Modeling the Removal and Retention of Radioiodine by TEDA-Impregnated Charcoal under Reactor Accident Conditions

The performance of charcoal filters for removing radioiodine from airstreams has been studied under conditions associated with routine reactor operations, as well as under conditions expected following an accident. These studies have led to the development of a physical model that can predict the time-dependent behavior of iodine release from triethylenediamine (TEDA)-impregnated charcoal filters under postaccident conditions. The charcoal filter model and the experimental studies performed to obtain appropriate values for the parameters used in the model are described.The model is a one-dimensional mass balance equation that includes convection, diffusion, and adsorption-desorption processes. The adsorption-desorption kinetics for CH3I on TEDA-impregnated charcoal is based on a two-step process: physical adsorption on the charcoal surface followed by chemisorption on TEDA impregnants, the rate of this chemisorption depending on the concentration of the physically adsorbed CH3I. Experiments were performed to determine the temperature and relative humidity dependences of the parameters used in the model, i.e., the adsorption and desorption rate constants and adsorption capacities. For a given charcoal, it was assumed that the rate constants depend only on temperature, whereas the adsorption capacities depend only on relative humidity. The observed rate constants for the physical and chemical adsorption and desorption processes all show Arrhenius temperature dependences. The observed dependence of adsorption capacity on relative humidity is consistent with the assumption that the adsorption sites are reduced as a result of capillary condensation. The full CH3I breakthrough curves, calculated using the model, reproduced the experimental data very well, supporting the assumption of a two-step adsorption-desorption mechanism. Some of the simulation results are also presented.