Tome Architecture and management of a geological repository - Andra

6 – C waste repository zoneTo achieve such properties, the swelling clay formulation (percentage of "MX80" clay or equivalent tosand) must also take into account the site groundwater’s geochemical properties. Andra’s studies [37],along with those carried out for the Swedish KBS-3 project, open up a wide enough range ofpossibilities to adapt this formulation to the chemical composition of the waters of a specific granitemassif.Steel corrosion (over-pack and sleeve) produces iron which is likely to interact with the swelling clay:smectites (swelling) are transformed into chlorites (non-swelling). This transformation spreads radiallyin the clay engineered barrier from the surface where it is in contact with the sleeve. Chloritisationlikely to disturb the swelling power of the clay only affects a few centimetres’ thickness of the claywhere it is in contact with the steel: further, the smectites are only very slightly transformed and theengineered barrier retains its swelling capacity [49].Furthermore alkaline disturbance is avoided by the absence or very small amount of concrete in andaround the cells.• Disposal cell endThe borehole bottom comprises a flat surface that receives the first elements of the engineered barrier.The option under consideration at this stage is to cast a low pH concrete base about fifteen centimetresthick. Other materials that are inert to clay (sand, bentonite pellets or powder) may also be envisaged.An alternative is granite surfacing at the borehole bottom.• A steel sleeveA steel sleeve, of the same grade as the over-packs, is placed in the engineered barrier axis. Its purposeis to receive disposal packages. This sleeve protects the engineered barrier mechanically duringpackage emplacement and makes package emplacement and retrieval easier. At this stage of thestudies, sleeve thickness is set at 25 mm for S235-grade steel. It allows for both 12 MPa of pressureapplied at the extrados (just like the over-pack, see above) and a corrosion allowance of about 5 mm.The sleeve base consists of a welded blind flange. The upper part is designed to take a steelradiological protection plug. This plug, made from S235-grade steel or equivalent, is dimensioned toprotect operators from external exposure while emplacing or retrieving packages from the sleeve. Ithas grooves at the top to enable the same grapnel to be used as for the package handling operations.• A limited number of packagesAt the time of disposal the thermicity of the C waste reference packages governs the number ofpackages to be emlaced in each disposal borehole.For the most exothermic reference packages, C1 to C4, the main overriding factor is the temperaturecriterion of 90° at the hottest point of the clay engineered barrier (so that its mineralogical andgeochemical characteristics remain unaltered). A design based on placing two superposed packages issuitable for adapting to the characteristics of most French granites with an interim storage period of60-70 years for reference packages C1 and C2, and century-long for reference packages C3 and C4,before being accepted by the repository [42].As C0 waste presents lower thermicity, a greater number of packages can be placed in each disposalborehole. However increasing cell capacity, which entails boring a deeper disposal borehole, is limitedby the higher statistical risk of encountering minor potentially water-conducting fractures. At thisstage, the number of C0 packages per disposal borehole is limited to five to allow for this risk.Dossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY177/228