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Theses in progress

Modeling by discrete element methods of long term hydromechanical behavior of vertical sealing systems - Impact of gas migration

Host laboratory: Research Laboratory for Migration and Interactions in the Geosphere (LETIS)

Beginning of the thesis: October 2018

Student name: Robert CAULK

Subject description

The vertical sealing systems of a deep geological repository are one of the key elements of this containment, as they are the main potential pathway between nuclear waste and the biosphere. Understanding the migration processes of gases produced by metallic corrosion, microbial degradation and radiolysis of water through these sealing systems is of great importance for the performance assessment and long-term evolution of these facilities. One of the candidate materials for these seals consists of a swellable clay mixture in the form of a polydispersed assembly of highly compacted granules and crushed granules, in an initial highly desaturated state.

In order to study the hydromechanical behaviour of the mixture on a macro and microstructural scale under hydraulic and gaseous load, IRSN has launched a series of in situ and laboratory tests (small-scale tests combined with X-ray microtomography, observations by tomodensitometry) within the Sealex and Vseal projects.  One of the main results is free swelling due to steam transport, with compacted clay granules revealing complex crack patterns.

This work aims to better understand the complex behaviour of this multi-scale material under asymmetrical, hydraulic and gaseous loads. Following a multi-scale approach, the first objective is to construct a discrete element based framework for the examination of the cracking patterns of MX80 clay granules developed during free swelling. In this framework, the clay granulate is discretized by discrete elements that are linked together by cohesive laws. The contact stiffness and the volumetric deformation of the discrete elements evolve according to an empirical suction function reported respectively by Darde (2018) and Molinero (2018). Suction is determined by a pressure saturation curve obtained for the MX80 clay and the partially saturated pore pressures are modelled using a Pore Finite Volume scheme. Spatial distributions of numerically simulated fractures are compared with experimental measurements and CT-scan imagery reported by Molinero (2018).

The results indicate that refinements to the model may be required, including consideration of microstress, micro-macroporosity fluid flows and changes in void ratio.


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