Geochemical impact of a low-pH cement liner on the near field of a repository for spent fuel and high-level radioactive waste

摘要

In Switzerland the geological storage in the Opalinus Clay formation is the preferred option for the disposal of spent fuel (SF) and high-level radioactive waste (HLW). The waste will be encapsulated in steel canisters and emplaced into long tunnels that are backfilled with bentonite. Due to uncertainties in the depth of the repository and the associated stress state, a concrete liner might be used for support of emplacement tunnels. Numerical reactive transport calculations are presented that investigate the influence of a concrete liner on the adjacent barrier materials, namely bentonite and Opalinus Clay. The geochemical setup was tailored to the specific materials foreseen in the Swiss repository concept, namely MX-80 bentonite, low-pH concrete (ESDRED) and Opalinus Clay. The heart of the bentonite model is a new conceptual approach for representing thermodynamic properties of montmorillonite which is formulated as a multi-component solid solution comprised of several end-members. The presented calculations provide information on the extent of pH fronts, on the sequence and extent of mineral phase transformations, and on porosity changes on cement–clay interfaces. It was found that the thickness of the zone containing significant mineralogical alterations is at most a few tens of centimeters thick in both the bentonite and the Opalinus Clay adjacent to the liner. Near both interfaces, bentonite–concrete liner and concrete liner–Opalinus Clay, the precipitation of minerals causes a reduction in the porosity. The effect is more pronounced and faster at the concrete liner–Opalinus Clay interface. The simulations reveal that significant pH-changes (i.e. pH > 9) in bentonite and Opalinus Clay are limited to small zones, less than 10 cm thick at the end of the simulations. It is not to be expected that the zone of elevated pH will extend much further at longer times.