Uranium carbides formation

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 ... 

Uranium Hexafluoride, Uranic Fluoride, UFg, is the only known compound of hexavalent uranium (with the possible exception of the boride in which the condition of the uranium is not established) which does not contain oxygen. It was first prepared by Ruff and Heinzel-mann by the action of fluorine on uranium pentachloride at —40° C. The action proceed.s as already described (see equation above), and the volatile hexafluoride is distilled off from the tetrafiuoride. The penta-ehloride, Avhen acted upon by dry hydrogen fluoride, yields a compound, UF5.a HF, which breaks up on distillation into the tetra- and hexafluorides, but this method of preparation is less convenient than the preceding one owing to the difficulty of separating the hexafluoride from hydrogen fluoride. Uranium carbide reacts with fluorine in presence of a little chlorine at —70° C., vith formation of the hexafluoride. 

Nitrogen combines directly with uranium at a temperature of 1000° 1 nitrogen or ammonia reacts with the carbide, yielding a nitride dry ammonia also produces a nitride when it reacts with uranium tetrachloride. The formula of the nitride is usually U N . The catalytic influence of uranium carbide in the manufacture of ammonia by the Haber process is attributed to the formation of the nitride. 

Possible leductants. Elements that might be considered for reducing UO2, UF4, or UCI4 to metallic uranium are hydrogen, sodium, magnesium, or calcium. Carbon is impractical because of formation of uranium carbide, and aluminum is undesirable because it forms an intermetallic compound with uranium. Sodium, magnesium, and calcium do not do this. 

Surface oxidation behaviour of uranium carbide (UC) in air was studied. It is seen that formation of uranium oxide/hydroxide and free carbon can happen as a consequence of the reaction between UC and oxygen/moisture. It is noted that the sutl ce of the oxidised UC consists of a top contamination layer of adsorbed oxygen/moisture, followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The bahaviour of sintered and arc-melted samples is found to be similar. Thermal conductivity of mixed oxide fuel (45%U - 55%Pu) was measured using laser flash technique covering a temperature range from 700 to 1550 K. 

Another promising uranium compound that can be used in nuclear fuels is uranium carbide that has a high melting point and better thermal conductivity than the oxide and in addition does not form oxygen when radiolyzed. Uranium nitride can also be used, but formation of from N could be problematic. In addition, other uranium compounds that can be used as a fuel in a nuclear reactor, ranging from aqueous solutions to molten salts that are brought to a high temperature in order to keep them in a molten state. MOX of uranium and plutonium also serve as a nuclear fuel in some reactors. 

The limitations on possible linear rating and burn-up levels imposed by the characteristics of metallic uranium have led to the development of ceramic forms of fuel such as uranium oxide (UO2) and uranium carbide (UC) pellets. While fuel performance is considerably enhanced by the use of the ceramic form, problems arise due to densification of the fuel and fuelcladding interaction in a radiation environment, leading to the formation of interpellet gaps and clad flattening. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

The most effective catalyst for accelerating the velocity of formation of ammonia was found to be osmium but it is too scarce for commercial work. Next came uranium, which, in the form of carbide, crumbles to a fine powder under the conditions, and then at 500° has a high catalytic activity provided water be absent. 

Uranium and plutonium carbide fuels have also been investigated and their dissolution in nitric acid results in the formation of organic acids as well as In the case of (Uo.gPuo.2)C... 

EXPLOSION and FIRE CONCERNS combustible solid NFPA rating (NA) reacts to form explosive products with metal amides contact with acids may cause formation of poisonous hydrogen selenide gas incompatible or reacts violently with barium carbide, bromine pen-tafluoride, chromic oxide, fluorine, lithium carbide, lithium silicon, metals, nickel, sodium, nitric acid, nitrogen trichloride, oxygen, potassium, potassium bromate, rubidium carbide, zinc, silver bromate, uranium, strontium carbide, and thorium carbide toxic gases and vapors may be released in a fire involving selenium, sodium selenite, sodium selenate, and selenium dioxide use water for firefighting purposes. 

The free energies of formation of some chromium carbides and ternary carbides of uranium with Cr, Mo and W were measured by the methane hydrogen equilibration technique developed earlier. 

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