Oxidation of UO2(s) in aqueous solution

In this review the kinetics and mechanism of oxidative dissolution of UO₂(s), mainly under conditions of relevance for the safety assessment of a deep geological repository for spent nuclear fuel, are discussed. Rate constants for the elementary processes involved (oxidation of UO₂ and dissolution of oxidized UO₂) are used to calculate the rates of oxidative UO₂(s) dissolution under various conditions (type of oxidant, oxidant concentration and HCO₃ ⁻/CO₃ ²⁻ concentration) for which experimental data are also available. The calculated rates are compared to the corresponding experimental values under the assumption that the experimental numbers reflect the steady-state conditions of the system. The agreement between the calculated rates and the corresponding experimental ones is very good, in particular for the higher rates. In general, the calculated rates are somewhat higher than the experimental numbers. This can be due partly to the use of initial concentrations rather than steady-state concentrations in the calculations. The kinetic data are also used to quantitatively discuss the dynamics of spent nuclear fuel dissolution under deep geological repository conditions.      

Leaching of spent nuclear fuel in the presence of siderophores

Metal species that are dissolved in water can be transported in the environment, because they can be mobile. Microorganisms can affect metal mobility by excreting organic ligands with high metal affinity. Siderophores are organic ligands with high affinities for Fe³⁺. They are also able to form complexes with other metals such as actinides. Many countries plan to deposit spent nuclear fuel in deep geological repositories. Microorganisms are present in these subterranean environments and could potentially affect the repository. In this study, the effect of microbial siderophores on the dissolution behavior of two fragments of a spent nuclear fuel pellet was investigated. The commercial hydroxamate siderophore, deferoxamine mesylate (DFAM), and pyoverdin siderophores, isolated from cultures of Pseudomonas fluorescens (CCUG 32456A), were used. DFAM and lyophilized pyoverdins were dissolved in synthetic groundwater to final concentrations of 10 μM and 2.5·10⁻² g·L⁻¹, respectively. The fuel pellet fragments were kept in sealed pressure vessels at 10 bars of H₂. The pyoverdin solution was first tested, followed by the DFAM solution and the pure synthetic groundwater. Samples were taken on 0, 1, 5, 9 and 14 days after changing the solution in the pressure vessels. The elemental composition of samples was analyzed by means of ICP-MS. The pyoverdin solution maintained significantly higher concentrations of Np and Pu than the pure synthetic groundwater. On the 14th day the concentrations of Np and Pu in the pure synthetic groundwater were 0.01 nM and 0.13 nM, respectively, compared to 0.02 nM and 0.31 nM in the pyoverdin solution. Furthermore, spent nuclear fuel samples were observed to release Ru in the presence of both pyoverdin and DFAM. Hence, it seems that siderophores can form complexes with elements present in spent nuclear fuel.     

IC-ICP-MS applied to the separation and determination of traces of plutonium and uranium in aqueous leachates and acid rinse solutions of UO2 doped with Pu

It is expected that spent nuclear fuel, today mainly UO₂, may become exposed to groundwater after extended storage in a deep geologic repository. After 1000 years, the radioactivity of the fuel will be constituted essentially by α-emissions by long-lived actinides. The α-emissions play a significant role in determining the dissolution behavior of uranium, because the radiolysis of water results in the formation of oxidizing chemical species near the fuel surface. In order to study this effect, UO₂ doped with 0.1 and 10 wt.% of a strong α-emitter (namely ²³⁸Pu) was subject to leaching at room temperature (RT) in deionized water. In order to study the mechanisms of leaching in simulated conditions, very precise and accurate techniques need to be employed. In this paper, the results obtained by inductively coupled plasma mass spectrometry coupled with ion chromatography for the determination of traces of ²³⁸Pu and uranium in aqueous leachates’ solutions are illustrated.     

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.     

Influence of γ-radiation on the reactivity of montmorillonite towards H2O2

Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH^•, e⁻_(aq), H^•, H₂O₂, H₂, HO₂^•, H₃O⁺), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe_Tot ratio in montmorillonite and its reactivity towards hydrogen peroxide (H₂O₂) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/Fe_Tot ratio of dispersed Montmorillonite increased from ≤3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/Fe_Tot ratio and the H₂O₂ decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H₂O₂ additions, since the structural Fe(II)/Fe_Tot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H₂O₂.     

Dissolution of uranium oxides under alkaline oxidizing conditions

Bench scale experiments were conducted to determine the dissolution characteristics of UO₂, U₃O₈, and UO₃ in aqueous peroxide-containing carbonate solutions. The experimental parameters investigated included carbonate countercation (NH₄ ⁺, Na⁺, K⁺, and Rb⁺) and H₂O₂ concentration. The carbonate countercation had a dramatic influence on the dissolution behavior of UO₂ in 1 M carbonate solutions containing 0.1 M H₂O₂, with the most rapid dissolution occurring in (NH₄)₂CO₃ solution. The initial dissolution rate (y) of UO₂ in 1 M (NH₄)₂CO₃ increased linearly with peroxide concentration (x) ranging from 0.05 to 2 M according to: y = 2.41x + 1.14. The trend in initial dissolution rates for the three U oxides under study was UO₃ ≫ U₃O₈ > UO₂.     

Conversion of neodymium oxide with N<Subscript>2</Subscript>O<Subscript>4</Subscript> into nitrate followed by supercritical fluid extraction of nitrate

A novel method that spent nuclear fuel is converted into nitrates with N₂O₄, and then nitrates are extracted with TBP in supercritical CO₂ (SC-CO₂), has been developed for reprocessing of spent nuclear fuel, which has a potential prospect because of its potential to decrease generation of the secondary liquid waste. In this paper, conversion of Nd₂O₃ with N₂O₄ into its nitrate under various conditions and extraction of the conversion product with TBP in SC-CO₂ were investigated. When temperature was 60–120 °C, the molar ratio of H₂O to Nd₂O₃ was from 1 to 6, and molar ratio of N₂O₄ to Nd₂O₃ was above 8, complete conversion of Nd₂O₃ into its nitrate was achieved. The conversion product was characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR) and Raman spectroscopy. Quantitative extraction of the conversion product with TBP in supercritical CO₂ was also achieved under experimental conditions.     

Oxidative Dissolution of Silver Nanoparticles by Biologically Relevant Oxidants: A Kinetic and Mechanistic Study

The oxidative dissolution of silver nanoparticles (AgNPs) plays an important role in the synthesis of well‐defined nanostructured materials, and may be responsible for their activities in biological systems. In this study, we use stopped‐flow spectrophotometry to investigate the kinetics and mechanism of the oxidative dissolution of AgNPs by H₂O₂ in quasi‐physiological conditions. Our results show that the reaction is first order with respect to both [Ag⁰] and [H₂O₂], and parallel pathways that involve the oxidation of H₂O₂ and HO₂^? are proposed. The order of the reaction is independent of the size of the AgNPs (≈5–20 nm). The rate of dissolution increases with increasing pH from 6.0 to 8.5. At 298 K and I=0.1 M , the value of k_b is five orders of magnitude higher than that of k_a (where k_a and k_b are the rate constants for the oxidative dissolution of AgNPs by H₂O₂ and HO₂^?, respectively). In addition, the effects of surface coating and the presence of halide ions on the dissolution rates are investigated. A possible mechanism for the oxidative dissolution of AgNPs by H₂O₂ is proposed. We further demonstrate that the toxicities of AgNPs in both bacteria and mammalian cells are enhanced in the presence of H₂O₂, thereby highlighting the biological relevance of investigating the oxidative dissolution of AgNPs.     

Buffer gas effect on the detection of iodine by laser induced fluorescence

Laser-induced fluorescence (LIF) coupled with photon-counting technique to detect molecular iodine at ultratrace level is reported. Electronic quenching rate constants for N₂, NO₂ and H₂O, as well as for the mixture of NO₂ and H₂O has been measured. The application of the LIF method to monitoring¹²⁹I₂ in spent fuel reprocessing off-gas streams is evaluated.     

Oxidative Dissolution of Silver Nanoparticles by Dioxygen: A Kinetic and Mechanistic Study

We have recently reported a kinetic and mechanistic study on oxidative dissolution of silver nanoparticles (AgNPs) by H₂O₂. In the present study, the parameters that govern the dissolution of AgNPs by O₂ were revealed by using UV/Vis spectrophotometry. Under the same reaction conditions (Tris‐HOAc, pH 8.5, I=0.1 M at 25 °C) the apparent dissolution rate (k_app) of AgNPs (10±2.8 nm) by O₂ is about 100‐fold slower than that of H₂O₂. The reaction rate is first‐order with respect to [Ag⁰], [O₂], and [Tris]_T, and inverse first‐order with respect to [Ag⁺] (where [Ag⁰]=total concentration of Ag metal and [Tris]_T=total concentration of Tris). The rate constant is dependent on the size of AgNPs. No free superoxide (O₂⁻) and hydroxyl radical (⋅OH) were detected by trapping experiments. On the basis of kinetic and trapping experiments, an amine‐activated pathway for the oxidation of AgNPs by O₂ is proposed.     

Glutarimidedioxime: A Complexing and Reducing Reagent for Plutonium Recovery from Spent Nuclear Fuel Reprocessing

Efficient separation processes for recovering uranium and plutonium from spent nuclear fuel are essential to the development of advanced nuclear fuel cycles. The performance characteristics of a new salt‐free complexing and reducing reagent, glutarimidedioxime (H₂A), are reported for recovering plutonium in a PUREX process. With a phase ratio of organic to aqueous of up to 10:1, plutonium can be effectively stripped from 30 % tributyl phosphate (TBP) in kerosene into 1 m HNO₃ with H₂A. The complexation‐reduction mechanism is illustrated with the combination of UV/Vis absorption spectra and the crystal structure of a Pu^IV complex with the reagent. The fast stripping rate and the high efficiency for stripping Pu^IV, through the complexation‐reduction mechanism, is suitable for use in centrifugal contactors with very short contact/resident times, thereby offering significant advantages over conventional processes.     

LiF–CaH2 alumina electrolytes for intermediate temperature fuel cell applications

Hydrofluoride-based electrolytes with proton conduction have been successfully used in intermediate temperature fuel cell applications. Among the various hydrofluoride electrolytes, LiF–CaH₂ and its composite with Al₂O₃, i.e., LiF–CaH₂–Al₂O₃, are the most promising candidates which show more advantages than the other hydrofluorides. In this communication, we put our emphasis on the LiF–CaH₂–Al₂O₃ electrolytes and their applications for intermediate temperature fuel cells. Furthermore, new fuel cell processes from hydride ions, H⁻, and the electrochemical behaviour of LiF–CaH₂–Al₂O₃ electrolytes and fuel cells are discussed in more detail.     

Enhanced electrochemical reduction of rare earth oxides in simulated oxide fuel via co-reduction of NiO in Li2O–LiCl salt

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li₂O–LiCl electrolyte for the electrochemical reduction of a simulated oxide fuel (simfuel). First, the electrochemical behaviors of Nd₂O₃, NiO, and Nd₂O₃–NiO were studied by cyclic voltammetry and voltage control electrolysis. Then, the electrochemical reduction of the simfuel containing UO₂ and rare earth oxides (Nd₂O₃, La₂O₃, and CeO₂) was conducted in molten LiCl salt with 1 wt.% Li₂O via the co-reduction of NiO. The extent of reduction of the rare earth oxides was found to be significantly improved.     

H2O2-photosensitized emulsion copolymerization of tetrafluoroethylene with propylene

The H₂O₂-photosensitized emulsion copolymerization of tetrafluoroethylene with propylene was carried out at room temperature in the presence of gaseous monomers of 50 mole-% tetrafluoroethylene content. The conversion increased almost linearly with irradiation time. The rate of polymerization was proportional to the 1.0 power of H₂O₂ concentration up to 3.5 × 10^?3M H₂O₂ and the 0.46 power of H₂O₂ concentration above 3.5 × 10^?3M H₂O₂. The result obtained at low H₂O₂ concentration was almost consistent with that obtained in the radiation-induced method. The rate of polymerization was proportional to the 0.58 power of the emulsifier concentration, and the degree of polymerization was independent of the emulsifier concentration. The H₂O₂-photosensitized emulsion copolymerization of tetrafluoroethylene with propylene is terminated mainly by degradative chain transfer of the propagating radical to propylene at low H₂O₂ concentration and by the reaction of the propagating radical with OH radical from photolysis of H₂O₂–aqueous solution at high H₂O₂ concentration.     

Amplification effects of magnetic field on hydroxylamine-promoted ZVI/H2O2 near-neutral Fenton like system

This study has demonstrated an interesting amplification effect of magnetic field（MF） on the hydroxylamine（HA）-promoted zero valent iron（ZVI）/H2 O₂ Fenton-like system.Sulfamethoxazole（SMX） could be efficiently degraded at near neutral pH.Conditional parameters affecting the SMX degradation in the ZVI/H₂ O₂/HA/MF system,e.g.,pH and the dosages of ZVI,HA and H₂ O₂,were investigated.Unlike the acid-favorable ZVI/H₂ O₂ and ...     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

Optimization and Degradation Mechanism of Photocatalytic Removal of Bisphenol A Using Zn0.9Fe0.1S Synthesized by Microwave‐assisted Method

The sulfide photocatalyst of Zn_0.9Fe_0.1S was successfully synthesized by a facile microwave‐assisted method, and Zn_0.9Fe_0.1S photocatalysts were characterized using SEM, EDX, XRD and BET. The specific surface area of synthesized Zn_0.9Fe_0.1S is 78.1 m² g^?1, and total pore volume is 0.4 cm³ g^?1. With bisphenol A (BPA) as a target pollutant, photocatalytic system of UV + Zn_0.9Fe_0.1S + H₂O₂ was set up. Some influencing parameters, including H₂O₂ dosage, initial pH value, initial concentration of BPA and Zn_0.9Fe_0.1S dosage, were investigated, and the stability of the Zn_0.9Fe_0.1S was also studied during the photocatalysis. The optimum values of operating parameters were found at an initial pH value of 5.0, a H₂O₂ dosage of 0.15 mmol L^?1 and a Zn_0.9Fe_0.1S dosage of 0.08 g when the initial concentration of BPA was 10 mg L^?1. Under the optimal conditions, the highest removal rate of BPA achieved 95%. After seven consecutive reaction cycles, the degradation efficiency of BPA could still reach 85% and there was only a little dissolution of Zn²⁺ and Fe²⁺. Compared with the traditional photo‐Fenton system, the UV + Zn_0.9Fe_0.1S + H₂O₂ system can not only improve the degradation efficiency of BPA, but also reduce the dosage of H₂O₂ and thus reduce the processing cost.     

Proton chemical shift tensors in neutral and ionic OH⋯O hydrogen bonds

Proton nuclear magnetic shielding tensors are calculated for some OH?O hydrogen bonds: (H₃O₂)^?, (H₂O)₂, and (H₅O₂)⁺. The effects of charge, geometry, and basis set are studied. Agreement with single crystal pulsed NMR experiments is obtained. A linear dependence between the proton chemical shift and the O?O separation is observed, correcting a previous misinterpretation of the data.     

Catalytic activity of a zirconium(IV) oxide surface supported with transition metal ions

The kinetics of H₂O₂ decomposition have been investigated using ZrO₂ supported with transition metal ions including Cu^II, Ag^I, Hg^II, Co^II, Mn^II, Ni^II and Fe^III. At pH = 6.8, the reaction rate exhibits a first order dependence on the initial H₂O₂ concentration at low concentrations. The order of activity of the different catalysts is strongly dependent on the [H₂O₂]₀ used. The reaction proceed via the formation of the peroxo-intermediate which has an inhibiting effect on the reaction rate. The rate increases with increasing pH, and attains a limiting rate at higher pH's. A reaction mechanism is proposed involving liberation of HO₂ radicals from the peroxo-intermediate as the rate-determining step.     

A Robust One‐Compartment Fuel Cell with a Polynuclear Cyanide Complex as a Cathode for Utilizing H2O2 as a Sustainable Fuel at Ambient Conditions

A robust one‐compartment H₂O₂ fuel cell, which operates without membranes at room temperature, has been constructed by using a series of polynuclear cyanide complexes that contain Fe, Co, Mn, and Cr as cathodes, in sharp contrast to conventional H₂ and MeOH fuel cells, which require membranes and high temperatures. A high open‐circuit potential of 0.68 V was achieved by using Fe₃[{Co^III(CN)₆}₂] on a carbon cloth as the cathode and a Ni mesh as the anode of a H₂O₂ fuel cell by using an aqueous solution of H₂O₂ (0.30 M , pH 3) with a maximum power density of 0.45 mW cm^?2. The open‐circuit potential and maximum power density of the H₂O₂ fuel cell were further increased to 0.78 V and 1.2 mW cm^?2, respectively, by operation under these conditions at pH 1. No catalytic activity of Co₃[{Co^III(CN)₆}₂] and Co₃[{Fe^III(CN)₆}₂] towards H₂O₂ reduction suggests that the N‐bound Fe ions are active species for H₂O₂ reduction. H₂O₂ fuel cells that used Fe₃[{Mn^III(CN)₆}₂] and Fe₃[{Cr^III(CN)₆}₂] as the cathode exhibited lower performance compared with that using Fe₃[{Co^III(CN)₆}₂] as a cathode, because ligand isomerization of Fe₃[{M^III(CN)₆}₂] into (FeM₂)[{Fe^II(CN)₆}₂] (M=Cr or Mn) occurred to form inactive Fe? C bonds under ambient conditions, whereas no ligand isomerization of Fe₃[{Co^III(CN)₆}₂] occurred under the same reaction conditions. The importance of stable Fe²⁺? N bonds was further indicated by the high performance of the H₂O₂ fuel cells with Fe₃[{Ir^III(CN)₆}₂] and Fe₃[{Rh^III(CN)₆}₂], which also contained stable Fe²⁺? N bonds. The stable Fe²⁺? N bonds in Fe₃[{Co^III(CN)₆}₂], which lead to high activity for the electrocatalytic reduction of H₂O₂, allow Fe₃[{Co^III(CN)₆}₂] to act as a superior cathode in one‐compartment H₂O₂ fuel cells.