Residents from a 30-kilometer radius of the stricken Fukushima No. 1 nuclear power plant were exposed to up to 50 millisieverts of radiation for two months after the onset of the crisis, according to researchers at Hirosaki University.
Residents from a 30-kilometer radius of the stricken Fukushima No. 1 nuclear power plant were exposed to up to 50 millisieverts of radiation for two months after the onset of the crisis, according to researchers at Hirosaki University.
When including exposure after they were evacuated, some are expected to have readings of 68 millisieverts a year, or more than three times the figure set by the central government when it ordered the evacuations.
The researchers estimated that radiation levels in the area during the two months before the evacuations came to 50 millisieverts.
Had residents of the Akougi area of Namie in the prefecture stayed, they would be exposed to radiation at a level of 190 millisieverts a year, according to the team's assessment.
A research paper on this investigation was released in the science report of the British science journal Nature, published Sept. 7.
The Akougi area, which lies just outside the 30-kilometer radius of the plant, was designated a "planned evacuation zone" in mid-April by the central government. All residents were required to evacuate by the end of May.
The estimate assumes that residents from the area moved to other towns in the prefecture two months after the March 11 onset of crisis at the Fukushima plant.
"Evacuation cut the radiation exposure to one-third," said Shinji Tokonami, a medical radiation expert who led the Hirosaki University team. "The government made the right decision from the perspective of avoiding exposure."
The research team measured airborne radiation at 1,623 locations--all of which were more than 20 kilometers northwest of the plant--in mid-April. The Koakuto district of the Akougi area had the highest level among residential areas at 32 microsieverts per hour.
Taking the half-life periods of cesium-134 and cesium-137 into account, the accumulated external exposure in the area was assessed at about 190 millisieverts a year, on the assumption that residents spent eight hours outside each day.
The team then estimated the exposure levels of people who relocated to Fukushima, Koriyama and Nihonmatsu cities in the prefecture at 57-68 millisieverts, 57-59 millisieverts and 59-64 millisieverts, respectively. Some areas of Fukushima city were measured at 3.2 microsieverts per hour.
According to radiation monitoring by the science ministry, the Akougi area is among areas with the highest radiation levels in the town of Namie. The upper limit of radiation to which people can be safely exposed is 1 millisievert a year, with 50 millisieverts for male workers handling radiation-related work. The planned evacuation zone was set for areas with radiation levels exceeding 20 millisieverts a year.
When the crisis flared at the Fukushima plant, about 360 people lived in the Akougi area, including 20 residents in the Koakuto district.