A physics-based mesoscale phase-field model for predicting the uptake kinetics of radionuclides in hierarchical nuclear wasteform materials

• Published in: Computational materials science 2019-03, Vol.159 (C), p.103-109
• Main Authors: Li, Y.L., Zeidman, B.D., Hu, S.Y., Henager, C.H., Besmann, T.M., Grandjean, A.
• Format: Article
• Language: English
• Subjects: Hierarchical zeolite; Ion exchange; Nuclear wasteforms; Phase-field model; Phase-field model, ionic exchange, porous silica, porous materials, radioactive waste storage, Hierarchically Porous Materials; Porous solids
• ISSN: 0927-0256; 1879-0801
• DOI: 10.1016/j.commatsci.2018.11.041

[Display omitted] Multiscale, hierarchical, porous materials are promising nuclear wasteform materials with potential for efficiently adsorbing and sequestering radionuclides. However, fundamental understanding of such complex micro- and meso-structures on radionuclide diffusion and uptake kinetics is lacking, which is essential for designing advanced nuclear wasteform materials. In this work we develop a microstructural-dependent diffusion model that accounts for three-dimensional (3D) microstructural features, including heterogeneous thermodynamic and kinetic properties, to predict radionuclide uptake kinetics in complex porous structures. Na+ and Sr2+ ionic exchange in LTA zeolite multiscale materials in batch tests is taken as a model system to demonstrate the model and its application to hierarchical materials with multiscale porosity. It is found that the reduction of ionic mobility associated with chemistry, structure, and/or phase changes has more profound impact on the uptake kinetics in a large 3D porous particle than that in a small one. Uptake kinetics in large particles are limited by two diffusion processes: bulk diffusion and surface layer transport. Decreasing particle size and increasing mesoscale pore volume fraction changes the uptake kinetics from two diffusion processes to a single process and dramatically increases the uptake kinetics. Predicted uptake kinetics and the effect of microstructures on the effective capacity of radionuclides and uptake efficiency are consistent with results observed in Na+/Sr2+ exchange experiments. The results demonstrate that the developed model should be adaptable to transport problems in other hierarchical material systems.