Development of mud-crushing methods for farmland-derived clayey soil and their application to particle-size separation for decontamination of cesium-contaminated soil

The volume of radioactive cesium-contaminated soil being removed during the cleanup process after the explosion of Fukushima nuclear power plant 1 is estimated to be 13 million m³. Soil contaminated with more than 20,000 Bq/kg of radioactivity is targeted for decontamination by the particle-size separation method to pass the criteria for reuse in Japan. Because more than 80% of the targeted soil is composed of fine silt and clay derived from farmland, a system was developed to increase the amount of recyclable soil by a combination of mud crushing and fine-particle separation with the classification point set lower than about 20 μm. Mud crushing is one of the key technologies for effective decontamination because clay forms aggregates due to the presence of organics such as humus. Four types of mud-crushing equipment were examined on site, followed by pebble milling or ultrasonic dispersion, both of which showed promising results. Using the selected equipment, the proposed soil-size separation was carried out. Overall, the proposed system was proven effective by the results of radioactivity reduction ratio and volume of soil for recycling. Furthermore, lab-scale pebble mill tests using a pot-type wet mill were carried out to confirm the basic characteristics of mud crushing for organic aggregates. (author)