Modelling simultaneous transport of natural and anthropogenic radionuclides in fractured media — Diffusion into the heterogeneous layered rock matrix for an arbitrary length decay chain

Transport models used for assessing the safety of radioactive waste repositories hosted in fractured bedrock typically do not consider fluxes of naturally occurring radionuclides in the rock and their further migration in flow-bearing fractures. A consistent model that simultaneously describes the transport of radionuclides from both natural and anthropogenic sources has been developed, where decay chains and rock heterogeneity are accounted for. The model accounts for advective flow in the fracture, a decay chain of arbitrary length, and diffusion into and out of the adjacent rock matrix composed of different geological layers. The proposed solution has been verified against a previously published steady state case which considers a homogeneous rock matrix of infinite extent where porewater ingrowth is not accounted for. The model is also applied to some different calculation examples for both transient and limiting steady state cases to represent typical applications of the model as well as to illustrate the effect of different parameters and processes on the transport of natural radionuclides in fractured rocks. This study presents a novel and powerful tool to simulate migration of both anthropogenic and natural radionuclides in and from crystalline rocks to the biosphere. The presented modelling is essential in safety and performance assessment of deep geological disposal of radioactive waste in fractured rocks. The obtained analytical solution can be used to compare relative fluxes of natural and anthropogenic radionuclides, which is useful for validation of the radionuclide transport parameters obtained from field and laboratory experiments.