Plutonium Oxalate Research Articles

Ab initio modeling and thermodynamics of hydrated plutonium oxalates

An ab initio study on the plutonium oxalate hydrates: Pu2(C2O4)3⋅10H2O and Pu(C2O4)2⋅6H2O, using PBE exchange-correlation with D3 dispersion correction and Hubbard correction for the plutonium atoms was performed and compared to experimental vibrational spectral and thermodynamic property values. We demonstrated that this technique can accurately predict the experimental infrared spectra Pu(III) oxalate hydrate, as well as the Raman peak of PuO2 (used to calculate the thermodynamic properties of the oxalates). For Pu(IV) oxalate hydrate, we found that our predicted structure agreed qualitatively with PXRD measurements, the only available experimental determination of the structure. Using this method at standard temperature and pressure, we predicted standard enthalpies of formation of -6,755 kJ mol−1 and -3,923 kJ mol−1 and standard Gibbs free energy of formation of -5,899 kJ mol−1 and -3,386 kJ mol−1 for Pu2(C2O4)3⋅10H2O and Pu(C2O4)2⋅6H2O, respectively.

Raman and infrared spectra of plutonium (IV) oxalate and its thermal degradation products

For over 80 years, plutonium dioxide has been routinely produced via thermal decomposition of hydrated plutonium(IV) oxalate. Despite the longstanding utility of this process, the chemical structures of starting materials and intermediates produced during this thermal conversion remain ill-defined. To help resolve this uncertainty, we measured high-resolution Raman and infrared spectra of Pu(C2O4)2·6H2O that was heated to 25, 100, 220, 250, 350, and 450 °C in air. Our measurements show that Pu(C2O4)2·6H2O has a rich vibrational spectrum with at least 15 Raman bands between 180 cm−1 and 1900 cm−1 and 9 infrared bands between 800 cm−1 and 4000 cm−1. As Pu(C2O4)2·6H2O is heated, water is liberated, and the oxalate ligand decomposes to produce plutonium oxycarbide species. When heated to 350 °C or higher, vibrational spectra are consistent with PuO2 with some residual carbon-containing species. Full vibrational spectra, powder X-ray diffraction, and scanning electron microscopy measurements of Pu(C2O4)2·6H2O and its thermal degradation products are presented herein along with approximate assignments for observed spectral bands. These data can be used to validate and potentially improve existing computational models that describe the chemical structure of compounds produced during thermal degradation of plutonium (IV) oxalate. Given the utility of plutonium (IV) oxalate in synthesizing plutonium dioxide, these results are expected to provide value in the fields of nuclear fuel processing, nuclear nonproliferation, and nuclear forensics.

Studies on nucleation and crystal growth kinetics of plutonium(IV) oxalatex

Abstract A new method is developed to calculate the dilution ratio N of the two reactant solutions during nucleation rate determination. When the initial apparent supersaturation ratio S N = f(N) in the dilution tank is controlled between 1.66 and 1.67, the counted nuclei is the most, both nuclei dissolving and secondary nucleation avoided satisfactorily. Based on this methoed, Plutonium(IV) oxalate is precipitated by mixing equal volumes of tetravalent plutonium nitrate and oxalic acid solutions. Experiments are carried out by varying the supersaturation ratio from 8.37 to 22.47 and temperature from 25 to 50 °C. The experimental results show that the nucleation rate of plutonium(IV) oxalate in the supersaturation range cited above can be expressed by the equation R N = A N exp(−E a /RT)exp[−B/(ln S)2], where A N = 4.8 × 1023 m−3 s−1 , and E a = 36.2 kJ mol−1, and B = 20.2. The crystal growth rate of plutonium(IV) oxalate is determined by adding seed crystals into a batch crystallizer. The crystal growth rate can be expressed by equation G(t) = k g exp(−E’ a /RT) (c − c eq) g , where k g = 7.3 × 10−7 (mol/L)−1.1(m/s), E’ a = 25.7 kJ mol−1, and g = 1.1.

Electrolytic and ozone aided destruction of oxalate ions in plutonium oxalate supernatant of the PUREX process: A comparative study

The article describes an efficient and straight forward method for the destruction of oxalate ions in the plutonium oxalate supernatant stream generated during the reprocessing of FBTR spent fuel, using ozone and electrolysis without any redox intermediates/catalyst at room temperature. The influencing parameters such as concentration of nitric acid, current density and temperature were studied. The destruction was achieved in 4 and 4.5 h, respectively using ozone and electrolysis. These methods have significant advantages due to the chemically benign nature of the oxalate destructed stream for all the downstream fuel cycle processes and the feasibility to automate these process steps in a radiochemical plant.

Insights into the thermal decomposition of plutonium(IV) oxalate – a DFT study of the intermediate structures

The thermal decomposition of plutonium oxalate to oxide is one of the most studied reactions in actinide chemistry but the intermediates have been the subject of debate for decades. Recent experimental data suggest that the decomposition of Pu(IV) oxalate in air undergoes dehydration first, then reduction to Pu(III) oxalate. The precise structural modifications that take place are unknown as experiments have not been able to fully characterize the intermediates at the microscopic level. To rectify this, we employed solid state density functional theory calculations at the PBE-D3 level with a Hubbard U correction to model the structures and energetics of potential dehydrated Pu(IV) and Pu(III) oxalate intermediate compounds. Based on the theoretical study presented here, the anhydrous analogues of the known hydrated Pu(IV) and Pu(III) oxalates are the preferred crystal structures formed through an overall exothermic reaction process. However, decomposition could proceed through the formation of a higher energy, more complicated 3D lattice structure with frustrated oxalate binding. It is expected that the intermediates presented here could be identified using spectroscopic techniques to enable further insight into the reaction mechanism.

Thermal Processing of Chloride-Contaminated Plutonium Dioxide.

Over 80 heat treatment experiments have been made on samples of chloride-contaminated plutonium dioxide retrieved from two packages in storage at Sellafield. These packages dated from 1974 and 1980 and were produced in a batch process by conversion of plutonium oxalate in a furnace at around 550 °C. The storage package contained a poly(vinyl chloride) (PVC) bag between the screw top inner and outer metal cans. Degradation of the PVC has led to adsorption of hydrogen chloride together with other atmospheric gases onto the PuO2 surface. Analysis by caustic leaching and ion chromatography gave chloride contents of ∼2000 to >5000 ppm Cl (i.e., μgCl g–1 of the original sample). Although there are some subtle differences, in general, there is surprisingly good agreement in results from heat treatment experiments for all the samples from both cans. Mass loss on heating (LOH) plateaus at nearly 3 wt % above 700 °C, although samples that were long stored under an air atmosphere or preexposed to 95% relative humidity atmospheres, gave higher LOH up to ∼4 wt %. The majority of the mass loss is due to adsorbed water and other atmospheric gases rather than chloride. Heating volatilizes chloride only above ∼400 °C implying that simple physisorption of HCl is not the main cause of contamination. Interestingly, above 700 °C, >100% of the initial leachable chloride can be volatilized. Surface (leachable) chloride decreases quickly with heat treatment temperatures up to ∼600 °C but only slowly above this temperature. Storage in air atmosphere post-heat treatment apparently leads to a reequilibration as leachable chloride increases. The presence of a “nonleachable” form of chloride was thus inferred and subsequently confirmed in PuO2 samples (pre- and post-heat treatment) that were fully dissolved and analyzed for the total chloride inventory. Reheating samples in either air or argon at temperatures up to the first heat treatment temperature did not volatilize significant amounts of additional chloride. With regard to a thermal stabilization process, heat treatment in flowing air at 800 °C with cooling and packaging under dry argon appears optimal, particularly, if thinner powder beds can be maintained. From electron microscopy, heat treatment appeared to have the most effect on degrading the square platelet particles compared to those with the trapezoidal morphology.

Microstructural characterization of plutonium oxalate and oxide particles by three-dimensional focused ion beam tomography

Focused ion beam (FIB) tomography was used to characterize three-dimensional (3D) internal microstructures of plutonium oxalate and oxide particles. Sequential two-dimensional (2D) sectioning and imaging using FIB and 3D image reconstruction were used to elucidate and distinguish internal pore microstructure and distribution in plutonium oxide particles calcined from plutonium oxalate precipitates. The results show that the plutonium oxide particles have a significant increase in pore volume fraction while retaining the exterior morphological shape of the corresponding oxalate precipitates. This technique enables the examination of the internal structure of radioactive microparticles for nuclear forensics that is not available with conventional microscopy.

Demonstration of Hollow Fiber Membrane Technique for the Recovery of Plutonium from Analytical Laboratory Waste

Laboratory-scale studies were carried out to develop an analytical methodology for the processing of plutonium-bearing analytical laboratory waste at liter scale using hollow fiber–supported liquid membrane (HFSLM) technique by selective recovery of plutonium from uranium, americium, and other laboratory chemicals. In the first stage, uranium and plutonium were selectively transported from the feed to the receiver phase using 30% tri-n-butyl phosphate/n-dodecane which was used as the carrier in HFSLM. From the thus separated uranium and plutonium mixture, Pu(III) was selectively precipitated as ammonium plutonium(III)-oxalate [NH4Pu(C2O4)2 · 3H2O], leaving most of the uranium in the supernatant solution. A combination of HFSLM method followed by ammonium plutonium–oxalate precipitation is faster, gives lower radiation exposure to working personnel, and generates lesser volume of secondary waste as compared to traditional precipitation/ion-exchange technique. Furthermore, the present methodology signifies its importance in providing a very good yield of Pu recovery (>99%) from waste solution.

ChemInform Abstract: Crystal Growth and First Crystallographic Characterization of Mixed Uranium(IV)—Plutonium(III) Oxalates.

AbstractThe mixed‐actinide UIV‐PuIII oxalates (NH4)0.5 [Pu0.5U0.5 (C2O4)2·H2O] ·nH2O (I) and (NH4)2.7Pu0.7U1.3 (C2O4)5·nH2O (II) are prepared by diffusion of Pu3+ and U4+ through glass fiber membranes separating three compartments of a glass cell containing 1.

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.

Insights into the Thermal Decomposition of Lanthanide(III) and Actinide(III) Oxalates – from Neodymium and Cerium to Plutonium

AbstractLanthanides are often used as surrogates to study the properties of actinide compounds. Their behaviour is considered to be quite similar as they both possess f valence electrons and are close in size and chemical properties. This study examines the potential of using two lanthanides (neodymium and cerium) as surrogates for plutonium during the thermal decomposition of isomorphic oxalate compounds, in the trivalent oxidation state, into oxides. Thus, the thermal decomposition of neodymium, cerium and plutonium(III) oxalates are investigated by several coupled thermal analyses (TG–DTA/DSC–MS/FTIR) and complementary characterisation techniques [XRD, UV/Vis and FTIR spectroscopy, scanning electron microscopy (SEM), carbon analysis] under both oxidizing and inert atmospheres. The thermal decomposition mechanisms determined in this study confirmed some previously reported results, among diverging propositions, and also elucidated some original mechanisms not previously considered. The calculated thermodynamic and kinetic parameters for the studied systems under both atmospheres are reported and compared with available literature data. Similarities and differences between the thermal behaviour of plutonium(III) and lanthanide(III) oxalates are outlined.

Preparation, characterization and application of thorium oxalate for sorption of plutonium and americium from aqueous solution

Synthetic inorganic exchangers exhibit good thermal and radiation stability. Thorium oxalate precipitate shows potential for co-precipitation of plutonium and americium from oxalate supernatant generated during plutonium oxalate precipitation. In the present study, efforts were made to prepare thorium oxalate precipitate to be used for column operation. Distribution ratios were determined to optimize conditions for sorption of plutonium and americium on thorium oxalate from nitric acid + oxalic acid solutions with composition similar to that of oxalate supernatant. Column experiments were also performed to evaluate the sorption capacity of thorium oxalate for plutonium and americium from the same medium. The result showed that, thorium oxalate prepared in 1.75M HNO3 at 70 °C is suitable for column operations. These studies showed that plutonium and americium could be simultaneously removed from aqueous solutions with composition similar to plutonium oxalate waste using glass column packed with thorium oxalate and these nuclides could be recovered by eluting with 3M HNO3.

Separation of Am(III) and Pu(IV) at Their Joint Oxalate Precipitation

A procedure was developed for purification of plutonium to remove americium by oxalate precipitation using diethyl oxalate as a precipitant. The developed procedure was tested under laboratory conditions and on an enlarged installation. It was shown that the decontamination factor of plutonium from americium can exceed 2 ×103. The plutonium oxalate precipitate can be dissolved by heating with HNO3 in the presence of V(V) catalyst and also by electrochemical dissolution at alternating current.

Synthesis and investigation of plutonium oxide cluster ions: Pu xO y+ ( x≤18)

The plutonium–oxygen system is fundamentally and technologically important. The primary goal of the present study was to prepare and carry out initial chemical studies on gas-phase plutonium oxide cluster ions, which would contribute to the general understanding of PuO x chemistry. In this effort, several plutonium oxide, oxide-hydroxide and hydroxide cluster ions, Pu x O y (OH) z +, were synthesized by pulsed laser ablation of hydrated plutonium oxalate in vacuum. Cerium was studied for comparison, and a chemically less diversified oxide cluster system was observed. The large variety of Pu x O y (OH) z + compositions found is attributed to the accessibility of multiple oxidation states for Pu, and the results demonstrate the feasibility of preparing clusters with diverse stoichiometries and variable plutonium oxidation states. These clusters may be studied by various gas-phase techniques to illuminate the behavior of plutonium as a function of key variables, such as oxidation state, structure/coordination and bonding. In addition, some gas-phase reaction studies were carried out and at least one significant cluster reaction product with dimethylether was identified. This result demonstrates the desirability of studying plutonium cluster chemistry in ion traps where longer residence times, and more ion–molecule collisions will result in greater reaction efficiencies. The largest primary oxide cluster identified here comprised six Pu atoms, but two distinctive ‘magic number’ clusters were also produced, which incorporated 16 and 18 Pu atoms, respectively. These ‘magic number’ clusters suggest that special stabilities obtain for these species and that it may be possible to prepare plutonium oxide ‘nanocrystals’ in the gas phase for examination by techniques complementary to those applicable to bulk materials. The plutonium cluster compositions and abundances observed in this work are discussed in the context of the oxidation states, structures, and bonding exhibited by plutonium.

Separation and Recovery of Uranium and Plutonium from Oxalate Supernatant

The separation of uranium and plutonium from oxalate supernatant, obtained after precipitating plutonium oxalate, containing ~10 g/l uranium and 30–100 mg/l plutonium in 3M HNO3 and 0.10–0.18M oxalic acid solution has been carried out. In one extraction step with 30% TBP in dodecane: ~92% of uranium and ~7% of Pu is extracted. The raffinate containing the remaining U and Pu is extracted with 0.2M CMPO+1.2 M TBP in dodecane and near complete extraction of both the metal ions is achieved. The metal ions are back extracted from organic phases using suitable stripping agents. The recovery of both the metal ions separately is >99%. The uranium species extracted into the TBP phase from the HNO3+oxalic acid medium was identified as UO2(NO3)2·2TBP.

Removal of plutonium and americium from oxalate supernatants by co-precipitation with thorium oxalate

As a part of treatment of low level active waste, co-precipitation of plutonium (Pu) and americium (Am), with thorium oxalate, from oxalate supernatants generated during plutonium oxalate precipitation has been investigated. A simple method for the simultaneous removal of both Pu and Am from oxalate supernatants could be developed. This simple process achieves incorporation of these alpha active nuclides into small volumes of solid matrix from large volumes of aqueous waste.

Studies on the recovery of Pu(IV) from hydrochloric acid-oxalic acid solutions using an anion exchange method

Sorption of Pu(IV) from hydrochloric acid-oxalic acid solutions has been investigated using different anion exchangers, viz., Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) for the recovery of plutonium from plutonium oxalate solutions. Distribution ratios of Pu(IV) for its sorption on these anion exchangers have been determined. The sorption of Pu(IV) from hydrochloric acid solutions decreases drastically in the presence of oxalic acid. However, addition of aluminium chloride enhances the sorption of plutonium in the presence of oxalic acid, indicating the feasibility of recovery of plutonium. Pu(IV) breakthrough capacities have been determined with a 10 ml resin bed of each of these anion exchangers at a flow rate of 60 ml per hour using a solution of Pu(IV) with the composition: 6M HCl+0.05M HNO3+0.1M H2C2O4+0.5M AlCl3+∼100 mg.l−1 Pu(IV). The 10% Pu(IV) breakthrough capacities for Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) are 15.0, 8.9 and 6.2 g of Pu(IV) l−1 of resin respectively.

Studies on plutonium(IV) hydroxosuccinate

The precipitation behaviour of basic plutonium(IV) compounds from dilute nitric acid medium with aliphatic dicarboxylic acids has been investigated. Unlike plutonium(IV) oxalate, which is precipitated in acid medium (1–4M), the higher dicarboxylates precipitate in 0.9–2.5 pH range and their elemental analysis indicate Pu/dicarboxylate ratio of 1∶1. The mode of formation, composition, solubility and thermal degradation behaviour for plutonium(IV) hydroxosuccinate has been studied. The hydroxy group determination by kinetic titration using fluoride complexing revealed that the compound has a Pu:OH ratio of 1∶1 and determination of bridging oxygen group gave a Pu:O ratio of 1∶0.5 suggest its formation as hydroxysuccinate having an oxobridged formula.

Dynamic Process Model of a Plutonium Oxalate Precipitator

A dynamic model of a plutonium oxalate precipitator has been developed to provide a means of predicting plutonium inventory on a continuous basis. The model is based on state-of-the-art crystallization equations, which describe nucleation and growth phenomena. The model parameters have been obtained through the use of batch experimental data. For any time-dependent input concentrations and flow rates, the model permits one to calculate the output flow rate of the precipitate phase, the output concentration and flow rate of the filtrate phase, the degree of conversion, and the average particle size. The model has been used to study the approach to steady state, to investigate the response to input transients, and to simulate the control of the precipitation process.

An investigation of plutonium-halogen chemistry in ethyl acetate solution

Plutonium may be dissolved in chlorine-saturated ethyl acetate by a mechanism involving chlorination of the solvent to produce chloroethyl acetate and HCl as well as direct oxidation by chlorine. The resulting solution contains plutonium primarily as the PuCl 6 2− anion, with charge balance being provided by protons. Bromine-ethyl acetate also dissolves plutonium, but in this case the mechanism apparently involves only direct oxidation by bromine rather than bromination of the solvent, and the resulting solution contains the trivalent species. Addition of a solution of oxalic acid in ethyl acetate precipitates plutonium oxalate quantitatively from the chlorine solutions and less completely from the bromine solutions. The resulting oxalate may be calcined, fluorinated and the reduced to metal in high yield.