TOKYO—The first comprehensive survey of soil contamination from the Fukushima Daiichi nuclear plant showed that 33 locations spread over a wide area have been contaminated with long-lasting radioactive cesium, the government said Tuesday.
The survey of 2,200 locations within a 100-kilometer (62-mile) radius of the crippled plant found that those 33 locations had cesium-137 in excess of 1.48 million becquerels per square meter, the level set by the Soviet Union for forced resettlement after the 1986 Chernobyl disaster.
Another 132 locations had a combined amount of cesium 137/134 over 555,000 becquerels per square meter, the level at which the Soviet authorities called for voluntary evacuation and imposed a ban on farming.
Authorities said that all of the highest levels are within the current evacuation zone, which is generally 20 kilometers (12 miles) around the plant plus some specific towns to the northwest that have already been found to have high levels of contamination.
Cesium-137 has a half life of 30 years, meaning that its radioactive emissions will decline only by half after 30 years and affect the environment over several generations. Cesium-134 is considered somewhat less of a long-term problem because it has a half-life of two years.
More than 400 researchers from across the country took part in the survey, conducted between June and July, collecting samples from every two square kilometers (1.2 miles) of land within the 100-kilometer radius of the crippled plant. Until then, only estimates were available about the extent of soil contamination through aerial surveys and airborne dust samplings.
"The results of the soil analysis have confirmed our estimates about contamination," an official of the education ministry said at a press briefing.
Japanese authorities said last week they expected the levels of radiation to fall by half in areas around the plant in two years through natural decay and cleanup efforts. But the latest data point to the possibility that cesium could also be washing away and spreading to other areas, potentially contaminating rivers, lower-lying land and the ocean.
Gunma prefecture, north of Tokyo, reported Monday that a fish containing more than the legally allowed amount of cesium was caught in a river in the prefecture, the first such case outside Fukushima prefecture, where the plant is located.
Also Tuesday, plant operator Tokyo Electric Power Co. said that a 40-year-old worker died of acute leukemia after working for seven days at the plant. The worker's cumulative radiation exposure was 0.5 millisievert, far below the legal limit. Tepco said that his death is unlikely to be related to his work at the plant.
The Health and Labor Ministry separately said that it may again lower the radiation exposure limit for workers at the plant from 250 millisieverts per year to 100 millisieverts, a level that is applied to other nuclear plants in Japan in emergency situations. The higher level had been set in March as an emergency level for workers only at Fukushima Daiichi.
Write to Mitsuru Obe at mitsuru.obe@dowjones.com
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