Tertiary evaluation of the committed effective dose of emergency workers that responded to the Fukushima Daiichi NPP accident

Abstract

ABSTRACTIn January 2014, Tokyo Electric Power Company (TEPCO) learned that the committed effective dose (CED) for nine emergency workers at the Fukushima Daiichi Nuclear Power Plant accident had been assessed by a method other than the standard assessment methods, established by the Ministry of Health, Labour and Welfare (MHLW) in a secondary evaluation conducted in July 2013. The MHLW requested that the TEPCO and primary contractors review all CED data for 6,245 workers who engaged in emergency work in March and April 2011 except those previously reviewed in the 2013 secondary evaluation. This tertiary evaluation revealed that the recorded CED for 1,536 workers had more than 0.1 mSv discrepancy with the CED evaluated by the standard method. The MHLW requested that TEPCO and primary contractors revise CED data for 142 workers whose CED was 2 mSv or greater that required a CED revision of 1 mSv or greater. The average CED revision was 5.86 mSv. The revised effective dose ranged from 2.17–180.10 mSv. In addition, the number of workers whose CED exceeded 100 mSv increased by one. New issues addressed during the tertiary evaluation included the following: (a) setting of calibration coefficients to convert the CED value from whole body counters equipped with NaI scintillator (WBC(NaI)) to a CED value from WBCs with Ge semiconductor detector; (b) estimation methods for the cases where 131I was not detectable by WBC (NaI) and where 137Cs was not detectable but 134Cs was detected; (c) effects of stable iodine (KI) tablets to block the uptake of 131I by thyroid gland; and (d) complications in determining additional doses during stand-by in the seismically isolated building. To prevent the future use of non-uniform CED assessment methods in the dose assessment for workers, the MHLW issued administrative guidance documents to TEPCO and primary contractors on March 25, 2014.