(376bd) Thermal Aging of Ag-MOR and Ag-Aerogel in Nuclear Off-Gas Streams Containing H2O and NOx

Reprocessing used nuclear fuel (UNF) generates several volatile radioactive species, including ³H, ¹⁴C, ⁸⁵Kr, and ¹²⁹I, into various process off-gas streams. Retention of these radioactive species is required to meet the regulations on volatile radioisotopes emissions to the environment. Ag-exchanged mordenite (Ag^­­_­-MOR) and Ag functionalized silica aerogel have shown outstanding performances for radioactive iodine removal from UNF reprocessing off-gases. However, long-term exposure of the Ag-containing adsorbents to the off-gas stream containing air, H₂O, and NO_X at elevated temperatures would cause a thermal aging of the materials. It is essential to understand the aging effects and mechanisms in order to address the issue. This study presents our investigations on the aging of Ag^­­_­-MOR and Ag-aerogel by the constituents in UNF reprocessing off-gas streams. Through aging and iodine adsorption experiments, significant iodine adsorption capacity losses were observed when Ag^­­_­-MOR and Ag-aerogel were exposed to dry air, humid air (dew point -15 ^oC), 1% NO, and 2% NO₂ at 150 ^oC for up to 6 months. The aged samples were analyzed with a number of characterization techniques including scanning electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray absorption fine structure (XAFS) to determine physical and chemical changes. Mechanisms of the thermal aging process of Ag^­­_­-MOR and Ag-aerogel in the studied gas streams were determined.