DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE | ROCK TUNNELS
Surface facilities
Shafts Double ramp Left, figure 1:
Callovo Oxfordian formation
Intermediate long-lived radioactive waste (ILW-LL) disposal zone
High-level radioactive waste (HLW) disposal zone
Cigéo project overview (Andra)
is a potential sealing material in several sealing systems designs (Sellin and Leupin 2013). Andra’s current sealing concept is for a confined
expansive bentonite-based sealing core, occupying the entire cross-section area of the drift (see Figure. 2). The expansive core would be confined by two concrete plugs, one at either end. The remaining portion of the drift would be backfilled with disaggregated and recompacted excavated COx claystone. Swelling under confined conditions leads to the
development of a swelling pressure that the core exerts on the surrounding materials. In this way, as the EDZ is re-saturated it will be gradually compressed by the progressive long-term deformation of rock and the increasing pressure exerted by the expansive core. This will lead to a gradual closure of the fractures, favouring self-sealing, and leading to a gradual recovery of the rock’s low permeability (Bastiaens et al 2007; De La Vaissière 2015; Wang et al 2022a, b). The performance of this type of solution, therefore,
relies on the development and long-term stability of the core swelling pressure and the recompression of the EDZ. The retaining plugs and the backfill material must
provide the support necessary, so that the deformation at the ends of the core are limited and the swelling pressure remains within the target design range. There are several configurations of concrete retaining
plugs under examination (i.e., lengths, anchoring methods). In this study, the plugs are assumed to be constructed inside the lining but without any anchoring system, and their length is assumed to be equal to
about one diameter of the drift (~ 10m). On the other hand, the core length is designed to have a margin of safety against a possible deconfinement and reduction of swelling properties and increase of permeability at the ends of the core. In this case, core length is assumed
equal to twice the excavated diameter (~ 20m). The design of the bentonite-based material for
the core requires necessary low permeability and appropriate swelling potential. The placement method must minimise residual voids and ensure the most uniform core properties. This paper considers bentonite powder–pellets mixtures as the preferred option, with operational advantages including facilitating underground placement of large quantities to target density.
Shaft sealing
Undisturbed COx EDZ
Backfilled gallery
Sealing core length ~ 2D
Backfilled gallery
Left, figure 2:
Conceptual scheme of Cígeo sealing concept
Storage zone
Concrete plug length ~ D
Concrete plug length ~ D
Shaft
February 2025 | 21
D ~ 10m
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