EARLY INTAKE OF RADIOCESIUM BY RESIDENTS LIVING NEAR THE TEPCO FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT AFTER THE ACCIDENT. PART 1: INTERNAL DOSES BASED ON WHOLE-BODY MEASUREMENTS BY NIRS

The Tokyo Electric Power Company’s Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 resulted in a release of radionuclides into the environment (131I: 142.9 PBq, 137Cs:12.4 PBq). This study presents the results of internal doses to 174 residents living near the FDNPP at the time of the accident based on whole-body (WB) measurements performed by the National Institute of Radiological Sciences (NIRS) during the period between 27 June and 28 July, 2011. The 174 subjects consisted of 125 adults (≥ 18-y) and 49 children ( females) was observed in the adults, but not the children. In this study, the committed effective dose (CED) from 134Cs and 137Cs was calculated based on individual WB contents (134Cs) corrected against body size, the observed body content ratio of 137Cs to 134Cs, and the assumed intake scenario, namely acute inhalation of Type F compounds on 12 March, 2011 when the first explosive event occurred at the site of the FDNPP. The 90th-percentile CED value for the adults was around 0.1 mSv and the maximum CED (0.63 mSv) was found in an elderly male. Comparable CED results were obtained in other WB measurements subsequently performed by the Japan Atomic Energy Agency (JAEA) in a similar manner to that of the NIRS, suggesting that the contribution of ingestion to the WB content observed would be trivial for most of the JAEA subjects. The intake ratio of 131I to 134Cs was evaluated to be 3~5 based on the 131I thyroid measurement data of Tokonami et al. Using the average intake ratio of 3.8, the resulting median and maximum thyroid-equivalent doses to the adult subjects of this study were estimated at 3.5 mSv and 84 mSv, respectively