Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behavior of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the containment-providing rock zone in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. BARIK is the German acronym for the research project on which this paper is based. This contribution provides an overview of the development and verification of the BARIK constitutive model, an extended Hoek–Brown model for fractured crystalline hard rock that takes into account up to three fracture systems. The model enables the consideration of the matrix and joint behavior of the rock separately, with each component having unique strength characteristics and failure criteria. These criteria are formulated such that suitable consideration of the strength-reducing properties of the respective fracture systems during barrier integrity verification is possible. The BARIK model has been implemented into two computer codes, FLAC3D and MFront for OpenGeoSys, allowing for the identification and evaluation of any inaccuracies that may arise from the use of different codes. The model enables isotropic–elastic, orthotropic–elastic, isotropic–elasto-plastic, and orthotropic–elasto-plastic calculations of the matrix, making it a valuable tool for the site selection process and for the construction and long-term safety of underground repositories. Furthermore, this poster presentation will show how the constitutive model was evaluated in relation to the dilatancy criterion and how the BARIK constitutive model's suitability for conducting an integrity assessment was validated. In conclusion, the development of BARIK is a significant step forward in the understanding and modeling of the complex material behavior of fractured crystalline hard rock. This contribution will provide insights into the development and verification of this model for the safe disposal of radioactive waste.
Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behaviour of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the “containment effective rock area” in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. This contribution provides an overview and first results of the laboratory programme, which was performed as a part of the research project. The aim was to generate a fundamental and extensive dataset for an anisotropic material used for verification and validation purposes of the newly developed constitutive model. The rock material for the test programme was “Freiberger gneiss” because of its pronounced anisotropic properties regarding deformation and strength as well as the good access to obtain a larger amount of sample material. A wide range of basic tests have been performed, e.g. determination of density and porosity, measurement of ultrasonic wave velocities to get dynamic elastic properties, Brazilian tests, and uniaxial compression tests to obtain strength data. Additionally, a large number of more complex multi-stage triaxial compression tests with examination of the post-failure region and hydromechanical coupled triaxial compression tests have been conducted or are in progress. The hydromechanical part, in particular, plays an important role in examining and quantifying the evolution of the micromechanical damage process, which changes the permeability and Biot coefficient and therefore the effective stresses inside a saturated rock material. Tests were conducted with different orientations of the structural planes in relation to the loading directions.
To rate rock salt barrier integrity, two local criteria are typically used – the dilatancy and the fluid pressure criterion. However, these local criteria are not suitable to assess the rock salt barrier as a whole. So, even if both criteria are locally exceeded, a tight rock salt barrier may be preserved if an intact rock salt envelope remains that encloses the mine openings. Thus, a non-local criterion of effective barrier thickness is proposed as a criterion for containment capability. The effective thickness of the intact rock salt barrier evolves as a function of time, taking into account present and future uncertainties. The influence of different technical concepts on barrier integrity can be assessed. The effective thickness of the intact rock salt barrier is defined as the shortest distance that a fluid must theoretically pass to intrude from the surrounding, potentially water-bearing rock into the mine openings and comprises different sections that may either be interconnected or may be intersected by one or more possible flow paths. The advantage of the use of this non-local criterion of effective thickness is its capability to describe rock salt barrier performance by a single comprehensible quantity regarding geology and the mine openings that implicitly covers the dilatancy and fluid pressure criteria.
Abstract. For the underground disposal of high-level nuclear waste in rock salt formations, the safety concept includes the backfilling of open cavities with crushed salt. For the prognosis of the sealing function of the backfill for the safe containment of the radioactive waste, it is crucial to have a comprehensive process understanding of the crushed-salt compaction behavior. The crushed-salt compaction process is influenced by internal properties (e.g., grain size, mineralogy, and moisture content) and boundary conditions (e.g., temperature, stress state, and compaction rate) and, therefore, involves several coupled thermal–hydro–mechanical (THM) processes (Hansen et al., 2014; Kröhn et al., 2017). With the paradigm shift from the limited release of radionuclides to safe containment due to the German Repository Site Selection Act passed in 2017, the importance of crushed salt as geotechnical barrier has increased, with a focus on the evolution of its hydraulic properties. Based on the knowledge gaps in the current process understanding, the “Compaction of crushed salt for safe containment” (KOMPASS) projects were initiated to improve the scientific basis behind using crushed salt for the long-term isolation of high-level nuclear waste within rock salt repositories. The efforts to improve the prediction of crushed-salt compaction begun during the first phase of the KOMPASS projects (Czaikowski et al., 2020) and were followed up in a second phase ending in June 2023. The primary achievements of the projects are as follows (Czaikowski et al., 2020; Friedenberg et al., 2022): specification of the KOMPASS reference material, an easily available and reproducible synthetic crushed-salt material, for generic investigations; development of pre-compaction methods and successful production of samples in the short term and under in situ loading conditions; formulation of an extended laboratory program addressing the isolated investigation of known relevant factors influencing the compaction behavior of crushed salt (Düsterloh et al., 2022); execution of long-term compaction tests addressing isotropic and deviatoric load changes, temperature, and compaction state; construction of a backfill body using the KOMPASS reference material in the Sondershausen mine through collaboration with the SAVER (Entwicklung eines salzgrusbasierten Versatzkonzepts unter der Option Rückholbarkeit) project (Schaarschmidt and Friedenberg, 2022); advancement of the tools for microstructure investigation methods (Svensson and Laurich, 2022); generation (first stages) of a microphysical process list combining literature research with our own findings; benchmarking of long-term compaction test for model development and optimization of various existing models as well as the development of new models; application of a virtual demonstrator (2D model representing a backfilled drift in rock salt) for the visualization of developments and the quantification of the models (Rabbel, 2022). In summary, the KOMPASS projects contributed to the reduction of uncertainties and the strengthening of the safety case for using crushed salt within rock salt repositories.
ABSTRACT Crushed salt may be used as backfill material and sealing measures of open cavities, drifts and shafts in nuclear waste repositories located in rock salt formations. The host rock salt will gradually compact the crushed over time due to salt creep. A comprehensive understanding of the crushed salt compaction process is essential, yet crushed salt compaction involves several thermal-hydraulic-mechanical (THM) coupled processes and is influenced by internal, as well as external conditions. The current understanding has some important knowledge gaps. For example, the porosity/permeability evolution, especially for the low porosity range is not known in its entireness and the calibration of numerical models is not finished yet. The KOMPASS projects were initiated with the aim to reduce these knowledge gaps and improve predictions of crushed salt behavior. This paper gives an insight in the KOMPASS projects with focus on the outcomings of the second phase. The experimental program for a detailed investigation of crushed salt compaction is presented. In the microstructural investigations, microscale indicators were found that can clearly be related to pre-compaction treatment. The numerical work showed that deficits still exists in the simulation of crushed salt compaction with the available constitutive models. INTRODUCTION Rock salt is considered as a possible host rock formation for the underground disposal of high-level nuclear waste in several countries. The safety concept for a salt repository in Germany is based on a multi-barrier system including the rock salt as geological barrier, geotechnical seals ensuring the safe containment and the waste canisters (Bertrams et al., 2020). For backfilling and sealing measures of open cavities crushed salt will be used due to its favorable properties as mined-off material, its easy availability and its lithological characteristics which guarantee a maximum compatibility with the host rock. The use of crushed salt for barriers and seals represent an important paradigm shift in repository design in Germany, since crushed salt previously served only as stabilizing backfill.
The KOMPASS project strives to improve the scientific basis behind using crushed salt for long-term isolation of high-level nuclear waste within rock salt repositories. Efforts to improve the prediction of crushed salt compaction began during the first phase of the KOMPASS project (KOMPASS-I, 2020). The second project phase (KOMPASS-II) just started in 2021. Its aim is foremost to quantify the effect of isolated experimental influencing factors on the compaction. Such influencing factors are for instance temperature, moisture or the chosen stress path. Used methods are laboratory tests, microstructural investigations and numerical simulations.
