Durability of Iodine Waste Forms

Radioiodine can be captured from off-gas and other waste streams by reaction with metallic silver to generate AgI, which can then be immobilized by microencapsulation in a durable matrix material. Waste forms can be generated by densifying the material used to support the silver or by vitrification in a low-temperature glass. Both the intrinsic durability of AgI and the durability of the encapsulating matrix will contribute to degradation resistance of iodine waste forms in a high-level waste disposal facility and should be taken into account in degradation models. Although AgI is only sparingly soluble under oxidizing conditions, reduction of ionic silver to metallic silver under even moderately reducing conditions will increase the solubility of iodide. The degradation model that is used to represent iodine waste form behavior in simulations used for repository assessments should include a redox-dependence term. Argonne is using a combination of electrochemical and immersion test methods to quantify the dissolution behavior of AgI alone, in galvanic couples, and encapsulated in inert matrix materials. The conceptual basis, testing methods and results, and mechanistically-based model being developed for iodide waste forms will be presented.