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 80 deg. C, extensive organic iodide 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 80 deg. C 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. The volatility of iodine generally decreases with increasing pH, although this trend can be reversed for solutions in contact with epoxy paint at higher temperatures. This reversal is at least partially due to the extensive formation of organic iodides under basic pH conditions, through aqueous phase reaction(s) with material released from the epoxy paint. Furthermore, under acidic conditions, interaction with the epoxy paint results in the reduction of I, to I-. Under irradiation, the long-term partition coefficient does not differ between buffered solutions prepared with I2, instead of I- but does depend strongly on pH, particularly between pH 5 and 6. The initial rate of deposition onto epoxy paint of gas phase I2 in dry air at 80 deg. C may be dictated by the rate of gas phase mass transfer. Over the long term, the rate is much slower, likely limited by a slow chemical reaction. The rate of desorption is extremely slow, also indicative of chemical bonding. However, the amount of iodine retained by the epoxy paint over the long term depends on the gas phase concentration, suggesting that an equilibrium is achieved. These apparently conflicting trends have been resolved through a model that reasonably represents both the current experimental results and data from other studies. This research, although quite fruitful, has raised as many questions as it has answered. Considerable further work is required in order to attain a satisfactory understanding of the interaction of iodine with epoxy painted surfaces. Specifically, the effects of steam, substrate and temperature on the adsorption of I2 (g) on epoxy paint should be investigated in order to evaluate the validity of the assumption used to extent the proposed model to the results of other laboratories. Attempts should be made apply the model to other paints although this may require further experimental measurements. Further work on the reduction of I2 (aq) by epoxy paint could prove valuable to rationalizing the distribution of iodine between submerged and dry surfaces. Also the component(s) of epoxy paint involved in the production of organic iodides in hot, basic solutions should be identified as this may contribute or suppress long term iodine volatility. (authors)