Plutonium Uranium Reduction Extraction Research Articles

Kinetic Study and Process Optimization of Plutonium Barrier Units for Enhanced Plutonium Stripping in the PUREX Process

In the PUREX (the plutonium uranium reduction extraction) process, a plutonium barrier unit (1BXX) is used to achieve deep plutonium stripping. According to the operating experience of the French reprocessing plant, after the separation of uranium and plutonium in the first cycle (1B + 1BXX), the plutonium barrier unit has excellent stripping effect, such that the removal of plutonium from uranium can already be achieved in the first cycle, and the second cycle only needs to focus on the removal of neptunium from uranium in order to obtain a qualified uranium product. In recent decades, China has also been actively conducting research on the plutonium barrier unit process to reduce the plutonium concentration in the primary uranium product in the first cycle to avoid the need to remove neptunium and plutonium at the same time in the second cycle, and to improve the efficiency and feasibility of reprocessing. Due to the lack of design basis for plutonium barriers to achieve deep plutonium stripping at present, this study conducts a basic study on the plutonium barrier unit, aiming to provide data for the optimization of plutonium barriers in the actual reprocessing process at a later date. In this work, a kinetic study on the reduction and stripping of trace plutonium from dibutyl phosphate-containing organic phases was carried out first, and the kinetic equations for the reduction and stripping of Pu(IV) by U(IV) under flow process conditions were obtained. The effects of U(IV) addition on the extraction loss of U(IV) and the concentration distribution of U(IV) at various stages were investigated by process simulation. Additionally, the oxidation of U(IV) under process conditions was investigated to clarify the process chemistry of U(IV) oxidation and to provide a reference for the oxidation consumption of U(IV). Finally, the process parameters of the plutonium barrier unit were preliminarily designed based on the above research.

Physicochemical properties of extraction solvents for the advanced recycling of spent nuclear fuel

A comprehensive understanding of solvent physicochemical properties is essential to developing advanced solvent extraction processes, such as Advanced PUREX (Plutonium Uranium Reduction Extraction) and GANEX (Group Actinide Extraction), that can safely and efficiently recycle spent nuclear fuel. The densities, viscosities and surface tensions of binary mixtures of tributyl phosphate (TBP) (0.92–3.66 M) in n-dodecane and N,N-di (2-ethyl hexyl)isobutyramide (DEHiBA) (0.69–2.78 M) in n-dodecane were measured at atmospheric pressure from 278.15 to 333.15 K. Through these measurements, empirical expressions to predict the density and viscosity at any molar composition of these solvents within the studied temperature range were constructed and shown to be in good correlation with all experimental data across the studied variables. The relationship between temperature and viscosity of pure TBP and DEHiBA can be described by the Arrhenius equation for viscosity in the temperature range studied. Surface tension measurements (air-liquid interface) were obtained for varying extractant concentrations between 278.15 and 333.15 K, which showed that DEHiBA acts as a surfactant when mixed with n-dodecane whilst TBP exhibits a higher surface tension than n-dodecane.

Comparison of Raman and mid-infrared spectroscopy for quantification of nitric acid in PUREX-relevant mixtures

During the plutonium uranium reduction extraction (PUREX) process, nitric acid facilitates the extraction of actinides from the aqueous phase into the organic phase by forming neutral, organic soluble complexes with tri-n-butyl phosphate (TBP). The concentration of nitric acid is generally measured by titration; however, titration is a time-consuming method that generates significant volumes of additional waste. Optical spectroscopic techniques can be used to perform fast, automated measurements off-line or on-line, without generating any waste. In this work, the effectiveness of Raman and mid-infrared (MIR) spectroscopy has been compared for the first time as an alternative to titration for the quantification of nitric acid in PUREX-relevant mixtures. Samples of 0–12 M nitric acid in the aqueous phase and 0–1.10 M nitric acid in the organic phase (TBP/odourless kerosene (OK)–H2O–HNO3 model system) were analysed and partial least squares (PLS) regression models were built to predict nitric acid concentration. MIR spectra required less pre-processing than Raman spectra and more accurate predictions of nitric acid concentration were obtained for MIR spectroscopy than for Raman spectroscopy, with root mean square error of prediction (RMSEP) values of 0.099 M versus 0.148 M obtained for the aqueous phase and root mean square error of cross validation (RMSECV) values of 0.006 M versus 0.013 M obtained for the organic phase. To investigate the ability to predict nitric acid concentration in the presence of uranyl nitrate, samples containing uranium (0–100 g/L) and nitric acid (0.15–0.64 M) in the organic phase (U-TBP/OK-H2O–HNO3 model system) were analysed by Raman and MIR spectroscopy. The RMSECV was 0.027 M and 0.066 M for MIR and Raman spectroscopy, respectively; these values are higher than those obtained in the absence of uranyl nitrate owing to differences in the experimental approaches employed. Therefore, the results obtained demonstrate that MIR or Raman spectroscopy could be used to measure the concentration of nitric acid in the organic and aqueous phases in the PUREX process.

Evolution of the chemical form of zirconium in aqueous solution during denitration and its influence on extraction by TRPO

Denitration by formic acid is a process that can reduce the acidity of High-Level Liquid Waste (HLLW) generated by the Plutonium Uranium Reduction EXtraction (PUREX) process, making it compatible with the feed requirements of the TRPO (trialkylphosphine oxide) process. Unfortunately, the denitration leads to the formation of interfacial crud in the TRPO extraction due to the presence of zirconium; however, the mechanism remains elusive. In this work, the evolution of the chemical form of Zr(IV) during the denitration process by formic acid was investigated. The aggregates formed in the Zr(IV) solutions after denitration were characterized by dynamic and static light scattering (DLS and SLS). Afterwards, the solutions were vacuum-dried and the solids were characterized by elemental analysis, scanning electron microscope (SEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Zr(IV) is present in solutions with different forms such as the tetramers of Zr4(OH)8(NO3)8 and Zr4(OH)8(HCOO)8 and their aggregates, depending on the [HCOOH]/[HNO3] ratio. The evolution of the chemical form of Zr(IV) can impact the extraction of Zr(IV) by TRPO and is responsible for the formation of interfacial crud.

Unveiling the Reduction Mechanism of Pu(IV) by Acetaldoxime.

The separation of plutonium (Pu) from spent nuclear fuel was achieved by effectively adjusting the oxidation state of Pu from +IV to +III in the plutonium uranium reduction extraction (PUREX) process. Acetaldoxime (CH3CHNOH) as a free salt reductant can rapidly reduce Pu(IV), but the reduction mechanism remains indistinct. Herein, we explore the reduction mechanism of two Pu(IV) ions by one CH3CHNOH molecule, where the second Pu(IV) reduction is the rate-determining step with the energy barrier of 19.24 kcal mol-1, which is in line with the experimental activation energy (20.95 ± 2.34 kcal mol-1). Additionally, the results of structure and spin density analyses demonstrate that the first and second Pu(IV) reduction is attributed to hydrogen atom transfer and hydroxyl ligand transfer, respectively. Analysis of localized molecular orbitals unveils that the reduction process is accompanied by the breaking of the Pu-OOH bond and the formation of the OOH-H and C-OOH bonds. The reaction energies confirm that the reduction of Pu(IV) by acetaldoxime is both thermodynamically and kinetically accessible. In this work, we elucidate the reduction mechanism of Pu(IV) with CH3CHNOH, which provides a theoretical understanding of the rapid reduction of Pu(IV).

Direct Dissolution of Used Nuclear Fuel in PUREX Solvent: Review and Flowsheet Development

This study explores the potential benefit to flowsheets for recovering actinides from used nuclear fuel of a process simplification in the head-end portion. The process simplification replaces acid dissolution of used nuclear fuel with dissolution in the tri-butyl phosphate solvent used in the industrially mature Plutonium Uranium Reduction Extraction flowsheet. Though characterized by considerable uncertainty, simplified flowsheets appear feasible and potentially offer significant reductions in process complexity, nitrate inventory, secondary liquid effluent generation, and plant footprint.

Electrochemistry of PUREX: R is for reduction and ion transfer

ABSTRACT The PUREX (Plutonium, Uranium, Reduction, EXtraction) process is the foundation of all industrial activities that involve the recycling of plutonium and uranium from used nuclear fuel. With over 70 years of research, engineering, and operations, it can be argued that little opportunity is left for further discovery and new fundamental understandings regarding PUREX, in general, and tri-n-butyl phosphate (TBP), in particular. Through use of an advanced electroanalytical approach, molecular-level insights regarding the back-extraction of Pu(IV) by reductive separation are reported. Studies of model PUREX systems, with lanthanide ions extracted into 20% TBP in n-dodecane, are used to demonstrate the approach. These studies reveal knowledge pertinent to mass-transfer aspects of the biphasic reductive stripping reaction central to the separation of U and Pu in PUREX. Two lanthanide ions – Ce(IV) and Yb(III) – were extracted from their concentrated solutions in aqueous 3 M HNO3 to expressly provide electrically conducting third phases to facilitate electrochemical data acquisition. To simplify the diabolically-complicated reductive separation aspects of PUREX, the controlled polarization of electrode surfaces was used to reduce Ce(IV) and Yb(III) in place of chemical reductants. Electroanalyses of the biphasic systems with Ce(IV) and Yb(III) using three-phase electrode differential pulse voltammetry demonstrate the effects of mass transfer across water–oil interfaces on the electrode potentials for the reduction to Ce(III) and Yb(II). These reductions trigger nitrate transfer out of the oil phase for Ce and proton transfer into the organic phase for Yb. This is in correlation with the transfer of the charge-neutral ion pairs Ce·3NO3 and Yb·2NO3 out of the oil phases. The differences in ion-transfer steps reflect significant variations in the equilibrium speciation of multinuclear Ce(IV) and mononuclear Yb(III) solvates of TBP in the oil phases and the loading of the organic phase.

An overview of solvent extraction processes developed in Europe for advanced nuclear fuel recycling, part 1 — heterogeneous recycling

ABSTRACT The hydrometallurgical separation concepts for the heterogeneous recycling of irradiated nuclear fuel developed in Europe are presented and discussed. Most of these concepts were developed within European collaborative projects and involve solvent extraction processes separating trivalent minor actinides (with a focus on americium) from the raffinate solution from processes such as PUREX (Plutonium Uranium Reduction Extraction) or an evolution of PUREX. Depending on the process chemistry applied, process schemes each consisting of one, two or three solvent extraction cycles are required to obtain a pure americium product. The various solvent extraction processes are briefly introduced. The most suitable choices are selected, and the process schemes are compared to one another.

Influence of Light and Temperature on the Extractability of Cerium(IV) as a Surrogate of Plutonium(IV) and its Effect on the Simulation of an Accidental Fire in the PUREX Process.

Modeling of plutonium(IV) behavior during an accidental fire in a reprocessing plant was considered using various non-radioactive metallic surrogates. Among those elements, cerium(IV) was supposed to be a suitable candidate due to possible formation of a complex with TPB, but its extractability and stability have not been studied previously under representative plutonium uranium reduction extraction (PUREX) conditions. In this work, we investigated the chemical analogy between cerium(IV) and plutonium(IV) in this extractive process and combustion thereof. Distribution ratios are reported for acidities of 1–4 mol L–1 in equal volumes of nitric acid and a 30:70 mixture of tributylphosphate and hydrogenated tetrapropylene. The influences of light, temperature, and extraction time were studied by UV–vis spectroscopy. The results showed that cerium(IV) is extracted quantitatively but is reduced over time to cerium(III) in the organic mixture. Spectrophotometric investigations of this reaction kinetics revealed an apparent rate constant k of 0.021 ± 0.002 mol0.5 L0.5 min–1 at 298 K and an apparent fractional reaction order of 0.5. The activation energy of this reduction was found to be around 82 ± 2 kJ mol–1 by the Arrhenius plot method. The combustion of mono- and biphasic solutions prepared with a cerium(IV) concentration of 10 g L–1 revealed that the extracted complexes, Ce2O·6NO3·3TBP(org) or Ce4O4·8NO3·6TBP(org), are reduced during the combustion. Compositions of the resulting ashes and soot were analyzed and highlighted the presence of pyrophosphates and polycyclic aromatic hydrocarbons, with some traces of cerium. Ce(IV) is not suitable to represent Pu(IV) from a chemical point of view in HNO3/TBP–HTP solutions.

Micro-Raman Technology to Interrogate Two-Phase Extraction on a Microfluidic Device.

Microfluidic devices provide ideal environments to study solvent extraction. When droplets form and generate plug flow down the microfluidic channel, the device acts as a microreactor in which the kinetics of chemical reactions and interfacial transfer can be examined. Here, we present a methodology that combines chemometric analysis with online micro-Raman spectroscopy to monitor biphasic extractions within a microfluidic device. Among the many benefits of microreactors is the ability to maintain small sample volumes, which is especially important when studying solvent extraction in harsh environments, such as in separations related to the nuclear fuel cycle. In solvent extraction, the efficiency of the process depends on complex formation and rates of transfer in biphasic systems. Thus, it is important to understand the kinetic parameters in an extraction system to maintain a high efficiency and effectivity of the process. This monitoring provided concentration measurements in both organic and aqueous plugs as they were pumped through the microfluidic channel. The biphasic system studied was comprised of HNO3 as the aqueous phase and 30% (v/v) tributyl phosphate in n-dodecane comprised the organic phase, which simulated the plutonium uranium reduction extraction (PUREX) process. Using pre-equilibrated solutions (post extraction), the validity of the technique and methodology is illustrated. Following this validation, solutions that were not equilibrated were examined and the kinetics of interfacial mass transfer within the biphasic system were established. Kinetic results of extraction were compared to kinetics already determined on a macro scale to prove the efficacy of the technique.

A Novel Microemulsion Phase Transition: Toward the Elucidation of Third-Phase Formation in Spent Nuclear Fuel Reprocessing.

We present evidence that the transition between organic and third phases, which can be observed in the plutonium uranium reduction extraction (PUREX) process at high metal loading, is an unusual transition between two isotropic bicontinuous microemulsion phases. As this system contains so many components, however, we have been seeking first to investigate the properties of a simpler system, namely, the related metal-free, quaternary water/n-dodecane/nitric acid/tributyl phosphate (TBP) system. This quaternary system has been shown to exhibit, under appropriate conditions, three coexisting phases: a light organic phase, an aqueous phase, and the so-called third phase. In the current work, we focused on the coexistence of the light organic phase with the third phase. Using Gibbs ensemble Monte Carlo (GEMC) simulations, we found coexistence of a phase rich in nitric acid and dilute in n-dodecane (the third phase) with a phase more dilute in nitric acid but rich in n-dodecane (the light organic phase). The compositions and densities of these two coexisting phases determined using the simulations were in good agreement with those determined experimentally. Because such systems are generally dense and the molecules involved are not simple, the particle exchange rate in their GEMC simulations can be rather low. To test whether a system having a composition between those of the observed third and organic phases is indeed unstable with respect to phase separation, we used the Bennett acceptance ratio method to calculate the Gibbs energies of the homogeneous phase and the weighted average of the two coexisting phases, where the compositions of these phases were taken both from experimental results and from the results of the GEMC simulations. Both demixed states were determined to have statistically significant lower Gibbs energies than the uniform, mixed phase, providing confirmation that the GEMC simulations correctly predicted the phase separation. Snapshots from the simulations and a cluster analysis of the organic and third phases revealed structures akin to bicontinuous microemulsion phases, with the polar species residing within a mesh and with the surface of the mesh formed by amphiphilic TBP molecules. The nonpolar n-dodecane molecules were observed in these snapshots to be outside this mesh. The only large-scale structural differences observed between the two phases were the dimensions of the mesh. Evidence for the correctness of these structures was provided by the results of small-angle X-ray scattering (SAXS) studies, where the profiles obtained for both the organic and third phases agreed well with those calculated from simulations. Finally, we looked at the microscopic structures of the two phases. In the organic phase, the basic motif was observed to be one nitric acid molecule hydrogen-bonded to a TBP molecule. In the third phase, the most common structure was that of the hydrogen-bonded TBP-HNO3-HNO3 chain. A cluster analysis provided evidence for TBP forming an extended, connected network in both phases. Studies of the effects of metal ions on these systems will be presented elsewhere.

Structural insights into the multinuclear speciation of tetravalent cerium in the tri-n-butyl phosphate-n-dodecane solvent extraction system.

X-ray and electrochemical studies of organic phases obtained by the extraction of tetravalent cerium, Ce(iv), from aqueous nitric acid (3 M) with tri-n-butyl phosphate (TBP) in n-dodecane reveal a tetranuclear Ce(iv) structural motif. This finding is consistent with the results of previous liquid-liquid extraction (LLE) studies that implicate the aggregation of (Ce-O-Ce)6+ dimers into multinuclear Ce(iv)·TBP solvates. The organic solution structures elaborated here for the Ce(iv)-HNO3-20% TBP-n-C12H26 system are correlated with multiscale phenomena-from the atomic level of the cerium coordination environment to the supramolecular scale of solute aggregates-in the organic phases, which are of relevance to the PUREX (Plutonium Uranium Reduction EXtraction) process. The combination of XANES, EXAFS, and SAXS results indicate the presence of tetranuclear cerium(iv)-oxo core structures in each of the organic phases investigated. In addition to the use of X-ray spectroscopy and scattering for direct metrical details about the organic phase solute speciation, three-phase-electrode differential pulse voltammetry (DPV) of the third phase reveals a wave attributable to Ce(iv) reduction. The electrode potential is consistent with values for the reduction of Ce(iv) in (Ce-O-Ce)6+ dimers in aqueous electrolytes. The Ce(iv) coordination chemistry of the organic solvates is independent of the bulk phenomenon of phase splitting, namely third phase formation. The local, molecular environment of Ce in the organic phase before splitting is identical to those in the two organic phases (the dense third phase and the light phase) after splitting. SAXS data are consistent with the formation of small spherical reverse micelles with core diameters (approx. 6 Å) that can accommodate a tetranuclear Ce(iv) oxo-cluster solvate of TBP. Sticky sphere modeling of the SAXS data for the organic phases with low cerium concentrations (<0.14 M) is consistent with the presence of randomly- and homogenously-dispersed micelles in combination with short-range percolated, associated micelles. At high cerium concentrations (approx. 1.5 M) in the third phase, the SAXS modeling is consistent with correlated, long-range percolated micellar aggregates. The presence of strong inter-micellar interactions (-3 to -5kBT) in all organic phases of the Ce(iv)-HNO3-TBP-n-C12H26 LLE system suggests that the phenomena of phase splitting and third phase inversion are due to liquid precipitation that is dependent solely on the concentration of the tetranuclear Ce solvate.

Elecrochemistry of Uranium and Plutonium in Ionic Liquids

Actinides can exist in many oxidation states. The redox potentials for various oxidation states of actinides strongly depend on their electronic structures, but are also affected by the medium properties. The differences between the redox potentials of actinides, most notably uranium and plutonium, have been utilized in the traditional PUREX process (Plutonium Uranium Reduction EXtraction) developed during the 1947. PUREX is based on a nitric acid dissolution of used nuclear fuel followed by liquid-liquid extraction and utilizes hazardous chemicals such as concentrated nitric acid and volatile and hazardous organics like tributyl phosphate and dodecane or kerosene. In spite of the differences in redox chemistries of the actinides, electrochemical methods have never been applied for large scale used nuclear fuel processing. Molten salts (in particular halides) have been demonstrated to be better suited for actinide electrochemistry; however, the practical application of pyroprocessing is limited by the high temperatures and strongly corroding properties of molten salts. Ionic Liquids (ILs) are relatively new media that offer the benefits of molten salts, e.g., high ionic conductivity required for practical operations and wide electrochemical windows, but are also free of the molten salt shortcomings such as the high temperature and corroding properties. Their high ionic conductivity and low reactivity make them especially well-suited for electrochemical applications. While the high conductivity enables for electrochemical reactions at the high current densities required for practical operations, the low reactivity typically translates into a wide potential window, which allows access to electrochemical processes that cannot be accomplished in conventional media. Moreover, ILs offer a wide range of tunable physical properties, which could potentially be utilized not only in the electrochemical but also in numerous other separation techniques relevant to the nuclear industry, including solvent extraction, ion exchange, dissolution, crystallization, etc. The development of separation technologies for spent nuclear fuel in ILs could complement or replace current PUREX and/or high-temperature pyrochemical processing technologies. The current state of understanding of the electrochemical behavior of actinides in ILs is far from satisfactory. As part of our multi-faceted approach to study the structural, thermodynamic, and kinetic behavior of actinides, we undertook electrochemical studies of uranium and plutonium in selected chloride and bistrifluorosulfonamide based-ILs in order to demonstrate the feasibility of using ILs for electrochemical actinide separations. In this presentation we will summarize our most recent results of those studies.

Initiating Events Study of the First Extraction Cycle Process in a Model Reprocessing Plant

Background: Definition and grouping of initiating events (IEs) are important basics for probabilistic safety assessment (PSA). An IE in a spent fuel reprocessing plant (SFRP) is an event that probably leads to the release of dangerous material to jeopardize workers, public and environment. The main difference between SFRPs and nuclear power plants (NPPs) is that hazard materials spread diffusely in a SFRP and radioactive material is just one kind of hazard material. Materials and Methods: Since the research on IEs for NPPs is in-depth around the world, there are several general methods to identify IEs: reference of lists in existence, review of experience feedback, qualitative analysis method, and deductive analysis method. While failure mode and effect analysis (FMEA) is an important qualitative analysis method, master logic diagram (MLD) method is the deductive analysis method. IE identification in SFRPs should be consulted with the experience of NPPs, however the differences between SFRPs and NPPs should be considered seriously. Results and Discussion: The plutonium uranium reduction extraction (Purex) process is adopted by the technics in a model reprocessing plant. The first extraction cycle (FEC) is the pivotal process in the Purex process. Whether the FEC can function safely and steadily would directly influence the production process of the whole plant-production quality. Important facilities of the FEC are installed in the equipment cells (ECs). In this work, IEs in the FEC process were identified and categorized by FMEA and MLD two methods, based on the fact that ECs are containments in the plant. Conclusion: The results show that only two ECs in the FEC do not need to be concerned particularly with safety problems, and criticality, fire and red oil explosion are IEs which should be emphatically analyzed. The results are accordant with the references.

Reduction of uranium (VI to IV) by hydrogenation using Adams’ catalyst

Plutonium uranium reduction extraction using U(IV) as universal reductant for Pu partitioning is the only technology practiced internationally to recover U and Pu from spent nuclear fuels. Uranous requirement of Indian reprocessing plants is met by the electrolytic reduction of uranyl nitrate with 50–60% conversion. Though the current requirement can be met with this method, it increases the load on uranium purification cycle. In addition, it is a batch process with slow kinetics. In order to achieve higher conversion of uranyl nitrate to uranous nitrate, catalytic reduction method using hydrogen in presence of Adams’ catalyst (PtO2) was tried. Parametric studies have been performed in an autoclave to evaluate the effect of U(VI) concentration, the role of hydrazine nitrate and pressure. It is observed that kinetics is improved at higher pressures. The studies revealed that near total conversion of uranium from (VI to IV) can be achieved by the catalytic reduction route.

The Attractiveness of Materials in Advanced Nuclear Fuel Cycles for Various Proliferation and Theft Scenarios

We must anticipate that the day is approaching when details of nuclear weapons design and fabrication will become common knowledge. On that day we must be particularly certain that all special nuclear materials (SNM) are adequately accounted for and protected and that we have a clear understanding of the utility of nuclear materials to potential adversaries. To this end, this paper examines the attractiveness of materials mixtures containing SNM and alternate nuclear materials associated with the plutonium-uranium reduction extraction (Purex), uranium extraction (UREX), coextraction (COEX), thorium extraction (THOREX), and PYROX (an electrochemical refining method) reprocessing schemes. This paper provides a set of figures of merit for evaluating material attractiveness that covers a broad range of proliferant state and subnational group capabilities. The primary conclusion of this paper is that all fissile material must be rigorously safeguarded to detect diversion by a state and must be provided the highest levels of physical protection to prevent theft by subnational groups; no “silver bullet” fuel cycle has been found that will permit the relaxation of current international safeguards or national physical security protection levels. The work reported herein has been performed at the request of the U.S. Department of Energy (DOE) and is based on the calculation of “attractiveness levels” that are expressed in terms consistent with, but normally reserved for, the nuclear materials in DOE nuclear facilities. The methodology and findings are presented. Additionally, how these attractiveness levels relate to proliferation resistance and physical security is discussed.

Purification and recovery of plutonium from uranium oxide obtained in the fast reactor fuel reprocessing plant using hydroxyurea as reductant

Spent fuel discharged from Fast Breeder Test Reactor (FBTR) in Kalpakkam is being reprocessed by modified plutonium uranium reduction extraction (PUREX) process using 30% TBP (tributylphosphate) as extractant in the presence of heavy normal paraffin (HNP) as diluent. Partitioning of uranium (U) and plutonium (Pu) is carried out using oxalate precipitation method. Uranium oxide product obtained by this method contains appreciable amount of plutonium which has to be recovered. Recovery of plutonium from this uranium oxide product is carried out by reducing Pu to inextractable Pu(III) using hydroxyurea (HU) and then uranium is extracted into 30% TBP. A small amount of Pu which is extracted in the organic phase is stripped back to aqueous phase by scrubbing with scrubbing agent containing 0.1 M HU in 4 M nitric acid. Similarly U and Pu are co-extracted into 30% TBP and then Pu is removed by scrubbing with 0.1 M HU in 4 M nitric acid. Further decontamination from Pu is obtained in the stripping stages. By this method Pu contamination in the uranium oxide is brought from 7300 ppm to 0.4–3 ppm (wt/wt). This uranium product obtained can be handled on table top.

Oxidation of Hydroxylamine by Nitrous and Nitric Acids. Model Development from First Principle SCRF Calculations

Ab initio molecular orbital calculations have been performed to develop an elementary reaction mechanism for the autocatalytic and scavenging reactions of hydroxylamine in an aqueous nitric acid medium. An improved understanding of the titled reactions is needed to determine the "stability boundary of hydroxylamine" for safe operations of the plutonium-uranium reduction extraction (PUREX) process. Under the operating conditions of the PUREX process, namely, 6 M nitric acid, the reactive forms of hydroxylamine are NH2OH, NH3OH+, and the complex NH3OH.NO3, and those of nitrous acid are NO+, H2ONO+, N2O4, N2O3, NO2, and NO. High-level CBSQB3/IEFPCM and CBSQB3/COSMO calculations were performed using GAUSSIAN03 to investigate the energy landscape and to explore a large number of possible ion-ion, ion-radical, ion-molecule, radical-radical, radical-molecule, and molecule-molecule pathways available to the reactive forms of the reactants in solution. It was found that in solution the autocatalytic generation of nitrous acid proceeds through free radical pathways at low-hydroxylamine concentrations from unprotonated NH2OH via hydrogen abstraction. At high [NH3OH+], we suggest a possible involvement of the NH3ONO+ intermediate via the reaction NH2ONO + NO2 --> HNO + HONO + NO. The NH3ONO+ intermediate, in turn, is formed favorably via the ion-ion reactions of NH3OH+ with NO+ and/or the reaction between NO+ and hydroxylammonium nitrate (HAN). The intermediates involved in the scavenging reaction of nitrous acid by hydroxylamine are NH3ONO+, NH2ONO, NH2(NO)O, NH(NO)OH, and HONNOH and the rate-determining step is the 1,2-NO migration in NH2ONO leading to NH2(NO)O. Reactions NH2ONO --> NH2(NO)O and NH2(NO)O --> NH(NO)OH were studied with two explicit water molecules and the results are discussed in the context of the importance of the explicit treatment of solvent in the determination of the energetics and mechanism of these processes. The rate constants for the reactions were estimated using transition-state theory and other traditional techniques. The kinetic parameters obtained at the B3LYP/CBSB7/IEFPCM level are in reasonable agreement with the limited experimental value. IEFPCM results on free energy of undelocalized polar ions such as NO3-, NO2-, and NH3OH+ are not very accurate and have difficulties in predicting the right direction of acid dissociation equilibrium of HONO2, HONO, and NH3OH+. Explicit incorporation of a solvation shell to these ions improves the theoretical descriptions of acid ionization equilibria as it captures some of the nonlocal effects of these ions. Additional work is needed to correctly describe the solvation shell and to introduce consistency in the theoretical treatment involving explicit solvent molecules. Nevertheless, this systematic exploration of reactions in solution and mechanism development for a solution phase process based on self-consistent reaction field (SCRF) results is likely to be one of the first of its kind.