Development of Experimental Techniques/Facilities at JAEA R&D Centers

Nuclear Fuel Cycle Engineering Laboratories

13-2 Radiocesium in the Environment－ Radiation Situation in Tokai Two Years after the 1F Accident －(a)(c)
Cs concentration (one-month average)
in airborne dust (Bq/m3)

30

10000
3100 nGy/h

25100020
15Rainfall
ST2ST4P1P3P5
P7100(32〜45) nGy/h10131415 16

17 18 19 20

ST1ST3ST5P2
P4P6P821 22 23 24 25 26105137

2100 nGy/h

Rainfall (mm)Dose rate (one-hour average)(nGy/h)4800 nGy/h010
On site (MP7)
Off site (ST2;Tokai-mura)

10.1

0.01

0.001

0.0001

Detection limit
0.0001 Bq/m3

Jan-2011

Jul-2011

Jan-2012

Jul-2012

Jan-2013

Date in March, 2011

(d)

(b)

1000
540 nGy/hOn site (MP7)
Off site (ST2;Tokai-mura)

340 nGy/h(May)

One-month average in March, 2013

190 nGy/h100

Cs concentration in foodstuff(Bq/kg fresh)

One-month average in March, 2011

160 nGy/h

99 nGy/h(May)59 nGy/hDose rate before Fukushima accident 30-40 nGy/h

Fish100

Shellfishlimit in foods1010.1
Detection limit : 0.08 Bq/kg (vegetable)
0.04 Bq/kg (seafood)
0.01

10
Jan-2011

SeaweedLeafy vegetable

137

Dose rate (one-month average)(nGy/h)1000

Jul-2011

Jan-2012

Jul-2012

Apr-2010

Jan-2013

Apr-2011

Apr-2012

Apr-2013

Fig.13-3 Chronological change in dose rate and 137Cs concentration observed in Tokai for two years around 1F accident

Because of the release of radioactive materials during theaccident at the TEPCO’s Fukushima Daiichi NPS (1F), an
increase in the dose rate and radiocesium concentration wereobserved in the Nuclear Fuel Cycle Engineering Laboratories(Tokai, Ibaraki Prefecture), which is located about 115 km
from 1F, after March 15, 2011. This was reported by the Japan
Atomic Energy Agency (JAEA-Review 2011-035). The doserates (1-h averages) just after the 1F accident are presented
in Fig.13-3(a). We report the chronological change in theradiation in Tokai for two years after the 1F accident.
The dose rates (one-month averages) are presented
in Fig.13-3(b) for monitoring post-7 (MP7; on site) andmonitoring station-2 (ST2; off site) as representative locations.
Because of the differences in the environments (e.g., thepresence or absence of trees), the observed dose rates
varied by a factor of two or three. Because the half-lives ofcesium-134 (134Cs) and cesium-137 (137Cs) are about 2 and30 years, respectively, they contributed most of the dose
rate. From May 2011, when the contribution from shortlife nuclides almost disappeared, to March 2013, the dose
rate decreased by around 60%. Thus, 25% of the 134Cs andCs were transported elsewhere or shielded by immersion,in addition to the decrease by radiological decay (at a rate of
35% in 22 months).
The 137Cs concentrations in airborne dust (one-month averages)
also tended to decline (Fig.13-3(c)). The concentration off site
is likely to be slightly higher than that on site. The origin ofrecent 137Cs in airborne dust is believed to be not 1F itself but
soil on which 137Cs was deposited two years ago. Outside of
the Laboratories, strong winds could resuspend and transportsoil particles from cultivated fields.
The 137Cs concentrations in foodstuffs are presented inFig.13-3(d). Taking into account 134Cs, the concentrationof 134Cs plus 137Cs was less than half of the limit in foods(100 Bq/kg fresh for 134Cs plus 137Cs, 500 Bq/kg fresh before
April 2012). The 137Cs concentration generally decreased,
but the concentration in demersal fish, which could beinfluenced by 137Cs in seabed sediment, did not decrease asmuch. Because the transfer of Cs from the soil to vegetables
is smaller than that from sea water to seafood, the 137Csconcentration in vegetables was less than 1 Bq/kg fresh.
137

ReferenceFujita, H. et al., Special Environmental Monitoring around Tokai-Mura after the Accident of the Fukushima Dai-Ichi Nuclear Power Station, Proceedings ofthe 13th International Congress of the International Radiation Protection Association (IRPA-13), Glasgow, Scotland, U.K., 2012, P12.31, 7p.
JAEA R&D Review 2013111