Kenji Ohse

Radiation dose rates now and in the future for residents neighboring restricted areas of the Fukushima Daiichi Nuclear Power Plant

Significance There is a potential risk of human exposure to radiation owing to the March 2011 Fukushima Daiichi Nuclear Power Plant accident. In this study, we evaluated radiation dose rates from deposited radiocesium in three areas neighboring the restricted and evacuation areas in Fukushima. The mean annual radiation dose rate in 2012 associated with the accident was 0.89–2.51 mSv/y. The mean dose rate estimates in 2022 are comparable with variations of the average 2 mSv/y background radiation exposure from natural radionuclides in Japan. Furthermore, the extra lifetime integrated dose after 2012 is estimated to elevate lifetime risk of cancer incidence by a factor of 1.03 to 1.05 at most, which is unlikely to be epidemiologically detectable. Radiation dose rates were evaluated in three areas neighboring a restricted area within a 20- to 50-km radius of the Fukushima Daiichi Nuclear Power Plant in August–September 2012 and projected to 2022 and 2062. Study participants wore personal dosimeters measuring external dose equivalents, almost entirely from deposited radionuclides (groundshine). External dose rate equivalents owing to the accident averaged 1.03, 2.75, and 1.66 mSv/y in the village of Kawauchi, the Tamano area of Soma, and the Haramachi area of Minamisoma, respectively. Internal dose rates estimated from dietary intake of radiocesium averaged 0.0058, 0.019, and 0.0088 mSv/y in Kawauchi, Tamano, and Haramachi, respectively. Dose rates from inhalation of resuspended radiocesium were lower than 0.001 mSv/y. In 2012, the average annual doses from radiocesium were close to the average background radiation exposure (2 mSv/y) in Japan. Accounting only for the physical decay of radiocesium, mean annual dose rates in 2022 were estimated as 0.31, 0.87, and 0.53 mSv/y in Kawauchi, Tamano, and Haramachi, respectively. The simple and conservative estimates are comparable with variations in the background dose, and unlikely to exceed the ordinary permissible dose rate (1 mSv/y) for the majority of the Fukushima population. Health risk assessment indicates that post-2012 doses will increase lifetime solid cancer, leukemia, and breast cancer incidences by 1.06%, 0.03% and 0.28% respectively, in Tamano. This assessment was derived from short-term observation with uncertainties and did not evaluate the first-year dose and radioiodine exposure. Nevertheless, this estimate provides perspective on the long-term radiation exposure levels in the three regions.

Concentration of radiocesium in rice, vegetables, and fruits cultivated in the evacuation area in Okuma Town, Fukushima

Radiocesium concentration in the crops cultivated in contaminated and decontaminated soils in the evacuation area were measured. The radiocesium concentration in the decontaminated soil decreased approximately one-quarter to that in the contaminated soil. The radiocesium concentration in the rice and vegetables cultivated in the decontaminated soil decreased to one-half to tenth of that in the contaminated soil. Except for eggplant and pumpkin cultivated in the decontaminated field, the radiocesium concentration in crops was higher than the standard limit in foods in Japan. The obtained transfer factors of most crops were within the reported values.

Response of β-glucosidase and proteases to cadmium addition in Andosols from a forest and a cultivated field.

Cadmium (Cd) is considered to be the metal with the most serious adverse effects on microbial and enzyme activities in soils polluted with heavy metals. To investigate its effects, we conducted a short-term laboratory experiment at five levels of Cd addition to the soils (0-, 5-, 50-, 100-, and 250-mg of Cd kg−1 soil) using surface soil samples from Andosols obtained from a forest and a cultivated field in an upland area of Japans Kanto District. We determined the activities of &bgr;-glucosidase and proteases in the soils after 0, 2, 10, 20, 40, and 80 days of incubation. Availability of Cd was determined by measuring water-soluble Cd. We also measured these enzyme activities after the addition of Cd to commercial crude &bgr;-glucosidase and proteases and after the addition of these crude enzymes and Cd to autoclaved soils to evaluate the stability of the enzymes and to detect their changes in response to Cd. During the incubation experiment, Cd inhibited protease activity by up to 47% in the forest soil and 31% in the cultivated soil, where negative correlation was found between soluble Cd content and protease activity on each incubation day. In contrast, &bgr;-glucosidase activity showed little or no significant inhibition. For crude enzyme experiments, Cd did not significantly affect &bgr;-glucosidase activity, but the four proteases we tested were strongly inhibited at different levels of Cd concentrations. Thus, the responses of the &bgr;-glucosidase and protease activities of the crude enzymes to Cd addition were similar to the results for the Andosols with Cd addition. As a result, the apparent direct inhibitory effect of Cd on protease stability would result in a greater decrease in N mineralization than in cellulose decomposition by &bgr;-glucosidase.

Influences of forest decline on various properties of soils on Mt. Hirugatake, Tanzawa Mountains, Kanto District, Japan: III. Changes in microbial biomass and enzyme activities of surface soils

Abstract To evaluate the influences of forest decline on microbial properties, the biomass carbon content and the activities of several enzymes in surface soils under different degrees of forest decline at Mt. Hirugatake, Tanzawa Mountains, Kanto district, Japan were investigated. The biomass carbon content as well as the activities of the dehydrogenase, β-glucosidase, and protease enzymes was significantly decreased with forest decline, and the correlation analysis suggested that these decreases were caused by the decrease in the soil organic matter content associated with forest decline. Moreover, the changes in the composition of soil organic matter might exert a significant influence on the microbial properties, since the proportions of the decrease in the biomass carbon content and enzyme activities were higher than those of the decrease in the total carbon content when a healthy forest site and other five sites with forest decline were compared. Furthermore, stepwise multiple regression analysis strongly indicated that the microbial properties were also affected by the changes in the water-holding capacity and porosity of the soils associated with forest decline in the study area.

Concentrations of 134, 137Cs and 90Sr in Agricultural Products Collected in Fukushima Prefecture

On April 1, 2012, new Standard Limits for radionuclide concentrations in food were promulgated, superseding the Provisional Regulation Values in Japan set in 2011. The new Standard Limits are calculated based on 1 mSv y−1 of annual internal radiation dose through food ingestion of 134Cs, 137Cs, 90Sr, Pu and 106Ru, which were detected or possibly released into the environment from the accident at the TEPCO Fukushima Daiichi Nuclear Power Stations (FDNPS). The concentrations of the radionuclides were based on the values of radiocesium (134, 137Cs) and of the other radionuclides (90Sr, Pu and 106Ru); the ratio observed in the determination or predicted concentrations in the soils from the FDNPS accident was used for estimating the concentration of the other radionuclides by means of the ratio against 137Cs. The new Standard Limit of radiocesium in general foods was defined to be 100 Bq kg−1 fresh weight by the Ministry of Health, Labour and Welfare. In the present study the concentration of radiocesium was measured in agricultural products collected mostly in Fukushima-shi and Date-shi, Fukushima Prefecture, in 2012 and 2013. The average concentration of radiocesium in agricultural plants in 2012 was 7.6 (<0.2–40) Bq kg−1 fresh weight, decreasing to 2.0 (<0.1–14) Bq kg−1 fresh weight in 2013, which was approximately one-fourth of the concentration in 2012. The concentration of 90Sr in agricultural products collected in Fukushima Prefecture in 2013 was 0.0047–0.31 Bq kg−1 fresh weight, which was a similar range to those collected throughout Japan. The concentration ratio of 90Sr/137Cs in the agricultural plants collected from the area 5 km west from the Nuclear Power Stations (difficult-to-return zone) was lower than the predicted 90Sr/137Cs ratio, which was calculated using the ratio in the soils and soil-to-plant transfer factors.

Changes in concentrations of 131I, 134Cs and 137Cs in leafy vegetables, soil and precipitation in Tsukuba city, Ibaraki, Japan, in the first 4 months after the Fukushima Daiichi nuclear power plant accident

Abstract Concentrations of 131I: iodine-131, 134Cs: cesium-134 and 137Cs: cesium-137 in leafy vegetables, soil and precipitation after the Fukushima Daiichi nuclear power plant accident were investigated at Tsukuba city, approximately 170 km from the nuclear power plant. The first precipitation after the accident was on March 21–23, 2011, and contained an amount of radionuclides. The concentration of 131I in leafy vegetables, particularly spinach (Spinacia oleracea L.), was significantly increased because of dry deposition of the first radioactive plume. The concentrations of 131I, 134Cs and 137Cs in soil were significantly increased by the first precipitation on March 21–23; however, the contribution of precipitation to the concentrations of 131I, 134Cs and 137Cs in leafy vegetables was smaller than that in soil. Changes in the concentrations of 131I, 134Cs and 137Cs differed between spinach and komatsuna (Japanese mustard; Brassica campestris var. peruviridis).