Architecture and management of a geological repository - Andra

5 – Repository Modules• Description of and justification for the dimensioning of swelling clay ringsThe expected properties of the clay buffer are as low a permeability and high a thermal conductivity aspossible.The aim of the low permeability is to create a diffusive medium around the packages; this is the firstproperty to be achieved. Studies have shown that a permeability of around 10 -13 m/s is achievable on asmall scale, in the knowledge that the full scale permeability objective of 10 -11 m/s would suffice.A high thermal conductivity makes it possible to dissipate the heat released by the waste. Studies haveshown that it is possible to achieve a long-term thermal conductivity in excess of 1.5 W/m/°C (resaturatedmaterial) and in excess of 1.2 W/m/°C during the thermal peak (unsaturated material).In order to achieve the aforementioned properties, the following characteristics are envisaged [65],[70]:- mixture of swelling clay (approximately 70%) and sand (approximately 30%), the latter enablingthe thermal conductivity to be improved;- choice of an “MX80” type clay or equivalent (low permeability even at low density);- swelling pressure: between 1 MPa (or even 0.5 MPa) and 7 Mpa, effective stress;- dry density at time of placing (prefabricated block: approximately 1.8 for the clay/sand mixture:approximately 1.6 for clay alone (it can change to a figure of 1.5 as the cell evolves) ;- degree of clay saturation at the time of placing: approximately 80%;- water content: approximately 15% for the clay/sand mixture; approximately 20% for clay alone;- mechanical strength of blocks: approximately 10 MPa under compressive stress, approximately1 MPa under tensile stress.The target maximum swelling pressure is 7Mpa : this is the final value which will establish itself in thelong term, in equilibrium with the argilite; refer to section 5.2.6.1 for a discussion of this point.The addition of swelling clay enables the thermal conductivity to be improved and the swelling abilityto be reduced if necessary. A sand content of 30% makes it possible to considerably improve themixture’s thermal characteristics without adversely affecting the permeability.The thermalperformance of the core of the buffer is of considerable importance here: it affects the packagedisposal compactness and earliest disposal age.The dry density indicated enables the swelling pressure to be maintained later in the desired range: ittakes into account the clearances established.The figures indicated for the degree of saturation and water content at the time of placing the packagesin the repository are a compromise between the following characteristics: dry density at the time ofplacing, compactability and mechanical strength of the parts.It should be noted that the clay materials envisaged have good resistance properties with respect to thedisturbances resulting from interactions with the other materials. On the one hand, these interactionsare disturbances due to iron and, on the other, alkaline disturbance.- The corrosion of steels (package, sleeve, lining) frees iron, liable to interact with swelling clay andargilite: smectites (swelling) are transformed into chlorites (non-swelling). This transformationspreads radially in the clay buffer from its internal and external faces. The studies assess thepossible extension from a surface in contact with the steel to a distance of 30 to 50 cm; it canaffect the entire buffer. However, other than in the area in contact, it is limited in intensity (beyond5 cm from the interface, the percentage of smectites transformed is less than 10% and less than 1%beyond 30 cm). The buffer thus retains most of its swelling ability (Figure 3.3.3).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM243/495