David Fellhauer

The role of the equatorial ligands and their influence on the electronic structures and bonding properties of uranyl and other actinyls are not well understood and are thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F^- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl^- and uranyl-Br^-. The good energy match between uranyl(VI) and F^- likely enhances the stability of the uranyl-F^- bond, contributing to the higher U-F^- affinity in aqueous solution compared to uranyl-Cl^-/Br^-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides: NaRb₈(UO₂)₅F₁₉·2H₂O, Rb₂UO₂Cl₄·2H₂O, and Rb₂UO₂Br₄·2H₂O. We describe covalent uranium-halide interactions, following the trend Br^- ≈ Cl^- > F^-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M₄ edge HR-XANES and RIXS exhibit unexpected σ* peak trends not directly correlated with U═O bond lengths but well explained by LFDFT RIXS calculations.

The role of the equatorial ligands and their influence on the electronic structure and bonding properties of uranyl and other actinyls are not well understood and thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl- and uranyl-Br-. The good energy match between uranyl(VI) and F- likely enhances the stability of the uranyl-F- bond, contributing to the higher U-F- affinity in aqueous solution compared to uranyl-Cl-/Br-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides—NaRb8(UO2)5F19·2H2O, Rb2UO2Cl4·2H2O, and Rb2UO2Br4·2H2O. We describe covalent uranium-halide interactions, following the trend Br- ≈ Cl- > F-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M4 edge HR-XANES and RIXS exhibit unexpected σ* peak trends, not directly correlated with U=O bond lengths but well explained by LFDFT RIXS calculations.

In actinide chemistry, the formal number of open-shell 5f electrons, n_open, is a well-defined quantity with an integer value. The effective 5f occupation, n_f, additionally takes donation and back-donation into account, generally has a non-integer value, and has varying numerical definitions. The present study explores the important distinction between n_f and n_open in actinide chemistry with the example of PuO₂, by using electronic structure methods with a relativistic Hamiltonian in combination with experimental Pu M₅-edge high-energy-resolution X-ray absorption and emission spectroscopic data. The total donation to the metal in PuO₂ is between 3.1 and 2.4 electrons, depending on the type of calculation, most of which is to the Pu 6d and 5f shells. The donation into 5f is sensitive to the approximations in the electronic structure model but likely amounts to 1.6/0.8 electrons when the diffuse regions of the 5f shell are included/excluded. Valence band resonant inelastic X-ray scattering experiments demonstrate that Pu 5f electron density is present in the valence band; thus, there is a clear experimental signature of covalent bonding in PuO₂. Pu M₅-edge and M₃-edge high-energy-resolution X-ray absorption near-edge structures for Pu³⁺ and Pu⁴⁺ in an aqueous solution are compared to PuO₂, showing that Pu in PuO₂ has an n_f closer to Pu⁴⁺(aq).

In actinide chemistry, the formal number of open‐shell 5f electrons, n(open), is a well‐defined quantity with an integer value. The effective 5f occupation, n(f), additionally takes donation and back‐donation into account, generally has a non‐integer value, and varying numerical definitions. The present study explores the important distinction between n(f) and n(open) in actinide chemistry with the example of PuO2, by using electronic structure methods with a relativistic Hamiltonian in combination with experimental Pu M5‐edge high energy resolution X‐ray absorption and emission spectroscopic data. The total donation to the metal in PuO2 is between 3.1 and 2.4 electrons, depending on the type of calculation, most of which is to the Pu 6d and 5f shells. The donation into 5f is sensitive to the approximations in the electronic structure model but likely amounts to 1.6/0.8 electrons when the diffuse regions of the 5f shell are included/excluded. Valence band resonant inelastic X‐ray scattering (VB‐RIXS) experiments demonstrate that Pu 5f electron density is present in the valence band; thus, there is a clear experimental signature of covalent bonding in PuO2. Pu M5‐edge and M3‐edge high energy resolution X‐ray absorption near edge structure (HR‐XANES) for Pu3+ and Pu4+ in aqueous solution are compared to PuO2 showing that Pu in PuO2 has n(f) closer to Pu4+(aq).

The ternary neptunium(V) (Np(V)) hydroxides Na_0.5[NpO₂(OH)_1.5]·0.5H₂O (I) and Na[NpO₂(OH)₂] (II) were synthesized in aqueous NaOH solutions at T = 80 °C, and their crystal structures were determined to be monoclinic, P2₁, Z = 2, a = 5.9859(2), b = 10.1932(3), c = 12.1524(4) Å, β = 98.864(1)°, V = 732.63(4) ų for (I) and orthorhombic, P2₁2₁2₁, Z = 4, a = 5.856(7), b = 7.621(9), c = 8.174(9) Å, V = 364.8(7) ų for (II). By combining the detailed structural information with results from systematic solubility investigations, a comprehensive chemical and thermodynamic model of the Np(V) behavior in NaCl-NaOH solutions was evaluated. The results reveal a great stability of the ternary Na-Np(V)-OH solid phases that significantly enhances the predominance field of the entire Np(V) redox state to high alkalinity.

Deep geological disposal is the internationally favoured option to isolate high-level nuclear waste (HLW) from the biosphere and to minimise the potential radiological risk for future generations. Potentially contacting aqueous solutions such as groundwater may, however, lead to the corrosion of the solid casks containing the nuclear waste, and the formation of aqueous radionuclide systems in the near-field of the emplacement rooms. As dissolved species, radionuclides can in principle further migrate into the far-field and finally reach the biosphere on medium and long timescales. Like all chemical species, the radionuclides are subject to fundamental (geo)chemical laws. Relevant reactions that control retention and release, and hence, the migration behaviour and fate of radionuclides in a repository, are solubility equilibria, formation of soluble complexes, redox reactions, sorption on and incorporation into mineral surfaces, transport phenomena etc. These processes depend directly on the (geo)chemical boundary conditions, and, consequently, can differ greatly for various host rock systems such as clay rock, rock salt, and crystalline rock. Many of the radionuclides in HLW are heavy metals that are sparingly soluble under various repository-relevant conditions, e.g. actinides, lanthanides, transition metals, so that only partial dissolution (mobilisation) from the solid waste matrices is expected. This underlines the importance of evaluating the radionuclide solubility within a geochemically based safety assessment for repositories as it provides reliable upper-limit concentrations of the mobile, potentially migrating radionuclide fraction in the near-field. In this contribution, we discuss relevant aspects related to the topic radionuclide solubility and thermodynamics in a HLW repository. This includes a summary of recent laboratory studies on the solubility behaviour and speciation of key radionuclides in repository-relevant solutions, which are an important basis for obtaining (geo)chemical information and models, and the corresponding fundamental thermodynamic constants on aqueous radionuclide systems. National and international thermodynamic database projects, where quality-assured thermodynamic data (solubility products, complex formation constants, and ion-interaction parameters) are evaluated and compiled, e.g. the Nuclear Energy Agency Thermochemical Database (http://www.oecd-nea.org, last access: 1 November 2021) or the Thermodynamic Reference Database (http://www.thereda.de, last access: 1 November 2021), are highlighted and the main remaining uncertainties discussed. The experimental information and the quantitative thermodynamic data are applied within a generic case study to demonstrate the impact of different geochemical solution conditions representing different host rock systems considered as HLW repositories in Germany on the solubility and speciation of selected radionuclides.

For more than 120 years, systematic studies of X-ray interaction with matter have been the basis for our understanding of materials—both of natural or man-made origin—and their structure-function relationships. Beginning with simple radiographic imaging at the end of the 19th century, X-ray based analytical tools such as X-ray diffraction, X-ray fluorescence and photoemission or X-ray absorption techniques are indispensable in almost any field of chemical and material sciences—including basic and applied actinide and radionuclide studies. The advent of dedicated synchrotron radiation (SR) sources in the second half of the last century has revolutionized the analytical power of X-ray probes, while—with increasing number of SR facilities—beamline instrumentation followed a trend towards increasing specialization and adaption to a major research topic. The INE-Beamline and ACT station at the KIT synchrotron source belong to the exclusive club of a few synchrotron beamline facilities—mostly located in Europe—dedicated to the investigation of highly radioactive materials. Since commissioning of the INE-Beamline in 2005, capabilities for synchrotron-based radionuclide and actinide sciences at KIT have been continuously expanded, driven by in-house research programs and external user needs.