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The overview of our research projects for Fukushima is presented ­including how they were derived. Then, where the fallout was found, right after the accident, is briefly summarized for soil, plants, trees, etc. The time of the accident was late winter, there were hardly any plants growing except for the wheat in the farming field. Most of the fallout was found at the surface of soil, tree barks, etc., which were exposed to the air at the time of the accident. The fallout found was firmly adsorbed to anything and did not move for months from the site when they first touched. Therefore, the newly emerged tissue after the accident showed very low radioactivity. The fallout contamination was not uniform, therefore, when radiograph of contaminated soil or leaves were taken, fallout was shown as spots. Generally, plants could not absorb radiocesium adsorbed to soil. Further findings are described more in detail in the following sections.

The behavior of radiocesium in winter wheat after the accident in March 2011 was characterized on the basis of an investigation of radiocesium in wheat grown in open fields. The distribution of the radiocesium contamination of wheat was analyzed by determining the radiocesium concentration in each part approximately 2 months after the fallout occurred in agricultural fields, which was a short period after the hydrogen explosion occurred at Fukushima Daiichi nuclear power plant. At that time, only the leaves and stems, but not the grains, were contaminated directly by the fallout because the wheat growing in the fields was in juvenile phase, before heading. The radioactivity was more than 1,000 times higher in the leaves growing at the time of the accident than that in the panicles that developed later. Autoradiographic images captured using an imaging plate showed that the highest radioactivity was found in many spots on the old leaves, suggesting that the radionuclides were strongly bound to the leaves. Moreover, effects of the seed sowing date, which varied from October 8th to November 20th, 2010, on the radiocesium concentration in grains were investigated. The radiocesium concentration in the grains after harvest was correlated with the plant height measured on May 28th. These results suggest that the radiocesium in the grains was derived from leaves and stems where it had accumulated when the fallout occurred.

Although most of the radiocesium fallout that deposited in paddy fields after the Fukushima nuclear disaster in March 2011 was expected to be bound to clay in the soil resulting in a very low soil-to-plant transfer function, a radiocesium contamination level of >500 Bq/kg was detected in brown rice grown in several hilly areas of Fukushima Prefecture in the autumn of the same year. The likely source of the radiocesium was fallout deposited on organic matter in the paddy fields and litter in mountain forests, from which runoff water flowed into irrigation channels that ultimately lead to the paddy fields. This problem appears to have been caused by conditions specific to lowland rice paddy fields, which are wetland ecosystems. Integrated studies of the soil, water, and plants from an ecological viewpoint are necessary to understand the mechanism of radiocesium absorption by rice before commercial rice production in the affected areas can be resumed.

Here, we review cesium uptake and accumulation in rice. Cesium is an alkaline metal, and its uptake is affected by potassium nutrition. Several transporters for cesium have been described. The distribution of cesium in the rice plant body differs from that of potassium, suggesting differential transport/storage mechanisms for cesium and potassium in rice. Cesium concentration in rice differs among cultivars, and it would be possible to determine the gene(s) responsible for cesium uptake/accumulation in future. This knowledge will form the foundation for the development of low radiocesium-accumulating cultivars.

The pattern of real-time Cs uptake by plants was visualized using a macroscopic real-time radioisotope imaging system. We found that Cs was easily taken up by rice plants only when it was dissolved in a liquid medium. In contrast, only a small amount of Cs was taken up when the same liquid medium containing Cs was added to the soil and supplied to the rice plants. This result demonstrates the intensive Cs adsorptive property of the soil. When rice was grown hydroponically in K-sufficient environment followed by K withdrawal for 2 days, Cs was taken up easily compared with the rice without K deficiency. The application of K was shown to be an effective method for preventing radiocesium uptake by rice plants. However, K-deficiency was found to have little effect on the xylem loading process of Cs. Understanding the Cs translocation processes and evaluating the factors affecting each process based on experimental evidence could facilitate the development of an agricultural technique to reduce the radiocesium content in the edible parts of plants.

The vertical migration of radiocesium fallout in the soil was monitored for 1 year at several locations in Fukushima after the nuclear power plant explosion. We determined the vertical gamma ray intensity profiles in boreholes in the soil using a scintillation survey meter with a lead collimator to restrict the incoming radiation, only allowing horizontal detection. The average migration distances of radiocesium at two time points were accurately determined based on the difference in the depth of the centroids of two gamma ray intensity profiles. The results showed that although the convective velocity of radiocesium was unexpectedly as high as 1/10th of the velocity of the infiltrating rainfall water 2–3 months after the nuclear plant accident, the velocity decreased to 1/100th–1/200th of that of the water after 6–12 months. This indicated that strong fixation of radiocesium to clay particles occurred during the initial 2–3 months. Radiocesium uptake by plant roots may have decreased remarkably along with the mobility of radiocesium in the soil.

Vegetables and field soils about 60 and 230 km away from the Fukushima Daiichi nuclear power plant were examined for Cs and Cs radioactivity. The total Cs and Cs transferred was <7 Bq/kg wet weight in potato tubers grown in fields where the total Cs and Cs concentration in the soil was ≤1,235 Bq/kg dry weight. For the edible parts of lettuce and cabbage, the total Cs and Cs concentrations were lower than the detection limit. In this case, the maximum value in soil was 651.2 Bq/kg dry weight.

The radioactivity measurements performed by Fukushima local ­government, from March 2011 to March 2012, is presented. Agricultural products (cereals, vegetables, and fruit trees), forest products (mushrooms and edible wild plants), marine products (saltwater and freshwater fish), and stock farm products (beef, pork, and raw milk) were measured for radioactivity before shipment. In March 2011, 19% of the samples contained >500 Bq/kg of radiocesium, followed by 12% in April, 10% in May, 7% in June, and 1–2% after July. Only 3% of the samples investigated contained >500 Bq/kg, whereas the remaining 97% contained less than the provisional regulation level of 500 Bq/kg in 2011.

Changes in the radioactivity of I, Cs, and Cs in milk produced by cows given pasture that was contaminated with these radioactive nuclides caused by the Fukushima Daiichi nuclear power plant accident on 11 March 2011, were examined between 16 May and 26 June 2011. Pasture (Italian ryegrass) was seeded on September 2010, and cultivated in the Animal Resource Science Center of the University of Tokyo (about 140 km south-west of the power plant). Pasture was harvested 2 months after the accident and prepared for fermented grass forage (haylage). The cows examined were born and kept in the Animal Resource Science Center and were given commercial mixed feed (total mixed ration forage: TMR) that contained no radioactive I, Cs, or Cs, for 2 weeks before being examined. They were given haylage and TMR (10 and 25 kg/600 kg of body weight/day, respectively) for 2 weeks, and then were given only TMR (35 kg/600 kg of body weight/day) for 2 weeks. During the examination, milk was collected twice a day and mixed in each cow. The weight and radioactivity of I, Cs, and Cs of the mixed milk in each cow were measured daily. No radioactive I was detected in either the milk or haylage. The radioactivity of Cs and Cs contained in the mixed feed of haylage and TMR was 380 Bq/kg (radiocesium radioactivity was represented as total concentrations of Cs and Cs). Radiocesium radioactivity concentrations in the milk rapidly increased to 30 Bq/kg after 4 days from the start of feeding and equilibrated to 36 Bq/kg after 12 days. After that, cows were given TMR containing no radiocesium, and radioactivity concentrations of radiocesium in the milk rapidly decreased. Two weeks after stopping radiocesium feeding, radioactivity concentrations were less than 5 Bq/kg (background level). In summary, when the cow (approximately 600 kg of body weight) was given feed with radioactive Cs and Cs (12,600 Bq/600 kg body weight/day), 5.71% of Cs and Cs was secreted into the milk (720 Bq/20 kg milk/day). Radioactivity concentrations of radiocesium in the milk were lower than new standard (50 Bq/kg) of Japan.

High concentrations of radiocesium (Cs, Cs, and Cs combined) were detected in several fish species such as nibe croaker, Pacific cod, and brown hakeling, which were collected from the Pacific Ocean off the coast of the Fukushima and Ibaraki Prefectures. High levels of radiocesium accumulated in fish muscle, but the radioactivity levels of naturally occurring radioactive K in some contaminated fish exceeded the levels of Cs and Cs. Three washes with 0.1% NaCl solution effectively removed the radiocesium from contaminated fish meat. This can be applied in the production of surimi-based products and other processed seafood such as boiled, dried, or seasoned products.