PNNL

Abstract

Porous materials present compelling advantages for absorbing radioactive isotopes in nuclear waste streams. To en-hance absorption efficiency in nuclear waste treatment, a thorough comprehension of the diffusion-advection process within porous structures is essential for material design. In this study, we present advancements in the volumetric lattice Boltzmann method (VLBM) for modeling and simulating pore-scale diffusion-advection of radioactive iso-topes within geopolymer porous structures. These structures are created using the phase field method (PFM) to pre-cisely control pore architectures. In our VLBM approach, we introduce a concentration field of an isotope seamlessly coupled with the velocity field, solving it through the time evolution of its particle population function. To address the computational intensity inherent in the coupled lattice Boltzmann equations for velocity and concentration fields, we implement GPU (Graphics Processing Unit) parallelization. Validation of the developed model involves examin-ing the flow field and diffusion field in porous structures. Remarkably, favorable agreements are observed for both the velocity field from VLBM and Multiphysics Object-Oriented Simulation Environment (MOOSE), and the con-centration field from VLBM and the finite difference method (FDM). Further, we investigate the effects of back-ground flow, species diffusivity, and porosity on diffusion-advection behavior by varying background flow velocity, diffusion coefficient, and pore volume fraction, respectively. Notably, all three parameters exert impacts on the diffu-sion-advection process. Increased background flow and diffusivity markedly accelerate the process, attributed to heightened advection intensity and enhanced diffusion capability, respectively. Conversely, increasing porosity has a less significant effect, causing a slight slowdown in the diffusion-advection process due to the expanded pore vol-ume. This comprehensive parametric study yields valuable insights into the kinetics of isotope uptake in porous structures, facilitating the progression of porous materials for nuclear waste treatment applications.