Researchers have found that small amounts of plutonium were included inside cesium-rich microparticles (CsMPs) emitted during the Fukushima Daiichi accident in 2011, the University of Helsinki reports. 

CsMPs are microscopic radioactive particles that formed inside the Fukushima reactors when the melting nuclear fuel interacted with the reactor's structural concrete. Due to loss of containment in the reactors, the particles were released into the atmosphere; many were then deposited across Japan.

Studies have shown that the CsMPs are incredibly radioactive and that they are primarily composed of glass (with silica from the concrete) and radio-caesium (a volatile fission product formed in the reactors). Whilst the environmental impact and distribution of the CsMPs is still an active subject of debate, learning about the chemical composition of the CsMPs has been shown to offer a much-needed insight into the nature and extent of the meltdowns at Fukushima.

The study published in Science of the Total Environment, involving scientists from Japan, Finland, France, Switzerland, the UK, and USA, was led by Dr. Satoshi Utsunomiya and graduate student Eitaro Kurihara (Department of Chemistry, Kyushu University). The team used a combination of advanced analytical techniques (synchrotron-based micro-X-ray analysis, secondary ion mass spectrometry, and high-resolution transmission electron microscopy) to find and characterize the Pu that was present in the CsMP samples.

The researchers initially discovered incredibly small uranium-dioxide inclusions, of less than 10 nanometers in diameter, inside the CsMPs; this indicated possible inclusion of nuclear fuel inside the particles. Detailed analysis then revealed, for the first-time, that Pu-oxide concentrates were associated with the uranium, and that the isotopic composition of the U and Pu matched that calculated for the Fukushima Daiichi NPP irradiated fuel inventory.

"These results strongly suggest that the nano-scale heterogeneity that is common in normal nuclear fuels is still present in the fuel debris that remains inside the site's damaged reactors. This is important information as it tells us about the extent / severity of the melt-down. Further, this is important information for the eventual decommissioning of the damaged reactors and the long-term management of their wastes," said Utsunomiya.

With regards environmental impact, Utsunomiya noted that CsMPs were distributed over a wide region in Japan (up to 230km from Fukushima), and that small amounts of plutonium "were likely dispersed in the same way."

Professor Gareth Law, a co-author on the paper from the University of Helsinki, indicated that the team "will continue to characterise and experiment with the CsMPs, in an effort to better understand their long-term behaviour and environmental impact. It is clear that CsMPs are an important vector of radioactive contamination from nuclear accidents."

Professor Bernd Grambow, a co-author from Nantes/France, states that "while the plutonium released from the damaged reactors is low compared to that of cesium; the investigation provides crucial information for studying the associated health impact."

Professor Rod Ewing at Stanford University emphasised that "the study used an extraordinary array of analytical techniques in order to complete the description of the particles at the atomic-scale. This is the type of information required to describe the mobility of plutonium in the environment."

Utsunomiya described the publication of the study as "a great achievement of international collaboration.

"It's been almost ten years since the nuclear disaster at Fukushima, but research on Fukushima's environmental impact and its decommissioning are a long way from being over," he concluded.

Team taking samples in Japan (Photo credit: Kyushu University)