Architecture and management of a geological repository - Andra

5 – Repository Modules5.3.2.2 Comparison of possible solutions and justification of choice• Comparison with or without swelling clay bufferFollowing the deterioration of the spent fuel container, the control of the release of radioactivenuclides essentially depends on the existence of a diffusive barrier around the package and on thephysico-chemical characteristics of the components of the spent fuel (uranium oxide pellets andstructures). Furthermore, when compared with type C waste, the decrease in thermal activity of spentfuel is slower due to actinides present in large quantities. In order to guard against thermo-hydromechanical(THM) effects on the argilites over periods in excess of 1 000 years, it has been consideredpreferable to resort to the use of a clay engineered barrier 93 . This provides resistance to possibledamage associated with these long-term THM effects Its plasticity allows it to repair its own fissuring.Studies have in fact shown that, after a thermal phase, it regains its plasticity, swelling properties andlow permeability.• Comparison between “horizontal” and “vertical” cellsAs for type C waste, a comparison between the horizontal and vertical configurations has led topreference being given to the horizontal configuration. It offers the possibility of optimising thermaldimensioning and reducing the excavated volume.• Comparison between “disposal cells” and “disposal drifts”The “disposal cell” option consists of selecting a diameter appropriate to that of the disposal package;the “disposal drift” option has additional space for handling and ventilation. As for type C waste, thedisposal cell is the preferred option. The disposal drift is in fact penalised by the requirement for asignificantly larger excavated volume. Furthermore, the capability to ventilate that it offers is validonly during the operational phase but not for the post-closing phase (the package heat dissipationtherein is purely passive in nature). Finally, it should be noted that the use of concrete in the vicinity ofspent fuels does not appear favourable at this stage (alkaline pH, risk of increasing the long-termpermeability).5.3.2.3 The solution adoptedThe spent fuel disposal cell has a number of similar features to that of the type C waste cell. The maindifference is the presence of an swelling clay buffer which leads to its diameter being increased (toapproximately 3.3 m for type CU1 waste and to approximately 2.6 m for type CU2).As for waste cells, spacers are used to space the packages for improved heat distribution within therock.The various components of the spent fuel cell and their associated functions are illustrated in thediagram at Figure 5.3.4 and Table 5.3.1.93It has been decided not to use concrete as the engineered barrier material for two reasons: its permeability and the alkaline pH that itinduces Its permeability cannot be reduced to that of swelling clay and cannot be maintained at its initial level due to the deterioration ofthe material (of chemical or thermal origin). The concrete gives its immediate vicinity an alkaline pH; it would be outside the validityrange of the spent fuel dissolving model currently available (pH 6 to 12).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM232/495