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Comparative study of convective and diffusive transport phenomena within the Opalinus Clay at Mont Terri

​Catherine Ji Yu has defended her thesis on 9th November 2017 at IRSN, in Fontenay-aux-Roses.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PRP-DGE/SRTG/LETIS

Authors > JI YU Catherine

Publication Date > 09/11/2017

Summary

​The safety of radioactive waste disposal facilities in deep geological formation depends on the evaluation of the rock confining properties and the processes governing radionuclides transfer. The thesis is conducted in the framework of Deep Borehole experiment of the Mont Terri project. The purpose of this research is to build confidence with regard to understanding relative importance of diffusive and convective phenomena withine the Opalinus Clay (OPA) and to identify the impact of a hydraulic and chemical transient behaviour on the transfers of fluid and solutes, and anomalous pressures generation.


A first experimental stage enabled to acquire the advective and diffusive transport parameters, and the associated driving forces, necessary to the evaluation of fluid and solute fluxes between the OPA and its adjacent aquifers. Temperature and pressure measurements revealed a geothermal gradient of

8.5 °C/100 m and an excess of hydraulic head of at least 60 m.


The chloride profile inversion by a Bayesian method with a Monte Carlo Markov Chain algorithm validates the paleohydrological evolution proposed in the litterature, considering a pure diffusive transport through the argillaceous formation. The contribution of osmotic transport phenomena was assessed by interpreting the pressure profile, using transient simulations that takes into account the temporal evolution of chlorinity and pressure during the geological scenario, and pseudo steady-state simulations. This profile is reproduced by evaluating the coupled advective flux, including pure advection, chemo-osmosis and thermo-osmosis, with a major contribution of the latter process.


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