Post-Fukushima Radiation Mapped

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 Sciencesconfirm 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.

Teppei Yasunari, a researcher at the Universities Space Research Association, in Columbia, Md., who led the PNAS study, hopes that Japanese authorities can use their cesium deposition maps to carry out more-detailed assessments and plan cleanup efforts. “Eastern Fukushima is highly impaired for farming,” he says. “For other areas, we need to keep watching.”

The other PNAS study, conducted by researchers at the University of Tsukuba, in Ibaraki, and at Nihon University, in Tokyo, measured the spread of radioactive iodine and tellurium in addition to cesium in Fukushima and the five prefectures to its south and southwest by measuring the energy of gamma rays emitted by soil samples. Each radionuclide produces gamma rays with unique energy signatures. The researchers found high levels of all three materials throughout most of Fukushima, with the highest levels in its northeastern regions directly north of the plant. Cesium-137 in northeastern Fukushima ranges from 100 000 Bq/m² to 1 million Bq/m².

Cleanup could take years, and the radiation has already affected agriculture. Levels of cesium exceeding the government’s safe limit of 500 Bq/kg have been found in food products, including leafy vegetables, mushrooms, milk, and beef. The government has suspended shipments of these foods, but some of it has found its way into supermarkets and restaurants.

Rice, Japan’s staple, most recently took a hit. In mid- and late November, high cesium levels were detected in rice samples from six farms in Fukushima prefecture, the highest level being 1270 Bq/kg. The farms are in Onami district, 60 km from the power plant. The government has banned rice shipments from all 154 farms in the district while it analyzes samples from each.

“There’s really no good way to clean up cesium-137 from a large area,” Moulder says. “To decontaminate a playground, you can scoop up the soil and lay down new asphalt, but you can’t scoop up a whole rice field. You’ll then have to dispose all that radioactive waste. These areas could become inhabitable but still couldn’t be used for agriculture.”

For all the gloom, one bit of positive news comes from a preliminary report published in the journal PLoS One. The report is by researchers at Hirosaki University, located 355 km north of Fukushima City. Between 15 March and 20 June, the university sent staff members to monitor contamination at the shelters for evacuated residents who had lived within a 20-km radius of the power plant.

Thirteen teams surveyed more than 5000 residents using Geiger counters and also kept track of their own exposure levels. They found that only 10 people among those surveyed had high radiation levels, and even those measurements, at less than 100 000 counts per minute, were not high enough to merit decontamination. Nearly everyone else showed levels low enough to classify them as not contaminated.