Le site de vulgarisation scientifique de l’Université de Liège. ULg, Université de Liège

Modelling the behaviour of host rock for nuclear waste
2/19/16

The experiments carried out at the ANDRA site, in particular, showed that during excavation, damaged zones and fracture zones develop around the galleries. These zones take the form of elliptical rings oriented either vertically or horizontally in relation to the direction of the excavation. Something that is difficult to explain. As part of his thesis, Benoît Pardoen was able to model the appearance of these phenomena, both the vertical (the easiest to model) and horizontal ellipses. "This won’t influence the excavation", the young researcher concludes, "but it helps to explain what is happening and, depending on how we excavate, to assess the risk of interactions between fracture zones and the increase of the permeability of the rock surrounding the gallery."

Benoît Pardoen also focused on hydromechanical coupling, i.e. the influence of fracturing on permeability. If the rock cracks, there is an increase in permeability and therefore the creation of preferential pathways for radionuclide migration through the rock. Here too, the aim is to reproduce large-scale measurements. With modelling, it is possible to deal with time scales, which can’t be done in a laboratory: at test sites, such as those in Bure or Mol, sensors record a series of parameters but this has only been happening over the past few decades. And in geological time, that is very short. "We’re sometimes doing calculations over 100,000 years”, Benoît Pardoen points out.
Thousands of years during which a whole range of phenomena will occur. "Radioactive decay produces heat which will dissipate over thousands of years”, Robert Charlier explains. "If we want to know how this heat will be dissipated and affect the rock mass, we have to build models over periods of several thousands of years. The waste loses its toxicity after several tens of thousands of years, up to 100,000 years. Another issue: the waste is sealed in stainless steel canisters… which nevertheless oxidise in the end! In a thousand years, the steel will have corroded, which requires oxygen! They are stored in an airtight environment which will be saturated in water in a thousand years time. Therefore, the steel will draw oxygen from the water to oxidise and will release hydrogen; gases will be produced which can increase the pressure. So we have to know how these gases are going to be able to migrate…” Which Benoît’s thesis helps to explain.

"Which", as Professor Charlier points out, "leads us to another problem: the plugs that are supposed to seal the galleries and fill the cracks. If we use a plug that is impermeable to gas, the gallery will become like a champagne bottle and may cause problems. So, should the plug prevent the gas from escaping or not? This question certainly goes against the preconception that has been the norm for a long time: nothing should ever escape from these galleries! But over time, this way of thinking has evolved and the idea of recoverability currently prevails. Maybe in 50 years time, when knowledge will have evolved, we will be able to make the waste inert and no longer toxic. In that case, we have to be able to recuperate it! Therefore, we have to reseal the galleries in the most impermeable way possible but, at the same time, we want to be able to reopen them without them collapsing. It is a major stake and our modelling work helps to take the necessary decisions."

The thesis defended by Benoît Pardoen also deals with another important problem, that of the ventilation required during excavation works and also while the waste is being stored, before the galleries are closed. This ventilation dries out the rock but once the galleries have been resealed, the rock will be saturated in water again.  How does the clay behave with regard to this desaturation and resaturation?  To understand it, it was necessary to model the hydraulic behaviour of the rock.

The concrete blocks in Mol

Fatemeh Salehnia’s thesis(2) also deals with the modelling of the rock behaviour although there are two major differences compared with the previous one. First of all, this thesis uses data from the Belgian site in Mol. Also excavated in clay… but one type of clay isn’t the same as another: the one in Mol is more “superficial” than the one in Bure (200 m depth compared with 500 m), the age isn’t the same, nor the porosity (7-8 % in Bure, almost 50% in Mol), and the permeability is low in Mol but it is 10 to 100 times lower in Bure. All differences that require special studies. The second major difference: in Mol, there are supports, which is not the case in Bure. "My work", the young researcher explains, "consisted of studying not just the behaviour of the rock but also that of the supports and the interaction between the two types of material". The rock in Mol is far more deformable that the one in Bure. It is therefore essential that these galleries are provided with supports, whose design has evolved over the years and according to various studies. The solution that was finally adopted was a support in the form of concrete blocks.

Supercontainer concept

(2) From some obscurity to clarity in Boom clay behavior: Analysis of its coupled hydro-mechanical response in the presence of strain localization, Salehnia Fatemeh, University of Liège, 2015, doctoral thesis.

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