Radioactive Waste Repository Research Articles

Analytical solution on temperature evolution in three-layer barrier high-level radioactive waste repository and required pre-cooling time determination

An analytical model of the temperature field in a high-level radioactive waste (HLW) disposal repository containing three-layer barrier of bentonite blocks, bentonite pellets and surrounding rock was established, and a series of mathematical methods were used to obtain a fully analytical solution for the model, which can visually represent the relationships between temperature and various parameters and provide a reference for designing repository. Through comparisons to other existing analytical and numerical simulation results, the validity of our solution was verified. Subsequently, temperature evolution analyses of one-canister and one-panel conditions were performed using the derived solution. Additionally, the temperature evolution at the point where peak temperatures are expected (i.e., mid-height of central canister's side surface in one disposal panel) was investigated with different geometric design dimensions, and it was observed that temperature remains virtually unchanged when the tunnel spacing exceeds 40 m. The required pre-cooling times for four types of HLW under different geometric and thermal parameters were analyzed using the obtained solution, with all canisters considered in one disposal panel.

Regional groundwater flow pattern in fractured bedrock with a high-level radioactive waste repository: numerical simulation of the Xinchang site, northwestern China

Regional groundwater flow pattern in fractured bedrock with a high-level radioactive waste repository: numerical simulation of the Xinchang site, northwestern China

Determining the Macrostructural Stability of Compacted Wyoming Bentonites by a Disaggregation Method

The use of compacted bentonites in radioactive waste repository barriers is a relevant application of geoenvironmental engineering. The on-site structural characteristics of the bentonites determine the performance and integrity of the barrier. The present work addresses the adaptation of the standardized sand equivalent shaking method for the controlled disaggregation of Wyoming bentonite specimens prepared at low, medium, and high compaction. The evolution of the macrostructural units’ size distribution was determined by sieving at different shaking times. The stability of the compacted material increases with dry density. However, if enough energy is applied in the disaggregation process, the size distribution of the macrostructural units after disaggregation has the same characteristics as that of the uncompacted starting material, regardless of the applied degree of compaction. Since the disaggregation rate is a function of the aggregation level (compaction), it follows that compaction energy is not only spent on reducing porosity but also on generating more stable macrostructural units. These findings pave the way for future research with different materials and test conditions (compaction, moisture, etc.). In addition, the proposed shaking method is adaptable and could also be used in other sectors, such as agriculture, to determine the structural stability of natural soils.

Numerical modelling of a large-diameter sealing structure in a deep radioactive waste repository

Numerical modelling of a large-diameter sealing structure in a deep radioactive waste repository

Inspecting the role of vapour loss and other model strategies in the mod-elling of a bentonite thermo-hydraulic cell

Inspecting the role of vapour loss and other model strategies in the mod-elling of a bentonite thermo-hydraulic cell

Multi-scale investigation on gas transport behaviour of compacted granular bentonite under partially saturated states

Compacted bentonites with high swelling potential and low permeability are under consideration as buffer/backfill materials in deep geological repositories of radioactive nuclear waste [1]. Within the short-term service period, the bentonite undergoes progressive saturation developing the swelling pressure, due to the water received from the host rock. Since the progressive saturation first occurs at the boundaries near the host rock, the bentonite close to the fuel canister could remain partially saturated for a long time. Within the long-term running stage, bentonite will withstand the accumulation and transport of gases such as H2 (mainly generated by the anaerobic corrosion of the metallic overpack of canisters) [2]. The accumulated volume of gas can produce a gas pressure build-up that could break through the bentonite, damaging its microstructural integrity and isolation capacity for radionuclides and contaminants [3]. This issue may be improved by adopting granular bentonite (GB) with an extended particle size distribution (maximum grain sizes of the order of mm) [4]. The compacted GB has a high initial macroporosity, which might permit gas transport at low pressures with less impact on the microstructure. However, up to now, few investigations have focused on the gas transport behaviour of partially saturated compacted GB within a multi-scale perspective.
 To this end, gas injection tests were performed on partially saturated compacted MX-80 type GB at different hydro-mechanical states. Effective gas permeability was measured in samples at the as-compacted state and in samples prepared at the as-compacted state and then partially saturated under isochoric conditions without additional vertical stress apart from the generated due to swelling. Additionally, the tests were also conducted on these two types of samples but subjected to initial vertical stress of 3.8 MPa. X-ray micro-tomography in combination with mercury intrusion porosimetry was employed to observe the microstructural characteristics of samples before and after gas injection. The results highlight that the pore network of compacted GB is constituted of inter-granular, inter-aggregate and intra-granular/aggregate pores. Increasing the as-compacted degree of saturation Sr reduces the proportion of inter-granular pores and the effective gas permeability Ka.eff (Figure 1(a)). For the as-compacted sample, gas transport can induce the size of inter-granular/aggregate pores to extend (Figure 1(b)). When the initial vertical stress is applied on the as-compacted sample, Ka.eff decreases, with the closure of inter-granular/aggregate pores. Although the progressive saturation of the as-compacted samples under constant volume causes the reduction of inter-granular/aggregate pores, they are mainly filled with the low-density bentonite gel and the accumulated gas pressure is able to reopen and connect them to form fissures (Figure 1(b)). Thereby, at a given Sr resulting from this saturation process, Ka.eff is similar to that at the same as-compacted Sr (Figure 1(a)). However, the decrease in Ka.eff is significant, when the progressive saturation is conducted under vertical stress. This is because many inter-granular/aggregate pores collapse during the saturation process, and the vertical stress restrains the formation of fissures driven by gas pressure. On the other hand, gas transport does not affect the distribution of intra-granular/aggregate pores within the studied hydro-mechanical states.
 The current outcomes provide multi-scale insights into the gas transport behaviour of partially saturated compacted GB and emphasise its microstructural response to gas transport under different hydro-mechanical states. The values of Ka.eff measured in the tested GB are higher than those obtained in powder bentonites but comparable to those of sand/bentonite mixtures at an equivalent porosity [5, 6]. The reason is the GB material presents the granular-type microstructure, with many interconnected inter-granular/aggregate pores, even at high Sr. Once the progressive saturation increases Sr to a value close to 1, Ka.eff will sharply drop, attributed to the significant microstructural modification [7]. Therefore, the initial microstructure (before gas injection) of compacted GB plays a critical role in the gas transport behaviour. This microstructural effect was also confirmed in previous investigations of other soil types [8]. Furthermore, the present work underlines the microstructure of compacted GB is modified during gas transport, dependent on hydro-mechanical states, which can further affect the gas transport behaviour.

Air and hydrogen injection tests on saturated compacted bentonite

Air and hydrogen injection tests on saturated compacted bentonite

Multiphase flow gas transport in a deep geological repository

Gas transport in porous media is of interest in many industrial applications, such as the oil and gas industry, geological storage, and deep geological repositories for radioactive waste. In a deep geological repository, gas will be generated due to the corrosion of metallic components and the degradation of organic materials. This leads to a build-up of gas pressure, which may activate gas transport through the host rock as well as the excavation-damaged zone around backfilled galleries.
 In order to understand different transport mechanisms involved, numerical simulations were performed, and the results were compared with laboratory data. In the framework of the EURAD/GAS project, a gas pressure-dependent permeability model was implemented into the finite element code OpenGeoSys-6 (OGS-6) [1]. The permeability alteration in this model is a function of gas pressure. The laboratory experiments showed that the rate of permeability change are different at low and high gas pressures. Therefore, the permeability model employed a threshold pressure ( to categorize this behaviour.
 and are empirical parameters. Moreover, two other permeability models were employed to study the hydro-mechanical behaviour of the host rock and permeability changes. In the strain-dependent permeability model, the permeability change was related to the elasto-plastic behaviour of the host rock, and in the embedded fracture model, it was related to the opening and closure of fractures [3] [4]. Thus, volumetric elastic strain and equivalent plastic strain are employed to be the controlling variables.
 The initial permeability of the intact rock samples were determined by applying a constant pressure at the upstream and downstream of the samples (i.e. constant pressure gradient). The imperical parameters were determined by matching experimental and numerical results.
 Two types of gas injection tests carried out by the Institute for Rock Mechanics (IFG GmbH, See Figure 1) were used to investigate the gas transport through Opalinus Clay and to examine the permeability models [2]. The first experiment demonstrates an advective-diffusiion gas transport through the sample and an elastic deformation. The second experiment highlights the formation of a tensile fracture (plastic deformation and preferred flow path). In both experiments the advective transport is the dominant transport mechanism. The strain-dependent permeability model was successfully applied to reproduce the hydro-mechanical behaviour of the host rock in both elastic deformation test and tensile fracture test (see Figures 2 and 3). The hydro-mechanical response of a saturated single phase flow model was compared with the behaviour of a saturated two-phase flow model. Both single phase and two-phase flow models were able to describe the hydraulic as well as mechanical behaviour of the experiments performed. Therefore, one can conclude that in these experiments the water phase (wet-phase) was immobile.
 A gas injection test under triaxial conditions was performed by École Polytechnique Fédérale de Lausanne (EPFL) in saturated Opalinus Clay (Fig. 4). The numerical simulations reproduced the hydro-mechanical behaviour of the sample during the gas injection test. A two-phase flow model was applied to simulate the experiment. The relative permeabilities and capillary pressure functions followed Mualem approach and van Genuchten formulation, respectively. The outflow volume and mechanical response of the sample were measured. The experimental results were in good agreement with the numerical ones. The results of the modelling illustrated the penetration of gas into the sample and hence, the displacement of water (see Figures 5 and 6).

Discrete particle methods for granular bentonite material simulation

Bentonite clays are strategic materials to ensure the sealing capacity of engineered barrier systems (EBS) in deep geological repositories of high-level radioactive waste. Formerly installed in the form of compacted blocks, in recent years, there has been growing interest in EBS design using granular bentonite materials (GBM) due to improved construction processes [1]. GBM range from the powdered and granulated forms, both natural and obtained from crushed blocks, to compressed bentonite pellets, and the mixtures thereof. The effect of thermo-hydro-chemo-mechanical (THCM) conditions on the evolving gap-filling performance following installation has been investigated in full-scale experiments [2] and in laboratory-scale tests [3], with particular focus on bentonite pellet-powder mixtures [4, 5, 6].
 Numerical modelling of coupled THCM expansive soil behaviour is widespread using continuum methods, namely the finite element method [7, 8], applicable up to full-scale systems even though at the expense of resolution. At the laboratory scale, discrete particle-based methods accounting for the granular nature of bentonite materials in the early stages of hydration provide insight into pellet shape and local effects on the mechanical behaviour of dry pellet-powder systems as well as hydro-mechanical effects on the swelling capacity of GBM systems.
 The compressibility of dry pellet packings and pellet-powder mixture samples is modelled by the discrete element method (DEM). This method has been applied to study the swelling behaviour of pellets represented by spherical particles [9]. The DEM is a powerful tool for reproducing complex granular shapes by non-spherical particle surfaces [10] and also with clumps of spheres (Figure 1(a)), allowing to define mixed material and mechanical contact properties within the pellets. The deformation and fissuring of pellets under oedometer compression is tracked using a cohesive particle model [11], including damage for interparticle contacts in tension, which captures deformation associated phenomena such as surface spalling and pellet reorganisation occurring at stress levels below the diametral fracture. In addition, bentonite powder is represented by loose packings of upscaled spheres matching
 the experimental porosity of the fillings, interacting by a classical linear spring-dashpot contact model resulting in the reduced overall compressibility observed in pellet-powder mixtures [5].
 Flow through GBM systems is modelled by the new discrete particle-based XMm method [12]. In the XMm, the material is discretised into a set of bentonite units (BU) or particles acting as representative elementary volumes of a homogeneous continuum inside which the mass balance equations are solved at the megastructural (X), macrostructural (M), and microstructural (m) pore levels (Figure 1(b)). Moreover, the mechanical compatibility of the system is obtained from the coupling between strain increments and centroid displacements in each BU, where the contact stress and overlap between neighbouring particles are updated similarly to the DEM approach. The hydro-mechanical problem is posed as a set of ordinary differential equations (ODE) to be solved for each BU, and thus the efficiency of the XMm is a compromise between the typically reduced computational cost of ODE solvers and the number of BU considered. The XMm has been used to simulate the hydration of a powder sample with low heterogeneity [4], successfully capturing the evolution of the displacement field upon saturation in isochoric conditions. This method is applied to reproduce the confined hydration of a heterogeneous pellet-powder system [6]. Assuming 2D axisymmetry, the XMm allows defining different initial material properties in each BU introducing the heterogeneity of the sample. The evolving swelling pressure is reproduced as well as the hydration velocities at the different structural levels, showing the decreasing heterogeneity of the mixture.

A coupled fluid-mechanical interaction model for controlled gas migration mechanism by dilatancy effect in saturated bentonite

A gas breakthrough in saturated bentonite is relevant to the safety of high-level radioactive waste repositories. The study of gas transport mechanisms in saturated bentonite is very important for the safety assessment of repositories. This paper proposed a coupled fluid-mechanical interaction model for predicting and simulating the path of gas transport and gas breakthrough in saturated Gaomiaozi bentonite. The model considered the effect of deformation and damage of bentonite on its permeability and introduced pore pressure into the deformation equation of bentonite. The damage coefficient was also introduced into the permeability evolution equation by combining the Mohr–Coulomb criterion, the maximum tensile stress criterion and the damage evolution. In addition, considering the heterogeneity of the soil, the Weibull distribution function was introduced to assign differential values to material parameters of the cells in the model. The numerical simulation of the bentonite stress field and seepage field was realized by the joint MATLAB and COMSOL secondary development, and the evolution law of the pore path in bentonite was explored under a flexible boundary. The gas breakthrough pressure and permeability pressures were calculated at various gas injection from a gas injection experiment into bentonite with flexible boundaries. Finally, the rationality and applicability of the model were verified by comparing the numerically calculated gas breakthrough pressure and permeability with experimental values.

Stability of modified technetium-containing magnesium potassium phosphate matrix under repository conditions with regard to biogeochemical effects

In this work, the biological stability of magnesium potassium phosphate (MPP) matrix was assessed for the first time under conditions modelling near-surface and deep repositories of radioactive waste (RW) containing technetium. It was found that MPP-matrix is potentially susceptible to microbial degradation, so it is necessary to introduce modifying additives. The efficiency of introduction of modifying additives - tetraphenylphosphonium chloride (TPP), thiourea (THIO), polyhexamethyleneguanidine hydrochloride (PHMG), polyethyleneimine (PEI), shungite (SHU), chalcopyrite (CP), orpiment (ORP) at immobilisation of pertechnetate ion in MPP-matrix has been estimated. The mechanical strength and water resistance of the obtained compounds were investigated. The maximum leaching rate of technetium was found to be 0.34 g cm-2 day-1 from the blank sample without additives. The addition of ORP and PHMG reduces the leaching rate of technetium by 90% and 62%, respectively, relative to the pure sample. The addition of other modifying additives reduced the leaching rate of pertechnetate ions by 15–25% on average. At the same time, the addition of PEI and THIO led to a decrease in compressive strength below the standard requirement of 4.9 MPa.a. Samples containing PHMG and ORP up to 0.5 wt% showed higher safety margin compared to the blank sample. ORP, PHMG and TPP can be proposed as modifying additives to reduce microbial activity during immobilisation of pertechnetate ion using MPP matrix.

Europium(III) as luminescence probe for interactions of a sulfate-reducing microorganism with potentially toxic metals

Microorganisms show a high affinity for trivalent actinides and lanthanides, which play an important role in the safe disposal of high-level radioactive waste as well as in the mining of various rare earth elements. The interaction of the lanthanide Eu(III) with the sulfate-reducing microorganism Desulfosporosinus hippei DSM 8344T, a representative of the genus Desulfosporosinus that naturally occurs in clay rock and bentonite, was investigated. Eu(III) is often used as a non-radioactive analogue for the trivalent actinides Pu(III), Am(III), and Cm(III), which contribute to a major part of the radiotoxicity of the nuclear waste. D. hippei DSM 8344T showed a weak interaction with Eu(III), most likely due to a complexation with lactate in artificial Opalinus Clay pore water. Hence, a low removal of the lanthanide from the supernatant was observed. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed a bioprecipitation of Eu(III) with phosphates potentially excreted from the cells. This demonstrates that the ongoing interaction mechanisms are more complex than a simple biosorption process. The bioprecipitation was also verified by luminescence spectroscopy, which showed that the formation of the Eu(III) phosphate compounds starts almost immediately after the addition of the cells. Moreover, chemical microscopy provided information on the local distribution of the different Eu(III) species in the formed cell aggregates. These results provide first insights into the interaction mechanisms of Eu(III) with sulfate-reducing bacteria and contribute to a comprehensive safety concept for a high-level radioactive waste repository, as well as to a better understanding of the fate of heavy metals (especially rare earth elements) in the environment.

Effect of hydrothermal alteration of Ca-bentonite in K-rich solutions with different pHs on the physicochemical, swelling, and Cs adsorption properties

Understanding the stability of the bentonite buffer when exposed to cement leachates at temperatures exceeding 100°C is crucial for optimizing the design of deep geological repositories (DGRs) for high-level radioactive wastes (HLRWs) and minimizing the required site area. Experiments were conducted under hydrothermal conditions at 150°C for 30–150 days using deionized (DI) water (pH 6) and K-rich solutions (1 mol/L KCl, pH 6 and 1 mol/L KOH, pH 13) on Ca-bentonite. The 1 mol K concentration is comparable to 15 times the cation exchange capacity (CEC) of raw bentonite. Using various analytical techniques, the mineralogical, physicochemical, swelling, and Cs adsorption characteristics of raw and reacted bentonite samples were determined. Almost no changes in the properties of bentonite reacted with DI water were found for a given reaction time. For bentonite reacted with a 1 mol/L KCl solution, the cation exchange of Ca by K was a primary alteration process, resulting in a slight decrease in swelling capacity. However, almost no mineralogical changes were observed and consequently, there was minimal change in the Cs adsorption capacity. In contrast, for the bentonite reacted with a 1 mol/L KOH solution, the dominant alteration process was the transformation of minerals in bentonite into zeolite minerals, which resulted in significant changes in physicochemical properties, in particular, a decrease in the swelling capacity. On the other hand, the Cs adsorption capacity increased up to 2 times compared with the raw bentonite.

Effects of initial compaction condition and temperature on swelling characteristics of compacted bentonite

Given that isolated high-level radioactive waste repository cannot be modified during operation, understanding the influence of the buffer's initial conditions is critical. This study investigates the influence of the initial dry density and water content on the swelling pressure and consolidation characteristics of Korean bentonite, which is a buffer candidate in the engineered barriers of the repository. In order to consider the high temperature repository condition, samples of various dry densities and water contents were tested at different temperature. Prediction models were developed using the test data and the models were used to analyze the effects of initial conditions. The research findings indicate that the influence of dry density, water content, and temperature on swelling pressure is not absolute and can show different tendencies depending on the case. In terms of consolidation behavior, the compression index and pre-consolidation pressure differ with dry density and temperature, while recompression is unaffected. The study's results align with prior research on compacted expansive clays, with the double structure of clay serving as a suitable explanation for the swelling phenomenon's dependence on the initial compaction condition.

The 14C dose assessment model chain – 14C source term definition and uncertainty quantification

Abstract. Carbon-14 (14C) is an important, dose-determining contributing radionuclide in a deep geological repository for low and intermediate level radioactive waste (L/ILW). 14C is relevant for safety assessments as it can be present both as dissolved and gaseous species in the disposal facility and the host rock, and 14C-organic compounds migrate in the near field and the geosphere due to weak interaction with mineral surfaces in alkaline to near-neutral conditions. Thus, the chemical form of the 14C-bearing carbon species dictates the routes of 14C migration in the engineered barrier system of a deep geological repository and the surrounding host rock and therefore determines the long-term contribution of 14C to dose release from a repository for radioactive waste. Furthermore, 14C released from a deep repository would be subject to a certain accumulation in the biosphere, given the role of carbon as main building block of any life form on earth. The contribution will illustrate Nagra's recent progress within the 14C dose assessment model chain for a L/ILW deep geological repository to address the quantification of uncertainties. The systematic approach began by incorporating a realistic best-estimate plus uncertainty description of 14C activities within the Swiss model inventory of radioactive materials assembled specifically for the purpose of the general licence application. In this context state-of-the-art activation calculations for expected reactor and decommissioning wastes as well as extensive inquiries into origin and use of 14C within medicine, research, and industry played a key role. Uncertainties were screened for relevance and main contributing factors taken into account, differentiating between the characterisation methods of the relevant waste streams. Based on detailed knowledge of the expected activity and material inventory, release rates for the release pathways through congruent and instantaneous release processes as well as resulting chemical species could be defined with confidence. Key findings and characteristics of the 14C source term will be highlighted and discussed.

Using a multi-criteria approach for a regional differentiation of the likelihood of future volcanic activity in Germany

Abstract. Future volcanic activity, which might influence the long-term stability of radioactive waste repositories, must be evaluated critically. In Germany, according to the Repository Site Selection Act (StandAG), areas in which Quaternary volcanism is either present or future volcanic activity is expected within the next 1 Ma must be excluded from the site selection process for a high-level radioactive waste repository. However, while regions with Quaternary volcanism are reasonably well known in Germany, forecasting volcanic activity in intraplate fields is difficult, as many processes and their interaction control the spatial distribution of the volcanic centres. To develop an appropriate method to forecast possible regions of volcanic activity, a semi-quantitative, multi-criteria approach is used that combines a variety of indicators including, amongst others, seismic anomalies in the mantle, gravity data, tectonic activity, sutures, ground motion, earthquakes, mantle degassing centres and the geochronological data of volcanic rocks. The used indicators provide possible information about the source of melting in the mantle, the ascent of melt through the lithosphere and the eruption process at the surface. In total, 30 indicators were defined and ranked according to their presumed relevance for the long-term prediction of volcanism, following the results of two expert surveys. Quantifiable parameters were selected for the individual indicators, and threshold values were assigned and related to levels of significance. The parameters were uniformly applied for Germany in order to enable a transparent and equal risk assessment of future volcanic activity in all of the federal states. To consider the uncertainties regarding the distribution and manifestation of parameters in addition to their mostly unknown underlying large-scale processes, buffer zones were defined in which the values of relevance decrease with increasing distance from the initial parameter extent. By combining all parameters into an overall index, the presented results allow the regional differentiation of the likelihood of future volcanic eruptions in Germany. The robustness of the results is shown by varying the weights of the used parameters in the Germany-wide risk map. Different categories for the probability of future volcanic activity can be defined and used, depending on the needs and assessment of the volcanic hazard potential.

Retention of trivalent actinides (Am, Cm) and lanthanides (Eu) by Ca feldspars

Abstract. The transport of radionuclides in the environment is a major problem for the safety assessment of radioactive waste repositories. Storage in deep geological repositories is considered a safe disposal strategy because of their ability to isolate hazardous components from the biosphere for hundreds of thousands of years. Minor actinides (i.e. Am, Cm and Np) dominate the radiotoxicity of spent nuclear fuel over geological timescales. In underground repositories, reducing conditions are expected, and therefore the trivalent oxidation state is dominant for Am and Cm as well as possibly for Pu. For investigations of the mobility of the trivalent actinides Am3+ and Cm3+, the less toxic trivalent rare earth elements, in particular Eu3+, are commonly used. Besides clay and salt, crystalline rock is considered a possible host rock for deep geological repositories. Crystalline rock (e.g. granite) consists mainly of quartz, mica and feldspar. The latter forms common aluminosilicates making up ∼60 vol. % of the earth's crust, but their sorption behaviour is not well understood, especially for the Ca-bearing members of the group. Here, we study the sorption of trivalent actinides and their rare earth element homologues on plagioclase which are Ca-bearing feldspars, quantitatively and mechanistically. Zeta potentials of various Ca feldspars show an unexpected increase at pH 4–7, which becomes more pronounced as the amount of Ca in the crystal lattice increases. This can be interpreted by assuming uptake of Al3+ and/or the precipitation of an Al phase, where Al originates from feldspar dissolution at different pH values. Nevertheless, only minor differences were found in the retention and surface speciation of Cm3+ on Ca and K feldspars (Neumann et al., 2021). Ca feldspar has a slightly higher potential to retain trivalent metal ions compared to K feldspar. An inner-sphere (IS) complex and its two hydrolysis forms have been identified on both minerals, but the hydrolysis of the IS complex is stronger in the Ca-rich mineral. A surface complexation model for Ca feldspar was developed by combining the batch sorption data and the spectroscopically identified surface complexes to describe the experimental data. These data will be the basis for the improvement of transport simulations for a reliable safety assessment of potential radioactive waste repositories in crystalline rock.

Cross-scale perspective on the Opalinus Clay: insights from the GeomInt2 project

Abstract. Understanding complex systems, such as radioactive waste repositories, involves the study of cross-scale coupled processes. We discuss some important concepts and their mutual interactions for interpreting such systems based on complementary model-based analyses at various scales (Fig. 1). These points are linked with practical examples that pertain to the hydromechanical effects and cracking of the Opalinus Clay. In the Federal Ministry of Education and Research (BMBF)-funded project “Geomechanical integrity of host and barrier rocks – experiment, modeling and analysis of discontinuities (GeomInt2)” (Cajuhi et al., 2023a in Kolditz et al., 2023), these effects have been investigated experimentally and numerically, both at laboratory and at field scales. While interpretation influences the conceptualization of experimental and/or numerical models, a clear goal definition is required to delimit the complex system. One goal statement in the context of this contribution is to explain the formation of drying cracks, where essential processes and boundary conditions must be pinned down. Monitoring data are critical for determining the correlation between experimental and numerical setups. This is achieved by quantifying and questioning the assignability of information derived from data obtained from core samples tested in the laboratory to field-scale analyses. Numerical modeling can help validate model ideas by reproducing measured data and making predictions beyond the experimentally observable range. The detailed information gained from these studies can be used for interpretation and simplification. In a recent study, we used the phase-field approach to model the formation and evolution of cracks and stated the conditions under which desiccation cracks will develop at the field scale as well as how deep they propagate into the rock (Cajuhi et al., 2023b). This information about the near field can be used to determine how detailed repository far-field models must be and, for example, whether cracks need to be taken into account. Consequently, the cross-scale study of complex systems can lead to more robust analysis results.

Climate scenarios, groundwater models, and uncertainties in long-term safety

Abstract. Assessing future climate scenarios plays a fundamental role in evaluating the long-term safety of a repository for high-level radioactive waste (HAW). Over the assessment period of 1 million years, the repository is affected by various processes and developments that must be considered in the design of the repository. Predictions are complicated by the dynamics of the climate and are subject to uncertainties. The aim of this study is to evaluate uncertainties for various climate-induced processes that are relevant for the safety assessment of a repository for HAW. This research addresses fundamental questions concerning the impact of future cold stages on different regions of Germany, the differences between past and future cold stages, the transfer of future climate developments to stylized climate models, and the uncertainties resulting from these models. The study identifies the primary climate processes that could potentially affect the evolution of a HAW repository, including the formation of glaciers or ice sheets, the development of permafrost conditions, fluctuations in sea level, erosion, and isostatic adjustment. These processes can be represented in numerical groundwater models through parameter variations. In this project, these parameter variations and the impact of climatic states on different processes will be modeled for a sensitivity study. Finally, the results will be analyzed with respect to uncertainties, and possible improvements to dealing with uncertainties will be discussed. The focus of the presentation is on the climate impacts and the implementation in the model.

How do uncertain parameters affect the analysis of host rock integrity? Methods and preliminary results for a generic clay rock site

Abstract. The thermal, hydraulic, and mechanical effects of excavating the repository mine and the thermal loading of nuclear waste on the safety of the geological barrier system are addressed in the regulations as integrity analyses. For repositories in clay rock, a modelling and assessment methodology has been developed in the ANSICHT (Methodik und Anwendungsbezug eines Sicherheits- und Nachweiskonzeptes für ein HAW-Endlager im Tonstein) project based on thermo-hydro-mechanically (THM) coupled simulations using the open-source software project OpenGeoSys and has been illustrated by its application to a generic clay rock site in northern Germany. However, the assessment can be significantly influenced by uncertainties in the model inputs, which result, on the one hand, from the limited ability to characterize the geological barrier and, on the other hand, from fundamental difficulties and inaccuracies in the measurements. Hence, the results of these established integrity analyses have to be enriched by statistical information. The developed workflow comprises different steps beginning with a realistic characterization of parameter uncertainties. A surrogate model based on mathematically sound techniques is built to capture the propagation of these uncertainties through the THM simulation and the integrity criteria evaluation. The model can then be evaluated to obtain probabilistic results for rock integrity assessment and a sensitivity analysis to identify the inputs that primarily impact the results. In this contribution, we present preliminary results based on the generic repository system from the ANSICHT project and the developed tools for stochastic analyses. This work is done as part of the BGE-funded research cluster URS (Uncertainties and Robustness with regard to the Safety of a repository for high-level radioactive waste).