A spectrophotometric study of the impact of pH and metal-to-ligand ratio on the speciation of the Pu(VI)-oxalate system

The oxalate ligand is prevalent throughout the nuclear fuel cycle. While the Pu(III)- and Pu(IV)-oxalate systems are well studied due to their use in plutonium metal and PuO2 production, the effect of oxalate on Pu(VI) remains understudied. Absorption spectroscopy was employed to probe the solution behavior of the Pu(VI)-oxalate system as a function of pH (1, 3, 7) and metal-to-ligand ratio (M/L; 10 : 1-1 : 10). Peak changes in the UV-vis-NIR spectra were associated with the formation of multiple Pu(VI)-oxalate species with increasing oxalate concentration. Some insight into identification of species present in solution was gained from the limited Pu(VI)-oxalate literature and comparisons with the assumed isostructural U(VI)-oxalate system. A peak in the UV-vis-NIR spectrum at 839 nm, which corresponds to the formation of a 1 : 1 PuO2(C2O4)(aq) complex, was observed and used to determine the formation constant (log β° = 4.64 ± 0.06). A higher coordinated Pu(VI)-oxalate peak at 846 nm was tentatively assigned as the 1 : 2 complex PuO2(C2O4)22- and a preliminary formation constant was determined (log β° = 9.30 ± 0.08). The predominance of both complexes was shown in speciation diagrams calculated from the formation constants, illustrating the importance of considering the Pu(VI)-oxalate system in the nuclear fuel cycle.

In situ beam reduction of Pu(IV) and Bk(IV) as a route to trivalent transuranic coordination complexes with hydroxypyridinone chelators

The solution-state interactions of plutonium and berkelium with the octadentate chelator 3,4,3-LI(1,2-HOPO) (343-HOPO) were investigated and characterized by X-ray absorption spectroscopy, which revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X-ray absorption near-edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white-line position energies for Pu-343-HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk-343-HOPO results were found to mirror the XANES behavior of Bk(III)-DTPA. Extended X-ray absorption fine structure results revealed An-O_HOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343-HOPO via density functional theory calculations. Pu(III)- and Bk(III)-343-HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)-, Cm(III)-, Cf(III)-, and Es(III)-343-HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series.

Multiple investigations of aqueous Eu(III)-oxalate complexes: the reduction in coordination number and validation of spectral linear correlation

Detailed information on the An(iii)/Ln(iii) complexation properties in solution is essential for separation chemistry and the prediction of their potential for radionuclide migration from nuclear waste repositories into natural aquifers. In the present study, to better reveal and confirm the structural information of [Eu(Ox)x (H2O)h-2x]3-2x (h = 8, 9; x = 0-3) aqueous species, especially the variable coordination number (CN), and explore the validity of the spectral linear correlation between the luminescence lifetime and the residual hydration number in the first coordination sphere of Eu(iii) compounds in solution, a comparison between the spectral results and the theoretical calculations in a wide parametric space in terms of the pH value and oxalate concentration was carried out by combining time-resolved luminescence spectroscopy (TRLS) with speciation modelling and density functional theory (DFT) calculations. We have found direct and clear evidence for the 9-fold to 8-fold coordination number reduction of Eu(iii) atoms upon coordination with more than one oxalate in an aqueous medium, and as well systematically validated the applicability of the spectral linear correlation in an aqueous system (otherwise solid state) involving multiple species with the support of relatively reliable and clear speciation modelling.

Kinetic study on the grain growth of PuO2 nanocrystals

Results on the kinetics of PuO₂ nanocrystals are presented in the temperature range below 1100 °C showing the activation energy for the particle growth is ∼350 kJ mol⁻¹. Particle growth proceeds from the nano- to the micrometer scale.

A low-temperature synthesis method for AnO2 nanocrystals (An = Th, U, Np, and Pu) and associate solid solutions

Actinide oxalate decomposition under hot compressed water is proposed as a milder production route for nanometric sized (mixed) actinide oxides.

Evidence of Trivalent Am Substitution into U3O8

U₃O₈ is considered to be the most stable phase for uranium oxide. Its structural properties must be accurately understood to foresee and manage aspects such as its leaching behavior when spent nuclear fuel is stored in an oxidative environment. Moreover, as fuel irradiation causes the formation of fission products and activation products such as plutonium and minor actinides, it is probable that U₃O₈ will be mixed with other chemical elements under real conditions of oxidation. The storage issue can be extended to americium transmutation, where the irradiated compounds are mixed oxides composed of uranium and americium. This study thus focused on determining the structural properties of a solid solution containing uranium and trivalent americium (U/Am ratio = 90/10) and synthesized so as to obtain conventional U₃O₈ oxide. This paper presents the possibility of combining trivalent americium with uranium in a U₃O₈ mixed oxide for the first time, despite the high valence and atomic ratio differences, and proposes novel structural arrangements. X-ray diffraction measurements reveal americium substitution in U₃O₈ uranium cationic sites, leading to phase transformation into a U₃O₈ high-temperature structure and general lattice swelling. X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure experiments highlight an excess of U^+VI organized in uranyl units as the main consequence of accommodation.