Novel concept for a filtered containment venting system with an ionic liquid to remove organic iodine (2) – Decomposition mechanism of an ionic liquid by heat and gamma-ray irradiation

A filtered containment venting system (FCVS) is being developed using an ionic liquid (IL), trihexyltetradecylphosphonium chloride ([P₆₆₆₁₄][Cl]), as a measure to reduce the release of radioactive organic iodine during severe accidents at nuclear power plants. To investigate the decomposition mechanism of [P₆₆₆₁₄][Cl] in alkaline solution (AS) contained in the FCVS and its temperature dependence under gamma-ray irradiation, a qualitative evaluation was conducted by varying the temperature in both irradiation and non-irradiation tests. In a comparison for 0.1 MGy absorbed dose at 433 K with the non-irradiation test condition, the concentrations of irradiated and non-irradiated [P₆₆₆₁₄][Cl] were almost the same values. It was confirmed that irradiation had a minimal impact on [P₆₆₆₁₄][Cl] decomposition. Decomposition of [P₆₆₆₁₄][Cl] primarily occurred through hydrolysis via its reaction with OH⁻ in the AS at 433 K. At higher NaOH concentrations, hydrolysis converted [P₆₆₆₁₄] into an intermediate, phosphonium acid in the first stage. It was then converted into phosphine oxide which led to an increased amount of these hydrolysates. The decontamination factor (DF) decreased due to the formation of phosphine oxide, but it is possible to achieve a high DF by controlling the amount of phosphine oxide formed by optimizing the NaOH concentration of AS.