The Behavior of Iodine in a Simulated Spent-Fuel Solution

A method to expel radioiodine from a spent-fuel solution is important for iodine control in reprocessing plants. Many authors have investigated the procedure without considering the influence of other fission products on that procedure. The present work studies the behavior of iodine in a simulated spent-fuel solution containing fission products. When an iodide (1 mg I^-) is put into a simulated spent fuel-3.4 M HNO₃ solution (100 ml) at 100°C, it is 93.3 to 98.5% volatilized as I₂, depending on the carrier gas, the presence of NO₂, and the solute concentration. Colloidal iodine constitutes a significant part of the nonvolatile iodine species in this solution, whereas is predominant in a similar solution without fission products. The colloidal iodine varies from 0.4 to 2.9% of the initial iodine, depending on the foregoing experimental conditions. The colloidal iodine consists of such iodides as Pdl₂ and Agl, which do not react with NO₂ but are decomposed by such iodates as KIO₃ and HIO₃. Besides acting as carrier , these iodates are able to dissolve the colloid by oxidizing its iodine to I₂. A high concentration is required to minimize the colloidal iodine. Increased HNO₃ concentration (e.g., 6.1 M) increases the proportion of . The presence of NO₂ increases the amount of colloid. Bubbling the solution with a N₂ flow retards the formation of the colloid, probably because it prevents the aging of the colloid. Expelling >99% of the iodine from the solution requires additional , besides the action of NO₂. These results indicate that the chemical reactions of fission products with iodine can interfere with the volatilization of iodine from the dissolver.