March 2016

Radionuclide concentrations in the air in Japan, five years after theFukushima Daiichi accident

In 2015, measurement results relating to the monitoring of radionuclides present in theatmosphere - obtained during 2014 - show that there are still perceptible traces in the ambient
air five years after the accident at the Fukushima Dai-Ichi nuclear power plant.
The main driving force behind this atmospheric remanence is linked to the re-suspension by winderosion of particles from the soils upon which radionuclides released at the time of the accident
were deposited. Biomass fires (more or less contaminated at the time of the accident) are also partof this remanence through the ash that can be dispersed in the atmosphere. These particles andash, more generally called "aerosols", can be transported by the wind towards territories wheredeposits were lower and thus lead to a temporary increase in concentrations in the air.
Among the different sites where measurements are carried out continuously on the levels ofradionuclide concentrations in the air1, here we have chosen to represent the values obtained, on
the one hand, in the outskirts of Tokyo city, in Tsukuba (170 km south-west of the nuclearpower plant) and, on the other hand, in a rural area, in Tsushima (40 km north-west of thenuclear power plant) in the zone evacuated due to high levels of contamination (Fig. 1).

40N

kBq/m239N

1000
Tsushima, Namie

38N

600
300

Centrale Daiichi
Nucléaire
Fukushima
NPPdeFukushima Dai-Ichi

37N

100
60
30

36N
Tsukuba

Tokyo

35N

138E

10137Cs

139E 140E 141E 142E 143E

Fig. 1: Map of 137Cs deposits (airborne measurements according to Torii et al., 2012) andlocalisation of two of the measurement sites of 137Cs concentrations in the air.
Figure 2 below shows that, in Tsukuba, the concentration of 137Cs in the air fell by around amillion-fold compared to the maximum value (38 Bq/m3) found in March 2011. Over the period
March to August 2014, the average is 12 µBq/m3 or around 10 times the value measured on the eveof the accident in 2011. The level can temporarily rise to around 50 µBq/m3. These increases can beexplained by re-suspension from the most affected territories and via the transport of the air mass
to places less exposed to fallout during the days following the releases.

1Research conducted by Pr. K. Kita (University of Ibaraki) and Dr. Y. Igarashi (JMA-MRI, Tsukuba)
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Levels of137

Cs and134

Cs (mBq/m3)

March 2016

Fig. 2: Levels of 137Cs and 134Cs in the air in Tsukuba, Ibaraki prefecture.
(With the kind permission of Pr. K. Kita and Dr. Y. Igarashi)

Tsushima
1011373)

137Cs (mBq/m3)
Concentration
Levelsof Csen(mBq/mIn Tsushima2, within the evacuated zone, measurements began at the end of 2012. They showthat concentrations are distinctly higher than in Tsukuba - by around factor 100 - and vary fromless than 100 µBq/m3 to 1,000 µBq/m3 with, from time to time, as is the case of Tsukuba,
concentration peaks linked to episodes of re-suspension of radionuclides deposited previously aswell as to the origin of the air masses. The presence of biomass fires surrounding this rural site canalso influence concentration levels. Similarly, the production of aerosols from plants (in particularspores) or from mushrooms seems to have an effect on the concentration level.

0,10,01030

60

90

120

150

180

210

240

270300330360

DayJourof theyear
de l’annéeFig. 3: 137Cs levels in the air at Tsushima, Namie: Fukushima prefecture, between 2012 and 2014.
(With the kind permission of Pr. K. Kita)
Reference
Torii T. et al. (2012). Investigation of radionuclide distribution using aircraft for surrounding
environmental survey from Fukushima Dai-ichi Nuclear Power Plant. JAEA-Technologoy 2012-036,
Japan Atomic Energy Agency, 182 pp (in Japanese, with English summary).

2Tsushima, Namie; district of Futaba; Fukushima prefecture
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