Radiocesium released from Fukushima Daiichi nuclear power plant due to the accident in 2011 remains in soil over northeast Japan. Most of radiocesium was emitted from the broken nuclear reactors as soluble forms and then strongly adsorbed on weathered micaceous minerals after deposition on soil. However, insoluble spherical microparticles containing radiocesium were discovered from aerosol filters collected on March 14, 2011. Subsequently, similar insoluble Cs-bearing radioactive microparticles (CsMPs) were found in soil, river sediment, dust, plant tissues collected from Fukushima and surrounding areas and they were characterized with SR-XRF/XAS or SEM/TEM/STEM by many researchers. The main body of CsMPs is silicate glass. Analyses by SR-XRF revealed that CsMPs contained fission products such as Cs, Sn, Mo, Te and U. The spherical CsMPs concentrate 0.1 to 3 Bq of radiocesium in the particles of a few microns in diameter and have stronger radioactivity than clay minerals contaminated by soluble radiocesium. The fate of CsMPs in the environment is not well-understood. Since silicate glass is slowly dissolved in water, CsMP is expected to disappear from the environment at some point in future. The purpose of this study is to investigate how CsMPs are modified in the environment by weathering. We compared shape and structure in CsMPs before and after dissolution in water by SEM/TEM/STEM. We also estimated the dissolution rate of CsMPs in water.

CsMPs were collected from non-woven cloth (NWC) that had covered the fields in Fukushima when radiocesium was deposited in March 2011. CsMPs were isolated from the other unrelated particles and observed by SEM-EDS. After observation, CsMPs were immersed in ion-exchange water at 120, 90 and 60 °C until their radiocesium activity became around a half by dissolution. CsMP after the dissolution treatment was observed by SEM-EDS again and then also observed by STEM-EDS after being thinned so as to be electron-transparent using an FIB system. We also observed CsMPs captured by dust filters collected from Fukushima in 2015.

Before the dissolution treatment, CsMPs had a spherical shape. After dissolution treatment in water, the forms of CsMPs were considerably altered from spherules, suggesting that the dissolution proceeded intricately without maintaining geometric homogeneity. Tin oxide and iron oxide nanoparticulates were formed on the surfaces of CsMPs after dissolution. These features were consistent with a CsMP collected from dust filters in 2015, indicating that CsMPs were not stable in the environment but collapsed with aging. The dissolution rate of CsMPs in water was estimated to be 0.011 µm/year at 13 °C, mean annual temperature in Fukushima.