Research Highlights

Local distributions of radioactive Cs deposited on plant andsoil from Fukushima Daiichi Nuclear Power Plants
F. Sakamoto1), T. Ohnuki1), N. Kozai1), S. Igarashi1,2), S. Yamasaki1), Z. Yoshida3), and S. Tanaka4)
1) R. G. for Bioactinide, ASRC, JAEA 2) Ibaraki University 3) JAEA
4) NPO Radiation Safety Forum

After the accident of Fukushima Daiichi Nuclear PowerPlant (FDNPP), the fallout radioactive Cs were dispersed
from FDNPP to ocean [1,2] and land [1]. Some of the released
radioactive Cs was deposited on the ground of the area locatednorth-west direction from FDNPP. The spatial concentration
distribution and depth profiles of radioactive Cs were measured
to estimate dose rate and to estimate the fate in the terrestrial
environment.
For the estimation of migration of radioactive Cs, chemical
states of the deposited radioactive Cs should be clarified.
We first sampled the soils, plants, and paddy area at Iitatemura, Fukushima on June, 2011 to estimate the chemical
states of radioactive Cs deposited [3]. We have measured local
distribution on/in trees, plants, and surface soil beneath theplants using autoradiography analysis.
The trees sampled were Cryptomeria japonica, Torreya
nucifera, and Prunus mume. The grass at meadow area and rice
(Orysa sative) stubble at paddy area were collected with soil
attached around roots. The local distribution of radioactive Cs
was measured by autoradiography technique using Bioimaging
analyzer of BAS2500 (Fuji Film, Japan). The samples were notpretreated for the autoradiography analysis. The shape and colorof the samples were recorded by optical photograph.
The autoradiography images of Torreya nucifera is shown inFig. 1 along with optical photograph. The black points in theautoradiograph image showed the area of radioactive Cs wasaccumulated. The black spots sized about 2 mm were distributedon the branch and leaves of Torreya nucifera, indicating thatradioactive Cs was distributed heterogeneously on the branch
and leaves of trees. The area of red circles in Fig. 1 showed noblack spot on the branch and the leaves. The optical photographshowed that the color of the leaves in the red circles sickly
green. This indicates that the branch and leaves in the red circles
were grown in this year probably after the FDNPP accident.
These results suggest that only small fraction of deposited Cs
may be transported to new branch and leaves grown after theaccident. Little fraction of the deposited radioactive Cs wastransported from the grown branch and leaves to new bornones. Similar results were obtained for Cryptomeria japonica
and Prunus mume. By the treatment of the branch and leaves
with a 1 M CH3COOH solution showed approximately 5% of
the radioactive Cs was dissolved from the branch and leaves
into the acetic acid solution, indicating that radioactive Cs wastightly associated with the branch and leaves.
The autoradiograph image of grass at the meadow (Fig. 2)
indicated the black spots were distributed on the grass above theground level (yellow dashed line), but not with soil root beneaththe ground level (red circles). The black spots on the stubble ofrice showed similar distribution. These results indicated that theradioactive Cs was deposited on the grass and the rice plant. In
addition, the ratio of the radioactive Cs penetrated into soil layer
by weathering was very small for two months after accident.
These results indicate that trees and plant would be the reservoirof the fallout Cs and function for retardation of the fallout Cs
7 ASRC

migration with rain water.

Fig. 1 Autoradiograph image (a) and optical photograph (b) of
Torreya nucifera sampled at Iitate, Fukushima, about 30
km from FDNPP.

Fig. 2 Autoradiograph image (a) and optical photograph (b)of cross section of meadow grass with soil beneath thegrass.
References[1] M. Chino et al., J. Nucl. Sci. Technol. 48, 1129 (2011).
[2] A. Omoto, Prog. ICAPP2011 Special Japan Session:presentation (2-6 May, 2011, Nice, France).
[3] F. Sakamoto et al., Trans. Atomic Energy Soc. Japan, 11, 1(2012) (in Japanese).