A total of 32,760 workers at the Fukushima No. 1 nuclear plant had an annual radiation dose exceeding 5 millisieverts as of the end of January, according to an analysis of Tokyo Electric Power Co. data.
A reading of 5 millisieverts is one of the thresholds of whether nuclear plant workers suffering from leukemia can be eligible for compensation benefits for work-related injuries and illnesses.
Of those workers, 174 had a cumulative radiation dose of more than 100 millisieverts, a level considered to raise the risk of dying after developing cancer by 0.5 percent. Most of the exposure appears to have stemmed from work just after the start of the crisis on March 11, 2011.
The highest reading was 678.8 millisieverts.
Overall, a total of 46,490 workers were exposed to radiation, with the average at 12.7 millisieverts.
The number of workers with an annual dose of over 5 millisieverts increased 34 percent from fiscal 2013 to 6,600 in fiscal 2014, when workloads grew to address the increase in radiation-tainted water at the plant. The number was at 4,223 in the first 10 months of fiscal 2015, which ends this month, on track to mark an annual decline.
A labor standards supervision office in Fukushima Prefecture last October accepted a claim for workers compensation by a man who developed leukemia after working at the plant, the first recognition of cancer linked to work after the meltdowns as a work-related illness. Similar compensation claims have been rejected in three cases so far, according to the labor ministry.
The average radiation dose was higher among Tepco workers at the plant than among workers from subcontractors in fiscal 2010 and 2011. Starting in fiscal 2012, the reading was higher among subcontractor workers than among Tepco workers.
The average dose for subcontractor workers was 1.7 times the level of Tepco workers in fiscal 2013, 2.3 times in fiscal 2014 and 2.5 times in fiscal 2015 as of the end of January.
A separate analysis of data from the Nuclear Regulation Authority showed that the average radiation dose of workers at 15 nuclear power plants across the country, excluding the Fukushima No. 1 and No. 2 plants, fell to 0.22 millisievert in fiscal 2014, when none of the plants was in operation, down 78 percent from 0.99 millisievert in fiscal 2010.