Coprecipitation of actinide peroxide salts in the U-Th and U-Pu systems and their thermal decomposition.

The uranium and plutonium co-conversion process constitutes a continuous subject of interest for MOx fuel fabrication. Among the various routes considered, chemical coprecipitation by the salt effect has been widely investigated regarding its simplicity of integration between the partitioning and purification steps of the PUREX process, and the straightforward recovery of precursors that are easily converted into oxide phases by thermal decomposition. The present study focuses on the coprecipitation behavior of U-Th and U-Pu actinide peroxide mixed systems by examining the precipitation yields and settling properties for nitric acidity in the range of 1 to 3 M and hydrogen peroxide concentration in the range of 4.5 to 7 M. The precipitated solids have been characterized by powder XRD, IR and Raman spectroscopy, laser granulometry and SEM-EDS analyses revealing the synthesis of studtite and actinide(IV) peroxo-nitrates as aggregated particles. The actinide solid phases are uniformly distributed within the filtered cakes. The precursor thermal decomposition results in the formation of oxide phases at low temperature according to a sequential release of water molecules, peroxide ligands and nitrate ions. The calcination step has a limited effect on the morphology of the powders which remain highly divided. The high precipitation rate of actinides makes this chemical route potentially interesting as a co-precipitation process.

Quantitative Precipitation of Uranyl or Plutonyl Nitrate with N-(1-Adamantyl)acetamide in Nitric Acid Aqueous Solution.

The reactivity of the N-(1-adamantyl)acetamide ligand (L = adam) has been evaluated as precipitating agent for the hexavalent uranyl cation ([U] = 20-60 g L-1) in concentrated nitric acid aqueous solution (0.5-5 M). It results in the formation of a crystalline complex (UO2)(adam)2(NO3)2·2(adam) (1), in which the uranyl center is 8-fold coordinated to two chelating nitrate groups and two N-(1-adamantyl)acetamide (= adam) ligands through the oxygen atom of the amide function. Two other noncoordinating adam moieties are also observed in the crystal structure packing and interact through a hydrogen-bond scheme with the uranyl-centered species. A similar molecular assembly has been obtained with the plutonyl(VI) cation, in the complex (PuO2)(adam)2(NO3)2·2(adam) (2). Precipitation studies indicate high (UO2)(adam)2(NO3)2·2(adam) formation yields (up to 99%U for an L/U molecular ratio of 5/1) for HNO3 concentration in the 0.5-5 M range. However, the precipitation kinetics is rather slow and the reaction is completed after several hours (3-4 h). The calcination of the resulting solid under an air atmosphere led to the formation of the U3O8 oxide from 400 °C through a transient phase UO2 fluorite-type (from 200 °C).

Crystal Growth in the Thorium-TEDGA-Oxalate-Nitrate System: Description and Comparison of the Main Structural Features.

This paper reports the synthesis and characterization of four new compounds based on thorium and tetraethyldiglycolamide (TEDGA), [Th(TEDGA)2(C2O4)][NO3]2[H2C2O4].6H2O (1), [Th(TEDGA)2(C2O4)2][H2C2O4]2.2H2O (2), [Th(TEDGA)4][NO3]4.4H2O (3), and [Th2(C2O4)3(TEDGA)4][NO3][HC2O4][H2C2O4]4.7H2O (4). All of them are obtained by successive crystallization from a unique medium containing thorium nitrate and TEDGA, in the presence of oxalic acid. Compound (1) ( a = b = 18.7140(12) Å, c = 12.9212(9) Å, S.G. P42212) crystallized at first from a gel obtained by slow evaporation of the medium. When compound (1) is left in gel for a period of some weeks, it tends to disappear and to be replaced by crystals of (2) ( a = 12.246(2) Å, b = 32.253(5) Å, c = 12.256(2) Å, ß = 106.741(12)°, S.G. P21/ n) and (3) ( a = 26.5966(13) Å, b = 15.4489(7) Å, c = 18.5582(9) Å, ß = 116.528(1)°, S.G. C2/ c). In their turn, solids (2) and (3) disappear from the gel left for some months, and compound (1) crystallizes in mixture with compound (4) ( a = 15.6611(7) Å, b = 17.9082(9) Å, c = 18.1814(7) Å, α = 89.896(2)°, ß = 65.549(2)°, γ = 87.623(2), S.G. P-1). Solving the crystal structure by single crystal diffraction reveals that TEDGA is always coordinated to thorium through its three oxygen atoms. In the mixed-ligands compounds (1), (2), and (4), Th4+ is surrounded by two oxalate ligands and two TEDGA, leading to a 10-fold coordination. The dimensionality of the networks changes from linear chains (1D) (1) to isolated entities (0D) (2) or dimeric units (0D) (4). Compound (3) is formed by the assembly of 12-fold coordinated monomeric entities (0D) in which the thorium cation is surrounded by four TEDGA. This compound is the first example of such a coordination number without nitrate anion included in the coordination sphere of Th.

Crystal growth and first crystallographic characterization of mixed uranium(IV)-plutonium(III) oxalates.

The mixed-actinide uranium(IV)-plutonium(III) oxalate single crystals (NH4)0.5[Pu(III)0.5U(IV)0.5(C2O4)2·H2O]·nH2O (1) and (NH4)2.7Pu(III)0.7U(IV)1.3(C2O4)5·nH2O (2) have been prepared by the diffusion of different ions through membranes separating compartments of a triple cell. UV-vis, Raman, and thermal ionization mass spectrometry analyses demonstrate the presence of both uranium and plutonium metal cations with conservation of the initial oxidation state, U(IV) and Pu(III), and the formation of mixed-valence, mixed-actinide oxalate compounds. The structure of 1 and an average structure of 2 were determined by single-crystal X-ray diffraction and were solved by direct methods and Fourier difference techniques. Compounds 1 and 2 are the first mixed uranium(IV)-plutonium(III) compounds to be structurally characterized by single-crystal X-ray diffraction. The structure of 1, space group P4/n, a = 8.8558(3) Å, b = 7.8963(2) Å, Z = 2, consists of layers formed by four-membered rings of the two actinide metals occupying the same crystallographic site connected through oxalate ions. The actinide atoms are nine-coordinated by oxygen atoms from four bidentate oxalate ligands and one water molecule, which alternates up and down the layer. The single-charged cations and nonbonded water molecules are disordered in the same crystallographic site. For compound 2, an average structure has been determined in space group P6/mmm with a = 11.158(2) Å and c = 6.400(1) Å. The honeycomb-like framework [Pu(III)0.7U(IV)1.3(C2O4)5](2.7-) results from a three-dimensional arrangement of mixed (U0.65Pu0.35)O10 polyhedra connected by five bis-bidentate µ(2)-oxalate ions in a trigonal-bipyramidal configuration.

First synthesis of uranyl aluminate nanoparticles.

This paper describes, for the first time, a simple method for the synthesis of uranyl aluminate (URAL) nanoparticles. URAL was prepared by U(VI) hydrolytic precipitation with ammonia at pH = 11 in the presence of mesoporous alumina MSU-X under 20 kHz of sonication followed by annealing of the obtained solids at 800 degrees C. TEM, XAFS, powder XRD, and (27)Al MAS NMR studies revealed that the speciation of uranium in this system strongly depends on uranium concentration. The sample with 5 wt % of uranium yields air-stable nanoparticles ( approximately 5 nm) of URAL. Presumably, UO(2)(2+) cations in this compound are coordinated with bidentate AlO(2)(-) groups. The increase of uranium concentration to 30 wt % causes mostly formation of U(3)O(8) fine particles ( approximately 50 nm) and small amounts of URAL.