3 Avril 2014
A United Nations report on the effects of radiation released in the Fukushima nuclear disaster said findings by the plant operator, Tokyo Electric Power Co., may have underestimated the levels of exposure by contract workers.
In its report on April 2, the U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) also said there was no discernible rise in overall cancer rates among residents of Fukushima Prefecture.
As part of the study, the U.N. committee reviewed data on about 4,000 TEPCO workers and 21,000 other workers employed by its contractors and subcontractors at the Fukushima No. 1 nuclear plant up to October 2012.
It also asked TEPCO and the government to submit internal radiation exposure records on workers and the actual measurement values and density of radioactive materials, on which dose values are based, for its experts to compare their estimates to. Experts from various countries reviewed the data.
Comparison of the data found reasonable agreement between the committee’s independent assessments of effective doses related to TEPCO workers and those reported by TEPCO.
However, the internal dose levels among subcontract company workers that were reported tended to be lower compared with the estimates of the U.N. experts.
In several cases concerning dose levels detected in the thyroid glands of contracted workers, the report said TEPCO’s levels were less than half of those calculated by the committee.
The committee also pointed out that no consideration was given to the possibility that TEPCO or contract workers were exposed to the shorter-lived isotopes of iodine than iodine-131 through inhalation.
“As a result, the assessed doses from internal exposure could have been underestimated by about 20 percent,” the report said.
Doubts raised by UNSCEAR on the effective exposure by workers from affiliated companies reported in an earlier study prompted the Japanese government last year to instruct TEPCO and its contractors to re-evaluate the internal dose levels.
Based on the re-evaluation, TEPCO revised dose levels upward for 500 workers last July.
In March, the utility revised the exposure level upward again for an additional 140 workers.
In terms of impact on the health of Fukushima Prefecture residents, UNSCEAR chair Carl-Magnus Larsson told a news conference on April 2 that cases of thyroid cancer may increase among infants who were 1 year old and lived within 30 kilometers of the nuclear plant at the time of the triple meltdown.
He added, however, that the committee is not sure if that will be reflected in thyroid cancer statistics in the future.
The committee emphasized that Fukushima Prefecture should continue its thyroid cancer screening on young people who were 18 or younger at the time of the nuclear accident.
The data should be analyzed in the future, it said.
The committee also said data collected from similar screening procedures on children in areas not affected by the accident would help determine the influence of radiation exposure on thyroid gland cancer.