Fukushima: Fallout fell as glass rain

Granules of molten reactor material enclose the radioactive csium

Fukushima Daiichi nuclear power plant on March 14, 2011: Radioactive smoke is rising. © Digital Globe
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Radioactive glass granules: In the nuclear accident in Fukushima, the radioactive cesium was released in a more durable form than previously thought. It rained mostly as tiny glass particles over the area and down Tokyo, as Japanese researchers have noted. In the glass granules, however, the radionuclide is insoluble in water and is hardly washed away by the rain - the radioactivity could therefore be retained longer in the environment and organisms.

The nuclear accident in the Japanese nuclear power plant Fukushima Daiichi has consequences to this day. Even more than five years later, large parts of the environment are contaminated, the nuclear reactors only with difficulty under control and still radioactivity is released. Especially the radionuclides cesium-134 and cesium-137 got into the environment.

The study by Satoshi Utsonomiya of Kyushu University begins at the Fukushima cesium fallout: For the first time, he and his team have studied in detail the physical form in which the radioactive isotopes have spread in the environment. For this they analyzed soil samples from a 230 kilometer radius around the nuclear power plant, but also samples of air filters, were in Tokyo on March 15, 2011 in use - four days after the nuclear accident.

Wrapped in molten glass

The surprising result: The largest part of the radioactive cesium in the fallout was not soluble in water, as previously assumed. Because the liberated radionuclides could not connect with the water droplets of the air, as Utsonomiya reports. Instead, the cesium was still in the reactor with iron-zinc nanoparticles together and was enclosed with them in hot suspended droplets of molten silica - glass.

As the researchers explain, this glass was formed during the meltdown in the reactors. The heat of the more than 2, 000 degrees hot nuclear fuel melted the inner walls of the reactor pressure vessel. The molten concrete - and with it also melted silicon dioxide - splashed up in tiny droplets during the explosions, enclosing the cesium particles floating around and then cooling off into the ambient air as it exited. display

Radioactive contamination one year after the Fukushima nuclear accident. Roulex_45 / CC-by-sa 3.0

89 percent glass particles

In Tokyo alone, these radioactive glass particles accounted for 89 percent of the total radioactivity trapped in air filters, Utsonomiya reports at the Goldschmidt Conference in Yokohama. Even in the soil around Fukushima, most of the radioactive cesium is present in the form of such glass microparticles. "That changes some of our assumptions about the fallout from Fukushima, " notes the researcher.

Because if the radioactive c sium is not present in water-soluble form, then it is also less easily washed away by the rain. It remains, for example, in the soil, longer lasting. Wherever the floor was not replaced during decontamination or streets and houses were not cleaned with high-pressure cleaners, the radioactive glass particles could therefore still be present almost unchanged,

Radioactivity highly concentrated

And one more thing turned out: radioactivity is sometimes highly concentrated in the glass particles. The researchers found in the tiny grains values ​​of up to 440 billion becquerels per gram which is between 107 and 108 times higher than the average contamination by C sium in the soils around Fukushima as they report. The reason for this is the accumulation of a particularly large amount of radioactive C sium in a small space in these particles.

"This concentration of radioactive csium in the microparticles means that radioactive fallout may have been much stronger or weaker than expected in places, " says Utsunomiya. "It also requires us to modify our assumptions about the health effects." Depending on the stress, some people and animals may have inhaled more radionuclides with glass dust than others.

In addition, because the cesium trapped in the glass is protected from environmental influences, it could also be retained in organisms and in the food chain for a longer time. "The biological half-life of insoluble C sium microparticles could be much longer than that of soluble cosium, " commented Bernd Grambow of the Japanese Atomic Energy Agency JAEA. (Goldschmidt Conference 2016)

(Goldschmidt Conference, 27.06.2016 - NPO)