Structure, Composition, and Physicochemical Properties of Radiocesium-Bearing Microparticles Emitted by the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

During the accident at TEPCO’s Fukushima Daiichi Nuclear Power Plant, radiocesium-bearing microparticles (CsMPs) were released from damaged reactors into the environment. These micron-sized spherical particles with high specific radioactivity have not been reported in previous nuclear accidents. Herein, the current understanding of the structure, composition, and physicochemical properties of CsMPs is summarized. Electron microscopy revealed that the CsMP matrix is composed of silicate glass containing Na, Cl, K, Fe, Zn, Rb, Sn, and Cs as major constituents. These elements are often inhomogeneously distributed, depending on the particle radius, and Cs was concentrated around the outer side of the particles. In addition, nanocrystals including Cr-rich oxides and chalcogenides were frequently found inside CsMPs. The average valence state of Fe in the CsMP glass matrix was almost Fe2+, indicating formation under a reducing atmosphere through condensation from the gas phase. Radiocesium diffused away from the CsMPs when heated to >600 °C. Accordingly, CsMPs may lose their high specific radioactivity when related radiation-contaminated waste is incinerated at sufficiently high temperatures. Although CsMP solubility is low, they cannot be regarded as “insoluble” materials owing to their small size. CsMP dissolution rates depend on the pH and dissolved species in the solution, and their dissolution behavior is comparable to that of silica-rich glass. Based on these dissolution properties, a method for estimating CsMP abundance and spatial distribution in the environment was proposed. The findings detailed herein contribute to the comprehensive elucidation of CsMP environmental dynamics.