Dissolution Of UO2 Research Articles

The effect of [Fe]TOT on the dissolution of synthetic Pb-doped UO2 and Th-doped UO2

The dissolution of synthetic Pb-doped UO2 and Th-doped UO2 was systematically studied to determine the influence of leach parameters [Fe]TOT and ORP under standard leach conditions of: T=50°C, [H2SO4]=15g/L (0.15M), and UO2=100mg/L. Results demonstrated reduced uranium dissolution in both systems compared to pure UO2. This effect was greatest for Th-doped UO2. The decrease in uranium dissolution between the doped systems and pure UO2 was attributed to the formation of precipitate layers at the surface of the solid, slowing down or blocking uranium release. In the case of Pb-doped UO2, the formation of a Pb sulphate phase was directly detected but in the case of Th-doped UO2, no layer was found. For the latter system it was postulated that passivation of the Th-doped UO2 surface occurs due to the formation of oxidised Th-rich phases Th(OH)4, ThO2 and ThO2·nH2O at the surface of grains preventing uranium release. In tests varying the ORP, there was an approximately linear dependence of the dissolution rate on [Fe]TOT for both systems however the rate orders indicated a step change between an ORP of 420 and 460mV. The specific influence of FeII showed that both Pb–UO2 and Th–UO2 exhibited two distinct regions of dissolution rate dependency similar to that previously noted for pure UO2.

The Anodic Reactions on Simulated Spent Fuel (SIMFUEL) in H2O2 Solutions: Effect of Carbonate/Bicarbonate

The anodic behavior of simulated nuclear fuel (SIMFUEL) in solutions containing H2O2 and HCO3−/CO32− has been studied electrochemically and using surface analytical techniques, in particular X-ray photoelectron spectroscopy. Two anodic reactions are possible, the oxidative dissolution of UO2 and H2O2 oxidation. The rates of both reactions are controlled by the chemical release of UVI surface species, and can both be increased by the addition of HCO3−/CO32−. Under anodic conditions the dominant reaction is H2O2 oxidation, although UO2 dissolution may also be accelerated by the formation of a uranyl peroxocarbonate complex. Similarly, under open circuit (corrosion) conditions both UO2 corrosion and H2O2 decomposition are also controlled by the rate of release of UVI surface species which blocks access of H2O2 to the underlying conductive UIV1–2xUV2xO2+x surface. Presently, the balance between these two reactions is not known.

Nano-FeS Inhibits UO2 Reoxidation under Varied Oxic Conditions

Bioreductive in situ treatment of U-contaminated groundwater can convert soluble U(VI) species to immobile reduced U(IV) solid phases such as UO2(s) to contain U movement. Once active bioremediation is halted, UO2 may be subsequently reoxidized if oxidants such as oxygen enter the reducing zone. However, iron sulfide minerals that form during bioreduction may serve as electron sources or oxygen scavengers and inhibit UO2 reoxidation upon oxygen intrusion. In this study, flow-through reactor experiments examined the abiotic kinetics of UO2 oxidative dissolution in the presence of oxygen and nanoparticulate FeS as a function of pH, dissolved oxygen (DO) concentration, and FeS content. The UO2 dissolution rates in the presence of FeS were over 1 order of magnitude lower than those in the absence of FeS under otherwise comparable oxic conditions. FeS effectively scavenged DO and preferentially reacted with oxygen, contributing to a largely unreacted UO2 solid phase during an "inhibition period" as determined by X-ray absorption spectroscopy (XAS). The removal of DO by FeS was significant but incomplete during the inhibition period, resulting in surface-oxidation-limited dissolution and greater UO2 dissolution rate with increasing influent DO concentration and decreasing FeS content. Although the rate was independent of solution pH in the range of 6.1-8.1, the length of the inhibition period was shortened by substantial FeS dissolution at slightly acidic pH. The reducing capacity of FeS was greatest at basic pH where surface-mediated FeS oxidation dominated.

Effect of bentonite on radiation induced dissolution of UO2 in an aqueous system

In order to elucidate the impact of bentonite on the process of radiation induced oxidative dissolution of UO2 in an aqueous system, the dissolution of U(VI) and consumption of H2O2 over time has been studied. In addition, γ-irradiation experiments were performed to study a more relevant and complex system, serving as a comparison with the previously stated system. In both cases, the experiments revealed that the presence of bentonite in water could either delay or prevent in part the release of uranium to the environment. The cause is mainly attributed to the scavenging of radiolytic oxidants rather than to the adsorption of uranium onto bentonite.

Oxidative UO2 Dissolution Induced by Soluble Mn(III)

The stability of UO2 is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO2 and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than that by a comparable concentration of dissolved O2. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO2. The ability of Mn(III) complexes to oxidize UO2 was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO2 surface to facilitate electron transfer. Systematic investigation into the kinetics of UO2 oxidative dissolution by the Mn(III)-PP complex suggested that Mn(III) could directly oxidize UO2 without involving particulate Mn species (e.g., MnO2). The expected 2:1 reaction stoichiometry between Mn(III) and UO2 was observed. The reactivity of soluble Mn(III) in oxidizing UO2 was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)-pyrophosphate complexes. Disproportionation of Mn(III)-PP occurred at pH 9.0, and the oxidation of UO2 was then driven by both MnO2 and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.

Electrowinning of UO2 from Ionic Liquid Medium

The room temperature ionic liquid, 1-butyl-3-methylimidazolium chloride (C4mimCl), was prepared and studied for the feasibility of using C4mimCl for the dissolution of uranium oxide and subsequent recovery by electrodeposition. The dissolution behavior of uranium oxides, UO3 and UO2, in C4mimCl medium by chlorination reaction was studied as a function of uranium loading, at 353 K. The electrochemical behavior of U(VI) in the resultant solution was investigated by cyclic voltammetry at tungsten electrode. The cyclic voltammogram consisted of a prominent cathodic wave occurring at the potential of −1.0 V (Vs Pt) was due to the reduction of . Controlled potential electrolysis of the solution results in a deposition of UO2. The electrodeposition of UO2 from C4mimCl medium was studied as a function of uranium loading. The results indicated that the recovery of uranium was quantitative within eight hours, when the solution obtained after the dissolution of UO3 was subjected to electrolysis. However, the recovery of uranium was less when the solution obtained after the dissolution of UO2 was subjected to electrolysis under similar conditions.

Magmatic and Metamorphic Uraninite Mineralization in the Western Margin of the Trans-Hudson Orogen (Saskatchewan, Canada): A Uranium Source for Unconformity-Related Uranium Deposits?

AbstractThe genetic model for the giant unconformity-related uranium deposits of the Athabasca Basin is still being debated, with one of the main issues being the source of the uranium concentrated by Mesoproterozoic era (ca. 1.60–1.00 Ga) diagenetic-hydrothermal events at the interface between the Athabasca Basin and the underlying Archean/Paleoproterozoic basement rocks. Currently, accessory minerals like monazite, zircon, and/or apatite from the sedimentary basin and basement rocks are proposed as the primary uranium source for these high-grade uranium deposits. Numerous occurrences of U mineralization of Hudsonian age have been documented for decades all around the Athabasca Basin; however, so far these have not been regarded as viable U sources. Here, a systematic and detailed study of two areas of basement rocks near the eastern part of the Athabasca Basin is presented (i.e., the Way Lake property, lying outside the current margin of the basin, and the Moore Lakes property, currently covered by the basin). This study highlights the significant and widespread occurrence of Hudsonian (ca. 1.81–1.76 Ga) uranium oxide (UO2) mineralization in these zones. Two types of mineralization are identified and documented here: magmatic uranium oxides related to granitic pegmatites and leucogranites, which are more common, and high-temperature, vein-hosted uranium oxides, which have the highest grades. The two types were formed during the peak (1.82–1.81 Ga) and/or postthermal peak (1.81–1.72 Ga) events related to the evolution of the Trans-Hudson orogeny. The magmatic uranium oxides formed by partial melting of Wollaston Group metasedimentary rocks. The origin of the vein-type occurrences is unclear, but their high thorium and rare earth element (REE) contents suggest a high-temperature process associated with Ca and/or Na metasomatism. The uranium oxides are associated with other U-, Th-, and REE-bearing accessory minerals like U-rich thorite, thorite, zircon, and/or monazite, adding to the exceptional U contents (100–2,460 ppm) of these UO2-bearing rocks (up to 200 times more primarily enriched in U than other basement or basin rock types). A 3-D model of a 1,300 × 630 × 200-m basement zone from the Way Lake property indicates that uraninite-bearing granitic pegmatites and leucogranites represent 7% of the total volume of crystalline rock. Within this rock volume are approximately 8,121 (assuming a mean U content of 250 ppm) to 16,242 (assuming a mean U content of 500 ppm) tonnes U. The U tonnage of this limited rock volume, contained mainly by the Hudsonian-age UO2, corresponds to between 4% (for McArthur River) and 103% (for Rabbit Lake) of the U tonnage of known unconformity-related U deposits of the basin.Some of the studied rock samples, even macroscopically fresh and located far away from any known unconformity-related U deposit, present clear evidence of alteration, including clay minerals, aluminophosphate-sulfate (APS) minerals, and UO2 dissolution, indicating the percolation of the brines associated with the formation of unconformity-related uranium deposits when the basin was far more geographically extensive. Due to geologic similarities between the studied zones and the basement domains from the eastern part of the Athabasca Basin, i.e., the Hearne province, it is proposed that these domains hosted widespread Hudsonian-age uranium oxide protores. These protores provided easily leachable uranium for the metal enrichment of basinal brines during their percolation within the basement and the formation of the unconformity-related U deposits. These observations bring new insight to the debate about the genetic model of unconformity-related U deposits, and reinforce the metal source potential of the basement compared to that of the sedimentary basin.

H2O2 and radiation induced dissolution of UO2 and SIMFUEL in [formula omitted] deficient aqueous solution

Understanding UO2 matrix dissolution is of major importance for the safety assessment of a future deep geological repository. Oxidative dissolution of UO2 and SIMFUEL pellets have been extensively studied in HCO3- solutions, while less is known about systems with no or very low HCO3- concentrations. The aim of this work is to elucidate the oxidative dissolution of UO2 and SIMFUEL pellets in HCO3- free solutions by studying the dissolution of U (VI) and consumption of H2O2 over time. The results are compared with previous experiments performed in HCO3- solutions. The oxidative dissolution rate is higher for the UO2 pellet in HCO3- compared to the other systems. It is evident that the kinetics of the reaction with H2O2 is qualitatively different for SIMFUEL in comparison with pure UO2. For the UO2 pellet in pure water, the presence of a secondary phase (meta) studtite, on the surface of the pellet is confirmed by Raman spectroscopy. The kinetic impact of the secondary phase is evaluated in separate UO2 powder experiments. The (meta) studtite (surface) precipitation leads to a slower release of uranium into the solution. Numerical simulations using experimentally determined rate constants are used to evaluate a simple mechanism of surface precipitation. The numerical results are in fair agreement with the experimental observations given certain criteria. In addition, the γ-radiation induced dissolution of UO2 and SIMFUEL pellets were investigated in pure water, and compared with HCO3- systems. Also here the dissolution rate of uranium is higher for UO2 in HCO3- compared to pure water, while for SIMFUEL longer irradiation times are needed to observe any difference between pure and HCO3- containing water.

Microwave-assisted dissolution of ceramic uranium dioxide in TBP–HNO3 complex

Microwave-assisted dissolution of ceramic uranium dioxide in tri-n-butyl phosphate (TBP)–HNO3 complex was investigated. The research on dissolution of ceramic uranium dioxide in TBP–HNO3 inclusion complex under microwave heating showed the efficiency of the use of this method. Nitric acid present in the inclusion complex participates both dissolution of UO2, and oxidation of U(IV)–U(VI), the resulting UO2(NO3)2 extracted with tri-n-butyl phosphate. Dissolution rate depends on both temperature of microwave dissolution process, and concentration of nitric acid present in the inclusion complex. The most intensive dissolution process is when the concentration of nitric acid ≥2 mol/L and the temperature of 120 °C. From the experimental data obtained by two kinetic models activation energies were calculated. At the average activation energy of UO2 dissolution in TBP–HNO3 complex equal 70 kJ/mol, and reaction order is close to one, i.e. the reaction takes place in an area close to kinetic.

An investigation on the effects of several anions on the dissolution of synthetic uraninite (UO2)

With the increase in the demand for energy, there is an increased interest in nuclear power sourced from a number of uranium minerals, primarily tetravalent UO2. During the leaching of uranium from its ores, a number of gangue minerals are leached which lead to a number of different ions in the leach slurry. The current work undertaken focused on the effect of anions that typically leach from gangue minerals found in uranium bearing ore bodies on the dissolution of synthetic UO2. The anions studied were F−, Cl−, Br−, SO42−, PO43− and NO3−. Cl− and Br− were found to have no significant influence on UO2 dissolution over the concentration range studied (0–1.8g/L Cl−; 0–4g/L Br−), whilst F− was found to have a detrimental effect on UO2 dissolution at low–medium concentration (0–0.3g/L) and a positive effect on UO2 dissolution when present in medium–high concentrations (0.5–1g/L), when compared to the dissolution obtained in the presence of the other halides studied. Of the oxo-anions studied NO3− had a positive effect on UO2 dissolution over the entire concentration range studied and PO43− had a negative effect when present at medium–high concentrations. This was due to precipitation of FePO4 at medium–high PO43− concentrations. SO42− had a negative effect when present at medium–high concentrations. This was most likely due to the formation of less effective FeIII based complexes, where the FeIII species present in solution play an important role in the UO2 oxidation/dissolution mechanism. Results obtained on the influence of all anions investigated simultaneously are also presented and discussed.

Fe-containing ionic liquids as effective and recoverable oxidants for dissolution of UO2 in the presence of imidazolium chlorides

Imidazolium-based Fe-containing ionic liquids (ILs) can directly dissolve UO2 in the presence of their corresponding imidazolium chlorides without additional oxidants. The dissolution process follows pseudo first-order kinetics initially. Raman spectroscopic studies indicate that FeCl4(2-) is the predominant reduction product after UO2 dissolution, and attenuated total reflection-Fourier transform infrared spectroscopy indicates that the UO2(2+) complex is the principal product in the ILs. The dissolved uranyl species can be successfully separated from the Fe-containing ILs via a combination of centrifugation and solvent extraction, and also, the Fe-containing ILs can be recovered easily. In conclusion, imidazolium-based Fe-containing ionic liquids in the presence of imidazolium chlorides could be used as effective and recoverable oxidants for the dissolution of UO2.

Influence of Chelating Agents on Biogenic Uraninite Reoxidation by Fe(III) (Hydr)oxides

Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO(2(S)), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO(2) may be susceptible to reoxidation by environmental factors, with Fe(III) (hydr)oxides playing a significant role. Little is known about the role that organic compounds such as Fe(III) chelators play in the stability of reduced U. Here, we investigate the impact of citrate, DFB, EDTA, and NTA on biogenic UO(2) reoxidation with ferrihydrite, goethite, and hematite. Experiments were conducted in anaerobic batch systems in PIPES buffer (10 mM, pH 7) with bicarbonate for approximately 80 days. Results showed EDTA accelerated UO(2) reoxidation the most at an initial rate of 9.5 μM day(-1) with ferrihydrite, 8.6 μM day(-1) with goethite, and 8.8 μM day(-1) with hematite. NTA accelerated UO(2) reoxidation with ferrihydrite at a rate of 4.8 μM day(-1); rates were less with goethite and hematite (0.66 and 0.71 μM day(-1), respectively). Citrate increased UO(2) reoxidation with ferrihydrite at a rate of 1.8 μM day(-1), but did not increase the extent of reaction with goethite or hematite, with no reoxidation in this case. In all cases, bicarbonate increased the rate and extent of UO(2) reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO(2) reoxidation occurred when the chelator promoted both UO(2) and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO(2) dissolution did not occur, UO(2) reoxidation likely proceeded through an aqueous Fe(III) intermediate with lower reoxidation rates observed. Reaction modeling suggests that strong Fe(II) chelators promote reoxidation whereas strong Fe(III) chelators impede it. These results indicate that chelators found in U contaminated sites may play a significant role in mobilizing U, potentially affecting bioremediation efforts.

Inhibition of radiation induced dissolution of UO2 by sulfide – A comparison with the hydrogen effect

In this work we have studied the influence of H2S on radiation induced dissolution of spent nuclear fuel using simple model systems. The reaction between H2O2 and H2S/HS− has been studied experimentally as well as the effect of H2S/HS− on γ-radiation induced dissolution of a UO2 pellet. The experiments clearly show that the reaction of H2O2 and H2S/HS− is fairly rapid and that H2O2 and H2S/HS− stoichiometry is favorable for inhibition. Radiolysis experiments show that H2S/HS− can effectively protect UO2 from oxidative dissolution. The effect depends on sulfide concentration in combination with dose rate. Autoclave experiments were also conducted to study the role of H2S/HS− in the reduction of U(VI) in the presence and absence of H2 and Pd particles in anoxic aqueous solution. The aqueous solutions were pressurized with H2 or N2 and two different concentrations of H2S/HS− were used in the presence and absence of Pd. No catalytic effect of Pd on the U(VI) reduction by H2S/HS− could be found in N2 atmosphere. U(VI) reduction was found to be proportional to H2S/HS− concentration in H2 and N2 atmosphere. It is clearly shown the Pd catalyzed H2 effect is more powerful than the effect of H2S/HS−. H2S/HS− poisoning of the Pd catalyst is not observed under the present conditions.

Oxidative dissolution of UO2 in a simulated groundwater containing synthetic nanocrystalline mackinawite

The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, and nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(IV) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH=7, PO2=0.02atm, and PCO2=0.05atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for about 51h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mössbauer, and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of rm=1.2±0.4×10−8molg−1s−1, higher than rm=5.4±0.3×10−9molg−1s−1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.

Uraninite oxidation and dissolution induced by manganese oxide: A redox reaction between two insoluble minerals

The longevity of subsurface U(IV) produced by reduction of U(VI) during in situ bioremediation can be limited by reoxidation to more mobile U(VI) species. Coupling of the biogeochemical cycles of U and Mn may affect the fate and transport of uranium. Manganese oxides can act as a powerful oxidant that accelerates the oxidative dissolution of UO2. This study investigated the physical and chemical factors controlling the interaction between UO2 and MnO2, which are both poorly soluble minerals. A multi-chamber reactor with a permeable membrane was used to eliminate direct contact of the two minerals while still allowing transport of aqueous species. The oxidation of UO2 was not significantly enhanced by MnO2 if the two solids were physically separated. Complete mixing of MnO2 with UO2 led to a much greater extent and rate of U oxidation. When direct contact is not possible, the reaction slowly progresses through release of soluble U(IV) with its adsorption and oxidation on MnO2. Continuously-stirred tank reactors (CSTRs) were used to quantify the steady-state rates of UO2 dissolution induced by MnO2. MnO2 dramatically promoted UO2 dissolution, but the degree of promotion leveled off once the MnO2:UO2 ratio exceeded a critical value. Substantial amounts of U(VI) and Mn(II) were retained on MnO2 surfaces. The total production of Mn(II) was less than that of U(VI), indicating that the fate of Mn products and their impact on UO2–MnO2 reaction kinetics were complicated and may involve formation of Mn(III) phases. At higher dissolved inorganic carbon concentrations, UO2 oxidation by MnO2 was faster and less U(VI) was adsorbed to MnO2. Such an inverse relationship suggested that U(VI) may passivate MnO2 surfaces. A conceptual model was developed to describe the oxidation rate of UO2 by MnO2. This model is potentially applicable to a broad range of water chemistry conditions and is relevant to other environmental redox processes involving two poorly soluble minerals.

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.

Effect of diffusive transport limitations on UO2 dissolution

The effects of diffusive transport limitations on the dissolution of UO2 were investigated using an artificial groundwater prepared to simulate the conditions at the Old Rifle aquifer site in Colorado, USA. Controlled batch, continuously-stirred tank (CSTR), and plug flow reactors were used to study UO2 dissolution in the absence and presence of diffusive limitations exerted by permeable sample cells. The net rate of uranium release following oxidative UO2 dissolution obtained from diffusion-limited batch experiments was ten times lower than that obtained for UO2 dissolution with no permeable sample cells. The release rate of uranium to bulk solution from UO2 contained in permeable sample cells under advective flow conditions was more than 100 times lower than that obtained from CSTR experiments without diffusive limitations. A 1-dimensional transport model was developed that could successfully simulate diffusion-limited release of U following oxidative UO2 dissolution with the dominant rate-limiting process being the transport of U(VI) out of the cells. Scanning electron microscopy, X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS) characterization of the UO2 solids recovered from batch experiments suggest that oxidative dissolution was more evident in the absence of diffusive limitations. Ca-EXAFS spectra indicate the presence of Ca in the reacted UO2 solids with a coordination environment similar to that of a Ca–O–Si mineral. The findings from this study advance our overall understanding of the coupling of geochemical and transport processes that can lead to differences in dissolution rates measured in the field and in laboratory experiments.

Combined effects of Fe(II) and oxidizing radiolysis products on UO2 and PuO2 dissolution in a system containing solid UO2 and PuO2

The stability of UO2 spent nuclear fuel in an oxygen-free geological repository depends on the absence of oxidizing reaction partners in the near field. This work investigates the reactions between the products of water radiolysis by alpha radiation and Fe(II) an the effect on UO2 dissolution. Solid 238PuO2 powder and UO2 pellet were allowed to react in Fe(II) solution in oxygen-free batch reactor tests and kinetics of the subsequent redox reactions were measured. Depending on the concentration of Fe(II) (tests with 10−5 and 10−4molL−1 were made), the induced redox reactions took place between 20 and 400h. Dissolved uranium concentrations went first through a minimum caused by reduction, followed by a maximum caused by radiolytic oxidation, and eventually reached another minimum, probably due to sorption on precipitated Fe(III). Plutonium concentrations were decreasing steadily after going through a maximum about 70h from the start of the experiments. The results show that in the presence of the strong alpha-radiolytic field induced by the presence of solid 238Pu, the behavior of the system is largely governed by Fe(II) as it controls the H2O2 concentration, reduces U(VI) in solution and drives the Fenton reaction leading to the oxidation of Pu(IV).

Kinetics of oxidation and dissolution of uranium dioxide in aqueous acid solutions

The oxidation and dissolution of UO2 has been studied using electrochemical methods with an UO2 rotating disc electrode in acidic (pH 3) and non-complexing (trifluoromethanesulfonate: 0.1molL−1 NaCF3SO3) media. The effect of the experimental parameters such as scan rate (v) and rotation rate (ω) on the electrochemical signal has been studied. The rotation rate of the electrode does not influence the resulting signal, which indicates that only a charge transfer is involved in the UO2 oxidation kinetic. However, scan rate variations show different reactions involved in the UO2 oxidation. Linear sweep voltammetry and cyclic voltammetry coupled to X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements suggest two successive electrochemical reactions with an exchange of one electron for each of them and the formation of one intermediate species of U(V).

Direct dissolution of intact UO2 pellet and in situ extraction by organic solutions of TBP-HNO3 at atmospheric pressure: role of solvate composition

Recently authors demonstrated direct dissolution of g-level PHWR UO2 fuel pellet fragments and in situ extraction by TBP-HNO3 and TiAP-HNO3 solutions at atmospheric pressures. Extending the work, similar studies were performed on intact unirradiated PHWR UO2 fuel pellets (~15 g U) with varying compositions of organic solvate of tri-n-butyl phosphate (TBP). It was observed that extent of dissolution was a strong function of organic solution composition TBP·(HNO3) x (H2O) y . Complete dissolution of intact UO2 pellet in a reasonable time was observed only in case of a particular solvate composition.