Tome Architecture and management of a geological repository - Andra

6 – C waste repository zone6.4.3 Distance between handling driftsC waste disposal compactness in a module is governed by the distance between two disposal cells in adrift and the distance between two parallel handling drifts. These distances depend on the applicationof mechanical and thermal criteria.From the mechanical angle, the distance between two neighbouring cells must be enough to preventdisturbances occurring in the volume of rock separating them. In granite, this basically depends on themechanical characteristics of the rock and the state of the natural stresses. At this stage a reasonablebase would appear to be a distance two to three times the thickness of the disposal borehole diameter,that is 4-6 m.From the thermal angle, the distance between two neighbouring cells must be enough to preventthermal interference occurring which would probably damage the clay engineered barrier. Preliminarycalculations to test the sensitivity of this distance at various foreseeable thermal characteristics forFrench granites have resulted in very close results: for interaxial distances between cells in the range6-8 m and interim storage periods of 60-70 years (reference packages C1 and C2) to century-long(reference packages C3 and C4), the thermal interference is only at its maximum at the midpointbetween two disposal boreholes (less than 60°C) after a century. When it is observed, the calculatedtemperature of the package is below 90°C [42].At the current stage, the presence of minor fracturing of the rock inside the module cannot be ruledout, but its hydraulic conductivity must be low enough not to affect the confinement functions of thevarious repository components: the rock itself at the cell sites, the disposal packages, the engineeredbarriers and backfill. This potential fracturing presence is currently managed by considering that anaverage of 10% of all possible cell locations will not be used.Thus about fifty reference packages C1-C4 (two packages per cell) and roughly 130 referencepackages C0 (five packages per cell) can be accommodated by a configuration characterised by driftsaveraging 250 m in length and with cells sited every 6-8 m6.5 Architecture of the C waste repository zoneThe C waste repository zone includes the repository modules for reference packages C0 and C1-C4distributed over two levels and their connecting infrastructures.Each repository module includes the handling drifts served by a shared cluster of connecting drifts.The number of handling drifts in a module depends both on the host block geometry and the distancethat separates two neighbouring drifts.This section presents the module dimensioning elements, followed by a description of them and therepository zone. It gives the preliminary assessments on the basis of the inventory model, of howmany modules need to be constructed and the underground footprints they will take up.6.5.1 Dimensioning elements of a repository module• Mechanical dimensioning elementsAs with the distance (pitch) between two disposal cells, there must be sufficient distance between twoneighbouring drifts to prevent perturbations occurring in the volume of rock separating them. At thisstage a pillar approaching three times the width of the drift, that is about fifteen metres wide, appearsto be the right basis for the mechanical characteristics of all French granites.Dossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY179/228