Radiation estimates for No. 1 workers likely 20% too low: U.N.

Kyodo

The radiation doses workers received in the initial phase of the Fukushima disaster may have underestimated by 20 percent, a report by a U.N. panel says.

The U.N. Scientific Committee on the Effects of Atomic Radiation said in a summary report on its website that the Japanese government and Tokyo Electric Power Co., known as Tepco, used tests that failed to take into account some types of radiation released by the three meltdowns at the Fukushima No. 1 power plant in March 2011.

The report said the committee analyzed the doses received by some 25,000 people working at the plant on or before October 2012, using data from Japan, Tepco and others to assess the amount of substances discharged during the crisis after the March 2011 earthquake and tsunami.

It also noted that workers were tested for radioactive iodine absorbed by their thyroid glands after a significant delay, with no account taken of “the potential contribution from intakes of shorter-lived isotopes of iodine, in particular iodine 133,” which have a short half-life of 20 hours.

It said that “as a result, the assessed doses from internal exposure could have been underestimated by about 20 percent.”

Increased exposure to such iodine is linked to increase risk of cancer and thyroid disorders.

If the estimates of the U.N. committee are accurate, more Fukushima plant workers will be eligible for free health checks from the government and Tepco, which says about 2,000 workers whose thyroids got doses of 100 millisieverts or more qualify for cervical ultrasound inspection.

The committee also said that, for the 12 workers estimated to have received 2 to 12 gray of thyroid exposure from iodine-131 alone, “an increased risk of developing thyroid cancer and other thyroid disorders can be inferred.”

Higher cancer risk is expected for more than 160 additional workers who received over 100 millisieverts from external exposure, but the incidence is expected to be “indiscernible,” it said.

The committee said that although the doses received by residents near the plant are low, continued research is needed to identify the full scope and expression of the differences in effects, mechanisms and risk from exposure to ionizing radiation for children and for adults.