Architecture and management of a geological repository - Andra

5 – Repository Modules5.2.1 Presentation of the main issuesThe factors which supported the design choices for the C waste repository modules are primarilylinked to managing the heat given off by the waste, creating a favourable physical and chemicalenvironment and controlling hydraulic phenomena.It is of particular importance to be able to dissipate the heat given off by the waste to minimisedamage to the argilite and remain within a range of operating parameters in which the phenomena areknown and can be modelled.To these considerations can be added the requirements linked to module separation and reversiblemanagement of the repository.5.2.1.1 Managing the heat given off by the wasteThe heat given off by the waste is due to radioactive decay. It drops off with time, depending on theradioactive half-life of the isotopes that cause it.An initial criteria is linked to controlling the phenomena involved in the repository. This entailslimiting the temperature in the cell and the argilite to less than 100°C. Above this temperature, thephenomena and their interactions are more complex. They are not fully understood today. Inparticular, it would be impossible to demonstrate that there would be no short-term damage to theargilite. For thermal design calculations, we have opted for a maximum cell wall temperature of 90°C(in contact with the argilite).A second criterion is to avoid mineralogical transformation of the argilites under the effect of thethermal load applied over the longer term. Knowledge acquired on the basis of experiments shows thatthe most important mineralogical transformation processes with respect to the hydrodynamic,hydromechanical and retention properties, concern the smectites [37]. The experiments carried out onthis subject show that the smectites are only transformed when large amounts of energy aretransmitted to them (combination of both the heating period and the temperature reached).Furthermore, experimentation with potassium smectites (more reactive than those associated with theCallovo-Oxfordian argilites) indicates that less than 50% of the smectites would be transformed whensubjected to a temperature of 80°C for a period of 10,000 years. These experiments show that athermal loading of 70°C for 10,000 years would lead to only extremely limited irreversibletransformation of the Callovo-Oxfordian argilites. We therefore aim to dip below the value of 70°Cwell before 10,000 years. It was conservatively checked that the temperature falls below the 70°Cthreshold after 1,000 years.5.2.1.2 Control of release of radionuclides, through a physical and chemical environmentfavourable to the packagesC waste repository packages comprise an over-pack which protects the primary package; it preventswater contacting the glass until its temperature has fallen below about 50°C. No release ofradionuclides occurs as long as the over-pack remains watertight.After deterioration of the over-pack, release of radionuclides is limited by restricting the speed ofdissolution of the glass. This is achieved by favourable environmental conditions.• Contribution to controlling corrosion of the over-packFirst of all, we must limit the risks of local or galvanic corrosion of the over-pack at the interface withthe cell.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM190/495