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POSIVA Report 2012-48

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Name:

Thermo-Hydraulic Modelling of Buffer and Backfill.

Writer:

Pintado, X. & Rautioaho, E.

Language:

English

Page count:

164

ISBN:

ISBN 978-951-652-230-5

Summary:

The temporal evolution of saturation, liquid pressure and temperature in the components of the engineered barrier system was studied using numerical methods. A set of laboratory tests was conducted to calibrate the parameters employed in the models. The modelling consisted of thermal, hydraulic and thermo-hydraulic analysis in which the significant thermo-hydraulic processes, parameters and features were identified. CODE_BRIGHT was used for the finite element modelling and supplementary calculations were conducted with analytical methods.

The main objective in this report is to improve understanding of the thermo-hydraulic processes and material properties that affect buffer behaviour in the Olkiluoto repository and to determine the parametric requirements of models for the accurate prediction of this behaviour. The analyses consisted of evaluating the influence of initial canister temperature and gaps in the buffer, and the role played by fractures and the rock mass located between fractures in supplying water for buffer and backfill saturation. In the thermo-hydraulic analysis, the primary processes examined were the effects of buffer drying near the canister on temperature evolution and the manner in which heat flow affects the buffer saturation process. Uncertainties in parameters and variations in the boundary conditions, modelling geometry and thermo-hydraulic phenomena were assessed with a sensitivity analysis.

The material parameters, constitutive models, and assumptions made were carefully selected for all the modelling cases. The reference parameters selected for the simulations were compared and evaluated against laboratory measurements. The modelling results highlight the importance of understanding groundwater flow through the rock mass and from fractures in the rock in order to achieve reliable predictions regarding buffer saturation, since saturation times could range from a few years to tens of thousands of years depending on the hydrogeological conditions in the rock. In addition to the rock hydraulic conductivity and fracture transmissivity, the saturation process was significantly affected by the material properties of the buffer and backfill components. The effects of heat flow and vapour transportation were less significant. In connection with thermal evolution, the thermal conductivity of repository components and the behaviour of air gaps in the buffer were the key variables.

Keywords:

Repository, spent fuel, thermal, hydraulic, thermo-hydraulic, saturation, buffer, backfill, rock, fracture, finite element method, CODE_BRIGHT.

File(s):

Thermo-Hydraulic Modelling of Buffer and Backfill (pdf) (14.3 MB)


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