The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Thermomechanics of Swelling Unsaturated Porous Media. Compacted Bentonite Clay in Spent Fuel Disposal
Petri Jussila
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 13th of November, 2007, at 12 noon.
Dissertation in PDF format (ISBN 978-952-478-266-1) [3226 KB]
Dissertation is also available in print (ISBN 978-952-478-265-4)
Abstract
A barrier of compacted bentonite clay is planned to be used in geological disposal of spent nuclear fuel. In addition to providing mechanical stability to the waste containers the barrier is supposed to prevent or delay the movement of groundwater and the consequential transport of radionuclides from the repository. Fluid flow, phase changes, mechanical behavior of the buffer, rock, and the containers, and the heat produced by the radioactive waste constitute a coupled thermo–hydro–mechanical (THM) system.
The objective of the thesis is to model the coupled THM behaviour of the bentonite buffer. For this purpose a general thermomechanical and mixture theoretical model is derived and applied to the fully coupled THM description of swelling compacted bentonite. The particular form of the free energy of the system is chosen to take into account interactions of the mixture components, namely, mixing of the gaseous components (water vapor and air) and adsorption and swelling interactions between the liquid water and the solid skeleton. The mechanical part of the model is limited to reversible behavior within the limit of small strains. Numerical implementation is done with the multi-purpose finite element method software ELMER.
The model is applied to various coupled experiments: two kinds of laboratory scale tests for Febex bentonite, larger scale mock-up and in-situ tests for Febex bentonite, and to three kinds of laboratory scale experiments for MX-80 bentonite. In addition, a brief consideration of the difference of the large scale Febex experiments and the real disposal situation is done by incorporating more realistic temperature evolutions of the containers.
The inclusion of the mixing interaction yields Clausius-Clapeyron equations which are valid both for the total pressure (i.e. the boiling pressure) and for the partial pressure of saturated vapor. Additionally, together with an appropriate dissipation function the mixing interaction yields a common form of the Fick law. The adsorption interaction together with the mixing interaction yields a modified Clausius-Clapeyron equation for the vapor-liquid equilibrium inside the porous medium with suction as the macroscopic result. The swelling interaction yields the macroscopic swelling deformation and swelling pressure in confined condition. Together with the adsorption interaction function and the appropriate dissipation function a modified form of the Darcy law is obtained.
The model is validated by the simulated experiments to reproduce the main coupled features of unsaturated swelling porous medium satisfactorily.
The main results are related to the important questions of the evolution of resaturation and the final hydration stage.
- The observations for the Febex in-situ and, especially, Febex mock-up experiment exhibit an unexpected transient behaviour that the continuum model does not reproduce. This phenomenon is the fast initial wetting of the internal points of the buffer. This phenomenon is attributed here to the prewetting and, especially, to the anisotropic brick-wall like structure of the Febex buffer. Validation of this claim is based on the facts that the phenomenon is not encountered in the continuum mechanical simulations found in the literature, or in other experiments having a more homogeneous structure. Furthermore, by neglecting the initial transient by assuming a higher initial water content the simulation results are consistent with the observations after the transient.
- The simulations of the THM experiments predict a steady unsaturated state. Despite the artificial wetting a Febex type bentonite buffer will not fully saturate if a high temperature gradient prevails as in the experimental setups considered.
- In realistic disposal conditions the simulations do predict a fully saturated state. However, the predicted time to achieve the full saturation is longer than commonly expected for a Febex type buffer.
The difference between the results for the experiments and for the realistic disposal conditions is due to the different heating powers and, consequently, different temperature profiles involved.
Keywords: engineered barrier, nuclear waste, coupled behavior, swelling, suction, adsorption, phase change, mixture theory, constitutive equations, mathematical modeling
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© 2007 Helsinki University of Technology
Last update 2011-05-26