External radiation exposure via car-borne survey among returnees in former restricted regions in Fukushima a decade after the nuclear accident

Prolonged radiation exposure continues in Fukushima Prefecture, Japan, due to the presence of long-lived artificial radionuclides (mostly 134Cs and 137Cs) discharged from the Fukushima Daiichi Nuclear Power Plant in 2011. Extensive decontamination has reduced the ambient gamma dose rates, allowing the government to lift evacuation orders. However, the external effective doses additionally received from the discharged radionuclides should be evaluated by considering the heterogeneous spatial distribution of ambient gamma dose rates from primordial radionuclides (238U- and 232Th-series elements and 40K) in the affected areas. The present study aimed to comparatively evaluate the doses of artificial and primordial radionuclides in Kawauchi Village and Tomioka and Okuma Towns by discriminatively measuring the dose rates associated with these radionuclides using a car-borne survey technique, thus elucidating the prolonged radiological impact over a decade after the nuclear accident. The results showed that the annual external effective doses of 134Cs and 137Cs ranged from 0 to 1.7 mSv (median range: 0.04–0.55 mSv), whereas those of primordial radionuclides ranged from 0.11 to 0.35 mSv. At more than 90% of the 938 measurement points, the annual doses of 134Cs and 137Cs were well below 1 mSv, which represent the lowest value of the reference level band (1–20 mSv) used for existing exposure situations conceptualised by the International Commission on Radiological Protection. The additional external dose remained below 1 mSv and was of the same order of magnitude as the external dose from the primordial radionuclides because the evacuation order was lifted. Government bodies can incorporate such data to promote public understanding of radiation in the affected areas to reduce concern surrounding radiation exposure on returnees, migrants, and evacuees.