Long-term performance of zeolite Na A-X blend as backfill material in near surface disposal vault

Abstract

This study investigates the feasibility of using synthetic zeolite Na A-X blend prepared from fly ash as near surface disposal backfill material. Tests were conducted at laboratory scale to evaluate the physical and chemical properties of the prepared zeolite. The zeolite density, porosity, and particle size distribution were measured. The distribution coefficient (Kd) value of Cs ions was evaluated using batch sorption experiment in synthetic groundwater to simulate possible conditions for near surface disposal. The transient behavior of the batch sorption experimental data were analyzed using Lagergren, Ho and Mckay, and Morris–Weber rate models to assess the controlling mechanism of the sorption process. It was found that the sorption process is chemisorption and controlled by diffusion mechanism. The dispersional behavior of Cs ions on the prepared material was studied using column experiment and the hydrodynamic dispersion coefficient was determined. To provide an overall functional performance of the proposed backfill material, the long-term behavior of the prepared zeolite has been evaluated using computer model. This model consists of two modules that has been developed to study the migration of Cs radionuclides from bare cementitious waste form through the backfill. The study compares the release rate from bentonite–crushed rock mixture to that from the prepared zeolite. The result demonstrates that synthetic zeolite Na A-X blend shows a better performance in terms of radionuclide containment.