Radioactive Cs in the severely contaminated soils near the Fukushima Daiichi nuclear power plant

Radioactive Cs isotopes (137Cs, t1/2 = 30.07 y and 134Cs, t1/2 = 2.062 y) occur in severely contaminated soils within a few km of the Fukushima Dai-ichi nuclear power plant at concentrations that range from 4×10^5 to 5×10^7 Bq/kg. In order to understand the mobility of Cs in these soils, both bulk and submicron-sized particles elutriated from four surface soils have been investigated using a variety of analytical techniques, including powder X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and analysis of the amount of radioactivity in sequential chemical extractions. Major minerals in bulk soil samples were quartz, feldspar, and minor clays. The submicron-sized particles elutriated from the same soil consist mainly of mica, vermiculite, and smectite and occassional gibbsite. Autoradiography in conjunction with SEM analysis confirmed the association of radioactive Cs mainly with the submicron-sized particles. Up to ~3 MBq/kg of 137Cs are associated with the colloidal size fraction (98% of Cs within top ~5 cm of the soil. These results suggest that the mobility of the aggregates of submicron-sized sheet aluminosilicate in the surface environment is a key factor controlling the current Cs migration in Fukushima.