Uranium dioxide production

Uranium Dioxide Production. The majority of the world s nuclear reactors are fueled with slightly enriched UOg prepared in the form of dense sintered pellets that are encapsulated in small bore tubes of zirconium alloy or stainless steel. Hex at the required enrichment(s) is produced specifically for a given reactor charge and the first process step with the UFg is to convert it to UOg having the desired ceramic-grade quality. This means an oxide which after granulation and pelleting can be sintered quickly and uniformly to pellets of near stoichiometric density. 

Uranium dioxide fuel is irradiated in a reactor for periods of one to two years to produce fission energy. Upon removal, the used or spent fuel contains a large inventory of fission products. These are largely contained in the oxide matrix and the sealed fuel tubing. 

The main technological uses for UO2 are found in the nuclear fuel cycle as the principal component for light and heavy water reactor fuels. Uranium dioxide is also a starting material for the synthesis of UF [10049-14-6] 6 (both critical for the production of pure uranium metal and... 

Since the uranium from the milling process is still in an unusable form, the yellow cake is broken down once again. The uranium trioxide is reduced to uranium dioxide at veiy high temperatures. Refining of the product also takes place. Now the uranium product consists almost entirely of UO,. 

Uranium tarnishes readily in the atmosphere at room temperature. Electropolishing inhibits the process whilst etching in nitric acid activates the surface. Uranium dioxide and hydrated UO3 are the principal solid products. 

The solid corrosion products in carbon dioxide and carbon monoxide are uranium dioxide, uranium carbides and carbon. The major reaction with carbon dioxide results in the formation of carbon monoxide ... 

Another, more modern, route of processing the yellow cake is shown in Figure 5.38, accomplishes the production of enriched uranium oxide entirely by pyroprocessing. Thus, uranium is finally obtained in three forms metallic uranium, enriched uranium dioxide, and natural uranium dioxide. As the flowsheet shows, and as briefly described herein, these are essentially the products of hydro and pyro-based processing schemes. 

Table 4 summarizes the geographical behavior of fission products at the Oklo reactors compared with the behavior of fission products from neutron-irradiated uranium dioxide. The reactor zones RZ 1-9 are exposed on the ground, while RZ 10-16 are situated below the ground, all quite deep except RZ 15. 

Table 4 Comparison of the characteristic behavior of fission products in the Oklo ores with those in irradiated uranium dioxide...

Since the water movement will be very slow compared with the rate at which the wastes dissolve, we are concerned first and foremost with equilibrium solubility. Also, if only to relate behaviour on the geological time scale to that on the laboratory time scale, we will need to know about the mechanisms and kinetics of dissolution and leaching. The waste forms envisaged at present are glass blocks containing separated fission products and residual actinides fused into the glass and, alternatively, the uranium dioxide matrix of the used fuel containing unseparated fission products and plutonium. In the... 

Uranium dioxide occurs in mineral uraninite. Purified oxide may be obtained from uraninite after purification. The commercial material, however, also is recovered from other uranium sources. Uranium dioxide is obtained as an intermediate during production of uranium metal (See Uranium). Uranyl nitrate, U02(N03)2, obtained from digesting the mineral uraninite or pitchblende with concentrated nitric acid and separated by solvent extraction, is reduced with hydrogen at high temperatures to yield the dioxide. 

Childs, B. G. 1963. Fission product effects in uranium dioxide. Journal of Nuclear Materials, 9, 217 -244. 

The dehydrogenation of ethylbenzene is an important process used for styrene manufacture, and uranium oxide catalysts have been inveshgated for this reaction. A catalyst of uranium dioxide supported on alumina showed high selectivity to styrene of 96% at high conversion [62, 63]. The catalyst was synthesized as a higher oxide of uranium and inihally it was not UO2. Consequently, over the initial onstream period only carbon dioxide and water were observed, as the catalyst produced total oxidahon products. However, as the reachon proceeded the uranium oxide was reduced in situ by the ethylbenzene and hydrogen to form the active UO2 phase. It was only when the uranium oxide was fully reduced to UO2 that styrene was produced with high selectivity. 

The results from a series of flames at different temperatures are analyzed by Equation 5 in Figure 2 where the function kT In (Ue/AT ) is plotted against rie. The intercept (3.6 eV) agrees well with the literature value for uranium dioxide, and the slope gives a value of 3.3 X 10 m for the product n a. The sprayer had been calibrated with cesium, and it was known thus that the total number density of uranium atoms in the flame was 1.6 X 10 m" which corresponds to a relative volume (47rnpfl /3) of 6.5 X 10" of UO2 (density 10.97 g cm ). This leads to separate values for a (the mean particle radius = 6.8 X 10 m) and (particle number density = 4.8 X 10 m ) and to the number of charges per particle (rie/rip = 7.2). 

The results of the test showed that the uranium dioxide produced a vigorous reaction with the nitrate melt, beginning at approximately 400°C. At this time the melt was open to the atmosphere, and no nitric acid was being added. Nitrogen dioxide was evolved as a gaseous product of the reaction, but there was no visual indication of any uranium solubility in the molten nitrate eutectic. Complete reaction, indicated by the cessation of gas evolution, required approximately 2.5 hours at 500°C. 

The product of this reaction appeared to have smaller particles than the original uranium dioxide, which settled rapidly to the bottom of the reaction tube. These particles were recovered by aqueous dissolution of the solidified eutectic and filtration of the insoluble uranium-containing species. No hydrolysis of the insoluble uranium-containing species is believed to occur with aqueous dissolution of the salt matrix. To preclude hydrolysis in experiments where it was more likely, the nitrate salts were dissolved in 0.5 M HNO3. 

Empirical formulas for the products from each melt system were obtained we assumed that oxygen was the other constituent of the compounds. The elemental ratios are subject to some variation because of uncertainties in the analytical data. The uranium analyses are estimated to be valid within 2%. Independent analytical determinations have shown that the original uranium dioxide contained approximately 0.5% iron, plus a trace of silica. Adjustment of the analytical data for these minor impurities was not done. 

The behavior of uranium dioxide, containing the addition of the synthetic fission-product mixture, was investigated in equimolar NaNC -KNC. The mass ratio of the fission product mixture to uranium dioxide was 1 10 the mass ratio of uranium dioxide to total alkali metal nitrate was 1 100. A complete experimental cycle was conducted, testing the reaction with the... 

Redox Reactions. The initial chemical reaction is oxidation of uranium dioxide. In the equimolar sodium-potassium nitrate system the product is sodium diuranate. The following reactions are believed to be the only valid oxidation reactions that are possible. 

Pyrochemical processes have the potential for low waste volume, but only if materials are recycled. No major problems are foreseen for recycle of the greatest bulk component, sodium nitrate. Regeneration will be required, but the presence of a considerable amount of nitrite is not a problem since nitrite also oxidizes uranium dioxide. Removal of the highly soluble fission products, such as cesium and iodine, will eventually require either a separation step or a bleed-off of the nitrate stream. 

Non-aqueous Process. A halide volatility process has been extensively studied among the dry reprocessing processes. The chloride distillation using carbon tetrachloride has been studied in applying to the treatment of irradiated uranium dioxide (32). In a proposed flow-sheet, chlorination and distillation processes are followed by the sorption and desorption process of uranium chloride on a barium chloride bed. Fundamental data of decontamination for the fission products have been accumulated, showing that excellent purification of uranium is achieved. 

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