Analysis of radionuclide release from spent ion-exchange resins

Ion-exchange resins represent one of the most important waste streams in low-level waste management due to the unstabilized nature of the waste form and the large amount of radioactivity contained. To describe the release of radionuclides from ion-exchange resins stored in a disposal facility, a mechanistic release model was developed. The model is based on description of radionuclide migration both in the resin bead phase and the bulk pore water phase within waste containers. This modeling setup provides the capability to describe all the major physical processes taking place for the release of radionuclides. Because of the difficulty in obtaining analytical solutions, the numerical solution approach was employed in this model. The new resin release model was used to examine key processes and parameters in describing radionuclide release. These were found to be diffusion within the bulk pore water phase, flow rate of infiltrating leachant water, concentration of counterions of the leachant water, and sorption during the transport in the bulk pore water phase. Some parameters were found to have little impact in describing the release. These include the interdiffusion coefficient within resin beads and the density and radius of resin beads. Existing simplified modeling approaches were also compared with the new resin release model, and validities of using these simplified models are discussed.