(415f) Photodecomposition of Organoiodides to Molecular Iodine As Pretreatment for Adsorption in Used Nuclear Fuel Recycling Operations

Capture and immobilization of volatile radionuclides such as ¹²⁹I, ⁸⁵Kr, ³H, and ¹⁴C resulting from used nuclear fuel (UNF) recycling operations is an essential component of the advanced fuel cycle. Particular attention is given to radioiodine due to its high mobility, half-life greater than 15 million years, and tendency to bio-accumulate. The dissolver off-gas (DOG) and the vessel off-gas (VOG) streams account for the majority of the radioiodine emissions with the VOG expected to contain iodine at parts per billion (ppb) levels. The chemical speciation of radioiodine significantly affects its capture by sorption, which is the preferred treatment technology. Organic iodide species are substantially more challenging to remove by adsorption compared to molecular iodine. We therefore demonstrate that by first decomposing organic iodide to molecular iodine and organics before sending to an adsorption column, the challenges can be significantly lessened. Specifically, we are investigating UV photolysis of methyl iodide in typical VOG conditions as a pretreatment step to adsorption.

Novel photochemical reactors for the UV photodecomposition of gaseous methyl iodide are presented in this work. The reactors are continuous and constructed of fluorinated ethylene propylene to allow transmission of UV light into the reactor while also providing resistance to the corrosive nature of methyl iodide. Integration of a titania nanotube photocatalyst increases reaction rate by two orders of magnitude compared to that of photolysis of methyl iodide with UV light alone. The UV light source is a low-pressure mercury vapor lamp and has a primary emission at 254 nm, a wavelength in the range of highest cross-sectional absorbance for methyl iodide. The titania photocatalyst is significantly active at low light intensity where without catalyst no detectable reaction occurs. 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.

Parametric analysis was carried out based on light intensity (UV lamp distance), gas composition (N₂ only vs air), and moisture. The titania photocatalyst was largely inactive without the presence of oxygen from air. Titania powder was shown to also significantly increase reaction rates compared to reaction without catalyst, however titania nanotubes produce higher reaction rates and offer easier methods for mechanically supporting the catalyst and integrating with the tubular reactors. The photolysis is affected by moisture, with reaction rates decreasing by a factor of six when the stream has 50% relative humidity compared to dry stream regardless of UV light intensity. The study performed demonstrates methyl iodide presence at ppb concentrations in VOG streams can effectively be completely and continuously decomposed with a common photocatalytic system and simple coil tube reactor. This supports the notion that VOG streams can be catalytically pretreated to facilitate the capture of radioiodine by downstream sorption systems.