FKO-03

APSORC’13 - 5th Asia-Pacific Symposium on Radiochemistry
Kanazawa, Japan, 22-27 September 2013

Two years observation results of radiocesium fate in some trees after theFukushima Daiichi nuclear power plant accident
Keiko Tagami, Shigeo Uchida
Office of Biospheric Assessments for Waste Disposal, National Institute of Radiological Sciences

Keywords – Cesium-137, translocation, foliar uptake, root uptake

I.

INTRODUCTIONRadiocesium (134Cs and 137Cs) released from the
Fukushima Daiichi nuclear power plant due to the reactor
failures after the great earthquake and tsunami of March 11,2011 is recognized as the only major contaminant in theterrestrial environment now. Decontamination of forests isthe most difficult task because they occupy large areas incontaminated areas. Therefore, radiocesium deposited onforests will move in the forest ecosystems, and in the future,
it will reach to an equilibrium condition as it was observed
for the global fallout 137Cs. Until that time, theconcentration of radiocesium will decrease in trees due toplant grow and weathering. It is important to provide theinformation on the radiocesium behaviors in trees for thedecision making process, and for the use of forest products.
In this study, therefore, concentrations of radiocesium intree tissues, e.g., leaves, branches and fruits, weremeasured for about two years after the releases ofradionuclides at NIRS Chiba.
II.

MATERIALS AND METHODS

Six species of trees, i.e., three deciduous trees(Someiyoshino cherry, persimmon, and chestnut) and threeevergreen trees (red robin, loquat and azalea) were selected,and tree tissues have been collected from late April 2011.
The collected tissues were usually leaves and branches, andif the tree has fruits, then they were collected. The
evergreen trees had leaves at the time of accident occurred,
therefore, direct deposition effect was found for old green
leaves: these results were not used in this study.
In April to June 2011, these samples were measured inraw, however, since the concentration in samples decreased
rapidly, collected samples from July 2011 were dried at80˚C for 3 days at least to concentrate. Each dried sample
was mixed well and transferred into a plastic container tomeasure 137Cs concentrations with a Ge detecting system(Seiko EG&G). Some of the branch samples wereseparated into barks, phloem, and xylem parts. For the caseof fruit (persimmon, chestnut and loquat), whole or fruit
flesh was measured.
III.

RESULTS AND DISCUSSION

The radiocesium concentrations in leaf samples of all six
plant species showed a decreasing trend. Figure 1 shows
radiocesium concentration changes in leaves of thepersimmon tree as an example. In 2012 and 2013 spring,when new shoot emerged, a slight concentration increase
was observed for the deciduous trees, however, evergreen
trees has different new shoot growing and defoliationseasons, the increase trend was not seen in spring.
Branch samples also decreased in radiocesium
concentration with time. When branches were separated
into three parts, radiocesium concentration was high in thefollowing order: bark>phloem>xylem. Radiocesium
behaves like potassium, which is essential elements, andthus the concentration in phloem is higher than in woodyparts, however, higher concentration in barks may imply
the radiocesium elimination from living part of trees.
Radiocesium concentrations in fruits decreased yearly.
The concentrations in edible part of persimmon and loquat
were smaller than that in leaves.
Acknowledgement: This study was partially supported by
the Agency for Natural Resources and Energy, Ministry ofEconomy, Trade and Industry (METI), Japan.

10000

Concentration, Bq/kg-dry

Abstract – To measure the decreasing trend of radiocesium insome trees after the Fukushima Daiichi nuclear power plant
accident, a continuous sampling of tree tissues (leaves, branchesand fruits) was carried out in Chiba, Japan for about two years.
Radiocesium concentration in each tissue tends to decrease,
however, since radiocesium can translocate in a tree, slightincrease in new leaves in early spring was observed for some tree
species. Because radiocesium mobility was low in soil, root
uptake pathway would provide limited contribution to the total
radiocesium amounts in trees; the radiocesium by foliar uptake
after the accident still remains in trees.

Cs-137
Cs-1341000

1001010200

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Days after March 11, 2011

Figure 1. Radiocesium concentration change with time inthe leaves of persimmon tree.