Investigation of Impurity Gas Cracking Catalysts for Tokamak Exhaust Processing Systems

This study aims to simplify the impurity gas conversion system in the tokamak exhaust processing (TEP) of the fuel cycle system by minimizing the use of additive gases. Conventional systems require additive gases tailored to various impurity species, such as methane and ammonia, resulting in multistage configurations and increased complexity.However, controlling the type and concentration of additive gases for impurity streams with variable compositions is challenging, making their elimination desirable. To address this, we investigated catalytic reactions among the major impurity gases—methane, oxygen, and water vapor—and analyzed competitive reactions and product formation to identify conditions for system rationalization. Since tritiated methane is the most difficult species to convert, its decomposition is key to simplification.Experiments demonstrated that steam reforming is effective for recovering hydrogen-form tritium, but in the presence of sufficient oxygen, competitive hydrogen oxidation occurs, leading to methane conversion into water vapor. Furthermore, rhodium-supported catalysts were found to be the most effective for methane decomposition, and performance improvement may be achievable through the control of noble metal particle size.This study provides a guideline for catalyst selection and presents conceptual ideas for rationalizing the TEP, with potential applicability to broader impurity gas conversion challenges in future fusion fuel cycles.