(245b) Aging Processes of Ag-Exchanged Mordenite and Ag-Functionalized Silica Aerogel in Spent Nuclear Fuel Reprocessing Off Gases

Yue Nan,¹ Carter W. Abney², Seungrag Choi,¹ Jiuxu Liu¹, Lawrence L. Tavlarides^1*

¹Department of Biomedical and Chemical Engineering, Syracuse University, 329 Link Hall, Syracuse, NY 13244, USA

²Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, USA.

Email: lltavlar@syr.edu (Tavlarides); ynan@syr.edu (Nan)

The aging issue is one major problem in the removal of radioactive iodine vapors from spent nuclear fuel reprocessing off-gas streams using silver containing adsorbents. During the off-gas treatment, columns packed with adsorbents are expected to be exposed to off-gas streams containing air, iodine vapors, H₂O vapor, NO and NO₂ over extended periods of time (up to 6 months). Consequently, the iodine adsorption capacity of the adsorbents would decrease over time due to a series of chemical and physical changes to the adsorbents. Therefore, it is of importance to understand the aging processes of the silver containing adsorbents in off-gas streams for better designing of the treatment systems.

As the most outstanding adsorbents for iodine retention in the off-gas treatment, aging processes of Ag-exchanged mordenite (Ag⁰Z) and Ag-functionalized silica aerogel (Ag⁰-aerogel) have been studied over the past few years. Previous studies have experimentally investigated the aging effects of Ag⁰Z and Ag⁰-aerogel in different off-gas species including dry-air, humid-air, 2% NO₂/air, and 1% NO/air, and found that the iodine adsorption capacity of Ag⁰Z and Ag⁰-aerogel decreased significantly under most of the studied aging conditions. For example, Ag⁰Z lost 60% of iodine adsorption capacity after aging in a humid-air flow for 4 months and lost 78% of the capacity after aging in a batch reactor filled with 1% NO/air for 2 months, while Ag⁰-aerogel lost 22% and 43% of iodine capacity, respectively, under the same conditions. Also, 30% iodine capacity loss was observed for Ag⁰Z after 2 months aging in a 2% NO₂/air batch, but no significant capacity loss was observed for Ag⁰-aerogel. These results from previous studies indicated that both adsorbents had different capacity losses in different aging gases, and under the same aging conditions Ag⁰Z and Ag⁰-aerogel had different relative capacity losses. However, reasons for these differences are not clear and, therefore, the mechanisms of the aging processes need to be determined.

In this study, experimental and analytical work has been proposed and is on-going to determine the mechanisms of aging processes of Ag⁰Z and Ag⁰-aerogel in different aging gases. A continues-flow aging experimental system has been constructed to produce aging samples of Ag⁰Z, Ag⁰-aerogel and Ag foil in dry-air, humid-air, 2% NO₂/air and 1% NO/N₂ flows at 150 ^oC for periods of time up to 6 months. The continuous-flow aging experiments with 2% NO₂/air and 1% NO/N₂are expected to provide more information than the batch experiments in previous studies. Iodine adsorption capacities of the aged samples are determined by iodine loading experiments. Chemical and physical changes to the aged samples are determined by analytical methods including X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), pair density function (PDF), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis. Chemical forms of Ag (Ag⁰, Ag₂O, AgNO₂, AgNO₃) and corresponding quantities in the adsorbents, as well as changes to the surface area, pore volume and size, and morphologies of the adsorbents will be determined via these analyses. The mechanisms of the aging processes are expected to be determined.

At this writing, samples of Ag⁰Z, Ag⁰-aerogel and Ag foil aged in dry-air and humid-air for 1 month have been obtained. Results of iodine loading experiments showed that Ag⁰Z lost 10% and 45% iodine capacity after aging in dry-air and humid-air, respectively, and Ag⁰-aerogel lost 13% capacity after aging in dry-air (the iodine adsorption experiment with humid-air aged sample is in progress). Characterizations of the samples by the analytical methods mentioned above is in progress. In addition, the aging experiments of Ag⁰Z, Ag⁰-aerogel and Ag foil in dry-air, humid-air, 2% NO₂/air, and 1% NO/N₂ flows for up to 6 months are on-going and those results will also be reported.