Uranyl Peroxides Research Articles

Effect of temperature on studtite stability: Thermogravimetry and differential scanning calorimetry investigations

The main objective of this work is the study of the influence of temperature on the stability of the uranyl peroxide tetrahydrate (UO2O2·4H2O) studtite, which may form on the spent nuclear fuel surface as a secondary solid phase. Preliminary results on the synthesis of studtite in the laboratory at different temperatures have shown that the solid phases formed when mixing hydrogen peroxide and uranyl nitrate depends on temperature. Studtite is obtained at 298K, meta-studtite (UO2O2·2H2O) at 373K, and meta-schoepite (UO3·nH2O, with n<2) at 423K. Because of the temperature effect on the stability of uranyl peroxides, a thermogravimetric (TG) study of studtite has been performed. The main results obtained are that three transformations occur depending on temperature. At 403K, studtite transforms to meta-studtite, at 504K, meta-studtite transforms to meta-schoepite, and, finally, at 840K, meta-schoepite transforms to U3O8. By means of the differential scanning calorimetry the molar enthalpies of the transformations occurring at 403 and 504K have been determined to be −42±10 and −46±2kJmol−1, respectively.

Expanding the Crystal Chemistry of Uranyl Peroxides: Synthesis and Structures of Di‐ and Triperoxodioxouranium(VI) Complexes.

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Expanding the Crystal Chemistry of Uranyl Peroxides: Synthesis and Structures of Di- and Triperoxodioxouranium(VI) Complexes

Four compounds containing tri- and diperoxodioxouranium(VI) complexes have been synthesized under ambient conditions and structurally characterized. The crystal structures of Na4(UO2)(O2)3(H2O)12 (monoclinic, P21/c, a=6.7883(6) A, b=16.001(2) A, c=16.562(2) A, beta=91.917(2) degrees, V=1797.9(3) A3, Z=4) and Ca2(UO2)(O2)3(H2O)9 (orthorhombic, Pbcn, a=9.576(3) A, b=12.172(3) A, c=12.314(2) A, V=1435.4(6) A3, Z=4) contain clusters of triperoxodioxouranium(VI). These clusters are bonded through a network of H bonding to H2O groups and in the Ca compound by bonds to Ca2+ cations. In the crystal structure of Na2Rb4(UO2)2(O2)5(H2O)14 (orthorhombic, Pbcm, a=6.808(2) A, b=16.888(6) A, c=23.286(8) A, V=2677.5(16) A3, Z=4), triperoxodioxouranium(VI) polyhedra share a peroxide edge, forming dimers of polyhedra of composition (UO2)2(O2)5(6-). Adjacent dimers are linked through bonding to Rb+ cations and by H bonds to H2O groups. The crystal structure of K6[(UO2)(O2)2(OH)]2(H2O)7 (orthorhombic, Pcca, a=15.078(8) A, b=6.669(4) A, c=23.526(13) A, V=2366(2) A3, Z=4) contains diperoxodioxouranium(VI) polyhedra that include two OH groups. These polyhedra share an OH-OH edge, forming dimers of composition (UO2)2(O2)4(OH)2(6-). The dimers are linked by bonds to K+ cations and by H bonding to H2O groups.

Ab initiostudy of uranyl peroxides: Electronic factors behind the phase stability

<i>Ab initio</i>study of uranyl peroxides: Electronic factors behind the phase stability

Affects of Hydrogen Peroxide on the Stability of Becquerelite

AbstractWhile the majority of studies of alteration of UO2 and commercial spent nuclear fuel under simulated geological repository conditions have emphasized the importance of uranyl oxide hydrates and uranyl silicates, the influence of peroxide on repository performance has been largely overlooked. There is considerable evidence that uranyl peroxides will be important alteration phases of nuclear waste, and that these phases may impact the long-term performance of a geologic repository such as Yucca Mountain. Here we report the thermodynamics and kinetics of becquerelite, Ca[(UO2)6O4(OH)6](H2O)8, in the presence of solutions containing hydrogen peroxide. Thermodynamic calculations reveal that in solutions containing 3.5 × 10-6 M hydrogen peroxide, studtite is thermodynamically favorable over becquerelite at 298 K. To access the kinetics of this reaction, batch experiments were conducted by the reaction of becquerelite and solutions containing hydrogen peroxide. In the presence of 0.1 M hydrogen peroxide, becquerelite altered to studtite within eight hours.

Presence and Persistence of Uranyl Peroxide Nanoclusters in Contact with Geological Media

AbstractRecently uranyl peroxide nanoclusters containing 24, 28, and 32 uranyl polyhedra were chemically and structurally characterized under alkaline conditions. Such nanoclusters could conceivable form from oxidative alteration of nuclear waste in a geological repository by incorporating peroxide formed by alpha-radiolysis of water or in tanks where high-level waste is stored. The stability and persistence of uranyl peroxides in the vadose zone will be impacted by their interaction with geological media. Here we report batch experiments of solutions containing monodisperse nanoclusters in contact with crushed welded tuff. Within the first 72 hours, U concentrations in solution remained unchanged; however concentrations of Si, Al, Ca, Mg, Na, and Fe increased due to the dissolution of calcium-bearing aluminum silicate minerals in the welded tuff. Despite the presence of excess Li+ in solution, within two weeks crystals precipitated in which Ca2+ replaced Li+ in the nanocluster cage.