(632a) Diffusion Behavior of Inert Gases in Activated Carbon Columns

Activated carbon beds to adsorb and retain inert gases have been widely used in the nuclear industry to allow sufficient decay time for radioactive isotopes such as xenon-131m, xenon-133, and krypton-88 before atmospheric discharge. Newer simplified designs of nuclear plants require much less frequent purging of radioactive waste gas from coolant water, such that adsorbed gases remain stationary in the adsorption bed for long periods of time, with the attendant concerns for diffusion of radioactive gases within the bed during this static period. This contribution describes experimental work with non-radioactive gases and a mass spectrometer detector to measure zone diffusion behavior of inert gases such as xenon, krypton, and argon. The inert gas is eluted through either an open tube or a packed column of activated carbon by a hydrogen or helium carrier gas.  During elution, the flow is stopped for a fixed wait time and the resulting broadening of the inert gas zone is measured. Methodology and concepts from chromatography are found to be useful in this work. A theoretical model that includes the free gas diffusivity, the void fraction of the column, a tortuosity or pore size factor, and the adsorption equilibrium constant is found to provide a reasonably good match to the experimental data.  Effective transport   parameters are estimated using this model and compared with their equivalent obtained from experimental data.