Architecture and management of a geological repository - Andra

6 – Overall underground architecture6.1.1.2 A structure layout that helps to delay and reduce the migration of radioactivenuclidesIn order to delay and reduce the migration of radioactive nuclides released by the packages, the clayformation forms the essential retention barrier. The transport of species in solution occurs principallyby diffusion in the water present in the pores of the argilite : physically, molecular agitation displacesthe solutes very slowly from the zones in which the concentration in the water is the highest (thedisposal cells) to the low concentration zones. For most species, this displacement is delayed bychemical sorption phenomena on the surface of minerals.Another objective of placing structures in the clay formation is to maximise the transit and diffusiontimes. That makes it possible (i) by favouring dispersion, to limit the flow of solutes reaching theboundaries of the clay formation and, consequently, to reduce the concentrations there and (ii) tobenefit from the radioactive decay of the radioactive nuclides.At this stage of the design of an architecture, only the Callovo-Oxfordian clay formation has beenconsidered as a retention barrier, without taking into account the additional contributions of the overand underlying formations.6.1.1.3 Limiting mechanical disturbances in the geological environmentMechanical disturbances induced in the adjacent argilite during excavation work must be limited. Thatwill prevent any damage in the argilite from increasing the water flows in the repository orsignificantly reducing the migration time of radioactive nuclides released by the waste packages.The first point concerns, in particular, the seals for which it is best to prepare against a risk ofhydraulic short-circuit by the damaged argilite zone. In order to do that, at this stage, we are looking toinstall structures that can be sealed as well as possible taking into account the argilite’s mechanicalresponse. As a reminder, Chapter 5 indicated how such provisions can be integrated into the design ofeach disposal module.More generally, around repository structures, the thickness of the argilite potentially damaged by theconstruction work must remain small when compared with the overall thickness of the formationstudied. This point has already been raised in Chapter 5.6.1.1.4 Limiting thermal disturbancesVitrified type C waste and, it applicable, spent fuels, are characterised by the large amount of heat thatthey give off. In Chapter 5, an explanation was given of the temperature criteria used for the design ofa repository, in order to remain within a field currently understood concerning (i) changes to materialsand (ii) a knowledge and modelling of phenomena: as a reminder, we are seeking to maintain adisposal temperature of under 100°C; in concrete terms, we have adopted a criterion of 90 °C in theargilite wall (or, where applicable, in the clay buffer). Chapter 5 also described the dimensioningelements for disposal modules enabling these criteria to be met.The maximum temperatures in the exothermic waste disposal modules are rapidly reached afterinsertion of the packages, when the heat flows are still confined to the inside of the same module. Thusin practice, these maxima depend solely on the design of each module. In the longer term, thetemperature in the cells reduces as the waste’s radioactivity decays.However, the volume which is thermally influenced by each structure gradually extends with time,with the heat being diffused in the geological environment. Thus, the relative layout of the variousmodules within the overall architecture affects the repository’s long-term thermal behaviour and thetemperature distribution over a large spatial scale.The link should also be noted between the duration of a repository’s thermal phase and the design ofthe disposal package, explained in Chapter 4.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM253/495