Agricultural implications of the Fukushima nuclear accident

Journal of Radiation Research, Vol. 57, No. S1, 2016, pp. i47–i52 doi: 10.1093/jrr/rrw042 Review, Supplement – ICRR highlights Agricultural implications of the Fukushima nuclear accident Tomoko M. Nakanishi Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan Corresponding author: Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan. Email: Received December 7, 2015; Revised March 2, 2016; Accepted March 14, 2016 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. KE YWOR DS: Fukushima nuclear accident, agricultural implications, 137 Cs, soil, crop, forest, tree, animal, decontamination I N T RO D U C T I O N O N TA M I N AT 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. 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 © The Author 2016. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. • i47 i48 • Nakanishi Fig. 1. Fukushima projects conducted at our faculty. Fig. 2. Radioactive Cs in soil [1,2]. Radioactivity measurement and soil collection was performed at the Fukushima Agricultural Technology Center at Koriyama 21 April 2011, located ∼50 km from the site of the accident. Left: The 134Cs and 137Cs radioactivity profile of the soil: soil contamination was limited to the top few centimeters of the field. Right: Autoradiography of the soils: ∼15 g of soil was collected from each of four kinds of field at the Center and spread on paper, and autoradiography was performed on an imaging plate with a 24-h exposure. (1) Farming land, 50 000 Bq/kg; (2) soil for vegetables, 8000 Bq/kg; (3) soil for wheat, 7000 Bq/kg; (4) paddy soil, 37 000 Bq/kg (Sho Shiozawa et al.) 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. Agricultural implications of the Fukushima nuclear accident • i49 Fig. 3. Autoradiography of trees in the forest [1,2]: (A) pine tree (Pinus dinsiflora) and (B) sugi (Cryptomeria japonica). The height of the sugi and pine tree were 23.2 m and 22.2 m, respectively. Wood disks were harvested from 1.3 m, 4.0 m, 7.0 m, 10 m, 13 m, 16 m and 19 m above the surface of the soil, and an autoradiograph of each disk was taken by an imaging plate: (a) autoradiograph and (b) photograph. The radioactivity of the wood disks was high at the surface of the bark in both trees. In the case of sugi (B), a large amount of radioactive Cs was transf (...truncated)