Radioactive fallout of Fukushima accident concentrated in 'glassy soot'

The flooding of the Fukushima Daiichi Nuclear Power Plant after the 2011 earthquake had caused the release of radioactive material

Most of the radioactive fallout, which landed on downtown Tokyo a few days after the Fukushima accident, was concentrated and deposited in non-soluble glass microparticles, as a type of 'glassy soot'.

According to a new research, it meant that most of the radioactive material was not dissolved in rain and running water, and probably stayed in the environment until removed by direct washing or physical removal.

The particles also concentrated the radioactive caesium (Cs), meaning that in some cases dose effects of the fallout are still unclear. These results were announced at the Goldschmidt geochemistry conference in Yokohama, Japan.

The flooding of the Fukushima Daiichi Nuclear Power Plant (FDNPP) after the disastrous earthquake on March 11, 2011 caused the release of significant amounts of radioactive material, including caesium (Cs) isotopes 134Cs (half-life, 2 years) and 137Cs (half-life, 30 years).

Japanese geochemists, headed by Dr Satoshi Utsunomiya (Kyushu University, Japan), analysed samples collected from within an area up to 230 km from the FDNPP. As caesium is water-soluble, it had been anticipated that most of the radioactive fallout would have been flushed from the environment by rainwater.

However, analysis with state-of-the-art electron microscopy in conjunction with autoradiography techniques showed that most of the radioactive caesium in fact fell to the ground enclosed in glassy microparticles, formed at the time of the reactor meltdown.

The analysis shows that these particles mainly consist of Fe-Zn-oxides nanoparticles, which, along with the caesium were embedded in Si oxide glass that formed during the molten core-concrete interaction inside the primary containment vessel in the Fukushima reactor units 1 and/or 3.

Closer microparticle structural and geochemical analysis also revealed what happened during the accident at FDNPP. Radioactive Cs was released and formed airborne Cs nanoparticles. Nuclear fuel, at temperatures of above 2200 K (about as hot as a blowtorch), melted the reactor pressure vessel resulting in failure of the vessel.

The airborne Cs nanoparticles were condensed along with the Fe-Zn nanoparticles and the gas from the molten concrete, to form the SiO2 glass nanoparticles, which were then dispersed.

Analysis from several air filters collected in Tokyo on 15 March, 2011 showed that 89% of the total radioactivity was present as a result of these caesium-rich microparticles, rather than the soluble Cs, as had originally been supposed.

The paper was presented at Goldschmidt Conference, 2016.