New, large and highly radioactive particles have been identified from among the fallout of the 2011 Fukushima Daiichi nuclear disaster in Japan. An international team of researchers has characterized the particles using nuclear forensic techniques and their results shine further light on the nature of the accident while helping to inform clean-up and decommissioning efforts.
This year marks the tenth anniversary of the Fukushima Daiichi disaster, which occurred as a result of a powerful earthquake that struck off of Japan’s east coast, generating a tsunami that reached some 14 m high when it reached the nearby shoreline. Breaching sea defences, the water from the wave shut down emergency generators that were cooling the reactor cores. The result was a series of nuclear meltdowns and hydrogen explosions that released a large amount of radioactive material into the surrounding environment — including microparticles rich in radioactive caesium that reached as far Tokyo, 225 km away.
Recent studies have revealed that the fall-out from reactor unit 1 also included larger caesium-bearing particles, each greater than 300 micron in diameter, which have higher levels of activity in the order of 105 Bq per particle. These particles were found to have been deposited in a narrow zone stretching around 8 km north-northwest from the reactor site.
Surface soil samples
In their study, chemist and environmental scientist Satoshi Utsunomiya of Japan’s Kyushu University and colleagues have analyzed 31 of these particles, which were collected from surface soil taken from roadsides in radiation hotspots.
“[We] discovered a new type of radioactive particle 3.9 km north northwest of the Fukushima Daiichi Nuclear Power Plant, which has the highest caesium-134 and caesium-137 activity yet documented in Fukushima, 105–106 Bq per particle,” Utsunomiya says.
Alongside the record-breaking radioactivity seen in two of the particles (6.1×105 and 2.5×106 Bq, after correction to the date of the accident) the team also found that they had characteristic compositions and textures that differed from those previously seen in the reactor unit 1 fall-out.
Reactor building materials
A combination of techniques including synchrotron-based nano-focus X-ray analysis and transmission electron microscopy indicated that one of the particles was found to be an aggregate of smaller silicate nanoparticles each with a glass-like structure. This is thought to be the remnants of reactor building materials that were first damaged in the explosion and then picked up caesium that had been volatized from the reactor fuel.
The other particle had a glassy carbon core and a surface peppered with other microparticles of various compositions, which are thought to reflect a forensic snapshot of the particles that were airborne within the reactor unit 1 building at the moment of the hydrogen explosion and the physio-chemical phenomena they were subjected to.
“Owing to their large size, the health effects of the new particles are likely limited to external radiation hazards during static contact with skin,” explained Utsunomiya — with the two record-breaking particles thought too large to be inhaled into the respiratory tract.
Impact on wildlife
However, the researchers note that further work is needed to determine the impact on the wildlife living around the Fukushima Daiichi facility — such as, for example, filter feeding marine molluscs which have previously been found susceptible to DNA damage and necrosis on exposure to radioactive particles.
“The half-life of caesium-137 is around 30 years,” Utsunomiya continued, adding: “So, the activity in the newly found highly radioactive particles has not yet decayed significantly. As such, they will remain [radioactive] in the environment for many decades to come, and this type of particle could occasionally still be found in radiation hot spots.”
Nuclear material corrosion expert Claire Corkhill of the University of Sheffield – who was not involved in the study – says that the team have offered new insights into the events that unfurled during the accident. “Although the two particles selected [for analysis] were small, a mighty amount of chemical information was yielded,” she said, noting that some of the boron isotopes the researchers identified could only have come from the nuclear control rods damaged in the accident.
Ongoing clean-up
“This work is important to the ongoing clean-up at Fukushima, not only to the decontamination of the local area, but in defining a baseline understanding of radioactive contamination surrounding the power plant, to ensure that any materials accidentally released during the fuel retrieval operations can be quickly identified and removed,” she adds.
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With this study complete, the researchers are now using the particles to better understand the conditions involved in the reactor meltdown, alongside looking quantify the distribution of this fallout across Fukushima, with a focus on identifying resulting radiation hot spots.
“If we can find and remove these particles, we can efficiently lower the radiation dose in the local environment,” Utsunomiya concluded.
The study is described in Science of the Total Environment.