Impurities, uranium oxide

Action of chlorine on uranium oxide to recover volatile uranium chloride Removal of iron oxide impurity from titanium oxide by volatilization hy action of chlorine... 

The major use for zirconium is in the nuclear industry. Zirconium alloys (zircaloys) are used extensively as a cladding for nuclear (uranium oxide) fuel rods in water cooled reactors. Zircaloys were favoured over stainless steel cladding because they had a considerably lower neutron cross-section, appropriate thermal conductivity and both corrosion and mechanical resistance. As indicated, hafnium is an impurity in nearly all zirconium ores. Hafnium, however, has a much higher neutron cross-section than zirconium and, as such, the two elements must be separated prior to using zirconium in fuel rod cladding. For many years the separation was very difficult due to the chemical similarity of the two elements. Zirconium hydride is used as a moderator in nuclear reactors. 

A large variety of secondary uranium minerals are known, many are brilliantly colored and fluorescent. The commonest are gummite (a general term like limonite for mixtures of various secondary hydrated uranium oxides with impurities) hydrated uranium phosphates of the phosphuranylite type, including autunite (with calcium), saleeite (magnesium), and torbernite (with copper) and hydrated uranium silicates such as cof-finite, uranophane (with calcium), and sklodowskite (magnesium). 

Satyanarayana, K. and Durani, S. (2010). Separation and indnetively eonpled plasma optical emission spectrometric (ICP-OES) of trace impurities in nuclear grade uranium oxide, J. Radioanal. Nucl. Chem. 285, 659-665. 

Crain, J. S., and Gallimore, D. L. (1992). Determination of trace impurities in uranium oxides by laser ablation inductively coupled plasma mass spectrometry.J./lna/./lt. Spectrom. 7(4), 605-610. 

The analyses reported above assumed that the UO3, D2O, and graphite were pure with no contaminants that would impact the neutronic performance or the plutonium production. The ASTM standards provide guidelines on impurity levels for uranyl nitrate and uranium oxides that are readily achievable with known chemical processes. The impurity levels in these standards would have no impact on the neutronic performance of any conceived reactor model Similarly, very pure graphite is needed because normal graphite contains minute amounts of boron. One of the boron isotopes ( °B) has an extremely high neutron-capture cross section. 

Emission spectrography is used to determine the content of impurities in samples of uranium and plutonium oxide powders and pellets. 

Plutonium purification proceeds by reducing the aqueous phase pH that oxidizes the plutonium to Pu" +, which then extracts into the TBP phase. Impurities stay in the aqueous phase. The TBP phase strip-ping/extraction cycle is repeated to complete the plutonium purification. The uranium is purified using the same TBP/nitric acid extraction/stripping cycle. Careful control of the each element s oxidation state in the extraction cascade produces the plant-scale separations of uranium from plutonium of 10 . Fission product decontamination factor was 10. The plutonium and uranium recovery is about 99.9% with 95% of the nitric acid values and 99.7 /o of the organic solvent recycled. ... 

The first step in the conventional process for refining manium is dissolution in nitric acid. When the concentrates have been produced by chemical leaching and are in the form of diuranates, dissolution proceeds rapidly and leaves little solid residue. When the concentrates have been separated mechanically and are in the form of the original uranium mineral, dissolution may require more concentrated acid, higher temperatures, longer times, and addition of oxidants such as MnO. Also, filtration to remove undissolved residues is usually required. In either case, dissolution produces an aqueous solution of uranyl nitrate hexahydrate U02(N03)2 6H2 0, containing excess nitric acid and variable amounts of nitrates of metallic impurities present in the concentrates. 

Production of uranium metal suffidently pure for use in nuclear reactors is difficult. Uranium forms very stable compounds with oxygen, nitrogen, and carbon, and it reduces the oxides of many common refractories. Methods that yield uranium metal at temperatures below its melting point result in a fine powder that oxidizes rapidly in air and is difficult to consolidate into massive metal. Uranium cannot be deposited electrolytically from aqueous solution. It is not practical to purify uranitun by distillation because of its very high boiling point, 3900° C. Any nonvolatile impurities introduced into uranium during production will remain in it during subsequent operations and contaminate the final product. 

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