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1.
Mechanochemical effects in U3O8   总被引:1,自引:0,他引:1  
The effect of the mechanical treatment of U3O8 in a planetary ball mill in air or as a suspension in benzene solution of thyolilthreefluoroacetone (TTA) on the nature of the oxide and on the leaching of U and 234Th into diluted aqueous solutions of HCl, Na2EDTA and NaCl has been studied. Transformation of U3O8 to UO2, is much stronger expressed when the mechanoactivation is performed in air is established. The leaching behavior of U and Th depends significantly on the activation mode and on the leaching reagent nature. The role of mechanochemically enhanced UO2-ThO2 solid solution formation for the observed effects is discussed.  相似文献   

2.
The effect of the mechanoactivation on UO3 and U3O8 in agate or stainless steel vessels in air or in toluene is studied. UO2(OH)2 is the main product of UO3·H2O activation in steel vessel in air. The presence of toluene leads to strong amorphization and dispersity increase and, probably, to the formation of U2O5. The activation of U3O8 leads to its reduction to U3O7 which relative content in the reaction mixture depends on the mechanoactivation conditions.  相似文献   

3.
A feasibility and basic study to find a possibility to develop such a process for recovering U alone from spent fuel by using the methods of an oxidative leaching and a precipitation of U in high alkaline carbonate media was newly suggested with the characteristics of a highly enhanced proliferation-resistance and more environmental friendliness. This study has focused on the examination of an oxidative leaching of uranium from SIMFUEL powders contained 16 elements (U, Ce, Gd, La, Nd, Pr, Sm, Eu, Y, Mo, Pd, Ru, Zr, Ba, Sr, and Te) using a Na2CO3 solution with hydrogen peroxide. U3O8 was dissolved more rapidly than UO2 in a carbonate solution. However, in the presence of H2O2, we can find out that the leaching rates of the reduced SIMFUEL powder are faster than the oxidized SIMFUEL powder. In carbonate solutions with hydrogen peroxide, uranium oxides were dissolved in the form of uranyl peroxo-carbonato complexes. UO2(O2) x (CO3) y 2−2x−2y , where x/y has 1/2, 2/1.  相似文献   

4.
For the disposal of a high efficiency particulate air (HEPA) glass filter into the environment, the glass fiber should be leached to lower its radioactive concentration to the clearance level. To derive an optimum method for the removal of uranium series from a HEPA glass fiber, five methods were applied in this study. That is, chemical leaching by a 4.0?M HNO3?C0.1?M Ce(IV) solution, chemical leaching by a 5 wt% NaOH solution, chemical leaching by a 0.5?M H2O2?C1.0?M Na2CO3 solution, chemical consecutive chemical leaching by a 4.0?M HNO3 solution, and repeated chemical leaching by a 4.0?M HNO3 solution were used to remove the uranium series. The residual radioactivity concentrations of 238U, 235U, 226Ra, and 234Th in glass after leaching for 5?h by the 4.0?M HNO3?C0.1?M Ce(IV) solution were 2.1, 0.3, 1.1, and 1.2?Bq/g. The residual radioactivity concentrations of 238U, 235U, 226Ra, and 234Th in glass after leaching for 36?h by 4.0?M HNO3?C0.1?M Ce(IV) solution were 76.9, 3.4, 63.7, and 71.9?Bq/g. The residual radioactivity concentrations of 238U, 235U, 226Ra, and 234Th in glass after leaching for 8?h by a 0.5?M H2O2?C1.0?M Na2CO3 solution were 8.9, 0.0, 1.91, and 6.4?Bq/g. The residual radioactivity concentrations of 238U, 235U, 226Ra, and 234Th in glass after consecutive leaching for 8?h by the 4.0?M HNO3 solution were 2.08, 0.12, 1.55, and 2.0?Bq/g. The residual radioactivity concentrations of 238U, 235U, 226Ra, and 234Th in glass after three repetitions of leaching for 3?h by the 4.0?M HNO3 solution were 0.02, 0.02, 0.29, and 0.26?Bq/g. Meanwhile, the removal efficiencies of 238U, 235U, 226Ra, and 234Th from the waste solution after its precipitation?Cfiltration treatment with NaOH and alum for reuse of the 4.0?M HNO3 waste solution were 100, 100, 93.3, and 100%.  相似文献   

5.
The effect of the oxidation temperature of sintered UO2 pellets on the powder properties of U3O8 was studied in the temperature range 250–900 °C in air. The U3O8 was obtained at 450 °C after 180 min and its particle size and surface area are respectively, 35 µm and 0.7 m2/g. The reduction of the U3O8 powder resulted in UO2 after 30 min with a surface area of 0.8 m2/g. This value was improved more than 3.5 times by applying five alternating oxidation–reduction cycles.  相似文献   

6.
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.  相似文献   

7.
The oxidation of UO2 was investigated by TG, DSC and X-ray diffraction . UO2 samples were prepared by the reduction of UO3 at PH2 + PN2 = 100 + 50 mm Hg and 5°C min?1 up to 800°C. In order to obtain six UO2 samples with different preparative histories, UNH, UAH and ADU were used as starting materials and their thermal decomposition was carried out at 450–625°C for 0–9 h at an air flow rate of 100 ml min?1. α-UO3, γ-UO3, UO3 - 2 H2O, and their mixtures were obtained. The reduction of UO3 gave β-UO2+x with different x values from 0.030 to 0.055. The oxidation carried out at PO2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO2 - U3O7 (Step I) and U3O7 → U3O8 (Step II). TG and DSC curves of the oxidation showed two plateaus and two exothermic peaks corresponding to Steps I and II. In the case of two of the samples, the DSC peak of Step II split into two substeps, which were assumed to be due to the different reactivities of U3O- formed from α-CO3 and that from other types of UO3. The increase in O/U ratio due to the oxygen uptake at room temperature changed from 0.010 to 0.042 except for a sample prepared from ADU which showed an extraordinarily large value of 0.445. TG curves showed an increase in O/U from room temperature to near 250°C for Step I and the plateau at 250–350°C where O/U was about 2.42, and showed a sharp increase in O/U above 350°C for Step II and the plateau above 100°C where O/U was 2.72–2.75. It is thought that the prepared UO2 had a defective structure with a large interstitial volume to accommodate the excess oxygen.  相似文献   

8.
The nature of the chemical bond in UO2 was analyzed taking into account the X-ray photoelectron spectroscopy (XPS) structure parameters of the valence and core electrons, as well as the relativistic discrete variation electronic structure calculation results for this oxide. The ionic/covalent nature of the chemical bond was determined for the UO8 (D4h) cluster, reflecting uranium's close environment in UO2, and the U13O56 and U63O216 clusters, reflecting the bulk of solid uranium dioxide. The bar graph of the theoretical valence band (from 0 to ~35 eV) of XPS spectrum was built such that it was in satisfactory agreement with the experimental spectrum of a UO2 single crystalline thin film. It was shown that unlike the crystal field theory results, the covalence effects in UO2 are significant due to the strong overlap of the U 6p and U 5f atomic orbitals with the ligand orbitals, in addition to the U 6d atomic orbital (AO). A quantitative molecular orbital (MO) scheme for UO2 was built. The contribution of the MO electrons to the chemical bond covalence component was evaluated on the basis of the bond population values. It was found that the electrons of inner valence molecular orbitals (IVMO) weaken the chemical bond formed by the electrons of outer valence molecular orbitals (OVMO) by 32% in UO8 and by 25% in U63O216.  相似文献   

9.
It was established that heating to 90 °C of nitrate solutions of U, Np and Pu in the presence of hydrazine hydrate results in the formation of hydrated dioxides of these elements. On ignition under inert or reducing conditions in the temperature range of 280–800 °C hydrated uranium dioxide transmogrify into crystalline UO2. On ignition in air atmosphere UO2·nH2O turns into UO3 at 440 °C and into U3O8 at 570–800 °C. It was shown that thermolysis of the solution containing a mixture of uranium, neptunium and plutonium nitrates at 90 °C in the presence of hydrazine hydrate allows one to prepare hydrated dioxides (U, Np, Pu)O2·nH2O which on heating to ~300 °C transmogrify into crystalline product of UO2, NpO2 and PuO2 solid solution. The technique of preparation of solid solutions of U and Pu dioxides is very promising as simple and effective method of production of MOX-fuel for.  相似文献   

10.
Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO2 and doped UO2 [(U, M)O2; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U4O9, U3O8) are shown against oxygen partial pressure. From the results of doped UO2, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po2 (x: deviation from the stoichiometric composition and Po2: oxygen partial pressure) and log vs. log Po2 (: electrical conductivity) relations. The defect structures of UO2 and doped UO2 are derived based on the Willis model for UO2+x. The detect structure of U4O9 phase is similar to that of UO2+x, but the defect structures of U3O8 phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO2 doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U4O9 and U3O8 clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.  相似文献   

11.
The thermal decomposition of (UO2)3(PO4)2 and U(HPO4)2 ·xH2O in the temperature range 25–1600?, was investigated. (UO2)3(PO4)2 decomposed first to 1/3[U3O8 + 3U2O3P2O7] and then to U3O5P2O7 before a loss of phosphorus was observed above 1350?. Decomposition in air and in inert atmospheres was nearly identical. Reduction with H2 or with carbon black in argon gave U3O5P2O7 and [UO2 + + (UO)2P2O7] before pure UO2 was formed. U(HPO4)2 ·xH2O decomposed to UP2O7 in argon. It oxidized partly in air before the same product was obtained. The high temperature stability of UP2O7 and U3(PO4)4 was also investigated.  相似文献   

12.
In severe nuclear accident scenarios (in air environments and high temperatures) UO2 fuel pellets oxidise to produce uranium oxides with higher oxygen content, e.g., U4O9 or U3O8. As a first step in investigating the microstructural changes following UO2 oxidation to hexagonal high temperature phase of U3O8, density functional quantum mechanical calculations of the structure, elastic properties and electronic structure of U3O8 have been performed. The calculated properties of hexagonal phase of U3O8 are compared to those of the orthorhombic pseudo-hexagonal phase which is stable at room temperature. The total energy technique based on the local density approximation plus Hubbard U as implemented in the CASTEP code is used to investigate changes in the lattice constants. The first-principles calculations predict a 35–42% increase in volume per uranium atom as a result of the transformation from UO2 to U3O8, in agreement with experimental data. The implications of this prediction on the linear expansion and fragmentation of fuel are discussed.  相似文献   

13.
Thermal decomposition of U(C2O4)2·6H2O was studied using TG method in nitrogen, air, and oxygen atmospheres. The decomposition proceeded in five stages. The first three stages were dehydration reactions and corresponded to removal of four, one, and one mole water, respectively. Anhydrous salt decomposed to oxide products in two stages. The decomposition products in nitrogen atmosphere were different from those in air and oxygen atmospheres. In nitrogen atmosphere UO1.5(CO3)0.5 was the first product and U2O5 was the second product, while these in air and oxygen atmospheres were UO(CO3) and UO3, respectively. The second decomposition products were not stable and converted to stable oxides (nitrogen: UO2, air–oxygen: U3O8). The kinetics of each reaction was investigated with using Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods. These methods were combined with modeling equations for thermodynamic functions, the effective models were investigated and thermodynamic values were calculated.  相似文献   

14.
Chemiluminescence (CL) accompanying the reaction of U4+ with O2 in 0.0004–0.1M HClO4 was studied. It was found that the electron-excited uranyl ion (UO2 2+)* is the CL emitter. The fact that the reaction rate and the CL yield increase as the solution acidity decreases was explained by different reactivities of the U aq 4+ aquation and the products of its stepwise hydrolysis, UOH3+ and U(OH)2 2+, toward O2. Based on the results of analysis of the chain-radical mechanism of the reaction between U4+ and O2, it was concluded that transfer of an electron from the UO2 + ion to the oxidizing agent (a ·OH radical) is the most plausible elementary step of the reaction of (UO2 2+)* formation. It was found that the reaction rate, as well as the CL yield, increase substantially in the presence of uranyl ion. Catalytic action of UO2 2+ was explained by the formation of a UO2 2+·UO2 + complex, which reduces the rate of the UO2 + disproportionation reaction (UO2 + is an intermediate of the reaction and is involved in chain propagation), and by regeneration of the active center, UO2 +, in the reaction of UO2 2+ with U4+. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1522–1528, September, 2000.  相似文献   

15.
For 11 years now, the structural diversity and aesthetic beauty of uranyl–peroxide capsules have fascinated researchers from the diverse fields of mineralogy, polyoxometalate chemistry, and nuclear fuel technologies. There is still much to be learned about the mechanisms of the self‐assembly process, and the role of solution parameters including pH, alkali template, temperature, time, and others. Here we have exploited the high solubility of the UO22+/H2O2/LiOH aqueous system to address the effect of the hydroxide concentration. Important techniques of this study are single‐crystal X‐ray diffraction, small‐angle X‐ray scattering, and Raman spectroscopy. Three key phases dominate the solution speciation as a function of time and the LiOH/UO22+ ratio: the uranyl–triperoxide monomer [UO2(O2)3]4?and the two capsules [(UO2)(O2)(OH)]2424?(U24) and [(UO2)(O2)1.5]2828?(U28). When the LiOH/U ratio is around three, U28 forms rapidly and this cluster can be isolated in high yield and purity. This result was most surprising and challenges the hypothesis that alkali templating is the most important determinant in the cluster geometry. Moreover, analogous experiments with KOH, NH4OH, and TEAOH (TEA=tetraethylammonium) also rapidly yield U28, which suggests that U28 is the kinetically favored species. Complete mapping of the pH–time phase space reveals only a narrow window of the U28 dominance, which is why it was previously overlooked as an important kinetic species in this chemical system, as well as others with different counterions.  相似文献   

16.
The applicability of a gravimetric method based on alkaline earth metal addition for the determination of oxygen in ternary uranium oxides of the type M—U—O (M=La, Ce and Th) is described. The oxide sample is mixed with MgO or Ba2.8UO5.8 and heated in air under suitable conditions. Because uranium is completely oxidized to the hexavalent state during the reaction, oxygen can be determined from the weight change. Oxygen in LayU1-y O2+x is determined up to y = 0.8 with a standard deviation for x of ±0.006 with MgO. For ThyU1-y O2+x, the value of x is determined with Ba2.8UO5.8 with a standard deviation of ±0.01 at y = 0.8. For CeyU1-y O2+x, the method can be applied only for low cerium concentrations where y = 0–0.2; the value for x with Ba2.8UO5.8 at y = 0.2 showed a standard deviation of ±0.002.  相似文献   

17.
Chronic exposure to uranium compounds led to the development of a methodology in order to characterize those compounds. This methodology, based on the recommendation of the I.C.R.P,1 involves three main steps: the measurement of the uranium concentration and the particle size distribution at workstations; the characterization of the industrial compound, i.e. its physico-chemical properties; and the study of in-vitro solubility using chemical and cellular tests. Different methods for uranium analysis are presented. Results and comments on UF4, UO3, U3O8, UO2 and U+UO2 are given.  相似文献   

18.
Zusammenfassung Es werden die Resultate des isothermen Zerfalles und der Reduktion vonstandardisiertem Ammoniumpolyuranat im Bereich von 285 bis 463° C (in Wasserstoff) wiedergegeben. Der Zerfall zu UO3 wurde schon bei einer Temp. unter 290° C festgestellt, diese Phase blieb jedoch darauf stabil bis 320° C. Zwischen 320° C und 380° C verläuft die Reduktion zu U3O8, über 380° C aber zu UO2. Die Aktivierungsenergien bei der Reduktion von UO3 zu U3O8 und von U3O8 zu UO2 wurden berechnet, und zwar 32,2 kcal/g-mol und 41,7 kcal/g-mol. Die Ergebnisse können mit den Literaturangaben für die Reduktion der einzelnen Phasen UO3 und U3O8 verglichen werden. Die beobachteten Unterschiede weisen auf den Einfluß der Aktivität der Präparate hin.
The isothermal decomposition and reduction of ammonium polyuranate (ADU) was investigated in the temperature interval 285–463° C in hydrogen. The formation of UO3 was noticed below 290° C and this product was stable up to 320° C. U3O8 was stable from this temperature on up to 380° C, where the reduction to UO2 was observed. The activation energies 32,2 Kcal/mole and 41.7 Kcal/mole were calculated for the reduction of UO3 to U3O8 and for the reduction of U3O8 to UO2, respectively. The results are comparable with the published data on reduction of separate phases UO3 and U3O8. Some differences noticed show the influence of the activities of the products.


Mit 4 Abbildungen  相似文献   

19.
Summary The heteropoly anions [UIVMo12O42]8–(UMo12), [UIVW10O36]8– (UW10), [UIV(PW11O39)2]10 [U(PW11)2] and [UIV(SiW11O39)2]12 [U(SiW11)2] were examined by cyclic voltammetry on a wax-impregnated carbon electrode. Reversible one-electron oxidations were observed for UMo12 (E = +0.91 V vs see at pH = ca. 0), U(PW11 )2 (E = +0.60 V at pH 4.4) and U(SiW11)2 (E = +0.19 V at pH 4.4). No oxidation of UW10 was detected at potentials prior to oxygen discharge (ca. +0.9 V at pH 7). Controlled potential oxidation of aqueous solutions of UMo12 gave unstable solutions of [UVMo12O42]7–. Oxidation of U(PW11)2 was achieved in aqueous and nonaqueous (acetonitrile, propylene carbonate) solution. The electronic spectra of UVMo12 and UV(PW11)2 are reported and are discussed in terms of UO12 (/y) and UO8 (D4d) chromophores respectively. Possibilities for geometrical and optical isomers of U(XW 11)2 anions are considered. Solutions of brucinium salts of U(PWI I)2 and UW10 in dimethyl formamide show induced Cotton effects at wavelengths corresponding to the f-f transitions of UIV. The voltammograms of UMo12, ThMo12 and CeMo12 show an irreversible twelve-electron reduction at -0.25 V. The pale brown reduced solutions cannot be reoxidized to the original heteropoly anions.Taken from the Ph. D. Thesis of S.C.T., Georgetown University, 1977. Presented in part at the 17th International Conference on Coordination Chemistry, Hamburg, September 1976.  相似文献   

20.
The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U38, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO2 from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U38 occurred from t=4 h at the expense of UO2, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a UIV/UVI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a UIV/UVI ratio of 25/75, when the precipitation of UO2 occurs. Thorough SEM observations of the U38 crystallites showed that the UO2 aggregates are embedded within. This may indicate that UO2 acts as reservoir of uranium(IV), for the formation of U38, stabilized by benzoate and THF ligands. During the early stages of the U38 crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U12), based on the inner hexanuclear core of {U6O8} type, decorated by three additional pairs of dinuclear U2 units. The U12 motif is stabilized by benzoate, oxalates, and glycolate ligands.  相似文献   

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