Editor's Note: This is part of the IEEE Spectrum special report: Fukushima and the Future of Nuclear Power.
2 December 2011—More than eight months after a tsunami triggered a meltdown at the Fukushima Dai-ichi nuclear power plant, the broad impact of the accident on Japan’s people and lands is becoming clear.
Three recently published academic studies show that while direct radiation exposure of Fukushima residents isn’t as high as was initially feared, soils across northeastern Japan are contaminated and could affect public health for decades through the produce farmed there. The research, combined with a map of soil radiation—which was based on measurements made during helicopter flights and released by Japan's science ministry—shows substantial soil contamination in the prefectures of Fukushima and its neighbors: Miyagi and Iwate to the north, Ibaraki and Chiba to the south, and Tochigi and Gunma to the southwest.
The Fukushima Dai-ichi accident—rated 7, the highest possible on the International Nuclear Event Scale released 160 petabecquerels of iodine-131 and 15 PBq of cesium-137, according to Japan’s Nuclear and Industrial Safety Agency. Both radioactive nuclides cause an increased risk of cancer, but cesium-137, with its half-life of about 30 years (compared with eight days for I-131) poses the most concern over the long term: The isotope is still responsible for radiation in the dead zone surrounding the Chernobyl site. The Japanese government considers a total cesium level—the sum of cesium-137 and cesium-134, which has a half-life of two years—higher than 5000 becquerels per kilogram of soil unsafe for farming.
“It is not trivial to convert dose in the dirt to dose in the food and then in people,” says John Moulder, a professor of radiation oncology at the Medical College of Wisconsin. “But some of the sampling around Fukushima indicates that the dose in the ground is well over what you want in agricultural lands.”
The government’s radiation map shows high levels of radioactive cesium in Fukushima and surrounding prefectures. Some spots have levels between 100 000 and 600 000 becquerels per square meter (148 000 was the standard used for mandatory resettlement after the Chernobyl disaster). That the cesium is mostly in chloride form makes matters worse, Moulder says: “It’s water soluble, easily taken up by the body, and very well distributed in the body—all the things you don’t want.” The contamination could also irradiate anyone who walks on the ground, he adds.
According to the Japanese newspaper The Asahi Shimbun, the science ministry says that about 8 percent of the country’s land has been contaminated with levels higher than 10 000 Bq/m2, a threshold that Japan’s science ministry defines as affected by a nuclear accident. The newspaper also reports that the government has confirmed radioactive materials from the meltdown in all prefectures, including Okinawa, which is 1700 kilometers from the power plant.
Two studies published this month online in the journal Proceedings of the National Academy of Sciences confirm the extent and intensity of soil contamination across Japan. In the first study, researchers from Japan, Norway, and the United States combined the Japanese government’s daily measurements of cesium-137 levels in soil from all prefectures between 20 March and 19 April with a computer model that calculated the transport of particles in the atmosphere and their deposition in the ground through rain and snow. The model allowed the researchers to calculate soil concentrations that confirmed and added detail to the readings the science ministry took by helicopter.
They estimate that the area around the nuclear power plant has more than 100 000 Bq/m2 of cesium-137. Miyagi and Ibaraki have levels higher than 25 000 Bq/m2. Using a conversion factor of 53 kilograms of soil per square meter, they calculate that soils in the eastern part of Fukushima prefecture exceed the government’s safe limit of 2500 Bq/kg of cesium-137 (about half of the total radioactive cesium is composed of cesium-137).
The researchers believe that some spots in neighboring Miyagi, Tochigi, and Ibaraki are likely to exceed the limit. Mountain ranges prevented the flow of radioactive plumes to the western parts of the country, the researchers say.