By Katherine Kornei
Originally Published in Science
Mar. 10, 2017
Six years after Japan’s Fukushima Daiichi Nuclear Power Plant disaster, thousands of former residents evacuated from the region are returning home. But is it safe? A new study says yes, finding that inhabitants of a never-evacuated village just 60 kilometers away don’t have to worry about dangerous levels of radiation over their lifetime. The team also found that natural radioactive decay and weathering from rain deserve far more credit for reducing radiation levels than do expensive decontamination efforts, like topsoil removal.
These results should help citizens decide whether to return to areas where evacuation orders are being lifted, says Deborah Oughton, an environmental chemist at the Norwegian University of Life Sciences in Ås, who was not involved in the study. Later this month, roughly 52,000 evacuees will be invited back to Fukushima prefecture.
The new effort was conducted by Makoto Miyazaki, a radiologist at Fukushima Medical University, and Ryugo Hayano, a University of Tokyo physicist. They analyzed radiation measurements collected by helicopters to track how levels of radioactivity above Date, the unevacuated village near the Daiichi reactors, changed over time. The researchers found that radiation levels—primarily from radioactive cesium—fell by 60% between 2011 and 2013. By estimating how that level would continue to decrease over the next 70 years and converting the results to ground-based radiation levels, the team inferred cumulative radiation doses in different parts of Date.
The median lifetime radiation dose due to the accident was 18 millisieverts in Zone A, the most contaminated part of the city, the team will report in an upcoming issue of the Journal of Radiological Protection. That’s extremely low considering that the International Commission on Radiological Protection considers a dose of between 1 and 20 millisieverts per year acceptable. Furthermore, 18 millisieverts is less than a person receives naturally over an entire lifetime from radioactive elements in Earth’s crust and high-energy particles bombarding the atmosphere.
Miyazaki and Hayano also investigated whether costly decontamination efforts such as topsoil removal and roof washing had any measureable impact on radiation levels in Date. Using personal radiation measurements collected by 425 people living in Zone A, the researchers found no evidence of a drop in radiation around the time of the decontamination work, which began in October 2012.
“Population-wise, we didn’t see a big decrease in the individual dose,” says Hayano. That’s not to say that decontamination shouldn’t have been carried out, the researchers note. “There may be individuals or families for whom the decontamination was effective,” he says. But he and Miyazaki conclude that radiation levels fell primarily because the radioactive cesium physically decayed and was washed away by rain and snow.
Kathryn Higley, a certified health physicist at the School of Nuclear Science and Engineering at Oregon State University in Corvallis, isn’t surprised that the decontamination made little difference. Only the areas about people’s houses were decontaminated, she says, and “people do not spend all of their time at home.”
The findings will come as a relief to Date citizens, and to evacuees returning to other parts of Fukushima prefecture. Miyazaki and Hayano also hope their results will help guide the response to future nuclear disasters. “If there’s ever another accident—and we hope there won’t be—[our work] may be useful for estimating lifetime radiation doses and setting the schedule of decontamination,” Miyazaki says.
*Update, 13 March, 1:16 p.m.: The headline of this story has been modified to make it clear that not all of Fukushima may be safe to return to.