Structures uranium oxides

CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

The crystal structures of a number of diphosphine disulphides (121) and (122) show a remarkable constancy in the bond lengths. Two types of molecule are observed in the crystal of the tetramethyl compound (121, X = Y = Me). The crystal structure of triphenylphosphine oxide (P—C 176 pm, P—O 164 pm) varies little from that observed in the uranium oxide complexes, and does not confirm P—O bond lengthening in complexes, as indicated by vp=.o (see Section 3C). 

Uranium oxides are of importance in the nuclear industry, and for this reason considerable effort has been put into understanding their nonstoichiometric behavior. The dioxide, U02 crystallizes with the fluorite structure with an ideal composition MX2 (Fig. 4.7a) but is readily prepared in an oxygen-rich form. In this state it is... 

Thermal decomposition of Pa20F8 (217) at 220-290°C yields a white solid, Pa02F, which also decomposes to Pa3OvF, which is also white, at 560-580°C. Thermal decomposition of U2OF8 does not follow the same path as that of Pa20F8, and UO2F is not formed (215). However, uranium oxide phases close to this composition have been reported, and their structures have been shown to contain O-U-O chain structures (219). 

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

Structure of Uranium Oxides 541 Table 13.1 Crystallographic data for the known uranium oxides [2],... 

T1 ecent investigations have shown that chromium, manganese, cobalt, nickel, copper, and zinc oxides react with uranium oxides at elevated temperatures to form double oxides with the formulas MUO4 and MU3O10. Table I lists eight compounds for which some structural and thermal stability information has been reported. 

There are a number of structurally interesting mixed-ligand uranyl hydroxides. For example, the basic compound of composition Zn(U02)2S04(00)4 1.5020, has a structure based on chains of 1102(011)302 pentagonal bipyramids containing tridentate bridging OH groups. Species of this type have also been studied in solution, but the complexity of the system has precluded structural characterization. There are many hydrated binary and ternary uranium oxides, such as the uraninites, that contain uranyl hydroxide complexes within their structure. 

Continued interest in the uranyl tellurite system stems from the fact that these compounds defy normal topological trends in uranium oxides. The expectation is that extended structures containing uranyl cations will be layered, and contain U(VI) in the form of a UOy pentagonal bipyramid with an approximately linear U02, uranyl, core [56, 57]. However this is simply not the case for uranyl tellurites. For example, K[U02Te205(0H)], P-Tl2[U02(Te03)2] and... 

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). 

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