Architecture and management of a geological repository - Andra

5 – Repository Modules• Limiting disturbances of argillitesDisturbance of argillites due to disposal are largely mechanical in nature. Digging out of cells causesdisturbances of argillites around the structures,. and structure design must aim to control extension ofsuch disturbances, and particularly to prevent it occurring in the short and long term. Disturbances ofmechanical origin are micro-cracks, fractures even, appearing in the excavation walls during thedigging of cells in argillites. They can have a considerable effect on the permeability of the immediategeological environment. Such disturbances, located on the walls to an approximate depth of a halfradius 58 after digging [57], may spread in the long term, following lining rupture (after a thousandyears or so). Such spreading, caused by delayed action (of a viscous nature) on the part of theargillites, only occurs when deformation of argillites on the walls reaches threshold level. Controllingspread involves limiting maximum possible rock deformation.In order to do this, it would appear necessary to limit spacing within cells (chiefly between disposalpackages), as well as limiting voids within the disposal packages themselves. Furthermore, areas ofrock disturbed may be limited by appropriate choice of excavating methods, and by disposal cellgeometry and orientation [57].It should be pointed out that disposal may also disturb rock chemically, due to introduction of air or ofexogenous materials. Sizes of such disturbances, especially those of alkaline type, have been assessedin order to verify their limited nature [7] [58].• Temperature controlAs is the case for disposal packages, temperature limitation within the cells is caused by:- the need to control changes in cell lining materials,- the need to be able to predict behaviour of radionuclides in the repository.In the case of concrete, temperature control is of considerable importance. Here, progressive changesdue to temperature depend jointly upon temperature levels and temperature rise duration. Study ofphysics and chemical properties of cement materials show the existence of a temperature thresholdaffecting the material chemical changes (carbonation, hydrolysis). As long as temperature remainsbelow this threshold, physics and chemical properties, especially permeability, change in a reversiblemanner or remain constant. Above the threshold, certain cement paste hydrates become unstable:major mineralogical modification would therefore lead to irreversible changes in concrete physics andchemical properties. Moreover, there are uncertainties regarding chemical properties at thesetemperature [48].Temperature and overheating duration must be limited as much as possible. A maximum temperatureof 70°C has been adopted.In the special case of cells containing B2 waste (bitumens), a much lower temperature limit, 30°C, hasbeen adopted to maintain the rheological properties of the bituminous bed .It is worth mentioning that the above criteria are in line with the findings of current thermohydromechanicalmodelling, as well as with knowledge of radionuclide behaviour as regards solubilityand sorption.• Maintaining sub-critical states for B packages containing traces of fissile elementsSome B waste contains traces of fissile elements. Dimensioning of cells related to that of disposalpackages should guarantee that all packages and installations stay sub-critical, not only by initialgeometrical configuration, but also by taking into account long-term alteration phenomena in materialsthat could lead to local increases in concentration of fissile material. Dimensioning parametersinvolved are largely governed by spacing between packagesand their arrangement.58Zone with micro-cracking at 500m in depth for circular excavations and oriented in parallel to major horizontal stress.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM162/495