Biggest radiation dose outside Fukushima zone

Residents outside the Fukushima exclusion zone were likely most exposed to radiation in the four months after the nuclear plant was wrecked by the March 11 earthquake and tsunami, the local government said on Tuesday.

Their exposure of 19 millisieverts is just below the annual limit set by an international nuclear safety body, it said.

The greatest exposure was in the town of Iitate, where residents were allowed to take their time to leave, located 40 km (25 miles) northwest of the plant and outside the 20 km evacuation zone imposed by the government.

The March disaster knocked out reactor cooling systems at the Fukushima Daiichi plant, operated by Tokyo Electric Power Co and located 240 km (150 miles) northeast of the capital, triggering radiation leaks over a large swathe of northern and eastern Japan, contaminating vegetables, tea, milk, seafood and water.

Japanese authorities have faced widespread criticism over a perceived slow response in evacuating residents from near the plant.

According to the estimates, those living within the 20 km zone, who were urged to leave quickly, were likely exposed to 0.18 to 2.3 millisieverts in the four months after the disaster.

That is less than the estimated 0.84-19 millisieverts for those living near the plant but outside the evacuation zone.

Most residents left the zone within a few days of the disaster.

"The research categorises residents' behaviour into 18 patterns to come up with the estimated exposure for each group. There is a possibility some people have been subject to heavier exposure," an official at the Fukushima local government said.

"But as long as exposure stays somewhere near 19, there should be no health risks."

The International Commission on Radiological Protection recommends that exposure be kept below 20 millisieverts per year for a period that follows the emergency phase of an incident.