Agricultural implications of the Fukushima nuclear accident
Journal of Radiation Research
Agricultural implications of the Fukushima nuclear accident
Tomoko M. Nakanishi 0
0 Graduate School of Agricultural and Life Sciences, The University of Tokyo , Tokyo , Japan
A B S T R AC T More than 4 years has passed since the accident at the Fukushima Nuclear Power Plant. Immediately after the accident, 40 to 50 academic staff of the Graduate School of Agricultural and Life Sciences at the University of Tokyo created an independent team to monitor the behavior of the radioactive materials in the field and their effects on agricultural farm lands, forests, rivers, animals, etc. When the radioactive nuclides from the nuclear power plant fell, they were instantly adsorbed at the site where they first touched; consequently, the fallout was found as scattered spots on the surface of anything that was exposed to the air at the time of the accident. The adsorption has become stronger over time, so the radioactive nuclides are now difficult to remove. The findings of our study regarding the wide range of effects on agricultural fields are summarized in this report.
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I N T R O D U C T I O N
After the Fukushima nuclear accident, the agricultural industries, the
Fukushima prefecture and the government agricultural agency
commenced strategies to assist in decontamination of the agricultural land
and its crops, e.g. use of potassium fertilizer or removal of the soil.
The Fukushima prefecture started to measure the radioactivity of all
kinds of agricultural products. Especially in the case of the rice, they
inspected entire crops prior to shipment. Through these
countermeasures, all of the products that were introduced into the market were
below the threshold levels for safe radiation exposure.
Immediately after the accident, the Graduate School of
Agricultural and Life Sciences at the University of Tokyo created an
independent team consisting of a wide range of specialists—for soil,
crops, wild and domestic animals, fish, forest, etc. (Fig. 1). The team
member entered the contaminated sites immediately after the
accident and commenced research to monitor the radioactive materials
integrated into the agricultural environment. The results of these
studies have been useful in the recovery of the affected area. This
report summarizes the findings of our group.
O N TA M I N AT I O N
The fallout was found as scattered spots on the surface of everything
that was exposed to the air at the time of the accident (Fig. 2) [
1,2
].
The salient feature, with regard to the fallout, is that the radioactive
Cs has largely remained at the initial contact site, with little
movement, and is difficult to remove.
The downward movement of the radioactive fallout in the soil is
currently ∼1–2 mm/year, whereas in the first 3 months after the
accident it moved ∼20 mm, and during the following 3 months it moved
∼6 mm. The speed of the movement is now much slower than
directly after the accident.
Since the nuclear power plant accident occurred late in winter,
there were no leaves on the deciduous trees, but there were leaves on
the evergreen trees in the mountains, and these needle-like leaves
were highly contaminated by the fallout. After a few years, these
leaves fell to the ground and were gradually decomposed by
microorganisms. During the decomposition process, the radioactive Cs
adsorbed on the leaves was transferred to the soil and then firmly
adsorbed onto the soil particles. The radioactive Cs was only
adsorbed onto the very fine particles of the clay. The mineral in the
clay was recently identified as weathered biotite [
3
].
The river water flowing from the mountain shows very low
radioactivity (<10 Bq/l). It was found that the water itself was hardly
radioactive at all after filtering out the clay. The amount of the
radioactive Cs flowing from the mountain was in the order of 0.1% of the
total fallout amount per year.
However, the radioactivity of mushrooms growing in the forest
did not decrease with time to any great extent. Some of the
mushrooms harvested more than 300 km from the site of the accident were
found to contain 137Cs only, indicating that they are still collecting
the global fallout of the nuclear bomb tested during the 1960s. Since
the half-life of 137Cs, 30 years, is much longer than that of 134Cs,
2 years, all of the 134Cs from the global fallout in the 1960s was
decayed out after 50 years. This means when only 137Cs was detected
in the mushroom, the 137Cs found was not from the Fukushima
nuclear accident. In the case of the fallout from the Fukushima
nuclear accident, the initial radioactivity ratio of 137Cs to 134Cs was
the same. Therefore, currently, both nuclides should be detectable in
mushrooms collecting fallout from Fukushima.
In the case of trees, the radioactive Cs was moved directly from
the surface of the trunk to the interior. Two kinds of trees in the
forest were cut down in Minami–Soma City in 2012; the radioactivity
of each whole tree was measured, and ra (...truncated)