Travel time simulation of radionuclides in a 200 m deep heterogeneous clay formation locally disturbed by excavation

In the North of Belgium the Boom Clay Formation, at a depth of 200m 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.
In a first phase, 100 simulations of the hydraulic conductivity of the clay are generated by conditional stochastic simulation, using measurements of hydraulic conductivity and 4 types of secondary variables: resistivity logs, gamma ray logs, grain size measurements and descriptions of the lithology.
Thereafter, 100 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 galleries.
Subsequently, the hydraulic conductivity simulations and the fracture simulations are randomly combined and used as input for a transport model that calculates the transport by advection, diffusion, dispersion, adsorption and decay through the heterogeneous and fractured medium. The effect of “self healing” of the clay formation as a function of time after excavation is taken into account.
Finally, this results in 100 breakthrough curves of radionuclides in the aquifers surrounding the Boom Clay, reflecting the uncertainty of travel time through the clay resulting from the uncertainty of the hydraulic conductivity and the fracture distribution.
The breakthrough curves can 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.