The reactivity of Cs and fly ash and the immobilization of Cs in the reaction products were investigated under hydrothermal conditions with NaOH solution. Crystalline phases of reaction products varied with hydrothermal conditions such as reaction temperature, concentration of NaOH solution and reaction time. The amount of immobilized Cs increased with the crystallization of pollucite, but its value decreased with the formation of other phases as Cs0.84Na2.16Al3Si3O12•0.32H2O, hydroxysodalite

more »

... d hydroxycancrinite. A part of Cs was exchanged to Na on Na-P1 zeolite, which was formed in a dilute NaOH solution at low temperature for a long reaction time. The synthesized pollucite was the solid solution between pollucite and analcime, and the atomic ratio of Cs/(Cs+Na) in obtained products was larger than the initial ratio of Cs and NaOH solution in the system. This result suggests that Cs is more easily immobilized in the structure of synthesized pollucite than Na under hydrothermal treatments.