Nearly ten years after meltdown at the Fukushima Daiichi Nuclear Power Plant caused a nuclear disaster, researchers have uncovered important new information about the extent and severity of the meltdown and the distribution patterns of the plutonium that have broad implications for understanding the mobility of plutonium during a nuclear accident.

According to a paper published July 8 in Science of the Total Environment, microscopic particles emitted during the disaster contained not only high concentrations of radioactive cesium, as previously reported, but also the toxic metal plutonium. These microscopic radioactive particles formed inside the Fukushima reactors when the melting nuclear fuel interacted with the reactor’s structural concrete.

“The study used an extraordinary array of analytical techniques in order to complete the description of the particles at the atomic-scale,” said Rod Ewing, co-director of the Center for International Security and Cooperation (CISAC) at Stanford University.

Ewing collaborated with researchers from Kyushu University, University of Tsukuba, Tokyo Institute of Technology, National Institute of Polar Research, University of Helsinki, Paul Scherrer Institute, Diamond Light Source and SUBATECH (IMT Atlantique, CNRS, University of Nantes).

The researchers found that, due to loss of containment in the reactors, the particles were released into the atmosphere and many were then deposited many kilometers from the reactor sites. Studies have shown that the cesium-rich microparticles, or CsMPs, are highly radioactive and primarily composed of glass (with silica from concrete) and radio-cesium (a volatile fission product formed in the reactors). But the environmental impact and their distribution is still an active subject of research and debate. The new work offers a much-needed insight into the Fukushima Daiichi Nuclear Power Plant, (FDNPP) meltdowns.

Geochemist Satoshi Utsunomiya and graduate student Eitaro Kurihara of Kyushu University led the team that used a combination of advanced analytical techniques, including synchrotron-based micro-X-ray analysis, secondary ion mass spectrometry, and high-resolution transmission electron microscopy, to find and characterize the plutonium 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 revealed, for the first-time, that plutonium-oxide concentrates were associated with the uranium, and that the isotopic composition of the uranium and plutonium 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,” said Utsunomiya. “This is important information as it tells us about the extent [and] severity of the meltdown. Further, this is important information for the eventual decommissioning of the damaged reactors and the long-term management of their wastes.”

With regards to environmental impact, Utsunomiya said, “as we already know that the CsMPs were distributed over a wide region in Japan, small amounts of plutonium were likely dispersed in the same way.”

Gareth T. W. Law, a co-author on the paper from the University of Helsinki, said the team “will continue to experiment with the CsMPs, in an effort to better understand their long-term behavior and environmental impact. It is now clear that CsMPs are an important vector of radioactive contamination from nuclear accidents.”

Bernd Grambow, a coauthor from Nantes/France, said, “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.”

Utsunomiya emphasized that this is a great achievement of international collaboration. “It’s been almost ten years since the nuclear disaster at Fukushima,” he said, “but research on Fukushima’s environmental impact and its decommissioning are a long way from being over.”

Ewing is also the Frank Stanton Professor in Nuclear Security, a Senior Fellow of the Precourt Institute for Energy, Senior Fellow at the Freeman Spoglie Institute for International Studies and. Professor of Geological Sciences in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). Co-authors of the paper include 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, and Satoshi Utsunomiya

Integration of analytical techniques was accomplished through an 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.  

This article was adapted from a press release produced by Kyushu University.

Read Particulate plutonium released from the Fukushima Daiichi meltdowns

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Josie Garthwaite

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Jody Berger

Center for International Security and Cooperation

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