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Abstract  

The two step oxidation of UO2+x and reduction of U3O8 powders observed during Differential Thermal Analysis (DTA) has been exploited to determine their Specific Surface Areas (SSAs). The results obtained by this method have been compared with the Braunauer, Emmett and Teller (BET) method and are found to be in good agreement in the SSA range of 2–4 m2/gm in the case of UO2+x obtained from ADU route and 4–8 m2/gm in the case of AUC route. A precision of ±0.1 m2/gm is obtained. The maximum temperature of oxidation and reduction of these oxides are dependent upon their preparative routes such as Ammonium Diuranate (ADU) and Ammonium Uranyl Carbonate (AUC).

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Abstract  

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.

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Abstract  

The effects of iron on uranium oxidation states during sample dissolution were studied. A mineral acid mixture in anaerobic conditions was used for the dissolution the sample and the uranium oxidation states were determined by ion exchange. The first experiments were performed with pure iron chloride compounds. In the second stage, study was made of common iron-containing minerals. Uranium oxidation states were affected when the content of iron compound was as low as 10-5M. In the case of the natural minerals, pyrite caused uranium to change to an increasingly reduced state, whereas goethite caused it to change to an increasingly oxidized state as the amount of mineral was increased. The interferences of the silicates fell between those of pyrite and goethite. The results indicate that a wide range of common bulk rocks with less than 20 wt% of iron-containing minerals can be reliable chemically analyzed for uranium oxidation state.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: S. Ravi, S. Ravi, A. K. Deepa, A. K. Deepa, S. Susheela, S. Susheela, P. V. Achuthan, P. V. Achuthan, S. Anil Kumar, and U. Jambunathan

Summary  

A method has been developed for the estimation of 90Sr in reprocessed uranium oxide samples obtained from the Purex processing of natural uranium spent fuel discharged from the research reactor. The method employs a combination of precipitation and solvent extraction procedure to eliminate other beta-impurities prior to resorting to the estimation of 90Sr by beta-counting. 106Ru was eliminated by volatalizing with perchloric acid, uranium was removed by carrier precipitation with strontium as sulphate. The sulphate precipitate was converted to carbonate and dissolved in nitric acid. 234Th and 234Pa were eliminated by synergistic solvent extraction using tri-n-butyl phosphate and thenoyl trifluoroacetone extractant mixture in xylene. An iron scavenging step was included to remove any residual impurities. Finally, strontium is precipitated as SrC2O4 . H2O. The separated 90Sr activity was followed to check the equilibrium growth of 90Y.

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Abstract  

Identifying both physical and chemical characteristics of Special Nuclear Material (SNM) production processes is the corner stone of nuclear forensics. Typically, processing markers are based on measuring an interdicted sample’s bulk chemical properties, such as the elemental or isotopic composition, or focusing on the chemical and physical morphology of only a few particles. Therefore, it is imperative that known SNM processes be fully characterized from bulk to trace level for each particle size range. This report outlines a series of particle size measurements and fractionation techniques that can be applied to a bulk SNM powders, categorizing both chemical and physical properties in discrete particle size fractions. This will be demonstrated by characterizing the process signatures of a series of different depleted uranium oxides prepared at increasing firing temperatures (350–1100 °C). Results will demonstrate how each oxides’ material density, particle size distribution, and morphology varies.

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Abstract  

A new method for determining the O/U ratio in uranium oxide is described. The method consists of dissolving an accurately weighed sample in a known excess of ceric ammonium nitrate in 2N perchloric acid at room temperature and estimating the unreacted Ce/IV/ with benzohydroxamic acid. A standard deviation of 0.0016 for ten estimations is observed.

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Abstract  

This work studied a way to reclaim uranium from contaminated UO2 oxide scraps as a sinterable UO2 powder for UO2 fuel pellet fabrication, which included a dissolution of the uranium oxide scraps in a carbonate solution with hydrogen peroxide and a UO4 precipitation step. Dissolution characteristics of reduced and oxidized uranium oxides were evaluated in a carbonate solution with hydrogen peroxide, and the UO4 precipitation were confirmed by acidification of uranyl peroxo–carbonate complex solution. An agglomerated UO4 powder obtained by the dissolution and precipitation of uranium in the carbonate solution could not be pulverized into fine UO2 powder by the OREOX process, because of submicron-sized individual UO4 particles forming the agglomerated UO4 precipitate. The UO2 powder prepared from the UO4 precipitate could meet the UO2 powder specifications for UO2 fuel pellet fabrication by a series of steps such as dehydration of UO4 precipitate, reduction, and milling. The sinterability of the reclaimed UO2 powder for fuel pellet fabrication was improved by adding virgin UO2 powder in the reclaimed UO2 powder. A process to reclaim the contaminated uranium scraps as UO2 fuel powder using a carbonate solution was finally suggested.

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Abstract  

Interferences in uranium oxidation states due to iron during acid dissolution of solid samples were studied. Hydrazine and different carboxylic acids were investigated for their suitability to minimize these interferences. Polyacrylic acid (PAA) was selected for further testing with common iron-containing minerals, pyrite and biotite. The interfering effect of iron decreased with increasing PAA concentration; 2.5% acid concentration was observed to be a suitable level to minimize interferences due to the iron-containing minerals. PAA and polyacrylic-co-maleic acid (PAMA) were tested for their effect on goethite. The oxidation of uranium by goethite was not supressed with the acids but further a slight oxidation was observed. Further experiments are required to find a suitable redox buffer for goethite and iron minerals. However, the promising results for pyrite and biotite widen the range of solid materials that can be considered suitable for the determination of uranium oxidation states.

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Abstract  

Laser desorption ionization (LDI) mode of matrix-assisted laser desorptionionization time-of-flight mass spectrometry (MALDI-TOFMS) analysis of uranium(VI)leads to the formation of uranium oxides clusters, as with fast atom bombardment(FAB). Different uranium clusters than those with FAB were observed. Threedifferent families of formula (UO2)x Oy 2+, and two of formula (UO2)x Oy 2+ were found.

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Abstract  

Preparation of thin film deposits of lanthanide, thorium and uranium oxides has been studied by chemical vapor deposition (CVD) method using -diketonate metal chelates with 2,2,6,6-tetramethyl-3,5-heptanedione and some reactant gases as starting materials. The deposition process was carried out using a special apparatus designed for the CVD method at atmospheric pressure and temperatures as low as 400–600°C.As a result, it was demonstrated that each chelate used was well suited for the above purpose by its high volatility and reactivity with the reactant, especially with water vapor.

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