Architecture and management of a geological repository - Andra

The radioactive elements can thus be confined within the packages on a long-term basis (except forgaseous radioactive nuclides as mentioned above).Additionally, from a phenomenological point of view, the insertion of certain packages induces theproduction of gas or heat.Packages containing bituminous coated materials (type B2 packages) and radioactive packagescontaining organic materials (type B5.1 packages for example) emit gases by radiolysis of theconstituent materials, principally hydrogen, but also carbon dioxide or methane. The quantities of gasemitted outside the disposal packages are of the order of several litres to several tens of litres of gasper package per year. In addition, certain packages (type B5.1 packages for example) may also emittraces of radioactive gases in very small quantities [62]. The architecture described in Chapter 5 showsthat it is possible to place the packages emitting gases within dedicated cells. The small quantities ofgas (radioactive or otherwise) are evacuated by ventilation. They are sufficiently small for it to bepossible to stop the ventilation for a few weeks if necessary.Only certain packages give off heat, principally type B5.1, B5.2 and B1 packages. The amounts aremodest compared with those given off by type C waste but cannot be ignored. However, theventilation inside cells allows approximately 80 to 90% of the heat given off to be evacuated, evenwith a relatively small air flow of 3 m 3 /s. The impact of this heat emission on the cell is thus limited:the air at the end of the cell, having passed through the entire cell, thus has a maximum temperature ofslightly under 40°C and the maximum disposal package temperature remains under 40 °C [10].It should be noted that maintaining the cell in this state and, in particular, ventilating for severaldecades or centuries does not noticeably increase the disturbances induced by excavation. In fact, evenif the de-saturation front can reach the un-fissured rock, the latter de-saturates very little and the extentof the micro-fissured rock zone does not vary [8], [57]. In addition, the oxidation of the pyrites andorganic material present in the rock, by the oxygen in the air, and the associated chemical disturbancesare small and remain limited to the fractured rock zone that may be created by excavation [84]. Thusthe chemical and hydro-mechanical disturbances induced by maintaining the ventilation remainlimited to the excavation damaged zone (EDZ), whose extent does not increase as long as the liningremains intact 134 .• Possibilities available for taking action and possible duration of the stageThe package disposal conditions at this stage are similar to those during pre-disposal storage. Thedeformations of the lining are sufficiently small that they do not modify the geometry of the disposalchamber. It would therefore be possible to retrieve the packages, simply by reversing the placementprocess (see §10.4). The ventilation evacuates any radiolysis gases and heat given off by the packages.Maintaining the ventilation in the cells requires only a small additional air flow at the entrance to therepository zone. It should be noted that the conditions in the repository are less sensitive to seasonalvariations than in a surface installation. In fact at the entrance to the cells, the air temperature is alwaysat the temperature of the rock at the disposal position (22°C) and the relative humidity varies around50% (30 to 80% of the relative humidity variation amplitude) [8].134 More precisely, the delayed deformation of the argilites tends to close and recompress the fissures in the rock in the presence of a rigidcoating [66].DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM371/495