Most of the radioactive fallout that descended upon downtown Tokyo in the days after the March 2011 accident at the

Daiichi nuclear plant was concentrated and deposited in non-soluble glass microparticles -- essentially, glass-filled soot.

As a result, the fallout, which contained concentrated radioactive cesium, wasn't dissolved by rainfall, and probably lingered in the environment. Its effects remain unclear, according to a newly-published research.

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In the study, released at the Goldschmidt geochemistry conference in Yokohama, a team of Japanese geochemists analyzed samples collected from within a 142 mile (230 kilometer) radius of the stricken nuclear plant, which suffered a partial meltdown of three reactors after an offshore earthquake triggered a devastating tsunami, which in turn disabled the power supply to the reactors' cooling systems.

The accident released a plume containing cesium-137, a radioactive isotope with a 30-year half-life. Once inhaled or ingested in the body, the substance can remain in the body for weeks or months, according to the U.S. Centers for Disease Control and Prevention. Exposure can damage cells and lead to an elevated risk of cancer, according to the CDC.

But since cesium is water-soluble, the Japanese scientists thought that most of it would be washed away by rainwater. Instead, analysis with electron microscopes and autoradiography, a technique that uses photographic film to record molecules and fragments of them, revealed that most of the radioactive cesium in fact fell to the ground enclosed in glassy microparticles. The latter were formed at the time of the reactor meltdown.

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"This work changes some of our assumptions about the Fukushima fallout," explained the lead researcher, Satoshi Utsunomiya of Kyushu University, in a press release. "It looks like the clean-up procedure, which consisted of washing and removal of top soils, was the correct thing to do. However, the concentration of radioactive cesium in microparticles means that, at an extremely localized and focused level, the radioactive fallout may have been more (or less) concentrated than anticipated.

"This may mean that our ideas of the health implications should be modified," he noted.

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