ACE bench-scale studies of iodine volatility and interaction with epoxy painted surfaces

The effects of pH, temperature, gamma irradiation, initial speciation and epoxy paint on iodine behaviour were studied in bench-scale experiments in support of the Advanced Containment Experiments (ACE) Phase B research program. The water/air partition coefficient and the retention of iodine on submerged epoxy painted coupons were evaluated. Additional information was obtained through radiochemical measurements of the aqueous and gas phase iodine speciation. Following irradiation, the overall iodine volatility depended strongly on pH, however, it did not differ significantly between solutions prepared with I2 and I-. At 80degC, extensive organic iodine formation occurred in unirradiated basic-solutions containing epoxy painted coupons, resulting in up to 10 % of the liquid phase iodine being converted to higher molecular weight organic iodides of low volatility. In a related study, the adsorption of I2 (g) on epoxy painted coupons at 80degC was investigated. Although the initial rate of deposition appeared to be limited by gas phase mass transfer, the rate over the longer term was suggestive of a slower chemical reaction. Desorption of the deposited iodine was extremely slow, indicating that the majority of the iodine on the epoxy was irreversibly bound. A semi-empirical model was developed and found to represent reasonably both the present results and those of other studies. (author)