Architecture and management of a geological repository - Andra

5 – Repository Modules5.3.1.1 Heat release managementFission products and actinides are the sources of the heat released from spent fuel packages. Thisrelease decreases as a function of the half-life of the radioactive nuclides. The presence of a largequantity of actinides may induce a slower temperature decrease than for type C waste (as in the case oftype CU1 and CU2 packages). On the other hand, type CU3 spent fuels have a level more akin to thatof medium heat generating type C0s. Such considerations lead to the disposal of the different types ofspent fuel in separate zones.Two thermal criteria must be observed, as for type C waste:- the first criterion consists of maintaining a temperature of under 100°C in the cell (the cell thusremains in a domain in which the combined thermal, hydro, mechanical and chemical phenomenaare known and understood). As for type C cells, and for the same reasons concerning themanagement of modelling uncertainties, this criterion results in the requirement to ensure that theclay wall temperature does not exceed 90 C. This criterion, localised to argilite walls in the case oftype C waste cells, applies to the interior wall of the clay engineered barrier in the case of spentfuel cells.- the second criterion is aimed at preventing significant mineralogical changes in the argilite and, inparticular, in the swelling clay of the engineered barrier, which would reduce its ability to swell(transformation of the swelling smectites into non-swelling illites). As for type C waste cells, acheck is made to ensure that the temperature drops back below 70°C after around a thousandyears.5.3.1.2 A favourable physico-chemical environment for the packagesThe immediate environment of the packages should form a diffusive hydraulic barrier (homogeneousmaterials with low permeability and with a diffusion coefficient that is as low as possible), so that thecell functions as a closed chemical system. This situation provides a chemical balance between thepackage’s components (envelopes and ceramic pellets forming the fuel) and the water in contact withthem. It makes it possible to limit the alteration of the packages and assemblies.The pH must be maintained in the range 6-9, the range in which the pellet dissolution model is valid.These properties are provided jointly by the swelling clay engineered barrier and the surroundingargilite (reference solution for type CU1 and CU2 cells) or by the argilite protected from damage, forcells without a clay engineered barrier (CU3).5.3.2 Design principles usedThe key parameters used are the same as for type C waste. The cells in a dead end configuration (tocontrol water circulation), with a single row of packages in each section (to dissipate the heat releasedby the packages); they are placed in a middle section of the formation (to preserve undisturbed argilitethickness).The main design questions concern the orientation of the cells (horizontal or vertical), the presence orotherwise of an swelling clay buffer, its fitting (integrated into the package or separate from it), thechoice of cross-section (as near as possible to the package or providing handling room or ventilationspace) and the choice of materials (concrete or steel for linings; concrete or swelling clay for theengineered barrier).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM230/495