Properties of Uranium Metal

For physics reasons, uranium in the form of metal rods was extensively employed as fuel for the first generation of nuclear reactors. The requirement for metallic fuel for a natural uranium graphite-moderated reactor is based on the need for a high fuel density and a fuel rod of sufficiently large diameter to reduce the resonance capture to a level where criticality may be achieved. Only uranium in metalhc form has sufficiently high thermal conductivity to permit adequate heat removal from rods of the required diameter. 

Metallic uranium has one of three possible crystal structures in the solid state, depending on temperature. The structures and corresponding temperature ranges are listed in Table 5.2. 

Distortion under temperature cycling is particularly marked when the structure contains grains with a preferred orientation in the [010] direction, as is typical of cold-worked uranium. On heating above about 350°C, stress relaxation takes place mainly by grain boundary flow. On cooling, the lattice is too rigid to allow the process to be reversed, and the stress relaxation which 

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In the sohd state, uranium metal exists in three aHotropic modifications. The transformation temperatures and the enthalpies of transformation are given in Table 5. The thermodynamic properties of uranium metal have been deterrnined with great accuracy and have been discussed (50). 

Table 5. Some Physical Properties of Uranium Metal ...

To take advantage of some recent measurements, the high temperature thermal properties of uranium metal have been reviewed. Figure 60 shows a summary of the heat content data. The various studies are in excellent agreement except in the y region. In the absence of any reason why this difference should exist, the reported values were combined and... 

In 1896, Becquerel discovered that uranium was radioactive (3). Becquerel was studying the duorescence behavior of potassium uranyl sulfate, and observed that a photographic plate had been darkened by exposure to the uranyl salt. Further investigation showed that all uranium minerals and metallic uranium behaved in this same manner, suggesting that this new radioactivity was a property of uranium itself In 1934, Fermi bombarded uranium with neutrons to produce new radioactive elements (4). 

The element was discovered in the pitchblende ores by the German chemist M.S. Klaproth in 1789. He named this new element uranium after the planet Uranus which had just been discovered eight years earlier in 1781. The metal was isolated first in 1841 by Pehgot by reducing the anhydrous chloride with potassium. Its radioactivity was discovered by Henry Becquerel in 1896. Then in the 1930 s and 40 s there were several revolutionary discoveries of nuclear properties of uranium. In 1934, Enrico Fermi and co-workers observed the beta radioactivity of uranium, following neutron bombardment and in 1939, Lise Meitner, Otto Hahn, and Fritz Strassmann discovered fission of uranium nucleus when bombarded with thermal neutrons to produce radioactive iso-... 

Reduction of halides. Table 5.30 lists pertinent properties of substances that might take part in the reduction of the uranium halides UF4 or UCI4. As this table shows, the heat available per mole of uranium produced is much higher than in the reduction of UO2. In addition, the melting point of the halide by-product is much lower than that of CaO and is near that of uranium metal. Consequently, the reaction temperature can be raised enough to melt both the uranium metal and the halide by-product, so that a clean separation between metal and slag can be obtained. 

With macro concentrations of uranium this equilibrium is shifted to the right with the observation of polymers with properties rather different than that of the uranyl ion. At a uranium concentration of approximately 0.001 M, more than 50% of the uranium is polymerized at pH 6, while for uranium concentrations less than 10 M the polymerization is negligible. This condition can be used to advantage trace metal concentrations can be used if one wishes to study the properties of a metal ion at relatively high pH s without... 

The chemical properties of uranium are derived from its electronic structure 92 protons and 92 electrons (the number of neutrons varies from 125 to 149). Six electrons are in its outer shell with an electron configuration of [Rn]7s 5U6d so the two most common valence states are +6 with [Rn]5f° configuration and +4 with [Rn]7s configuration. Trivalent and pentavalent compounds are also known, but their commercial importance is quite negligible. When uranium metal is exposed to air, an oxide layer is formed. Finely divided metal is pyrophoric and can ignite spontaneously in air. The chemistry, production, and reactions of some of the most commercially important uranium compounds, especially those that play a role in the NFC, are discussed later. 

In the assessment of the refining performance of uranium, systematic data has been reported for the chemical properties of uranium complex in various alkali chlorides such as LiCl-RbCl and LiCl-CsCl mixtures [3-5], Information on the coordination circumstance of solute ions is also important since it should be correlated with stability. The polarizing power of electrolyte cations controls the local structure around neodymium trivalent Nd " " as an example of f-elements and the degree of its distortion from octahedral symmetry is correlated with thermodynamic properties of NdClg " complex in molten alkali chlorides [6]. On the other hand, when F coexists with Cr in melts, it is well-known that the coordination circumstances of solute ions are drastically changed because of the formation of fluoro-complexes [7-9]. A small amount of F stabilizes the higher oxidation states of titanium and induces a negative shift in the standard potentials of the Ti(IV)ITi(ni) and Ti(III)ITi(II) couples [7, 8], The shift in redox potentials sometimes causes specific electrochemical behavior, for example, the addition of F to the LiCl-KCl eutectic leads to the disproportionation of americium Am into Am " and Am metal [9],... 

The actinoid series encompasses the fourteen chemical elements with atomic numbers from 90 to 103, thorium (Th) to lawrencium (Lr). The actinoid series derives its name from the group-IIla element actinium (Ac) which can be included in the series for the purpose of comparison. Only Th and uranium (U) occur in usable quantities in nature. The other actinoids are man-made elements. Pure Th is a silvery-white metal which is air-stable and retains its luster for several months. U exhibits three crystallographic modifications as follows a (688°C) —> P (776°C) —> U is a heavy, silvery-white metal. The luster of freshly prepared americium (Am) is white and more silvery than neptunium (Np) or plutonium (Pu) prepared in the same manner. All actinoid elements are radioactive. Table 2.113 sutnmarizes some physical properties of actinoid metals (Th, U and Am). 

Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world s nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. The various nuclear applications of plutonium are well known. 238Pu has been used in the Apollo lunar missions to power seismic and other equipment on the lunar surface. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals. 

The properties of hydrated titanium dioxide as an ion-exchange (qv) medium have been widely studied (51—55). Separations include those of alkaH and alkaline-earth metals, zinc, copper, cobalt, cesium, strontium, and barium. The use of hydrated titanium dioxide to separate uranium from seawater and also for the treatment of radioactive wastes from nuclear-reactor installations has been proposed (56). 

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