Architecture and management of a geological repository - Andra

5 – Repository Modules• Controlling water circulation around and within cellsConstruction and use of underground structures create disturbances of natural hydraulic systems. Theresaturation phase for B cells lasts several tens of thousands of years and more [8]. After establishmentof a new hydraulic balance, control of water circulation seeks to minimise flow of water likely tocirculate in the cell and to limit flow speed. Design seeks in particular to reduce flows of water comingfrom access structures. Cell geometry along with means of access and closure must help to minimisesuch flows.• Splitting up disposalSplitting up disposal space into modules limits the effect on population and environment of anypossible failure after repository closure.• Protecting waste from physico-chemical attackProtection of waste is aimed at limiting release of radionuclides. In practice, this consists of limitingcorrosion of B waste containing activated parts made of stainless steel or alloys containing nickel,zirconium or magnesium (i.e. B1, B4, B5, B6 and B7 packages) and of limiting alteration of bitumenin B2 packages.When water is present (that is to say, after cell closure and resaturation), physico-chemical conditionsof alkaline pH within the cell help limit anoxic corrosion of metallic waste.In addition, the right environmental conditions lead to pH levels below 12.5 in cells containing B2packages, limiting alteration of bituminous beds, in conjunction with the temperature limitationalready mentioned above.• Contribute to retention of certain radionuclidesIn conjunction with the rock itself, lining materials employed within the cells can also help immobilisecertain radionuclides in the repository, through the physico-chemical conditions they impose in thecell 59 .5.1.1.4 Taking reversibility into accountReversibility is taken into account in structure design, which must incorporate factors to ensureflexible repository management, as well as to simplify any possible future package retrieval operation.Such factors mainly concern structure durability and functional clearances.Structure durability guarantees maintenance of functional clearances between packages and cell wallsfor at least a century. It also contributes to disposal package durability (already mentioned in Chapter4). Maintenance of handling clearances and of package integrity provides great cell managementflexibility, and makes any possible future package retrieval operation as simple as their originalplacement, as long as the cell is not sealed.5.1.2 Cell design principles adopted5.1.2.1 Several options considered and comparedKey design parameters for a B waste disposal cell concern cell geometry, arrangement of packageswithin the cell and handling of packages. Disposal cell design is inseparable from disposal packagedesign and handling systems.59Thus, alkaline pH encourages insolubility of nickel, americium and curium. Cement also has a capacity to retain neptunium andplutonium.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM163/495