Clusters uranium oxides

The ions in the tetrahedral sites have been shown by Mossbauer spectroscopy to have an oxidation state of +3. In a way similar to the uranium oxide structure, the distance between the closest octahedral holes and the tetrahedral hole is too short to allow both sites to be occupied simultaneously. In this case, four vacancies on the octahedral sites are created for every interstitial tetrahedral ion, as shown in Figure 6.8. This type of cluster occurs at low values of x. As x increases, larger clusters form in which there are thirteen vacancies and four interstitial ions. This is called a Koch-Cohen cluster (Figure 6.9). 

N. F. Baumann and E. J. Hennelly, Buckling Measurements of I>20 Moderated Clusters of Natural Uranium Oxide Rods, TRANSACTIONS OF THE AMERICAN NUCLEAR SOCIETY, 4, 1 (1961), p. 48. 

CGAR fuel takes the form of a 36-pin cluster of uranium oxide enriched up to 3% surrounded by a graphite sleeve. Each fuel element is 1 m in length and contains around SO kg of uranium. 

In a modern reactor, almost 100 tubes with uranium oxide pellets are assembled into a fuel cluster, which in turn is placed in a casing, known as a fuel rod, prior to insertion into the reactor. A modem reactor contains several hundred of these fuel as-sembhes. Approximately one-fifth of the fuel is replaced each year. Control rods are used to start and stop the reaction and to regulate the power output. They contain substances, e.g. boron carbide, that have the abiUty to absorb neutrons. 

The fuel is uranium dioxide mixed with plutonium dioxide (U02 - Pu02). It is contained in 103 subassemblies, each containing 217 pins, which in turn consist of a stack of sintered oxide pellets, S.S mm in diameter, in stainless steel cladding. The pins are assembled in clusters in a stainless steel outer shell, which also contains the upper and lower fertile blanket pins (depleted uranium oxide) and the upper neutron shielding. 

A nonstandard synthesis of the first uranium cluster containing an isopolyoxo-metalate core was reported recently [310b], Reduction of [Cp UCl2] (Cp7 — 1,2,4-/-Bu3C5H2) with two equivalents of KC8 in THF, followed by an addition of two equivalents of pyridine N-oxide, was conducted in an attempt to produce the organometallic dioxo species [Cp7U02]. However, the cluster compound [Cp (bipy)2][U6013] was isolated instead as the main product (54% yield) from this reaction. 

Allen PG, Conradson SD, Wilson MS, Gottesfeld S, Raistrick ID (1993) Real time structural electrochemistry of platinum clusters using dispersive XAFS. Mat Res Soc Symp Proc 307 (Applications of Synchrotron Radiation Techniques to Materials Science) 51-56 Allen PG, Conradson SD, Wilson MS, Gottesfeld S, Raistrick ID, Valerio J, Lovato M (1995b) Direct observation of surface oxide formation and reduction on platinum clusters by time-resolved X-ray absorption spectroscopy. J. Electroanal Chem 384 99-103 Allen PG, Shuh DK, Bucher JJ, Edelstein NM, Palmer CEA, Silva RJ, Nguyen SN, Marquez LN, Hudson EA (1996b) Determinations of uranium structures by EXAFS schoepite and other U(VT) oxide precipitates. Radiochim Acta 75 47-53... 

Uranium is often associated with molybdenum in its deposits, and several uranium-molybdenum minerals are known. Two of these minerals contain U —mourite and sedovite—which were reported by Kppchenova et al. and Skvortsova et al. to occur in the supergene zone over a uraninite-molybdenite deposit. Sedovite is U (Mo 04)2 and forms the core of sedovite-mourite clusters. Its colour is brown to reddish brown, which indicates that the uranium is in one valence state. Mourite is given as U Mo5 Oi2(OH)io and shows a deep violet colour. The valence states of U and Mo were not determined and the deep colour indicates some electron exchange suggestive of partial oxidation of the U or reduction of the Mo. Both... 

Brthet, J. C., Thuery, R, and Ephritikhine, M. 2005. Unprecedented reduction of the uranyl ion [U02] + into a polyoxo uranium(IV) cluster Synthesis and crystal structure of the first f-element oxide with a M6( i3-0)8 core. Chem. Commun. 3415-3417. 

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