Pelletization with Spark Plasma Sintering and Characterization of Metal Iodides: An Assessment of Long-Term Radioiodine Immobilization Options

Four promising iodine “getter” materials (Ag, Cu, Bi, and Sn) for radioiodine capture were assessed in their pure metal-iodide (MIx) pelletized forms to compare relative chemical durabilities. To study chemical durability, commercial MIx compounds of AgI, BiI3, BiOI, CuI, and SnI4 were converted to dense monolithic pellets using spark plasma sintering. Semidynamic leach testing in the form of modified ASTM C1308 tests was then performed on the pellets in two different forms including unmounted (as-pressed) specimens (i.e., “U”) and epoxy-mounted specimens (i.e., “M”) with polished surfaces. The chemical durability results and sample characterizations showed that three of the five MIx compounds tested (i.e., AgI, CuI, and BiOI) displayed moderate to high leach resistances. The remaining two MIx compounds (i.e., BiI3 and SnI4), which are both desirable iodine waste forms due to their high iodine loading capacities, readily decomposed during leach testing, indicated by crystallographic changes in the specimens as well as large amounts of iodine detected in the leachate solutions. The instabilities of BiI3 and SnI4 raise uncertainties for using the base metals/cations (i.e., Bi0/Bi3+ and Sn0/Sn4+, respectively) as viable getters for radioiodine capture due to likely poor waste form chemical durabilities after capture and consolidation into waste forms.