Semi-dynamic leach testing of densified silicon-based iodine waste forms

A contaminant of concern in nuclear waste, iodine must be immobilized in an iodine waste form (IWF) prior to long-term disposal. A conceptual corrosion release model (CCRM) is required to provide iodine release rates in performance modeling of nuclear waste disposal repositories. To develop a CCRM, corrosion mechanisms of the IWFs and data from consistent test methods are needed to parameterize the model. The present study advances both by assessing the corrosion resistance of IWFs with a series of semi-dynamic leach tests using monolithic IWFs in deionized water (leachant). Experiments were conducted under various test conditions with changes to temperature, leachant replacement, leachant pH, leachant volume, masking, and surface finish to determine if varying these conditions impacted IWF corrosion behaviors. Tests were conducted on two classes of IWFs: (1) iodine-bearing silver-mordenite (AgZ) materials processed by hot isostatic pressing (HIP) at different temperatures, pressures, sizes, and times; and (2) iodine-bearing silver-functionalized silica aerogels (SFA) processed by either HIP or spark plasma sintering (SPS). The corrosion susceptibility of AgZ samples was influenced by HIP temperature and pressure. The SPS SFAs retained iodine far better than HIP SFAs. Additional findings in this study include: (1) the iodine dissolution rate decreased with decreasing temperature, (2) a common ion effect may occur and slow dissolution of the host phase if the leachant is not regularly replaced, (3) pH affects the dissolution rate, and (4) the iodine dissolution rate slows with extended test time (up to 224 days). These parameters should thus be represented when developing a CCRM.