves that grew after the pollution event had extremely low concentration ofradionuclides; however, the plant/soil radiocaesium ratio was 0.023+++++0.006. It is anticipated that decomposition of fallenleaves will promote recycling of radionuclides in the environment.
A catastrophic earthquake (9.0 Mw) and tsunamioccurred on 11 March 2011, which caused major de-struction in northeastern Japan and severe damageat the Fukushima Daiichi nuclear power plant.Electrical generators were shut off and three nuclearreactors suffered explosions due to hydrogen gasafter the cooling system failed. Residents within 20-km radius of the Fukushima Daiichi nuclear powerplant were evacuated; densely populated areas suchas the cities of Fukushima and Koriyama werehighly contaminated. Major contaminants origi-nated from venting and hydrogen explosions at reac-tors from 12 March to 15 March 2011. Total 137Csdepositions over the Japanese islands were estimatedto be .1.0 PBq(1, 2). Fission products were trans-ported northwestward and deposited on the groundvia precipitation, which resulted in heterogeneouscontamination and geographical distribution ofratios of radionuclides such as 131I/137Cs, 134Cs/137Cs and 110mAg/137Cs(3). In addition, hydrogenexplosions and metrological conditions such aswind direction and precipitation caused 137Cs, 131I,110mAg and 129mTe in the surface soil to vary.
MATERIALS AND METHODS
Surface soils (01 cm) and plant leaves were col-lected from 64 sites in the Fukushima Prefecture(Fig. 1) on four sampling expeditions from 12 to 16April, 26 to 28 April, 6 to 10 June and 15 to 16June 2011, and then these samples were analysed forradionuclides. For the soil analysis, stones and plant
roots were removed by handpicking and soil wastransferred to a 100-ml polystyrene container. Tocompare the deposition to soils and leaves and theplant uptake after deposition, leaves of Japanesemugwort that grew after the pollution event werealso collected. We collected leaves from the upperpart of the plant to minimise contamination fromthe soil. Plant leaves were cut into 11 cm with ascissor. The concentrations of 134Cs, 136Cs, 137Cs,131I, 110mAg, 129Te and 129mTe were determined bygamma-ray spectroscopy. Gamma-ray emissions atenergies of 460 keV (129Te), 604 keV (134Cs), 636keV (131I), 662 keV (137Cs), 730 keV (129mTe) and885 keV (110mAg) were measured for 1000 up to80 000 s by a high-purity coaxial Ge gamma-ray de-tector (ORTEC GEM-40190, SEIKO-EG&G).Mixed gamma standard sources with differentsample heights distributed from the JapanRadioisotope Association were used for efficiencycorrection. Because the measurements were startedin the middle of May 2011, some short half-liferadionuclides such as 131I (T1/28.02 d) and 136Cs(T1/213.16 d) could not be detected.
RESULTS AND DISCUSSION
Heterogeneous deposition of radionuclides on theground
The highest concentration of 134Cs (84.4 kBq kg21)in surface soil was observed at the Nagadoro (S54)in Iidate village located 32 km northwest from theFukushima Daiichi nuclear power plant (Table 1).In addition, 137Cs (82.0 kBq kg21), 131I (924 kBq
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Radiation Protection Dosimetry (2012), Vol. 152, No. 13, pp. 198203 doi:10.1093/rpd/ncs222Advance Access publication 29 August 2012
at St Petersburg State University on February 3, 2014
kg21, concentration is corrected to March 11),110mAg (0.40 kBq kg21), 129Te (56.3 kBq kg21) and129mTe (89.4 kBq kg21) were detected for the samesample on March and April. Furthermore, the ex-tremely high-dose rates after dispersion from thenuclear power plant were thought to be mainlycaused by 132Te and its daughter 132I based on thein situ gamma-ray spectrum(3). High radiocaesiumconcentrations were observed in the northwesternNamie town (S46) and southwestern Minamisohmacity near the Tetsuzan dam (S12), which are locatednorthwest of the Fukushima Daiichi nuclear powerplant. This is consistent with the results of aircraftmonitoring by the Ministry of Education, Culture,Sports, Science and Technology, Japan (MEXT, http://www.mext.go.jp/english/incident/1304796.htm).Fukushima city is located on the major contamin-ation path that also shows heterogeneous geograph-ical distribution, particularly anomalous highconcentrations for S63 (Table 1). This resulted fromgeographical features such as the surrounding moun-tains and the setting of buildings and trees, becauseFukushima city is densely populated.
Concentration ratios against 137Cs for 134Cs and129mTe are mostly constant (Fig. 2), although thosefor the 131I and 129mTe are observed different trends.131I/137Cs ratios decrease from 32 to 10 with 137Csconcentration (,10 kBq kg21). On the contrary,110mAg/137Cs increase for high 137Cs samples (.50kBq kg21). These fractionations were caused by thephysical and/or chemical properties for each nuclideduring transportation and deposition processes.
Hosoda et al.(4) reported that the highestambient-dose rate in air was 36 mGy h21 atHirusone, Namie town (37.558N, 140.8478E), on thebasis of the carborne survey conducted on 12 April2011. Regional heterogeneity in radiocaesium
distribution was observed at Minami-Tsushima,Namie town (Fig. 3). Concentrations of 137Cs col-lected from the three paddies (S19, S27 and S36)widely vary, ranging from 0.24 to 9.9 kBq kg21. Inthe same paddy, much lower radiocaesium concen-trations are observed in samples S19-1 and S36-1relative to the other soil samples. The heterogeneousdistribution of 137Cs concentration can reflect boththe initial deposition in the surface soil and the post-depositional redistribution, such as transportationadsorbed soil particles by the rainfall, in the soil.
The soil sample from inside a greenhouse inTsushima, Namie town, had much lower 137Cs con-centration than the soil outside the greenhouse(Fig. 4). This indicates that the major deposition tookplace during wet precipitation and that the inside wasnot contaminated. The radiocaesium deposition pri-marily occurs in association with precipitation (3).This indicates that the intrusion of radiocaesiumindoors could be prevented easily.
Deposition and translocation of radionuclides to theplant
Figure 5 shows the concentration of radionuclides insoils (S12S14) and plant samples (L11L13) nearthe Tetsuzan dam, corrected to 12 April. Plantsamples L11, L12 and L13 are leaves of Japaneseceder, bamboo and clinopodium gracile, respectively.The concentrations of radionuclides in S12 arenearly twice of that in others. On the other hand,plant samples show much wider variation in radio-caesium concentration. Japanese ceder leaves havelow radiocaesium concentrations (134Cs1.18 kBqkg21 and 137Cs1.18 kBq kg21). Bamboo leaveshave concentrations several folds higher than thoseof soils and other plants. In cryptomeria, this could
Figure 1. Map of the soil sampling locations and total radioactive caesium concentrations (134Cs137Cs). The soilsamples observed high radiocaesium concentration (134Cs137Cs .100 kBq kg21) are shown on the map and Table 1.
RADIOACTIVE POLLUTION FROM FUKUSHIMA DAIICHI NUCLEAR POWER PLANT
at St Petersburg State University on February 3, 2014