It is likely that radioactive cesium from the disaster at Japan's Fukushima Daiichi nuclear power plant spread across much of northern and eastern Japan and could damage agriculture in several provinces, according to estimates released Monday.
The study by researchers based in the United States, Japan and Norway is the first Japan-wide estimate of the spread of contamination from the Fukushima Daiichi meltdowns, said Teppei Yasunari, its lead author.
Yasunari and his colleagues combined observations reported by Japanese government agencies with computer simulations on how cesium-137 particles would have been carried through the atmosphere. The study tracked data from March 20, eight days after the first hydrogen explosion at the tsunami-damaged plant, to April 19. The map is in the study.
"Most of the Japanese general public would like to understand the contaminated area in Japan," Yasunari, earth systems scientist with the Universities Space Research Association, told CNN. Previous studies focused only on contamination in a limited area around Fukushima or on the possible spread of contamination beyond Japan, he said.
Yasunari said he and his colleagues hope Japanese authorities use their projections to plan their cleanup effort.
"What we would like to say from our paper is that (the) Japanese government should carry out soil samplings once in all the prefectures, even if lower contamination is expected at a certain prefecture," Yasunari said.
The study was published Monday by the U.S. journal Proceedings of the National Academy of Sciences.
Cesium-137 has a radioactive half-life of 30 years, making it one of the longer-lived nuclear wastes spread by the Fukushima Daiichi disaster. The largest portion released by the plant probably was blown out to sea by the prevailing winds in the early days of the accident, while the highlands of central Honshu, the main Japanese island, protected the country's west, the researchers found.
"The mountain range in the middle of Japan probably worked like a shield and prevented the direct air flow from the Fukushima nuclear power plant to western areas," Yasunari said. "That is due to the topography in Japan and a fortunate thing."
But concentrations of radioactive cesium fell on eastern Fukushima Prefecture, the province that includes the nuclear plant, at levels that are likely to leave farming "severely impaired," the study found.
The researchers project that eastern Fukushima Prefecture received more than Japan's legal limit of 2,500 becquerels, a measurement of radioactive intensity, of cesium-137 per kilogram of soil. Neighboring prefectures are likely to have received about 250 becquerels per kilogram, the study found.
About 80,000 people remain displaced after the meltdowns in three reactors at Fukushima Daiichi, the worst nuclear disaster since the 1986 Chernobyl accident.
The nuclear plant, located about 240 kilometers (150 miles) north of Tokyo, was swamped by the tsunami that followed Japan's historic March 11 earthquake. The flooding knocked out coolant systems at the plant, leading to spectacular hydrogen explosions in two reactor buildings, a suspected explosion inside a third and extensive damage from the spent fuel housed in a fourth unit that had been shut down before the quake.
The plant's owner, the Tokyo Electric Power Company, says it has been able to keep the reactors cool for several weeks and has greatly reduced the amount of radioactive water that had piled up in the basements of the plant's turbine buildings. Japanese authorities took reporters clad in protective gear on a tour of the crippled plant over the weekend, allowing them the first close-up look at the extent of the damage since March.
Masao Yoshida, the man in charge of the plant, said that all the reactors have stabilized and predicted that Tokyo Electric is on track to have a cold shutdown by year's end. But he conceded that the danger is still far from over - especially for the thousands toiling to bring this nuclear nightmare to an end.
"Even saying it's stabilized doesn't mean that it is extremely safe," Yoshida said. "When working, the radiation remains high. So when it comes to working every day, there is still danger."