Thermodynamic databases play an important role in assessing the safety of a particular site selected or being investigated for a radioactive waste repository. They allow solubility limity to be considered when calculating the migration of radionuclides and other pollutants. In combination with sorption modelling, over-conservatism can be reduced, so that potential sites and concepts may kept in play.

Particularly for saline solutions, which are typical of many potential host rocks in Germany, there are still considerable data gaps despite many years of research. However, an adequate database needs to be made available in a timely manner. A step-by-step strategy is proposed to close data gaps within a reasonable timeframe:

1. Identify the range of geochemical boundary conditions (pH, redox level, concentration of major and minor components)
2. Expand the database with literature data from other databases, reviews and individual publications
3. Close remaining gaps primarily with estimated thermodynamic data
4. Check, where feasible, performance of new data set using experimental reference values
5. Identify long-term kinetic barriers
6. Assign numerical uncertainties to estimated data
7. Identify the most sensitive data gaps by probabilistic modelling of solubilities under different geochemical boundary conditions
8. Focus experimental and theoretical research on priority data gaps

The example of molybdenum is used to illustrate how the THEREDA database can be extended via steps 1 to 4. The element occurs in the repository mainly as an inactive alloy component and as a long-lived activation product. Under the geochemical conditions potentially relevant for a repository (pH between 6 and 13, Eh strongly reducing to moderately oxidising), molybdenum can occur in the oxidation states +III, +IV, +V and +VI. The following steps have been taken:

• Data from the JAEA database were used as a starting point
• They were supplemented with experimentally based literature data for Mo(VI) species, solid phases and Pitzer ion interaction coefficients

Major gaps in knowledge concerned the aquatic chemistry and solid phases of Mo(III), Mo(IV) and Mo(V). These have been addressed by estimation methods:

• Extrapolation approaches and analogies for aqueous Mo(III) and Mo(IV) species and solid phases
• Correlation methods and reference values for Pitzer ion interaction coefficients for Mo(III) and Mo(IV) species

Lack of thermodynamic data and lack of evidence for its presence in environmental media lead to the omission of Mo(V). In agreement with experimental results, the extended THEREDA database indicates that Mo(IV) is not stable under alkaline conditions and is partially displaced by the more soluble Mo(VI). Only in near-neutral solutions is Mo(IV) the only predominant oxidation state.

Further work is needed to understand the kinetic barriers that prevent the establishment of thermodynamic equilibria between Mo oxidation states (step 5). Steps 6 and 7 will be crucial in deciding which aspect of the rather complicated aqueous molybdenum chemistry requires closer attention.

Financial support

This research has been partly supported by the German Federal Ministry of Economic Affairs and Energy (BMWi) (grant No. 02E11365)

How to cite: Hagemann, S.: When time matters – accelerating the development of thermodynamic databases illustrated by the example of molybdenum, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-87, https://doi.org/10.5194/safend2025-87, 2025.

Technetium (₄₃Tc) is an inherent radioactive element with isotopes ranging from ⁸⁵Tc to ¹²⁰Tc [1]. Among them,⁹⁹Tc is the most environmentally relevant since it is a high-yield fission product of ²³⁵U and ²³⁹Pu, and the ground-state nuclear isomer of metastable ⁹⁹Tc (^99mTc)– the isotope worldwide most commonly used for radiodiagnosis. ⁹⁹Tc is a long-lived beta minus emitter (τ_½ = 2.13×10⁵ years) and its aqueous speciation and migration behavior is influenced by the (physico-)chemical conditions (e.g, pH, E_h, presence of ligands, etc.). It is known that Tc^VIIO₄⁻ barely interacts with minerals and, thus, its mobility in water is high. On the contrary, the migration of Tc^IV is limited since it forms low-soluble species (e.g., TcO₂ or Tc-sulfides), surface complexes on minerals, and/or it is incorporated into the mineral structures [2]. Studying Tc mobility is a matter of concern for the safety assessment of the nuclear waste repository and radioecology. Thus, several works focused on aqueous Tc speciation based on redox changes and the chemical composition of the solution [3–5].

In this work, we have studied the Tc speciation when KTc^VIIO₄ is electrochemically reduced in carbonate solutions at varying pH (8.2–10.0), Tc concentration (0.5–9.5 mM), carbonate concentration (5–1000 mM), and the applied potential. Tc^VII reduction was monitored in situ by UV-vis, by using a spectro-electrochemical cell. At −0.85 V a pink solution (λ_max = 512 nm) was obtained, corresponding to a Tc^IV carbonate species [3], whereas reduction at −0.95 V yields a bluish green solution (λ_max = 630 nm), associated with a Tc^III carbonate complex [3]. Additionally, the obtained solutions were then investigated by ⁹⁹Tc NMR. The −0.85 V specimen gives rise to a resonance at ~1600 ppm, characteristic for Tc^V [6]. The solution yielded at −0.95 V, besides the aforementioned Tc^V signal, reveals one additional signal at ~152 ppm, corresponding to the chemical shift range of Tc^III [6].

These unprecedented NMR data on aqueous Tc carbonate species, complemented by UV-Vis spectroscopical analysis, advance the mechanistic understanding of Tc redox behavior, and help to improve safety and risk analyses for nuclear waste management.

In addition, this work will show the further developments on spectroelectrochemical methods to study the structural behavior of Tc and other redox-active elements in solution (by in situ NMR) and in solid phase (by in situ IR) as a function of the redox potential. These advanced techniques will help to determine the mobility of redox-active elements under varying redox conditions, which in turn will be useful for the safety assessment of the nuclear waste repository.

Acknowledgements:

The authors acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of NukSiFutur TecRad young investigator group (02NUK072)).

References:

[1] E.V. Johnstone, J. Chem. Educ. (2017) 320–326. [2] A.H. Meena, Env. Chem Lett. (2017) 241–263. [3]           J. Paquette, Can. J. Chem. (1985) 2369–2373 [4] M. Chotkowski, J. Electroanal. Chem. (2018) 83–90. [5] D.M. Rodríguez, Inorg. Chem. (2022) 10159–10166. [6] V.A. Mikhalev, Radiochemistry (2005) 319–333.

How to cite: Bureika, A., Kretzschmar, J., Müller, K., and Mayordomo, N.: Technetium speciation in carbonate media and further developments on spectroelectrochemical methods, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-27, https://doi.org/10.5194/safend2025-27, 2025.

The environmental fate of radionuclides (RN), such as actinides and fission products, disposed of in underground nuclear waste repositories is a major concern. Long-term safety assessments of these disposal sites depend on the ability of geochemical models and thermodynamic databases (TDBs) to reliably predict the mobility of RNs over very long time scales. One example where TDBs still have large data gaps is related to the complexation of trivalent actinides and lanthanides with aqueous phosphates. Indeed, solid phosphate monazites are one of the candidate phases for the immobilization of specific high-level waste streams for future safe storage in deep underground disposal facilities, therefore potentially and locally increasing the presence of phosphate at the final disposal site.

Recent work [1-3] obtained reliable complexation constants at 25 °C and at elevated temperatures and thus, closed some knowledge gaps. Laser-induced luminescence spectroscopy was used to study the complexation of Cm(III) and Eu(III) as a function of total phosphate concentration in the temperature regime 25-90 °C, using NaClO₄ as a background electrolyte (I = 0.5 to 3.0 M). These studies have been conducted in the acidic pH-range (−log₁₀ [H⁺] = 1.00, 2.52, 3.44, and 3.65) to avoid precipitation of solid Cm and Eu rhabdophane. For the first time, in addition to the presence of the 1:1 CmH₂PO₄²⁺/EuH₂PO₄²⁺ species [1-3], the formation of Cm(H₂PO₄)₂⁺ [2] and Eu(H₂PO₄)₂⁺ [3] was unambiguously established from the collected luminescence spectroscopic data. The conditional complexation constants of all aqueous complexes were found to increase with increasing ionic strength and temperature [1-3]. Extrapolation of the obtained conditional complexation constants to infinite dilution at 25 °C and elevated temperature was performed by applying the Specific Ion Interaction Theory (SIT). Using the integrated van´t Hoff equation, both the molar enthalpy of reaction Δ_rH_m° and entropy of reaction Δ_rS_m° values were derived.

During complexation, depending on the concentration of phosphate, monodentate or bidentate Cm(III)/Eu(III)-phosphate complexes can form with different overall coordination numbers (8,9). Obtaining additional information about the complex structures from spectroscopic data only is often challenging, especially at the low metal-ion concentrations used in the experiments. Thus, relativistic quantum chemical (QC) methods can be considered as an additional tool to complement experimental observations. In this study, the structural properties, electronic structures, and thermodynamics of the 1:1 and 1:2 Cm(III) and Eu(III) phosphate complexes were solved using state-of-the-art QC calculations. In particular, the QC methods allowed i) to investigate the complexation strength of Cm(III) and Eu(III) with aqueous phosphate, ii) to understand the possible change of the coordination number with increasing temperature and iii) to investigate the nature (ionic/covalent) of the Cm/Eu bonds with water and phosphate. Combining the information obtained from quantum chemical calculations with the observed spectral changes facilitates the decisive determination of the structures of the formed phosphate complexes and their overall coordination [2,3].

[1] Jordan, N., et al., Inorg. Chem. (2018), 57:7015−7024.

[2] Huittinen, N., et al., Inorg. Chem. (2021), 60:10656−10673.

[3] Jessat, I., et al., Inorg. Chem. (in preparation).

How to cite: Jordan, N., Jessat, I., Huittinen, N., Réal, F., and Vallet, V.: Cm(III) and Eu(III) complexation with aqueous phosphates: an experimental, thermodynamic, and ab initio study, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-118, https://doi.org/10.5194/safend2025-118, 2025.

The safe long-term disposal of high-level radioactive waste in deep geological repositories requires a thorough understanding of radionuclide (RN) migration at interfaces between host rocks and geoengineered barriers, such as bentonite, particularly under geochemically relevant conditions. As part of the international Colloid Formation and Migration (CFM) project [1], laboratory-scale experiments are being conducted to investigate interactions between RNs, bentonite colloids, and host rock materials under the geochemical conditions (pH = 9.6, Eh = -220 mV) of the Grimsel groundwater (GGW) system. Colloid-mediated RN migration must be assessed in the context of repository scenarios in crystalline host rock, where bentonite erosion and container failure are considered, and advective groundwater flow is assumed.

To our knowledge, this is the first attempt to study the sorption properties of lamprophyre dykes from the Grimsel Test Site (GTS), NAGRA’s generic underground research laboratory in crystalline rock, and to compare them with granodiorite [2] to assess their capacity to immobilize key RNs relevant to colloid-mediated transport, including U, Np, Pu, Am, and Tc.

A two-phase experimental approach is being implemented in parallel to assess various sorption properties of the rocks. In Phase 1, batch sorption experiments using powdered lamprophyre and granodiorite samples (<63 µm) are being conducted under controlled conditions. Characterization techniques, including XRD, XRF, BET, SEM-EDS imaging, and ICP-OES/MS, have been employed to determine the composition and surface properties of the materials. An essential aspect of this phase is the selection of an optimal method to maintain reducing in-situ conditions, which is crucial for studying the behavior of redox-sensitive RNs. Rongalite and hydrazine are currently being tested in batch experiments without RNs, with Eh, pH, and elemental composition monitored over time.

Phase 2 focuses on RN surface sorption behavior onto bulk rock material using autoradiography after equilibration with GGW in the presence and absence of bentonite colloids. Prior to these sorption experiments, micro-CT imaging was conducted to examine mineralogical differences between the two rock types, revealing a layered structure in lamprophyre and an isotropic mineral distribution in granodiorite. Following imaging, the rock samples were cut into representative sections, and SEM-EDS analyses are being used to further characterize their mineralogical composition. Sorption experiments and subsequent autoradiography are planned, with ICP-MS analysis being used to quantify residual RN concentrations in solution post-sorption.

Preliminary mineralogical and sorption results will be presented. The outcomes of this study will support the design and execution of an upcoming in-situ RN tracer test in a region of the GTS where granodiorite contains fractured lamprophyre intrusions. This will provide valuable data to quantify radionuclide and colloid retention/retardation properties on fractured granitic rock surfaces. In turn, these data will help parameterize models for assessing radionuclide migration in crystalline host rock systems.

Acknowledgements:

This study is supported by the BMUV funded EVIDENT Project “Erosion von Bentonit unter In-situ Bedingungen durch Einwirkung natürlicher Wässer in geologischen Tiefenlagern”. Förderkennzeichen: 02 E 12153B.

References:

[1] https://www.grimsel.com/gts-projects/cfm-section/cfm-introduction

[2] Huber, F., Kunze, P., Geckeis, H., Schäfer, T. Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material. Applied Geochemistry, 2011, 26(12), pp. 2226–2237

How to cite: Shelyug, A., Quinto, F., Palina, N., Schneeberger, R., Metz, V., and Geckeis, H.: Sorption behavior of radionuclides in lamprophyre and granodiorite: implications for colloid-mediated transport in crystalline host rocks of Grimsel Test Site, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-125, https://doi.org/10.5194/safend2025-125, 2025.

We are convinced that a sound understanding of retardation and migration processes is essential for the development of any sorption databases. In this context discussions often raise the question, "Haven't we already collected enough data and knowledge on sorption processes over the past decades?" Indeed, as an example, many studies^[1,2] have focused on the sorption of trivalent actinides (e.g., Am, Cm) and lanthanides (e.g., Eu, Y) onto various mineral surfaces. However, these studies typically investigate geochemically simple systems, often in binary configurations with single surfaces and single sorbates, to gain a fundamental understanding of surface processes.

Natural systems are far more complex, with the possibility of e.g. competitive sorption, bulk and surface precipitation, incorporation, and co-precipitation processes that all potentially affect radionuclide (RN) retardation, not to mention possible organic interactions. Therefore, in an ongoing research project we focus on retardation and mobilization processes under more complex, closer-to-nature geochemical conditions. Here, the challenge lies in the unique combination of spectroscopic, microscopic, chemical, and electrochemical techniques, as along with the corresponding thermodynamic modeling. Consequently, we couple batch, column, and surface charge (streaming potential) experiments with atomic force microscopy (AFM), crystal truncation rod scattering/resonant anomalous X-ray reflectivity (CTR/RAXR), powder X-ray diffraction (PXRD), and quartz-crystal-microbalance (QCM) measurements. Our focus is on competing surface reactions, dissolution kinetics, and surface precipitation. The systems investigated involve trivalent lanthanides competing with Al(III) and Ga(III) on K- and Ca-rich feldspars, as well as on hematite and quartz surfaces.

So far, AFM measurements have confirmed the presence of an Al layer on the surface of K-rich feldspar (orthoclase), revealing the formation of a low-density amorphous Al layer, which varies with pH. It is assumed that this layer might influence the mineral’s surface charge behavior as previously reported in [2]. In terms of hematite, competitive sorption studies revealed that Al(III) does not significantly affect Eu(III) retention, except for a slight increase at low pH, suggesting limited competition under environmentally relevant conditions. For the investigated quartz surfaces, Al(III) retardation is relevant at lower pH-values compared to Eu(III) sorption processes resulting in potential competing surface reactions under relevant pH conditions which will be studies in detail. QCM measurements with feldspar-coated sensors have shown that it is possible to observe feldspar dissolution and Al as well as Eu uptake, albeit for relatively high solute concentrations. A new cell design was developed offering the opportunity to directly couple QCM and streaming potential measurements.

The experimental data will be used to develop sound and robust thermodynamic surface complexation models (SCMs) consistent with dissolution/precipitation patterns. These models will then be validated using data from column experiments to assess the applicability, advantages, and limitations of derived parameters.

[1] J. Neumann et al., J. Colloid Interface Sci. (2020) (https://doi.org/10.1016/j.jcis.2020.11.041) [2] J. Lessing et al., Colloids and Surfaces A (2024) (https://doi.org/10.1016/j.colsurfa.2024.133529)

How to cite: Britz, S., Brendler, V., Fricke, J., Lessing, J., Hilpmann, S., Schmidt, M., Lützenkirchen, J., and Hosseinimonjezi, B.: Exploring competing surface reactions in radionuclide retention: Experimental and modelling insights, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-165, https://doi.org/10.5194/safend2025-165, 2025.

In the context of the Mont Terri Project, the DR-C experiment investigates the impact of a thermal gradient on the diffusion of radionuclides in the sandy facies of Opalinus Clay. While current safety assessments for high-level radioactive waste disposal often assume containment throughout the thermal phase, this experiment addresses a more conservative scenario: radionuclide release during the thermal phase due to premature canister failure.

The experiment compares two in situ setups: one reference borehole at ambient temperature and one heated borehole maintained at 80°C. Both are equipped with tracer injection systems and monitoring tools. A suite of tracers, including HTO, ¹²⁹I⁻, ⁶⁰Co²⁺, ¹³⁷Cs⁺, ²²Na⁺, and ¹³³Ba²⁺, will be used to study diffusion under these contrasting thermal conditions. Observation boreholes monitor temperature, strain, and porewater pressure to capture thermo-hydro-mechanical effects.

Heating is expected to stabilize by Q3 2025, with tracer injection planned thereafter. The diffusion phase will last a minimum of two years, followed by over-coring and analysis. This experiment aims to generate critical in situ data to improve understanding of temperature effects on radionuclide migration, supporting more robust safety cases and addressing concerns linked to retrievability and public acceptance.

How to cite: Kerleguer, V., Pochet, G., Bernier, F., Barroo, C., Deissmann, G., Yang, Y., Herberling, F., Ratnayake, S., Montoya, V., Dietl, C., Borkel, C., Magri, F., Agnel, M. I., Graupner, B., and Jaeggi, D.: The DR-C Mont Terri Project: Inferring Effects of a Thermal Gradient on the Diffusion of Radionuclides in Opalinus Clay, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-115, https://doi.org/10.5194/safend2025-115, 2025.

Abstract. Part of the process to ensure the safety of radioactive waste disposal is the predictive modeling of the solubility of all relevant toxic components in a complex aqueous solution. To ensure the reliability of thermodynamic equilibrium modeling as well as to facilitate the comparison of such calculations done by different institutions it is necessary to create a mutually accepted thermodynamic reference database. To meet this demand several institutions in Germany joined efforts and created THEREDA (Moog et al., 2015).

THEREDA is a suite of programs at the base of which resides a relational databank. Special emphasis is put on thermodynamic data along with suitable Pitzer coefficients which allow for the calculation of solubilities in high-saline solutions. Registered users may either download single thermodynamic data or ready-to-use parameter files for the geochemical speciation codes PHREEQC and Geochemist’s Workbench. Data can also be downloaded in a generic JSON-format to allow for the import into other codes. The database can be accessed via the world wide web: www.thereda.de

Prior to release, the released part of the database is subjected to many tests. Results are compared to results from earlier releases and among the different codes. This is to ensure that by additions of new and modification of existing data no adverse side effects on calculations are caused. Furthermore, our website offers an increasing number of examples for applications, including graphical representation, which can be filtered by components of the calculated system.

References

Moog, H. C., Bok, F., Marquardt, C. M., and Brendler, V.: Disposal of Nuclear Waste in Host Rock formations featuring high-saline solutions - Implementation of a Thermodynamic Reference Database (THEREDA). Appl. Geochem., 55, 72-84, doi: 10.1016/j.apgeochem.2014.12.016, 2015.

How to cite: Wissmeier, L., Moog, H., Seher, H., Kozlowski, A., Hagemann, S., Brendler, V., Bok, F., Richter, A., Gaona, X., Altmaier, M., Kiefer, C., Marquardt, C., Freyer, D., Sohr, J., Pannach, M., and Voigt, W.: THEREDA - Thermodynamic Reference Database, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-169, https://doi.org/10.5194/safend2025-169, 2025.

Radioactive materials are utilized for a variety of applications, including nuclear power plants, medical radiography, and material testing technologies. These uses result in radioactive waste, which poses a risk to human health and the environment due to its radiotoxic and chemotoxic properties. The widely recognized approach for the ultimate disposal of high-level waste involves the containment within deep geological repositories (DGR). A reliable modeling of potential release scenarios for radionuclides (RNs) is essential in Long-Term Safety Analysis (LSA). In addition to hydrogeological information, element-specific thermodynamic data sets are required for all RNs relevant to long-term safety. Hence, a comprehensive understanding of the molecular processes of RN potentially occurring in the near and far field of a DGR is mandatory for the improvement of the existing codes for LSA.

The purpose of this study is to investigate RN retention mechanisms such as sorption, incorporation, surface complexation, and precipitation, including their mutual interdependencies using a combination of batch sorption experiments and spectroscopic techniques. Starting with solid albite feldspar as a mineral surface and Eu(III) as an analogue of trivalent lanthanides, batch sorption experiments are performed to evaluate the extent and mechanisms of Eu(III) uptake on albite surfaces under varying conditions. These experiments provide quantitative insights into the sorption behavior, including sorption capacity, kinetics, and the influence of pH and ionic strength. In a subsequent step, this information is utilized in investigations of more complex rocks containing feldspars. Our research will identify preferred mineral surfaces for sorption and characterizes binding forms under near-natural geochemical circumstances.

The experimental data derived from batch sorption studies are complemented by the application of advanced spectroscopic techniques, including time-resolved laser induced fluorescence spectroscopy (TRLFS), infrared (IR) spectroscopy, and Raman microscopy. These techniques potentially provide the identification of radionuclide species formed at the mineral’s surface. The combination of sorption data and spectroscopic analysis provides a comprehensive understanding of the molecular-level mechanisms driving the retention of trivalent lanthanides on mineral surface.

The data, which will be derived from this study contribute to improving thermodynamic models and reaction equations for RN migration. Furthermore, we are aiming to extend the method to additional trivalent actinides such as Am(III) and Cm(III). This work will not only enhance the understanding of radionuclide behavior in complex geochemical systems but also provide crucial insights for the safe disposal of nuclear waste.

How to cite: Sarkar, R., Jordan, N., Brendler, V., and Foerstendorf, H.: Identification of interfacial processes and species of trivalent lanthanides and actinides on mineral surfaces, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-176, https://doi.org/10.5194/safend2025-176, 2025.