Clay Host Rock Research Articles

Relating the Cation Exchange Properties of the Boom Clay (Belgium) to Mineralogy and Pore-Water Chemistry

AbstractThe Boom Clay in northern Belgium has been studied intensively over recent decades as a potential host rock in the context of disposal of radioactive waste. One of the parameters of interest is the cation exchange capacity (CEC) as it is related to the sorption potential of radionuclides to the clay host rock. In the past, the CEC was determined using various methods on a limited number of samples, leading to significant variations. To constrain the CEC of the Boom Clay better, a sample set covering the entire stratigraphy was measured using the quick copper(II) triethylenetetramine method. Part of the sample set was also measured using the cobalt(III) hexamine method, as a quality control for the results of the former method. In addition, the exchangeable cation population of the Boom Clay was quantified systematically for the first time and these results were compared to the in situ pore-water chemistry, indicating a strong coupling between the pore-water composition and the exchangeable sites of clay minerals.

Ion irradiation used as surrogate of neutron irradiation in graphite: Consequences on 14C and 36Cl behavior and structural evolution

Ion irradiation used as surrogate of neutron irradiation in graphite: Consequences on 14C and 36Cl behavior and structural evolution

A new view on iron-claystone interactions under hydrothermal conditions (90 °C) by monitoring in situ pH evolution and H2 generation

Many countries are considering the geological disposal of high-level radioactive waste combining natural clayey and engineered barriers (e.g. steel canister, bentonite, clay host rock, concrete). This configuration can lead to metallic iron-claystone interactions. Although a lot of experimental data on iron/clay interactions already exists, none of it allows the understanding and quantifying of the close interactions between the mineralogical transformations and the iron corrosion process. To fill this gap, a parametric kinetic investigation of iron/claystone interactions at 90°C in a water saturated-anoxic medium was carried out. A new approach is adopted to monitor a series of original experiments with various Fe0/claystone mass ratios. The continuous in situ monitoring of H2 production and of pH time-evolution allows to precisely follow the reaction rate and the chemical changes induced by the mineralogical transformations. In addition, gas composition and solid run products were characterized. We highlight that the Callovo-Oxfordian claystone of the Paris Basin (France) investigated here, in contact with metallic iron, drives the iron corrosion process, by controlling some medium parameters (like pH) and by acting as a Fe-consumer incorporating the released iron into neoformed Fe-rich clays minerals.

Impact of the electron donor on in situ microbial nitrate reduction in Opalinus Clay: results from the Mont Terri rock laboratory (Switzerland)

At the Mont Terri rock laboratory (Switzerland), an in situ experiment is being carried out to examine the fate of nitrate leaching from nitrate-containing bituminized radioactive waste, in a clay host rock for geological disposal. Such a release of nitrate may cause a geochemical perturbation of the clay, possibly affecting some of the favorable characteristics of the host rock. In this in situ experiment, combined transport and reactivity of nitrate is studied inside anoxic and water-saturated chambers in a borehole in the Opalinus Clay. Continuous circulation of the solution from the borehole to the surface equipment allows a regular sampling and online monitoring of its chemical composition. In this paper, in situ microbial nitrate reduction in the Opalinus Clay is discussed, in the presence or absence of additional electron donors relevant for the disposal concept and likely to be released from nitrate-containing bituminized radioactive waste: acetate (simulating bitumen degradation products) and H2 (originating from radiolysis and corrosion in the repository). The results of these tests indicate that—in case microorganisms would be active in the repository or the surrounding clay—microbial nitrate reduction can occur using electron donors naturally present in the clay (e.g. pyrite, dissolved organic matter). Nevertheless, non-reactive transport of nitrate in the clay is expected to be the main process. In contrast, when easily oxidizable electron donors would be available (e.g. acetate and H2), the microbial activity will be strongly stimulated. Both in the presence of H2 and acetate, nitrite and nitrogenous gases are predominantly produced, although some ammonium can also be formed when H2 is present. The reduction of nitrate in the clay could have an impact on the redox conditions in the pore-water and might also lead to a gas-related perturbation of the host rock, depending on the electron donor used during denitrification.

The stress–strain–permeability behaviour of clay rock during damage and recompaction

Characterisation and understanding of the stress–strain–permeability behaviour of a clay host rock during damage and recompaction are essential for prediction of excavation damaged zone and for assessment of its impact on the repository safety. This important issue has been experimentally studied in triaxial compression tests on the Callovo-Oxfordian clay rock in this study. The samples were sequentially loaded by (1) hydrostatic precompaction to close up sampling-induced microcracks, (2) applying deviatoric stresses to determine damage and permeability changes, and (3) recompression along different loading paths to examine reversibility of the damage. The critical stress conditions at the onset of dilatancy, permeability percolation, failure strength, and residual strength are determined. An empirical model is established for fracturing-induced permeability by considering the effects of connectivity and conductivity of microcracks. The cubic law is validated for the variation of permeability of connected fractures with closure. The experiments and results are also presented and discussed.

Evaluation of temperature-induced effects on safety-relevant properties of clay host rocks with regard to HLW/SF disposal

AbstractDBE TECHNOLOGY, BGR and GRS are developing a methodology to demonstrate the safety of a repository for high-level waste and spent fuel (HLW/SF) in clays according to the requirements of the German regulating body. In particular, these requirements prescribe that the barrier effect of host rocks must not be compromised by a thermal impact resulting from HLW/SF emplacement. To substantiate and quantify this requirement, we carried out a literature survey of research on thermally-induced changes on clay properties. Effects thus compiled can be divided into thermo-hydro-mechanical and chemical-biological-mineralogical effects and were analysed with regard to their relevance to the integrity of clay host rocks. This analysis identified one effect of major influence within each group: thermal expansion and compaction as well as results of microbial activities. Importantly, it further revealed that a moderate temperature increase above 100°C cannot be expected to compromise the integrity of the geological barrier according to the current knowledge state. Evidence is presented in this paper that temperature increases up to 150°C can actually contribute to an improved performance of a radioactive waste repository by increasing the consolidation of the clay and sterilizing the repository's near-field to depress the deteriorative microbial effects. A quantitative temperature criterion for thermal impact of HLW/SF on clay host rocks is accordingly proposed.

On the impact of temperatures up to 200 °C in clay repositories with bentonite engineer barrier systems: A study with coupled thermal, hydrological, chemical, and mechanical modeling

One of the most important design variables for a geological nuclear waste repository is the temperature limit up to which the engineered barrier system (EBS) and the natural geologic environment can be exposed. Up to now, almost all design concepts that involve bentonite-backfilled emplacement tunnels have chosen a maximum allowable temperature of about 100°C. Such a choice is largely based on the consideration that in clay-based materials illitization and the associated mechanical changes in the bentonite (and perhaps the clay host rock) could affect the barrier attributes of the EBS. However, existing experimental and modeling studies on the occurrence of illitization and related performance impacts are not conclusive, in part because the relevant couplings between the thermal, hydrological, chemical, and mechanical (THMC) processes have not been fully represented in the models. This paper presents a fully coupled THMC simulation of a nuclear waste repository in a clay formation with a bentonite-backfilled EBS for 1000years. Two scenarios were simulated for comparison: a case in which the temperature in the bentonite near the waste canister can reach about 200°C and a case in which the temperature in the bentonite near the waste canister peaks at about 100°C.The model simulations demonstrate some degree of illitization in both the bentonite buffer and the surrounding clay formation. Other chemical alterations include the dissolution of K-feldspar and calcite, and precipitation of quartz, chlorite, and kaolinite. In general, illitization in the bentonite and the clay formation is enhanced at higher temperature. However, the quantity of illitization is affected by many chemical factors and therefore varies a great deal. The most important chemical factors are the concentration of K in the pore water as well as the abundance and dissolution rate of K-feldspar; less important ones are the concentration of sodium and the quartz precipitation rate. In our modeling scenarios, the calculated decrease in smectite volume fraction in bentonite ranges from 1 to 8% of the initial volume fraction of smectite in the 100°C scenario and 1–27% in the 200°C scenario. Chemical changes in the 200°C scenario could also lead to a reduction in swelling stress up to 15–18% whereas those in the 100°C scenario result in about 14–15% reduction in swelling stress for the base case scenario. Model results also show that the 200°C scenario results in a much higher total stress than the 100°C scenario, mostly due to thermal pressurization. While cautions should be taken regarding the model results due to some limitations in the models, the modeling work is illustrative in light of the relative importance of different processes occurring in EBS bentonite and clay formation at higher than 100°C conditions, and could be of greater use when site specific data are available.

Coupled modeling of Excavation Damaged Zone in Boom clay: Strain localization in rock and distribution of contact pressure on the gallery’s lining

Around galleries excavated at depth in geological media, the creation of a damaged zone with significant irreversible deformation is generally unavoidable. In the case of a geological disposal system for high-level radioactive waste, the resulting change in the host rock properties in this damaged zone may potentially be important with respect to the long-term evolution and the performance of that system. In this context, predicting the extent of the so-called Excavation Damaged Zone (EDZ) and, possibly, the fractures’ network topology remains a challenge. This study is aimed to simulate numerically the extension of this zone at the large scale’s excavation, around the Connecting gallery (HADES URL, Mol, Belgium), in Boom clay host rock through analyzing the evolution of strain localization in shear bands mode. To realistically model the involved phenomena, the concrete lining is considered on the gallery wall highlighting its impacts on the evolution of convergence and EDZ around the gallery. The focus of the current paper is made on analyzing the coupled hydro-mechanical behavior of Boom clay host rock during and after the gallery excavation with respect to the evolution of localized shear bands around the gallery. This study is accompanied by the analysis of the contact mechanism on the interface between the clay massive and the lining. The obtained results reveal some interesting features regarding the contact phenomenon relatively to the evolution pattern of shear bands within the clay around the gallery. To assess the reliability of the proposed approach, a discussion on some in-situ observations during the gallery’s construction is also performed based on which a good agreement is found between the in-situ evidence and simulated results.

Three-dimensional numerical simulation by upscaling of gas migration through engineered and geological barriers for a deep repository for radioactive waste

Abstract This paper presents the results of a benchmark study that compares a number of numerical models applied to a specific problem in the context of hydrogen flow and transport in a nuclear waste repository. The processes modelled are two-phase (water and hydrogen) immiscible compressible two-component transient flow in a heterogeneous porous medium under isothermal conditions. The three-dimensional (3D) model represents a module of a repository for high-level waste in a clay host rock. An upscaling technique and a vertex-centred finite-volume method are employed to yield very accurate solutions. Since the full range of results required in the benchmark is too large to be displayed in this paper, we focus on the evolution of the pressures, the saturations, the fluxes and the comparison of the numerical results with the other participants. A homemade C++ upscaling code and the parallel multiphase flow simulator DuMu X have been adopted for this study.

Extension and Tuning of TOUGH2-MP EOS7R for the Assessment of Deep Geological Repositories for Nuclear Waste: Hydrogen, Arbitrarily Long Decay Chains, and Solubility Limits

Numerical simulation of multiphase flow and transport processes forms an important base for the assessment of deep geological repositories for radioactive waste. The finite volume simulation code TOUGH2-MP with its EOS7R equation-of-state module is a good starting point for large-scale simulations of the relevant processes, including solute transport of radionuclides, in and around a geological repository. On this base, we developed the equation-of-state module EOS75Rx that contains optimizations and specific extensions allowing for a much more efficient treatment of the problem at hand. First, hydrogen, which is formed by corrosion of waste containers and by radiolysis of organic wastes, replaces air as the main component of the gas phase. Second, an arbitrary number of variably long decay chains with branching can be considered. Third, solubility limitation and associated precipitation of chemical elements are modeled. Finally, a bug fix related to the source terms has been implemented. The new TOUGH2-MP EOS75Rx module has been validated using unitary tests and benchmark problems for the single- and two-phase flow and transport of radionuclides through porous media and soils. Its performance has been demonstrated by a large-scale, three-dimensional simulation of the performance of a generic deep geological repository in clay host rock.

EBS behaviour immediately after repository closure in a clay host rock: HE-E experiment (Mont Terri URL)

Abstract The evolution of the clay-based engineered barrier system (EBS) of geological repositories for radioactive waste has been the subject of many research programmes during the last decade. The early post-closure thermal behaviour is elucidated by the HE-E experiment, a 1:2 scale heating experiment (at the Mont Terri Rock Laboratory), which was implemented in the first semester of 2011, with the initiation of the heating phase in June 2011. A maximum temperature of 140 °C was reached in June 2012. After 15 months of heating, the temperature evolution in the EBS and the Opalinus Clay reflects the design calculations, and thermally induced porewater overpressures are being measured at a few metres’ distance in the Opalinus Clay. Seismic methods proved to be a sensitive tool for the continuous characterization of changes of EBS and Opalinus Clay properties. Design modelling and predictive modelling based on the as-built parameter dataset with established coupled codes (TOUGH, CODE_BRIGHT; using various geometries) are described. The results indicate that the models are generally in agreement with the observations and capable of capturing the evolution of the experiment.

Characterization of gas flow through low-permeability claystone: laboratory experiments and two-phase flow analyses

Abstract For the characterization of gas migration through a low-permeability clay host rock for deep underground repositories, a comprehensive understanding of the relevant phenomena of gas and fluid flow through low-permeability clay is required. The National Cooperative for the Disposal of Radioactive Waste (Nagra) in Switzerland has developed a comprehensive programme to characterize gas flow in low-permeability Opalinus Clay through laboratory tests and detailed numerical analyses for developing appropriate constitutive models. Laboratory tests were performed on cores by two different laboratories, the Laboratory for Soil Mechanics at EPFL and the Department of Geotechnical Engineering and Geosciences at UPC. Loading tests were performed by both laboratories to study rock compressibility at different stress levels and water permeability dependence on void ratio. The water retention behaviour demonstrated by EPFL and UPC produced comparable results. Water permeability tests and fast controlled-volume air injection experiments were performed in a triaxial cell under isotropic stress conditions on two samples with flow parallel and normal to the bedding planes. A confining stress of 15 MPa was applied during gas testing, corresponding to a lithostatic pressure at a depth of c. 600 m below ground. For detailed analyses, the two-phase flow code TOUGH2 ( Pruess et al. 1999 ) was used. This considers fluid flow in both liquid and gas phases under the influence of pressure, viscous and gravity forces, according to Darcy's law. The standard analyses could not reproduce the measured pressure responses well, and the calibrated hydraulic and two-phase parameters were not consistent with the preceding water test and laboratory analyses. Implementing the non-linear behaviour in terms of the observed relationship between changes in void ratio and associated changes in permeability under different stress conditions significantly improved the simulated results, resulting in a conceptual model that well reproduced the observed injection pressure and outflow responses for both tests, parallel and normal to bedding, using a consistent parameter set.

Thermo-Hydro-Mechanical Effects in Clay Host Rocks for Radioactive Waste Repositories

In all nuclear power-generating countries, the management of spent nuclear fuel and long-lived radioactive waste is an important environmental issue. Disposal in deep clay geological formations is one of the promising options recommended at the international level. In this concept of the geological disposal system, the host clay formation is considered as a principal barrier on which rests the fulfilment of key safety functions. Hence, preventing unnecessary damage to the host formation is one of the objectives of repository design. A proper evaluation of the excavation-damaged zone (EDZ) in the host formation is, thus, an important issue for the long-term safety of underground disposal. As a side effect of radioactive decay, vitrified high-level wastes and spent fuel release a significant amount of heat, even after several decades of cooling in surface facilities. The influence of the thermal load generated by the radioactive waste on the performance of the disposal system is particularly important, since the early transient thermohydro-mechanical (THM) perturbation might be the most severe impact on the repository system on a large spatial scale and in a relatively short period of time. Assessing the consequences of the thermal transient on the performance of the disposal system was the main objective of the TIMODAZ European project. Between 2006 and 2010, the TIMODAZ project (Thermal Impact on the Damaged Zone Around a Radioactive Waste Disposal in Clay Host Rocks, http:// www.timodaz.eu/), which gathered 15 partners from eight countries, has investigated the coupled THM effects in clay formations that are being studied as potential host rocks for the geological disposal of radioactive waste. Three types of clay and clay rocks corresponding to the three potential host sites for the Belgian, Swiss, and French disposal facilities were investigated: the Boom Clay (Belgium), the Opalinus Clay (Switzerland), and the Callovo–Oxfordian argillite (France). The project focused on the combined effect of the EDZ and the thermal output from the waste on the repository host rock. The influences of the temperature increase on the evolution of the EDZ as well as the possible additional damage created by the thermal load were also the main issues of the study. The chemical evolution as well as its interaction with the THM processes around the underground repository were also addressed in the project, but to limited extent. Intensive experimental (laboratory and in situ in underground research laboratories) and numerical studies have been performed in order to better understand the impact of the coupled THM processes on the long-term evolution and safety of repositories. On 6–7 February 2012, an international post-TIMODAZ workshop was organized at Mont Terri Rock Laboratory, Switzerland, in order to present the main TIMODAZ results, and also other contributions in the area of the THM analysis of nuclear waste disposal systems and to foster informal discussions and exchanges on THM issues in clay host rocks. This special issue includes 15 selected papers which expand on the technical contributions presented at this International post-TIMODAZ workshop. The submitted papers underwent a complete review process following the publication rules of Rock Mechanics and Rock Engineering under the supervision of the guest editors. J. Sulem (&) UR Navier-CERMES, Ecole des Ponts Paris Tech, IFSTTAR, CNRS, Universite Paris-Est, Marne-la-Vallee, Paris, France e-mail: jean.sulem@enpc.fr

TIMODAZ: A successful international cooperation project to investigate the thermal impact on the EDZ around a radioactive waste disposal in clay host rocks

Disposal of spent nuclear fuel and long lived radioactive waste in deep clay geological formations is one of the promising options worldwide. In this concept of the geological disposal system, the host clay formation is considered as a principal barrier on which the fulfillment of key safety functions rests. Between 2006 and 2010, the European Commission project TIMODAZ, which gathered 15 partners from 8 countries, has investigated the coupled thermo-hydro-mechanical (THM) effects on clay formations for geological disposal of radioactive waste, and specific attention was paid to investigating the thermal effect on the evolution of the damaged zone (DZ). Three types of potential host clay formations were investigated: the Boom Clay (Belgium), the Opalinus Clay (Switzerland) and the Callovo-Oxfordian argillite (France). Intensive experimental (laboratory and in situ in underground research laboratories) and numerical studies have been performed. Multi-scale approach was used in the course of the project. High degree of similarities between the failure modes, sealing process, stress paths, deformation, etc., observed in laboratories and in situ has been obtained, which increased the confidence in the applicability of laboratory test results and up-scaling perspective. The results of the laboratory and in situ tests obtained allowed the parameters for numerical models at various scales to be derived and provided the basis for the simplified performance assessment models that are used to assess the long-term safety of a repository. The good cooperation between the numerical modeler and experimenters has allowed an in-depth analysis of the experimental results and thus better understanding the underlying processes, and consequently increased the capabilities to model the THM effects in claystones. This paper presents the main achievements obtained by TIMODAZ project and shows how a European scientific community investigates a problem of concern in a collaborative way and how the obtained main results are applied to the performance assessment of a geological repository.

Consequences of the Thermal Transient on the Evolution of the Damaged Zone Around a Repository for Heat-Emitting High-Level Radioactive Waste in a Clay Formation: a Performance Assessment Perspective

A proper evaluation of the perturbations of the host rock induced by the excavation and the emplacement of exothermic wastes is essential for the assessment of the long-term safety of high-level radioactive waste disposals in clay formations. The impact of the thermal transient on the evolution of the damaged zone (DZ) has been explored in the European Commission project TIMODAZ (thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks, 2006–2010). This paper integrates the scientific results of the TIMODAZ project from a performance assessment (PA) point of view, showing how these results support and justify key PA assumptions and the values of PA model parameters. This paper also contextualises the significance of the thermal impact on the DZ from a safety case perspective, highlighting how the project outcomes result into an improved understanding of the thermo–hydro–mechanical behaviour of the clay host rocks. The results obtained in the TIMODAZ project strengthen the assessment basis of the safety evaluation of the current repository designs. There was no evidence throughout the TIMODAZ experimental observations of a temperature-induced additional opening of fractures nor of a significant permeability increase of the DZ. Instead, thermally induced plasticity, swelling and creep seem to be beneficial to the sealing of fractures and to the recovery of a very low permeability in the DZ, close to that of an undisturbed clay host rock. Results from the TIMODAZ project indicate that the favourable properties of the clay host rock, which guarantee the effectiveness of the safety functions of the repository system, are expected to be maintained after the heating–cooling cycle. Hence, the basic assumptions usually made in PA calculations so far are expected to remain valid, and the performance of the system should not be affected in a negative way by the thermal evolution of the DZ around a radioactive waste repository in clay host rock.

Modeling of Coupled Thermo-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonite-Backfilled Repository Tunnels in Clay Formations

This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS- rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in hor- izontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geo- mechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our ini- tial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low- permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribu- tion. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabil- ization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a pos- sible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.

Mineralogical Evolution of a Claystone After Reaction With Iron Under Thermal Gradient

AbstractThe design of the repository for high-level nuclear waste (HLW) in France consists of a multiple-barrier system including steel canisters in a clay host rock. The system will undergo temperature variations in time and space, the heat source being the HLW within the canisters. The effect of a thermal gradient in space on the Fe-claystone interaction was investigated here by applying a thermal gradient (150–300°C and 80–150°C) to a mix of claystone, Fe, and an aqueous chloride solution over periods of 3 and 6 months. Following the reaction, the starting clay minerals (mostly illite and mixed-layer illite-smectite) evolved toward chlorite, Fe-serpentine, Fe-saponite, mixed-layer chlorite-smectite, or mixed-layer serpentine-smectite as a function of temperature. Iron corrosion made the medium basic and reductive. Magnesium enrichment of clay minerals was observed in the hottest part of the experiment due to Mg migration under the thermal gradient. Reaction progress was enhanced at the lowest temperatures, compared to batch experiments.

Experimental interactions of the Opalinus Clay and Boom Clay with various repository relevant solutions at 90 °C under closed conditions

A temperature increase coupled with other geochemical perturbations is expected to occur during the heat phase of the geological repository due to the presence of heat-emitting radioactive waste and engineered barrier system. To study the impact of a temperature increase on the geochemistry of some candidate clay host rocks considered in Europe, we tried to simulate such conditions by means of batch tests followed by detailed mineralogy analysis and geochemical modelling. The batch tests were performed on whole rock Boom Clay (WRBC) and whole rock Opalinus Clay (WROPA) samples in contact with some representative pore waters at 90°C, the maximum temperature expected as a result of heat-emitting waste. The same experimental settings were applied to run parallel tests on the separated clay fractions (CFBC and CFOPA). The aim was to assess the impact of coupled heat and chemical processes on their mineral stability in the presence and absence of the organic matter and carbonates. At specific time intervals, the solids were separated from solutions and subject to (Q)XRD, FTIR, UV–VIS, TSA and CEC determinations. The whole rock samples were found stable irrespectively of the solution composition with no negative impact on their physico-chemical properties. In line with the experimental observations, the geochemical modelling suggests that minerals are able to reequilibrate relatively fast with the ambient solutions. The clay mineralogy was modified only in the case of Boom Clay deprived of carbonates and organic matter. The clay mineralogical alterations are associated with the increase of the layer charge, TSA and CEC. The experimental and modelling results indicate that coupled heat and perturbed geochemical conditions did not jeopardise the stability of the constituent minerals in the studied clays. Altogether, the presented work highlights very good buffer capacity of both candidate host rocks towards geochemical perturbations.

Spent fuel dissolution in Belgian Supercontainer conditions: source term and compatibility

ABSTRACTIn the frame of the Safety and Feasibility Case 1 development, the Belgian agency for radioactive waste (ONDRAF/NIRAS) has to demonstrate that the geological disposal of spent nuclear fuel in the Boom Clay host rock following the Supercontainer design provides sufficient radiological safety in the long term. This safety relies on the containment of the radionuclides by the overpack, the limitation of radionuclide release from the waste matrix, the diffusion controlled transport of the radionuclides, and their low solubility and sorption on the Boom Clay host rock. This paper presents an evaluation of the main processes considered for the limitation of radionuclide release by the Spent Fuel in Supercontainer conditions, characterized by the presence of a concrete buffer and hyperalkaline pore water. We present a description of the main expected processes with their potential impact on the fuel alteration mechanisms, and potential reference fuel matrix alteration rates and effective fuel surface areas for use in safety and performance assessment models.

FUNMIG Integrated Project results and conclusions from a safety case perspective

Abstract The scope of the FUNMIG Integrated Project (IP) was to improve the knowledge base on biogeochemical processes in the geosphere which are relevant for the safety of radioactive waste repositories. An important part of this project involved the interaction between data producers (research) and data users (radioactive waste management organisations in Europe). The aim thereof was to foster the benefits of the research work for performance assessment (PA), and in a broader sense, for the safety case of radioactive waste repositories. For this purpose a specifically adapted procedure was elaborated. Thus, relevant features, events and processes (FEPs) for the three host rock types, clay, crystalline and salt, were taken from internationally accepted catalogues and mapped onto each of the 108 research tasks conducted during the FUNMIG project by a standardised procedure. The main outcome thereof was a host-rock specific tool (Task Evaluation Table) in which the relevance and benefits of the research results were evaluated both from the PA and research perspective. Virtually all generated data within FUNMIG are related to the safety-relevant FEP-groups “transport mechanisms” and “retardation”. Generally speaking, much of the work within FUNMIG helped to support and to increase confidence in the simplified PA transport and retardation models used for calculating radionuclide (RN) transport through the host rock. Some of the studies on retardation processes (e.g. coupled sorption-redox processes at the mineral–water interface) yielded valuable data for all three rock types dealt within the IP. However, most of the studies provided improved insight regarding host-rock specific features and processes, the majority of this work being dedicated to clay-rich and crystalline host rocks. For both of these host rock types, FUNMIG has significantly contributed to improving understanding on a conceptual level, both by providing new experimental data at different spatial scales and by developing improved modelling approaches. The disposal concept in salt host rocks differs from those in other host rock types in that the host rock is, at least in part, regarded as near-field from which under normal scenarios no release of radionuclides is possible. Corresponding investigations in FUNMIG concerned therefore not salt host rocks but an example of an overburden under the premise of radionuclide escape from the salt barrier. Selected highlights with regard to FUNMIG’s achievements include: For argillaceous host rocks, the systematic effort of investigating and comparing diffusion and sorption processes at different scales in different clay rocks using a variety of methods has substantially increased the knowledge basis for future safety cases. For crystalline host rocks, valuable data on the generation, transport and filtration of clay colloids from the near-field and their impact on RN transport under realistic conditions have been obtained. The results from studies on organic colloids and on biofilms, including their interaction with radionuclides, have been shown to be of interest for future safety cases of salt-host rocks. Among the main research issues from a PA perspective that need to be addressed in the future, the following are noted: (i) the question of irreversibility of RN sorption to colloids in fractures of crystalline rocks, (ii) a comprehensive model for cation and anion diffusion in clays for different scales and (iii) the applicability of mechanistic retardation models for strongly sorbing radionuclides in intact clay and crystalline host rocks. An important lesson learnt from the interaction between research and PA is that it would be desirable to apply a similar evaluation procedure for proposed research tasks before the start of the research work. In this regard, the procedures developed within FUNMIG are a useful tool for planning future Integrated Projects.