Study finds wide variation in radiation levels within no-entry zone

BY AKIKO OKAZAKI STAFF WRITER

2011/04/23

A study of radiation levels within a 20-kilometer radius of the crippled Fukushima No. 1 nuclear power plant revealed contamination levels in many locations would not currently pose a major threat to human health.

The research, conducted by the Ministry of Education, Culture, Sports, Science and Technology and the Fukushima plant's operator, Tokyo Electric Power Co. (TEPCO), assumed that current radiation levels would be unchanged for a year and that exposed individuals would remain outdoors for eight hours a day.

Of 128 locations within the 20-km zone, researchers projected that annual radiation exposure would exceed 100 millisieverts in only 17 places.

According to the International Commission on Radiological Protection (ICRP), annual radiation exposure exceeding 100 millisieverts poses dangers to health. Annual exposure exceeding 500 millisieverts is associated with a drop in lymphocyte cells in the blood.

The worst-contaminated location had a radiation level that might expose individuals to more than 500 millisieverts in a year. But nearly half of the sample locations had radiation levels below the 20 millisieverts standard used by the central government as a guideline for issuing evacuation orders.

Because of the huge impact of evacuation on the lives of residents, some experts argued the exclusion rules should be reviewed in the light of further research.

Shunichi Yamashita, a professor of radiation medicine at Nagasaki University and adviser to Fukushima Prefecture on radiation health risk control, said: "The ICRP has set 100 millisieverts as a limit so ordinary people will not be placed under the risk of radiation exposure. It will be difficult for residents to return soon to areas that have high levels of radiation."

But areas believed to pose that level of risk were mainly within a radius of between 1 and 6 kilometers of the Fukushima plant and to the northwest of the plant, including Namie town.

Wind direction and local topography has had a major influence on the dispersal of radioactive materials.

In Minami-Soma, a majority of study locations had radiation exposure levels less than 20 millisieverts. Even within a 10-kilometer radius of the Fukushima plant, there were 16 locations where projected radiation exposure levels were under 20 millisieverts.

Yutaka Kukita, deputy chairman of the Nuclear Safety Commission of Japan, told an April 20 news conference: "The reactors (at the Fukushima No. 1 nuclear power plant) continue to be in an unstable condition. We cannot ignore the possibility of radioactive material being emitted once again, so we decided to designate an area within a 20-kilometer radius as the no-entry zone for the time being."

But Yoshiya Shimada, a project leader on medical radiation research at the National Institute of Radiological Sciences, said: "There is almost no risk of cancer if the radiation exposure level is about 20 millisieverts. For the elderly, the stress from living as an evacuee will have a much greater impact on their health than the risks from radiation exposure. Rather than look just at radiation levels, a more flexible response should be adopted in the future taking into account social, economic and psychological factors."