Effect of excavation induced fractures on radionuclide migration through the Boom Clay (Belgium)

in Applied Clay Science (2006), 33(3-4), 207-218

In Belgium, the Boom Clay is being evaluated as a potential host formation for the disposal of high-level nuclear waste. In order to investigate this option, an underground research facility composed of ... [more ▼]

In Belgium, the Boom Clay is being evaluated as a potential host formation for the disposal of high-level nuclear waste. In order to investigate this option, an underground research facility composed of two access shafts and 200m of galleries was excavated. Excavation induced fractures were observed in a zone of approximately 1 m around the galleries. In this study, the potential effect of these fractures on radionuclide migration in the Boom Clay is investigated. Therefore, a hydrogeological model of the clay is built with a radionuclide source in the middle of the clay layer surrounded by different fracture configurations. Two types of fracture configurations are inserted in the model. The first type of fracture properties is drawn stochastically from the probability distributions of the properties of the fractures observed around previously excavated galleries. These fracture patterns are considered to be realistic although in this study it is conservatively assumed that no self-sealing occurs. The model is run for a large number of stochastically drawn fracture configurations and the results are compared to a model without fractures. These calculations show that the radionuclide fluxes through the clay are not significantly influenced by these fractures. For the second type of fracture configurations, the fracture properties are varied over a much larger range. Hypothetical fractures with much higher values of fracture extent, aperture, dip and frequency than observed are modeled, With these hypothetical fracture configurations, the critical values of the fracture parameters are determined that must be exceeded to have a significant effect on the radionuclide fluxes through the clay. These calculations show that the extent of the fractured zone has the largest effect on radionuclide migration. The other fracture parameters (aperture, spacing and dip) have a limited effect on the radionuclide fluxes. To obtain a total radionuclide flux through the lower clay boundary that is respectively 10%, 50% and 100% larger than without a fractured zone, the extent of the hypothetical fractured zone should be respectively 27.97 m, 43.86 m and 46.92 m. Such fractures are not expected to be generated by mechanical excavation in the Boom Clay. The obtained critical extent values of the hypothetical fractures are a factor 30 to 40 higher than the measured values of the extent of the excavation disturbed zone. These calculations thus indicate that it is very unlikely that the extent of the fractured zone around the galleries will be large enough to have a significant effect on the radionuclide fluxes through the Boom Clay. This conclusion is further supported by the conservative assumption that no self-sealing occurs. These calculations can function as a preliminary robustness test in ongoing safety analysis studies. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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

in Seventh International Geostatistics Congress (2005)

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 ... [more ▼]

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. [less ▲]