Capture of Organic Iodides from Vessel Off-Gas Streams

Several gaseous radionuclides, produced during used nuclear fuel (UNF) reprocessing, are present in the off-gas streams of UNF reprocessing facilities. For instance, the dissolver off-gas (DOG) contains a mixture of 3H (as tritiated water), 129I (as 129I2 and CH3I), 14C (as 14CO2), 85Kr, and 135Xe. The DOG contains roughly 98% of the total iodine in UNF, while the remaining 2% of total iodine is found in the vessel off-gas (VOG) stream in a mixture of elemental (I2) and organic forms (CH3I, etc.). In total, a reprocessing plant needs to capture >99.9% of all iodine to meet federal regulations on plant emissions. Therefore, it is necessary to capture iodine from both the DOG and VOG streams.Capture of iodine from VOG is complicated by a number of factors including: (i) high flow-rates (roughly 10-100 higher than in DOG), (ii) low concentrations (between 5-100 ppb compared to ppm levels in DOG), and (iii) presence of volatile organic species from solvent extraction. These conditions create a very dilute system where the iodine present in the off-gas stream is made up of a complex mixture of organic iodides ranging from methyl iodide (CH3I) to iodo-dodecane (C12H25I). To efficiently capture the organic iodides from VOG, studies must be carried out experimentally and analytically to determine an effective adsorbent and understand the adsorption properties. Silver adsorbents have long been considered for iodine adsorption due to the high strength of the Ag-I chemical bond, which helps to prevent desorption of iodine after capture. Additionally, studies have shown that adsorbents impregnated with reduced-silver (Ag0) can achieve higher decontamination factors (DFs) and iodine loading than their unreduced (Ag+) counterparts. Therefore, this study proposes to first compare the adsorption capacity of three Ag-containing adsorbents: reduced Ag mordenite (Ag0Z), reduced Ag-functionalized aerogel (Ag0-aerogel) and Ag alumina. With the selected adsorbent, adsorption experiments and material characterizations will be performed to determine the adsorption kinetics and reaction pathways for the organic iodides. In addition, the oxidation or 'aging' of the reduced-Ag adsorbents in streams containing H2O and NOX will also be investigated.