Architecture and management of a geological repository - Andra

4 – Waste disposal Packages• "Parallelepipedal, mid-size concrete package" optionThis option consists of a parallelepipedal, mid-size concrete box with cylindrical housings adjusted tothe primary packages. This package may be stacked over several levels in the disposal cell. In the caseof bitumen B2.1 reference packages, this type of disposal package grouping four primary packages hasdimensions close to a cube with sides of 1.5 metres and weighs approximately 7 tonnes.• "Parallelepipedal, large concrete package" optionThis option is for a large parallelepipedal concrete box that cannot be stacked in the cell.The thickness of the external wall could be sized to reduce the dose rate on contact to a valuecompatible with operator presence around the disposal packages; Andra has therefore been able to usethis option to examine an alternative to irradiating cells, whilst seeking the simplification of operatingprocesses.In the case of the bitumen B2.1 packages, this type of non-irradiating disposal package, containing60 primary packages (superposed over three levels) weighs in the order of 110 tonnes. It will be4.4 metres long, 3.7 metres wide and 3.9 metres high.4.1.2.2 Comparison of options• Waste disposal package shapeA cylindrical shape has the major disadvantage of leaving fairly large residual voids between thedisposal packages, even when they are in contact with one another. A parallelepipedal shape, however,responds more favourable to concerns over compactness and minimising voids. Regularity inclearance between the packages after emplacement limits the void rate and avoids the need forbackfilling. This assumes that the waste disposal package is a solid object, limiting the internal voidsaround the primary packages.• Material used in the waste disposal packageThe primary waste packages are surrounded by concrete in all the options presented above. This is anideal material for large volume use, given its strength and relatively low density.The question is raised over the interest in adding a metallic envelope, as in the first option above. Thistype of addition is not justified with respect to the mechanical integrity of the package, given thedurability and strength of concrete. Such an option could be effective with respect to the containmentof gaseous radionuclides released by some primary packages; a leaktight seal for the primary containerwould appear simpler, however.Note that the alkaline environment produced by concrete seems overall favourable to limiting therelease of radionuclides into the geological medium. This assumes nevertheless limiting the pH to 12.5to remain within the validity domain of the models describing the release. In addition, the concreteformulation should limit the amount of water and organic matter contained in the concrete, particularlywith respect to the radiolysis mechanism and resultant gas production (hydrogen).• Dimensions and weightLarge containers offer several advantages. The number of objects requiring handling underground canbe reduced and transfer flows in the cell minimised. In addition, as these packages are not stacked inthe cell, the overhead clearance remains low during handling operations. There is virtually no risk offalling. Lastly, the incorporation of radiological protection as in the option presented above negates theneed for a transfer transfer cask in the connecting drifts and the installation of a radiological protectionair lock at the cell head.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM111/495