Salt Repository Research Articles

Thermo-Hydro-Mechanical Modeling of Brine Migration in a Heated Borehole Test in Bedded Salt

AbstractThis research paper focuses on the thermo-hydro-mechanical (THM) modeling of brine migration in a heated borehole test conducted as part of the ongoing Brine Availability Test in Salt (BATS) at the Waste Isolation Pilot Plant (WIPP) in New Mexico. It is a component of the international collaboration project DECOVALEX-2023 (DEvelopment of COupled models and their VALidation against EXperiments), which aims to understand the THM processes governing brine flow in heated rock salt repositories through collaborative analysis by multiple research teams. Using the TOUGH–FLAC simulator, THM simulations were performed and compared with data from the BATS phase 1a. This experiment involved two identical horizontal-borehole arrays, one heated and one serving as a control, both equipped with sensor arrays. Analysis of measurements revealed water flow rate surges during heater power transitions, with the highest jump observed during cooling. Acoustic emission activity exhibited distinct patterns in response to heater power changes, suggesting that damage to rock salt is particularly pronounced during the cooling phase. The THM simulations successfully captured these phenomena, highlighting the significance of thermal effects, brine migration, and mechanical behavior in predicting brine availability in heated and damaged rock salt. Our modeling also revealed the critical interplay between heating and cooling-induced damage and its influence on flow properties, particularly affecting brine inflow estimation. Notably, we found that cooling-induced brine inflow spikes result from increased permeability due to tensile dilatancy. These findings have important implications for the development of robust containment strategies and enhance our understanding of the complex processes involved in repository performance.

Systematic laboratory investigations concerning the reduction of uncertainties related to the modeling of the material behavior of crushed salt

Abstract. The planned use of crushed salt as the backfill and sealing material in the context of the final disposal of radioactive waste in rock salt is justified and favored by the realization that the material properties of loosely deposited crushed salt will be transformed into the material properties of in situ rock salt in the long term by the convergence of the viscous rock salt mass. In other words, the convergence-induced compaction of crushed salt ultimately results in a nearly pore-free material, with a tightness and creep capacity corresponding to that of the ambient rock salt mass. The computational prognosis of the load-bearing and sealing behavior required for the long-term safety assessment of underground repositories requires a sufficiently proven physical modeling of the, in general, three-dimensional load bearing and permeability behavior of crushed salt. This requirement can only be met if the influencing variables on the load bearing and permeability behavior are sufficiently quantified and represented by mathematical–physical functions on the basis of suitable laboratory and field investigations. In order to eliminate or reduce the currently (still) existing scientific deficits and uncertainties concerning the general three-dimensional load bearing and permeability behavior of crushed salt, a systematized laboratory program was developed within the framework of the joint project KOMPASS (compaction of crushed salt for the safe containment), with the aim of ensuring an isolated recording of individual processes and influencing factors on the compaction behavior. Based on this, triaxial long-term compaction tests, TUC-V2 and TUC-V4, with a test duration of 750 and 180 d, respectively, were carried out within the framework of the joint project KOMPASS and of the follow-up project KOMPASS II. Both tests, but especially TUC-V2 with its 2-year duration, represent a sequence of several loading phases for separate analysis of the effects of individual influencing factors on the compaction behavior of crushed salt. By integrating several loading phases in one long-term test each, it was possible, on the one hand, to eliminate the scattering of the material behavior that would have been expected if several individual tests had been carried out as an alternative and, on the other hand, to provide metrological evidence of the range of validity of the compaction behavior as a function of individual influencing factors for a comparatively wide range of different porosities in each case. The computational reanalysis of the long-term tests opens up the possibility of validating and strengthening the currently available constitutive model approaches with regard to their ability to influence several parameters on the compaction behavior, while eliminating the scatter band of the material behavior. The improvement of the existing constitutive model approaches up to the sufficient reproduction of the measurement results equals an increase in the validation status or the credibility of the constitutive models or, vice versa, a reduction in the uncertainties remaining due to (not yet) validated model assumptions. Within the framework of the KOMPASS II joint project, a benchmark was carried out against the background outlined above for the comparative analysis of the constitutive models currently available to the project partners for depicting the compaction behavior of crushed salt with regard to their ability to computationally represent the measurement results generated within the TUC-V2 experiment. Specifically, the focus is on constitutive model functions that control the viscous compaction and shear deformation behavior and, within these functions, the influencing factors of mean stress, deviatoric stress, and temperature, which were recorded by laboratory tests for medium ranges of porosities from ≈ 16 % to very low porosities of ≈ 2.5 %. Within the framework of the continuation of the investigations planned under the acronym MEASURES (Multi-scale Experimental and numerical Analysis of crushed Salt material Used as engineered backfill for a nuclear waste REpository in rock Salt), it is intended to continue the benchmark study with a more detailed investigation of the influence of the deviatoric stress, which includes considering the data from TUC-V4. It is also intended to supplement the laboratory data base with further experiments by (a) investigations on the compaction behavior in the range of high-to-medium porosities and (b) investigations on the influence of the moisture content on the compaction capability (to be performed with KOMPASS reference material).

Compaction of crushed salt for safe containment – a summary of the KOMPASS projects

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.

Impact of the compaction behavior of crushed salt on the thermo-hydro-mechanical response of a generic salt repository for heat-generating nuclear waste

Salt formations are a promising host rock for the disposal of heat-emitting nuclear waste due to their ability to heal fractures, high thermal conductivity, and near-zero permeability. This study investigates the concept of waste emplacement in salt formations by placing waste packages in drifts and backfilling them with crushed salt. The aim is to minimize fracturing and restore the initial properties of the host rock.One of the key issues in assessing the safety and performance of nuclear waste repositories in salt is determining the reconsolidation rate of the crushed salt backfill, which has been traditionally predicted using the C-WIPP model. However, this model was calibrated using data from uniaxial oedometer lab tests, where specimens with high initial porosity values have been compacted to medium porosity values. The observed compaction behavior has then been extrapolated to the low porosity range, leading to a prediction of rapid full reconsolidation of the backfill in less than 20 years. In contrast, the new C-WIPP/TUC model used in this study was calibrated for the medium porosity range, between 17% and 8%, based on more suitable triaxial lab tests. Using coupled Thermo-Hydro-Mechanical (THM) simulations with the TOUGH-FLAC simulator, it was found that the C-WIPP/TUC model predicts a slower reconsolidation rate of the backfill compared to the C-WIPP model, with an average porosity of the backfill remaining roughly 10 times that of natural salt after 10,000 years. The effects of the relatively slow reconsolidation of the crushed salt backfill on the THM behavior of the repository are also examined, highlighting the importance of accurately capturing the behavior of crushed salt for the study of the long-term integrity of a nuclear waste repository in salt.

Thermal Assessment of Repository Footprint Requirement for Advanced Fuel Cycles

Decay heat is an important constraint for repository size and design because it can drive processes that affect performance and compromise critical materials. This paper investigates how compliance with repository thermal limits is affected by three decay heat management strategies: waste package loading, waste package spacing, and surface storage time. In particular, this paper focuses on how repository area, a result of package spacing, is impacted by waste loading and surface storage time. A two-part analytical heat transfer model is presented and executed iteratively to determine the minimum allowable repository area. The analysis considers three generic close-contact repository designs along with the wastes generated from the 40 fuel cycle analysis examples used to generate metric data for the Fuel Cycle Evaluation and Screening study. Detailed results are presented for two fuel cycles: the once-through use of low-enriched uranium fuel in light water reactors and the continuous recycling of U and Pu in sodium fast reactors. Two limits for surface storage time are identified: the time required for disposal to be possible at all and the time at which further surface storage time yields no gains. The impact of waste loading is also diminished with increasing surface storage time. In general, the generic salt repository is most flexible to accept high-heat-generating wastes with less surface storage time than other repository environments. Limited-recycle fuel cycles are shown to pose a disposal challenge because of elevated, sustained decay heat generation in the waste.

Numerical investigation of heating and cooling-induced damage and brine migration in geologic rock salt : Insights from coupled THM modeling of a controlled block scale experiment

Controlling brine flow is important for the safe disposal of radioactive waste in salt rocks. Thermal and mechanical processes can have a significant impact on brine flow, although this has never been thoroughly investigated. In this study, we conduct fully coupled THM modeling for analyzing brine flow in rock salt, considering non-isothermal two-phase flow through deformable porous media. To rigorously represent rock salt behavior, we incorporate suitable phenomenological models for creep and shear and tensile-induced dilatancy/damage, and their effect on the flow properties. To validate such a complex model, we use it to analyze an experiment on a meter-scale salt block subjected to multistage heating and cooling under controlled laboratory conditions. The experimental data and our model predictions of temperature and brine inflow show good agreement. Our modeling shows that it is important to consider the coupling between the heating- and cooling-induced damage and the flow properties for rigorously estimating brine inflow. Based on our modeling, a cooling-induced brine inflow spike is caused by a permeability increase due to tensile dilatancy. Thus, our modeling will be useful for a proper design of a cooling phase in the salt repositories to avoid or minimize damage and the resulting brine inflow.

Password Management Against Versatile Attacks

Passwords play a very critical role in authentication, information and security of web applications. Securing and managing passwords is a significant problem for most of the people in the modern world. As the number of accounts increases managing and securing of the passwords gets complicated. As a consequence, many users tend to adopt weak password management schemes which can significantly reduce the security of the systems. Prior to the research it confirms that security experts recommend Password managers to help users generate, store and enter strong unique passwords. In this research, after checking Password habits of users accounts, it allowed us to understand the password habits of users according to sensitive data of their account, and also revealed some critical issues associated with password choice. So we propose PasswordAgent(password hashing mechanism).It utilizes both salt repository(salt-sequence of randomly generated bytes) and a browser plug in to secure logins with strong passwords. Password hashing is a technique that allows user to remember simple low entropy passwords and have them hashed to create high-entropy secure passwords. PasswordAgent is less vulnerable to offline attacks, and it provides stronger protection against password theft.

Methodology of structured development and validation of multiphysical constitutive models using the example of crushed salt compaction under 3D THM load conditions

Abstract. The conceptual plans for the final underground disposal of radioactive waste in rock salt formations are based on extensive backfilling with crushed salt of the residual cavities left after waste deposition. It is therefore of particular importance for the historical and prognostic analysis of the load-bearing behavior and impermeability of a final repository in rock salt to demonstrate that compaction of the crushed salt backfill, which progresses over time, is suitable to seal the breaches in the geological barrier created during the underground excavation of the cavity in the long term such that safe containment of the waste is ensured. Relevant investigations on the thermal-hydraulic-mechanical (THM) behavior of crushed salt revealed that the constitutive models for the description of crushed salt compaction, which have regularly been based on the evaluation of oedometer tests, are not suitable for a sufficiently realistic representation of the essentially three-dimensional stress-strain behavior of crushed salt depending on the external load in space and time. Evidence for the above statement lies in particular in the fact that even when standardized mixtures of crushed salt are used, a computational reanalysis of compaction tests using a standardized set of parameters has hitherto been unsuccessful when different loading scenarios were specified for these laboratory tests. This means that deformations and porosities measured in the context of one individual laboratory tests can currently only be reanalyzed in sufficient quantity, irrespective of the choice of constitutive model, if the model parameters are determined in relation to this test. As a result, it must be stated that, on the one hand, the compaction behavior of crushed salt in space and time is not yet definitively understood, while, on the other hand, to ensure reliable, robust and sufficiently realistic statements to be made on compaction behavior, and thus to prove the safe containment of radioactive waste in rock salt, the availability of extensive systematically and sufficiently validated constitutive models is indispensable. This presentation introduces a methodological approach for the systematic and structured development and validation of multiphysical constitutive models, an approach that has meanwhile been successfully tested many times. The practical application of this methodology will be presented here using the example of a constitutive model that takes into account the triaxial stress-strain behavior of crushed salt. The individual development and validation steps are documented for the crushed salt model, EXPO-COM, newly developed at the Chair for Waste Disposal Technologies and Geomechanics. Validation of the constitutive model is performed by means of a back-analysis of triaxial long-term crushed salt compaction tests as follows: Test TK-031 of the German Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe, BGR) for isotropic load conditions Tests V1 (dry), V2 (w=0.1 %), and V3 (wet) of the German Society for Plant and Reactor Safety (Gesellschaft für Anlagen- und Reaktorsicherheit gGmbH, GRS) for different stresses and temperature levels as well as humidity Test TUC_V2 of the Clausthal University of Technology (TUC) for isotropic and deviatoric stress conditions. The TUC_V2 test characterizes, in the context of the methodology for the structured development and validation of multiphysical constitutive models, an innovative test method geared towards constitutive model development, in which the loading boundary conditions specified in the test guarantee the isolated analysis of individual factors influencing compaction behavior (Fig. 1). A description of the tests and test techniques that are still required for the full development and validation of the EXPO-COM constitutive model planned as part of the KOMPASS II research project is given together with a description of methodological guidelines relating to requirements on reliability, functionality, practicability, and validity ranges of the EXPO-COM constitutive model (Fig. 2). As a result of the subsequently possible comparison of experimentally validated and not yet validated dependencies or process variables, a validation status is defined for the constitutive model EXPO-COM. This validation status shows which factors influencing the THM-coupled material behavior of crushed salt are currently sufficiently realistically taken into account, and which influencing factors cannot yet be validated by the constitutive model. The main objectives of the tests to be carried out as part of the KOMPASS II research project include: Continued validation based on the systematized database to be created in KOMPASS II. Testing of the constitutive model in the context of numerical analyses of the predictive quality and numerical stability of the constitutive model for in situ relevant stress boundary conditions, prediction times and material properties.

Deformation mechanisms and their microstructural indicators in the compaction of crushed salt as a geotechnical barrier

Abstract. Following 30 years of research, it is common sense that crushed salt is the most suitable geotechnical material for encapsulating radioactive waste in a rock salt repository (e.g., Chaikowski et al., 2020). After emplacement, it provides sufficient permeability to allow outflow of unwanted canister-corrosion gases. In the long term, however, when it becomes compacted by converging cavity walls, it safely hinders any fluid flow from and to the waste. Hence, it is essential to know the evolution of (1) the material's key parameters during compaction, such as porosity and permeability, backfill resistance and viscosity; (2) the material's response to environmental controls, such as temperature, humidity, and stress; and (3) the material's long-term rheology. Here we align microstructural deformation indicators with physical processes that underlie compaction (Mills et al., 2018a). We strive to identify and – where feasible – to quantify the dominant deformation mechanisms (Blenkinsop, 2002; Jackson and Hudec, 2017). As a preliminary result, we show that the abundancy of deformation indicators increases with increasing compaction state. In early compaction, we observe more brittle mechanisms, such as grain fracturing (Fig. 1a) and cataclastic flow. At later stages, especially in the presence of moisture, plastic deformation overtakes. Therein, we observe an increased presence of indicators for pressure solution precipitation (grain boundary seams) and dislocation creep (subgrain formation, Fig. 1b), with progressing deformation. In our upcoming work, we aim at linking the observed indicators to environmental controls, such as moisture content, temperature, and strain rate by applying our approach to larger suits of samples compacted under best-known controlled conditions. Final goal is the joint interpretation with findings from in situ-compacted material (Mills et al., 2018b). Do lab tests mimic in situ processes of crushed salt compaction adequately? Can we learn how to do better by means of microstructural investigations?

Over 30 years of research on crushed salt as a barrier material: fundamental findings and open questions

Abstract. Repositories for high-level radioactive waste in geological formations require knowledge on thermal, mechanical and fluid transport properties of the whole repository system, including the engineered barriers and backfill materials. For about 30 years, crushed salt has been considered the most suitable geotechnical barrier material to backfill cavities and encapsulate radioactive waste in rock salt repository sites (e.g., Czaikowski et al., 2020). Over time, when the surrounding cavity walls converge by the creep of salt, it can become strongly compacted and safely encapsulates radioactive waste from any fluid flow. Hence, crushed salt has been characterized in detail for its physical material properties and its response to environmental controls (stress, temperature and moisture). This characterisation provides a basis for long-term numerical simulations (e.g., Liu et al., 2018), which verify so-called safety cases in radioactive waste disposal. Displacement-controlled oedometric compaction tests mimic the long-term in situ behaviour of crushed salt. The tests show that it can be compacted to a state comprising physical rock properties similar to natural rock salt. In general, compaction is easier with an increase in humidity and temperature (e.g., Stührenberg, 2007; Kröhn, et al., 2017). Triaxial test series address the compactions' response to differing confining pressures and help to identify generalized constitutive equations for crushed salt. Both BGR procedures, the oedometric and the triaxial compaction, are verified by the German accreditation body (DAkkS). Figure 1 illustrates the history of oedometric tests at the BGR laboratory since 1993, which examined crushed salt from various origins and differing temperature conditions. Most tests focused on material from the Asse mine, revealing the compactions' response to the materials' humidity and to brine flow. Moreover, systematic test series with synthetic grain size distributions and bentonite additives provided a basis for barrier material design. More recent tests on bedded salt formations (e.g., Teutschenthal and Sondershausen mines) allow the differentiation from characteristics from domal salt deposits (e.g. Gorleben). The current research continues the history of oedometric and triaxial tests, but has a new focus on late compaction stages with marginal remaining porosities (<5 %). The approach of systematic material characterization under best-controlled conditions essentially benefits from the international research collaboration in the KOMPASS project (Czaikowski et al., 2020). The aim of its current phase two is to synthetically generate, identify and quantify dominant grain-scale deformation processes in response to changes in environmental controls. Subsequently, these laboratory results will be embedded in numerical models on the long-term in situ rheology of crushed salt.

"СЛУЖБЫ" КУЗНЕЦКИХ ГОРОДОВЫХ КАЗАКОВ В КОНЦЕ XVIII - НАЧАЛЕ XIX В. НА ТЕРРИТОРИИ БИЙСКОГО УЕЗДА

In the article the author used “Monthly Reports and Lists of Kuznetsk Town-Service Cossack Party Personnel Conditions” for 1812 to tell about missions of its members on the territory of the Byisk district. In this period in Western Siberia there were several fortified defense lines. They intended for protection the state territory from outward enemy. The district town Kuznetsk was a part of the structure of Siberian fortified defense lines. It had the permanent town-service Cossack party nearly from the early 19th century. By the facts according to 1812 the most members of the Cossack party did their service in the Biysk district. The Cossacks guarded the salt extraction territory on the territory of numerous salt lakes. Kuznetsk town-service Cossacks also did their service in towns and defense points of the district. Their responsibilities there were connected with guarding salt and other foodstuffs storages. By the facts according to 1812 the missions of the Kuznetsk Cossacks to Biysk district were continuous and lasted very often 5 and more years. There were some cases when Cossacks spent more than 15 years in such missions. Keywords: Siberian fortified defense lines, line-service and town-service Cossacks, near and distant missions, salt lakes, provision and salt repositories, salt storekeepers

Potential for biocolloid transport of cesium at high ionic strength

In subsurface repositories, active bacterial populations may directly influence the fate and transport of radionuclides including in salt repository systems like the Waste Isolation Pilot Plant in Carlsbad, NM. This research quantified the potential for transport and interaction between Chromohalobacter sp. and Cs in a high ionic strength system (2.6 M NaCl) containing natural minerals. Mini-column experiments showed that Chromohalobacter moved nearly un-retarded under these conditions and that there was neither association of Cs with microbes nor dolomite despite changes in bacterial metabolic phases. Growth batch experiments that monitored the potential uptake of Cs into the microbes confirmed results in column experiments where intracellular uptake of Cs by Chromohalobacter was not observed. These results show that Cs may be highly mobile if released in high ionic strength systems and/or carbonate minerals with negligible inhibition by these microbes.

Heat‐Generating Nuclear Waste in Salt: Field Testing and Simulation

Core Ideas A field‐scale experiment and numerical simulations confirm salt backfill behavior. Simulations closely match temperature around and under the piled salt backfill. Results indicate limited dissolution–precipitation reactions around the heat source. Alteration of backfill is unlikely if the drift is allowed to dry before emplacement. Investigations relating to in‐drift disposal of heat‐generating nuclear waste in salt have raised questions about heat–brine interactions in the unsaturated run‐of‐mine (RoM) salt pile used as backfill. These interactions have the potential to change the structure of the RoM salt surrounding the canister, possibly altering long‐term containment of the source. An experiment is in progress at the Waste Isolation Pilot Plant (WIPP), New Mexico, in which a heated canister was placed on the floor of an open drift, covered in a pile of RoM salt, and energized with 1000 W. Temperature in the RoM salt pile had stabilized after about 15 d, allowing evaluation of the heat‐up period of the ongoing experiment. Using a multiphase porous flow simulator that has been modified to handle salt‐specific coupled processes, we examined coupled thermal–hydrological–chemical behavior in the RoM salt pile. Our simulations suggest that for the relatively dry cases examined, porosity changes within RoM salt in a generic salt repository are likely to be minor in the period between waste emplacement and plastic closure of the drift. The primary sensitivity for porosity change is to the early moisture content of the RoM salt used to cover the canister. Secondary influences include moisture availability from the disturbed rock zone (DRZ) surrounding the drift and the capillary pressure ratio between the DRZ and the RoM salt. Early changes in porosity and permeability may be affected by moisture content, but this was not observed in the test. Such changes would be most likely to occur when using damp RoM salt or if waste is emplaced in a drift immediately following opening of the drift before evaporative dewatering of the drift walls occurs.

Coupled Processes Modeling in Rock Salt and Crushed Salt Including Halite Solubility Constraints: Application to Disposal of Heat-Generating Nuclear Waste

This paper presents numerical modeling of coupled thermal, hydraulic and mechanical processes in rock salt and crushed salt considering halite solubility constraints. The TOUGH-FLAC simulator is used, with a recently enhanced Equation-Of-State module that includes the thermodynamic properties of aqueous fluids of variable salinity. Laboratory and field scale tests performed on rock salt and crushed salt under temperature gradients are modeled first to evaluate the capabilities of the simulator to reproduce important features, such as porosity changes induced by halite dissolution/precipitation, and brine and heat migration. Since the results are quite satisfactory, the simulator is used to predict the long-term response of a generic salt repository for heat-generating nuclear waste. To evaluate the impacts of halite solubility on the predictions, two simulations that respectively consider or neglect solubility constraints are performed. In the scenario studied, the results are not significantly affected by dissolution/precipitation, and only some differences are observed due to changes in porosity, but the dominating processes remain the same. With the new provisions, TOUGH-FLAC is more complete in terms of processes occurring around a heat-releasing nuclear waste package and can therefore provide more accurate predictions of the long-term performance of a nuclear waste repository in salt formations.

Magmatic pulse driven by sea-level changes associated with the Messinian salinity crisis.

Between 5 and 6 million years ago, during the so-called Messinian salinity crisis, the Mediterranean basin became a giant salt repository. The possibility of abrupt and kilometre-scale sea-level changes during this extreme event is debated. Messinian evaporites could signify either deep- or shallow-marine deposits, and ubiquitous erosional surfaces could indicate either subaerial or submarine features. Significant and fast reductions in sea level unload the lithosphere, which can increase the production and eruption of magma. Here we calculate variations in surface load associated with the Messinian salinity crisis and compile the available time constraints for pan-Mediterranean magmatism. We show that scenarios involving a kilometre-scale drawdown of sea level imply a phase of net overall lithospheric unloading at a time that appears synchronous with a magmatic pulse from the pan-Mediterranean igneous provinces. We verify the viability of a mechanistic link between unloading and magmatism using numerical modelling of decompression partial mantle melting and dike formation in response to surface load variations. We conclude that the Mediterranean magmatic record provides an independent validation of the controversial kilometre-scale evaporative drawdown and sheds new light on the sensitivity of magmatic systems to the surface forcing.

Enthalpies of formation of polyhalite: A mineral relevant to salt repository

Polyhalite is an important coexisting mineral with halite in salt repositories for nuclear waste disposal, such as Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The thermal stability of this mineral is a key knowledge in evaluating the integrity of a salt repository in the long term, as water may release due to thermal decomposition of polyhalite. Previous studies on structural evolution of polyhalite at elevated temperatures laid the basis for detailed calorimetric measurements. Using high-temperature oxide-melt drop-solution calorimetry at 975K with sodium molybdate as the solvent, we have determined the standard enthalpies of formation from constituent sulfates (ΔH°f,sul), oxides (ΔH°f,ox) and elements (ΔH°f,ele) of a polyhalite sample with the composition of K2Ca2Mg(SO4)4·1.95H2O from the Salado formation at the WIPP site. The obtained results are: ΔH°f,sul=−152.5±5.3kJ/mol, ΔH°f,ox=−1926.1±10.5kJ/mol, and ΔH°f,ele=−6301.2±9.9kJ/mol. Furthermore, based on the estimated formation entropies of polyhalite, its standard Gibbs free energy of formation has been derived to be in the range of −5715.3±9.9kJ/mol to −5739.3±9.9kJ/mol. These determined thermodynamic properties provide fundamental parameters for modeling the stability behavior of polyhalite in salt repositories.

Investigation on the Permeability Evolution of Gypsum Interlayer Under High Temperature and Triaxial Pressure

The permeability of the surrounding rock is a critical parameter for the designing and assessment of radioactive waste disposal repositories in the rock salt. Generally, in the locations that are chosen for radioactive waste storage, the bedded rock salt is a sedimentary rock that contains NaCl and Na2SO4. Most likely, there are also layers of gypsum \(( {\text{CaSO}}_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O)}}\) present in the salt deposit. Radioactive wastes emit a large amount of heat and hydrogen during the process of disposal, which may result in thermal damage of the surrounding rocks and cause a great change in their permeability and tightness. Therefore, it is necessary to investigate the permeability evolution of the gypsum interlayer under high temperature and high pressure in order to evaluate the tightness and security of the nuclear waste repositories in bedded rock salt. In this study, a self-designed rock triaxial testing system by which high temperature and pressure can be applied is used; the μCT225kVFCB micro-CT system is also employed to investigate the permeability and microstructure of gypsum specimens under a constant hydrostatic pressure of 25 MPa, an increasing temperature (ranging from 20 to 650 °C), and a variable inlet gas pressure (1, 2, 4, 6 MPa). The experimental results show: (a) the maximum permeability measured during the whole experiment is less than 10−17 m2, which indicates that the gypsum interlayer has low permeability under high temperature and pressure that meet the requirements for radioactive waste repository. (b) Under the same temperature, the permeability of the gypsum specimen decreases at the beginning and then increases as the pore pressure elevates. When the inlet gas pressure is between 0 and 2 MPa, the Klinkenberg effect is very pronounced. Then, as the pore pressure increases, the movement behavior of gas molecules gradually changes from free motion to forced directional motion. So the role of free movement of gas molecules gradually reduced, which eventually leads to a decrease in permeability. When the inlet gas pressure is between 2 and 6 MPa, the Klinkenberg effect dribbles away, and the gas flow gradually obeys to the Darcy’s law. Hence, the permeability increased with the increase in inlet gas pressure. (c) The curve of permeability versus temperature is divided into five stages based on its gradient. In the temperature range of 20–100 °C, the permeability of gypsum decreased slowly when the temperature decreased. From 100 to 200 °C, the permeability of gypsum increased dramatically when the temperature increased. However, a dramatic increase in permeability was observed from 200 to 450 °C. Subsequently, in the temperature range of 450–550 °C, due to closure of pores and fractures, the permeability of the specimens slowly lessened when the temperature increased. From 550 to 650 °C, the permeability of gypsum slightly increased when the temperature increased; (d) the micro-cracks and porosity obtained from the CT images show a high degree of consistency to the permeability evolution; (e) when compared to the permeability evolutions of sandstone, granite, and lignite, gypsum exhibits a stable evolution trend of permeability and has a much greater threshold temperature when its permeability increases sharply. The results of the paper may provide essential and valuable references for the design and construction of high-level radioactive wastes repository in bedded salt rock containing gypsum interlayers.

Deformation-assisted fluid percolation in rock salt.

Deep geological storage sites for nuclear waste are commonly located in rock salt to ensure hydrological isolation from groundwater. The low permeability of static rock salt is due to a percolation threshold. However, deformation may be able to overcome this threshold and allow fluid flow. We confirm the percolation threshold in static experiments on synthetic salt samples with x-ray microtomography. We then analyze wells penetrating salt deposits in the Gulf of Mexico. The observed hydrocarbon distributions in rock salt require that percolation occurred at porosities considerably below the static threshold due to deformation-assisted percolation. Therefore, the design of nuclear waste repositories in salt should guard against deformation-driven fluid percolation. In general, static percolation thresholds may not always limit fluid flow in deforming environments.

Mechanical Response and Microprocesses of Reconsolidating Crushed Salt at Elevated Temperature

Design, analysis and performance assessment of potential salt repositories for heat-generating nuclear waste require knowledge of thermal, mechanical, and fluid transport properties of reconsolidating granular salt. To inform salt repository evaluations, we have undertaken an experimental program to determine Bulk and Young’s moduli and Poisson’s ratio of reconsolidated granular salt as a function of porosity and temperature and to establish the deformational processes by which the salt reconsolidates. Tests were conducted at 100, 175, and 250 °C. In hydrostatic tests, confining pressure is increased to 20 MPa with periodic unload/reload loops to determine K. Volume strain increases with increasing temperature. In shear tests at 2.5 and 5 MPa confining pressure, after confining pressure is applied, the crushed salt is subjected to a differential stress, with periodic unload/reload loops to determine E and ν. At predetermined differential stress levels the stress is held constant and the salt consolidates. Displacement gages mounted on the samples show little lateral deformation until the samples reach a porosity of ~10 %. Interestingly, vapor is vented only for 250 °C tests and condenses at the vent port. It is hypothesized that the brine originates from fluid inclusions, which were made accessible by heating and intragranular deformational processes including decrepitation. Identification and documentation of consolidation processes are inferred from optical and scanning electron microstructural observations. Densification at low porosity is enhanced by water film on grain boundaries that enables solution-precipitation phenomena.

Thermal and physical properties of reconsolidated crushed rock salt as a function of porosity and temperature

Crushed salt can be used as backfill to bury and conduct heat away from radioactive waste in salt repositories. As the crushed salt compacts during reconsolidation, its thermal, mechanical and hydrologic properties will change in a manner related to the porosity. Measurements of crushed salt thermal properties are conducted to evaluate such relationships. A simple mixture theory model is presented to predict thermal conductivity of consolidating salt in repository conditions. Experimental work was completed to evaluate the model by measuring thermal conductivity, thermal diffusivity and specific heat of crushed salt as a function of porosity and temperature. Sample porosity ranged from 0 to 46 %, and measurements were made at ambient pressure, from room temperature to 300 °C. These are the temperature conditions expected in a radioactive waste storage facility. Crushed salt thermal conductivity decreases with increasing porosity and temperature. Thermal diffusivity showed little porosity dependence but decreased with increasing temperature. Specific heat also shows little porosity dependence but increases with increasing temperature. Fracture porosity in deformed bedded salt cores appears to reduce thermal conductivity more dramatically than inter- and intra-granular porosity in consolidated salt. A long-term effort to dry crushed salt at high temperatures resulted in a 0.48 weight-percent loss of water that had resided at grain boundaries and in intra-granular fluid inclusions. While this loss does not significantly affect thermal properties, the release of this water volume could impact the mechanical response of the reconsolidating salt and host rock.