The adsorption process of iodine, a major volatile radionuclide in the off‐gas streams of spent nuclear fuel reprocessing, on hydrogen‐reduced silver‐exchanged mordenite (Ag0Z) was studied at the micro‐scale. The gas‐solid mass transfer and reaction involved in the adsorption process were investigated and evaluated with appropriate models. Optimal conditions for reducing the silver‐exchanged mordenite (AgZ) in a hydrogen stream were determined. Kinetic and equilibrium data of iodine adsorption on Ag0Z were obtained by performing single‐layer adsorption experiments with experimental systems of high precision at 373–473 K over various iodine concentrations. Results indicate approximately 91% to 97% of the iodine adsorption was through the silver‐iodine reaction. The effect of temperature on the iodine loading capacity of Ag0Z was discussed. The Shrinking Core model describes the data well, and the primary rate controlling mechanisms were macro‐pore diffusion and silver‐iodine reaction. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1024–1035, 2017

This work is part of our continuing efforts to address engineering issues related to the removal of tritiated water from off-gases produced in used nuclear fuel reprocessing facilities. In the current study, adsorption equilibrium of water on molecular sieve 3A beads was investigated. Adsorption isotherms for water on the UOP molecular sieve 3A were measured by a continuous-flow adsorption system at 298, 313, 333, and 353 K. Experimental data collected were analyzed by the Generalized Statistical Thermodynamic Adsorption (GSTA) isotherm model. The K + /Na + molar ratio of this particular type of molecular sieve 3A was ∼4:6. Our results showed that the GSTA isotherm model worked very well to describe the equilibrium behavior of water adsorption on molecular sieve 3A. The optimum number of parameters for the current experimental data was determined to be a set of four equilibrium parameters. This result suggests that the adsorbent crystals contain four energetically distinct adsorption sites. In addition, it was found that water adsorption on molecular sieve 3A follows a three-stage adsorption process. This three-stage adsorption process confirmed different water adsorption sites in molecular sieve crystals. The second adsorption stage is significantly affected by the K + /Na + molar ratio. In this stage, the equilibrium adsorption capacity at a given water vapor pressure increases as the K + /Na + molar ratio increases.

The objective of the current work was to shorten the gap for fundamental adsorption kinetic data required for the development of advanced adsorption unit-operation models to be incorporated into an overall plant-level model for spent nuclear fuel reprocessing. The kinetics of water-vapor adsorption on molecular sieve 3A was investigated at 25–80 °C and water dew points from −69 to 17 °C. Water uptake curves were fitted with three kinetic models including the linear-driving-force model, the shrinking-core model, and the Langmuir kinetic model. The results suggest that the water-vapor adsorption on molecular sieve 3A under the investigated experimental conditions was controlled by both external film resistance and internal macropore resistance. The contribution of the external film resistance varied from 25% to 50% of the total mass-transfer resistance depending on the adsorption temperature. It was also found that the Langmuir kinetic model fitted individual sets of kinetic data very well, but the Langmuir adsorption constant obtained from curve fitting decreased with increasing adsorption temperature and with increasing water vapor pressure. This result indicates a significant surface heterogeneity of molecular sieve 3A and also implicitly verifies that the Langmuir isotherm model is unable to represent isotherms of water adsorption on molecular sieve 3A.

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Capture of radioactive iodine is a significant consideration during reprocessing of spent nuclear fuel and disposal of legacy wastes. While silver-exchanged mordenite (AgZ) is widely regarded as a benchmark material for assessing iodine adsorption performance, previous research efforts have largely focused on bulk material properties rather than the underpinning molecular interactions that achieve effective iodine capture. As a result, the fundamental understanding necessary to identify and mitigate deactivation pathways for the recycle of AgZ is not available. We applied X-ray Absorption Fine Structure (XAFS) spectroscopy to investigate AgZ following activation, adsorption of iodine, regeneration, and recycle, observing no appreciable degradation in performance due to the highly controlled conditions under which the AgZ was maintained. Fits of the extended XAFS (EXAFS) data reveal complete formation of Ag0 nanoparticles upon treatment with H2, and confirm the formation of α-AgI within the mordenite channels in addition to surface γ/β-AgI nanoparticles following iodine exposure. Analysis of the nanoparticle size and fractional composition of α-AgI to γ/β-AgI supports ripening of surface nanoparticles as a function of recycle. This work provides a foundation for future investigation of AgZ deactivation under conditions relevant to spent nuclear fuel reprocessing.