Synthesis of Gd-Fe garnet-based ceramic for immobilization of hydrous ferric oxide with actinide-containing waste

Abstract

Liquid waste generated in nuclear facilities has a large volume and is prone to leak into the environment so it should be immobilized in the physiochemically stable solid waste form before final disposal. Radionuclides in the liquid phase must be separated prior to the immobilization process. This study aims to investigate the removal efficiency of actinides from the liquid phase using iron-based inorganic adsorbent and immobilize actinides and iron into a durable matrix. Simulated liquid waste was prepared from uranium which is a representative actinide, and then uranium removal efficiency was evaluated using HFO (Hydrous ferric oxide) that amorphous iron hydroxide with a high specific surface area. As a result, HFO showed a high uranium removal efficiency near-neutral pH, and it was confirmed that the pH range where adsorption occurs is broadened when the temperature is increased through the adsorption experiments. Afterward, Gd-Fe garnet ceramic waste form was synthesized to immobilize previous residual waste. Cerium was used as a surrogate for actinides, and garnet was fabricated using cold pressing and conventional sintering method at 1375℃. Sintered pellet showed high relative density up to 95% of theoretical and a single garnet lattice accommodated 0.4 atoms of cerium to its structure. The leachability index from the result of ANSI/ANS-16.1 standard leach test to 3 components showed greater than 6 which is the waste acceptance criteria of KORAD and U.S. NRC. The PCT-A and MCC-1 tests were also performed and the leach rate of all components in deionized water was 10$^{-3}$~10$^{-6}$ g/m$^2$·day which indicates garnet has good leaching resistance.