TSUKUBA, Ibaraki Prefecture--Nearly 30,000 becquerels per square meter of cesium-137 fell on Tsukuba in March as a result of the accident at the Fukushima No. 1 nuclear power plant, the government's Meteorological Research Institute said Dec. 1.
The amount was 50 times higher than the previous record level of 550 becquerels, which was measured in Tokyo in 1963 and was the result of deposits from atmospheric nuclear tests.
The MRI, affiliated with the Japan Meteorological Agency, said the cesium-137 deposits in Tsukuba in April fell to less than one-tenth the March level, and by summer fell further to several tens of becquerels per square meter, approximately the same levels found in the aftermath of the 1986 Chernobyl accident, researchers said.
"It may take decades for the figures to come down to levels before the Fukushima accident," said Yasuhito Igarashi, a laboratory head at the MRI's Atmospheric Environment and Applied Meteorology Research Department.
Meanwhile, analysis of seawater collected in April and May found that radioactive substances spewed by the stricken Fukushima No. 1 plant fell over broad areas in the North Pacific. Fallout was also detected near the West Coast of the United States.
The research institute estimated that the Fukushima plant discharged at least 3,500 trillion becquerels each of cesium-137 and cesium-134 into the ocean. It forecast that the radioactive material will spread east across the North Pacific on surface ocean current before drifting southwest on deeper ocean currents. Part of the radioactive materials carried by mid-depth ocean currents will return to seas near Japan's coast in 20-30 years, the scientists said.
"Continual surveys are necessary across all areas of the North Pacific," said Michio Aoyama, a laboratory head at the MRI's Geochemical Research Department.
The MRI has been engaged in radioactivity measurements since 1954.
On March 31, the budget for fiscal 2011, which was to start the next day, was abruptly frozen, and the researchers were told to suspend the measurements. The latest findings are a fruit of efforts by scientists who ignored that order and continued with the measurements.
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