Caesium fallout in Tokyo on 15$^{th}$ March, 2011 is dominated by highly radioactive, caesium-rich microparticles

In order to understand the chemical properties and environmental impacts oflow-solubility Cs-rich microparticles (CsMPs) derived from the FDNPP, the CsMPscollected from Tokyo were investigated at the atomic scale usinghigh-resolution transmission electron microscopy (HRTEM) and dissolutionexperiments were performed on the air filters. Remarkably, CsMPs 0.58-2.0micrometer in size constituted 80%-89% of the total Cs radioactivity during theinitial fallout events on 15th March, 2011. The CsMPs from Tokyo and Fukushimaexhibit the same texture at the nanoscale: aggregates of Zn-Fe-oxidenanoparticles embedded in amorphous SiO2 glass. The Cs is associated withZn-Fe-oxide nanoparticles or in the form of nanoscale inclusions of intrinsicCs species,rather than dissolved in the SiO2 matrix. The Cs concentration inCsMPs from Tokyo (0.55-10.9 wt%) is generally less than that in particles fromFukushima (8.5-12.9 wt%).The radioactivity per unit mass of CsMPs from Tokyo isstill as high as 1E11 Bq/g, which is extremely high for particles originatingfrom nuclear accidents. Thus, inhalation of the low-solubility CsMPs wouldresult in a high localized energy deposition by beta (0.51-12)*1E-3 Gy/h withinthe 100-micrometer-thick water layer on the CsMP surface) and may havelonger-term effects compared with those predicted for soluble Cs-species