Interaction of Aluminium (III) with the f-Family Ions Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) in the Course of Simultaneous Hydrolysis

The interaction of Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) with Al(III) in solutions at pH 0–4 was studied by the spectrophotometric method. It was shown that, in the range of pH 3–4, the hydrolyzed forms of neptunyl and plutonyl react with the hydrolyzed forms of aluminium. In the case of Pu(VI), the mixed hydroxoaqua complexes (H₂O)₃PuO₂(-OH)₂Al(OH)(H₂O)₃ ²⁺ or (H₂O)₄PuO₂OAl(OH)(H₂O)₄ ²⁺ are formed at the first stage of hydrolysis. Np(VI) also forms similar hydroxoaqua complexes with Al(III). The formation of the mixed hydroxoaqua complexes was also observed when Np(IV) or Pu(IV) was simultaneously hydrolyzed with Al(III) at pH 1.5–2.5. The Np(IV) complex with Al(III) has, most likely, the formula (H₂O) _n (OH)Np(-OH)₂Al(OH)(H₂O)₃ ³⁺. At pH from 2 to 4.1 (when aluminium hydroxide precipitates), the Np(V) or Nd(III) ions exist in solutions with or without Al(III) in similar forms. When pH is increased to 5–5.5, these ions are almost not captured by the aluminium hydroxide precipitate.     

Determination of aqueous plutonium oxidation states by solvent extraction

The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.Work supported by the U. S. Department of Energy under Contract DE-ACO6-76RLO 1830.     

Bicyclic and acyclic diamides: comparison of their aqueous phase binding constants with Nd(III), Am(III), Pu(IV), Np(V), Pu(VI), and U(VI)

This report describes affinity measurements for two, water-soluble, methyl-alkylated diamides incorporating the malonamide functionality, N,N,N',N' tetramethylmalonamide (TMMA) and a bicyclic diamide (1a), toward actinide metal cations (An) in acidic nitrate solutions. Ligand complexation to actinides possessing oxidation states ranging from +3 to +6 was monitored through optical absorbance spectroscopy, and formation constants were obtained from the refinement of the spectrophotometric titration data sets. Species analysis gives evidence for the formation of 1, 4, 1, and 2 spectrophotometrically observable complexes by TMMA to An(III, IV, V, and VI), respectively, while for 1a, the respective numbers are 3, 4, 2, and 2. Consistent with the preorganization of 1a toward actinide binding, a significant difference is found in the magnitudes of their respective formation constants at each complexation step. It has been found that the binding affinity for TMMA follows the well-established order An(V) < An(III) < An(VI) < An(IV). However, with 1a, Np(V) forms stronger complexes than Am(III). The complexation of 1a with Np(V) and Pu(VI) at an acidity of 1.0 M is followed by reduction to Np(IV) and Pu(IV), whereas TMMA does not perturb the initial oxidation state for these dioxocations. These measurements of diamide binding affinity mark the first time single-component optical absorbance spectra have been reported for a span of actinide-diamide complexes covering all common oxidation states in aqueous solution.     

Extraction of actinides with heterocyclic dicarboxamides

Extraction of actinides from aqueous nitric acid by three different heterocyclic dicarboxamides (2,6-pyridinedicarboxamide, 2,2′-bipyridine-6,6′-dicarboxamide and 1,10-phenanthroline-2,9-dicarboxamides) was studied. It was shown that all studied ligands extract actinides at different oxidation states (U(VI), Np(V), Pu(IV), Am(III), Cm(III)) from acidic solutions. All studied diamides extract Am(III) better than Cm(III). Et(pHexPh)ClPhen contains electron-withdrawing chlorine atoms at the positions 4 and 7 of the phenanthroline moiety (SF_Am/Cm = 4–6) and possesses the highest separation factor Am(III)/Cm(III). The studied ligands possess high extraction ability to all actinides present in HLW and therefore they could be used for simultaneous extraction of actinides in the GANEX-type process.     

Actinide speciation in the environment

Nuclear test explosions and nuclear reactor wastes and accidents have released large amounts of radioactivity into the environment. Actinideions in waters often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is related to the form in which the nuclides are introduced into the aquatic system. Chemical speciation, oxidation state, redox reactions, and sorption characteristics are necessary in predicting solubility of the different actinides, their migration behaviors and their potential effects on marine biota. The most significant of these variables is the oxidation state of the metal ion as the simultaneous presence of more than one oxidation state for some actinides in a solution complicates actinide environmental behavior. Both Np(V)O₂ ⁺ and Pu(V)O₂ ⁺, the most significant soluble states in natural oxic waters, are relatively noncomplexing and resistant to hydrolysis and subsequent precipitation. The solubility of NpO₂ ⁺ can be as high as 10⁻⁴M while that of PuO₂ ⁺ is much more limited by reduction to the insoluble tetravalent species, Pu(OH)₄, (pK_sp≥56) but which can be present in the pentavalent form in aqautic phases as colloidal material. The solubility of hexavalent UO₂ ²⁺ in sea water is relatively high due to formation of carbonate complexes. The insoluble trivalent americium hydroxocarbonate, Am(OH)(CO₃) is the limiting species for the solubility of Am(III) in sea water. Thorium(IV) is present as Th(OH)₄, in colloidal form. The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing the environmental behavior of actinides. Much is understood about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these radioactive species on marine biota, but much more understanding of the behavior of the actinides in the environment is needed to allow proper and reliable modeling needed for disposition of nuclear waste over many thousands of years. This paper has been presented as a Hevesy Medal Award Lecture by the medalist author at the 1st International Nuclear Chemistry Congress (1st-INCC), 22–29 May 2005, Kusadasi, Turkey     

Evaluation of a rapid technique for measuring actinide oxidation states in a ground water simulant

A system using an ion chromatograph coupled to a flow-cell scintillation detector for rapidly measuring the oxidation states of actinides at low concentrations (<10–6M) in aqueous solutions was evaluated. The key components of the system are a cation–anion separation column (Dionex, CS5) and a flow cell detector with scintillating cerium activated glass beads. The typical procedure was to introduce a 0.5 ml aliquot of sample spiked with actinides in the +III to +VI oxidation states into a 5 ml sample loop followed by 4 ml of synthetic groundwater simulant. Separation was achieved at a flow rate of 1 ml/min using an isocratic elution with oxalic, diglycolic, and nitric acids followed by distilled water. Tests were first conducted to determine elution times and recoveries for an acidic solution (pH 2) and a ground water simulant (pH 8) containing Am(III), Pu(IV), Th(IV), Pu(V), and U(VI). Then, an analysis was performed using a mixture of Pu(IV), Pu(V), and Pu(VI) in the ground water simulant and compared to results using the DBM extraction technique. Approximate elution times were the same for both the acidic solution and the ground water simulant. These were as follows: Pu(V) at 10 min, Am(III) at 15 min, Pu(IV) at 25 min, Th (IV) at 28 min and U(VI) at 36 min. Recoveries for the acidic solution were quantitative for U(VI) and Th(IV) and exceeded 80% for Am(III). Recoveries for the ground water simulant were quantitative for U(VI), but they were generally not quantitative for Th(IV), Pu(IV), and Am(III). For Th(IV) and Pu(IV), less than quantitative recoveries were attributed to the formation of neutral hydroxides and colloids; for Am(III) they were attributed to insoluble carbonates and/or hydroxycarbonates. When applied to the measurement of plutonium in the ground water simulant, the technique provided showed good agreement with the dibenzoylmethane (DBM) extraction technique, but it could not distinguish between Pu(V) and Pu(VI). This was likely due to the reduction of Pu(VI) to Pu(V) in the sample by the oxalic acid eluent. However, in spite of this limitation, the technique can be used to distinguish between Pu(IV) and Pu(V) in aqueous environmental samples within a pH range of 4 to 8 and an E _H range of -0.2 to 0.6 V, the predominance region for Pu(III), (IV), and (V). In addition, this technique can be used to corroborate oxidation state analysis from the dibenzoylmethane (DBM) extraction method for environmental samples.     

Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).     

Determination of neptunium in a reprocessing plant by an on-line solid-phase extraction/electrochemical detection system

In this study, a flow-based electrochemical detection system coupled to a solid-phase extraction column was developed for the determination of neptunium in the presence of Pu(IV). Np(V) in the sample solution was completely oxidized to Np(VI) via electrolysis using a column electrode composed of carbon fibers. The column electrode effluent was then loaded onto a TEVA^® column, and subsequently onto a UTEVA^® column using 3 mol L^?1 HNO₃. Pu(IV) was retained on the TEVA column and separated from Np(VI), while Np(VI) was retained on the UTEVA column. Np(VI) was eluted from the UTEVA column with 0.01 mol L^?1 HNO₃ and then introduced directly into a flow-through electrolysis cell. An electrochemical amperometric method with a working potential of +0.1 V (vs. Ag/AgCl) was used to detect Np(VI). The current produced due to the reduction of Np(VI) was continuously monitored and recorded, and the Np concentration was calculated from the peak area. The relative standard deviation of 10 analyses was 2.4 % for an Np solution (0.50 mg L^?1) containing 1.0 μg Np. The detection limit, which was determined to be three times the standard deviation, was 35 μg L^?1 (70 ng Np).     

Determination of stability constants of U(VI), Np(VI) and Pu(VI) with chloride ions by extraction chromatography

The complex formation of U(VI), Np(VI) and Pu(VI) with chloride ions was studied in HClO₄−HCl solutions at ionic strength of 2.0 and [H⁺]=2.0M by the method of extraction chromatography using dilute HDEHP as the stationary phase.     

Modeling Speciation and Solubility in Aqueous Systems Containing U(IV,VI), Np(IV,V, VI), Pu(III,IV, V,VI), Am(III), and Cm(III)

A comprehensive thermodynamic model, referred to as the Mixed-Solvent Electrolyte model, has been applied to calculate phase equilibria and chemical speciation in selected aqueous actinide systems. The solution chemistry of U(IV, VI), Np(IV, V, VI), Pu(III, IV, V, VI), Am(III), and Cm(III) has been analyzed to develop the parameters of the model. These parameters include the standard-state thermochemical properties of aqueous and solid actinide species as well as the ion interaction parameters that reflect the solution’s nonideality. The model reproduces the solubility behavior and accurately predicts the formation of competing solid phases as a function of pH (from 0 to 14 and higher), temperature (up to 573 K), partial pressure of CO₂ (up to \( p_{{{\text{CO}}_{2} }} \)  = 1 bar), and concentrations of acids (to 127 mol·kg^?1), bases (to 18 mol·kg^?1), carbonates (to 6 mol·kg^?1) and other ionic components (i.e., Na⁺, Ca²⁺, Mg²⁺, OH^?, Cl^?, \( {\text{ClO}}_{4}^{ - } \), and \( {\text{NO}}_{3}^{ - } \)). Redox effects on solubility and speciation have been incorporated into the model, as exemplified by the reductive and oxidative dissolution of Np(VI) and Pu(IV) solids, respectively. Thus, the model can be used to elucidate the phase and chemical equilibria for radionuclides in natural aquatic systems or in nuclear waste repository environments as a function of environmental conditions. Additionally, the model has been applied to systems relevant to nuclear fuel processing, in which nitric acid and nitrate salts of plutonium and uranium are present at high concentrations. The model reproduces speciation and solubility in the U(VI) + HNO₃ + H₂O and Pu(IV, VI) + HNO₃ + H₂O systems up to very high nitric acid concentrations (\( x_{{{\text{HNO}}_{3} }} \approx 0.70 \)). Furthermore, the similarities and differences in the solubility behavior of the actinides have been analyzed in terms of aqueous speciation.     

Complexation and redox chemistry of neptunium,plutonium and americium with a hydroxylaminato ligand

There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-(^tBuNO)py]⁻(interchangeable hereafter with [(^tBuNO)py]⁻), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[(^tBuNO)py]₄ (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, ¹H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [(^tBuNO)py]⁻ ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E_1/2 = −2.26 V vs. Fc^+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of U^IV[(^tBuNO)py]₄ that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [(^tBuNO)py]⁻ ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [Am^III(μ₂-(^tBuNO)py)((^tBuNO)py)₂]₂ (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-(^tBuNO)py]⁻, in good agreement with experimental observations.

The coordination modes and electronic properties of a strongly coordinating hydroxylaminato ligand with Np, Pu and Am were investigated.Complexes were characterized by a range of experimental and computational techniques.     

Sensitive redox speciation of neptunium by CE–ICP–MS

Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1?×?10^-9 and 5?×?10^-10 mol L^-1 for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10^-9 to 10^-6 mol L^-1. The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5?×?10^-7 mol L^-1 Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe²⁺ led to complete sorption of the Np onto the clay. After desorption with HClO₄, a mixture of Np(IV) and Np(V) was found in solution by CE–ICP–MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe²⁺.     

Reductive Dissolution of PuO2(am): The Effect of Fe(II) and Hydroquinone

PuO₂(am) solubility was investigated as a function of time, for pH from 0.5 to 11, and in the presence of 0.001 M FeCl₂ or 0.00052 M hydroquinone to determine the effect of environmentally important reducing agents on PuO₂(am) solubilization under geological conditions. Equilibrium was reached in <4 days. The observed PuO₂(am) solubilities were many orders of magnitude higher than the Pu(IV) concentrations predicted from thermodynamic data. Spectroscopic, solvent extraction, and thermodynamic analyses of data showed that Pu(III) was the dominant aqueous oxidation state. The experimental pH, pe, and Pu(III) concentrations from both the Fe(II) and hydroquinone systems provided a log K ⁰ value of 15.5 ± 0.7 for [PuO₂(am) + 4H⁺ + e ^– Pu³⁺ + 2H₂O]. The data show that reduction reactions involving Fe(II) and hydroquinone are relatively rapid and that reductive dissolution of PuO₂(am), hitherto ignored, may play an important role in controlling Pu behavior under reducing environmental conditions.     

Das Ionenaustauschverhalten von Neptunium in salpetersauren Lösungen

Zusammenfassung Das Verteilungsverhalten von²³⁷Np–²³⁹Np zwischen salpetersauren Lösungen verschiedener Konzentration und einem Anionenaustauscher (Dowex 1X8) wurde untersucht. Durch Reduktion mit Fe²⁺+Hydrazin konnte quantitativ das Np(IV) hergestellt und dessen Verteilungskoeffizient (D) in 1n-10n-HNO₃ bestimmt werden. Mit Hydrazin allein und bei Lösungen ohne Reduktionsmittel wurden stark schwankendeD-Werte gefunden, die auf unterschiedliche Prozentsätze an Np(IV), Np(V) und Np(VI) zurückzuführen waren. Durch die Bestimmung der Anteile der einzelnen Oxidationsstufen konnten jedoch die jeweiligenD-Werte [D(IV),D(V),D(VI)] berechnet werden. Die mit dieser Methode erhaltenen Werte stimmten gut mit den Daten überein, die an Systemen gewonnen wurden, in denen jeweils nur eine Neptuniumoxidations-stufe vorlag.

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The ion exchange behaviour of neptunium in nitric acid solutions

The distribution of²³⁷Np–²³⁹Np between nitric acid solutions of different concentration and an anion exchanger (Dowex 1X8) was investigated. By reduction with Fe²⁺+hydrazine, the Np(IV) was obtained quantitatively and its distribution coefficients (D) in 1n to 10n-HNO₃ were determined. With hydrazine alone and without any reduction media, strongly varyingD-values were found. This was due to different amounts of Np(IV), Np(V), and Np(VI) in the solutions. By determining the fractions of the individual oxidation states the correspondingD-values [D(IV),D(V), andD(VI)] could be calculated. The data obtained by this method agreed well with theD-values resulting from determinations of systems, in which the individual Np-oxidation state was the only component of the corresponding solution.

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Mit 4 Abbildungen

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Herrn Prof. Dr.Hans Nowotny gewidmet.     

Synergistic extraction of hexavalent actinides by HTTA and neutral donors-II

Synergistic extraction studies on Np(VI) and Pu(VI) have been carried out as a part of the programme on the synergistic extraction of hexavalent actinides. Extraction of Np(VI) and Pu(VI) were carried out by mixtures of HTTA and TBP in benzene from aqueous perchlorate and nitrate media. Equilibrium constant values for the various reaction equilibria involved were calculated from the data obtained by using slope-ratio as well as Job’s method. The extraction of Np(VI) by the synergistic mixture from 1M nitric acid indicated that the species NpO₂ (TTA) (NO₃). TBP was not involved in the extraction. The log values of K_A, K_AB and β_AB were −1.5, 2.92 and 4.43, respectively for Np(VI) and −1.63, 2.50 and 4.13 respectively for Pu(VI).     

Speciation of Pu ions by differential pulse polarography

By means of differential pulse polarography, Pu ions of different oxidation states have been investigated in 1M Na₂CO₃ solution. Redox reactions of Pu/III/, Pu/IV/, Pu/V/ and Pu/VI/, which are mostly of irreversible nature, have been observed within the potential range of the dropping mercury electrode /DME/, from 0 to –1.5 V, against a Ag/AgCl/NaCl (3M) reference electrode. Based on the peak potential observed for each reaction, the stability of a given oxidation state in the solution could be ascertained. The redox potential of the Pu/IV/–Pu/III/ pair, which was found to be –1.0 V, indicated that the Pu/IV/ carbonate complex was of high stability. The detection sensitivity of the Pu/IV/ ion was found to be 1×10^–6M.     

Reduction kinetics of Pu(IV) and Np(VI) by N,N-dimethylhydrazine,and its potential application in nuclear fuel reprocessing

Reduction kinetics of Pu(IV) by N,N-dimethylhydrazine (NNDMH) were studied by spectrophotometry, and the reduction rate equation in 3M (mol/dm³) nitric acid was obtained. The reduction properties of NNDMH for U(VI), Np(VI), and Pu(IV) was studied in the mixture solution of trin-butylphosphate diluted to 30 vol.% by n-dodecane (30% TBP) and 3M nitric acid. It was confirmed that NNDMH selectively reduce Np(VI) to Np(V) without affecting the valences of U(VI) and Pu(IV) in a few minutes. Numerical simulation indicated that 99.9% of Np was separated from U and Pu applying NNDMH for a mixer-settler.     

Oxidation State of Neptunium and Plutonium and Their Leaching from Sodium–Aluminum–(Iron) Phosphate Glasses

The oxidation states of neptunium and plutonium in doped sodium–aluminum–(iron) phosphate glasses have been determined by X-ray photoelectron spectroscopy. Neptunium is present in the form of Np(IV) as Np⁴⁺ and, in smaller amounts, in the form of Np(V) as neptunyl ions. Plutonium is present mainly in the form of Pu(IV) and additionally Pu(III). The most easily water-leachable element is neptunium, which is attributed to its existence in the mobile form of neptunyl (NpO₂)⁺. The leaching rates of plutonium and impurity americium are approximately two orders of magnitude lower.     

Separation of low levels of actinides by selective oxidation/reduction and co-precipitation with neodymium fluoride

A systematic study of separating the actinides from each other in 1 M hydrochloric acid media has been carried out using selective oxidation/reduction processes followed by coprecipitation with neodymium fluoride. We have optimized two such procedures, one with bromate and another with permanganate, for the sequential separation of Am, Pu, Np, and U isotopes. The first procedure involves oxidation of Pu, Np, and U to +6 state in 1 M HCl media at 85° C with 30% NaBrO₃ and separation from trivalent Am by collecting the latter on the first NdF₃ coprecipitated source. Plutonium is then reduced and converted to +4 oxidation state with 40% NaNO₂ at 85°C, while Np and U are kept oxidized with additional bromate in 50–70°C hot solution, thus separating Pu by collection on a second NdF₃ source. At this stage, Np present in the filtrate is reduced with hydroxylamine hydrochloride and separated from U by collecting on a third source. Subsequently, U is reduced with 30% TiCl₃ and co-precipitated on a final source. The second procedure, which employs KMnO₄ in 1 M HCl media at 60–85°C for oxidizing Pu, Np, and U, and separating from Am, produced MnO₂ which is collected along with Am on the coprecipitated NdF₃. This MnO₂ is dissolved on the filter itself with 1 mL of acidified 1.5% H₂O₂ without any degradation of the -spectra. After evaporating the filtrate to destroy H₂O₂, Pu, Np, and U are separated by following steps similar to those in the bromate procedure. The recoveries of the actinides with both procedurés are >99%. The decontamination factors are between 10³ and 10⁴. The precision and accuracy of measurements, as expressed by the relative standard deviation of replicate analyses, are within 5%. Absolute detection limits for a one-day count on a 600 mm² detector at 32% counting efficiency and 450 mm² detector at 27% counting efficiency are about 2.7×10^–4 and 3.2×10^–4 Bq, respectively. These procedures have been applied to the analysis of actinides in environmental samples.     

New approaches to reprocessing of oxide nuclear fuel

Dissolution of UO₂, U₃O₈, and solid solutions of actinides in UO₂ in subacid aqueous solutions (pH 0.9–1.4) of Fe(III) nitrate was studied. Complete dissolution of the oxides is attained at a molar ratio of ferric nitrate to uranium of 1.6. During this process actinides pass into the solution in the form of U(VI), Np(V), Pu(III), and Am(III). In the solutions obtained U(VI) is stable both at room temperature and at elevated temperatures (60 °C), and at high U concentrations (up to 300 mg mL^?1). Behavior of fission products corresponding to spent nuclear fuel of a WWER-1000 reactor in the process of dissolution the simulated spent nuclear fuel in ferric nitrate solutions was studied. Cs, Sr, Ba, Y, La, and Ce together with U pass quantitatively from the fuel into the solution, whereas Mo, Tc, and Ru remain in the resulting insoluble precipitate of basic Fe salt and do not pass into the solution. Nd, Zr, and Pd pass into the solution by approximately 50 %. The recovery of U or jointly U + Pu from the dissolution solution of the oxide nuclear fuel is performed by precipitation of their peroxides, which allows efficient separation of actinides from residues of fission products and iron.