During the Fukushima Daiichi Nuclear Power Plant (FDNPP) nuclear disaster, tiny, sparingly soluble particles a few micrometers in diameter, known as highly radioactive cesium-rich microparticles (CsMPs), were released into the environment. Because these particles contain highly concentrated radioactive cesium (Cs activity concentrations of 10¹⁰–10¹¹ Bq/g), understanding their potential health impacts following inhalation and their dispersion in the environment has become an important research priority. In addition, understanding the reactor meltdown processes and the properties of the nuclear fuel debris remaining within the damaged reactors remain critical concerns.
Professor Satoshi Utsunomiya of NTU Geosciences and Yushan Scholar has been leading an international research team consisting of Professor Shinya Yamasaki of the University of Tsukuba, Professor Bernd Grambow of the University of Nantes, Professor Gareth T.W. Law of the University of Helsinki, Professor Rodney C. Ewing of Stanford University, Professor Kenji Horie of the National Institute of Polar Research, and scientists from the Japan Atomic Energy Agency to investigate radioactive contamination resulting from the Fukushima accident.
Over the past decade, the team has conducted annual field investigations in areas surrounding the FDNPP and published more than 20 original research articles related to the Fukushima nuclear disaster. The team reported that CsMPs consist of a glassy silicate matrix containing Cs (0.55–30 wt.%), Fe, and Zn. Many particles also contain minor amounts of B, Na, Al, K, Ba, Ag, W, Te, Tc, Mo, Sn, U, Pu, and other trace elements. The ¹³⁴Cs/¹³⁷Cs activity ratios of individual CsMPs are approximately 1, confirming their origin from the FDNPP.
The team was the first to characterize, at the atomic scale, nanofragments of nuclear fuel released from the FDNPP into the environment. These nanoscale fuel fragments were either encapsulated within or attached to CsMPs and occurred in two different forms: (i) UO₂₊ₓ nanocrystals approximately 70 nm in size, embedded within magnetite and associated with Tc and Mo on their surfaces; and (ii) isometric (U,Zr)O₂₊ₓ nanocrystals approximately 200 nm in size, with U/(U+Zr) molar ratios ranging from 0.14 to 0.91 and containing intrinsic pores (~6 nm), indicating the entrapment of vapors or fission-product gases during crystallization. These findings revealed the heterogeneous physical and chemical properties of fuel debris at the nanoscale. The debris consists of mixtures of melted nuclear fuel and reactor materials and reflects the complex thermal processes that occurred within the FDNPP reactors during the meltdowns.
The team also discovered traces of plutonium associated with CsMPs using micro X-ray fluorescence and nanoscale analytical techniques. A Cs-pollucite-based CsMP was found to contain discrete U(IV)O₂ nanoparticles approximately 10 nm in size, one of which was enriched in plutonium and located adjacent to fragments of zirconium fuel cladding. The isotope ratios of ²³⁵U/²³⁸U, ²⁴⁰Pu/²³⁹Pu, and ²⁴²Pu/²³⁹Pu were determined to be approximately 0.0193, 0.347, and 0.065, respectively, consistent with the calculated isotopic compositions of irradiated nuclear fuel. These results suggest that the long-distance dispersion of plutonium from the FDNPP occurred through the transport of CsMPs containing nanoscale fuel fragments.
The team also successfully identified distinctive boron–lithium isotopic signatures in individual CsMPs. The particles contain 1,518–6,733 mg kg⁻¹ of total boron and 11.99–1,213 mg kg⁻¹ of lithium. The ¹¹B/¹⁰B ratios (4.15–4.21) and ⁷Li/⁶Li ratios (213–406) are significantly higher than the corresponding natural abundance ratios (~4.05 and ~12.5, respectively), indicating that ¹⁰B(n,α)⁷Li reactions occurred in B₄C control materials prior to the reactor meltdowns. The total amount of boron released with CsMPs was estimated to be only 0.024–62 g, suggesting that nearly all boron remains within reactor Units 2 and/or 3. However, the heterogeneous distribution of boron should be carefully considered during future decommissioning activities.
Their recent work has revealed the atmospheric dispersion mechanism of CsMPs released during the 2011 Fukushima Daiichi Nuclear Power Plant accident. The study was published in the Journal of Hazardous Materials (https://doi.org/10.1016/j.jhazmat.2026.142180).
In this recent study, the team analyzed soil samples collected from 100 locations across Fukushima Prefecture to investigate the abundance and distribution of CsMPs. The results showed that CsMP abundance varied greatly among regions and that, at some locations, more than 60% of the total radioactivity in the soil was attributable to CsMPs.
By combining these measurements with atmospheric dispersion simulations, the researchers revealed that a large number of CsMPs were generated and released around 3:00 a.m. on 15 March 2011. A radioactive plume containing up to 4,700 CsMPs per cubic meter of air was transported across a wide area of Fukushima Prefecture through the clockwise movement of the plume from south to northwest on 15 March. In contrast, radioactive cesium released after midnight on 16 March contained no CsMPs and instead existed primarily in water-soluble forms. The study also demonstrated that the deposition of CsMPs onto soil strongly depended on rainfall patterns and the abundance of airborne particles.
“These findings provide important insights into how radioactive microparticles released during nuclear accidents disperse in the environment and are expected to contribute to future environmental risk assessments for nuclear disasters. Furthermore, the knowledge and research approach established in this study represent an important step toward the creation of a new research field, Advanced Radioactive Particle Science,” said Professor Satoshi Utsunomiya.
“International and interdisciplinary collaboration is vital to our research, not only for producing high-quality scientific outcomes but also for sustaining international interest and long-term research efforts related to Fukushima.”
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