Low-Temperature Condensation and Solidification of Radioactive Liquid Waste by Freeze Drying

Nuclear facilities such as laboratories have generated various radioactive liquid wastes containing not only radioactive species but also reactive chemicals, and those have been accumulating inside the facilities due to difficulty in handling and treatment. Japan Atomic Energy Agency is conducting fundamental studies for treatment of those liquids in Systematic Treatment of Radioactive liquid wastes for Decommissioning (STRAD) project. Immobilizing radioactive species and various chemicals is an indispensable technology in the project. One of the promising immobilization technologies is solidification such as vitrification, cementation, etc. Vitrification is a commercially available technology employed in reprocessing plants. However, the technology cannot be easily introduced in existing facilities due to the required high temperatures. The technology is also unsuitable for volatile elements and chemicals. Cementation is also a common technology for immobilization of elements and can be done at a lower temperature. However, resistance against the leaching of elements and chemicals into the water cannot be expected. Therefore, a new immobilization technology that can be applied to various chemical species is required to be developed. Our group is focusing on a new technology to condense radioactive liquid and encapsulate chemical species simultaneously using a freeze-drying method, where ingredients of a matrix for the encapsulation are added into a target liquid before the condensation. After the freeze-drying condensation, species in the liquid are trapped in the matrix. Our preliminary test on a metal ion solution succeeded in the separation of water and solutes and the encapsulation of the solute in the matrix. In this study, components of the matrix were parametrically changed in the solidification tests on metal ions in nitric acid solutions, and solidification performance was evaluated through structural analyses of the product by XRD and EXAFS, and leaching tests to show the fundamental performance and mechanism of this technology. The applicability of the technology to the solution containing various chemicals was also examined. An appropriate combination of the matrix was selected from Al, P, Fe, B, and Si, and systems with Al + Si or Fe + Si were revealed to be stable. The product had an amorphous structure, and cations were shown to be adsorbed on the matrix by electrostatic force. The leaching speed from the product was almost the same as that of the vitrified glass. Relatively large size chemical species such as ammonium ions and sulfamic acid molecules were also immobilized in the matrix. The new immobilization technology developed by this study is promising for the treatment of various radioactive liquid wastes stored in the facilities. The applicability of this technology for organic liquid wastes will also be investigated soon