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Post-Fukushima radioactivity levels in ocean, its impact assessed

The research team found that releases to the ocean peaked in April, one month after the March 11 earthquake and tsunami that precipitated the nuclear accident.

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The level of radioactivity discharged into the ocean after the Fukushima disaster and its impact has been measured by researchers.

The study was conducted by Woods Hole Oceanographic Institution chemist Ken Buesseler and two colleagues based in Japan, Michio Aoyama of the Meteorological Research Institute and Masao Fukasawa of the Japan Agency for Marine-Earth Science and Technology.

The team found that releases to the ocean peaked in April, one month after the March 11 earthquake and tsunami that precipitated the nuclear accident.

The attribute this fact to “the complicated pattern of discharge of seawater and freshwater used to cool the reactors and spent fuel rods, interactions with groundwater, and intentional and unintentional releases of mixed radioactive material from the reactor facility.”

The scientists also found that the releases decreased in May by a factor of 1,000, “a consequence of ocean mixing and a primary radionuclide source that had dramatically abated,” they report.

While concentrations of some radionuclides continued to decrease, by July they were still 10,000 times higher than levels measured in 2010 off the coast of Japan.

This indicates that the plants “remain a significant source of contamination to the coastal waters off Japan,” the researchers report.

“There is currently no data that allow us to distinguish between several possible sources of continued releases,” said Buessler.

He said that at levels indicated by these data, the releases are not likely to be a direct threat to humans or marine biota in the surrounding ocean waters.

There could be an issue, however, if the source remains high and radiation accumulates in marine sediments.

Though international collaborations for comprehensive field measurements to determine the full range of radioactive isotopes released are underway, it will take some time before results are available to fully evaluate the impacts of this accident on the ocean, Buessler said.

The research paper will be published in the journal Environmental Science and Technology.

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