Multispecies random walk simulations in radial symmetry: model concept, benchmark, and application to HTO, ²²Na and ³⁶Cl diffusion in clay

AbstractModelling of radionuclide transport in clay can be simplified and accelerated by exploiting radial symmetry of laboratory experiments or nuclear waste repositories design. Consequently, the multispecies reactive transport code MCOTAC has been extended to radial symmetry, exploiting the advantages of its random walk transport description. Random walk in radial r-symmetry is mimicked by two-dimensional (2D) random walk projected to a radial coordinate for geochemical calculations. This guarantees fast 2D(x, y) transport calculations, together with complex geochemical calculations in 1D(r) only. The new model concept has been benchmarked for simple geochemical systems and applied to laboratory diffusion experiments with HTO, 22Na and 36Cl in Opalinus Clay using higher spatial resolution for modelling than reported in the literature. This makes it possible to distinguish between the transport properties of the sample and the filters, and yields more careful determination of radionuclide transport parameters: for example, there was a difference of a factor of 2 between the diffusion coefficient and log Kd sorption coefficient in the case of 22Na compared to the former best-fit analysis.