Travel Time Simulation of Radionuclides in a 200 m Deep Heterogeneous Clay Formation Locally Disturbed by Excavation

Abstract

In Belgium, the Boom Clay Formation at a depth of 200 m below surface is being evaluated as a potential host formation for the disposal of vitrified nuclear waste. The aim of this study is to model the transport of radionuclides through the clay, taking into account the geological heterogeneity and the excavation induced fractures around the galleries in which the waste will be stored. This is achieved by combining a transport model with geostatistical techniques used to simulate the geological heterogeneity and fractures of the host rock formation. Two different geostatistical methods to calculate the spatially variable hydraulic conductivity of the clay are compared. In the first approach, one dimensional direct sequential co-simulations of hydraulic conductivity are generated, using measurements of hydraulic conductivity (K) and 4 types of secondary variables: resistivity logs, gamma ray logs, grain size measurements and descriptions of the lithology, all measured in one borehole. In the second approach, three dimensional cokriging was performed, using hydraulic conductivity measurements, gamma ray and resistivity logs from the same borehole and a gamma ray log from a second borehole at a distance of approximately 2000 m from the first borehole. For both methods, simulations of the fractures around the excavation are generated based on information about the extent, orientation, spacing and aperture of excavation induced fractures, measured around similar underground galleries. Subsequently, the obtained 3D cokriged and 1D simulated values of hydraulic conductivity are each randomly combined with the simulated fractures and used as input for a transport model that calculates the transport by advection, diffusion, dispersion, adsorption and decay through the clay formation. This results in breakthrough curves of the radionuclide Tc-99 in the aquifers surrounding the Boom Clay that reflect the uncertainty of travel time through the clay. The breakthrough curves serve as a risk management tool in the evaluation of the suitability of the Boom Clay Formation as a host rock for vitrified nuclear waste storage. The results confirm previous calculations and increase confidence and robustness for future safety assessments.