Abstract :
[en] Deep geological disposals have been selected as a reasonable option for the final management of high and medium-activity-level nuclear waste.
The safe isolation of the waste is ensured by the host rock and/or engineered barrier (EB). Among the other EB components, the role of bentonite is central. Bentonite presents very low permeability in saturated conditions and a certain swelling potential, namely swelling strain and swelling pressure development upon water saturation (respectively in unconstrained and constant volume conditions).
The bentonite volume involved in the sealing of underground disposals is huge. As a consequence, the time required for the full saturation of the material is extremely long (for example more than 1800 years are required for the full saturation of a bentonite plug of 10 m diameter and 20 m length).
In order to assess the long-term evolution of a geological repository, a good prediction of the hydro-mechanical response of bentonite subjected to various boundary conditions is needed.
For this aim, a number of constitutive models have been developed. In most cases, such models have been calibrated and validated on lab scale experiments. On the other hand, large scale experimental investigations highlighted a certain scale effect for bentonite blocks, evidencing, for instance, that the full-saturation time of the seal is much larger with respect to the one estimated via the lab scale results.
Moreover, most of the experimental studies and constitutive models have mainly focused on bentonite compacted blocks, whereas, in recent years, the use of high density bentonite pellets combined with powdered bentonite has also been considered.
The bentonite pellets mixtures present a prominent initial heterogeneity level as a result of the inter-pellets porosity (trimodal pore distribution instead of compacted bentonite bimodal structure).
Given the already-known complexity of the multiphysical and multiscale coupled processes taking place during bentonite hydration, this further level of heterogeneity may also affect the water flow mechanism in the first phase of the hydration process.
Anyway, despite this strong initial heterogeneity, once saturated and with the density homogenised, a swelling pressure equivalent to that reached by compacted blocks is obtained.
In the present work, a swelling pressure test for pellets mixture carried by CEA (France) has been analysed and modelled in the context of the European project BEACON (project receiving funding from the Euratom research and training programme 2014-2018 under grant agreement No 745942).
The experiment consists on a wetting in constant volume test on MX-80 pellets and powder mixture.
This test has been performed in order to study the behaviour of pellets mixture during water saturation and gives good hints concerning the material characterisation, which is not as exhaustive as the one available for bentonite compacted blocks.
In this work, the adopted modelling strategy is described: the Barcelona Basic model is considered for the bentonite mechanical behaviour, pressure dependence is taken into account for some mechanical parameter and the double porosity model proposed by Dieudonne¿ is used for the water retention behaviour.
The initial heterogeneous state, especially the distinction between pellets and powder and its effects, is not explicitly taken into account in the constitutive model and the overall assembly is considered homogeneous.
However, the obtained numerical results are in good agreement with experimental measurements. Especially, the non-monotonic evolution of the swelling pressure during the hydration phase is well captured by this model, which is always a challenge for this type of problem.
