Selection of a form for fixation of iodine-129

This report summarizes work on the selection of an 129I disposal form. Iodine compounds have been screened on the basis of solubilities, thermal stabilities, cost and availability, toxicity of the cation, and the thermodynamic resistance to oxidation and hydrolysis, and leaching of that compound in portland type III cement. Also considered were iodine capture technology, disposal criteria or guidelines, and the disposal site/strategy. The recommended iodine fixation forms, based on their leach resistance and chemical stability and contingent on the disposal strategy/site and capture technique, are silver iodide in cement and barium, calcium, or strontium, and mercuric iodates in cement. Iodine sodalite appears promising and merits further study. If compatible with disposal requirements, the recommended forms for Mercurex and Iodox are insoluble iodates in cement and for silver sorbents the sorbents in cement or AgI in cement. Conversion between the different oxidation states of iodine is feasible but complicates the iodine treatment. For the different disposal strategies, isotopic dilution or ocean disposal has the least stringent disposal form requirements. Any of the recommended forms should be suitable with proper site selection. Isolation in a geologic repository for thousands of years requires the disposal form to be thermally and chemically stable and resistant to leaching at elevated temperatures. Probably the best form studied for isolation is silver iodide in cement. For extraterrestrial disposal, the disposal form may have to withstand reentry impact and surface disposal in the event of an aborted mission; this assumes the capsule is not recovered. Thus the primary containment barrier is critical. The suggested iodine form for space disposal is a heavy metal iodide