By KEITARO FUKUCHI/ Staff Writer
December 31, 2024 at 07:00 JST
Visitors crowd a street lined with blooming cherry trees in Tomioka, Fukushima Prefecture, on April 1, 2023, the day an evacuation order was lifted for part of the town. (Sayuri Ide)
The Japan Atomic Energy Agency said it has developed a more accurate method to estimate radiation exposure doses among people who spend time around the stricken Fukushima No. 1 nuclear plant.
The JAEA has adapted the method, based on daily life patterns, into program format and is offering it for free on a municipal government website and elsewhere.
When the central government designated evacuation zones following the 2011 triple meltdown at the plant operated by Tokyo Electric Power Co., it estimated radiation doses among residents using a simple evaluation method that assumed they spent eight hours outdoors and 16 hours indoors a day.
That method allowed for quick estimation, but it tended to overestimate the doses.
Other existing evaluation methods also have shortcomings, including a failure to reflect the actual environment.
The JAEA began developing the new method in 2017.
JAEA researchers drew on data compiled by the Nuclear Regulation Authority to calculate average air dose rates for 100-meter-by-100-meter areas.
They also took into account where and for how long the residents and workers frequented near the plant, and how they moved between different locations, such as on foot or by car, the officials said.
The JAEA researchers also verified the accuracy of the evaluation method.
They asked around 30 people working in former and current evacuation zones to carry personal dosimeters and then compared the measurements and estimates for their exposure doses in 106 patterns.
The study showed the exposure dose estimates could be off by 10 percent or so under a dose rate of about 20 millisieverts per year, the threshold for lifting an evacuation order, the officials said.
A research article on the method, including the verification results, was published in November in the online edition of science journal Environment International (https://doi.org/10.1016/j.envint.2024.109148).
JAEA officials said forecasts derived from the method have been provided to cleanup review committees of different local governments, where they have been used to help decide whether evacuation orders should be lifted.
Dose evaluation systems based on the method are available for public access on online bulletin boards of municipal offices and in other locations in Namie, Tomioka and Katsurao in Fukushima Prefecture. The systems can also be used on a website operated by the Okuma town government, also in the prefecture (https://simulation.okuma-town.jp/seed/makePattern/07545/).
After information on daily life patterns is entered, the systems return estimates for exposure doses.
“Our method can be used to help officials decide whether evacuation orders should be lifted, thereby contributing to Fukushima Prefecture,” said Kazuya Yoshimura, a JAEA official who was on the research team. “It would also allow, in the event of another nuclear disaster, exposure doses to be evaluated instantly and evacuation routes to be optimized.”
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