The test records of 479 workers at the crippled Fukushima No. 1 nuclear power plant contained false documentation on the amount of internal radiation they were exposed to, the Ministry of Health, Labor and Welfare said July 5.
The records of 452 of them have since been revised upward by a maximum of 48.9 millisieverts, according to health officials. The records of the rest were revised downward.
The ministry said Tokyo Electric Power Co., the plant operator, failed to follow government instruction to make sure that its employees and contractors followed proper calculation protocol and that inadequate methods were employed to estimate the amount of internal exposure.
In March, records of external exposure were found to be in error by up to several millisieverts in 63 individuals.
The revised internal exposure calculations recorded 50 millisieverts in 24 people by the end of March 2013. Six people topped 100 millisieverts.
The maximum dose limit for nuclear plant workers by law is set at 100 millisieverts over a five-year period. At least two individuals continued to work after reaching that limit.
The latest findings increase concerns over the health effects from radiation following the revelations that workers received much greater exposure than originally reported.
About 20,000 individuals had worked at the stricken nuclear plant by the end of 2011, nine months after the reactor meltdowns.
Protocol calls for individuals to be tested at the first sign of internal exposure. But radiation levels were not taken for several months after initial exposure due to a shortage of measuring devices, though in principle, the workers should have been tested immediately.
According to the findings by the health ministry, the erroneous documentation partially assumed that radioactive substances entered the body at midpoint during the work period. But according to accepted protocol for nuclear emergencies, when the exact date of the first exposure is not available, the calculations should assume that the intake took place at the beginning of the work period.
Input errors were also found in other cases.
The now-defunct Nuclear and Industrial Safety Agency instructed TEPCO in July 2011 to set official calculation rules, which the utility failed to carry out scrupulously. Academics and others pointed out the possibility of inaccurate exposure calculations, which TEPCO never seriously investigated.
There was similar inaction on the part of the government. It began delving into the matter only this year after the United Nations organization tasked with studying the effects of radiation raised questions about the records.
Neither TEPCO nor the health ministry has come forward to take responsibility for the discrepancies.
"We have notified our employees and contractors of the rules, and we believed they were observing them," a TEPCO official said. "We have done all in our power we can do."
Health ministry officials say they believed that TEPCO was making sure the rules were observed.
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