Actinide Oxalates Research Articles

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Invited for the cover of this issue is the group of Amy Hixon at the University of Notre Dame. The image depicts the newly identified structure of a PuIV oxalate sheet compared to the historically assumed structure. Read the full text of the article at 10.1002/chem.202301164.

Front Cover: Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries (Chem. Eur. J. 47/2023)

Front Cover: Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries (Chem. Eur. J. 47/2023)

Insight into the structural ambiguity of actinide(IV) oxalate sheet structures: a case for alternate coordination geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2O4)2∙6H2O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation via precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2O4)-2∙6H2O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2O4)-2∙6H2O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2O4)2·6H2O (ThOx). This data, along with new characterization of UOx and NpOx, has resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we identify the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. This work exemplifiesthe need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Formation and growth of cerium (III) oxalate nanocrystals by liquid-cell transmission electron microscopy

Using rare earth elements as analogs for actinide series elements enables studying the chemistry of these radioactive materials without the inherent risks. A fundamental understanding of the nucleation and growth of actinide oxalates is crucial for processing nuclear waste. Using in-situ liquid cell TEM (LC-TEM), we show particle attachment processes occurring during the nucleation and growth of cerium oxalate. The early stages of the observed non-classical growth mechanisms have important implications for particle evolution and predictions of final particle morphology. We observed particle formation via monomer-by-monomer addition or the particle aggregation pathway depending on the selected precursor concentration. At the highest precursor concentration at a high dose rate, we observed particle alignment in a branch structure is due to the interfacial instability. It was demonstrated that in-situ LC-TEM is an invaluable tool in studying the cerium oxalate formation and ultimately a path to understanding actinide chemistry.

Experimental and modeling studies of PR and ND oxalate solubility to high ionic strengths: Insight into actinide(III) oxalates

Actinide oxalates are chemical compounds important to nuclear industry, ranging from actinide separation in waste reprocessing, to production of specialty actinides, and to disposal of high level nuclear waste (HLW) and spent nuclear fuel (SNF). In this study, the solubility constants for Pr2(C2O4)3·10H2O and Nd2(C2O4)3·10H2O by performing solubility experiments in HNO3 and mixtures of HNO3 and H2C2O4 at 23.0 ± 0.2 °C have been determined. The targeted starting materials, Pr2(C2O4)3·10H2O and Nd2(C2O4)3·10H2O, were successfully synthesized at room temperature using PrCl3, NdCl3 and oxalic acid as the source metrials. Then, we utilized the targeted solubility-controlling phases to conduct solubility measurements. There was no phase change over the entire periods of experiments, demonstrating that Pr2(C2O4)3·10H2O and Nd2(C2O4)3·10H2O were the solubility-controlling phases in our respective experiments.Based on our experimental data, we have developed a thermodynamic model for Pr2(C2O4)3·10H2O and Nd2(C2O4)3·10H2O in the mixtures of HNO3 and H2C2O4 to high ionic strengths. The model for Pr2(C2O4)3·10H2O reproduces well the reported experimental data for Pu2(C2O4)3·10H2O, which are not utilized for the model development, demonstrating that Pr(III) is an excellent analog for Pu(III). Similarly, the model for Nd2(C2O4)3·10H2O reproduces the solubility of Am2(C2O4)3·10H2O and Cm2(C2O4)3·10H2O. The Pitzer model was used for the calculation of activity coefficients. Based on the published, well established model for dissociation constants for oxalic acid and stability constants for actinide-oxalate complexes [i.e., AmC2O4+, and Am(C2O4)2−] to high ionic strengths, we have obtained the solubility constants (log10K0) for the following reactions at 25 °C,Pr2(C2O4)3·10H2O ⇌ 2Pr3+ + 3C2O42− + 10H2O(l)Nd2(C2O4)3·10H2O ⇌ 2Nd3+ + 3C2O42− + 10H2O(l) to be −30.82 ± 0.30 (2σ), and −31.14 ± 0.35 (2σ), respectively. These values can be directly applied to Pu2(C2O4)3·10H2O, Am2(C2O4)3·10H2O and Cm2(C2O4)3·10H2O.The model established for actinide oxalates by this study provides the needed knowledge with regard to solubilities of actinide/REE oxalates at various ionic strengths, and is expected to find applications in many fields, including the geological disposal of nuclear waste and the mobility of REE under the surface conditions, as Pr2(C2O4)3·10H2O and Nd2(C2O4)3·10H2O can be regarded as the pure Pr and Nd end-members of deveroite, a recently discovered natural REE oxalate with the following stoichiometry, (Ce1.01Nd0.33La0.32Pr0.11Y0.11Sm0.01Pb0.04U0.03Th0.01Ca0.04)2.01(C2O4)2.99·9.99H2O. Regarding its importance in the geological disposal of nuclear waste, Am2(C2O4)3·10H2O/Pu2(C2O4)3·10H2O/Cm2(C2O4)3·10H2O can be the source-term phase for actinides, as demonstrated by the instance in the disposal in clay/shale formations. This is exemplified by the stability of Am2(C2O4)3·10H2O in comparison with Am(OH)3(am), Am(OH)3(s) and AmCO3(OH)(s) under the relevant geological repository conditions.

Oxidation as an Early Stage in the Multistep Thermal Decomposition of Uranium(IV) Oxalate into U3O8.

The thermal decomposition of actinide oxalates is greatly dependent on the oxidation state of the cation, the gas involved, and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C2O4)2·6H2O can be viewed as a peculiar case, as its sensibility toward oxidation leads to a specific series of reactions when heating under an oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO3, we show here an extended characterization of the different intermediates through a combination of X-ray diffraction, vibrational spectroscopies and X-ray absorption near-edge spectroscopy. In this frame, uranium oxidation was found to occur at low temperature (200 °C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350 °C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to substoichiometric UO3-δ after heating at 550 °C. Finally, U3O8 was obtained as the final stable phase after heating above 660 °C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through high-temperature environmental scanning electron microscopy observations: decomposition led to a 20% reduction in the size of the aggregates, while uranium oxidation clearly promoted growth within the agglomerates.

First principles investigation of the structural and bonding properties of hydrated actinide (IV) oxalates, An(C2O4)2·6H2O (An = U, Pu)

The structural and bonding properties of actinide (IV) oxalates, U(C2O4)2·6H2O and Pu(C2O4)2·6H2O, are investigated using the generalized gradient approximation (GGA) to spin-polarized density functional theory (DFT) with van der Waals corrections. The GGA optimized structures, ground state magnetic moments, site-projected density of states, and Bader charges are reported. We calculate the energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations on the Pu or U sites to determine the preferred magnetic structure of these materials. The relaxed AFM-spin structure of Pu(C2O4)2·6H2O was found to be considerably lower in energy than the corresponding relaxed FM-spin structure; whereas, there was negligible energy difference in the relaxed AFM and FM-spin structures of U(C2O4)2·6H2O. Weak hybridization between the actinide (Pu or U) 5f and O (2p) states in the site-projected density of states suggests that these systems are ionic. Furthermore, Bader charge analysis reveals charges on the actinide and oxalate oxygen sites in both U(C2O4)2·6H2O and Pu(C2O4)2·6H2O that are similar to literature data on other actinide species which are ionic, in particular the actinide dioxides. Calculating the density of states using a Hubbard correction parameter of U = 4.0 eV based on the GGA + U method shows band gaps of ∼1 eV for Pu(C2O4)2·6H2O and ∼3 eV for U(C2O4)2·6H2O. Both systems are predicted to be charge-transfer insulators.

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.

Study on optimization of actinide oxalate precipitation process in a vortex flow reactor

ABSTRACTOxalate precipitation process is globally accepted and is the current choice for conversion of actinide nitrates to their oxides. A vortex flow reactor with magnetic stirrer is found to be a better option for a hassle-free operation of batch as well continuous actinide reconversion process. Utilization of magnetic stirrer as an alternate for the conventional mechanical stirrer eliminates corrosion-related issues as none of the metal parts are in contact with nitric acid as well as oxalic acid solution. Moreover, this alternative system ensures a maintenance-free process. Optimization of various process conditions in a vortex reactor was accomplished using cerium nitrate as a surrogate for actinide nitrate and the results obtained were compared with that of pitched blade mechanical stirrer. About 40 L of thorium nitrate solution was successfully precipitated by the optimized procedure in a vortex reactor.

Hydrothermal decomposition of actinide(IV) oxalates: a new aqueous route towards reactive actinide oxide nanocrystals

AbstractThe hydrothermal decomposition of actinide(IV) oxalates (An= Th, U, Pu) at temperatures between 95 and 250 °C is shown to lead to the production of highly crystalline, reactive actinide oxide nanocrystals (NCs). This aqueous process proved to be quantitative, reproducible and fast (depending on temperature). The NCs obtained were characterised by X-ray diffraction and TEM showing their size to be smaller than 15 nm. Attempts to extend this general approach towards transition metal or lanthanide oxalates failed in the 95–250 °C temperature range. The hydrothermal decomposition of actinide oxalates is therefore a clean, flexible and powerful approach towards NCs of AnO2 with possible scale-up potential.

Multiscale structural characterizations of mixed U(iv)-An(iii) oxalates (An(iii) = Pu or Am) combining XAS and XRD measurements.

Mixed actinide(III,IV) oxalates of the general formula M2.2UAn(C2O4)5·nH2O (An = Pu or Am and M = H3O(+) and N2H5(+)) have been quantitatively precipitated by oxalic precipitation in nitric acid medium (yield >99%). Thorough multiscale structural characterization using XRD and XAS measurements confirmed the existence of mixed actinide oxalate solid solutions. The XANES analysis confirmed that the oxidation states of the metallic cations, tetravalent for uranium and trivalent for plutonium and americium, are maintained during the precipitation step. EXAFS measurements show that the local environments around U(+IV), Pu(+III) and Am(+III) are comparable, and the actinides are surrounded by ten oxygen atoms from five bidentate oxalate anions. The mean metal-oxygen distances obtained by XAS measurements are in agreement with those calculated from XRD lattice parameters.

Crystal growth methods dedicated to low solubility actinide oxalates

Two novel crystal growth syntheses dedicated to low solubility actinide-oxalate systems and adapted to glove box handling are described. These methods based on the use of precursors of either actinide metal or oxalic acid have been optimized on lanthanide systems (analogue of actinides(III)) and then assessed on real actinide systems. They allow the synthesis of several actinide oxalate single crystals, Am2(C2O4)3(H2O)3·xH2O, Th(C2O4)2·6H2O, M2+x[PuIV2−xPuIIIx(C2O4)5]·nH2O and M1−x[PuIII1−xPuIVx(C2O4)2·H2O]·nH2O. It is the first time that these well-known compounds are formed by crystal growth methods, thus enabling direct structural studies on transuranic element systems and acquisition of basic data beyond deductions from isomorphic (or not) lanthanide compounds. Characterizations by X-ray diffraction, UV–visible solid spectroscopy, demonstrate the potentialities of these two crystal growth methods to obtain oxalate compounds.

An improved thermodynamic model for the complexation of trivalent actinides and lanthanide with oxalic acid valid to high ionic strength

The dissociation constants of oxalic acid (Ox), and the stability constants of Am3+, Cm3+ and Eu3+ with Ox2− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and extraction techniques, respectively. The experimental data support the formation of complexes, M(Ox)n3−2n, where (M=Am3+, Cm3+ and Eu3+ and n=1 and 2). The dissociation constant and the stability constant values measured as a function of NaClO4 concentration were used to estimate the Pitzer parameters for the respective interactions of Am3+, Cm3+ and Eu3+ with Ox. Furthermore, the stability constants data of Am3+–Ox measured in NaClO4 and in NaCl solutions from the literature were simultaneously fitted in order to refine the existing actinide–oxalate complexation model that can be used universally in the safety assessment of radioactive waste disposal. The thermodynamic stability constant: log β0101=6.30±0.06 and log β0102=10.84±0.06 for Am3+ was obtained by simultaneously fitting data in NaCl and NaClO4 media. Additionally, log β0101=6.72±0.08 and log β0102=11.05±0.09 for the Cm3+ and log β0101=6.67±0.08 and log β0102=11.15±0.09 for the Eu3+ were calculated by extrapolation of data to zero ionic strength in NaClO4 medium only. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The thermodynamic model developed in this work will be useful in accurately modeling the potential solubility of trivalent actinides and early lanthanides to ionic strength of 6.60m in low temperature environments in the presence of Ox. The work is also applicable to the accurate modeling transport of rare earth elements in various environments under the surface conditions.

Structural study of (N2H5,H)2.9U1.1Ce0.9(C2O4)5·10H2O from a conventional X-ray diffraction diagram obtained on a powder synthesized by a fast vortex process

In the context of research on U/minor actinides for nuclear fuel reprocessing in the transmutation process, developments are first studied with surrogates containing uranium and lanthanides to facilitate testing. The tests consist of precipitating and calcining a hydrazinium uranium/cerium oxalate. The structure of this oxalate had not been previously determined, but was necessary to validate the physicochemical mechanisms involved.The present study, firstly demonstrates the structural similarity of the U/Ce oxalate phase (N2H5,H)2.9U1.1Ce0.9(C2O4)5·10H2O, synthesized using a vortex precipitator for continuous synthesis of actinide oxalates, with previously known oxalates, crystallizing in P63/mmc symmetry, obtained by more classical methods.This fast precipitation process induces massive nucleation of fine powders. Their structural and microstructural determination confirms that the raw and dried phases belong to the same structural family as (NH4)2U2(C2O4)5·0.7H2O whose structure was described by Chapelet-Arab in P63/mmc symmetry, using single crystal data. However, they present an extended disorder inside the tunnels of the structure, even after drying at 100°C, between water and hydrazinium ions. This disorder is directly related to the fast vortex method.This structure determination can be used as a basis for further semi-quantitative analysis on the U/minor actinides products formed under various experimental conditions.

Actinide oxalate complexes formation as a function of temperature by capillary electrophoresis coupled with inductively coupled plasma mass spectrometry

Abstract Complexation of various actinides (U(VI), Np(V), Pu(V), Am(III)) by oxalato ligand was studied by capillary electrophoresis (ICPMS detection) in 0.1 mol L –1 NaClO4 ionic strength solutions at various temperatures (15, 25, 35, 45 and 55 ℃). For each solution a unique peak was observed as a result of a fast equilibrium between the free ions and the complexes (labile systems). The results confirmed the formation of the 1:1, 1:2 and 1:3 complexes for U(VI) and Am(III); the formation of the 1:1 and 1:2 complexes for Np(V) and the formation of only 1 complex for Pu(V). For each complex, the thermodynamic parameters (the Gibbs energy Δ r G(T), the molar entropy change Δ r S(T) and the molar enthalpy change Δ r H(T 0)) were fitted to the experimental data. The effect of the ionic medium was treated using the specific ion interaction theory and the thermodynamic parameters at zero ionic strength were compared to previously published data.

Direct carbothermic reduction of actinide oxalates: Example of Nd(III) oxalate–carbon mixtures conversion

Mixed uranium–plutonium monocarbide was obtained by carbothermic reduction of mixture of UO 2, PuO 2 and C at around 1500 °C under vacuum or argon for 4 h followed by sintering at around 1650 °C under mixture of Ar + 8%H 2 for 10 h. Moreover, PuO 2 was synthesized by oxalic precipitation followed by calcination at around 700 °C (PUREX process). This work deals with a process simplification, i.e. fabrication of carbide by calcination of actinide oxalate, precipitated in presence of carbon, at 1600 °C for 13 h under argon. In this preliminary study, neodymium was used as a surrogate of actinides. No carbon influence was noticed during calcination of neodymium oxalate into oxide. Carbon content was the same before and after calcination making possible conversion to carbide directly from mixture of neodymium oxalate and C.

Separation of Americium, Curium, and Trivalent Lanthanides from High-Level Wastes by Oxalate Precipitation: Experiments with Synthetic Waste Solutions

The separation of trivalent actinides and rare earths from other fission products in high-level nuclear wastes by oxalate precipitation followed by ion exchange (OPIX) was experimentally investigated using synthetic wastes and a small-scale, continuous-flow oxalic acid precipitation and solid-liquid separation system. Trivalent actinide and rare earth oxalates are relatively insoluble in 0.5 to 1.0 M HNO/sub 3/ whereas other fission product oxalates are not. The continuous-flow system consisted of one or two stirred-tank reactors in series for crystal growth. Oxalic acid and waste solutions were mixed in the first tank, with the product solid-liquid slurry leaving the second tank. Solid-liquid separation was tested by filters and by a gravity settler. The experiments determined the fraction of rare earths precipitated and separated from synthetic waste streams as a function of number of reactors, system temperature, oxalic acid concentration, liquid residence time in the process, power input to the stirred-tank reactors, and method of solid-liquid separation. The crystalline precipitate was characterized with respect to form, size, and chemical composition. These experiments are only the first step in converting a proposed chemical flowsheet into a process flowsheet suitable for large-scale remote operations at high activity levels.