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Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Copper coating of the steel disposal canister is considered as an option to ensure complete waste containment for a period of 100’000 years. Sulphide is a potential corroding agent to copper. While background sulphide concentrations in the OPA are very low (ca. 10−11 mol/L), it cannot be ruled out that sulphide reducing bacteria (SRB) thriving at discrete locations within the engineered barrier system (EBS) of the repository could generate significantly higher dissolved sulphide concentrations. The aim of this study is to conservatively evaluate the potential for SRB to generate and sustain elevated sulphide concentrations in the repository near-field by considering specific geochemical bottle-necks, to quantify the maximum sulphide fluxes towards the canister, and to assess their potential to corrode the copper canister coating. To address these objectives reactive transport calculations are performed, and main conceptual and parametric uncertainties of the model are assessed.

Hydrogen gas (H 2 ) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H 2 is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm −3 day −1 for H 2 , and 0.14 and 0.20 μmol cm −3 day −1 for sulfate. Based on the stoichiometry of the reaction, there is an excess of H 2 consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H 2 is lost via diffusion. These rate estimates are critical to evaluate whether biological H 2 consumption can negate H 2 production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.

Sorption is the most important process responsible for the retention of radionuclides or heavy metals in repositories. A major task in understanding sorption is to account for uncertainties that affect the long-term repository performance, and eventually, study their effect on the migration of radionuclides. This paper presents a novel comprehensive uncertainty and sensitivity analysis study of a detailed sorption chemistry model used in Cesium reactive transport modeling, based on the Morris and Sobol method. The analysis addresses the effects on the transport of Cesium through clay, exemplified for Cesium migration in the vicinity of a deep geological repository for nuclear waste, due to uncertainties in 1) site-specific detailed equilibrium sorption reactions constants and 2) clay pore water cations’ concentration, i.e. the pore water composition. The results show that the equilibrium sorption reaction constants on Type2 and FES sites in addition to the concentrations of the respective cations involved in these surface reactions are the most important uncertain parameters affecting the transport of Cesium. Furthermore, a classification tree is constructed to identify the combination of parameters values leading to the maximum Cesium concentrations at a specified location. Finally, a new Cesium isotherm and K d uncertainty range are constructed numerically based on our deterministic complex sorption chemistry calculations using the MCOTAC reactive transport code. These results can be used in large-scale repository performance assessment to inform the estimates for Cesium transport parameters, with values solidly based on detailed reactive transport modeling. The presented approach can be used for parameter uncertainty analysis of any complex reactive transport problem and can be applied to analyze any radionuclide of interest.

The near field of the planned deep geological repository for low- and intermediate-level radioactive waste (L/ILW) in Switzerland will consist of different waste materials with different reactivities that are conditioned in cementitious matrices. Geochemical modelling was applied to predict the temporal evolution of the chemical conditions in a cement-stabilised model waste form by simulating the degradation of the waste materials and the alteration of the cementitious matrix. The model waste form contains large amounts of metallic waste and a low amount of organic waste. The geochemical modelling considered the use of either siliceous or calcareous aggregates for fabrication of the cementitious matrix used to condition the waste form. The results show that the type of aggregates has a major effect on the temporal evolution of the chemical conditions in the waste form. The use of calcareous aggregate instead of the commonly used siliceous aggregate is expected to maintain highly alkaline conditions of the waste form over the period of concern for the L/ILW repository. The latter conditions prevent accelerated iron corrosion and, related to that, accelerated H2 production from occurring due to the high pH. The evolution of the waste form is decisively controlled by the availability of water. In the absence of water ingress from the near field, the degradation processes cease as free water in the waste package is exhausted. In the case of saturation achieved by water ingress with time, the degradation processes continue over the entire period of concern for the L/ILW repository. The present study aims to illustrate the potential of geochemical modelling for predicting the temporal evolution of the chemical conditions of a cement-stabilised waste form.

Redox buffering is one important factor to be considered when assessing the barrier function of potential host rocks for a deep geological repository for long-lived radioactive waste. If such a repository is to be sited in fractured crystalline host rock it must be demonstrated that waste will be emplaced deeper than the maximum depth to which oxidizing waters can penetrate from the earth’s surface via fractures, during the assessment timeframe (typically 1 Ma). An analogue for penetration of such oxidizing water occurs in the Cretaceous Toki Granite of central Japan. Here, a deep redox front is developed along water-conducting fractures at a depth of 210 m below the ground surface. Detailed petrographical studies and geochemical analyses were carried out on drill core specimens of this redox front. The aim was to determine the buffering processes and behavior of major and minor elements, including rare earth elements (REEs), during redox front development. The results are compared with analytical data from an oxidized zone found along shallow fractures (up to 20 m from the surface) in the same granitic rock, in order to understand differences in elemental migration according to the depth below the ground surface of redox-front formation. Geochemical analyses by XRF and ICP-MS of the oxidized zone at 210 m depth reveal clear changes in Fe(III)/Fe(II) ratios and Ca depletion across the front, while Fe concentrations vary little. In contrast, the redox front identified along shallow fractures shows strong enrichments of Fe, Mn and trace elements in the oxidized zone compared with the fresh rock matrix. The difference can be ascribed to the changing Eh and pH of groundwater as it flows downwards in the granite, due to reactions with rock forming minerals, in particular feldspar dissolution. These observations give important insights into the processes that control the rates of redox front penetration in fractured crystalline rock. The findings of the study can be used to help build confidence among stakeholders that radioactive waste would be emplaced in such rocks at greater depth than that to which oxidizing water is likely to penetrate in future.

The FEBEX experiment is a 1:1 simulation of a high level waste disposal facility in crystalline rock according to the Spanish radwaste disposal concept. This experiment has been performed in a gallery drilled in the underground laboratory Grimsel Test Site (Switzerland). Two boreholes parallel to the FEBEX drift were drilled 20 and 60 cm away from the granite–bentonite interface to provide data on potential bentonite–granite solutes transfer. Periodic sampling and analysis of the major ions showed: (a) the existence of solutes transfer from the bentonite porewater towards the granite groundwater, explaining the Cl− and Na+ contents of the latter; (b) that the concentration of the natural tracers coming into the granite groundwater from the bentonite porewater increased over time. This bentonite–granite solutes transfer was modelled in order to predict the increase in the Cl− and Na+ concentrations of the granite groundwater. The modelled results seem to confirm that the mechanism of solute migration in this scenario is that of diffusive transport. An effective diffusion coefficient of De = 5 × 10−11 m2/s was that which best fitted the data obtained.

Subsurface redox fronts control the mobilization and fixation of many trace elements, including potential pollutants such as certain radionuclides. Any safety assessment for a deep geological repository for radioactive wastes needs to take into account adequately the long-term redox processes in the geosphere surrounding the repository. To build confidence in understanding these processes, a redox front in a reduced siliceous sedimentary rock distributed in an uplifting area in Japan has been studied in detail. Geochemical analyses show increased concentrations of Fe and trace elements, including rare earth elements (REEs), at the redox front, even though concentrations of reduced rock matrix constituents show little change. Detailed SEM observations revealed that fossilized microorganisms composed of amorphous granules made exclusively of Fe and Si occur in the rock’s pore space. Microbial 16S rDNA analysis suggests that there is presently a zonation of different bacterial groups within the redox band, and bacterial zonation played an important role in the concentration of Fe-oxyhydroxides at the redox front. These water–rock–microbe interactions can be considered analogous to the processes occurring in the redox fronts that would develop around geological repositories for radioactive waste. Once formed, the Fe-oxyhydroxides within such a front would be preserved even after reducing conditions resume following repository closure.

Advanced plans for the construction of a deep geological repository for highly radioactive wastes from nuclear power plants have evolved during the past decades in many countries including Sweden. As part of the Swedish concept, the waste is to be encapsulated in canisters surrounded by low permeability backfill material. The copper canisters will be deposited at around 500 metres depth in granitic rock, which acts as a natural barrier for the transport of radionuclides to the ground surface. These natural and engineered barriers are chosen and designed to ensure the safety of the repository over hundred of thousands of years. One issue of interest for the safety assessment of such a repository is the redox evolution over long times. An oxidising environment would enhance the corrosion of the copper canisters, and increases the mobility of any released radionuclides. In the first part of the present thesis, the ability of the host rock to ensure a reducing environment at repository depth over long times was studied. A model framework was developed with the aim to capture all processes that are deemed to be important for the scavenging of intruding oxygen from the ground surface over long times. Simplifications allowing for analytical solutions were introduced for transparency reasons so that evaluation of results is straight-forward, and so that uncertain parameter values easily can be adjusted. More complex systems were solved numerically for cases when the analytical simplifications are not applicable, and to validate the simplifications underlying the analytical solutions. Results were presented for prevailing present day conditions as well as for conditions deemed to be likely during the melting phase of a period of glaciation. It was shown that the hydraulic properties have a great influence on the oxygen intrusion length downstream along flow-paths in the rock. An important parameter that determines the extent of interaction between the dissolved oxygen and the reducing minerals in the rock was shown to be the flow-wetted surface to flow-rate ratio. The results show that for an initial period of time, depending on the amount of reducing minerals and reaction rates, chemical reaction kinetics may control the rate of the overall depletion of oxygen. For longer times, internal diffusion resistance in large particles or in the rock matrix become rate limiting for the overall process. It was found that there are many uncertainties that have to be considered in order to make reliable quantitative predictions on the extent of oxygen intrusion. In the second part of the thesis, the impact of intruding oxygen on the corrosion of the copper canisters was explored. Also, a mechanism for the production of sulphide close to the deposition holes was studied. Sulphide is another corroding agent that may be produced microbially in a reducing environment from sulphate in the presence of organic reductants such as methane. From calculation results it was found that corrosion of more than 50 kg of copper is not likely over a period of one million years

One of the French options for the final disposal of high-level radioactive waste is a deep geological repository. The Callovo-Oxfordian formation in the Meuse and Haute-Marne (France) was proposed, by the French agency for the management of radioactive wastes (ANDRA), to be the site of a research laboratory in order to evaluate in situ the interaction between the host formation and the engineered barriers. In the previous experimental part, secondary minerals such as zeolites, tobermorite and katoite have been observed after the alteration of the Callovo-Oxfordian clay under alkaline conditions. Three different reactions of phyllosilicates alteration have been proposed as a function of both pH and chemical composition of the reacting solutions. The aim of this work was to simulate the reaction between the Callovo-Oxfordian clay and high-pH solutions in order to validate the proposed alteration reactions from experimental data. The thermodynamic modelling of stability relationships among minerals in the Callovo-Oxfordian clay was performed using the solution speciation solubility modelling code KINDIS at 120 °C for different chemical media (NaOH, KOH and Ca(OH)2). Input data were the mineralogical composition of the