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.