A Nuclear Magnetic Resonance Study on Ligand Exchange Reactions in La(III), Nd(III), and U(VI) Nitrato Complexes with n-Octyl(pheny1)-N,N-Diisobutylcarbamoylmethylphosphine Oxide in Non-aqueous Solvents

The exchange reactions of n-octyl(pheny1)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in La(III), Nd(III), and U(VI) nitrate complexes with CMPO (La(III)-, Nd(III)-, and U(VI)-CMPO complexes) have been studied in CD₃COCD₃ by means of ³¹P NMR method. The number of CMPO coordinated to the first coordination sphere of La(III) ion was directly determined to be 3 by the area integrations of ³¹P NMR signals of free and coordinated CMPO molecules. The same coordination number of 3 was also obtained for the U(VI)-CMPO complex. The coordination number was not determined for the Nd(III)-CMPO complex, because of its paramagnetic behavior. The exchange rate constants of CMPO in La(III)- and U(VI)- CMPO complexes were obtained by the two-site exchange model. Paramagnetic line broadening was observed in the Nd(III)-CMPO complex and the rate constant for the exchange of CMPO was determined by the line-broadening method. The exchange rates of CMPO in La(III)- and Nd(III)-CMPO complexes depend on the free CMPO concentration ([CMPO]), while that in U(VI)-CMPO complex is independent of [CMPO]. The dissociative (D) and dissociative interchange (I_d ) mechanisms were proposed for the exchange reactions in the La(III)- and Nd(III)-CMPO complexes, and dissociative (D) or I_d mechanism was proposed for the U(VI)-CMPO complex. The dissociative rate constants (s^?1) at 25°C and activation parameters ΔH^# (kJ·mol^?1) and ΔS^# (J·K^?1·mol^?1) are 4.76x10³, 28.7±0.1, ?78.4±0.2 for La(III)-CMPO complex, 4.72x10³, 42.6±0.4, ?31.7±1.3 for Nd(III)-CMPO complex, and 3.20x10³, 46.9±0.6, ?20.5±2.2 for U(VI)-CMPO complex, respectively.     

EPR Study of Uranium (V) Species in Photo-and Electrolytic Reduction Processes of UO2 NO3)2-2TBP

Abstract

Electron Paramagnetic Resonance (EPR) and optical spectra of uranium(V) species were observed in both processes of photo- and electrolytic reduction of UO₂(NO₃)₂-2tributylphosphate (TBP) in 80%TBP-n-dodecane solution. The formation of U(V) species was detected by an optical spectrum (λ_max: 770, 970 and 1,420 nm). EPR signal with the value of ff-factor –2.3 and a linewidth of approximately 1,100 Gs was observed during the electrolytic reduction. On the other hand, during the photoreduction the signal with the value of fil-factor –1.94 was observed and there was found a superhyperfine structure with the intensity ratio of 1:2:1, that is caused by the superhyperfine coupling with nuclear spin, I=1/2, of the strongly coordinated ³¹P to the central uranium through oxygen atom. The superhyperfine coupling constant was estimated to be 27 Gs. Moreover, the signal with the value of g-factor –2.00 due to an organic radical was observed. The residue after the thermo-gravimetric analysis of UO₂(NO₃)₂-2TBP was identified as α-UP₂O₇ by the powder X-ray diffraction analysis, indicating the strong coordination of TBP to the central uranium atom.     

Ab initio Quantum Chemical Study on Charge Distribution and Molecular Structure of Uranyl (VI) Species with Raman Frequency

Ab initio molecular orbital calculation was performed for uranyl (VI) monomer and dimer complexes with some water molecules and/or hydroxide ions. The Raman active frequencies were calculated for each complex after structural optimization in vacuum state, and investigated the molecular structure and the charge distribution. For uranyl monomer, the calculated Raman frequencies for uranyl with 5 or 6 water molecules show good agreement with experimental Raman frequencies for uranyl hydrates. On the contrary, the calculation underestimates the Raman frequency in case of hydroxide ions in uranyl complex. The calculation models for uranyl dimer were made from [UO₂(H₂O)₅]²⁺, then the hydroxide ions bridging model, [(UO₂)₂(OH)₂(H₂O)₆]²⁺, is more stable than water molecules bridging, [(UO₂)₂(H₂O)₈]⁴⁺, and the theoretical Raman frequency and uranyl bond lengths have the good coincidence with those of experiments. The calculated uranyl bond length of dimer is slightly longer than that of monomer. Also, the charge of oxygen atom in uranyl shows larger change than that of uranium atom between dimer and monomer. And this charge distribution is mostly influenced by the charge donation of ligands. If only same ligands are surrounding, the number of ligands influenced this charge distribution.     

Solubility of Uranyl Nitrate Precipitates with N-Alkyl-2-pyrrolidone Derivatives (Alkyl = n-Propyl,n-Butyl,iso-Butyl,and Cyclohexyl)

The solubility of UO₂(NO₃)₂(NRP)₂ (NRP = N-alkyl-2-pyrrolidone) in aqueous solutions with HNO₃ (0–5.0 M) and the corresponding NRP (0–0.50M) has been studied. As a result, the solubility of each speciesof UO₂(NO₃)₂(NRP)₂ generally decreases with increasing concentrations of HNO₃ and the corresponding NRP (C _HNO3 and C _NRP, respectively) in the supernatant. The solubility of UO₂(NO₃)₂(NRP)₂ also depends on the type of NRP; a higher hydrophobicity of NRP generally leads to a lower solubility of UO₂(NO₃)₂(NRP)₂. The logarithms of effective solubility products (K _eff) of UO₂(NO₃)₂(NProP)₂, UO₂(NO₃)₂(NBP)₂, UO₂(NO₃)₂(NiBP)₂, and UO₂(NO₃)₂(NCP)₂ at different C_HNO3 values and 293K were evaluated. For instance, at C_HNO3 = 3:0 M, logK ^NProP _eff = ?1:07 ± 0:03, log K ^NBP _eff = ?2:23 ± 0:02, log K ^NiBP _eff = ?2:59 ± 0:03, and log K ^NCP _eff = ?3:80 ± 0:05. The solubility of UO₂(NO₃)₂(NRP)₂ is determined by the balance among the common-ligand effect, ionic strength, and variation of log K _eff with C _HNO3.     

Photochemical Reaction of Bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato) (tetrahydrofuran)dioxouranium(VI) in Supercritical Carbon Dioxide

The emission lifetime of bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)(teterahydrofuran)dioxouranium(VI) complex, UO₂(hfac)₂thf, has been measured in sub and supercritical carbon dioxide (sc-CO₂) under the conditions of 10 to 60 MPa and 293.2 to 340.2 K. It was found that the decay rate constants of excited UO₂(hfac)₂thf species (*UO₂(hfac)₂thf) decrease with increasing the pressure of sc-CO₂. The rate constants for quenching reactions of *UO₂(hfac)₂thf with alkenes in sc-CO₂ were determined as 4.32×10⁹(for cyclohexene), 3.04×10⁹(for cycloheptene), 2.13×10⁹(for 1-octene), and 3.801×10⁸mol^-1·dms^-1·s^-(for 1-hexene). The activation volumes for the quenching reactions of *UO₂(hfac)₂thf with cyclohexene, cycloheptene, 1-octene, and 1-hexene at 308.2 K were determined to be 16.7, 28.7, 16.7, and 24.7cm³.mol^-1, respectively.     

Electrodeposition of metallic uranium at near ambient conditions from room temperature ionic liquid

The electrochemical behavior of U(IV) in the room temperature ionic liquid (RTIL), N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPiNTf₂), was investigated to evaluate the feasibility of using the RTIL for non-aqueous reprocessing application. In this context, the rate of dissolution of uranium oxide (UO₂) in HNTf₂ was studied at 353 K. The dissolution of UO₂ in HNTf₂ was rapid; nearly 50% of UO₂ dissolved within 3 h and more than 95% dissolved in 25 h. The resultant solution was dried, diluted with MPPiNTf₂ and the electrochemical behavior of U(IV) in MPPiNTf₂ was studied at 373 K at platinum, glassy carbon and stainless steel electrodes. The cyclic voltammograms of U(IV) in MPPiNTf₂ at platinum and glassy carbon electrodes consisted of four cathodic waves occurring at a peak potentials of −0.7 V (Fc/Fc⁺), −1.4 V, −2.2 V and −2.7 V. Controlled potential electrolysis of a solution of U(IV) in MPPiNTf₂ at −2.8 V (Fc/Fc⁺) resulted in the deposition of metallic uranium, which was confirmed by X-ray diffraction and scanning electron microscopy.     

Electrochemical Properties of Uranyl Ion in Ionic Liquids as Media for Pyrochemical Reprocessing

We have proposed a new reprocessing process by using ionic liquids (ILs) instead of molten salts of alkali chlorides in pyrochemical process. In the proposed process, spent nuclear fuels are dissolved in ILs by using Cl₂ as an oxidant, and UO₂ ²⁺ and PuO₂ ²⁺ ions in ILs are recovered as UO₂ and PuO₂ by electrochemical reduction. In order to examine applicability of ILs as media for reprocessing, we have studied electrochemical behavior of UO₂ ²⁺ in 1-butyl-3-methylimidazolium chloride (BMICl), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄), and 1-butyl-3-methylimidazolium nonafluorobutanesulfonate (BMINfO). Electrochemical properties of uranyl chloride dissolved into ILs were examined by cyclic voltammetry. In BMICl, an almost reversible redox couple was observed, and the formal potential and the diffusion coefficient were evaluated as _0:758V vs. Ag/AgCl and 4:8 × 10^?8 cm²s^?1, respectively. On the other hand, the electrochemical reactions of UO₂ ²⁺ in BMIBF₄ and BMINfO were irreversible. In BMINfO, some reduction peaks and one sharp oxidation peak were observed in the range of ?0:6～–0:2V and around 0.85V vs. Ag/AgCl, respectively. The reduction and oxidation peaks were assigned to multi step reduction of UO₂ ²⁺ to U(IV) via U(V) and/or direct reduction of UO₂ ²⁺ to U(IV), and the oxidative dissolution of the resulting U(IV) compounds, respectively. The electrochemical reduction of UO₂ ²⁺ in BMINfO at ?1:0V vs. Ag/AgCl produced the deposits on a carbon electrode as a cathode. Analyses of the deposits with the scanning electron microscope and the energy dispersive X-ray spectrometer indicated that the deposits are compounds containing uranium, oxygen, and chlorine. As a result, it is expected that the UO₂ ²⁺ in IL can be recovered electrolytically as uranium compounds such as UO₂ and uranium oxychlorides.     

Study on Redox Equilibrium of UO2 2+/U2 + in Molten NaCl-2CsCl by UV-Vis Spectrophotometry

In order to enhance the understanding of the redox equilibriums of uranyl ions in molten NaCl-2CsCl eutectic salt UV-Vis absorption spectrophotometry measurements were performed for UO₂ ²⁺ in molten NaCl-2CsCl at 923 K under simultaneous electrolytic control of their ratio. A prominent absorption band at 395 nm was assigned to UO₂ ⁺, and its molar absorptivity was determined to be 832±27 mol^-1·l·cm^-1. From the dependence of the rest potential of the melt on the spectrophotometrically determined ratio of [UO₂ ²⁺]/[UO₂ ⁺], the standard redox potential of the couple UO₂ ²⁺/UO₂ ⁺ was determined to be ?0.903±0.007 V vs. Cl₂/Cl^—.     

Extraction properties of diisoamyl methylphosphinite

The properties of one of the new, efficient extractants for uranium, diisoamyl methylphosphinate (DAMP), are described. It was shown that uranyl nitrate is extracted from nitric acid solutions by this extractant with considerably higher distribution coefficients than tributyl phosphate (TBP). The extraction of uranyl nitrate and HNO³ with DAMP solutions in hydrogenated kerosene was studied. It was shown that uranyl nitrate is extracted in the form of the complex [UO₂(NO₃)₂ (DAMP)₂], whose stability constant equals 2540±200, and HNO₃ is extracted in the form of the compound HNO₃ DAMP, whose stability constant is 0.30±0.03.     

Reaction Characteristics of β-UO3 in Aqueous NH4NO3

The influences of reaction time, temperature and NH₄NO₃ concentration on the reaction of solid β-UO₃ with aqueous NH₄NO₃ were experimentally studied. Both type I(UO₃·2H₂O) and type II(3UO₃·NH₃·5H₂O) uranyl compounds were observed in the X-ray diagrams of reaction product. High temperature (80°C) and long reaction time (2 h) favor the formation of type II uranyl compounds and generation of uranyl nitrate. Based on the experimental results, two possible models were proposed. The model that assumed pseudohomogeneous reactions conforms to the experimental data better than the model that assumed noncatalytical solid-fluid heterogeneous chemical reaction. The dependence of uranyl nitrate generation rate on reaction time, temperature and NH₄NO₃ concentration can be calculated from the pseudohomogeneous reaction model.     

Sorption of U(VI) on the 4-Mercaptopyridine Self-Assembled Monolayer

The sorption of U(VI) on the 4-mercaptopyridine self-assembled monolayer (4-PyS-SAM) on Au(111) was studied by cyclic voltammetry. Cyclic voltammograms (CVs) of the 4-PyS-SAM working electrode were obtained by contact with 1mM UO₂(NO₃)₂ solution, 1mM UO₂(NO₃)₂ and 50mM acetic acid solution, or 1mM UO₂(NO₃)₂ and 50mM oxalic acid solution for 6 h at pH 4. The reduction current of U(VI) to U(V) was detected in the CV. The CV of the U(VI) associated 4-PyS-SAM after transport to U(VI)-free 0.1M NaClO₄ solution showed that the reduction current was detected in the cases of UO₂(NO₃)₂ and U(VI)-acetate, but not in the case of U(VI)-oxalate solution, indicating that U(VI) was adsorbed on the 4-PyS-SAM from the UO₂(NO₃)₂ and U(VI)-acetate solutions, but not from U(VI)-oxalate solution. These results suggest that stability of U(VI)-4-PyS-SAM is not so high that U(VI)-4-PyS-SAM cannot be formed in the presence of 50mM oxalate.     

An investigation of the reaction of UO2(NO3)2 with Na2HPO4 in aqueous solution

The system UO₂(NO₃)₂- Na₂HPO₄-H₂O was investigated by determining the solubility, pH, electrical conductivity and apparent volume of the precipitates. It was found that the reaction in this system takes place in three stages, with successive formation of (UO2)a(PO4), a mixed sodium uranyl acid phosphate UO₂H_xNa_1–xPO₄ and, finally, NaUO₂PO₄·nH₂0.Coneiusions are drawn regarding the possibility of titrimetric determination of the uranyl ions and the free acidity in its salts, also the conditions for obtaining the most closely compacted umnyi phosphate precipitate.Translated from Atomnaya Énergiya, Vol. 14, No. 4, pp. 395–399, April, 1963.     

In-situ Measurement of UO2 2+ Concentration in Molten NaCl-2CsCl by Differential Pulse Voltammetry

In order to assess the applicability of the Differential Pulse Voltammetry technique to the in-situ measurement of UO₂ ²⁺ concentration in the oxide electro-winning process, DPV measurements for UO₂Cl₂ in molten NaCl-2CsCl were studied. DPV measurement of UO₂ ²⁺ in NaCl-2CsCl at 923 K, with a set of optimized parameters (potential sweep rate ?0.1 V/s, pulse cycle 0.1 s, pulse width 10 ms and pulse potential height 50 mV), showed a clear current peak at ?0.9 V vs. Cl₂/Cl^?. This was attributed to the reduction of UO₂ ²⁺ to UO₂ ⁺. The relation between the current peak height and the analytical concentration of the UO₂ ²⁺ showed good proportionality in the concentration region up to 0.06 mol.l ^?1, and the applicability of UO₂ ²⁺ concentration measurement by DPV was confirmed up to 0.4 mol.l ^?1. In order to assess the interference by the coexisting fission product elements to the measurement of UO₂ ²⁺ concentration, DPV measurements of UO₂ ²⁺ concentration in molten NaCl-2CsCl containing PdCl₂, NdCl₃, SmCl₃ and CeCl₃ were also performed. Even before removing Pd, the current peak at ?0.9 V vs. Cl₂/Cl^? by the reduction of UO₂ ²⁺ to UO₂ ⁺ was found to be distinguishable from the reduction currents of Pd²⁺ to Pd at ?0.7 V vs. Cl₂/Cl^?. As a result, the application of DPV measurement technique to the in-situ monitoring of UO₂ ²⁺ concentration in the oxide electro-winning method requires the improvement of the DPV measuring condition or the electrode structure on higher UO₂ ²⁺ concentration condition.     

Precipitation Reaction of Uranyl Ion in Molten Potassium Thiocyanate

The qualitative and the quantitative analyses of the reaction products obtained by heating uranyl solution of molten potassium thiocyanate were carried out. It was found that in the presence of water in the melt, uranyl ion is converted into UO₂ accompanied with the evolution of CO₂ and the formation of free sulfur. The molar ratios on these products were almost equal to each other.

By detecting ammonia in reaction products, it was concluded that the precipitation reaction of uranyl ion in the melt is expressed as follows:

UO₂ ⁺⁺ + SCN^? + 2H₂O = UO₂ + CO₂ +S+ NH₄ ⁺.     

Kinetics and Equilibrium of Reaction between Octakis (dimethyl sulfoxide) uranium(IV) and Nitrite Ion in Dimethyl Sulfoxide

The kinetics and equilibrium of the reaction between octakis (dimethyl sulfoxide) uranium (IV), U(dmso)₈ ⁴⁺, and nitrite ion N0₂ ^? were studied in dmso solutions by using stopped-flow, diode-array and UV-visible spectrophotometers. Three different reactions were observed under the condition of an excess NaNO₂, and U(IV) was finally oxidized to U(VI) by N0₂ ^?. The first reaction is the formation of 1: 2 adduct complex [U (dmso₈)(NO₂)₂ ²⁺), the rate of which was so fast that the rate constant k_l obs was not determined accurately even by the stopped-flow method. If the concentration of N0₂ ^? was as low as that of U(IV), 1:1 adduct complex was formed. However, 1:1 complex was inert for further redox process, i.e. U(IV) was not oxidized by N0₂ ^? under this condition. The second reaction represents the substitution of coordinated dmso with N0₂ ^? forming mixed ligand complex [U(dmso) _8-x(N0₂ ^?)_x+2(^{2-x +}) The rate constant k ₂0bs for the second reaction was measured by the conventional method using a diode-array spectrophotometer and k ₂0bs increased linearly with the concentration of free N0₂ ^?. The third reaction coincides with the intra-molecular oxygen transfer from coordinated N0₂ ^? to U(IV) and this process is accompanied by the oxidation of U(IV) to U(VI). The oxygen transfer mechanism was confirmed by ¹⁷O labeled experiments using ¹⁷O-NMR. It was found from acid dependence experiments that only nitrite ion oxidized U(IV) and that nitrous acid was inert as an oxidant for U(IV).     

The effect of radiation on the valence state of plutonium in nitric acid solutions

We investigated the effect of x -radiation on the valence state of plutonium in nitric acid solutions from 0.3 to 2.0 M and also in 0.3 M nitric acid solutions containing varying concentrations of UO₂(NO₃)₂ and K₂Cr₂O₇. The effect of radiation on nitric acid solutions, not containing UO₂(NO₃)₂, causes only oxidation of plutonium and the oxidation yield decreases with an increase in the concentration of NO ₃ ^– ions and acidity of the solution. We put forward the hypothesis that the oxidation is effected by OH radicals. Under certain conditions the reduction of plutonium was observed in the presence of UO₂(NO₃)₂. Apparently, it is not effected by atomic hydrogen but by UO ₂ ⁺ ions. Potassium dichromate causes an acceleration in radiation oxidation of plutonium but under the given conditions it does not retard its reduction.     

Extraction of Thorium and Uranium from Chloride Solutions by Tri-n-Butyl Phosphate and Tri-n-Octyl Phosphine Oxide

The extraction of thorium and uranium chlorides by TBP and TOPO was studied. The composition of complexes extracted from the chloride solutions of low acid concentration was established by partition study to be UO₂Cl₂ (TOPO)₂, UO₂Cl₄ (TOPO)₂, UO₂Cl₄ (TOPO)₂ and UCl₄ (TBP)₂. Composition of the thorium complex in the TBP phase free from hydrochloric acid was revealed by infrared study to be ThCl₄ (TBP)4. The extraction behavior of thorium chloride by TBP was different from that of U (N) and Pu(N) chloride, and the composition of the complex was presumed to be HThCl₅(TBP)₄ in the extraction from concentrated chloride solution containing hydrochloric acid.     

溶剂萃取法分离铀中钌的研究

一、前言 在核燃料后处理Purex流程中,铀纯化段的回收铀中常含有少量钌,使铀钌分离成为纯化铀的一个重要问题。实验表明,辐照铀经HNO_3溶解后,裂变产物钌主要以亚硝酰钌RuNO~(3+)状态存在。它与溶液中的NO_3~-,NO_2~-,OH~-和H_2O等离子或分子形成配位数为6的多种配合物。各种RuNO~(3+)的硝酸基配合物的比例取决于溶液中NO~-和H~+离子的浓度。而NO_2~-与RuNO~(3+)的配合能力比NO_3~-与RuNO~(3+)的强。在一定条件下,两类配合物之间可以相互转化。铀主要是以UO_2~(2+)的NO_3~-配合物存在。     

Recent Results in JT-60 Advanced Experiment JT-60 Team,JAERI (Presented by Naoyuki MIYA)

Major efforts in the recent JT-60 experiments have been concentrated on the improved confinement of plasmas with profile control and on the steady state operation study. Peaked density profiles were produced with the successive pellet injection. The energy confinement time was improved by 40% as large as that with the gas fuelled discharges. The fusion products n _e(0)τET _i(0)reached 1.2 × 10²⁰m^?3·s·keV, which was twice that of gas fuelled discharges. High-β_p, plasmas were obtained in low-I _p discharges with improved confinement and a high ion temperature T _i, (0) of 12 keV. The bootstrap current reached 80% of the total plasma current at β_p=3.2. The new concept of a steady-state tokamak power reactor has been proposed on the basis of this result. The maximum current drive efficiency ηCD of 3.4 × 10₁₉m^?2·MA/MW was obtained in the LH current drive experiments. Helium ash exhaust experiments using He-beam injection into H⁺ plasmas showed promising results for α-particle exhaust in a fusion power reactor.