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

Abstract

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.

Keywords: Fukushima nuclear disaster; Strontium-90; exposure assessment; food duplicate; forest contamination.

Fig. 1.

Former and current statuses of the restricted area and areas to which evacuation orders have been issued around the FDNPP (Left, April 22, 2011; Right, August 8, 2013). Municipalities belonging to the Sousou region are bordered by thick lines. The three study areas are in blue.

Fig. 2.

Cumulative probability plot of external, ingestion, and inhalation radiation dose rates in 2012.

Fig. 3.

Time course of total, ingestion, and inhalation radiation dose rates in the village of Kawauchi based on the physical decay of radiocesium. The 50th (solid line) and 90th percentile (dotted line) values were plotted. The total dose rate was calculated by Monte Carlo simulation. Ingestion and inhalation dose rates were derived from parameter estimates for a log-normal distribution.