Specific activity and vermiculite content of soils of various particle sizes contaminated by the Fukushima Daiichi nuclear power plant accident

Soil polluted by the Fukushima Daiichi nuclear power plant accident was collected and reduced in volume. We explored whether clay particles adsorbed radioactive cesium. Soil was sampled in a highly contaminated zone 16 km northwest of the power plant; soil particles 0.15–200 [Formula: see text]m in diameter were obtained by sieving and centrifugation, and specific activity was analyzed using a high-powered Ge detector. Vermiculite contents were quantified by X-ray diffraction. For soil particles[Formula: see text]m in diameter, the specific activity was constant, at about 381 kBq/kg, which is close to the 350 kBq/kg reported previously. For particles 10–200 [Formula: see text]m in diameter, the specific activity decreased by approximately the square root of the surface/entire volume ratio. The vermiculite content was identical for all soil particles [Formula: see text]2.5 [Formula: see text]m in diameter, but decreased in particles 2.5–200 [Formula: see text]m in diameter. Thus, the specific activity and vermiculite content were approximately proportionate. Vermiculite adhered to soil particles [Formula: see text]10 [Formula: see text]m in diameter, and radioactive cesium was uniformly distributed within the vermiculite.