Nuclear Waste Repository Research Articles

Machine Learning Surrogates of a Fuel Matrix Degradation Process Model for Performance Assessment of a Nuclear Waste Repository

Spent nuclear fuel repository simulations are currently not able to incorporate detailed fuel matrix degradation (FMD) process models due to their computational cost, especially when large numbers of waste packages breach. The current paper uses machine learning to develop artificial neural network and k-nearest neighbor regression surrogate models that approximate the detailed FMD process model while being computationally much faster to evaluate. Using fuel cask temperature, dose rate, and the environmental concentrations of CO3 2−, O2, Fe2+, and H2 as inputs, these surrogates show good agreement with the FMD process model predictions of the UO2 degradation rate for conditions within the range of the training data. A demonstration in a full-scale shale repository reference case simulation shows that the incorporation of the surrogate models captures local and temporal environmental effects on fuel degradation rates while retaining good computational efficiency.

Validation of the fully-analytical solution with temperature superposition for the nuclear waste repository

Temperature distributions and evolutions in high-level radioactive waste (HLW) disposal repository are usually obtained by numerical simulation or analytical solution. For the use of analytical solution to calculate the temperature distributions of one disposal panel in the repository, the temperatures from every nuclear waste canister were superposed for calculating the whole effects of all the canisters, and the disposal tunnels, which are filled with backfill materials, were ignored to simplify the deducing process. Yet the accuracy of the temperature superposition has not been validated and the influence of tunnels on temperature has not been evaluated. In this paper, a disposal panel with 1189 waste canisters was modelled by the finite element method for analyzing the temperature distributions and evolutions. The size of numerical domain was determined by studying the temperature distributions. When the distance between the outer edges and outermost canisters surface was set as 700 m or above, the influence of boundaries on calculated results can be ignored. A disposal panel with tunnels was modelled to study the influence of tunnels on temperature evolution, and the results show that the tunnels have practically no effect on the temperature evolution. Finally, the comparison of temperatures calculated by numerical simulation and analytical solution was done to validate the analytical solution with the superposition method.

Development of a 2D model for rapid estimation of sulfide corrosion of copper canisters in a spent nuclear fuel repository

AbstractReactive‐transport models (RTMs) have been an integral part of the safety case and design optimization for the nuclear waste repository in Finland. Such highly resolved, internally complex RTMs tend to be computationally demanding, thus their design is often a compromise between representativeness and computational efficiency. A modeling strategy is presented, demonstrating how a 2D simplified model can be constructed based upon and calibrated against a geometrically realistic 3D RTM. Key geometrical parameters of the 2D model are consistent with the 3D model, and the 2D model is verified by a series of benchmarks for various designs and conditions of the repository. Overall, the 2D model is proven suitable for fast scoping calculations, as (1) it reduces the mesh elements by 16 times and shortens the run time by about 40 times; (2) it has much more flexible remeshing possibilities; and (3) it provides highly consistent results with the 3D model.

Biogeochemical interactions between aged cementitious materials and sulfate reducing microbial community with propionate as electron donor in the context of nuclear waste repository

Heterotrophic microbial sulfate reduction can occur in both natural and engineered systems. The process can influence the radionuclide speciation and mobility in deep repository of radioactive waste (DRRW). DRRW are characterised by significant masses of concrete, imposing alkaline pH in the waste cell. This paper aims to evaluate microbial sulfate reduction coupled with propionate oxidation influenced by the alkalinity of the environment: moderate alkaline pH close to 9.0 without cement paste, and moderate alkaline pH which was then moderately increased (pH 9.0 to 9.4) by the presence of pre-aged solid cement pastes via advanced and moderate ageing protocols, respectively. Regardless of the degree of ageing of the cement pastes, the sulfate reduction rate decreased by up to 84% when the pH increased from 9.0 to 9.4 in the presence of cement paste and by up to 90% in the absence of cement paste. No sulfate reduction or propionate oxidation was observed for pH > 9.5. Microbial metabolites, sulfide, CO2 and acetate, produced from the reduction of sulfate and oxidation of propionate, and the presence of the microorganisms attached to the cement pastes (composed of up to 80% of sulfate reducing bacteria) led to their biodeterioration. Sulfide enrichment, precipitation of secondary ettringite, and intensified decalcification were notably detected. Self-healing like phenomena were also observed: calcium leached from the cementitious phases reacted with carbonate produced by microorganisms to form of calcium carbonate relocated either in the micro-cracks or on the surface of the pre-aged solid cement pastes.

Coupled Two-Phase Flow and Elastodamage Modeling of Laboratory and In Situ Gas Injection Experiments in Saturated Claystone as a Potential Host Rock for Nuclear Waste Repository

Understanding gas migration behavior in host rocks is of importance to the safety evaluation of deep geological repositories for nuclear waste. Experimental results reported in the literature show that gas flow in saturated claystone is through a highly localized network of dilatant pathways and that water is barely displaced. To explain the specific gas migration behavior, a two-scale approach is developed. A subcritical criterion for microcrack propagation is proposed to represent the time-dependent damage at the macroscale. The passage from microscale to macroscale is implemented through an asymptotic homogenization method. The solid mechanics is coupled with the fluid flow through pore pressure variation and an intrinsic permeability model, which implicitly accounts for the fracture opening induced permeability change. The developed model is tested against both laboratory and in situ gas injection experiments in the literature. Some key experimental findings, such as the development of preferential gas pathways and the fully saturated state are explicitly captured by the poroelastic damage model. Model results explain that the highly localized fracture pathways are the major places where gas and water interact with each other, and as a result the whole rock is almost kept fully saturated, which helps us get in-depth understanding of this gas transport mechanism.

Numerical simulation of long-term performance of deep geological repository placement rooms in crystalline and sedimentary rocks

Evaluation of the stability of placement rooms and damages in the geosphere surrounding a deep geological repository (DGR) for high-level nuclear waste are essential components for assessing the overall repository performance. Assessments of the performance of placement rooms for the duration of the regulatory period (typically 1 million years) usually involves simulations using numerical codes capable of modeling coupled thermo-hydro-mechanical (THM) processes induced by different natural and repository-induced perturbations over different time and length scales. The numerical constitutive model used to represent the host medium should be capable of capturing highly nonlinear responses to stress changes, including development of damage and fracturing. In this study, a novel approach for simulation of damage evolution and fracturing around the repository rooms, based on the bonded block model (BBM) devised for analyzing coupled THM processes, is used to perform a study of the long-term geomechanical performance of the Nuclear Waste Management Organization hypothetical DGR configurations in sedimentary and crystalline geological settings. The perturbation effects of different loading conditions, including in-situ stress changes by room excavation, heat generated from the used fuel canisters, and glaciation were analyzed. Each THM analysis was conducted using a 3D continuum numerical code to generate evolving temperature and pore pressure fields in the rock mass. The temperature and pore pressure data were then imported in the 2D BBM model for simulating evolution of damage around the placement rooms. Sensitivity analysis was conducted to investigate the effect of different poro-mechanical parameters, rock mass permeabilities, and different approaches for calibrating the BBM, on predictions of room stability and extent of the excavation damage zone around the placement room. The modeling results indicate that the confinement provided by the bentonite backfill inside the room clearly prevents any unravelling but also limits the extent of the damage to less than 1.6 m into the room walls in any of the analyzed cases considering different loading conditions and uncertainty of the material properties.

Using geophysical log data to predict the fracture density in a claystone host rock for storing high-level nuclear waste

Previously drilled boreholes of a host rock for a potential nuclear waste repository in Hungary revealed a highly fractured claystone rock body. A crucial step for characterizing the hydrodynamic behavior of such a fractured reservoir is fracture identification and accurate calculation of the fracture density. Although acoustic borehole televiewers provide a reliable base for determining the fracture density, older boreholes usually lack such data. However, conventional borehole geophysical measurements are often accessible in such cases. The aim of this study was to identify any correlations between well log data and fracture density. Multiple linear regression analysis was performed on data from two boreholes penetrating the Boda Claystone Formation in southwest Hungary. The upper section of the BAF-4 borehole was used for training, where the fracture density was estimated with a fit of R2 = 0.767. The computed regression function predicted the fracture density with high accuracy in both boreholes for all intervals with typical lithological features. However, in some sections where anomalous well log data indicated changes in the lithology, the prediction accuracy decreased. For example, the function underestimated the fracture density in sandy intervals.

Coupled flow and transport modelling of a large-scale in situ migration experiment with 14 C-labelled natural organic matter colloids in Boom Clay

Abstract Twenty-five years ago, a unique long-term and large-scale in situ experiment with 14 C-labelled natural organic matter (NOM) was set up at the HADES underground research facility in Mol (Belgium) to study its migration behaviour. Natural organic matter plays an important role in the mobility of various safety-relevant radionuclides, which is critical in the context of Safety & Performance Assessment (SA/PA) calculations for a possible nuclear waste repository. The objective of this work is to enlarge the confidence in current NOM transport models by validating them with the in situ experiment, which is still continued to this day. Stepwise adding more complexity to the model resulted in a 10-parameter model with which excellent fits to the data are obtained. The model considers two different fractions that are transported by advection and diffusion and can be subject to both irreversible and reversible immobilization processes. The associated fitted parameter values compare well with values determined on small-scale migration experiments. This builds confidence in the NOM transport model, which in turn contributes to the confidence in the outcome of the radionuclide migration calculations performed in the context of SA/PA. These results again highlight the incredible value of such long-running experiments at underground research facilities like HADES.

Influence of groundwater composition on the reductive precipitation of U(VI) on corroding iron foil surfaces

In order to assess the disposal of spent nuclear fuel in a deep geological nuclear waste repository, the interactions between U(VI) and corroded iron present in the canister material are of importance. It is important to correctly model the fate of the oxidatively dissolved uranium in order to correctly estimate radium releases from the canister in the long term. The release of radionuclides into the environment depends on the dissolution of the UO2 matrix which is dependent on the redox conditions at the fuel surface. The effect of metallic iron on the reduction of U(VI) was studied under anoxic conditions using synthetic groundwaters with different compositions, chosen to investigate the influence of calcium-uranyl-carbonato complexes on the thermodynamics and kinetics of U(VI) reduction on anoxically corroding iron. The corrosion products formed on the iron surface were investigated using SEM-EDS and XPS to identify elemental composition and oxidation states of uranium and iron on the surface. The iron foils efficiently reduced U(VI) to U(IV) causing its significant sorption and precipitation on the iron foil surfaces in the form of U(IV).

Engineered mineralogical interfaces as radionuclide repositories

Effective capture of fugitive actinides and daughter radionuclides constitutes a major remediation challenge at legacy or nuclear accident sites globally. The ability of double-layered, anionic clay minerals known as hydrotalcites (HTC) to contemporaneously sequester a range of contaminants from solution offers a unique remedy. However, HTC do not provide a robust repository for actinide isolation over the long term. In this study, we formed HTC by in-situ precipitation in a barren lixiviant from a uranium mine and thermally transformed the resulting radionuclide-laden, nanoscale HTC. Atomic-scale forensic examination of the amorphized/recrystallised product reveals segregation of U to nanometre-wide mineral interfaces and the local formation of interface-hosted mineral grains. This U-phase is enriched in rare earth elements, a geochemical analogue of actinides such as Np and Pu, and represents a previously unreported radionuclide interfacial segregation. U-rich phases associated with the mineral interfaces record a U concentration factor of ~ 50,000 relative to the original solute demonstrating high extraction and concentration efficiencies. In addition, the co-existing host mineral suite of periclase, spinel-, and olivine-group minerals that equate to a lower mantle, high P–T mineral assemblage have geochemical and geotechnical properties suitable for disposal in a nuclear waste repository. Our results record the efficient sequestering of radionuclides from contaminated water and this novel, broad-spectrum, nanoscale HTC capture and concentration process constitutes a rapid solute decontamination pathway and solids containment option in perpetuity.

Rock Mechanics for GeoEnergy Applications

The discipline of rock mechanics, which emerged in the 1960s and 1970s as a distinct scientific subject, is undergoing rapid development, including a significant shift within the traditional civil, transportation, and petroleum fields. This shift leans toward using Earth resources to mitigate adverse climate change and to boost sustainable and renewable energy production. Energy geotechnics, or “GeoEnergy,” focuses on novel geomechanics and geotechnical engineering strategies to meet emerging sustainability market demands. GeoEnergy applications that employ rock mechanics and engineering include geothermal systems, geologic carbon sequestration, underground hydrogen storage, compressed air energy storage, and nuclear waste repositories.

Molecular Dynamics Study of the Beryllium Interaction with C-S-H Phases

Beryllium has applications in fission and fusion reactors, and accordingly it is expected in specific waste streams in nuclear waste repositories. As a part of the multi-barrier system, cementitious materials were shown to strongly sorb beryllium, but the precise uptake mechanisms remain ill-defined. Computational simulations were used to study Be(II) uptake by calcium-silicate-hydrate (C-S-H) phases. Molecular dynamics (MD) calculations show that Be(II) sorbs on (001), (004), and (100) C-S-H surfaces through Ca-bridges and hydrogen bonds. Energy profiles indicate that surface complexes with the highest number of Ca-bridges are the most stable. MD simulations support also Be(II) retention in the C-S-H interlayer. Be(OH)3– is predominantly bound through the exchange of water molecules for deprotonated silanol groups or through multiple Ca-bridges, whereas Be(OH)42– is immobilized in the interlayer midplane. These results provide key inputs to understand the mechanisms driving Be(II) uptake by cementitious materials of relevance in the context of nuclear waste disposal.

TransPyREnd: a code for modelling the transport of radionuclides on geological timescales

Abstract. The German site selection procedure for a high-level nuclear waste repository is entering a stage in which preliminary safety assessments have to be conducted and the release of radionuclides has to be estimated for a large number of potential sites. Here, we present TransPyREnd, a 1D finite-differences code for modeling the transport of radionuclides in the subsurface at geological timescales. The code simulates the processes advection, diffusion, equilibrium sorption, decay of radionuclides, and the build-up of daughter nuclides. We summarize the modeled physical processes, their mathematical description and our numerical approach to solve the governing equations. Finally, two simple tests are shown, one considering diffusion, sorption, and radioactive decay, the other involving diffusion and a radioactive decay chain. In both tests, the code shows good agreement with the reference solutions. Caveats of the model and future additions are discussed.

Interseismic Strain Accumulation between the Colorado Plateau and the Eastern California Shear Zone: Implications for the Seismic Hazard near Las Vegas, Nevada

ABSTRACTCrustal deformation in the central Basin and Range between the Colorado plateau and the Eastern California Shear Zone is active but slow, making it a challenge to assess how strain is distributed and crustal motion transferred. However, knowledge of strain rates is very important, particularly for addressing the seismic hazard for both the Las Vegas urban area and the site of the proposed Yucca Mountain nuclear waste repository, in southern Nevada. Global Positioning System (GPS) data provide important constraints, particularly now that the GPS network in the area has substantially expanded in recent years. However, because deformation is slow, it is important to mitigate any transient tectonic and nontectonic signals to obtain the most accurate long-term interseismic motion and robust estimation of strain rates. We use data from all GPS stations in the region including both long-running continuous and semicontinuous stations. We model and remove postseismic displacements at these stations using source parameters for 41 events, dating back to the 1700 Cascadia megathrust earthquake, which contribute significantly to the deformation field within the central Basin and Range. We also remove correlated noise from the time series with the common-mode component imaging technique. We find that removal of both the postseismic transients and common-mode noise substantially reduces the uncertainties and spatial variation in the velocities. We find east–west extension across the Las Vegas Valley of 0.5–0.6 mm/yr. The interseismic strain rate field, calculated with the final velocities, reveals higher strain rates through southern Nevada than in previous studies, with rates within Las Vegas Valley of 8.5±2.4×10−9 yr−1. Our results also confirm shear along the Pahranagat shear zone, but the estimated amplitude is strongly affected by postseismic relaxation.

The final countdown to site selection for Canada’s nuclear waste geologic repository

ABSTRACTSelecting a site to build a deep geologic repository for high-level nuclear waste is a process fraught with social tension and few countries have managed to do so successfully. Canada is poised to be next, with a site selection decision due in 2024. Two potential sites are under consideration, one in northern Ontario and one in southern Ontario, each with different geologic settings and different social and economic landscapes. We review the consent-based process that has led to this point, the complexities involved in expressing consent in Canada, and some of the hurdles that remain.

Clay–rock fracturing risk assessment under high gas pressures in repository systems

At the interface between the steel canister and the bentonite in a nuclear waste repository, we expect generation of hydrogen gas because of corrosion processes. The pressurized gas might fracture the engineered or natural clay barrier system, enhancing radionuclide transport into the geosphere. To assess the long-term integrity of the clay host rock under various conditions and scenarios, we need a large number of numerical simulations. However, a simulation tool for complex fracture propagation is often prohibitively expensive to run many realizations. Here, we developed a risk analysis tool based on the Design of Experiments to overcome the computational challenges by generating a computationally inexpensive proxy fracture model using a set of critical factors known as heavy hitters. We provided parameters and their probability distributions that are subject to uncertainty, as well as an objective function that assesses the risk of fracturing due to high gas pressures. Through various scenarios, we found that the fracture toughness dominates the impact on the risk.

Homogenization of unsaturated bentonite during hydration

Bentonite barriers are a key component in may designs of deep geological repositories for high level nuclear waste. During the hydration stage, the bentonite undergoes non-uniform changes in dry density that may persist even after reaching a fully saturated state. Since dry density controls the properties of the bentonite that ensure the functions of safety of the barrier, the potential of bentonite for homogenization or otherwise is a matter of high relevance. The paper presents the results and the analyses of two hydration tests on initially heterogenous samples, especially designed to explore the phenomena and processes underlying the homogenisation of bentonite materials. The formulation and constitutive model used in the analyses are briefly presented followed by a description of the tests. Subsequently, the experimental dataobtained from the tests are discussed together with the results of the hydromechanical analyses performed. Particular attention is given to the degree of homogenization of the specimens achieved at the end of the tests. The simulations have resulted in a very satisfactory agreement with test observations, especially egarding the final state of the samples, thus enhancing the confidence in the numerical tool employed.

Implementation and validation of pressure-dependent gas permeability model for bentonite in FEM code Thebes

In an Engineered Barrier System of a nuclear waste repository, gas migrates through: a) diffusion/advection of dissolved gases, b) two-phase continuum flow, c) dilatant pathway flow and d) single-phase gas flow through macro-fractures in the soil. The gas production rate and the corresponding gas pressure accumulation affect the clay material behaviour and its properties such as air entry value. For the safe design of the EBS system, computational models need to account for the identified transport mechanisms. This study presents an enhancement in the finite element code Thebes [1, 2] that replicates the observed increase in permeability at higher gas pressures, e.g. due to pore dilatancy and gas fracture as proposed by Xu et al. [3]. The formulation links permeability to gas pressure and threshold/critical pressure. For model validation, the study utilizes a gas injection experiment carried out in IfG (Institute for Rock Mechanics, Germany) on Opalinus Clay [4]. The results show a good fit against the measurements while giving insight into gas flow through clays.

Safety analysis of near surface nuclear waste repository constituting an aquifer in proximity

Probabilistic safety assessment involves the calculation of risk associated with possible radioisotope contamination of Groundwater. The overall scenario and the consequences that may occur on the failure of the external barriers are identified using an Event Tree analysis to be the multi-barrier failure due to rainwater intrusion into the near-surface disposal site and leaching out of radionuclides into groundwater which is extracted as drinking water by critical groups present around the repository. A program is developed in Python 3.0 for such a scenario to assess necessary parameters such as radioactive release rate, groundwater radionuclide concentration, dose ingested by members and total risk associated with each radioisotope. The initiation is with the assessment of the failure density function of the multi-barrier system operating in redundancy, using the failure density functions. The radionuclide concentrations of the near-surface disposal facility are necessary to be calculated to know the probable release rate of each radionuclide into groundwater. The concentration depletion with Spatio-temporal variance is also considered a factor for radioisotope concentration variation in groundwater. The concentration of radionuclides in groundwater when ingested at a rate of 2.2 L/day helps us to calibrate the Radiation dose to humans through this pathway by using the Ingestion dose coefficient for each isotope fixed by International Atomic Energy Agency [IAEA]. The final risk assessment is accomplished by multiplying the radiation dose ingested by members of the critical group with a risk factor for fatal or non-cancers and critical hereditary effects by the International Commission on Radiological Protection. In the present work, a subjective analysis is done to calibrate the dosage of radiation, of five radioisotopes (3H, 60Co, 59Ni, 90Sr, 129I) for disposal. Radioactive Iodine delivered maximum risk despite a low concentration in the inventory of radioactive waste. However, much lower than the permissible dosage suggested by ICRP i.e.,1 mSv in a scenario where an exclusion zone is considered to be existing, and a drinking water well is established at a distance of 1.6 km from the repository and the risk is computed over a period of 150 years since the multi-barrier failure.

A Bayesian regression analysis of in situ stress using overcoring data

Characterising the state of in situ stress at a target depth is crucial for all underground engineering projects. Consequently, on critical projects such as nuclear waste repositories extensive campaigns are implemented with the goal of estimating the in situ stress state. These campaigns often comprise both direct measurement and indirect estimation methods, but the data obtained across a project volume may exhibit significant variability. This poses significant challenges in both quantifying the variability and uncertainty of in situ stress, and determining the stress state to be used for design purposes.It is often assumed that the state of in situ stress increases linearly with depth, and thus linear regression of principal stress magnitude against depth are often found in the literature. As such methods not honouring the tensorial nature of stress are, strictly, incorrect. To show how this limitation may be overcome, here we present a Bayesian regression analysis of in situ stress with depth that uses the Cartesian stress tensor. The analysis is performed using over 100 overcoring data obtained at the SKB Forsmark site in Sweden.A comparison between the customary and Bayesian approaches is presented, which shows the superiority of the tensorial technique.