Architecture and management of a geological repository - Andra

5 – Repository ModulesThe purpose of this chapter is to explain the design of repository modules as regards their designatedfunctions, and to demonstrate suitability in terms of construction, performances, monitoring,reversibility and closure. As a reminder, a repository module comprises one or more disposal cellsalong with drifts providing access to them, and that the term disposal cell refers to a basic unit inwhich disposal packages are placed.This chapter first of all explains the functions to be fulfilled and the design principles considered foreach category of waste. It shows how design takes into account long-term safety measures, inparticular, protection of packages within a favourable physical and chemical environment,immobilisation of radionuclides and any toxic matter they contain, and control of disturbances inargillites brought about by works in progress. This chapter also explains how specific design measuresallow for reversible management of modules.A specific set of principles governs choice of materials, their durability and long-term behaviour. Inthe various design options selected for disposal packages, concrete is the chosen material for B waste.Concrete is well suited to the construction of large-scale structures, and also helps increase retention inthe case of certain radionuclides. Concrete is not, however, the preferred material for C waste disposalcells (or for spent fuel). Metallic materials, whose behaviour at temperature is better understood overtime, are preferred.This chapter then provides detailed description of structures along with justification of their designbasis. It deals with the mechanical stability of structures excavated into argillites and control ofmechanical disturbances incurred. Dimensioning is based on a depth of 500 m. Greater depths (amaximum of 630 m in the transposition zone) raises no particular problems. Its consequences areexamined in the safety assessment [56]. Module dimensioning is also related to repositorycompactness objectives, with appropriate configuration being the goal as regards waste volume andcorresponding volumes of argillites to be excavated. The various possible construction techniques areexplained, along with the reasons why Andra has chosen the one it has.Finally, this chapter explains the method for closing modules, which reconciles (i) need for lowpermeability (ii) ability to carry out the operation with maximum ease, and (iii) retaining the ability toreverse the process.5.1 B waste repository modulesThis section presents basic design principles chosen for B waste repository modules and provides theirrationale. For purposes of comparison, it takes a look at concepts studied in other countries.Repository module components – a cell, its access drift and its equipment – are then described, alongwith results of calculations of the cell mechanical, thermal and thermo-mechanical dimensioning. Inparticular, it is shown how long-term safety measures as well as package emplacement andreversibility are taken into account in choice of materials and design, and the rationale behindarrangement of cells according to waste types is explained. Finally, this section provides a descriptionof construction and closing techniques.5.1.1 Presentation of main issuesThe elements underlying design of B waste repository modules are essentially governed by the numberof primary waste packages, the radiation property displayed by the majority of them and the physicochemicalcharacteristics of the waste and its conditioning.They also include operational safety and reversibility requirements.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM160/495