Photodecomposition of methyl iodide as pretreatment for adsorption of radioiodine species in used nuclear fuel recycling operations

Abstract

This work presents novel photochemical reactors with integrated titania photocatalyst for the UV photodecomposition of gaseous methyl iodide present in simulated vessel off-gas (VOG) in a nuclear fuel reprocessing facility. Continuous flow reactors composed of fluorinated ethylene propylene (FEP) were designed and constructed to allow transmission of UV light into the reactor while also providing resistance to the corrosive nature of methyl iodide. Titania nanotube photocatalyst increases methyl iodide photolysis rate by two orders of magnitude compared to that with UV light alone. The rate constant for methyl iodide photolysis with and without catalyst directly depends on the intensity of the light at the location of the reaction, which is inversely proportional to the distance from the linear light source. The titania photocatalyst is significantly active at low light intensity where no detectable reaction occurs in absence of catalyst. For example, at an initial concentration of 400 ppb methyl iodide, a residence time of 9 s, and a light intensity of 0.5 mW/cm2, conversions with and without titania photocatalyst are >99% and 0%, respectively. The UV light source is a low-pressure mercury vapor lamp with primary emission at 254 nm, the wavelength that is in the range of highest cross-sectional absorbance for methyl iodide. Parameters investigated in this work include UV lamp to reactor distance (light intensity), impact of humidity, gas phase composition, and catalyst form (nanotube vs powder). This work also demonstrates that titania possesses a characteristic activation period on the order of approximately one hour to reach its full steady-state activity.