Displays

In Canada, the Nuclear Waste Management Organization (NWMO) is responsible for the long-term management of spent nuclear fuel, which involves sealing used fuel bundles in copper-coated carbon steel used fuel containers (UFC) and emplacing them ~500 m underground in a deep geological repository (DGR). In this plan, copper plays a vital role in ensuring the safety of the DGR as it is intended to serve as a corrosion barrier for greater than one million years. Veins and pods of natural copper have long been known to occur in the Lake Superior region of North America where they have been culturally significant to Indigenous peoples for millennia. The natural Lake Superior copper deposits were emplaced close to one billion years ago in lithosphere which has since had a protracted history of glacial overrides and related isostatic adjustment events. In light of this longevity, structural history and exposure to crustal fluids, it has the potential to hold many lessons for DGR specialists and society in general. We present two aspects of our approach. The first is an outline of our efforts to increase cultural competency of non-indigenous scientists in our group while building an understanding of the Indigenous Knowledge system and how it can be respectfully and effectively applied to research. This includes respectful ways in which to collect, and learn from, copper samples. This work directly relates to the important relationships in the context of the NWMO Indigenous Knowledge policy, Reconciliation policy and how both policies apply to the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP). In this context we will also present our early field and laboratory observations of natural copper properties through an integrated analysis of chemical and orientation microstructure in tandem with electrochemical behaviour. Techniques in the laboratory workflow include electron microscopy (EDS, EBSD) and atom probe tomography to map impurities and microstructure relative to manufactured wrought copper (phosphorus-doped oxygen free); Auger electron and X-ray photoelectron spectroscopies to determine surface and near-surface composition and chemical environment; and electrochemical methods such as corrosion potential measurements and potentiodynamic polarization scans to probe the corrosion performance. These measurements may inform container design aspects such as optimal fabrication and the role of impurities in corrosion behaviour of electrodeposited and cold spray-deposited copper in a DGR over geological timescales. Taken together, our aim is to come to an appropriately comprehensive understanding of the cultural, geological and material corrosion aspects of natural copper that has persisted for a time span three orders of magnitude greater than the DGR design requirements. It is our hope that this learning approach to ancient natural copper will play a positive role in seeking social license for DGR planning, while having value for societal education in the global challenge of geostorage.

How to cite: Moser, D., Noël, J., Dobkowska, A., Zagidulin, D., Perritt, J., Keech, P., Behazin, M., Binns, J., Arcuri, G., and Langelier, B.: Learning from 1 billion year old copper, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12231, https://doi.org/10.5194/egusphere-egu2020-12231, 2020.

The Waste Isolation Pilot Plant (WIPP), located in southeastern New Mexico, has been developed by the U.S. Department of Energy (DOE) for the geologic (deep underground) disposal of transuranic (TRU) waste. Containment of TRU waste at the WIPP is regulated by the U.S. Environmental Protection Agency (EPA) according to the regulations set forth in Title 40 of the Code of Federal Regulations (CFR), Part 191. The DOE demonstrates compliance with the containment requirements according to the Certification Criteria in Title 40 CFR Part 194 by means of performance assessment (PA) calculations performed by Sandia National Laboratories (SNL). WIPP PA calculations estimate the probability and consequence of potential radionuclide releases from the repository to the accessible environment for a regulatory period of 10,000 years after facility closure.

The models used in PA are maintained and updated with new information as part of an ongoing process. Improved information regarding important WIPP features, events, and processes typically results in refinements and modifications to PA models and the parameters used in them. Planned changes to the repository and/or the components therein also result in updates to WIPP PA models. WIPP PA models are used to support the repository recertification process that occurs at five-year intervals following the receipt of the first waste shipment at the site in 1999.

The 2019 Compliance Recertification Application (CRA-2019) is the fourth WIPP recertification application submitted for approval by the EPA. A PA has been executed by SNL in support of the DOE submittal of the CRA-2019. Results found in the CRA-2019 PA are compared to those obtained in the 2014 Compliance Recertification Application (CRA-2014) PA in order to assess repository performance in terms of the current regulatory baseline. This presentation includes a summary of the changes modeled in the CRA-2019 PA, as well as the estimated releases over the assumed 10,000-year regulatory period. Changes incorporated into the CRA-2019 PA included repository planned changes, parameter updates, and refinements to PA implementation.

Overall, the total normalized releases for the CRA-2019 PA have increased at all probabilities compared to those from the CRA-2014 PA. Releases from each of the four potential release mechanisms tracked in WIPP PA (cuttings and cavings, spallings, releases from the Culebra formation, and direct brine releases) have also increased at all probability levels. Cuttings and cavings releases continue to dominate total releases at high probabilities and direct brine releases continue to dominate total releases at low probabilities. Although the calculated releases have increased, the total normalized releases continue to remain below regulatory limits. As a result, the CRA-2019 PA demonstrates that the WIPP remains in compliance with the containment requirements of 40 CFR Part 191.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.. This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy. SAND2020-0131A

How to cite: Zeitler, T., Bethune, J., Brunell, S., Kicker, D., and Long, J.: 2019 Performance Assessment Calculations for the Recertification of the Waste Isolation Pilot Plant, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2474, https://doi.org/10.5194/egusphere-egu2020-2474, 2020.

The global production of 12,000 metric tonnes of high-level radioactive waste (HLW) every year is a big challenge with respect to its safe long-term storage. In the favored multi-barrier system, bentonite is discussed as a geo-technical barrier in many disposal programs worldwide. The bentonite seals the space between the canister containing the HLW and the surrounding host rock, thereby fulfilling two major tasks: 1) slow down the process of corrosion when water enters the disposal site, and 2) hinder the discharge of radionuclides into the bio-geosphere in case of a leaking canister. Due to their metabolic activity, microorganisms could significantly influence the properties of the bentonite barrier. In order to investigate the metabolic potential of naturally occurring microorganisms, we conducted anaerobic bentonite-slurry experiments containing uncompacted bentonite and a synthetic Opalinus Clay pore water solution. Within one-year incubation at 30 and 60 °C, lactate- or H₂-stimulated microcosms at 30 °C showed the dominance and activity of strictly anaerobic, sulfate-reducing and spore-forming microorganisms. Consequently, hydrogen sulfide gas was generated in the respective set ups, leading to the formation of fractures and iron-sulfur precipitations. Experiments that incubated at 60 °C, showed the dominance of thermophilic bacteria, independent of the presence of substrates. The respective set-ups showed/revealed no significant changes in the analyzed bio-geochemical parameters. The obtained results clearly show that indigenous microorganisms evolve in a temperature- and substrate-dependent manner. The formed metabolites can potentially affect the dissolution behavior of minerals and ions within the bentonite as well as corrosion processes and require further investigations.

How to cite: Matschiavelli, N., Dressler, M., Neubert, T., Kluge, S., Schierz, A., and Cherkouk, A.: The role of microorganisms in the bentonite barrier of high-level radioactive waste repositories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2300, https://doi.org/10.5194/egusphere-egu2020-2300, 2020.

Clay minerals are widely used as materials for construction of engineered barriers for nuclear waste and spent fuel repositories all over the world due to perfect isolation properties and high sorption capacity. Unwanted microbiological processes that occur in geological repository can cause deterioration of clay barrier materials, which may significantly affect long-term safety of the repository. It is important to note that such unwanted processes could be caused both by native microbial population and bacteria brought in from outside during the construction of the repository.

This paper aims to develop a general concept that could be used to prove the risk of unwanted microbial processes’ occurrence in clay materials.  

Some features of mineral composition of clay materials, including the content of iron, sulphur, phosphorus, organic and mineral carbon, provide the basis for the concept. The ratios of free mono- and di-valent cations present in the solution (Na-K-Ca-Mg) are also taken into account. Another approach presumes microflora composition analysis by means of high-efficient 16S rRNA sequencing method. In addition, the results of several tests dedicated to microbial communities’ stimulation are discussed. These include tests on hydrogen or organic substance addition as electron donors with subsequent standard tests on metabolic activity evaluation, MTT test and respiration assessment of microbial population, which is represented by both planktonic cells and cells incorporated into biofilms. The developed concept was used to assess clay materials found in Russian Federation that could potentially be used to construct engineered safety barriers. These data formed the basis for the formation of a database of microbial safety of engineering barrier materials for radioactive waste storage.

How to cite: Safonov, A., Popova, N., Spirina, E., Abramova, E., Philippova, N., Jarkova, V., and Boldyrev, K.: Biogenic safety of clay barrier materials for radioactive waste repository database, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3966, https://doi.org/10.5194/egusphere-egu2020-3966, 2020.

Compacted bentonite is commonly considered for use as backfill material in emplacement tunnels of nuclear waste repositories because of its low permeability, high swelling pressure, and retardation capacity of radionuclide. To assess whether this material can maintain its favorable features when undergoing heating from the waste package and hydration from the host rock, we need a thorough understanding of the thermal, hydrological, mechanical, and chemical evolution under disposal conditions. Laboratory and field tests integrated with THMC modeling have provided an effective way to deepen such understanding; however, most of this work has been conducted for maximum temperatures around 100°C. In contrast, some international disposal programs have recently started investigations to understand whether local temperatures in the bentonite of up to 200°C could be tolerated with no significant changes in safety relevant properties. For example, the United States disposal program is evaluating the feasibility of geological disposal of large spent nuclear fuel canisters that are currently in dry storage. Direct disposal of these canisters is attractive for economical and safety reasons, but faces the challenge of exposing the bentonite to significant temperatures increases. As a result, strong thermal gradients may induce complex moisture transport processes and bentonite-rock interactions while cementation and perhaps also illitization effects may occur, all of which could  strongly affect the bentonite properties.

Here, we present initial investigations of bentonite behavior exposed to strongly elevated temperatures. We first show results from coupled thermal, hydrological, mechanical and chemical (THMC) simulations of a generic nuclear waste repository in a clay formation with a bentonite-based buffer exposed to a maximum temperature of 200°C. Modeling results illustrate possible performance impacts, such as the time frame and condition of the early unsaturated phase during bentonite hydration, the porosity and permeability after the bentonite becomes fully saturated, and changing in swelling properties. We then discuss preliminary data from a bench-scale laboratory mockup experiment which was designed to represents the strong THMC gradients occurring in a “hot” repository, and we briefly touch on a full-scale field experiment to be conducted soon in the Grimsel Test Site underground research laboratory in Switzerland (referred to as HotBENT, with bentonite exposure from up to 200^oC). 

How to cite: Birkholzer, J., Zheng, L., Rutqvist, J., Borglin, S., Chang, C., Chou, C., Wu, Y., and Kneafsey, T.: Investigating the Temperature Limits of Bentonite Backfilled Repositories: Coupled THMC Modeling, Lab Mockup Testing and Field Experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3191, https://doi.org/10.5194/egusphere-egu2020-3191, 2020.

When the recommencement of the search for and selection of a site for a disposal facility for HLRW in Germany was stipulated by the Site Selection Act (StandAG 2017) in 2017, a precautionary temperature limit of 100 °C on the outer surface of the containers with high-level radioactive waste in the disposal facility section was set. This precautionary temperature limit shall be applied in preliminary safety analyses provided that the “maximum physically possible temperatures” in the respective host rocks have not yet been determined due to pending research. Therefore, this issue is addressed and discussed in this paper, contributing to “pending research” by a review of the literature.

This presentation briefly discusses a few examples of thermohydraulical, mechanical, chemical and biological processes in a disposal facility, because temperature limits are derived based on safety impacts regarding THMCB-processes. The temperature-dependent processes have been extracted from databases for features, events and processes (FEP-databases). Furthermore, it is dicussed if the feasibility to retrieve and recover HLRW is hampered at high temperatures.

It is concluded that a design temperature concerning single components of a disposal facility for the preservation of their features can be derived when a safety concept is established. However, the interactions of all relevant processes in a disposal concept must be considered to determine a specific temperature limit for the outer surface of the containers. Therefore, applicable temperature limits may vary for particular safety and disposal concepts in the following host rocks: rock salt, clay stone and crystalline rock.

Technical solutions for retrieval and design options for recovery seem to be viable up to temperatures of 200 °C with different, sometimes severe, downsides according to expert judgement.

It is summarized that emperature limits regarding the outer surface of the containers can be derived specifically for each safety concept and design of the disposal facility in a host rock. General temperature limits without reference to specific safety concepts or the particular design of the disposal facility may narrow down the possibilities for optimisation of the disposal facility and could adversely affect the site selection process in finding the best suitable site.

How to cite: Bracke, G., Hartwig-Thurat, E., Larue, J., Meleshyn, A., Weyand, T., and Kock, I.: Which Processes could define Temperature Limits on the Outer Surface of a Container in a Disposal Facility ?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3336, https://doi.org/10.5194/egusphere-egu2020-3336, 2020.

In August 2015, the German government approved the national programme for the responsible and safe management of spent nuclear fuel (SNF) and radioactive waste proposed by the Federal Ministry for the Environment, Nature Conservation, Building and Reactor Safety (BMU). The assumption is that about ~ 1 100 storage casks (10 500 tons of heavy metal) in the form of spent fuel assemblies will be generated in nuclear power plants and will have to be disposed. However, a decision on the disposal concept for high-level waste is pending and an appropriate solution has to be developed with a balance in multiple aspects. All potential types of host rocks, clay and salt stones as well as crystalline formations are under consideration. In the decision process, evaluation of the risk of different waste management options and scenarios play an enormous role in the discussion. Coupled physical and chemical processes taking place within the engineered barrier system of a repository for high-level radioactive waste will define the radionuclide mobility/retention and the possible radiological impact. The objective of this work is to assess coupled processes occurring in the near-field of a generic repository for spent nuclear fuel in a high saline clay host rock, integrating complex geochemical processes at centimetre-scale. The scenario considers that radionuclides can be released during a period of thousands of years after full saturation of the bentonite barrier and the thermal phase.

Transport parameters and the discretization of the system, are implemented in a 2D axisymmetric geometry. The multi-barrier system is emplaced in clay and a solubility limited source term for the selected radionuclides is assumed. Kinetics and chemical equilibria reactions are simulated using parameters obtained from experiments. Additionally, porosity changes due to mineral precipitation/dissolution and feedback on the effective diffusion coefficient are taken into account. Protonation/deprotonation, ion exchange reactions and radionuclide inner-sphere sorption is considered.

Numerical simulations show, that, when the canister corrosion starts, the redox potential decreases, magnetite precipitates and H₂ is formed. Furthermore, the aqueous concentration of Fe(II) increases due to the presence of magnetite. By considering binding to montmorillonite via ion exchange reactions, the bentonite acts as a sink for Fe(II). Additionally, magnetite forms a chemical barrier offering significant sorption capacity for many radionuclides. Finally, a decrease of porosity in the bentonite/canister interface leads to a further deceleration of radionuclide migration. Due to the complexity of reactive transport processes in saline environments, benchmarking of reactive transport models (RTM) is important also to build confidence in those modelling approaches. Development of RTM benchmark procedures is part of the iCROSS project (Integrity of nuclear waste repository systems - Cross-scale system understanding and analysis) funded by both the Helmholtz Association and the Federal Ministry of Education and Research (BMBF).

How to cite: Montoya, V., Silva, O., Coene, E., Molinero, J., Lu, R., Shao, H., and Kolditz, O.: Near field evolution of a spent fuel repository in an argillaceous rock formation and impact on radionuclide migration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6079, https://doi.org/10.5194/egusphere-egu2020-6079, 2020.

Radionuclides usually migrate slower than the flowing water due to sorption and matrix diffusion. The performance assessment assumes that retention takes place mostly in the vicinity of the deposition holes. REPRO (REtention Properties of ROck matrix) experiments analyzed the matrix retention properties of the rock matrix under realistic conditions deep in the bedrock in ONKALO underground characterization facility at Olkiluoto, Finland. The objective was to investigate tracer transport in the rock matrix, which was representative to the near-field of the final disposal repository of the spent nuclear fuel, and to demonstrate that the assumptions made in the safety case of the deep geological spent fuel repository were in line with site evidence.

REPRO is composed of several supporting laboratory and in-situ experiments which investigate the retention properties under different experimental configurations. The first in-situ experiments were water phase diffusion experiments performed 2012-2013. Through Diffusion Experiment (TDE) studies diffusion and porosity properties of rock matrix in stress field of repository level and sorption properties of nuclides in intact rock circumstances.

The TDE experiment has been performed in three parallel drillholes drilled near to each other. Breakthrough of the radioactive tracer is monitored with on-line measurements and samplings along and perpendicular to the foliation. The non-sorbing radioactive isotope traces of HTO and ³⁶Cl, as well as slightly sorbing ²²Na and strongly sorbing ¹³³Ba and ¹³⁴Cs were used. TDE was designed to control advective flow, as it had caused problems in previous in-situ tests.

Supporting laboratory studies were performed for drillcore samples sampled from the experimental drillholes. In these laboratory experiments, i.e. porosity, permeability and diffusion coefficients of the drillcores were determined using different methods.

The TDE experiment was carried out from 2016 to 2019. A breakthrough was seen in the timeframe predicted by scoping calculations carried out. REPRO has produced data and knowledge to the safety case and the performance assessment. According to the preliminary results, values measured in the laboratory are applicable also in larger scale and in-situ conditions.

How to cite: Rahkola, K., Poteri, A., Koskinen, L., Andersson, P., Nilsson, K., Byegård, J., Siitari-Kauppi, M., and Helariutta, K.: REPRO: Through Diffusion Experiment (TDE) - Diffusion and Porosity Properties of Rock Matrix in Stress Field of Repository Level, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-74, https://doi.org/10.5194/egusphere-egu2020-74, 2020.

Sorption is one of the main processes, which determine the retention of radionuclides (RN) in a repository for nuclear waste. In a multi-barrier system, the host rock poses the ultimate barrier retarding the release of RN into the environment. Feldspars (e.g. orthoclase) are one of the main constituents of crystalline rock (e.g. granite), which is considered one potential host rock type in many countries (e.g. Finland, Sweden, Germany). In this study, the sorption of trivalent actinides (Cm, Am) and their rare earth analogues (Lu, Y, Eu, Nd, La) onto orthoclase (K‑feldspar) is investigated. For reliable predictions concerning the migration of RN, a process understanding on the molecular level of such processes is necessary. To achieve this, batch sorption experiments are combined with TRLFS and SCM.

Batch experiments were performed covering a broad range of experimental conditions (pH 4-11, oxic and anoxic conditions, [M³⁺] = 10^-6-10^-4 M, 3-50 gL^-1 orthoclase (grain size: < 21 and 63-200 µm; SSA: 4.2 and 0.2 m²g^-1)). Weak retardation below pH 5, followed by a strong increase between pH 5 and 7 and complete removal from solution at pH ≥ 8 was observed for all investigated metals. Cm- and Eu-TRLFS-measurements suggested the formation of an outer-sphere surface complex at lower (pH<5) and two different inner-sphere surface complexes at higher pH values (pH > 5 and pH > 7.5, respectively). Surface precipitation was observed for higher metal concentrations (10^-4 M). As the investigated metals revealed a similar behavior over a broad range of conditions, a generic approach was used for the SCM to describe the system as a whole. Experimental data of different series with different metals were simultaneously fitted by coupling PHREEQC with UCODE using the same underlying speciation model. Resulting generic stability constants for the involved surface complexes will be presented.

The identification of comparable processes and their unified description with one suitable model is important to map the complexity of natural systems onto simplified geochemical models. This step is crucial for large-scale reactive transport calculations needed for a reliable safety assessment of potential repository sites, as they require enormous computing efforts.

How to cite: Brinkmann, H., Neumann, J., Britz, S., Brendler, V., Stumpf, T., and Schmidt, M.: Sorption of trivalent actinides (Cm, Am) and their rare earth analogues (Lu, Y, Eu, Nd, La) onto orthoclase: Batch experiments, Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) and Surface Complexation Modeling (SCM), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7013, https://doi.org/10.5194/egusphere-egu2020-7013, 2020.

Alkali-activated materials (AAMs), hybrid cements (cements composed of Portland cement clinker, supplementary cementitious materials and an alkaline activator) and cements based on the principle of ‘Opus Caementicium’ (OC) potentially provide advantages over conventional cements for the use in sealing structures in rock salt, such as low heat of reaction and related thermal induced deformations (expansion and contraction) and autogenous shrinkage, all of which can lead to crack formation.
In this study several binders have been investigated by isothermal calorimetry and X-ray diffraction (XRD). Specific mix-designs of the AAMs were chosen for further investigations on the development of the mechanical strength of salt-saturated mortars, i.e. crushed rock salt was used as aggregates, under different conditions (23 °C/50 % r.H. and 40 °C/35 % r.H.) and compared to a ‘low-pH’-cement-based salt-saturated reference mortar.
After a reaction time of 7 days the heat of reaction of the hybrid cements was always lower than 70 % of that of an ordinary Portland cement (OPC), while the heat of reaction of the AAM and the OC was always lower than 20 %. For the hybrid cements Na₂SO₄ accelerated the early reaction of the Portland clinker, while Na₂CO₃ appeared to decrease the reaction and led to a shift of the second hydration peak (likely related to slag reaction) to later hydration times. Besides minor peaks in the heat flow after 4 days, the AAM and OC provided a rather continuous heat release over the considered reaction time.
The AAMs showed no signs of major crystalline reaction products; only traces of carbonates, alumina or zeolites were identified after 28 days. For the hybrid cements, semicrystalline C-A-S-H, portlandite and hydrotalcite were present in all samples as hydration products. Furthermore, in these systems the use of Na₂SO₄ lead to the formation of ettringite (AFt-phase), while the use of Na₂CO₃ lead to the formation of hemicarbonate (AFm-phase).
Compared to the ‘low-pH’-cement-based mortar, the AAM-based mortars had lower mechanical strength after 28 days. With increasing curing time the differences decreased. Increasing the temperature and decreasing the air humidity led to an acceleration of the strength development at early ages, but also to a decrease of the final strength for specific AAM-based formulations, suggesting the introduction of microcracks due to drying shrinkage. The exception was the two-part AAM-formulation, which provided the highest final strength in the considered period (91 days) independent of the storing conditions.

How to cite: Sturm, P., Gluth, G., Moye, J., Garel, S., and Kühne, H.-C.: Comparative Study of Alternative Binders for Concrete Sealing Structures in Rock Salt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4606, https://doi.org/10.5194/egusphere-egu2020-4606, 2020.

For underground storage facilities and future HAW repositories, a secure closure is indispensable. Within the scope of two consecutive research projects, three closure elements were installed in large-scale tests at the Teutschenthal mine in the Carnallitit Mountains between 2006 and 2008. Special mention should be made here of the large-scale test 2 (“GV2”), which was produced from MgO concrete with the 5-1-8 binder phase. This structure was made using the dry-mix shotcrete procedure. The low temperature development during the setting of the shotcrete was very advantageous. The 10.25 m long structure, with a height and width of 3.55 m each, consists of 104 concreting sections with an average layer thickness of 9.9 cm. It was of interest whether the concreting section boundaries (“BAG”) influence the permeability (negatively). The structure is equipped with pressure transmitters and TDR sensors in three measuring levels. After completion of the structure and injections in the contact area, the integral system permeability was 2*10^‑16 m². Liquid pressurization via pressure chamber was carried out on the test structure after a maturing period of about 10 years. After 8 years, the permeability with gas and with solution was determined in boreholes and on drill cores, especially with regard to the development over time. The determined in-situ gas permeability is on average 2.7*10^‑19 m², on compact concrete (without BAGs) on average 2.0*10^-20 m². Test areas containing BAGs showed a higher permeability of maximum three orders of magnitude in some measurements. The solution permeability was determined both with a saturated NaCl solution and with a NaCl-saturated solution containing MgCl₂ and is between 1.0*10^‑20 m² and 9.0*10^‑20 m², whereby this decreases by half a power of ten over the measurement period of 600 days. In further integral injection tests in 4.5 m and 4.8 m long boreholes, a significant decrease in permeability over time was also observed. From an initial 2*10^‑15 m² and 4*10^‑16 m², respectively, the integral permeability decreased to <10^‑19 m² over a measuring period of 2.5 years. The reason for this decrease is the reduction of pore space due to the recrystallization of MgO and the transformation of the metastable 5-1-8 phase to the long-term stable 3-1-8 phase due to the increase in volume that takes place when the solution is added. Potential weak points or defects at the technically determined concrete section boundaries, therefore, do not represent weak zones in the structure in the long term due to this self-healing effect.

This paper reports on the large-scale experiment GV2 made of MgO concrete with 5-1-8 phase and the comprehensive permeability and strength investigations in drillings and on drill cores. The test results are the precondition for a modeling of the long-term behaviour of MgO-concrete.

How to cite: Arendt, J., Kudla, W., Wilsnack, T., Popp, T., and Freyer, D.: Investigations on the permeability of MgO concrete with 5-1-8 phase at the GV2-drift-sealing in the Teutschenthal mine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12948, https://doi.org/10.5194/egusphere-egu2020-12948, 2020.

Geochemistry of groundwater within the host rock is one of the most crucial boundary conditions for radionuclide speciation and migration.

The typical porosity of crystalline rock is less 0,5 wt%. In addition to the circulation of groundwater in the cracks, different types of fluids may be present in the pore space. Total porosity ε_T - corresponds to the volume of rock not filled with mineral grains. It is often defined by the relation: ε_T = ε_F + ε_D + ε_R, where ε_F stands for effective porosity (the dominant fluid transport is advective flow). ε_D represents diffusive porosity (the predominant transport in water-filled pores is diffusion) and ε_R represents residual porosity (discontinuous pores in which no transport takes place), in which solutions may also be present in closed inclusions.

Most of the solutions contained in the pore space, the "pore fluid of the rock matrix", cannot be collected using conventional groundwater sampling techniques. Only limited number of techniques has been reported, e.g. Smellie et al., (2003); Waber and Smellie (2008) and Eichinger et al. (2008).

Therefore, determination of pore water chemistry in crystalline rock from underground laboratory Bukov (SÚRAO) in at least 500 m depth was in focus, testing different laboratory and in-situ techniques. Fresh crystalline rock samples from the 12^th (-550 m) and 24^th (-1 000 m) horizon of Bukov URL were used for the laboratory experiments. In-situ sampler was installed at undisturbed section of the borehole at URL Bukov.

Firstly, leaching experiments, inspired by methods mentioned above were performed.

Secondly, high pressure techniques were used in order to extract pore water from the rock samples.

Finally, in-situ extraction of rock fluids, using a sampling packer system, installed into the undisturbed rock section, was applied.

Paralelly, geochemical modelling, using PHREEQC code, considering long-term interaction of main rock constituting minerals with solution in pores, was ongoing.

The results of in-situ sampling, lab leaching and modelling indicated that the determination of the pore water composition of crystalline rocks is still an open issue, since only about 1.5 ml of groundwater was present in approximately 20 cm of drill core (porosity below 0.5%).

Its composition will be significantly influenced by the equilibration of the solutions, entering pores, with the main and minor rock components (e.g. sulphites). The composition will most probably move towards the Na-HCO₃-Cl type with increasing chloride component with increasing depth. Here close communication with depth specific groundwater can be found. However, chlorine source within quartz – plagioclase – biotite – amphibolites rock type is not clear. Geochemical modelling showed that Cl^- source other than rock forming minerals might have an influence, either residual solutions or fluid inclusions (containing NaCl according to the analyses).

References:

Smellie J. et al.. (2003): Technical Report, SKB, TR-03-18.

Waber, H., Smellie (2008):  Applied Geochemistry 23, 1834-1861.

Eichinger F. et al. (2008): Project Report, Nagra, NPB 08-16.

Eichinger F. at al. (2006): Posiva Working Report 2006-103.

Acknowledgements

The work described herein was funded by SÚRAO within Deep Horizons project (SO2017-023).

How to cite: Havlová, V., Zuna, M., Pecková, A., and Jankovský, F.: Evaluation of crystalline rock pore water geochemistry in DGR conditions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20326, https://doi.org/10.5194/egusphere-egu2020-20326, 2020.

Russian Federation has a selected site for Deep Geological Disposal in the Nizhnekansky massif (Krasnoyarsk territory). The current work is devoted to justification of its suitability. One of the main parts of the safety case of is the prediction of radionuclides migration in the environment which requires development and application of groundwater flow and transport models. This work presents the evolution of the hydrogeological model.

The granitoid rock of Nizhnekansky massif is complicated by presence of such geological structural elements as dykes, faults and crushing zones which influence significantly permeability features. Currently all available geological data are consolidated with the use of the MICROMINE program into a structural geological model. The three-dimensional model of the distribution of the main structural elements in the area of the DGD site is more detailed. The corresponding dykes appear to have a north trend and a steep fall (about 70 ° east).

Using the geometry of structural elements one can assess their role in the structure of groundwater flow on the basis of profile model. Verification of the model was carried out on the basis of measured hydraulic heads.

The preliminary calculations showed that including into the model additional structural elements (to a greater extent dykes and crushing zones) leads to a better matching between observed and model heads. This implies the need to take into account the structural elements  more accurately. The migration of a conservative tracer was calculated as well using the developed flow model.

Moreover, the heterogeneity near the Deep Geological Disposal is three-dimensional in nature and it is impossible to implement it accurately in a two-dimensional setting without approximations. This requires three-dimensional modeling, such 3D numerical flow and transport models are developed using the GeRa code.

How to cite: Neuvazhaev, G., Rastorguev, A., Morozov, O., Kapyrin, I., and Grigorev, F.: 3D hydrogeological modeling of Deep Geological Disposal in the Nizhnekansky Rock massif, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21509, https://doi.org/10.5194/egusphere-egu2020-21509, 2020.

In some countries, low carbon steel has been considered as the candidate material of the disposal container for high-level radioactive wastes (HLWs) due to its excellent anti-irradiation, high strength, low cost and fine processing performance. However, during the long-term geological disposal, the steel disposal container will suffer from the threat of corrosion damage under the coupled THMC conditions.

This work focused on the corrosion behavior of low carbon steel under different water conditions in compacted bentonite of China-Mock-Up by in situ electrochemical impedance spectroscopy (EIS) with the infiltration of groundwater from outside to inside. Based on the EIS results, the corresponding equivalent circuit models were proposed to interpret the evolution of electrochemical characteristics of low carbon steel with the increase of water content in compacted bentonite. In the initial stage of EIS measurement, water in bentonite around the electrochemical sensors from outside to inside was hygroscopic water and chemical bonding water successively. With the running of China-Mock-Up, water in outer bentonite transformed from hygroscopic water to free water. Meanwhile, the water in the inner bentonite blocks transformed from chemical bonding water to hygroscopic water, which caused a slight corrosion of low carbon steel. After China-Mock-Up running for 1202 days, the instantaneous corrosion rate of low carbon steel located in the inner bentonite blocks was just 0.002 mm/a. While in the outside bentonite blocks, the corrosion rate reached to 0.58 mm/a after 1155 days, indicating that the free water could cause a serious corrosion of low carbon steel.

How to cite: Wei, X., Liu, Y., and Dong, J.: Corrosion behavior of low carbon steel under different water conditions in compacted bentonite of China-Mock-Up, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6356, https://doi.org/10.5194/egusphere-egu2020-6356, 2020.

In Germany, salt deposits play an important role as industrial raw material as well as sites for energy storage. However, in geological fault zones, the contact with migrating groundwater can lead to the formation of geogenic caverns that are filled with gas and brine. These brine occurrences belong to the most significant risks in salt mining as they can cause mine flooding and land subsidence. Especially within highly soluble potash seams, the interactions between brine and salt rock result in cavernous structures surrounded by moisture penetration zones (hereinafter referred to as transition zones). In order to facilitate an early detection and a safe long-term retention of geogenic caverns, the temporal and spatial development of these transition zones was simulated.

In a first step, the software PHREEQC (Parkhurst & Appelo, 2013) and a polytherm dataset for the hexary system Na-K-Mg-Cl-SO₄-Ca-H₂O from THEREDA were used to investigate the dissolution behavior of different potash salts. A titration model based on thermodynamic equilibrium showed that the components within a potash seam are only partly converted into secondary minerals. Brine composition and precipitations mainly depend on the ratio between kieserite and sylvite and the dissolution process only stops if water, kieserite or sylvite is fully depleted. As a consequence, 1 kg of brine can influence several tens of kilograms of potash salt. A 1D model in PHREEQC implied that the transition zone between a cavernous structure and the unaffected rock can be divided into different mineralogical regions, containing secondary minerals like glaserite, leonite or kainite besides halite. A comparison with measured data from a natural brine occurrence validates the model results. However, these models do not include temporal or spacial scaling.

The titration model in PHREEQC was then used as a basis for the coupling of chemistry and hydraulics which is done in Python. Transport processes free and forced convection as well as diffusion are taken into account. A 2D model of the potash seam was built considering the stratification of the rock as well as changing permeabilities due to geological fault zones and dissolution and precipitation. Cavern and transition zone are assumed to be porous media which coincides with field measurements from K+S. In the area of the dissolution front, the amount of dry potash salt that is made available for chemical reactions is controlled by a dissolution rate. Apart from that, thermodynamic equilibrium is assumed within the transition zone but a temporal scaling is still given based on the exchange rate. Besides sensitivity analyses, several scenario analyses for varying initial and boundary conditions have been done. The results are compared to a natural transition zone in a german mine and provide important insights into the long-term development of natural cavern systems within potash seams.

References:

Parkhurst, D. L., Appelo, C. A. J. (2013): Description of input and examples for PHREEQC version 3 - a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. US Geological Survey Techniques and Methods, 6 (A43), 497 p.

How to cite: Steding, S., Kempka, T., Zirkler, A., and Kühn, M.: Geochemically coupled 2D models reproduce the formation of transition zones within potash seams, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1578, https://doi.org/10.5194/egusphere-egu2020-1578, 2020.

The disposal of heat generating nuclear waste is increasingly becoming a concern for several countries worldwide. This issue is of particular concern for the United States because of the 364,000 m³ of heat-generating nuclear waste currently in temporary storage. Numerous concepts for the disposal of heat generating nuclear waste have been investigated internationally, such as, mined repositories in crystalline, argillite, and salt formations, and deep borehole disposal. Currently, salt formations are being investigated as candidate disposal host rocks for heat-generating nuclear waste in the United States, Germany, the Netherlands, and the United Kingdom. Salt formations may be an ideal host media due to salt’s extremely low permeability, high thermal conductivity, and self-healing capability. Salt lacks circulating groundwater, but it is not dry. Brine availability in salt has multiple implications for the safety and design of a nuclear waste storage facility. Brine transport is a potential off-site radionuclide transport vector, brine leads to corrosion of metallic and glass waste forms and waste packages, chloride in brine can reduce criticality concerns, and accumulated brine can provide back-pressure that resists long-term creep closure of porosity associated with mining the repository. In order to improve understanding of brine migration in heated salt, borehole heater experiments are being conducted at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The salt heater test collaboration aims to collect data to gain a better understanding of brine availability, transport, and thermal evolution of salt in response to heating up to 140 °C. Due to the complex nature and coupled processes that take place within bedded salt, this study will utilize 1D, 2D, and 3D numerical simulations of the salt heater test to deconvolve the parametric controls on brine availability and migration. The purpose of this study is two-fold, in addition to understanding the hydrogeology of salt formations, we utilize two different subsurface flow codes in a code comparison study, TOUGH and PFLOTRAN. Preliminary results from this study illustrate the importance of understanding the host rock properties and the initial/boundary conditions of the salt and multiphase fluid flow near the excavation site.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

How to cite: Jayne, R. and Kuhlman, K.: Reservoir Response to Heat Generating Nuclear Waste Disposal in Bedded Salt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5705, https://doi.org/10.5194/egusphere-egu2020-5705, 2020.

Claystone is characterized by a complex, highly coupled hydraulic-mechanical behavior. The physical understanding of the related effects is of great importance concerning the stability during the construction phase as well as for the safety assessment of the integrity of a potential repository for high-level nuclear waste. The rock laboratory Mont Terri, Switzerland, provides the unique possibility to conduct in-situ experiments in the Opalinus Clay for a broad international community. The experiment on the influence of humidity on the cyclic and long-term deformation behavior (CD-A experiment) is conducted in the new part of the rock laboratory, which has been finalized in 2019.

To compare the coupled hydraulic-mechanical effects under different conditions, two parallel oriented niches, called twins, have been excavated in autumn 2019. The twins have a length of 11 m and a diameter of 2.3 m and no shotcrete support. The first twin remains under “natural conditions”. Here, the atmospheric conditions are characterized by a seasonal change in air humidity and temperature. This leads to a desaturation of the claystone around the niche. The second twin is locked. In this area, the air conditions imply a high humidity and the desaturation of the claystone will be avoided as much as possible.

In both twins, a geological characterization of drill cores and of rocks exposed in the niches have been carried out. Furthermore, a long-term measurement program of the related parameters has been launched. It includes measurements of the air humidity, the temperature, the deformation (extensometer), the convergence of the niches, the pore water pressure (piezometer) and the water content (Taupe). Additionally, periodic measurements of the permeability, electrical resistivity (ERT), and nuclear magnetic resonance (NMR) on the niche walls as well as petrophysical analyses of drilled cores are planned. Seismic borehole measurements will also be carried out. The measuring program will be accompanied by the numerical simulation of the coupled hydraulic-mechanical effects in the vicinity of the niches. The comparison of the measurements with simulation results considering different model approaches should support the identification of significant physical effects of the complex coupled material behavior.

This contribution will focus on the observations during the excavation of the twin niches and analysis of the first measured data as well as numerical investigations carried out with OpenGeoSys.

How to cite: Maßmann, J., Ziefle, G., Costabel, S., Furche, M., Graupner, B., Hesser, J., Jaeggi, D., Königer, F., Rink, K., Schuhmann, R., Vowinckel, B., and Wieczorek, K.: In-situ Experiment on the Influence of Humidity on the Cyclic and Long-Term Deformation Behavior (CD-A) of the Opalinus Clay at the Mont Terri Rock Laboratory, Switzerland: Excavation of the Twin Niches, First Measurements, Simulations and Analysis , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9314, https://doi.org/10.5194/egusphere-egu2020-9314, 2020.

A multi-barrier system in which the radioactive waste is encapsulated in metal containers surrounded by a geotechnical barrier (e.g. compacted bentonite) deep underground in a stable geological formation is one of the internationally accepted options for the disposal of highly radioactive waste. Bentonites have good properties such as high swelling capacity and low hydraulic conductivity, which makes them favorable as backfilling material. However, indigenous microorganisms may affect these properties. Bentonite samples were collected from the Full-scale Engineered Barrier Experiment (FEBEX) - Dismantling Project [1] at the Grimsel Test Site (Switzerland) to study their microbial diversity. For that, total DNA was extracted directly from the cores and from enrichments of sulfate- and iron-reducing microorganisms as well as microorganisms were isolated from those enrichments. The microbial communities of the bentonites, the enrichments, as well as the isolates were analyzed by 16S rRNA gene sequencing. The results showed that Actinobacteria and Alphaproteobacteria dominated the FEBEX bentonite microbial population, while the dominant phylum in both enrichments was Firmicutes: concretely, Bacilli and Clostridia classes. In addition, bacteria from the genera Desulfitobacterium, Desulfosporosinus and Clostridium were isolated from the enrichments. Desulfosporosinus hippei DSM 8344 as a phylogenetic close relative was selected to study its interactions with uranium and especially its potential to reduce U(VI) to U(IV). This study revealed that microorganisms are present in bentonite samples after a long-term continuous heating. Sulfate- and iron-reducing microbes were enriched by using favorable conditions in specific media and the potential of the sulfate-reducing microorganisms on the reduction of uranium was verified. Therefore, it is important to characterize the microbial population of the bentonite, because microbes might compromise the safety of the deep geological repository of highly radioactive waste.

[1]http://www.grimsel.com/gts-phase-vi/febex-dp/febex-dp-introduction

How to cite: Raff, J., Lopez-Fernandez, M., Hilpmann, S., Kluge, S., Steudtner, R., and Cherkouk, A.: Microbial activity in nuclear waste repository systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9654, https://doi.org/10.5194/egusphere-egu2020-9654, 2020.

In this study, we considered different the wind speeds affects the thermal performance when the dry storage cask for spent nuclear fuel , which used NAC- MAGNASTOR cask of system, and placed outdoors. We Ddiscuss the dry storage cask for spent nuclear fuel cooling by the thermal buoyancy ventilation of cask. Firstly, Compare the experimental data of low-speed wind tunnel experiments with the result of from a commercial software PHOENICS CFD (Computational Fluid Dynamics) for heat flow analysis, and confirm the reliability of the CFD simulation results of the software. Then we used the software to simulate higher wind speeds to understand the thermal performance of the cask for spent nuclear fuel by various wind speeds. With external wind speed is was more much faster, channel airflow of the cask hads increased, especially upper channel airflow of the cask and near steel cylindrical. The temperature of the windward side of the cask hads also been significantly reduced, The lee side hads a slower wind speed and a smaller temperature drop, and generates an eddy below the lee side, which helpeds to dissipate .heat,. However, as the wind speed gradually increases, the lee side changed to a downdraft, and the temperature drop was also slowed down. It is noticeable that the situation may occur when a typhoon comes.

How to cite: Cheng, L. and Lai, C.-M.: The effect of the cooling on a dry storage cask for spent nuclear fuel by different wind speed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13709, https://doi.org/10.5194/egusphere-egu2020-13709, 2020.

One important criterion for the characterization of a potential nuclear waste repository is the crustal stress field. However, stress data are sparse and usually incomplete regarding the six independent components of the stress tensor. The World Stress Map (WSM) is a valuable compilation of stress data, but it does not include information about stress magnitudes as only the orientation of the maximum horizontal stress (S_Hmax) is provided. To receive a comprehensive and continuous 3D description of the stress field in a particular area, geomechanical-numerical modelling is required. Key objectives of the SpannEnD project (Spannungsmodell Endlagerung Deutschland) is to provide such a model for Germany and to develop methods for robust stress predictions at the local scale.

The SpannEnD model is based on finite element techniques and comprises a 3D lithosphere-scale structural model of Germany. The lateral extent of the model covers a pentagon-shaped area of Central Europe with dimensions of 1000 x 1250 km². The model has been chosen significantly larger than Germany to reduce boundary effects in the study area. Furthermore, on the base of the observed stress orientation pattern, the boundaries have been defined parallel or perpendicular to the known orientation of S_Hmax to simplify the definition of the boundary conditions. The vertical extent of the model is from the surface to a depth of 100 km, incorporating several sedimentary layers, several basement units and the Mohorovičić discontinuity. The mesh is laterally homogenous with a resolution of about 4 km and vertically inhomogeneous with a decreasing resolution with increasing depth, to provide the finest mesh in the layers of the greatest interest, near the surface. These units also provide the most stress data measurements to calibrate the model. Furthermore, a selected number of important faults is implemented in the model. This structural model is discretized into about 4 million elements. For the calibration of the model we use a new compilation of stress magnitude data. We present the workflow, the model geometry, and some first results.

How to cite: Ahlers, S., Röckel, L., Henk, A., Reiter, K., Hergert, T., Müller, B., Schilling, F., Heidbach, O., Morawietz, S., Scheck-Wenderoth, M., and Anikiev, D.: The SpannEnD Project - Numerical modelling of the 3D stress state of Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16928, https://doi.org/10.5194/egusphere-egu2020-16928, 2020.

Numerical models of a generic repository for nuclear waste were created to analyze the long-term barrier integrity of bedded salt formations. This study focuses on sensitivity analysis and validation for numerical simulation by using 2D models. Under consideration of thermal loading caused by the disposal of heat generating nuclear waste, thermal and thermomechanical coupled calculations were performed using FEM code JIFE (Faust et al. 2016). Thermal analyses were conducted to investigate the influence of mesh density on the temperature results in both near and far field of disposal area. Thermomechanical coupled calculations were carried out to evaluate boundary influence of different model dimension on the results of temperature and vertical displacement. Furthermore, since the overburden are assumed to be elastic, variation of Young’s modulus shows sensitivity of overburden stiffness in thermomechanical coupled calculations of the generic model for bedded salt. The calculated stress and deformation were compared to investigate the influence of overburden stiffness on the results of geomechanical assessment of the salt barrier integrity. The observation of this study gives a better understanding of how to establish a 2D or 3D model in bedded salt formation for barrier integrity analysis.

How to cite: Liu, W., Eickemeier, R., and Fahland, S.: Sensitivity analysis for numerical modeling of a generic repository for nuclear waste in bedded salt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14981, https://doi.org/10.5194/egusphere-egu2020-14981, 2020.

The multi-barrier concept of HLW storage in geological formations relies on the thermo-, hydro-, mechanical (THM) and chemical properties of the potential host rock. In claystone, these properties are strongly influenced by its mineralogical composition and their spatial distribution.

As part of the first phase of the BASTION project, BGR carried out investigations in two areas. The geological part focused on the structural-lithological composition of various claystone formations. The numerical part investigated methods to integrate the drillcore-scale properties into formation-scale numerical THM models with the aim of being able to perform generic model studies at this scale. A major challenge in the first phase of the project was the conversion and transfer of data collected from structural and mineralogical studies to the input parameters required in the THM models for the simulation of generic site studies and future assessment cases.

In the current contribution, a further step is taken towards this purpose while retaining the focus on the determination of thermal parameters of claystone. The classification of the geological sub-units as facies types in claystone has been extended with the introduction of the sub-facies concept, thus allowing a finer resolution in the classification within each facies type. The introduction and standardization of this concept for claystone are foreseen to allow a more precise choice of samples for the experimental determination of (thermal) parameters. This workflow is presented as a proof-of-concept and is utilized in a simplified simulation to evaluate its benefits. Extension of the concept to other input parameters of numerical THM models will be touched upon. The presented concept contributes towards the continuing effort to integrate data from drillcore-scale measurements in formation-scale simulations.

How to cite: Kumar, V., Kneuker, T., and Maßmann, J.: Transferring geo-/mineralogical information to THM simulations of HLW storage in claystone formations (BASTION II), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17898, https://doi.org/10.5194/egusphere-egu2020-17898, 2020.

To date, the most secure, technically feasible and internationally accepted solution for the safe management of Radioactive Waste (RW) is burial in deep host rock units, also referred to as disposal in a deep geological repository (DGR). For this purpose, it is mandatory to select a site in a hydrogeological setting which provides sufficiently safe natural conditions for waste isolation from groundwater flow over long time periods (up to 1 Ma).

However, over such a long time period, external factors (e.g. climate change) and intrinsic basin features (e.g. tectonics), here referred to as changing conditions, will impact the hydrological (H), thermal (T), mechanical (M) and chemical (C) state of the entire system. Therefore, it is crucial to better understand the impacts of changing conditions on far-field radionuclide mobilization and behavior in order to select the most suitable DGR for RW disposal.

Multiphysics simulators offer powerful tools that couple groundwater flow (H), transport of heat (T), as well as geochemical reactions (C) in a deforming solid framework (M). These coupled THM-C numerical models can provide evaluations for performance and safety assessment of a DGR at different scales. However, a limited number of studies so far addressed the far-field evolution of radionuclides under the changing conditions listed above.

The newly funded German Science Foundation (DFG) and the Russian Foundation for Basic Research (RFBR) project “INFRA” (NA1528/2-1 and MA4450/5-1; 2020-2022) aims to investigate the impacts of (i) glaciation, (ii) permafrost and (iii) tectonic events on the coupled boundaries that control large-scale groundwater flow near hypothetical waste repositories. For this purpose, the open source simulator OpenGeoSys [1,2] will be applied using available data from selected areas of the Yeniseisky Site in Russia [3].

Though the context of this study is related to RW issues, the outcomes of INFRA will be of interest for any field of geosciences that deals with large-scale simulations of coupled processes under transient boundary conditions.

[1] Kolditz, O., Bauer, S., Bilke, L., Böttcher, N., Delfs, J. O., Fischer, T., … Zehner, B. (2012). OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media. Environmental Earth Sciences, 67(2), 589–599. https://doi.org/10.1007/s12665-012-1546-x

[2] Bilke, L., Flemisch, B., Kalbacher, T., Kolditz, O., Helmig, R., & Nagel, T. (2019). Development of Open-Source Porous Media Simulators: Principles and Experiences. Transport in Porous Media, 130(1), 337–361. https://doi.org/10.1007/s11242-019-01310-1.

[3] Laverov, N., Yudintsev, S., Kochkin, B., Malkovsky V. (2016). The Russian Strategy of using Crystalline Rock as a Repository for Nuclear Waste. Elements, 12(4), 253–256. https://doi: 10.2113/gselements.12.4.253  

How to cite: Magri, F., Nagel, T., Liebscher, A., and Malkovsky, V.: Impacts of changing conditions on far-field radionuclide evolution: background and goals of a newly funded DFG-RFBR project., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17712, https://doi.org/10.5194/egusphere-egu2020-17712, 2020.

This study aims at investigating the hydraulic and thermal properties which are important for buffer materials to be used for geological disposal of high-level radioactive wastes. MX-80 bentonite and Kunigel V1 bentonite, originated from Wyoming USA and Japan, respectively, were used in the experimental program. The characteristics of these 2 bentonites, including soil-water characteristic curve, swelling pressure, hydraulic conductivity, and thermal conductivity were determined in the laboratory. And these data are applied in the simulation of the resaturation processes of buffer material in a deposition hole, such that a comparison can be made on the thermo-hydro-mechanical coupling effects of the buffer material can be evaluated. It is found that the two bentonites do not behave very differently in terms of the moisture distribution and heat transfer characteristics with the same boundary conditions assumed. 

How to cite: Huang, W.-H.: Experimental study on the thermo- and hydro- properties of two bentonites as buffer materials, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19178, https://doi.org/10.5194/egusphere-egu2020-19178, 2020.

After some decades in applying boric acid with natural isotopic abundance (natural boric acid, NBA) solution as a neutron absorber, some nuclear facilities have started to use boric acid enriched in B-10 (enriched boric acid, EBA) to increase the control ability and parallelly, decrease the amount of liquid waste. Meanwhile, the stabilization condition of EBA in the cementitious matrix and durability of the waste form in disposal facilities have not been assessed or at least have not been reported yet. However, high relative mass difference between the two stable isotopes of boron (B-10 and B-11) implies a different leachability index for cementitious matrix prepared with NBA and EBA wastes.

In this study, the leachability (ASTM C1308-08 standard, 2017) of boron isotopes from cementitious matrix and its geochemical background will be assessed using ICP-OES, XRD, SEM-EDX and Raman-spectroscopy. The effects of parameters such as temperature, water to cement ratio (w/c), boric acid concentration and shape of the waste form will be studied. Geochemical modeling of the experiments will be done via PHREEQC software, which should support our understanding of the different geochemical behavior of NBA and EBA.

Based on the theoretical knowledge, a significant increase in leachability of boron from the cementitious matrix is expected when EBA is used instead of NBA because of the geochemical behavior of the two stable isotopes. Increase in leachability is expected when temperature and w/c increased, whereas the leachability will decrease when the normal cylindrical shape of samples are changed to spherical shape.

References:

• M. Saleh and H. A. Shatta; 2013; Immobilization of Simulated Borate Radioactive Waste Solution in Cement-Poly(methylmethacrylate) Composite:Mechanical and Chemical Characterizations; Journal of Nuclear Chemistry; Article ID 749505.
• Lucile Dezerald, Jorge J. Kohanoff, Alfredo A. Correa, Alfredo Caro, Roland J.-M. Pellenq, Franz J. Ulm and Andrés Saúl; 2015; Cement as a Waste Form for Nuclear Fission Products: The Case of 90Sr and Its Daughters; Journal of Environmental science and technology; 49; 13676−13683.
• ASTM C1308 - 08(2017); Standard Test Method for Accelerated Leach Test for Diffusive Releases from Solidified Waste and a Computer Program to Model Diffusive, Fractional Leaching from Cylindrical Waste Forms; west conshohocken, PA 19428, United state
• IAEA; 1996; Processing of nuclear power plant waste streams containing boric acid; IAEA-TECDOC-911, ISSN 1011-4289; Vienna, Austria.

How to cite: Rostamiparsa, M., Szabó-Krausz, Z., Fábián, M., Falus, G., Szabó, C., and Völgyesi, P.: Experimental assessment of interaction between boric acid enriched in boron-10 and cementitious matrix , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19441, https://doi.org/10.5194/egusphere-egu2020-19441, 2020.

Numerical modelling is an important method in the improvement of the understanding of a variety of geological processes such as the reactivation of faults and seismicity, orogeny or volcanism. Furthermore, it can be crucial for geotechnical activities such as geothermal use of the underground, oil and gas production or the use of dams. Geomechanical models enable stress predictions even in areas without stress data and can therefore greatly contribute to the long-term safety and productivity of underground activities.

As computational power is limited the geology of geomechanical models often needs to be simplified, especially for larger scale models. This is true not only for the resolution of the implemented stratigraphy but also for the implementation of faults as they severely increase the amount of required elements and influence the model stability. Furthermore, the implementation of faults often leads to artifacts and can therefore reduce the accuracy of the model results. Due to these limitations it is frequently necessary to distinguish between faults that are crucial for the model as they influence the stresses in magnitude and orientation on a large scale and faults that will only influence the model on a local scale and may therefore be neglected on a regional perspective. The impact of faults on a geomechanical model depends on various different factors such as geometry and mechanical properties of the fault itself, the tectonic regime or the scale of the model. As the relevance of a fault for a geomechanical model is not necessarily identical to its relevance in other geoscientific fields it can be challenging to identify relevant faults.

The SpannEnD project focuses on the generation of a 3-D geomechanical model of Germany and adjacent areas in the context of the disposal of nuclear waste in order to predict the tectonic stresses in areas without stress data. There is a multitude of faults known in the modelling area but due to their sheer amount not all faults can be incorporated. Criteria have to be found that drastically reduce the number of faults while keeping the change in the geomechanical stress pattern to a minimum. We will present different criteria that can be used for the fault selection which have being worked out in the framework of the SpannEnD project.

How to cite: Röckel, L., Ahlers, S., Morawietz, S., Reiter, K., and Müller, B. and the The SpannEnD Team: Criteria of fault selection for geomechanical models , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20210, https://doi.org/10.5194/egusphere-egu2020-20210, 2020.

The Federal Republic of Germany has decided to dispose its high-level radioactive waste in deep geological formations. Three types of host rock are considered: rock salt, clay rock and crystalline rock. The Site Selection Act (StandAG¹), which came into effect on the 16^th of May 2017, defines the successive steps of the repository siting process, which has to ensure the best possible safety conditions for a period of one million years. Based on precaution considerations, the StandAG (§27 (4) StandAG) sets a preliminary temperature limit of 100°C at the outer surface of a repository container for the preliminary safety assessment.

This contribution provides an overview about the state of the scientific and technical knowledge on the limiting temperatures in the repository site selection process of Germany. It also illustrates the different treatments of the definition of temperature limits within other European siting processes. The findings highlight that, in Europe, the proposed criteria which consider temperature at the outer surface of a repository container get more and more into focus of research and discussion especially for the three different types of host rocks.

After presenting the national regulatory frameworks, this contribution summarizes how the European countries address the different temperature related issues for their site selection, their repository concepts and how in turn these all can influence the German safety case strategy for the German site selection. Not at least, links to site selection criteria in other countries (e.g. USA, Japan, Russia) are provided.

Reference

¹  StandAG: Standortauswahlgesetz vom 5. Mai 2017 (BGBl. I S. 1074), das zuletzt durch Artikel 2 Absatz 16 des Gesetzes vom 20. Juli 2017 (BGBl. I S. 2808) geändert worden ist.

How to cite: Maurer-Rurack, U., Liebscher, A., and Magri, F.: State of the scientific and technical knowledge about limiting temperatures in the Repository Site Selection process of Germany with simultaneous consideration to Europe and other European repository concepts , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20302, https://doi.org/10.5194/egusphere-egu2020-20302, 2020.

Nuclear power generation became popular in the 1950s in industrialised countries as an alternative to fossil energy sources to provide large amounts of low cost, low carbon energy. Currently 6% of the world’s energy supply is produced in 451 nuclear reactors across 30 countries. However, nuclear power generation has a serious disadvantage and hidden cost: the accumulation and disposal of spent fuel or high level nuclear waste (HLW) - notably highly radioactive nuclear fission products and the absence of suitable long-term storage solutions, threatening livestock and the environment. Sustainable disposal of HLW holds many challenges: fluid and heat transfer may induce strongly coupled undesirable thermal, hydrological, mechanical and chemical processes.

A crystalline rock repository construction license has been accomplished by Finland in 2015 for the first long-term HLW repository worldwide. In Germany, a consortium of federal offices is exploring the opportunity of establishing a long-term underground repository in crystalline rock for HLW as an alternative to potential repositories in salt rock and mudrock.

The aim of this research is to de-risk hypothetical storage solutions for long-term HLW repositories in Germany in crystalline rock. As no geological site must be alluded to for legal reasons during the repository site investigation process at the time being, flow is modelled for a generic fractured rock site based on academic studies of crystalline rock. An inverse problem approach is applied to investigate hydraulic site requirements for the long-term storage of HLW and provide footing for the analysis of coupled thermal, hydrological, mechanical and chemical processes. 

This work demonstrates progress towards finding a long-term storage solution for HLW in Germany through evaluating hydrological processes in a generic crystalline rock site. Through Oda analysis and simulating steady-state flow and particle tracking in a synthetic discrete fracture network (DFN), degrees of fracture connectivity and hydraulic conductivity of fractures have been identified for the hydraulic (boundary) conditions in a repository in crystalline rock.

How to cite: Weihmann, S. and Maßmann, J.: Creating a generic model of a high level waste (HLW) repository in crystalline rock and determining hydraulic parameters to investigate minimum requirements to the host rock for a safe storage location according to national German law, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19930, https://doi.org/10.5194/egusphere-egu2020-19930, 2020.

The German Site Selection Act (Standortauswahlgesetz – StandAG) defines the search for and selection of the national German site with best possible safety for a disposal facility for high-level radioactive waste. The Federal Office for the Safety of Nuclear Waste Management (BASE) is the federal regulatory authority for radioactive waste disposal. BASE supervises the site selection process for a repository for high-level radioactive waste and is responsible for the accompanying public participation. To fulfill its tasks according to the state of science and technology, task related research forms an integral part of BASEs activities. Current research activities in the context of the site selection process address geoscientific questions, methodological aspects of the implementation of the site selection process, and public participation aspects. This contribution provides an overview on the current geoscientific and methodological research activities of BASE.

According to § 16 StandAG , the national implementer (Bundesgesellschaft für Endlagerung  mbH) has to execute surface-based exploration and BASE has to review and define the respective exploration program. Therefore, the two projects MessEr and übErStand compiled state of science and technology with regard to surface based exploration methods. The foci were on methods suitable for addressing the criteria and requirements set out in the German Site Selection Act.

The project KaStör reviewed the current knowledge on active faults and fault zones in Germany and studied methodological approaches to date and identify the activity of faulting. The results support BASE to review the application of the exclusion criteria for areas with “active faults zones” according to § 22 (2) StandAG.

For the time being, § 27 (4) StandAG defines 100 °C as precautionary maximum temperature at the outer surface of waste canisters for all host rocks. The project Grenztemperatur compiled and studied the temperature dependency of the different THMC/B processes according to available FEP catalogues for rock salt, clay stone, and crystalline rock. The project also identified open and pending research questions and describes ways to define host rock specific maximum temperatures based on specific disposal and safety concepts.

During the site selection process, safety oriented weighting of different criteria and comparison of different potential regions and sites have to be performed. The project MaBeSt studied and reviewed methodological approaches to this weighting and comparison problem with special emphasis on multi criteria analysis (MCA) and multi criteria decision analysis (MCDA).

Key requirement for safe geological disposal of nuclear waste is barrier integrity. The project PeTroS experimentally studied potential percolation mechanisms of fluids within rock salt at isotropic conditions at disposal relevant pressures and temperatures.

How to cite: Liebscher, A., Borkel, C., Maurer-Rurack, U., and Jendras, M.: Towards best possible safety - Current regulatory research for the German site selection process for high-level radioactive waste disposal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22601, https://doi.org/10.5194/egusphere-egu2020-22601, 2020.

Coupled thermo-hydro-mechanical (THM) models are used for the assessment of nuclear waste disposal, reservoir engineering, and other branches of geo-environmental engineering. Model-based decision-making and design optimization in these domains require sensitivity analyses (SA) and uncertainty quantification (UQ) methods that are suitable for coupled THM problems on an engineering scale. Due to different coupling levels, non-linearities, and large spatial and temporal extents, these analyses can often be challenging both conceptually and computationally.

For an initial evaluation in a setting relevant to nuclear waste disposal we start by employing an analytical solution for thermal consolidation around a point heat source which encompasses the most relevant primary couplings and allows us to cover the entire parameter space robustly and efficiently. For uncertainty quantification, we applied an experimental design (DoE-) based history-matching approach. This approach uses DoE methods to construct a proxy model, which is used later for efficient Monte Carlo sampling and subsequent filtering of the uncertainty space of the history-match error. As a result, we obtain a family of curves that is compatible with the prior parameter set and experimental data to match, which then enables further uncertainty quantification. In our work, we demonstrate the applicability of the workflow and discuss its particular suitability to this problem class, including its (in-)sensitivity to prior parameter distribution assumptions.

For SA, we contrast the conclusions drawn via two different approaches: local one variable at a time (OVAT) and global sensitivity analysis (GSA) based on Sobol indices for different spatio-temporal settings to observe near and far-field effects as well as early- and late-stage system response. The conducted studies can serve as a benchmark for UQ and SA software designed around numerical THM simulators.

How to cite: Buchwald, J., Chaudhry, A., Yoshioka, K., Kolditz, O., and Nagel, T.: Sensitivity analyses and uncertainty quantification in THM models: a benchmark study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22535, https://doi.org/10.5194/egusphere-egu2020-22535, 2020.

Geological disposal of High-Level Radioactive Waste (HLRW) is considered to be one of the best disposal methods that can stably and semi-permanently isolate high-level radioactive waste from the biosphere. In this study, three types of potential host rocks for HLRW disposal were selected and the hydrogeological characteristics were investigated using deep drilling cores collected at about 50 m intervals in the borehole of a depth of 1 km. The rocks used in this study were granites and gneiss which are crystalline rock, and mudstone which is a sedimentary rock. The results of the study showed that the average porosity of granite was 0.48% and the permeability ranged from 7.87 × 10^-19 m² to 1.39 × 10^-21 m² except for samples outside the measurement limit (4.04 × 10^-22 m²). The average porosity of gneiss was 0.49% and the permeability ranged from 3.62 × 10^-18 m² to 4.58 × 10^-22 m². The average porosity of mudstones was 3.62% and the values of permeability for most mudstone samples were lower than the measurement limit. For SEM-EDS analysis, many microcracks were observed in the crystalline rock samples having high permeability. On the other hand, there were almost no microcracks in crystalline rock samples having low permeability, and even if there were cracks, the cracks were filled with fillers such as clay minerals. These results indicate that the presence of microcracks or the filling of cracks in crystalline rocks has a significant effect on the flow of groundwater through the host rock.

How to cite: Park, J., Ko, K., Lee, M., and Yang, M.: Hydrogeological characteristics of candidate sites for high-level waste disposal in South Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21147, https://doi.org/10.5194/egusphere-egu2020-21147, 2020.

The undisturbed stress state of a potential site for nuclear waste disposal is of key importance for the assessment of long-term stability of the geotechnical installations and for seismic hazard assessment. In particular, the stability of pre-existing faults within and near a repository can only be evaluated with the knowledge of the initial stress state. Information on stress magnitudes is rare and unevenly distributed. Thus, 3D geomechanical-numerical modelling is used to estimate the stress state in an area of interest. However, due to the limitation of available data, the modelled stress state has a large uncertainty which has not been rigorously quantified yet. We present an approach to quantify the uncertainties in a 3D geomechanical-numerical modelled stress field. We combine the available S_Hmax and S_hmin data records to pairs. For each pair we compute an individual model scenario. At each location in the model each scenario contains the full stress tensor. Then, from all model scenarios we compute an average value and a standard deviation for each component of the full stress tensor at each location within the model. This provides a comprehensive assessment of the stress state and its uncertainties.

Furthermore, we present an approach to reduce the previously quantified uncertainties in the model results: We use additional borehole observables (Formation Integrity Tests) and observed seismicity and - if available - its focal mechanisms. These observables cannot provide any data records on the stress state. Yet, the information that can be extracted is valuable as it contains upper boundaries for the magnitudes of the minimum principal stress (Formation Integrity Tests) and the maximum principal stress/differential stress (seismicity), respectively. These boundaries are compared to the stress states in the individual model scenarios. Then, each scenario is assigned a weight based on its agreement with the additional data. This allows computing a weighted average and a standard deviation. The resulting standard deviation is clearly smaller compared to the unweighted approach and small changes in the average stress state are observed. Thus, even with only limited data record availability, a quantification and even a significant reduction of uncertainties in the modelling results is possible which increases the significance and value of the model.

How to cite: Ziegler, M. and Heidbach, O.: Quantification and Reduction of Uncertainties in 3D Stress Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2821, https://doi.org/10.5194/egusphere-egu2020-2821, 2020.

The Yanim plain hosts the national Israeli radioactive waste disposal site. The site is located on the Miocene aged Hazeva formation, comprised of loose sand, sandstone and dispersed clay layers. The current research examines the sorption capacity of the local sand to solutions doped with Cs ions. The sand contains ~95% quartz and ~5% of various clays, carbonates, and oxides. Batch sorption experiments were conducted at a liquid to solid ratio of 10. Two end-member solutions were used, fresh (MQ) and concentrated (Na-Nitrate solution). Both solutions were doped with 0.1, 1, 10, 100 and 1000 ppm of Cs (as a nitrate). For the MQ experiments Kd values ranged between ~2 and ~1300, where the highest Kds were registered for the 1 ppm doping level, and the smallest Kds were for the 1000 ppm doping level. For the concentrated solution Kd values ranged between ~0 and ~1.5, where the highest Kds were for the 1 ppm doping level, for all other doping levels Kds were <1. Freundlich and Langmuir isotherm calculations revealed a significantly better correlation on a linearized Freundlich isotherm, indicating a multi-layer and multi-site sorption model, with a similar slope for both solutions, indicating a common sorption mechanism. Column transport experiments (L=25cm, r=2cm, φ=30%, 1PV=180cc) have shown minimal retardation of the Cs in the concentrated solution flow experiments (R= ~2). A second, probably colloid-related peak, showed an early breakthrough with respect to a conservative color tracer.  On the other hand, when MQ was run in the column no breakthrough was observed within 10 column pore volumes. In one fresh experiment a very small colloid related peak was found with breakthrough similar to the conservative tracer. We used the CXTFIT model to calculated the dispersity (λ [L]) and normalized velocity (V) of the measured tracer. For the conservative tracer the values were λ=0.13 cm and V=0.9 cm. For Cs transport in the concentrated solution λ=0.22 cm and V=1.6. Thus, it may be concluded that even in similar doping levels in the same matrix the solution chemistry will play a major role in contaminant retardation. Thus, rain episodes which abruptly change the solution chemistry, can significantly affect solute and colloid mobility.

How to cite: Klein-BenDavid, O., Balaban, N., Gelfer, S., Komerian, R., Saban, D., and Rosenzweig, R.: Cs batch sorption and column transport experiments on sand from the Yamin plain, Israel – Experimental results and modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4078, https://doi.org/10.5194/egusphere-egu2020-4078, 2020.

Monitoring and Retrieval of High-Level Radioactive Waste

The retrievability of high-level radioactive waste (HAW) is defined as the option to retrieve previously emplaced waste from a respository. This is a design requirement in many countries, as for example in Germany, justified by the need to react on possible failures in the repository system.

Retrievability affects the footprint of the repository (Léon-Vargas et al., 2017) and requires a monitoring program (Stahlmann et al., 2018), as the decision on retrieval should be justified on sound basis. For a holistic analysis of the design consequences of retrievability of high-level radioactive waste it is necessary to get information about the retrieval process itself. In TRANSENS, a transdisciplinary research platform for HAW disposal research, the retrieval process will be analyzed in general.

The presentation will focus on a generic repository approach based upon Stahlmann et al. (2018) modified for the analysis of the retrieval process. Main impacts of the retrieval works on the host rock were identified, as the effects of the redriven emplacement drifts on the repository system. The presentation will focus on these processes and give a short outlook on their consequences for a monitoring program.

Leon Vargas, R.; Stahlmann, J.; Mintzlaff, V. (2017): Thermal impact in the geometrical settings in deep geological repositories for HLW with retrievability and monitoring. 16th International High-Level Radioactive Waste Management Conference (IHLRWM 2017), Charlotte, NC, April 9-13, 2017.

Stahlmann, J.; Mintzlaff, V.; León Vargas, R.P.; Epkenhans, I. (2018): Normalszenarien und Monitoringkonzepte für Tiefenlager mit der Option Rückholung. Generische Tiefenlagermodelle mit Option zur Rückholung der radioaktiven Reststoffe. ENTRIA-Arbeitsbericht-15. Braunschweig.

How to cite: Mintzlaff, V. and Stahlmann, J.: Monitoring and Retrieval of High-Level Radioactive Waste, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5738, https://doi.org/10.5194/egusphere-egu2020-5738, 2020.

Predictions of rock spalling around deep-drilled boreholes and tunnels in underground geologic repositories in crystalline rocks remain a significant challenge, due to the heterogeneities present in the rock mass, uncertain stress fields, and the complex thermo-mechanical behaviour of the rock mass at elevated temperatures.

This study presents a three-dimensional numerical analysis of multiple fracture growth leading to spalling around a deposition borehole. The mechanical spalling due to stress amplification after drilling is simulated using a finite element-based discrete fracture growth simulator. Fractures are grown by computing stress intensity factors at each fracture tip, and the mesh is adapted to accommodate the changing fracture geometries at every growth step. The model is validated using the Äspö Pillar Stability Experiment (APSE), calibrated to simulate the drilling of a borehole in the Forsmark granite, and subjected to a far-field anisotropic triaxial stress, corresponding to the in situ stress model from Forsmark. The deposition tunnel is implicitly simulated by attaching the deposition borehole to a free domain boundary.

The effect of borehole geometry on the predicted spalling around a typical deposition borehole is studied. The cylindrical borehole is modified at the top to provide an access ramp for the spent fuel canisters, which can effectively improve the repository design by reducing the height of the deposition tunnel. Three cases are investigated, in which the borehole top is cylindrical, conical, and wedge-shaped, respectively. Numerical results show that spalling occurs in all cases, but the borehole geometry affects fracture nucleation and growth patterns. The enlargement of the borehole top induces higher stress concentrations at the borehole-tunnel junction, increasing the severity of spalling at the top of the borehole. The final spalled zone and the fractures-borehole interaction are illustrated for each stress and geometry scenario.

How to cite: Saceanu, M. C., Paluszny, A., Zimmerman, R., and Mas Ivars, D.: Impact of deposition borehole geometry on mechanical spalling in nuclear waste repositories, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5936, https://doi.org/10.5194/egusphere-egu2020-5936, 2020.

In the multi-barrier system of HLW repository, overpack is the first barrier to isolate high-level radioactive nuclides from biosphere, and Low carbon steel has been considered to be a promising candidate material for manufacturing the oberpack due to its good mechanical performance and workability and weldability. However, during thousands of years of geological disposal, the corrosion resistance of low carbon steel and its corrosion evolution behavior are the first element that must be fully understood, because it determines the life cycle of the artificial barrier.

Conventional studies had suggested that the corrosion of low carbon steel under the deep geological environment was driven by hydrogen evolution reaction (HER) based on that the dissolved oxygen was completely depleted during the long term disposal. However, the residual oxygen content is a critical factor to determine the corrosion mode of cathodic reduction reaction. Thermodynamics data indicated that the initial ferrous corrosion products formed in the deaerated bicarbonate solution can be chemically oxidized into ferric substance by the trace content of dissolved oxygen, and the accumulated FeOOH as a cathodic depolarizer significantly increased the open circuit potential and enhanced the corrosion rate of the low carbon steel. Moreover, chloride and sulfate in the simulated groundwater can reduce the increase of open circuit potential but it still promotes the corrosion of the low carbon steel. As the environments contained aggressive anions and high concentration of dissolved oxygen, low carbon steel was prone to suffer from the localized corrosion and the corrosion rate was obviously increased. By alloying with some contents of Ni and Cu, the corrosion rate of low alloy steel was decreased by an order of magnitude and it was less prone to suffer from the localized corrosion.

Under the conditions of simulated groundwater with different content of GMZ bentonite，the bentonite colloidal particle layer attached to the surface of low carbon steel showed blocking effect on resisting oxygen diffusion to the steel substrate, which consequently decrease the further oxidation of ferrous to ferric substances and the corrosion rate of low carbon steel. However, the barrier performance of bentonite colloids would be deteriorated due to their coagulation caused by the ferrous ions dissolved from the steel substrate. High content of bentonite was beneficial to maintain and to prolong the stabilization of the barrier system. An equivalent circuit model which correlates with the interfacial structure between electrode substrate and rust and bentonite layer was proposed. The fitting results showed a very good match between the model and experimental data, and the evolution of the results was also in agreement with real changes.

How to cite: Dong, J.: Corrosion study of low carbon steel under simulated geological disposal environments for HLW in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6413, https://doi.org/10.5194/egusphere-egu2020-6413, 2020.

These abstract bases on the R & D project ELSA Phase II - Concept development for shaft seals and testing of sealing elements for HAW repositories, funded by the German Federal Ministry of Energy and Economic Affairs.

The installation of sealing elements in salt rock requires a mechanical support system, which is chemical compatible with the host rock. In future HAW-repositories abutments and sealing elements within the shafts and drifts could be made of magnesia building material with the long term stable 3-1-8 binder phase, if solution containing magnesium can attack the seal.

In 2014 a first large-scale experiment was performed in the Sondershausen salt mine in Germany. A vertical borehole with the depth of two meters and a diameter of 1.1 meter was filled with the magnesia-based concrete. Several sensors measured the development of temperature, comprehensive stress and expansion within the test construction for approximately one year. During the binding reaction the temperature increased by 55 K in the center. After 150 days, the expansion in axial direction reached 2.4 mm/m and 1.1 mm/m in radial direction.

In 2018 a second large-scale-experiment was performed in the Teutschenthal salt mine, Germany to continue the investigations. A new vertical borehole with the depth of 3.5 meters and a diameter of 1.3 meters was filled with the same material. The temperature in the center increased by 40 K during the binding reaction and decreased down to the ambient temperature after 20 days. In result of the first experiences, the stress sensor range was increased. After one year a comprehensive stress of 6.2 MPa was measured at the contour and is still evolving at this point (early 2020). The maximum axial expansion reached 7.9 mm/m and stays at this level. The maximum radial expansion reached 0.7 mm/m 20 days after concreting and decreased subsequently. This material behavior corresponds to the high comprehensive stress level.

The second experiment is equipped with a pressure chamber at the bottom. A first determination of the integral gas permeability revealed a value of approx. 3E^-18 m² to 3E^-17 m². In the near term a multistage pressurization of the construction is planned, using a saturated NaCl solution to evaluate the sealing ability.

This contribution reports on the measured parameters (temperature, stress, strain) of the two large-scale tests with long-term stable MgO-concrete and the composition requirements to obtain a long-term stable MgO concrete. Long-term stable MgO concrete with 3-1-8 phase has been used for the first time in these tests. The measured test-results are the foundation for modelling the behaviour of the MgO-concrete.

How to cite: Aurich, J., Freyer, D., Gruner, M., and Kudla, W.: Large-scale tests to investigate MgO concrete with a long-term stable 3-1-8 phase in the Sondershausen and Teutschenthal mines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7897, https://doi.org/10.5194/egusphere-egu2020-7897, 2020.

Studies on incorporation of radionuclides into the crystal structures of phosphate minerals strongly indicate that uranium and its mobile fission products can be efficiently immobilized through uptake from aqueous solution by formation of phosphate and carbonate minerals. Aiming at development of a new engineered backfill material, we have investigated uptake of uranium at a range of temperatures similar to those expected at waste repository sites, where thermal peak of the waste package (heated by radioactive decay) is ~300°C (Greenburg and Wen 2013). The available literature data on uranium uptake by phosphates are limited to ambient temperatures (e.g. Arey et al. 1999), and to our best knowledge no experimental studies on uranium uptake by phosphates and carbonate at hydrothermal conditions have been performed.

Experiments were conducted in the autoclaves at saturated water pressure and 200-350°C, where metastable phosphate (brushite) and carbonate (aragonite) were transformed to apatite/monetite and calcite in NaCl solutions. Uranium was introduced into autoclave in separate tubes in the forms of U₃O₈ and UO₃ in experiments at reduced and oxidized conditions. Oxidation state of dissolved uranium (U⁴⁺ or U⁶⁺) was controlled by addition of solid redox buffers into autoclaves.

X-ray diffraction (XRD) and backscattered electron diffraction (EBSD) of crystalline products allowed estimation of mineral transformation rate. Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS allowed obtaining uranium content in crystals and its concentration in co-existed solution. Partition coefficient (D) of uranium was calculated as the ratio of uranium content in the solid to uranium concentration in the solution. Selected solids were examined with synchrotron-based X-ray absorption spectroscopy (XAS).

Overall, our results showed: 1) brushite transforms to monetite and apatite mixture during 6 days, but up to 1 month is required for complete transformation to apatite; 2) mineralogy of the final phase (monetite or hydroxyapatite) depends on ionic strength of the solution (confirmed by thermodynamic calculations); 3) uranium is compatible with phosphate and carbonate minerals, where D could be as high as 1000; 4) uptake of U⁴⁺ by calcite is higher than that of U⁶⁺ by up to a factor of 100; 5) uranium incorporates into calcite structure as U⁶⁺ at oxidized conditions. Additional analyses are pending and results will be presented at EGU meeting.

References

Arey J.S., Seaman J.C., and Bertsch P.M. (1999) Environ. Sci. Technol.  33, 337-342.

Greenburg H.R. and Wen J. (2013) LLNL-TR639869-DRAFT, 38.

How to cite: Gabitov, R., Migdisov, A., Anh, N., Angel, J., Noah, V. H., Alberto, P.-H., Aleksey, S., Kirsten, S., Jason, B., Varun, P., Florie, C., Hongwu, X., and Robert, R.: Immobilization of uranium by phosphate and carbonate crystallization as an improvement of engineering barriers., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10772, https://doi.org/10.5194/egusphere-egu2020-10772, 2020.

The Israeli national site for radioactive waste is situated in the Yamin Plain, within the Negev desert. Estimation of  water recharge to the ~500 m deep vadose zone underlying the site  is crucial for assessing risks related to contaminants transport. However, estimation of water fluxes in deep arid vadose zones is a challenging task because of their small magnitude and the lack of a direct measurement technology. Studies conducted in a deep arid vadose zone in Nevada, USA point to complex transient flow dynamics, in which the direction of water flow in the top of the vadose zone is upward while in the rest of the section water flows downwards to the water table.    

            In this study we present a combination of techniques which are used to obtain an initial evaluation of the water dynamics in this environment. These techniques include direct and continuous measurements of water content at the upper 5.5 m of the vadose zone through a vadose zone monitoring system which contain FTDR water content sensors; profiles of water content, leachable chloride and soil texture; and numerical modeling.

            The monitoring of the upper 5.5 m of the vadose zone during the years 2014-2018 indicates that even after extreme rain events of ~ 50 mm (constituting more than a half of the annual rainfall) there is no water infiltration to the lower parts of the section. These results exemplified the need for an alternative method to detect low water fluxes that characterize this arid area. We therefore use an inverse modeling approach where numerical solutions of water movement in the vadose zone are fitted to measured profiles of chemical and physical parameters from two shallow boreholes in the Yamin Plain. The water content of both boreholes revealed an extremely dry environment, with low saturations and high pore-water chloride concentrations, above 15,000 mg/l, in certain depths. Peak chloride concentrations did not coincide in the two boreholes, raising the question whether these peaks are connected to water fluxes or to changes in soil texture, which can inhibit water infiltration.

             Numerical simulations were then used to solve water flow and solute transport. Input parameters, including chloride deposition rate, precipitation rate, and surface run-off fraction were varied to fit the measured chloride profiles. Results indicate very small water fluxes of less than 1 mm/yr in the bottom of the vadoze zone. The simulations also show that the mass of chloride in the profile is less than the one expected based on estimated chloride deposition rate and published records of paleo-rain. These results suggest either a delayed climate shift to dry conditions compared to previous estimates for the region (8000 yr BP), and/or a partial input of the 4 g/m²/yr of deposited chloride, possibly due to runoff.

How to cite: Balaban, N., Rosenzweig, R., Stauffer, P., Klein-BenDavid, O., Dody, A., Calvo, R., Kuluris, S., and Bussod, G.: Assessment of effective infiltration in the deep arid vadose zone of the Negev, Israel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13981, https://doi.org/10.5194/egusphere-egu2020-13981, 2020.

In Germany, the site selection for a repository for radioactive waste in deep geological formations was (re-) started in 2017 with the Repository Site Selection Act coming into force. The Site Selection Act envisages preliminary safety assessments as a measure to ensure the safety of a considered site.

The focus of the presentation will be the methodology of the preliminary safety assessments as it is derived from the legal requirements. In this context, the Federal Ministry for Environment, Nature Conservation and Nuclear Safety published the draft of the regulation on the safety requirements for the disposal of high-level radioactive waste in summer 2019. Article 2 of this regulation contains the requirements for the implementation of preliminary safety assessments in the site selection procedure. One essential aspect is the systematical identification and characterization of uncertainties. We will discuss the key features of the handling of uncertainties in the site selection procedure, especially with regard to numerical reactive transport modelling. The German Site Selection Act is divided into several steps with increasing level of detail. The identification and quantification of uncertainties plays a major role to improve quality and plausibility in each step. Well-prepared explorations for instance, need to be addressed in a way to minimise data uncertainties. In addition, the handling of uncertainties in safety assessments on an international level is evaluated.

How to cite: Rühaak, W., Kreye, P., Hoyer, E.-M., Wolf, J., Panitz, F., and Gawletta, D.: Methodology of preliminary safety assessments in the site selection procedure in Germany , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20522, https://doi.org/10.5194/egusphere-egu2020-20522, 2020.

Claystones are considered to represent an important barrier rock in the context of nuclear waste storage. When cavities are opened underground, the rock mass in the near vicinity of the constructed repository is strongly affected by unloading, which is generally referred to as the Excavation Disturbed Zone (EDZ). This area is primarily characterized by newly formed unloading fractures, leading to an enhanced hydraulic transmissivity of the EDZ in comparison to the intact host rock. This phenomenon can affect the integrity of a geologic barrier as open fractures provide possible flow paths and endanger the long-term safety of underground storage facilities. A precise characterization of the EDZ is therefore essential for risk assessment and strategy development in terms of radioactive waste disposal.

In this study the Excavation Disturbed Zone (EDZ) of the Mont Terri Rock Laboratory is investigated with regard to hydraulic, mechanical and geophysical properties by using three simple field measuring devices, (1) portable permeameter, (2) microscope camera and (3) needle penetration test (NPT). The hydraulic aperture of accessible joints within the Opalinus Clay formation in the EZ-B niche is measured by a portable transient-airflow permeameter. The instrument was validated by flow-through experiments and is able to accurately determine hydraulic fracture apertures down to about 10 µm. In-situ measurements were carried out at 43 points and show a mean hydraulic aperture of 84 ± 23 µm, extending over a range from 20 to 100 µm. Fracture apertures do not change with increasing distance to the gallery in the accessible area of uncovered claystone.

For the same set of measuring points, the mechanical fracture aperture was determined by a digital microscope camera. Mechanical fracture apertures in the EZ-B niche ranged between 16 and 1400 µm with a mean value of 268 ± 276 µm. As comparable hydraulic apertures can be derived from the measured mechanical aperture by using empirical relations based on estimated joint surface roughness, the microscope camera represents a valuable alternative besides the air permeameter. The hydraulic characterization of the EDZ proves the existence of accessible fluid pathways within the Opalinus Clay of the Mont Terri Rock Laboratory, even about 15 years after tunnel excavation.

The mechanical and geophysical properties of the EDZ are investigated by a needle penetration test (NPT). Whereas the needle penetration index (NPI) is strongly influenced by bedding anisotropy, the influence of the EDZ is negligible. The NPT proves to be a suitable tool for estimating mechanical properties by using different empirical relations. Especially for the uniaxial compressive strength, a high correlation with literature values is observed. In contrast, geophysical parameters such as P-wave velocity cannot be reliably determined with this method. The obtained field data could be used as a reasonable input for numerical models that aim at investigating swelling and shrinking behavior of the Opalinus Clay with regard to self-sealing processes within the EDZ.

How to cite: Hale, S., Ries, X., Jaeggi, D., and Blum, P.: Methods for in-situ HM characterization of claystone at the Mont Terri Rock Laboratory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20570, https://doi.org/10.5194/egusphere-egu2020-20570, 2020.

The safe disposal of nuclear and radioactive waste is one of the most challenging tasks in current and future environmental geosciences. In so-called deep geotechnical repositories nuclear waste barrels are either directly embedded in argillaceous buffers located in deep bedrock formations or the buffer is placed at distance to seal the deposit tunnel cavity. Due to their swelling capacity and low hydraulic permeability bentonite- or clay-based materials are widely regarded as suitable buffer materials.

The design of these deep geotechnical repositories is not a simple task and its evaluation and improvement is still subject of current research. During the design, finite element models can be used to simulate the behavior of the buffer and the bedrock subjected to hydro-mechanical loading. In order to achieve realistic predictions, these models have to meet several requirements: coupled hydro-mechanical simulation techniques are needed to capture the dependence of the swelling process of the buffer (or the bedrock) on the amount of supplied mountain water. Further, the swelling induced changes in the hydraulic permeability, strain and stress should be addressed in the simulations. The swelling process, however, is a path-dependent process which should also be taken into account by the numerical model.

Although several models capable of predicting the swelling process already exist, a hydro-mechanical model, which incorporates the capability of modelling swelling of an initially fully saturated material depending on its loading history, still lacks.

The proposed constitutive model is aimed to be suitable for application in both the dry and the swollen state. The swelling process is activated by a change in volumetric water content. We therefore introduce a swelling water content which defines how much of the pore water contributes to the swelling of the porous medium. The swelling water is assumed to be attached to the material particles and cannot be reduced by mechanical processes. Thus, the swelling process is regarded irreversible. To incorporate the path dependency of the swelling process, the evolution of the swelling water content depends on the effective stress state. Irreversible changes of the material due to swelling, e.g. a reduction of the stiffness, are modelled by introducing a swelling degree, which allows the transformation of material properties from the dry material to those of the swollen material.

The experimental studies which form the basis of the proposed constitutive model include oedometric swelling tests, standard oedometer tests and measurements for the determination of the suction-saturation relation and the hydraulic permeability. All tests are carried out on reconstituted samples of opalinus clay.

The proposed hydro-mechanical model is implemented using the finite element method and validated by numerical simulations. First simulations are in good agreement with the experimental results.

How to cite: Nitsch, A., Machacek, J., Leuthold, J., Grandas Tavera, C., and Wichtmann, T.: A novel hydro-mechanical model for swelling of argillaceous material, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22481, https://doi.org/10.5194/egusphere-egu2020-22481, 2020.