New research strongly suggests that the nano-scale heterogeneity that is common in normal nuclear fuels is still present in the fuel debris that remains inside the Fukushima's damaged reactors.
Small amounts of plutonium (Pu) were released from the damaged Fukushima Daiichi Nuclear Power Plant (FDNPP) reactors into the environment during the site's 2011 nuclear disaster. However, the physical, chemical and isotopic form of the released Pu has remained unknown.
Now, recent work published in the journal "Science of the Total Environment" has shown that Pu was included inside cesium-rich microparticles (CsMPs) that were emitted from the site.
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-cesium (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 much-needed insight into the nature and extent of the FDNPP meltdowns.
Pu-oxide concentrates associated with uranium
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 FDNPP 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 or 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", stated Dr Utsunomiya.
With regards environmental impact, Dr Utsunomiya states: "As we already know that the CsMPs were distributed over a wide region in Japan (up to 230 km from the FDNPP), small amounts of Pu were likely dispersed in the same way."
Understanding long-term impact
Professor Gareth Law, a co-author on the paper from the University of Helsinki, indicates that the team will continue to characterize and experiment with the CsMPs, in an effort to better understand their long-term behavior and environmental impact.
"It is clear that CsMPs are an important vector of radioactive contamination from nuclear accidents", Law says.
Professor Bernd Grambow, a co-author from Nantes, France, states that "while the Pu released from the damaged reactors is low compared to that of Cs; the investigation provides crucial information for studying the associated health impact."
Professor Rod Ewing at Stanford University emphasizes 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."
"It took a long time to publish results on particulate Pu from Fukushima. I would like to emphasize that this is 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", Utsunomiya concluded.
NOTE: Integration of the state-of-the-art analytical techniques was accomplished through a world-wide international network that included Kyushu University, University of Tsukuba, Tokyo Institute of Technology, National Institute of Polar Research, University of Helsinki, Paul Scherrer Institute, Diamond Light Source, SUBATECH (IMT Atlantique, CNRS, University of Nantes), and Stanford University.
Publication:
Eitaro Kurihara, Masato Takehara, Mizuki Suetake, Ryohei Ikehara, Tatsuki Komiya, Kazuya Morooka, Ryu Takami, Shinya Yamasaki, Toshihiko Ohnuki, Kenji Horie, Mami Takehara, Gareth T.W. Law, William Bower, J. Frederick W. Mosselmans, Peter Warnicke, Bernd Grambow, Rodney C. Ewing, Satoshi Utsunomiya, Particulate plutonium released from the Fukushima Daiichi meltdowns, Science of The Total Environment, Volume 743, 2020, 140539, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2020.140539.
Additional information
Gareth Law, Professor
Department of Chemistry, Radiochemistry Unit, University of Helsinki
