Why did Fukushima reactors explode

Fukushima: New type of fallout discovered

Radiant glass: Researchers have discovered a new form of radioactive fallout in Fukushima. They are larger, vitrified granules that contain highly radioactive cesium. They were released in the hydrogen explosion of reactor block 1. Because of their size, they cannot be inhaled, so their possible harmful effects are limited to external contact, say the researchers. At the same time, the particles remain in the environment for a long time.

Almost ten years have passed since the serious nuclear disaster in Fukushima, but the struggle with the consequences continues to this day. The damaged reactors of the nuclear power plant are still highly radioactive and have to be cooled. Groundwater, soil and sea sludge in the vicinity of the nuclear power plant are contaminated by radioactive fallout. The particles released in the explosions and leaks of the reactors mainly contain radioactive cesium, but also uranium, strontium and radioactive isotopes of the elements lithium, sulfur and zirconium.

It has long been known that tiny glass particles were also released from the damaged reactors. They arose when parts of the concrete reactor inner wall melted during the core meltdown and cesium and other radioactive particles were trapped. This rain of glass made of microparticles was carried as far as Tokyo by the wind.

"Highest cesium activity ever measured in Fukushima"

Kazuya Morooka of Kyushu University and his team have now tracked down a previously undetected form of glazed fallout. They had taken and analyzed soil samples from an area a few kilometers northwest of the Fukushima Daiichi nuclear power plant. They found radioactive glass particles that were more than 300 micrometers in diameter and were much larger than the well-known glass rain.

In addition, these glass grains were highly radioactive: "Two of these particles have the highest cesium radioactivity ever measured for particles from Fukushima," report Morooka and his team. The measured values ​​were still up to 2.5 million Becquerel per particle. To find out why, the scientists analyzed the radiant glass granules using state-of-the-art X-ray and spectrometry methods.

Particles from the reactor air

It turned out that some of the glass granules consist of an accumulation of amorphous silicate nanoparticles in which the radioactive cesium is embedded. The other type has a glassy core made of carbon, the surface of which bears numerous different microparticles, including cesium, lead-tin alloys, lime, quartz and aluminum silicate.

Based on the structure and isotope composition of the particles, the researchers were able to reconstruct where these glass granules came from: They were apparently released when the hydrogen gas formed in the overheated reactor block 1 at Fukushima Daiichi exploded. During the explosion, the particles floating around inside the reactor attached to the molten rock glass and thus preserved the air composition at the moment of the explosion.

Snapshot of the explosion

“These new particles come from areas close to the damaged reactor core and thus provide valuable information,” says Morooka's colleague Satoshi Utsunomiya. "They give us a snapshot of the atmospheric conditions in the reactor building at the time of the hydrogen explosion and the physico-chemical phenomena that occurred during the core meltdown."

As the scientists explain, this knowledge helps to break down the processes involved in the nuclear disaster, but also in decontamination. “Even if almost ten years have passed since the accident, scientific insights like this have never been more important,” emphasizes Utsunomiya. Understanding the various forms of contamination is important for cleaning the environment and assessing the dangers for returning residents.

In terms of health, only little concern

The newly discovered glass particles are probably only of limited concern for human health, as Moroooka and a team explain. "Because of their size, the health effects of the highly radioactive particles are likely to be limited to external radiation," they write. Specifically, this means that these granules are not inhaled and are also not soluble in water. As a result, there is little chance that they will get inside the body or stick to the skin for longer.

However, this does not mean that this fallout could not have significant consequences for nature around Fukushima: "We have to consider possible effects on other living things, for example filtering organisms in habitats in the vicinity of the nuclear power plant," says Utsunomiya. "These particles will remain in the environment for many decades and will cause radioactivity hotspots." (Science of The Total Environment, 2021; doi: 10.1016 / j.scitotenv.2021.145639)

Source: University of Helsinki

February 19, 2021

- Nadja Podbregar