Isolation of Actinides from Irradiated Neptunium and Uranium Targets and Purification with TRU Resin

Within the framework of nuclear-physical experiments on determination of the yields of plutonium and americium isotopes in nuclear reactions ²³⁷Np(³He,xnyp), and also of neptunium and plutonium isotopes in nuclear reactions ²³⁶U(³He,xnyp), a procedure was developed for preparing samples containing americium, plutonium, neptunium, and uranium with a high degree of separation from each other and from the fission and activation products. A new resin for extraction chromatography, TRU-Spec (Eichrom Industries, the United States) was used for isolation of americium, plutonium, neptunium, and uranium from irradiated targets. New data were obtained on the behavior of some fission products in the course of chemical purification with TRU-Spec resin. A procedure was suggested for separation of americium from cerium-group lanthanides.     

Crystal chemistry of transuranium tungsten bronzes TuxWO3 (Tu = Np,Pu, Am)

Three transuranium tungsten bronzes have been synthetised for the first time in the systems TuO₂-WO₃-W systems (Tu=Np, Pu, Am). They were identified using X-ray powder diffraction; their structures are of perovskite type. Both plutonium and americium bronzes show a trivalent oxidation state, whereas Mőssbauer spectra reveal clearly a Np^IV ion in Np_xWO₃.     

Supercritical Fluid Extraction of Actinide Complexes: II. SFE of Actinide β-Diketonates

The solubility of uranium, plutonium, neptunium, and americium -diketonates in supercritical carbon dioxide (SC-CO₂) was studied. The concentration of actidine -diketonates in SC-CO₂ can be as high as 10-100 g l^{- 1}. The complexes with dipivaloylmethane, trifluoroacetylacetone, and hexafluoroacetylacetone and especially their adducts with tributyl phosphate and water are the most soluble. The residues after treatment of the complexes with SC-CO₂ were studied by IR spectroscopy.     

Regular Trends in Variation of Crystal Structures of Actinide Metals and Their Alloys

The regular trends in variation of crystal structures of 5f actinide metals in their alloys, under theaction of a high pressure, also in alloys of 4f lanthanide metals with plutonium can by explained by par-ticipation of 5f electrons in formation of the interatomic bond. By analogy with the criterion R _m R _4f (the ratio of "metallic" radius R _m to the radius of 4f shell), used for lanthanide metals, the ratio of the metallic radius to the radius of the trivalent ion (R _m/R ^{3 +}) was selected as a measure of this participation. The changes occurring in crystal lattices of americium, curium, and samarium in their alloys with plutonium, phase transitions in transplutonium metals under a high pressure, and also the order of variation of metallic radii along the actinide series are considered.     

Extraction of Americium(III) from Nitric Acid Solutions with Neat Diphenyl(dibutylcarbamoylmethyl)phosphine Oxide in the Presence of Metal Salts

Extraction of Am(III) from nitric acid solutions with a liquid complex of diphenyl(dibutylcarbamoylmethyl)phosphine oxide (Ph₂Bu₂) with nitric acid as influenced by concentration of Al, Ca, Na, Ni, Co, Cr, Fe, Zr, and Mo nitrates was studied. Nonextractable (Al, Ca, Na) and poorly extractable (Ni, Co, Cr) nitrates at their widely varied concentration do not interfere with the exhaustive extraction of americium from 3 M HNO3. These salts facilitate both formation of the extraction-active liquid phase on contact of Ph₂Bu₂ with 1 M HNO₃ and exhaustive extraction of americium from the acid. Extraction of Fe(III) at its widely varied concentration insignificantly interferes with extraction of americium, but this effect increases with increasing acidity and phase contact time. Americium is extracted with a powdered reagent from 0.1 M HNO₃ when liquid extractant is not formed in the visible amounts. In extraction of americium from weakly acidic solutions in the presence of salts the solid reagent, apparently, partially transforms into the liquid extractant, and two different extraction mechanisms are operative. It was shown that the neat liquid complex of Ph₂Bu₂ with HNO₃ can be used for exhaustive extraction of americium from high-level saline wastes from plants for processing nuclear fuel.     

Effect of Humic Substances on Plutonium and Americium Speciation in Soils and Soil Solutions

The molecular-weight distribution of humic substances (HSs) of the organic and mineral soils and corresponding soil solutions was studied by gel chromatography, and their distribution over fractions of humus acids was evaluated by the Ponomareva-Plotnikova chemical fractionation technique. Distribution of ^239,240Pu and ²⁴¹Am over HS fractions of different dispersity was studied. In these fractions, HSs are associated with different mineral components and have different solubility and mobility. Plutonium and americium are more strongly associated with humic substances of organic soils as compared to those of mineral soils. Therefore, in passing from mineral to organic soils, the migration ability of radionuclides decreases. These effects are caused by qualitative difference in the HS chemical composition of the HSs of organic and mineral soils. As compared to plutonium, americium enters the composition of poorly soluble organomineral complexes to a smaller extent, forming to a greater extent mobile anionic complexes in the soil solutions. This difference can be responsible for enhanced americium mobility and its easier availability to plants.     

Application of novel extractants for actinide(III)/ lanthanide(III) separation in hollow-fibre modules

Separation tests using hollow-fibre modules were performed for the difficult selective extraction of trivalent actinides over fission lanthanides from acidic media. This article shows that with 2,6-di(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine as the extractant, up to 94% americium could be extracted from 1.0 kmol/m³ HNO₃, with minimal lanthanide co-extraction. Using a synergistic mixture of bis(chlorophenyl)dithiophosphinic acid and tri-n-octyl phosphine oxide, tests were performed on extraction, lanthanide scrubbing and stripping. In the extraction test, up to 99.99% americium could be extracted from 0.5 kmol/m³ HNO₃, with approximately one third of the lanthanides being co-extracted. Mass transfer calculations using a consistent set of input data showed good agreement with the experiments.     

Synthesis of New Crystalline Pu(V) Compounds from Solutions: II. Double Pu(V) Oxalates with Co(NH<Subscript>3</Subscript>)<Subscript>6</Subscript><Superscript>3+</Superscript> Ions in the Outer Sphere

Crystalline compounds of the general composition Co(NH³)⁶PuO₂(C₂O₄)²·nH₂O (n = 2, 3, 5) were isolated from freshly prepared neutral oxalate solutions of Pu(V) by addition of Co(NH₃)₆³⁺ ions. These compounds are fairly stable in storage in air and poorly soluble in water. Previously unknown double Np(V) oxalates Co(NH₃)₆NpO₂(C₂O₄)·nH₂O (n = 2, 5) were also synthesized and studied. All the compounds of Pu(V) and Np(V) of the same composition are mutually isostructural. The behavior of these compounds at heating was studied, and their IR spectra were measured. The optical spectra of new Np(V) compounds were measured.     

Double Np(V) Malonates with Co(NH3)6 3 + in the Outer Sphere

Light yellow crystalline products of the general formula Co(NH₃)₆(NpO₂L)₂A·xH₂O, where A = Cl, NO₃, or ClO₄, are precipitated with Co(NH₃)₆ ^{3 +} ions from weakly acidic chloride, nitrate, and perchlorate solutions of Np(V), containing up to 0.3 M CH₂(COO)₂ ^{2 -} (L^{2 -}). When only malonate ions are present in the solution, Np(V) precipitates in the form of Co(NH₃)₆(NpO₂L)₂HL·H₂O. These compounds are virtually insoluble in water and have similar X-ray patterns. Unlike oxalate and phthalate solutions, complex neptunyl(V) ion NpO₂L₂ ^{3 -} is not precipitated with Co(NH₃)₆ ^{3 +} ions from malonate media of the above composition.     

Synthesis and crystal structure of double Np(V) cesium oxalate CsNpO2C2O4 · nH2O

An X-ray diffraction study of CsNpO₂C₂O₄ · nH₂O was carried out. The structure consists of NpO ₂ ⁺ and Cs⁺ cations, oxalate anions, and water molecules. Dioxo cations are combined by cation-cation interaction into cyclic trimer in which each NpO ₂ ⁺ cation act as monodentate ligand. The coordination polyhedron of the Np(V) atom is a pentagonal bipyramid. The similarity of the structures of MNpO₂C₂O₄ · nH₂O (M = NH₄, Cs) based on cyclic trimers combined by oxalate ions into a three-dimensional framework was confirmed. Original Russian Text I.A. Charushnikova, N.N. Krot, I.N. Polyakova, 2006, published in Radiokhimiya, 2006, Vol. 48, No. 3, pp. 202–204.     

Reaction of ozone with uranium(IV) oxalate in water

Decomposition of aqueous suspensions of uranium(IV) oxalate under the action of an ozone–oxygen mixture was studied. The process occurs in two steps. In the first step, the U(IV) oxidation with the formation of oxalic acid uranyl solutions prevails. The second step involves decomposition of oxalate ions and hydrolysis of uranyl ions. An increase in temperature accelerates the transformation of uranium(IV) oxalate into uranium(VI) hydroxide compounds. In solutions containing KBr or UO₂Br₂, the following reaction occurs: O₃ + Br^– → O₂ + BrO^–. The arising hypobromite ions and hypobromous acid oxidize uranium(IV) oxalate extremely efficiently. The possible mechanism of ozonation of aqueous uranium(IV) oxalate suspensions is discussed.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.     

Synthesis and X-ray Diffraction Study of Mixed-Metal Orthophosphates with NaZr2(PO4)3 and CePO4 Structures

Mixed-metal orthophosphates with Ca^{2 +}, Cd^{2 +}, Gd^{3 +}, Ti^{4 +}, Hf^{4 +}, Ce^{4 +}, U^{4 +}, and Pu^{4 +} were synthesized and studied by X-ray diffraction. Crystalline compounds containing cerium or plutonium, or both these elements were prepared. Cerium was taken as an imitator for plutonium. The phosphates containing Ti^{4 +} and Hf^{4 +} formed two-phase systems. One of the phases was assigned to the Zr₂(PO₄)₃ structural type (NZP), and the other phase, to the CePO₄ type (monazite). In the other cases, single-phase products were formed. The effect of the cationic composition on the crystal structure of the complex orthophosphates, including the unit cell parameters of the crystalline phases, was studied.     

Crystal structure of a Np(V) sesquioxalate,Na<Subscript>4</Subscript>(NpO<Subscript>2</Subscript>)<Subscript>2</Subscript>(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

The crystal structure of a previously unknown Np(V) sesquioxalate, Na₄(NpO₂)₂(C₂O₄)₃·2H₂O was studied. The crystal structure consists of neptunyl(V) cations, sodium cations, oxalate anions, and water molecules of crystallization. Neptunyl(V) cations and oxalate ions form anionic chains [(NpO₂)₂(C₂O₄)₃] _n ^4n? . The coordination polyhedron (CP) of Np (pentagonal bipyramid) contains two apical “yl” oxygen atoms and five equatorial O atoms of three oxalate ions. The CP of Na(1) and Na(2) cations are combined through the common edges into zigzag chains in the [010] direction. Two independent oxalate ions are tridentate and tetradentate ligands.     

Kinetics of neptunium(V) reduction with iron(II) sulfamate in nitric acid solutions

The kinetics of chemical reduction of Np(V) with iron(II) sulfamate in HNO₃ solutions was studied by spectrophotometry. The reduction process can be described by the equation NpO ₂ ⁺ + Fe²⁺ + 4H⁺ ? Np⁴⁺ + Fe³⁺ + 2H₂O and follows the first- and zero-order rate law with respect to the Np(V) concentration. The reaction rate constants were determined in relation to the HNO₃ concentration (1–3 M), iron(II) sulfamate concentration [(0.59–2.94) × 10^?3 M], and temperature (298.2–319.2 K). The activation energy (E _a) and the thermodynamic functions of formation of the activated complex (activation free energy ΔG ^≠, enthalpy ΔH ^≠, entropy ΔS ^≠) were calculated.     

Improved chemical route for quantitative precipitation of lead zirconyl oxalate (PZO) leading to lead zirconate (PZ) powders

An improved chemical route is described to precipitate quantitatively a molecular precursor lead zirconyl oxalate tetrahydrate, PbZrO(C₂O₄)₂·4H₂O (PZO) at room temperature. In this route, soluble ammonium zirconyl oxalate, (NH₄)₂ZrO(C₂O₄)₂ (AZO), is initially prepared by chemical reaction of 0.1-M solution of zirconyl nitrate with 0.2-M solution of ammonium oxalate. The anionic species ZrO(C₂O₄)₂⁻² thus obtained is exchanged by adding equimolar solution of lead acetate to precipitate single-phase PZO with higher yield. The improvement in the presently described route is to drive exchange reaction to its completion by suitably manoeuvring the choice of precursor, pH and reaction conditions, in such a way that single-phase PZO is precipitated without any destabilization of anionic species ZrO(C₂O₄)₂⁻². The pyrolysis of PZO produced stoichiometric, monophasic, orthorhombic PbZrO₃ (PZ) powders at temperature T=700°C. Various physico-chemical techniques were used to characterize both molecular precursor PZO as well as PZ powders obtained by its pyrolysis. All our experimental results related to the synthesis and characterization of these powders are presented in this paper.     

Effect of complexation on crystallization of plutonium(IV) oxalate in the system Pu(NO<Subscript>3</Subscript>)<Subscript>4</Subscript>-HNO<Subscript>3</Subscript>-H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The published data on complexation in the system Pu(NO₃)₄-HNO₃-H₂C₂O₄ were treated on the basis of a unified approach to determination of the oxalate ion concentration. Because of discrepancies between results published by different researchers, additional experiments on crystallization of Pu(IV) oxalate were carried out at widely varied excess and deficiency of oxalic acid. These experiments confirmed high stability of the complex cations PuC₂O ₄ ²⁺ . The upper boundary of the field of metastable supersaturated solutions of Pu oxalate at the initial Pu concentration of 15–50 g l^?1 was determined.     

Preparation of LaNiO3 powder from coprecipitated lanthanum-nickel oxalates

Coprecipitation of lanthanum and nickel oxalates in a water-alcohol mixed solution of an oxalic acid resulted in a simultaneous and homogeneous deposition of the respective oxalate particles with a desired cation ratio. The coprecipitated oxalate could be readily converted to a fine powder (5 to 6 m² g^–1) of the desired LaNiO₃ by heating at 800 to 850° C. Detailed examination of some precipitation conditions established an optimum procedure needed for the powder synthesis of LaNiO₃. Thermal analysis showed that La₂NiO₄ (high temperature form) is transiently produced prior to the formation of LaNiO₃. Mixed valency of the nickel ion in the synthesized powders was quantitatively determined by means of the oxidation-reduction titration, suggesting that the chemical formula of the powders might be LaNiO_{2.85 to 2.90}.     

Effect of Inorganic Anions on the Physicochemical State of Radionuclides in Process Freshwater Basins at the Mayak Production Association

Published data on the speciation of microcomponents and radionuclides (calcium, strontium, plutonium, and americium) in the water and bottom sediments of selected process water basins at the Mayak Production Association were discussed in terms of the model of the sorption behavior of a sorbate existing in several forms. It was shown that distribution of calcium, ⁹⁰Sr, and natural isotopes of strontium does not contradict the views on formation of Ca^{2 +}, CaSO₄ ⁰, Sr^{2 +}, SrSO₄ ⁰, Sr[SO₄]₂ ^{2 -} ionic and molecular species and of colloidal sulfate (CaSO₄·2H₂O) at selected sites of the water basin. By combining these factors it is possible to explain the variation in the distribition coefficients of Ca, Sr, Pu, and Am in the water-bottom sediments system in terms of the model of ion-exchange sorption in the presence of nonsorbable nonequilibrium colloid.     

Redox Kinetics of U,Pu, and Np in TBP Solutions: VI. Reactions of Neptunium and Plutonium with Organic Reducing Agents: Determination of the Final Oxidation States and Reaction Rates

Reactions of Pu(IV) and Np(VI) with organic reducing agents of various types (substituted hydroxylamines, oximes, aldehydes, etc.) in tributyl phosphate solutions containing nitric acid were studied spectrophotometrically. The molar extinction coefficients of neptunium and plutonium in various oxidation states [Np(IV,V,VI), Pu(III,IV,VI)] in TBP solutions were determined as influenced by HNO₃ and H₂O concentrations and temperature. It was found that organic reducing agents at low HNO₃ concentration convert plutonium and neptunium to Pu(III) and Np(V), respectively. With increasing HNO₃ concentration Pu(III) and Np(V) are partly oxidized back to Pu(IV) and Np(VI), respectively, by reaction with nitrous acid. The rate constants of Pu(VI) and Np(VI) reduction and Np(V) oxidation as influenced by concentration of organic reducing agents and HNO₃ were evaluted from the kinetic data.