The MARS beamline at the SOLEIL synchrotron is dedicated to the characterization of radioactive material samples. One great advantage of the beamline is the possibility to characterize about 380 radionuclides by different X-ray techniques in the same place. This facility is unique in Europe. A wide energy range from around 3.5 keV to 36 keV K-edges from K to Cs, and L3 edges from Cd to Am and beyond can be used. The MARS beamline is optimized for X-ray absorption spectroscopy techniques (XANES/EXAFS), powder diffraction (XRD) but x-ray fluorescence (XRF) analysis, High Energy Resolution Fluorescence Detected -XAS (HERFD-XAS), X-ray Emission (XES) and -XAS/XRD are also possible. A description of the beamline as well as its performances are given in a first part. Then some scientific examples of XAS studies from users are presented which cover a wide variety of topics in radiochemistry and nuclear materials.

The formation of an aqueous ternary complex was investigated in the malonate-based Advanced TALSPEAK system by solvent extraction and optical spectroscopy. Features of the spectroscopy confirmed the presence of a ternary complex between malonate, HEDTA, and the trivalent americium or lanthanide ion. The ternary complex was then incorporated into a solvent extraction model to refine stability constants under Advanced TALSPEAK conditions. Good agreement was found in the equilibrium constants determined by both methods. These values were then used to model metal speciation and predict extraction behavior at different concentrations of malonate and HEDTA. Calculated metal distribution ratios show good correlation with experimental values. The apparent simplicity of the chemical interactions and predictability of malonate-buffered Advanced TALSPEAK provides a significant advantage over the less ideal interactions of conventional TALSPEAK.

The interaction of uranyl nitrate with the series of diamides Et2N(C=O)(CH2)n(C=O)NEt2 (0 ≤ n ≤ 6) was investigated to evaluate systematically the effect of the (CH2)n spacer on the solid‐state structures of the corresponding uranyl complexes. Under aerobic conditions, [UO2(NO3)2·6H2O] reacted with an excess amount of these diamides (L) in organic solvents to yield [UO2(κ2‐NO3)2(L)] {1 [n = 0, tetraethyloxalamide (TEOA)], 2 [n = 1, tetraethylmalonamide (TEMA)], 3 [n = 2, tetraethylsuccinamide (TESA)], 5 [n = 3, tetraethylglycolamide (TEGA)], 6 [n = 4, tetraethyladipicamide (TEAA)], 7 [n = 5, tetraethylpimelicamide (TEPA)], and 8 [n = 6, tetraethylsubericamide (TESUA)]}, which were isolated and characterized by 1H NMR, ESI‐MS, IR, and Raman spectroscopy. Under anhydrous and anaerobic conditions, [UO2(OTf)2] (OTf = trifluoromethanesulfonate) was treated with an excess amount of L to give [UO2(L)2][OTf]2, which was isolated for n = 1 (9) and n = 2 (10). The crystal structures of 2, 5, 6, 7, 8, and of the peroxido‐bridged complex [{UO2(κ2‐NO3)(L)}2(μ,η2,η2‐O2)] (4; n = 2) are presented. In all cases, the uranium ion is eight‐coordinated with a classical hexagonal‐bipyramidal configuration. The number of CH2 groups in the diamide central chain has considerable influence on the dimensionality of the complexes. They are monometallic (n = 1, 2), dimeric (n = 3, 4), or polymeric (n = 5, 6) with either a helical or zigzag structure, depending on the coordination mode of the bidentate diamide, which can be bridging or not, and the position cis or trans of the nitrate ions. DFT calculations in the gas phase show that the mono‐ and κ2‐bidentate coordination modes of the diamide (1 ≤ n ≤ 5) onto [UO2(κ2‐NO3)2] are energetically similar. By using the continuum solvation model, the binding affinity of the κ2‐bidentate diamide gradually decreases with the increase in n, but the most stable bidentate uranyl complex is obtained for n = 1 (TEMA). The structural differences in the series of [UO2(κ2‐NO3)2{Et2N(C=O)(CH2)n(C=O)NEt2}] complexes are directly related to the length of the (CH2)n spacer.