Uranium IV oxide

Synonyms uranium oxide uranic oxide urania uranium(IV) oxide... 

The reaction with oxygen in contrast to that with nitrous oxide doesn t lead to the formation of oxoalkoxides, due probably to the intermediate formation of peroxocomplexes in this case. The reaction products are then alkoxides of uranium (IV) and uranium (IV) oxide ... 

Uranium carbide, 0559 Uranium(III) nitride, 4727 Uranium(IV) oxide, 4837 Zinc, 4921... 

Uranium(VI) dioxydichloride, 5 148 Uranium (VI) hydrogen dioxyortho-phosphate 4-hydrate, 6 150 analysis of, 5 151 Uranium(IV) oxalate, 3 166 Uranium (IV) oxide, formation of, by uranyl chloride, 6 149 Uranium (IV) (VI) oxide, U3Oa, formation of, by uranyl chloride, 5 149... 

Leaching uranium ores requires an acid and an oxidant in order to convert the insoluble uranium(IV) oxide to a soluble uranium(VI) salt. Iron(III) is the usual oxidant (Figure 6.16). 

Pellets of U-235 enriched uranium)IV) oxide powder are manufactured using the usual dry pressing process at... 

Interest in the so-called sol-gel process for the remote-controlled manufacture of plutonium-containing fuel rods is increasing due to its high safety. In this process a filter cake of freshly precipitated uranium(IV) oxide is converted ultrasonically into a U02-gel, which after drying is fired at 1150°C. The resulting microspheres, 40 to 60 t,m in diameter, are then poured into casing tubes using vibratory techniques. 

Coated spherical Th02- or U02-particles are increasingly utilized in the fuel of gas-cooled high temperature reactors. Their 50 to 1500 pm core of uranium(IV) oxide is manufactured using conventional sintering techniques. This is then pyrolytically coated with many layers of carbon and silicon carbide (see Section 5.7.5.1). 

The most important fuel for currently operated nuclear power stations (mainly light-water reactors) is - U-enriched uranium(IV) oxide. Also of importance are metallic uranium for the Magnox reactors and a few research reactors and uranium-plutonium mixed oxides for light-water reactors. Fuel production comprises extraction and dressing of uranium ores to uranium concentrates, conversion into UF, the uranium compound used for enrichment of the BSy.jjjotope, enrichment of and production of fuel from enriched UF5 (reconversion). 

The uranium in primary pitch blend is mainly present as uranium(IV) oxide and must first be oxidized to hexavalent uranium. This is most easily achieved with the Fe- " ions, which come from the ore itself ... 

Two processes are employed for the production of uranium(VI) fluoride, namely the wet and dry processes. In both processes uranium(IV) oxide and uranium(lV) fluoride are formed as intermediates. In the wet process the uranium(IV) oxide is produced from the uranium concentrate by way of uranyl nitrate, whereas in the dry process the uranium concentrate is directly reduced to uranium(IV) oxide. The methods of purification used are also different in the wet process the purification proceeds at the uranyl nitrate stage, by solvent extraction, whereas in the dry process the end product uranium hexafluoride is itself distillatively purified. 

In the production of uranium(lV) oxide in the wet process, the uranium concentrate is first converted into a uranyl nitrate solution with nitric acid. After the purification of the uranyl nitrate by solvent extraction, it can be converted into uranium(IV) oxide by two different routes either by thermal denitration to uranium(VI) oxide which is then reduced to uranium(IV) oxide or by conversion of uranyl nitrate into ammonium diuranate which is reduced to uranium(IV) oxide. Purification proceeds by extraction of the uranyl nitrate hydrate from the acidic solution with tri-n-butylphosphate in kerosene and stripping this organic phase with water, whereupon uranium goes into the aqueous phase. 

The temperature must not exceed 400°C, to prevent the formation of U3O8. The nitrous gases produced are processed to nitrie aeid, whieh is recycled. The subsequent reduction of uranium(VI) oxide to uranium(IV) oxide with hydrogen at 500°C also proceeds in the fluidized bed furnace. 

The second route to uranium(IV) oxide eonsists of precipitation of ammonium diuranate from the solvent extraetion-purified aqueous uranyl nitrate solution by adding ammonia and then reducing it with hydrogen to uranium(lV) oxide (Comurhex process developed in France). 

Uranium(IV) oxide is the starting material for uranium(lV) fluoride production in which uranium(lV) oxide is generally reacted with anhydrous hydrogen fluoride. This difficult to carry out exothermic reaction proceeds either in a fluidized bed, in moving bed reactors, or in screw-reactors. To achieve as complete as possible reaction in fluidized bed reactors, two fluidized bed reactors are connected in series. Screw-reaetors are also preferably connected in series. In moving bed reactors the reduction zone and the hydrofluorination are arranged above one another in a plant. The uranium(IV) oxide produced by the reduction of uranium(VI) oxide with hydrogen is very reactive and is eompletely reaeted with HF at temperatures between 500 and 650°C to uranium(lV) fluoride. 

In the dry process, introduced by Allied Chemical Corp., the uranium concentrate is pelletized and directly reduced with hydrogen to uranium(IV) oxide at temperatures between 540 and 650°C in a fluidized bed reactor. Hydrofluorination to uranium(IV) fluoride proceeds in two fluidized bed reactors connected in series. After fluorinating the uranium(IV) fluoride formed in a production unit consisting of a flame-reactor and a fluidized bed reactor, the uranium(Vl) fluoride produced is purified in a two stage pressure distillation process. This distillative purification process is necessary, because, in contrast with the wet process, no purification is carried out in earlier stages. 

There are three processes which are industrially convert enriched uranium(VI) fluoride into sinterable uranium(IV) oxide two wet processes and one dry process. 

Ammonium diuranate (ADU) process This process was developed in the USA in the 1950 s and is currently still the most important process. However, the raw uranium(IV) oxide produced with this process contains up to 2% by weight of fluoride and hence requires aftertreatment before it is suitable for pressing into fuel pellets. This disadvantage is not shared by the other two processes. New reconversion plants do not, therefore, in the main, utilize the ADU-process. 

The precipitate is largely freed of fluoride ions after filtration by extraction or recrystallization. After drying at 200°C, the ammonium diuranate is reductively decomposed by a H2/H2O mixture at ca 500°C to UyOg, which is then reduced with hydrogen at 500 to 800°C to uranium(IV) oxide. The reductive decomposition to U Og and its reduction can be carried out in a single step e.g. in a rotary kiln. Since the uranium(IV) oxide formed can be pyrophoric, it is weakly reoxidized. 

The IDR (Integrated Dry Route) process consists of reacting gaseous uranium(VI) fluoride with superheated steam, whereupon solid UO2F2 is formed, which is reduced with hydrogen to uranium(IV) oxide. This reaction can be carried out in both a fluidized bed reactor and in a rotary kiln, whereby the latter appears more suitable. 

The uranium(IV) oxide produced by the above-described processes is used for the manufacture of uranium(IV) oxide sintered pellets. The uranium(IV) oxide is ground, pressed in e.g. hydraulic presses, then sintered at ca. 1700°C in the presence of hydrogen and thereby shrink to the desired... 

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