Plutonium particles scattered by a series of nuclear tests at Bikini Atoll in the 1950s have been accumulating in seas close to Japan, a research team has found.

The research group of the National Institute of Radiological Science said the contamination level is too weak to cause serious immediate impact on humans or the environment. But the group said its findings could provide useful data in a study on how radioactive fallout produced by past atmospheric nuclear tests flows throughout the world’s oceans.

Plutonium pollution from the Bikini tests had never been confirmed in seas around Japan. The particles are believed to have been carried by ocean currents even after 50 years, and some of the plutonium has sunk into the sea and become attached to dead plankton, according to the team.

“If we could figure out how such plutonium particles are flowing around the world, it would be useful at a time of a nuclear accident,” said Masatoshi Yamada, the research team’s leader.

Unlike underground testing, atmospheric nuclear tests release huge amounts of radioactive material, called “dead ash,” into the air. In 1963, Britain, the Soviet Union and the United States agreed to ban that type of nuclear testing. France and China however continued such testing.

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