Radionuclide Transport in Fractured Rock: Numerical Assessment for High Level Waste Repository

Deep and stable geological formations with low permeability have been considered for high level waste definitive repository. A common problem is the modeling of radionuclide migration in a fractured medium. Initially, we considered a system consisting of a rock matrix with a single planar fracture in water saturated porous rock. Transport in the fracture is assumed to obey an advection-diffusion equation, while molecular diffusion is considered the dominant mechanism of transport in porous matrix. The partial differential equations describing the movement of radionuclides were discretized by finite difference methods, namely, fully explicit, fully implicit, and Crank-Nicolson schemes. The convective term was discretized by the following numerical schemes: backward differences, centered differences, and forward differences. The model was validated using an analytical solution found in the literature. Finally, we carried out a simulation with relevant spent fuel nuclide data with a system consisting of a horizontal fracture and a vertical fracture for assessing the performance of a hypothetical repository inserted into the host rock. We have analysed the bentonite expanded performance at the beginning of fracture, the quantified radionuclide released from a borehole, and an estimated effective dose to an adult, obtained from ingestion of well water during one year. 1. Introduction With the advent of nuclear technology, countries that employ it received many benefits from the energy production increase, but, as occurs to any activity related to industrialization, there is inevitably a waste production, but the volume of radioactive waste produced is very small, when compared with other industrial residues. The relatively long half-lives of some radionuclides present in this material, as well as their high activity and toxicity, pose a burden for the present and future generation. Part of this waste, the high level waste resulting from reprocessing and spent fuel disposal (HLW—high level waste), remains hazardous for thousands of years and therefore should be carefully stored, using barriers that delay the migration of radioisotopes to the biosphere. Thus, the greatest challenge involves the management of high level radioactive waste, which must be isolated from the environment, and a disposal site for this waste, chosen to ensure safety for many thousands of years, OECD [1]. Although it is impossible to establish a safe depth, for which the biosphere could be effectively isolated from the buried radioactive wastes, it is recognized that a deep repository