An overview of internal dose estimation using whole-body counters in Fukushima Prefecture.

A large amount of radioactive cesium was released by the Fukushima Daiichi Nuclear Power Plant accident following the Great East Japan Earthquake. Due to the increasing concerns about internal exposure, more than 50 whole-body counters (WBCs) have been installed at various locations in Fukushima Prefecture. A study on around 10,000 subjects in the early stage after the accident revealed that very few received a committed effective dose of more than 0.3 mSv for subjects (age >13 years old). Another study on WBC results for one hospital showed that the ratio of cesium-positive was 1.0% among all the subjects. Assuming a constant daily intake, the detection limit of 300 Bq/body for a typical WBC corresponds to an effective dose of 21 μSv/y even for a subject of age up to 10. It was also seen out that the subjects with a significant amount of body cesium are likely to regularly eat wild products, which they harvested or caught themselves without testing for radioactive cesium. These study findings suggested that the internal exposure for most of the residents was controlled at a very low level. Future tasks regarding WBC measurements are how to personally explain the WBC results to each subject and how to disclose the statistically processed WBC data to the general public.