This major analysis, conducted by an international team from Japan, Finland, America, and France, examines materials from the damaged FDNPP reactors and offers new insights into the ongoing environmental and radioactive waste management challenges in Japan. The study, "Invisible radioactive cesium atoms revealed: Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant," is published in the Journal of Hazardous Materials.
The Fukushima Daiichi Meltdowns: A Continuing Challenge
In 2011, the Great Tohoku Earthquake and Tsunami caused meltdowns in three FDNPP reactors. This event led to extensive research on fuel debris, the mixture of melted nuclear fuels and structural materials found within the damaged reactors, which must be safely removed and disposed of.
However, uncertainties about the physical and chemical state of the fuel debris complicate retrieval efforts.
Understanding the Chemistry of Radioactive Cesium
A significant amount of radioactive Cs was released from the damaged reactors as Cs-rich microparticles (CsMPs). These particles are small (<5 um), glass-like, and poorly soluble. Prof. Satoshi Utsunomiya from Kyushu University, Japan, led the study. He explained, "The CsMPs formed in the bottom of the damaged reactors during the meltdowns, when molten nuclear fuel impacted concrete."
Prof. Gareth Law, from the University of Helsinki, noted, "This means we lack full information on the chemical form of Cs in the particles and fuel debris." Utsunomiya added, "While Cs in the particles is present at reasonably high concentrations, it is often still too low for successful atomic scale imaging using advanced electron microscopy techniques. When Cs is found at a high enough concentration, we have found that the electron beam damages the sample, rendering resulting data useless."
A Breakthrough in Imaging
In their previous work, using a high-resolution high-angle annular dark-field scanning transmission electron microscope (HR-HAADF-STEM), the team found inclusions of pollucite (a zeolite) within CsMPs. Law explained, "In past analysis, we showed that the iron-rich pollucite inclusions in the CsMPs contained >20 wt.% Cs. In nature, pollucite is generally aluminum-rich. The pollucite in the CsMPs was clearly different from that in nature, indicating it formed in the reactors."
Learning that some pollucite inclusions were stable in the electron beam, the team, including graduate student Kanako Miyazaki, finally imaged radioactive Cs atoms. Utsunomiya described, "It was incredibly exciting to see the beautiful pattern of Cs atoms in the pollucite structure, where about half of the atoms in the image correspond to radioactive Cs."
Beyond Imaging
Utsunomiya emphasized, "Our work sheds light on pollucite formation and the likely heterogeneity of Cs distribution within the FDNPP reactors and the environment." Law added, "We unequivocally demonstrate a new Cs occurrence associated with the materials emitted from the FDNPP reactors. Finding Cs containing pollucite in CsMPs likely means it also remains in the damaged reactors; as such, its properties can now be considered in reactor decommissioning and waste management strategies."
Emeritus Prof. Bernd Grambow from Subatech, IMT Atlantique Nantes University, noted, "We should now also begin to consider the environmental behavior or Cs-pollucite and its possible impacts. It likely behaves differently from other forms of Cs fallout documented thus far. Also, the effect on human health might have to be considered. The chemical reactivity of pollucite in the environment and in body fluids is certainly different than that of other forms of deposited radioactive Cs." Prof. Rod Ewing from Stanford University emphasized the need for continued research to inform debris removal strategies and environmental remediation: "Yet again, we see that the pain-staking analytical efforts of international scientists really can unlock the mysteries of nuclear accidents, aiding long-term recovery efforts."
The study is titled "Invisible radioactive cesium atoms revealed: Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant" and is published in the Journal of Hazardous Materials. The work was supported by bilateral funding from the Japan Society for the Promotion of Science and the Research Council of Finland.
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