Design Against Weathering of Iodine Filters

Control of the release of radioactive iodine is an important task of the air filtration system in a nuclear installation. The filtration efficiency of an active charcoal filter for methyl iodide is shown to decrease due to the poisoning of the filter by secondary materials in the gas stream. A model is developed that appears to reproduce the decrease in filtration efficiency. Lack of detailed information on the poison-filter interaction prevents definitive confirmation of the model The model facilitates the choice of the optimum values of certain micro-structural and operational parameters of the filter, to reduce the effect of secondary poisons on the filter and thereby to lengthen the usable life of the filter. The microstructural properties addressed by the model are catalyst pellet size, porosity, surface area per gram, and pore radius. The operational parameters of the filter are the gas transit time and the gas diffusivity in the catalyst pores as controlled by the gas temperature.