Architecture and management of a geological repository - Andra

5 – Repository Modules• Back-filling the access driftWhen all the cells of a repository module have been closed, the decision may be taken to back-fill theaccess drifts (see Chapter 10). Back-filling the drifts fulfils the same basic function of all back-filling:to limit in the long term the extent of the damaged argilite zone around the drift. As previouslyindicated, it must also be able to fulfil a support back-fill role at the cell location, i.e. resist themovement of cell retaining plug in the event of their deterioration.This latter function justifies carrying out this back-filling in the same way as drift sealing supportback-fills (see section 7.6). The back-fill can consist of a mixture of prepared excavated argilite (regroundand screened to approximately 20 mm) and sand, in similar proportions. At the bottom of thedrift it can be placed in compacted horizontal layers (20 to 30 cm thick); at the top it is placed insloping layers; for the latter, it is intended to compact the mixture using a pneumatic tamper.Pneumatic injection of powdered bentonite can be used in order to improve the concreting joints.Using these methods, it is possible to obtain a mean dry density in-situ of around 1.7; this figure iscompatible with the target characteristics (deformation modulus of 20 MPa and an internal frictionangle of 40°).Initially, the entire access drift could be back-filled in this way. Back-filling demonstration trialscurrently in progress may open the way to improvements: alternating several metres of support backfillingat the cell location and standard back-filling (without sand).5.3 Spent fuel repository modulesSpent fuel is not currently considered as waste. The possibility of one day placing it in a repository ina deep geological formation has nevertheless been examined. The feasibility of such disposal is thusassessed with respect to various scenarios considered for downstream management of the electronuclearcycle.This chapter describes spent fuel disposal cells, their equipment and their layout with respect to eachother. The main design options are examined by comparison with those of type C waste cells. For typeCU1 and CU2 spent fuels, whose heat rating is high, and for which the temperature decrease is slow,the reference solution is a cell with a swelling clay-based engineered barrier; the description belowconcentrates on this solution. Type CU3 spent fuels have a lower heat rating. Separate management ofthese fuels allows more freedom for finding the best individual solutions. Given their low heat rating,it would be possible to dispose type CU3 spent fuels in cells similar to those described above for typeC0 waste.5.3.1 Presentation of main questionsSpent fuel modules must accommodate various types of package in distinct zones. This type ofseparation makes it possible to take account of their individual thermal behaviours.The dimensioning requirements are generally similar to those of type C waste. In particular, theyconcern the management of heat given off and the limiting of transport possibilities in the immediatevicinity of the fuel. This latter point is aimed at reducing the dissolving of fuel pellets by checking thesolubility of their components.For type CU1 and CU2 packages, another question is the particularly large weight and dimensions ofthe packages (which impose handling constraints).Finally, the spent fuel poses a problem of criticality control due to the fissile material content. Subcriticalconditions are ensured by the design of the disposal packages (see Chapter 4).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM229/495