Lot A2 test, THC modelling of the bentonite buffer

Finnish spent nuclear fuel is planned to be disposed of deep in the crystalline bedrock of the Olkiluoto island. In such a repository, the role of the bentonite buffer is considered to be central. The initially unsaturated bentonite emplaced around a spent-fuel canister will become fully saturated by the groundwater from the host rock. In order to assess the long-term safety of a deep repository, it is essential to determine how temperature influences the chemical stability of bentonite. The aim of this study was to achieve an improved understanding of the factors governing the thermo-hydro-chemical evolution of the bentonite buffer subject to heat generation from the disposed fuel and in contact with a highly permeable rock fracture intersecting a canister deposition hole.TOUGHREACT was used to model a test known as the long-term test of buffer material adverse-2, which was conducted at the Äspö hard rock laboratory in Sweden. The results on the evolution of cation-exchange equilibria, bentonite porewater chemistry, mineralogy, and saturation of the buffer are presented and discussed. The calculated model results show similarity to the experimental results. In particular, the spatial differences in the saturation and porewater chemistry of the bentonite buffer were clearly visible in the model.

► The long-term test A2 of buffer material at the Äspö HRL in Sweden was modelled.
► TOUGHREACT was used to model the coupled thermo-hydro-chemical phenomena.
► Bentonite did not saturate fully due to the very high temperature of the heater.
► Enrichment of anhydrite took place at the hot end of the bentonite buffer.
► The model results were in agreement with the experiment except for cation exchange.