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Recent publications

Workreport 2017-32



Discrete Fracture Network Modelling (Version 3) in Support of Olkiluoto Site Description 2018


Hartley, L., Appleyard, P., Baxter, S., Hoek, J., Joyce, S., Mosley, K., Williams, T., Fox, A., Cottrell, M., La Pointe, P., Gehör, S., Darcel, C., Le Goc, R., Aaltonen, I., Vanhanarkaus, O., Löfman, J., Poteri, A.



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Olkiluoto Island, which is located in SW Finland, has been selected as the site for a deep geological repository for the high-level spent nuclear fuel generated by the nuclear power plants operated by the Finnish power companies TVO and Fortum.

On behalf TVO and Fortum, Posiva has continued updating the multidisciplinary site description, toward the last licencing stage of the repository – the application of the operating licence. Characterisation data comes from 58 deep cored drillholes, a wide variety of geophysical investigations, many outcrops, kilometres of underground mapping and testing in the ONKALO research facility, and groundwater pressure monitoring and sampling in both deep and shallow holes. A third generation of Olkiluoto discrete fracture network, ODFN3, model has been developed to provide an integrated description of geological, hydrogeological, geomechanical and transport properties of natural fractures and brittle fault zones at the site.

The purposes of the DFN include: providing a basis for flow and transport calculations from deposition holes to the far-field; simulations of how hydrogeological and hydrogeochemical conditions around the repository evolve over time; and as a decision support tool in the development of operational acceptance criteria for disposal at a specific location given local investigation data. Hence, the ODFN3 model provides both statistical distributions of properties over the whole repository area and predictions specific to local geological conditions. With these needs in mind, fracturing related to faults is represented as swarms of fractures within their damage zones, while background fracture patterns are correlated to tectonic unit, lithology and depth. Hydraulic properties of fractures are described along hydromechanical principles according to size, morphology/mineralogy characteristics and stress field.

This report provides a detailed traceable record of how each facet of the ODFN3 model has been conceptualised, represented in numerical DFN models and calibrated against site data. It presents numerical sensitivity tests to quantify the consequences of conceptual and parameter uncertainties. Confirmation of the suitability of the model is provided through simulation of hydraulic interference tests on two scales and palaeohydrogeological evolution of groundwater composition. Finally, a plan for validating that the model is fit for purpose is formulated as a series of prediction-outcome exercises using mapping and hydraulic testing data from new construction areas, with a first exercise to test and inform the forward plan documented here.


Geology, hydrogeology, rock mechanics, discrete fracture network, modelling


WR 2017-32_web (pdf) (79.5 MB)
AppendixA_web (pdf) (29 MB)


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