Search

The selective Am and Cm recovery from the highly active raffinate could be achieved by innovative-Selective ActiNide EXtraction (i-SANEX) process downstream major actinides removal. The TetraOctyl-DiGlycolAmide (TODGA) extractant and a hydrophilic complexing agent, 2,6-bis[1-(propan-1-ol)-1,2,3-triazol4-yl)]pyridine (PTD) being the most promising one, could be synergically used to separate Am and Cm from chemically similar lanthanides. Besides promising selectivity and efficiency for Am and Cm extraction, such systems must prove radiolytic and hydrolytic stability, given the high radionuclide content of the feed. In this work, PTD selectivity for trivalent Am and Cm towards Eu, chosen as representative lanthanides, has been studied at relevant conditions by different spectroscopic techniques. In order to mimic the process conditions, solutions of PTD were γ-irradiated up to 200 kGy by a 60Co source or aged up to 100 days in 0.44 mol/L HNO3 before complexometric titration experiments. The obtained UV-Vis and Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) results were compared with those of fresh PTD solutions, in order to evaluate the impact of radiolysis and hydrolysis on metal-ligand complex formation. In accordance with the stability outlined by previous studies, also the complexation capabilities resulted to be unaltered. These results further support PTD as a valid candidate for partitioning processes.

The complexation of NpO2+ with lactate in aqueous solution is studied as a function of the total ligand concentration ([Lac-]total), ionic strength (Im = 0.5 – 4.0 mol kg-1 Na+(Cl-/ClO4-)) and temperature (T = 20 – 85 °C) by Vis/NIR absorption spectroscopy. The formation of two NpO2+ lactate species with the stoichiometry NpO2(Lac)n1-n (n = 1, 2) is observed at the studied experimental conditions. The temperature dependent conditional stability constants log beta 'j(T) at different ionic strengths are calculated with the law of mass action. The conditional data are extrapolated to IUPAC reference state conditions (Im = 0) with the specific ion interaction theory (SIT). With increasing temperature up to 85 °C log beta 01(20 °C) = 1.92 ± 0.14 decreases by 0.12 and log beta 02(20 °C) = 2.10 ± 0.13 decreases by 0.17. The thermodynamic stability constants correlate linearly with the reciprocal temperature according to the integrated Van’t Hoff equation. Thus, linear regression analyses yield the standard reaction enthalpy delta rH0 and entropy delta rS0 for the complexation reactions. In addition, the sum of the SIT specific binary ion-ion interaction coefficients delta epsilon j,k(T) of the complexation reactions are determined by variation of the ionic strength. Structural parameters of the formed complex species and the coordination mode of lactate towards the NpO2+ ion are investigated as a function of pHc by extended x-ray absorption fine structure spectroscopy (EXAFS) and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FT IR). The results show, that the coordination mode of lactate changes from end-on (coordination via only the COO- group) to side-on (formation of chelate rings involving the OH-group) with increasing pHc. The experiments are supported by quantum chemical calculations.

In the advent of the 2nd International Workshop on Advanced Techniques in Actinide Spectroscopy (ATAS 2014), an inter-laboratory round-robin test (RRT) was initiated. The main goal of this RRT was the comprehensive molecular analysis of a simple aqueous com-plexing system – U(VI) acetate – which was selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods applied by leading laboratories in actinide or geochemical research. Ultimately, more than 40 scientists hosted at twenty insti-tutions in seven countries participated. The outcome of this RRT can be considered on two levels: First, conformities as well as discrepancies in the results of each spectroscopic technique and their sources are evaluated. The raw data from the experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was surprisingly high. By contrast, the data obtained using luminescence spectroscopy turned out to be strongly related to the chosen acquisition pa-rameters. Second, the potentials and limitations of coupling various spectroscopic and, in particular, theoretical approaches for the comprehensive study of actinide molecule com-plexes are assessed. The additional benefits of the combined approach with regard to the exploration of the aqueous speciation of the U(VI) acetate system are elaborated.

The necessity to reprocess spent nuclear fuel has arisen from increasing volumes of highly active and long-lived radionuclides associated with nuclear fission alongside increasing environmental impact concerns. In particular, one major challenge is the separation of the long-lived trivalent minor actinides (here, AmIII and CmIII) from the trivalent lanthanides in order to allow the future transmutation of the actinides into shorter lived radionuclides for the purpose of reducing the long-term radiotoxicity of the waste and the volume build up in storage. There are currently a number of different processes under development world-wide, all of which rely on the coordination chemistry and selective extraction of the minor actinides from the trivalent lanthanides using solvent partitioning. Of these, The TALSPEAK process (Trivalent Actinide Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexations) has showed great promise in separating the trivalent lanthanides from the minor actinides. The process uses DTPA (diethylenetriamine pentaacetic acid) as the chelating holdback reagent, HDEHP (bis-2-ethylhexyl phosphoric acid) as the extracting ligand and lactic acid as the buffer to prevent precipitation of DTPA that occurs at below pH 3.6. However, despite the considerable separation factors obtainable, and the fact the process exhibits promising radiolysis resistance, the process operates at sub optimal pH values. To overcome these limitations, we have developed a family of DTPA-amino acid conjugates that operate under TALSPEAK-like conditions over a much lower pH range of 1.5-2.5. Additionally, these ligands show comparable selectivity to the TALSPEAK process and are also relatively resistant to radiolysis. We will discuss the coordination chemistry of these ligands with the lanthanides and minor actinides using a combination of luminescence and NMR spectroscopies.

The pH-dependent (pH 2 - 4) formation of aqueous Am(III) complexes with formate (Form) is studied by EXAFS, iterative target transformation factor analysis (ITFA) and by quantum chemical calculations [1]. According to thermodynamical calculations three complexes (aq. Am3+, AmForm2+, AmForm2+) coexist and change their fractions, so that EXAFS spectral mixtures occur. A maximal spectral change of only 9% is observed between the different Am LIII-edge EXAFS spectra, which rules out the direct use of conventional shell fit analysis for structural investigation prior to the decomposition of the spectra into the single spectral components by ITFA. By combining pH-speciation calculations with quantum chemistry, the pH-dependent number of coordinated water and formate molecules is calculated and used as constraint for a modified ITFA-approach. The decomposition results in the separate spectral contributions of the exchanged molecules, hence the signal of a coordinated water and a monodentate coordinated formate molecule for which the structural parameters are determined by shell-fitting and are compared with twelve complex structures gained by quantum chemical calculations. Methodologically consistent, the prevailing coordination numbers are 9, 9 and 8 for the aq. Am3+, AmForm2+ and AmForm2+ complexes, respectively [2]. Low concentrations of species with other coordination numbers and modes cannot be excluded, as energy differences obtained by our quantum chemical calculations are small [2]. This work shows the power of the proposed ITFA-framework in obtaining structural information for weak ligand systems like formate, where conventional EXAFS data analysis fails due to the lower spectroscopic resolution in analysing mixtures of metal species. References [1] Rossberg et al., Anal. Bioanal. Chem. 376, 631-638 (2003). [2] Fröhlich et al., Inorg. Chem. submitted (2017).

The ThermAc project is extending the chemical understanding and available thermodynamic database for actinides, long-lived fission products and relevant matrix elements in aquatic systems at elevated temperatures. To this end, a systematic use of estimation methods, new experimental investigations and quantum-chemistry based information is used. ThermAc has started in March 2015 and is projected for three years, running until 28.02.2018. The project is funded by the German Federal Ministry for Education and Research (BMBF) and is coordinated by KIT-INE.

Partitioning and transmutation (P&T) facilitates a reduction of the long-term radiotoxicity and heat load of spent nuclear fuel by separation of the actinides and subsequent conversion into shorter-lived or stable nuclides. A chemically demanding key step in this process is the separation of the trivalent actinides from fission lanthanides. This can be achieved by liquid-liquid extraction using highly selective extraction agents, such as nPrBTP (1) or C5-BPP (2). These have high separation factors (>100) for trivalent americium over europium. However, the molecular origin of their selectivity is largely unclear. NMR spectroscopy on paramagnetic samples allows a separation of the observed paramagnetic shift into a part due to transferred electron spin density (Fermi contact shift, FCS) and a part due to dipolar coupling of electron and nuclear spin (Pseudo contact shift, PCS). Evaluation of the FCS thus allows an assessment of the share of covalence in the metal-ligand bond of the N-donor complexes. Several methods that enable the separation of the shift contributions have been proposed in literature. So-called “model free methods” that do not require structural models of the complexes appear most promising. We will present and compare the results of different temperature-dependent and temperature-independent model-free methods for complexes of both ligands. The merits, but also the limitations of currently available methods will be discussed in detail. Furthermore, we will evaluate the applicability of temperature-dependent methods for shift separation of Am(III) complexes and give a qualitative assessment on covalence in the bonding of these complexes.

Multi-dentate ligands are instrumental to extraction and separations chemistry associated with nuclear fuel reprocessing. Specifically, the TALSPEAK (Trivalent Actinide Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexations) process utilises DTPA to facilitate the separation of minor actinides, MA3+ (Am3+ and Cm3+), from Ln3+ and Y3+, allowing the MA3+ to be reprocessed further by transmutation. The TALSPEAK process involves the preferential extraction of the major component (Ln3+) into the organic phase using HDEHP, while the DTPA-derived ligands remain in the aqueous phase coordinating MA3+ which favour soft donor interactions. The process requires the use of lactic acid as a buffer to maintain pH 3.6 in order to prevent the precipitation of DTPA complexes at low pH, commonly experienced during the processing cycle. Amino acid conjugates derived from EDTA and DTPA present ideal candidates as self-buffering DTPA/EDTA ligands, therefore removing the need for lactic acid in the TALSPEAK process. The ligands (right) produce an internal buffer pH 1.5-2.5 at μM to mM concentrations. The synthesis, coordination chemistry, photophysical properties and separation behaviour of these new ligands and stability towards ionising radiation is presented.

The ThermAc project is extending the chemical understanding and available thermodynamic database for actinides, long-lived fission products and relevant matrix elements in aquatic systems at elevated temperatures. To this end, a systematic use of estimation methods, new experimental investigations and quantum-chemistry based information is used. ThermAc has started in March 2015 and is projected for three years, running until 28.02.2018. The project is funded by the German Federal Ministry for Education and Research (BMBF) and is coordinated by KIT-INE. The ThermAc project is developed with the aim of improving the scientific basis for assessing nuclear waste disposal scenarios at elevated temperature conditions. Adequate scientific tools must be available to assess the related chemical effects and their impact upon safety. A clear focus of ThermAc is on long-lived actinides in oxidation states III, V and VI, with selected fission products and important redox controlling matrix elements like Fe also receiving attention. Tetravalent actinides and detailed investigations of redox processes are excluded from the current ThermAc work programme. ThermAc mainly addresses the temperature range from ~5°C up to ~90°C, focusing on systems at low or intermediate ionic strength. Chemical analogs for the actinide elements will be used, especially in order to gain information on solid phase transformation processes. Ion-interactions are treated with the Specific Ion Interaction Theory (SIT), in agreement with the approach favored by the NEA-TDB project. Quantum chemical calculations are used to support the interpretation of experimental findings, and establish a fundamental understanding of chemical effects on a molecular level. Within the scope of ThermAc, a significant impact can be realized within a strong collaborative and integrated concept with the following strategic components: (1) Systematic use of estimation methods for thermodynamic data and model parameters. (2) Comprehensive experimental validation of the estimations. (3) Fundamental studies for improved process understanding of actinide chemistry at elevated T. (4) Comprehensive critical evaluation of the work performed within (1-3). A key result from the comparison of predictions based upon estimation methods with new experimental data derived within ThermAc will be the assessment of the use of estimations methods to set up a workable thermodynamic database for elevated temperatures with high applicability to nuclear waste disposal issues. In this context it will be clarified, to which extent systems will remain critical with regard to available thermodynamic data, and which relevant processes at elevated temperatures are still not sufficiently understood.