Governmental Re-evaluation of the Committed Effective Dose Received by Emergency Workers at the TEPCO Fukushima Daiichi NPP Accident

Abstract

In April 2013, the Ministry of Health, Labour and Welfare (MHLW) noticed that significant discrepancies were present between the committed effective dose (CED) data for emergency workers at the TEPCO Fukushima Daiichi Nuclear Power Plant provided by TEPCO and that reported by five primary contractors. The MHLW, based on re-evaluation of the data, provided administrative guidance which required TEPCO and the five primary contractors to readjust the CED data for 479 workers (2.5% of 19,346 emergency workers). Of these, data for 450 workers from the primary contractors were readjusted based on the change in assessment methods. Doses were readjusted to higher values for 431 workers and doses were readjusted to lower values for 19 workers, CED data were corrected due to calculation errors for 29 workers from seven contractors, 12 additional workers were found to have received more than 50 mSv but no more than 100 mSv, an increase of 1.7% over the previously established count of 723 workers who had received that dose. Six additional workers were found to have received more than 100 mSv, an increase of 3.6% compared with the previous count of 167 workers. Major issues addressed during re-evaluation included: a) selection of intake scenario for the calculation of the residual activity ratio; b) assumptions about the intake date; c) assessments of exposure to radiation from 132I and 132Te; and d) assumptions about 131I exposure in cases where 131I was undetectable. Regarding the divergent CED data of 138 workers, the MHLW also confirmed that the CEDs assessed by the primary contractors were valid. To prevent the recurrence of similar incidents, the MHLW issued administrative guidance documents to TEPCO and primary contractors to employ standardized CED assessment methods on July 5. 2013.