Thomas Vercouter

Determination of the ¹⁴⁴Ce/²³⁸U ratio in spent nuclear fuels using the double isotope dilution technique associated with thermal ionization mass spectrometry measurements.

Uranyl binitrate complexes have a particular interest in the nuclear industry, especially in the reprocessing of spent nuclear fuel. The modified PUREX extraction process is designed to extract U(VI) in the form of UO₂(NO₃)₂(L)₂ as has been confirmed by extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and time-resolved laser-induced fluorescence spectroscopy (TRLFS) measurements. In this study, the L ligands are two molecules of N,N-di-(ethyl-2-hexyl)isobutyramide (DEHiBA) monoamide used to bind uranyl in its first coordination sphere. DEHiBA ligands can coordinate uranyl in either trans- or cis-position with respect to the nitrate ligands, and these two conformers may coexist in solution. To use luminescence spectroscopy as a speciation technique, it is important to determine whether or not these conformers can be discriminated by their spectroscopic properties. To answer this question, the spectra of trans- and cis-UO₂(NO₃)₂(DEiBA)₂ conformers were modeled with ab initio methods and compared to the experimental time-resolved luminescence spectra on UO₂(NO₃)₂(DEHiBA)₂ systems. Moreover, the hydrated uranyl binitrate UO₂(NO₃)₂(H₂O)₂ complexes in the same trans and cis configurations were modeled to quantify the impact of organic DEHiBA on the luminescence properties.

The stability constants of ternary calcium uranyl tricarbonate complexes, CaUO₂(CO₃)₃²⁻ and Ca₂UO₂(CO₃)₃(aq), were determined in NaClO₄ medium at various ionic strengths using time-resolved laser-induced luminescence spectroscopy (TRLS).

The luminescence spectra of triscarbonatouranyl complexes were determined by experimental and theoretical methods. Time-resolved laser-induced fluorescence spectroscopy was used to monitor spectra of uranyl and bicarbonate solutions at 0.1 mol kg_w^–1 ionic strength and pH ca. 8. The concentrations of Mg²⁺ and Ca²⁺ in the samples were chosen in order to vary the proportions of the alkaline earth ternary uranyl complexes MgUO₂(CO₃)₃^2–, CaUO₂(CO₃)₃^2–, and Ca₂UO₂(CO₃)₃. The luminescence spectrum of each complex was determined by decomposition in order to compare it with the simulated spectra of model structures Na_mM_nUO₂(CO₃)₃^{(4–m–2n)–} (M = Mg, Ca; m, n = 0–2) obtained by quantum chemical methods. The density functional theory (DFT) and time-dependent (TD)-DFT methods were used with the PBE0 functional to optimize the structures in the ground and excited states, respectively, including relativistic effects at the spin-free level, and water solvent effects using a continuum polarizable conductor model. The changes in the structural parameters were quantified with respect to the nature and the amount of alkaline earth counterions to explain the luminescence spectra behavior. The first low-lying excited state was successfully computed, together with the vibrational harmonic frequencies. The DFT calculations confirmed that uranyl luminescence originates from electronic transitions from one of the four nonbonding 5f orbitals of uranium to an orbital that has a uranyl-σ (5f, 6d) character mixed with the 2p atomic orbitals of the carbonate oxygens. Additional single-point calculations using the more accurate TD-DFT/CAM-B3LYP allow one to determine the position of the luminescence “hot band” for each structure in the range 467–476 nm and compared fairly well with experimental reports at around 465 nm. The complete luminescence spectra were built from theoretical results with the corresponding assignment of the electronic transitions and vibronic modes involved, mainly the U–O_ax stretching mode. The resulting calculated spectra showed a very good agreement with experimental band positions and band spacing attributed to MgUO₂(CO₃)₃^2–, CaUO₂(CO₃)₃^2–, and Ca₂UO₂(CO₃)₃. The evolution of luminescence intensities with the number of alkaline earth metal ions in the structure was also correctly reproduced.

The luminescence properties of the [UO₂Cl₄]^2– complex in an organic phase, especially the influence of large organic countercations, have been studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and ab initio modeling. The experimental spectrum was assigned by vibronic Franck–Condon calculations on quantum chemical methods on the basis of a combination of relativistic density functional approaches. The shape of the luminescence spectrum of the uranyl tetrachloride complex is determined by symmetrical vibrations and geometrical change upon emission. The possible change in the luminescence properties depending on the first and second uranyl coordination spheres was predicted theoretically for the [UO₂Br₄]^2– and [R₄N]₂[UO₂Cl₄] ([R₄N] = [Bu₄N], [A336]) systems. The computations reveal that, for U(VI), the second coordination sphere has little influence on the spectrum shape, making speciation of uranyl complexes with identical first-coordination-sphere ligands tedious to discriminate. The computed structural changes agreed well with experimental trends; theoretical spectra and peak attributions are in good accordance with TRLFS and magnetic circular dichroism (MCD) data, respectively.

The minimization of the sample quantities required by analytical laboratories, as well as the increase of the fastness of the analytical operations are emerging axes for improved radiochemical analyses related to D&D issues. Two microsystem-based protocols were developed for the selective recovery of ⁵⁵Fe from radioactive samples by solvent extraction. Both protocols were tested on iron solutions in two different microchips. The yields of Fe extraction were compared with macroscale batch experiments. Better performances with more than 80% of iron extracted were obtained with the second protocol, which is based on a reactive transfer of the iron cation, and more suited to the use of microchannels and very low contact times. This study already demonstrate the high potential of microfluidic technology to improve analytical operations on D&D samples. This method will further be validated with radioactive samples.