Office for Radiation Protection of Workers,
Industrial Health Division,
Occupational Safety and Health Department,
Labour Standards Bureau
Results of the Additional Re-evaluation of Committed Effective Doses of Emergency Workers at TEPCO's Fukushima Daiichi Nuclear Power Plant
The research team funded by the Ministry of Health, Labour and Welfare (MHLW) requested the Tokyo Electric Power Company (TEPCO) to submit detailed data of measured committed effective doses and calculation procedures, etc. with the aim of utilizing such data for the epidemiological studies targeting emergency workers(Note 1), which have been conducted since FY2013. In the process, at the end of January 2014, it was found that TEPCO had data on committed effective doses assessed by a method other than the standard assessment methods.
Since February 2014, the MHLW has closely examined data on emergency workers' committed effective doses to ascertain whether there are any other similar cases. Examined data were for 6,245 emergency workers, excluding those covered by the previous re-evaluation(Note 2), from a total of 7,529 emergency workers (data for workers engaged in March and April 2011). This examination revealed that the data for 1,536 emergency workers are suspected to have been obtained by methods other than the standard assessment methods. On 6 March, 2014, the MHLW instructed TEPCO and primary contractors to re-evaluate these data. Consequently, the committed effective doses for 142 emergency workers were readjusted.
Workers to whom the emergency exposure dose limits (100mSv; 250mSv during the period between 14 March and 16 December, 2011) are applied (until 16 December, 2011, in principle)
Press release on 5 July, 2013; conducted with regard to workers whose committed effective doses assessed by TEPCO and the relevant primary contractor differed significantly
Overview of Additional Re-evaluation
Purpose and Principle
Purpose and principle
Under circumstances with uncertainty, the MHLW provided administrative guidance to choose assessment methods so as to ensure conservative assessment within a reasonable extent and to standardize assessment methods of committed effective doses to the extent possible.
Under circumstances with uncertainty, the MHLW provided administrative guidance to change assessment methods so as to ensure conservative assessment within a reasonable extent and to standardize assessment methods of committed doses to the extent possible.
Difference from the re-evaluation conducted in July 2013
The re-evaluation conducted in July 2013 only covered data in which assessment results by contractors were lower than those by TEPCO and whose validity was doubted. The purpose of the re-evaluation was to confirm the validity of such discrepancies in the data.
The current additional re-evaluation was demanded for the purpose of completely integrating detailed assessment procedures for each nuclide, which will be necessary for exposure assessment in epidemiological studies. This is administrative guidance to get involved in the details of assessment methods and covers all data on the committed effective doses of emergency workers, which is different from the previous re-evaluation.
Results of the additional re-evaluation
Subjects for close examination (emergency workers with internal exposure in March and April 2011; excluding 1,284 workers covered by the previous re-evaluation): 6,245 workers (1,845 at TEPCO and 4,400 at contractors)
Subjects for re-evaluation: 1,536 workers (608 at TEPCO and 928 at contractors)
Subjects for readjustment (whose committed effective doses were not less than 2mSv with a fluctuation range of + 1mSv or larger): 142 workers (24 at TEPCO and 118 at 18 contractors) (0.73% of 19,346 emergency workers)
Fluctuation range: 5.86mSv on average (1.01mSv to 89.83mSv)
Effective dose (emergency exposure dose): 2.17mSv to 180.10mSv
The MHLW provides TEPCO with guidance on the following matters. The MHLW also instructs primary contractors that independently assess committed effective doses about thorough preservation of all the records, etc.
The internal audit sector should inspect the sector in charge of the management of personal doses, check the flow of its operations and data management, etc., and take necessary remedial actions.
Before externally reporting or announcing radiation exposure doses, the data should be checked by a person in charge of radiation management in a quality management sector, in principle.
Furthermore, in accordance with the readjusted committed effective doses, TEPCO should provide health care for the emergency workers based on the MHLW Minister's guidelines.
Rigorous epidemiological studies, including surveys of the age structure, personal habits of smoking and drinking, medical histories, etc., are indispensable for identifying the health effects of radiation. For this purpose, the MHLW will steadily carry out required epidemiological studies.
(Note: mSv = millisieverts)
Increase in the number of workers whose emergency exposure dose exceeded 100mSv: One worker (one at TEPCO)
* 0.6% increase to 174 workers compared with 173 workers prior to the readjustment
TEPCO considered that stable iodine tablets taken by the workers had been effective to some extent and did not conduct estimation of exposure to I-131 by any of the standard methods. However, based on the recommendations from experts, the MHLW considered it preferable to ignore the effects of the stable iodine tablets, as long as the possibility of exposure to I-131 cannot be completely denied, even though this may result in overassessment. Consequently, the MHLW instructed TEPCO to estimate exposure to I-131 under the assumption that the minimum detected amount (MDA) of I-131 was detected and to add such estimated values to the data.
Increase in the number of workers whose emergency exposure dose exceeded 50mSv but did not exceed 100mSv: Two workers at two contractors.
* 0.3% increase to 731 workers compared with 730 workers (as of July 2013) prior to the readjustment