Architecture and management of a geological repository - Andra

4 – Waste disposal Packages• Safety-criticalitySome B waste (particularly reference packages B3, B4, B5 and B6) come from spent fuel reprocessingand therefore contain traces of fissile materials. The repository design must ensure sub-criticality in allcircumstances.In this context, the B5 reference packages present the most constructing design factorswith respect to their potential fissile material content. As for the heat transfer phenomenon, thepackage design is linked to the cell design from the sub-criticality perspective, particularly as pertainsto spacing between packages (see Chapter 5).4.1.1.3 Investigation into calling on the retention capability of the disposal packageAn analysis of the spread of radiological activity in the B waste shows the interest, for some of thewaste to study the possibility,of calling on the disposal package for the retention of certain activationor long-lived fission products (niobium-94 ( 94 Nb), zirconium-93 ( 93 Zr), césium-135 ( 135 Cs), etc.). Sucha performance in the long term (at the scale of 10,000 years) may be particularly interesting for theB5.2 (hulls and end caps without organic matter) and B1 (activated waste) reference packages, thatrepresent a major part of the B waste inventory for the radionuclides listed above; note that thesewaste primary packages do not require the disposal containers to be permeable to gas (see above), forthey contain no organic matter (see Section § 3.2).In order to reinforce the confinement capacity of the disposal container, the latter must limit solutetransportation. This means that it does not crack under the effect of the mechanical load liable to beapplied in the long term. Therfore, the mechanical properties of such a package must be greater thanthose of a standard package. The set objective is to withstand a mechanical pressure of 12 MPa,corresponding to the long-termground pressure, assuming there to be a rupture in the cell lining.4.1.2 Design principles adoptedThe design of the disposal package is linked to the disposal cell design and the related handlingequipment. Consistency in the "package-cell-handling" combination is justified as much by needs foroperating safety and reversibility as by long-term safety.4.1.2.1 Over-packing options envisagedSeveral key parameters govern the identification of technical options for B waste over-packing. Thedisposal package may be cylindrical or parallelepipedal. The package material may be concrete, steelor a "noble" metal (copper, titanium, etc.). The package size and weight may be limited, particularly toallow stacking over several levels in the disposal cell, or conversely without constraints in a repositorywhere no stacking exists. The possible combinations of these parameters are illustrated by the fouroptions presented in Figure 4.1.1:DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM109/495