Heat-generating Radioactive Waste Research Articles

Numerical assessment of the time-dependent behaviour of a tunnel with a yielding support system for a potential repository in claystone

For the disposal of heat-generating radioactive waste and spent fuel in claystone formations, support structures for underground excavations are essential for the operational safety of geological disposal facilities (GDF). Claystone formations of moderate strength, at considerable depths, are characterised by their squeezing, creeping, and, sometimes, swelling behaviour that results in continuous tunnel convergence over time. Under these conditions, a rigid support system can be subjected to very high loads, requiring a considerable thickness and/or high-performance concretes. Therefore, yielding support systems are being currently investigated as a promising alternative for GDF. This work presents a numerical study of the behaviour of a tunnel in a claystone formation with a yielding support system. The use of a compressible mortar between the rock and the lining is assumed, as a means of reducing loads and mitigating the effects of creep deformations. A key aspect of the analyses is that the host rock is characterised by a constitutive model that includes a number of features that are relevant for the satisfactory description of the hydro-mechanical behaviour of stiff clayey materials, such as mechanical anisotropy, creep, strain softening, and its ability to simulate localised deformations through a nonlocal regularisation. An elastoplastic constitutive model was also developed to represent the behaviour of the compressible mortar. Results provide relevant insights into the performance of the adopted yielding support system, particularly regarding the effect of time-dependent deformations and the additional relaxation of the rock on the fractured zone near the excavation.

BARIK: an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

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.

BARIK: laboratory programme within the framework of the development of an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

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.

Compositional-structural characterization of the Opalinus Clay and Passwang Formation: New insights from Rietveld refinement (Mont Terri URL, Switzerland)

Middle Jurassic Opalinus Clay (OPA) in Switzerland and Southern Germany is considered as host rock for the disposal of heat-generating radioactive waste. For the present study, core samples from the Mont Terri rock laboratory (Switzerland) were investigated using a facies-based approach including mineralogical and geochemical analyses. Special focus was on the analysis of the degree of ordering of irregular illite-smectite interstratified clay minerals (I-S) responsible for sorption of radionuclides and swelling properties of the rocks.The investigations support the classification of OPA into five main facies (instead of the established threefold division) and into further subfacies. The facies concept was extended to the Passwang Formation (PWF). The amount of clay fraction varies in samples from the different facies, which is consistent with variations in cation exchange capacity (CEC). X-ray diffraction (XRD) analyses of the <2 μm fraction revealed a homogenous composition of the main constituents, namely R1-ordered I-S, kaolinite, and illite. Crystal structure based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified illite-smectite minerals for all investigated facies. The I-S-phase can be described with the R1 model, which is together with the amount of illitic layers in the I-S an indicator for a maximum burial temperature between 90 and 105°C. The amount of illitic layer in the I-S varies from 73%–85% for all refinements and ordering types, and in all OPA and PW facies and subfacies. This, together with the homogenous composition of the clay size fraction points to (1) a homogenous overprint of the clay assemblage during burial and/or (2) a uniform siliciclastic sedimentary supply throughout the sedimentation period of the OPA. The results of this study highlight the need to combine lithofacial studies with mineralogical investigations in order to adequately capture the variability of clay-rich host rocks under consideration for radioactive waste disposal.

Colloidal silica as a grouting material for the temporary encapsulation of heat-generating radioactive waste during removal and transport operations: A proof of concept

Hazardous nuclear waste is produced at all stages during the nuclear fuel cycle. The removal operations of nuclear waste from nuclear reactors and/or storage facilities, such as spent fuel pools and storage silos, pose a hazard for the workforce and the environment, due to the potential release of radioactive particulates, and loss of radioactive debris. The development of innovative techniques to address this issue is desirable. A potential technology to inhibit particulate release during nuclear waste removal and transport operations is colloidal silica grouting. Colloidal silica is an aqueous suspension of silica (SiO2) nanoparticles, able to provide immobilisation of particulates within a hydrogel matrix. In this study, an experimental investigation was carried out to simulate colloidal silica grouting operations around objects at temperatures of 60°C and 120°C, to simulate radioactive waste in standard storage conditions, and during loss of cooling/loss of coolant accident scenarios. The results of the experimental campaign confirm the suitability of colloidal silica to safely remove and transport heat-generating radioactive waste. Critical parameters for designing the silica grout mix, in order to optimise the performance of the hydrogel upon exposure to temperature in different scenarios, are identified and discussed.

Предварительные оценки параметров инженерных барьеров в концепции ПГЗРО с горизонтальным размещением упаковок РАО в контейнерах с медным покрытием

The paper discusses a DGR concept providing for heat-generating radioactive waste (RW) disposal in relatively thinwalled steel containers with a copper coating in packages of compacted bentonite blocks installed into horizontal drifts. The study assumes that this DGR layout should provide safe disposal of historical RW Class 1 with relatively low activity and heat generation levels. The proposed concept imposes appropriate restrictions on the bentonite buffer, among which the paper quantitatively considers the thickness varying depending on the residual RW heat release, the minimum bulk density, as well as the content and type of montmorillonite. For further assessment purposes, the paper proposes to consider bentonite blocks made of compacted bentonite from Russian deposits with a predominant content of sodium montmorillonite characterized with minimum required bulk density. The authors also proposed to fill the gaps between RW packages made of compacted bentonite and drifts with pellets. Such bentonite pellets should have maximum achievable bulk density and a predominant content of calcium montmorillonite.

Модель термомиграции жидкого включения в монокристалле галитов в однородном поле градиента температуры

The paper focuses on an evolution model developed for a liquid inclusion in halites under a temperature gradient field, taking into account surface phenomena. To demonstrate the safety of heat-generating radioactive waste disposal in salt rocks (halites), thermally induced migration of liquid inclusions should be studied. Based on the study, a software package was developed providing numerical implementation of this model. The diffusion equation with initial and boundary conditions was approximated via the finite element method on triangular unstructured mobile grids. In case of weak formulation, the equations were discretized and solved in the FEniCS software. The study showed good agreement between the calculations and the experimental data.

Building the bridge between safety requirements and numerical modeling: an example considering crack development of Opalinus clay in laboratory and field scales

Abstract. Salt, crystalline and clay formations are under discussion as potential host rocks for storage of heat-generating radioactive waste. Each of these rocks has a different structure and composition, and consequently a different material behavior. The latter needs to be studied and evaluated with respect to the main aim: to find a place to store the waste in a safe and sustainable manner. Several requirements in the context of the safety of a repository need to be fulfilled, concerning the long-term as well as the operational phase. One key point in this matter is the integrity, which refers to retention of the isolating rock zone's containment capabilities. With the focus on some experimental and numerical investigations on the excavation influenced near-field behavior of Opalinus clay (OPA), this contribution aims to illustrate an example for the role of numerical modeling in safety assessment. Once, e.g. anthropogenic action such as excavation starts, the natural state of equilibrium in the formation is disturbed. Trying to restore it, the rock deforms (convergence) and/or releases energy in other ways such as cracking. This could lead to loss of integrity since crack nucleation and propagation can affect the mechanical stability and create paths to transport contaminants. During operation in the excavated rock, environmental changes, e.g. temperature and humidity, further affect its behavior. The understanding of these dynamic phenomena ideally needs to occur at the in situ scale; however, performing an experiment in the spatial and time scales of interest is not always possible. For this reason, the in situ problem needs to be formulated, abstracted and mathematically modeled. The interpretation of the results must take place with simplifying assumptions and complementary laboratory scale experiments can be used to improve understanding of the system. The real problem is approached stepwise, each step associated to the size of the model and its complexity. The gradually obtained knowledge is necessary to achieve a better understanding of the process and to evaluate the capacities and limitations of the models. This contribution aims at showing the basic practical steps for numerical modeling with particular focus on the preparation and interpretation of the models and results, e.g. model calibration, verification and validation. As an example, the OPA at the Mont Terri site is chosen. The material parameters are obtained either experimentally or from the literature. We choose and perform laboratory scale simulations that are related to nearly the same mechanism as in the in situ scale. To have a first impression on the latter, a simplified, large-scale numerical model is prepared. The mechanism in study is drying and wetting, which is associated with shrinkage and swelling. We analyze the pore pressure and stress development in both scales. Thus, hydraulic mechanically coupled approaches are essential. The concept of effective stress is used, which combines the contributions of the solid and fluid phases (gas and liquid). In the current modeling approach, the gas pressure remains constant (atmospheric pressure) and during drying, the liquid pressure induces capillary pressure development and decrease of saturation. The laboratory scale simulation is important to evaluate the model of choice and to assess potential numerical problems. Furthermore, it can be used to perform a sensitivity study of material and numerical parameters. This step is necessary during the development or extension of numerical models as well as to evaluate their applicability on new research questions. The simplified in situ scale numerical model is then extended. In this phase the numerical model is evaluated once again, especially with respect to its complexity. Furthermore, specific questions related to this scale are posed: overall behavior of the rock, influence of the excavation, seasonal and long-term effects. In this contribution we deal with the long-term cyclic deformation (CD-A) experiment. The CD-A experiment has been taking place in the Mont Terri Rock Laboratory since October 2019. It consists of twin niches, a closed and an open niche, subjected to either high air humidity or seasonal humidity changes leading to saturation/desaturation during summer/winter in the OPA, respectively. Several parameters are periodically or continuously measured, including relative air humidity, convergence and crack development. We attempt to transfer the knowledge and numerical models developed in the small scale to the large scale and to evaluate the possibilities and limitations of the chosen approaches by comparing the numerical and experimental results.

An interdisciplinary view of the long-term evolution of repository systems across scales: the iCROSS project

Abstract. The interdisciplinary project “Integrity of nuclear waste repository systems – Cross-scale system understanding and analysis (iCROSS)” combines research competencies of Helmholtz scientists related to the topics of nuclear, geosciences, biosciences and environmental simulations in collaborations overarching the research fields energy and earth and environment. The focus is to understand and analyze close-to-real long-term evolutionary pathways of radioactive waste repositories across nanoscales to repository scales. The project is subdivided into work packages dealing with laboratory studies, field experiments in underground research laboratories (URLs), advanced modelling studies and the integration and alignment of data and information using virtual reality methods. In this sense, the project structure aims at a holistic view on relevant processes across scales in order to comprehensively simulate potential repository evolutions. Within the multi-barrier system of a repository for heat-generating radioactive waste, a number of complex reactions proceed, including dissolution, redox processes, biochemical reactions, gas evolution and solid/liquid interface and (co)precipitation reactions. At the same time, thermal and external mechanical stress has an impact on the conditions in a deep geological repository. All those processes are highly coupled, with multiple interdependencies on various scales and have a strong impact on radionuclide mobility and retention. In recent years, substantial progress was achieved in describing coupled thermal-hydro-mechanical-chemical-biological (THM/CB) processes in numerical simulations. A realistic and concise description of these coupled processes on different time and spatial scales is, at present, a largely unresolved scientific and computational challenge. The close interaction of experimental and simulation teams aims at a more accurate quantification and assessment of processes and thus, the reduction of uncertainties and of conservative assumptions and eventually to a close-to-real perception of the repository evolution. One focus of iCROSS is directed to relevant processes in a clay rock repository. In this context, the iCROSS team became a full member of the international Mont Terri consortium and worked in close collaboration with international and German institutions in URL projects. Respective experiments specifically deal with coupled processes at the reactive interfaces in a repository near field (e.g. the steel/bentonite and bentonite/concrete interfaces). Within iCROSS, the impact of secondary phase formation on radionuclide transport is investigated. At Mont Terri, experiments are in preparation to study radionuclide transport phenomena in clay rock formations within temperature gradients and in facies exhibiting significant heterogeneities on different scales (nm to cm). Beside those studies, high resolution exploration methods for rock characterization are developed and tested and the effect of temperature and other boundary conditions on the strength, creep properties and healing of faults within Opalinus clay are quantified. Multiphysics models coupled to reactive transport simulation have been further developed and applied to laboratory and field experiments. Results are digitally analyzed and illustrated in a visualization center, in order to enhance the comprehension of coupled processes in repository systems across scales. The present contribution provides an overview on the project and reports selected results. The impact of considering complex coupled processes in repository subsystems for the assessment of the integrity of a given (generic) repository arrangement is discussed.

Structure, kinematics and composition of fluid-controlled brittle faults and veins in Lower Cretaceous claystones (Lower Saxony Basin, Northern Germany): Constraints from petrographic studies, microfabrics, stable isotopes and biomarker analyses

Investigation of fault rocks is crucial for the evaluation of sealing properties of rock formations considered as oil and gas storage or for the disposal of heat-generating radioactive waste. Even in tight rock formations, fluid flow may concentrate along brittle faults. The present study focuses on the sealing properties of faulted Lower Cretaceous clay- and siltstones encountered in drill cores from the central and eastern Lower Saxony Basin (Germany). Our investigations are based on a multidisciplinary approach including microstructural, mineralogical-geochemical, stable isotope and biomarker analyses.The new data suggest that preexisting discontinuities and/or sedimentary heterogeneities played a significant role for the increase in fluid pressure and formation of mode I veins within low permeability claystones. High-strain domains of fault cores consist of fine-grained fault gouge. A scaly fabric of varying deformation intensity is present around larger faults. Calcite-precipitation in faults and complex mineralized veins, and the differences in trace element concentrations of various veins, reflect several phases of palaeofluid activity. Calcite precipitation in the fault rocks led to elevated amounts of Mn, Ba and/or Sr. Isotope signatures indicate a local carbonate source for the origin of thin calcite coatings of fault planes, but a more complex palaeofluid interaction for the larger faults and for complex veins, consistent with the microfabrics. Biomarker signatures reveal the presence of mature hydrocarbons pointing to hydrocarbon migration in sections where steep, only weakly mineralized faults occur.The new results suggest that faulted claystones represent critical zones with a potentially reduced barrier capability consistent with published data.

The Role of Temperature in the Safety Case for High-Level Radioactive Waste Disposal: A Comparison of Design Concepts

The disposal of heat-generating radioactive waste in deep underground facilities requires a sparing use of spatial resources on the one side and favorable temperature conditions over the project lifetime on the other side. Under heat-sensitive conditions, these goals run in opposite directions and therefore a balance of some kind must be found. Often the elected strategy is to determine the size of the repository by capping the temperatures in the near-field, thus setting an upper limit to the deterioration of barrier materials. Alternatively, the spatial resources available in the siting area can be used to further reduce temperatures as long as supplementary benefits are returned from doing so. Using analytical modeling of the heat flow in the circumambient rock of a repository for high-level waste and spent fuel, this contribution examines possible obstacles in substantiating the safety case, namely the retrievability of waste during the operational lifetime of the facility, the representativeness of pilot disposal areas for monitoring, and the effect of thermal anomalies underground. The results indicate that there are, amongst the visited criteria, several benefits to the temperature-optimizing strategy over the prevailing space-optimizing concepts. The right balance between saving spatial resources and obtaining optimal temperature conditions is yet to be found.

German approach and feedback on experience of transportability of SNF packages after interim storage

AbstractIn Germany, the concept of dry interim storage of spent fuel in dual purpose metal casks is implemented, currently for periods of up to 40 years. The casks being used have an approved package design in accordance with the international transport regulations. The license for dry storage is granted on the German Atomic Energy Act with respect to the recently revised ‘Guidelines for dry interim storage of irradiated fuel assemblies and heat-generating radioactive waste in casks’ by the German Waste management Commission. For transport on public routes between or after long term interim storage periods, it has to be ensured that the transport and storage casks fulfil the specifications of the transport approval or other sufficient properties, which satisfy the proofs for the compliance of the safety objectives at that time. In recent years, the validation period of transport approval certificates for manufactured, loaded and stored packages were discussed among authorities and applicants. A case depen...

Self-disposal option for highly-radioactive waste reconsidered

ABSTRACTSelf-disposal option for heat-generating radioactive waste (HLW, spent fuel, sealed radioactive sources) known also as rock melting concept was considered in the 70s as a viable but alternative disposal option by both DOE in the USA and Atomic Industry Ministry in the USSR. Self-disposal is currently reconsidered with a novel purpose – to penetrate into the very deep Earth’s layers beneath the Moho’s discontinuity and to explore Earth interior. Self-descending heat generating capsules can be used for disposal of dangerous radioactive wastes in extremely deep layers of the Earth preventing any release of radionuclides into the biosphere. Descending of capsules continues until enough heat is generated by radionuclides to provide partial melting of surrounding rock. Estimates show that extreme depths of several tens and up to hundred km can be reached by capsules which could never be achieved by other techniques.

Fractal model for permeability calculation from porosity and pore radius information and application to excavation damaged zones surrounding waste emplacement boreholes in opalinus clay

An excavation damaged zone (EDZ) around emplacement boreholes for radioactive waste represents a potential pathway for radionuclides, due to its increased porosity and crack permeability. As clay is one of the potential host rocks for radioactive waste disposal, a methodical approach was developed for permeability calculation of damaged opalinus clay. The approach was based on a permeability–porosity relationship and on the theory of fractals, which allows the description of rock mass properties by fractal dimensions. It was calibrated against laboratory test data and provides the possibility to calculate the permeability from the pore size parameters "effective porosity" and "pore radius" as well as from the fractal dimension. An application of this relationship was given in terms of calculating the EDZ permeability around a vertical 50 m deep emplacement borehole in opalinus clay at a German generic repository site. The Particle Flow Code (PFC) was applied to simulate the fissure development in the EDZ. The results of the simulation provide the necessary pore size parameters. Considering the thermo-mechanical impact of adjacent, already filled, boreholes in an emplacement field for heat-generating radioactive waste, the crack occurrence and evolution over time in the EDZ of a newly drilled borehole was calculated on a 2D horizontal plane. A 3D calculation as well as a verification against in-situ data is still pending.

Influence of graphite and quartz addition on the thermo–physical properties of bentonite for sealing heat-generating radioactive waste

Bentonite is one of the most favored buffer materials for the deep geological disposal of waste in clay and granite formations all over the world. Buffer material, used to isolate heat emitting waste canisters, has to take up a strong heat load. This paper presents results of investigations on enhancing the heat conduction within the bentonite sealing. Admixtures of quartz and graphite accelerate the heat transfer into the host rock. Test samples consisting of different bentonite–quartz and bentonite–graphite mixtures were prepared. The thermal conductivity was determined as a function of admixture content, temperature, water content, and sample density within 35° to 140 °C at a uniaxial pressure of 2 MPa. The necessary conductivity could not be achieved with quartz, but the addition of graphite led to a suitable thermal conductivity. A set of equations was developed for the calculation of the thermal conductivity and the design of an engineered geotechnical barrier with heat conduction properties similar to those of the particular host rock.

Deformation of metavolcanics in the Karachay Lake area, Southern Urals: petrophysical and mineral-chemical aspects

AbstractThis paper is the result of laboratory tests aimed at comparative assessment of the filtration, elastic and mechanical properties of metavolcanics, as well as a preliminary investigation of the nature of characteristic variations in P–T parameters, simulating the conditions in the vicinity of an underground storage facility for heat-generating radioactive waste. The properties have been studied in cores collected from the deepest (1200 m) hole in the metavolcanics, focusing on intervals subjected to different strain intensities. Interpretation of the data obtained suggests that the identification of the most probable pathways for fluid flow and radionuclide transport depends on estimates of the retardation characteristics of zones of disjunctive dislocation. However, the widespread occurrence of linear zones of schistosity, mylonitization and brecciation may not be a decisive negative factor in the long-term performance assessment of the underground storage facility, because these minerals, which concentrate radionuclides such as epidote, chlorite, sericite, and Fe oxyhydroxides are widely distributed in these zones. Moreover, open microfractures and pores affected by heat will be filled with neoformed minerals (epidote, chlorite, carbonate) characterized by high sorption capacity with respect to radionuclides, due to release of intergranular water, as well as carbonatization reactions. Simultaneously, the system of open macro- and microfractures that resulted from stress affecting relatively undisturbed blocks may be one of the factors dramatically reducing country rock retention capability. Therefore, the results of petrophysical and mineral-chemical investigations must be taken into account in feasibility studies of designs for the construction of underground systems for final disposal of high-level wastes and/or long-term storage of spent nuclear fuel in the PA Mayak area.

Critical issues in modelling the long-term hydro-thermomechanical performance of natural clay barriers

Performance assessment of deep repositories for heat-generating radioactive waste requires the capability of predicting reliably the evolution of the system during a time period commensurate with the hazardous life of the waste. In many repository designs clay barriers represent important elements of the waste isolation system. In order to provide reasonable assurance that clay barriers will ensure long-term waste isolation, it is essential to understand their behaviour under a variety of conditions. Due to the variability of argillaceous materials, to the complexity of the phenomena that might take place in a waste repository and to the longevity of the required isolation, an adequate understanding of the behaviour and the capability to model the evolution of the clay barriers are not easy tasks. The factors that need to be understood and modelled include stress evolution, long-term strain or creep, thermal effects on solid skeleton, on interstitial fluids and on mineralogy. The difficulty of the task is increased by the facts that many effects are coupled, that their rates must be extremely low, in order to be realistic, and that the time period to be modelled defies the possibility of direct experimental observation. Several critical issues are identified and discussed briefly, such as: constitutive law to describe the thermo-mechanical behaviour of the clay skeleton, modelling of the fluid phase in clays and its response to heating, thermal fracturing and healing. Strategies are suggested for a rational approach to the experimental investigation of some relevant processes. The study of suitable natural analogues, for example the thermo-metamorphic halo occurring at Orciatico in Tuscany, could provide valuable insights in the thermal effects of heating clay barriers. It is conceivable that models describing a variety of relevant phenomena, such as dehydration, fracturing and permeability changes could be tested through the study of the Orciatico analogue. In the end performance assessments of clay barriers would benefit through improvements in modelling: this would involve progress in understanding the basic phenomena and their coupled nature, improved conceptual and mathematical models and increased reliability for their calibration/validation. The improved understanding of phenomena requires additional experimental activities on various levels: molecular, microscopic, macroscopic, medium scale and in situ.

高レベル放射性廃棄物と鉱物学

高レベル放射性廃棄物と鉱物学

The International Stripa Project: an overview

Abstract The International Stripa Project was a cooperative research and development project among several member countries of the Nuclear Energy Agency of the Organization for Economic Cooperation and Development. The scientific and technical objectives of the project were to investigate several aspects of technology concerned with the feasibility and safety of disposal of long-lived, heat-generating radioactive waste at depth in granitic rocks. In particular, the Stripa Project addressed: 1) the development of instruments and procedures to characterize candidate repository sites; 2) the understanding and modelling of groundwater flow and solute transport in fractured crystalline rock; and 3) the design of engineered barriers capable of contributing to waste isolation by restricting groundwater flow in proximity to the waste containers and in the surrounding host rock. This overview summarizes the work focussed on natural barriers and engineered barriers.

Objectives, Criteria, and Preliminary Planning for the Disposal of Heat-Generating Radioactive Waste in Germany

Abstract In Germany, heat-generating radioactive waste is intended to be disposed of in the Gorleben repository. With regard to planning and construction of this facility as well as its operational and postoperational phase, safety issues must be taken into account. Emphasis is on the preliminary planning for the Gorleben project and on radiation protection and safety criteria regarding the postclosure phase of this planned repository.