Modelling the behaviour of host rock for nuclear waste

Nuclear energy is nowadays a crucial issue and the storage of radioactive waste raises many questions. Over the past few decades, many countries have subsequently built underground laboratories allowing them to test hypotheses on a large  scale. In Belgium, a decision was taken in 1974 to create the HADES laboratory in the layer of clay situated under the Nuclear Energy Study Centre in Mol, with the first excavation works taking place in the 1980s. The various studies carried out over the past three decades have led to the development of various concepts. This is especially the case regarding the behaviour of the rock that will host the waste. This is due in particular to the development of calculation tools which currently allow the user to model these behaviours fairly accurately, by taking into account the most complex situations, especially in the long term over several tens of thousands of years.

The Laboratory of Geomechanics and Engineering Geology (ArGEnCo department at the Faculty of Applied Sciences, University Liège) is one of the few teams in Europe working on this numerical area. Three doctoral theses have just been defended, which demonstrate the laboratory’s competence in this field. Thanks to their input, several major steps have been accomplished in the modelling of the fracture zones that appear during excavation, the behaviour of rock under the effect of ventilation, and the behaviour of concrete supports and the plugs designed to seal off the galleries and fill in the technological gaps.

The Laboratory of Geomechanics and Engineering Geology (ArGEnCo department at the Faculty of Applied Sciences, University Liège) studies the behaviour of soil and rock under various stresses (excavation of galleries or foundations, for instance). Its internationally renowned expertise has led to the development and use of a finite element method that enables the numerical modelling of complex phenomena. Among the areas studied over the past 30 years is the burying of long-lived, highly toxic nuclear waste. Several countries are testing storage solutions in galleries excavated in deep geological repositories. In Belgium, this storage is studied in the underground HADES laboratory, located 225 metres beneath the Centre d’Etude de l’Energie Nucléaire (CEN) in Mol. This research is continuing in close collaboration with the Belgian universities that have developed competences in this field. Including ULg, where three doctoral theses were defended at the end of 2015 and at the beginning of 2016. "We studied the behaviour of the host rock and the materials that might be used around it – support, plugs – for efficient storage", says Professor Robert Charlier, co-director of the Laboratory of Geomechanics and Engineering Geology and promoter for these theses alongside Frédéric Collin.
Nuclear Waste Greenpeace

The type of host rock chosen by countries such as Belgium, France and Switzerland is argillaceous, while other countries, especially those in Scandinavia, prefer granite. The storage of long-lived, highly radioactive nuclear waste is governed by the law of “staying within one’s own borders”. Hence the increase in test sites, each with its own specificities that obviously have to be taken into account, although this doesn’t rule out international collaborations. The three theses from Liège fall within this scope, one of them referring to the Belgian site in Mol and the other two to the French site, where ANDRA (National Agency for Radioactive Waste Management) carries out experiments in Bure (Haute-Marne).

Rock fracturing

In the first recently-defended thesis(1), Benoît Pardoen focuses on the argillaceous rock that hosts the underground ANDRA laboratory at the site in Bure. He particularly studied the phenomena that occur when this clay is excavated, and during exploitation and maintenance phases. These galleries have to be ventilated, which dries out the rock and thus alters its state; as for the waste, it releases heat, another element to be taken into account during long-term simulation.
The first part of the thesis deals with phenomena that appear when the rock is excavated. "Clay, especially the one in Bure", Benoît Pardoen explains, "is an anisotropic material; the characteristics of the phenomena that appear will therefore be different depending on the direction of the excavation. This was one of the first steps in my research."

(1) Hydro-mechanical analysis of the fracturing induced by the excavation of nuclear waste repository galleries using shear banding, Pardoen Benoît, University of Liège, 2015, doctoral thesis.

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