Corrosion of uranium

For many years the corrosion of uranium has been of major interest in atomic energy programmes. The environments of importance are mainly those which could come into contact with the metal at high temperatures during the malfunction of reactors, viz. water, carbon dioxide, carbon monoxide, air and steam. In all instances the corrosion is favoured by large free energy and heat terms for the formation of uranium oxides. The major use of the metal in reactors cooled by carbon dioxide has resulted in considerable emphasis on the behaviour in this gas and to a lesser extent in carbon monoxide and air. 

Broczkowski ME, Noel JJ, Shoesmith DW. (2005) The inhibiting effects of hydrogen on the corrosion of uranium dioxide under nuclear waste disposal conditions. J Nucl Mater 346 16-23. 

Pipes, valves, fittings, heat exchangers, or magnetic, electrostatic, or other collectors made of graphite or coated in graphite, yttrium, or yttrium compounds resistant to the heat and corrosion of uranium vapour. 

Figure 37. Corrosion of uranium in aerated distilled water (26)...

Mr. Allison stressed the necessity of using helium of very high purity to avoid corrosion of uranium by impurities of the circulating helium. He referred in particular to recent experimental results that indicate that considerable amounts of gases are emanated by the graphite on heating after being previously evacuated. 

Properties. Uranium metal is a dense, bright silvery, ductile, and malleable metal. Uranium is highly electropositive, resembling magnesium, and tarnishes rapidly on exposure to air. Even a poHshed surface becomes coated with a dark-colored oxide layer in a short time upon exposure to air. At elevated temperatures, uranium metal reacts with most common metals and refractories. Finely divided uranium reacts, even at room temperature, with all components of the atmosphere except the noble gases. The silvery luster of freshly cleaned uranium metal is rapidly converted first to a golden yellow, and then to a black oxide—nitride film within three to four days. Powdered uranium is usually pyrophoric, an important safety consideration in the machining of uranium parts. The corrosion characteristics of uranium have been discussed in detail (28). 

Uranium hexafluoride [7783-81-5], UF, is an extremely corrosive, colorless, crystalline soHd, which sublimes with ease at room temperature and atmospheric pressure. The complex can be obtained by multiple routes, ie, fluorination of UF [10049-14-6] with F2, oxidation of UF with O2, or fluorination of UO [1344-58-7] by F2. The hexafluoride is monomeric in nature having an octahedral geometry. UF is soluble in H2O, CCl and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The importance of UF in isotopic enrichment and the subsequent apphcations of uranium metal cannot be overstated. The U.S. government has approximately 500,000 t of UF stockpiled for enrichment or quick conversion into nuclear weapons had the need arisen (57). With the change in pohtical tides and the downsizing of the nation s nuclear arsenal, debates over releasing the stockpiles for use in the production of fuel for civiUan nuclear reactors continue. 

The behaviour of uranium has been well characterised for a variety of environments of importance in the nuclear industry. The corrosion is governed by the constitution and physical character of the solid reaction products which in turn are determined mainly by the oxygen potential of the environment, the temperature and the presence of water. The mechanisms of attack are known in broad outline. A major area in need of more detailed study is the influence of irradiation both prior to and during oxidation. 

Extensive work into the corrosion and oxidation of uranium and its alloys has been undertaken over the past decade but much of this is in the form of Ministry and industrial reports which are not necessarily readily available. The present review concentrates on the work published in the normal scientific and technical press. 

Galvanic corrosion reports have emerged from two sources. In the first , the chemical compatibility of uranium carbides and Cr-Fe-Ni alloys was discussed. Evaluation was by thermodynamic modelling and experimental... 

Bourns, W. T., Corrosion Testing of Uranium Silicide Fuel Specimens, AECL-2 718 (1968)... 

There are a number of industrial gas separation systems that use the selective permeability of plastics to separate the constituents. In design problems relating to such applications, the designer must consider the environmental conditions to determine whether the materials having the desired properties will withstand the temperatures and physical and chemical stresses of the application. Frequently the application will call for elevated temperatures and pressures. In the case of uranium separation, the extreme corrosivity of the fluorine compounds precluded the use of any material but PTFE. The PTFE... 

Little was done with this new material until the military, working on the atomic bomb, needed a special material for gaskets that would resist the corrosive gas uranium hexafluoride, which was one of the materials used to make the atomic bomb. General Leslie Groves, responsible for the U.S. Army s part in the atomic bomb project, had learned of DuPont s new inert polymer and had DuPont manufacture it for them. 

Salbu et al. (2003) used micro-XAS to examine oxidation of depleted uranium (DU) munitions. Interestingly, these studies revealed the presence of U02 and U3Os but no U6+ oxide hydrate phases. Brock et al. (2003) examined the corrosion of DU penetrators in an arid environment. Using SEM, they observed aggregates of tabular, hexagonal schoepite and meta-schoepite crystals with clay/silt particles that were coated with amorphous silica. Brock et al. (2003) suggested that as the schoepite/meta-schoepite phases were coated with amorphous silica/clays, further dissolution was inhibited. 

Brock, A. L., Buck, B., Johnson, W. Ulery, A. L. 2003. Corrosion of depleted uranium in an arid environment Characterizaion with XRD, SEM/ EDS, and microprobe analyses. Geological... 

Uranium and Uranium Alloys , in J.J. Burke et al, Eds, Physical Metallurgy of Uranium Alloys , Brookhill Publ Co (1976) also see J.C. Bailer et al, Eds, Comprehensive Inorganic Chemistry , Vol 5, Pergamon Press, Oxford (1973), 40-42 26) LJ. Weirick, Corrosion Testing of the General Electric Mantech Gau 8/A Penetrator, SANDIA 76-8055 (1977) 27) W.C. Hanson... 

Fig. 1. Schematic flowsheet of uranium processing (acid leach and ion exchange) operation. Numbers refer to the numbers that appear in the boxes on the flowsheet. Operations (3), (6), (9), and (11) may be done by thickening or filtration. Most often, thickeners are used, followed by filters. The pH of the leach slurry <4) is elevated to reduce its corrosive effect and to improve the ion-exchange operation on the uranium liquor subsequently separated, In tile ion exchange operation (7), resin contained in closed columns is alternately loaded with uranium and then eluted. The resin adsorbs the complex anions, such as UC fSO 4-. in which the uranium is present in the leach solution. Ammonium nitrate is nsed for elution, obtained by recycling the uranium filtrate liquor after pH adjustment. Iron adsoibed with the uranium is eluted with it. Iron separation operation (8) is needed inasmuch as the iron hydroxide slurry is heavily contaminated with calcium sulfate and coprecipitated uranium salts. Therefore, the slurry is recycled to the watering stage (3). Washed solids from 1,6). the waste barren liquor from (7), and the uranium filtrate from (11) are combined. The pH is elevated to 7.5 by adding lime slurry before the mixture is pumped to the tailings disposal area. (Rio Algom Mines Limited, Toronto)...

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