Study on the immobilization of cesium absorbed by copper ferrocyanide using allophane through pressing/sintering method

The development of the Cs stable immobilization method contributes to the treatment of insoluble ferrocyanide sludge containing radioactive Cs (Cs–CuFC) generated by the Fukushima nuclear accident. Cs–CuFC begins to decompose at 200 °C, and Cs in Cs–CuFC volatilizes after being oxidized to Cs2O in air at high temperature. Therefore, Cs has an immobilization ratio of less than 4% at 1000 °C. Meanwhile, Cs–CuFC generates poisonous gases, such as HCN, NO, and NH3, and the release of HCN is suppressed in active atmosphere. For the stable immobilization of Cs, a pressing/sintering method that used allophane as an additive was examined. Allophane was mixed uniformly with Cs–CuFC in a mass ratio of 1:1, and the mixture was sintered at different temperatures to obtain solidified bodies. A stable crystal called pollucite was formed after sintering above 900 °C, and the immobilization ratio of Cs was approximately 100%. Part of pollucite was concentrated on some spots on the surface of solidified bodies. These bodies had good mechanical properties for geological storage. The leaching percentage of Cs for the solidified bodies sintered at 1100 °C in distilled water was less than 0.01% and 0.4% at 25 °C and 90 °C, respectively, thereby indicating that the solidified bodies had excellent immobilization properties and chemical stability.