Technical Report NTB 23-03

Abstract

The Swiss concept for the long-term disposal of radioactive waste foresees the construction of deep geological repositories for high-level waste (HLW), comprising spent fuel and vitrified HLW, as well as for low- and intermediate-level waste (L/ILW), from the operation and dismantling of the nuclear power plants and from applications in industry, research and medicine. Nagra announced in September 2022 that they would submit a general licence application for a combined repository for HLW and L/ILW in the Opalinus Clay in the Nördlich Lägern siting region.

The L/ILW repository is specifically designed to contain large amounts of cement-based materials (concrete, mortar) used for waste conditioning, tunnel support, infrastructure elements and the backfilling of cavities. The cement-based near-field thus constitutes a safety pillar of the repository, where the cement contributes to the retention and slow release of radionuclides and maintains the high-pH environment, thereby ensuring the slow corrosion of metals and low gas generation rates. The waste inventory in the L/ILW repository section consists of a wide range of organic and inorganic materials. This report describes the spatial and temporal geochemical evolution of the cementitious near-field, and the interactions between the engineered barriers and the surrounding host rock, by reviewing the different processes which are expected to occur.

The spatial and temporal evolution of the cementitious near-field is governed by several externally and internally induced processes that are coupled with each other. The water saturation state of the repository is of central importance for the temporal evolution of the repository near-field. Under desaturated or partially saturated conditions of the emplacement caverns, the adjacent host rock or the engineered barriers, the transport of dissolved species and the chemical reactions are suppressed as they only take place in water-films on minerals. The saturation evolution of the near-field is controlled by gas production, which causes a gas pressure increase and limits the inflow of water from the host rock. Gas pressures are reduced by the transport of dissolved gases from the near-field into the host rock and gas transport through cavern seals into the tunnel system. The production of gas by anoxic corrosion of metals and by microbial degradation of organic wastes also consumes water. In addition, the reactions that give rise to concrete degradation, such as aggregate-cement interaction or carbonation may also consume or produce water, respectively. However, these reactions require a minimum humidity in the gas phase to form the necessary water films at pore surfaces. With decreasing saturation, the vapour transport in the gas phase becomes important. The vapour is in equilibrium with liquid-filled small pores and causes the formation of liquid films along mineral surfaces in which dissolved substances may be transported and react with each other.

The description of cement degradation is consistent with that reported in previous studies. It is expected that the degradation of the cementitious near-field will occur in several phases:

The first phase of cement degradation is related to the hydration of cement phases. In this phase, the porewater has a pH of 13 or even higher due to the high content of dissolved alkali hydroxides. This phase is expected to persist only for a relatively short period, especially in situations and places where strong geochemical gradients foster transport of solutes.

The second phase of the cement degradation is controlled by the equilibrium of porewater with portlandite, which is buffered at a constant pH of 12.5. The alkali concentration is reduced by mineral reactions and/or solute transport.

In the third phase of the cement degradation, the portlandite is completely dissolved due to the reaction with silicates, aluminates or carbonates present in the near-field and interaction with the groundwater of the host rock or associated with reactive waste materials or concrete aggregates.

At this stage, the porewater is in equilibrium with calcium-silicate-hydrates (C-S-H), which leads to a pH value below 12.5 to near 11 or even lower. The Ca/Si ratio of C-S-H changes from high (Ca/Si ~ 1.6) towards lower values (~ 0.7) as the composition of the C-S-H phases evolves.

In a very late phase of the cement degradation, the formation of carbonates, clays or zeolites results in a decrease of the pH to near-neutral values.

Various processes influence the geochemical evolution of the cementitious near-field. The most important are the interactions with the host rock (i.e. cement-Opalinus Clay interaction), the interactions with waste, and the degradation of cement phases by aggregate-cement interaction and by cement carbonation. Interfaces between cement and clay materials can be characterised by strong (geo-)chemical differences. The diffusion-dominated exchange of porewater between the cementitious near-field and the host rock gives rise to mineral reactions and changes in the porewater pH. Typically, dissolution of cement and clay phases are observed in combination with precipitation of secondary mineral phases at or near the material interfaces. The reduction of the free connected pore space leads to a reduction of mass and solute fluxes across the interface, slowing down the geochemical alteration processes. Such mineral reactions at the interface were investigated with the help of numerical models. The clay minerals of the host rock are dissolved and transformed into secondary minerals (e.g. zeolites) up to a distance of a few decimetres in 100,000 years. Beyond this transition zone and further into the host rock, a zone with an elevated pH of 8 – 9 is expected to develop, but without significant mineralogical changes. Under the diffusive transport regime, this zone extends a few metres into the host rock.

Conservative estimates for a fully water-saturated near-field show that concrete with an average composition is degraded up to a distance of 2 metres from the near-field-host rock interface by diffusion-dominated porewater exchange with the Opalinus Clay. This results in portlandite dissolution and cement mineral transformations, causing a decrease in the concrete porewater pH to values corresponding to the third phase of cement degradation.

Cementitious materials that contain aggregates composed of aluminosilicate minerals will be subject to cement degradation by aggregate-cement reactions. Silicon dioxide from dissolution of concrete aggregates will react with portlandite and form C-S-H phases until the aggregates are in thermodynamic equilibrium with cement phases. These transformations will convert the material into degradation Stage III in the long term. The temporal progression of this reaction is poorly constrained. From available kinetic constants of mineral dissolution reactions, it is inferred that in particular the fine-grained aggregates with a high surface area could be dissolved quickly, i.e. concrete degradation might occur within some hundreds to thousands of years if the material is water-saturated. The full or partial desaturation of concrete materials and the imperfect contact of the cement paste with the aggregates might strongly delay this reaction.

The degradation of organics in an L/ILW repository will take place by a combination of abiotic (i.e. radiolysis and hydrolysis) and biotic processes, such as methanogenesis, which produces CH₄ and CO₂. Due to the unfavourable conditions (high pH), the microbial activity-driven degradation process is expected to be slow. The released CO₂ is expected to react with the surrounding concrete and form solid carbonates. An accurate prediction of the degradation rates of organic waste is difficult since the rates depend not only on pH but also on the availability of water and potential microbial activity which depends on the presence of certain nutrients such as phosphorus and nitrogen. All of these factors limit the microbially mediated degradation of organic compounds. Based on current knowledge, it can be assumed that low-molecular-weight organic materials degrade within a few thousand years. The degradation rates of the high-molecular-weight organic substances (polymeric materials) are rather uncertain due to limited availability of experimental data. It is expected that the degradation may take at least several tens or hundreds of thousands of years, if they degrade at all.

Some inorganic waste can react with the surrounding materials, assuming that water availability is not a limiting factor. Particularly important is the anoxic corrosion of metals, specifically steel, which can produce large amounts of H₂. Corrosion products could contribute to the degradation of the surrounding cement phases. Note that this again requires percolation of a water phase in all the pores to maintain the transport of dissolved species.

The general description of concrete degradation is rather similar to the previous study (Kosakowski et al. 2014). This work puts more emphasis on the heterogeneity of near-field materials and postulates that, even after long times, geochemical conditions in the repository might be quite heterogeneous. The heterogeneity of geochemical conditions depends on the spatial material distribution and the water saturation in space, which also induces a spatially different evolution in time.

The basic mechanisms of radionuclide transport (retention) and radionuclide solubility have not changed compared to the previous study (Kosakowski et al. 2014). The refined analysis of the processes and conditions confirm that the cementitious near-field is an effective barrier for the transport of radionuclides over very long timespans. The cementitious near-field can be considered to contribute effectively to the retention of radionuclides over tens of thousands of years. The occurrence of colloid-facilitated radionuclide transport is considered as very unlikely since the host rock and near-field porewater chemistries give rise to very low colloid stabilities and thus significantly reduce colloid concentrations.