Architecture and management of a geological repository - Andra

5 – Repository ModulesReservations are made in the slab for running cables. The slab is equipped with a concrete roadway fortransporters running on tyres. Depending on the type of transporter chosen, the slab could also befitted with rails.The concrete lining does not need to be reinforced in the standard section.5.2.3.3 Thermal design of the repository modulesThe thermal design of the module consists, for a given package heat rating, in determining thegeometries which meet the thermal criteria (defined in section 5.2.1.1).• Objectives of the thermal designThe first objective of the thermal design is to identify the foreseeable preliminary storage period priorto placing the waste in the repository, for the proposed concepts.For this preliminary storage period, the second objective is to identify the geometrical configuration ofthe repository modules corresponding to the smallest possible total excavated volume and meeting thetemperature limit value. This temperature limit is relative to a specific phenomenon called the"thermal peak". The arrangement of the packages in the cells and of the cells in the repository modulesis chiefly dependent on this phenomenon. It is recalled that the temperature limit used in thecalculations is 90°C in contact with the rock.• Thermal modellingThe thermal design is conducted by finite-elements 3D modelling [10].This modelling is also used toevaluate the duration of the thermal phase, in other words the time for which the temperature of thepackages exceeds a pre-defined threshold value. This latter aspect justifies the design of the packaging(see chapter 4). Thermal modelling gives the main time and space scales. The thermal effects linked tothe general architecture (general footprint of the modules and layout of the modules in relation to eachother, verification of thermal independence of the repository zones) are presented in chapter 6.Modelling data and hypothesesC1 to C4 disposal packages are highly exothermic. Their heat rating drops from 2600 watts to300 watts in 100 years (see chapter 3). For their part, C0 disposal packages are significantly lessexothermic. Their heat rating at the time they are packaged is about 10 times lower than that of a C1package.In the model, the heat transfer modes are conduction and radiation. Conduction is modelled for all thematerials making up the cell. Radiation is taken into account between all the components of the modelseparated by a film of air (functional clearance, cell ends, etc.).To simplify matters, it was also considered that all the cells in the repository zone concerned weresimultaneously loaded on the same date and that all the access drifts were backfilled after 30 years.This is a conservative approach, which ignores evacuation of some of the heat through drift ventilationduring the operating phase.Finally, the value chosen for the initial temperature of the Callovo-Oxfordian formation at the time ofdisposal was 22 °C.Modelling principleThe main problem with thermal modelling of the repository lies in the considerable disparity betweenspace and time scales.In terms of space, there is a significant contrast between the dimensions of cell components (forexample just a few centimetres for the functional clearance between packages and sleeve) and thedimensions of the repository as a whole.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM204/495