Accommodating transient velocities in time-domain particle tracking simulations of radionuclide transport

Particle tracking in the time domain has previously been shown to be a robust and efficient technique for simulating radionuclide transport in the subsurface. In time-domain particle methods, particles representing packets of radionuclide mass are moved along predefined pathways using random residence time distributions developed from the underlying physicochemical retention processes. Time-domain particle tracking is extended here to accommodate pathway properties that are piecewise constant in time. Advection in flowing fractures and diffusion/sorption in the surrounding rock matrix is considered. In the new algorithm, a particle's distance away from a flowing fracture is sampled at the end of a constant-velocity flow period. Given this position in the matrix, the time required to diffuse back to the flowing fracture is then sampled. The particle is then restarted in the fracture using the new pathway velocity. Numerical verification tests demonstrate that the new approach is sufficiently accurate for use in assessing geological barriers in high-level nuclear waste repositories.