Introduction
129I is one of the volatile radioactive species in the off-gas stream generated from spent nuclear fuel reprocessing processes.129I has a long half-life (1.6 x 107years) and it is fatal to the human body if released to the environment. Many studies have been carried out to capture radioactive iodine from spent nuclear fuel reprocessing off-gas streams. Liquid scrubbing methods have been reported1, 2 for radioactive iodine removal, but these methods have a low removal efficiency, and require a high construction cost and additional works to reprocess toxic and acidic solutions produced from the liquid scrubbing processes. As an alternative to wet scrubbing systems, solid adsorbents to capture gaseous iodine have been also suggested.1, 2 Of those studied solid adsorbents, hydrogen-reduced silver mordenite (Ag0Z) has been introduced as a representative adsorbent which can effectively capture gaseous iodine. Various studies on Ag0Z3-9 have been reported including mechanisms and performances on iodine loading capacity.
In previous studies, many researchers investigated Ag0Z reduced under a hydrogen environment, and discussed the reduction process, the mechanism, and the performance on the reduced silver mordenite.3, 4Ag0 was formed on the surface of mordenite as larger particles through the reduction reaction in H2, and it could capture gaseous iodine effectively in the form of crystalline AgI. These studies showed the formation of AgI on Ag0Z exposed to gaseous iodine. To study the impact of off-gas streams on silver containing adsorbents, a number of aging studies have been performed. In efforts to understand the long-term behaviors of Ag0Z in spent nuclear fuel reprocessing off-gas treatment, a number of aging studies on Ag0Z5-9 have been done for extended period of times up to 6 months. These studies showed the iodine loading capacities of Ag0Z aged in gas streams at 150oC for up to 6 months decreased. For example, iodine loading capacity was decreased by 40% after exposure to dry air for 6 months5 and by 60% after aging in humid air for 4 months.6 Though the adverse effects of off-gas streams on Ag0Z were investigated in previous studies, it is not clear why the iodine loading capacities of the adsorbents decreased when exposed to off-gas streams.
To understand the mechanisms of the aging processes on Ag0Z, characterization studies including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) were previously performed. It was found that Ag0 was oxidized to Ag+ by oxygen and water in dry air and humid air, then the oxidized Ag+ was migrated into the pores of mordenite crystalline structure. Subsequently, Ag+ was bound to the mordenite crystalline structure by replacing hydrogen detached from the mordenite. Finally, the replaced hydrogen was bound to oxygen or hydroxide to produce water.
The objective of this study is understanding aging processes of the selected adsorbent and to develop appropriate kinetic models to predict the effect of off-gas streams on silver containing adsorbents. To better understand the impact of off-gas streams including dry air and humid air on the silver containing adsorbent, Ag0Z, the kinetics of the aging processes on Ag0Z in off-gas streams were studied under experimental conditions including aging time (up to 6 months), temperatures (100 oC to 200oC), and water vapor concentrations (dew points: -40oC to +15 oC). The kinetic data from aging and iodine adsorption experiments were obtained, and a suitable reaction model was also recommended to predict the aging impacts on Ag0Z.