Dozens of workers at the crippled Fukushima No. 1 nuclear power plant were exposed to radiation levels higher than those registered in their personal records, according to a health ministry investigation.
Misleading information was kept in at least 63 cases.
The Ministry of Health, Labor and Welfare has instructed the plant operator, Tokyo Electric Power Co., and its contractors to correct their records, based on the finding.
Data for 49 workers was put right as of Feb. 25, but records for 14 have yet to be updated because their whereabouts are unknown, the ministry said.
The ministry instructed TEPCO and its 37 contractors report to it for the investigation, which looked into radiation exposure levels of those who worked at the plant between November 2011 and October 2012.
These companies introduced a system in November 2011 that used two types of dosimeters: one monitors the radiation dose a worker was exposed to daily and the other tallies cumulative radiation exposure.
The investigation found that 63 workers had the lower of the two readings registered in their individual records despite at least a 20-percent discrepancy between the two devices.
The ministry said the higher reading should be logged for the safety of the workers.
In the largest discrepancy, one worker's monthly exposure was revised from 4.4 millisieverts to 6.35 millisieverts.
Those whose records are not updated could end up working at a nuclear facility beyond the legal annual limits for radiation exposure.
The number of workers with inaccurate personal radiation exposure records is expected to increase because the ministry has yet to start an investigation into the eight-month period immediately following the disaster that unfolded in March 2011.
In those months, radiation levels were much higher than the period investigated and TEPCO and its contractors did not rigorously monitor radiation levels to which the workers were exposed.
Electric utilities and their contractors are obliged to issue personal records for workers at nuclear facilities and in cleanup work and monitor their radiation exposure to keep it within legal limits: up to 50 millisieverts annually and 100 millisieverts over five years.
The records also include the results of their health checks.
Workers receive their records when their contracts end, and new employers are supposed to use them to ascertain the accumulated radiation dose of each individual.
In addition to tracking the radiation exposure of their workers themselves, utilities are also supposed to submit the data in electronic form to the Radiation Effects Association, a public entity, for centralized management.
Earlier, TEPCO was found to have failed to send the association data for a total of 21,000 workers employed over a period from the nuclear accident through fiscal 2011.
The ministry's recent investigation followed reports by The Asahi Shimbun last summer that subcontractors manipulated readings of dosimeters for their workers by covering them with lead shields.
The ministry announced in October that it confirmed 19 instances of irregularities, including non-use of dosimeters.
(This article was written by Toshio Tada and Jun Sato.)
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