Chemical shifts (ChSh) of nine emission lines of the uranium L-series in uranium oxides UO2+x (x=0-1) with respect to UO2 were studied by using a precise crystal-diffraction X-ray spectrometer. ChSh of Lα1,α2 uranium and thorium lines in solid solutions yUO2·(1-y )ThO2 (y=0:1) were measured with respect to UO2 and ThO2, respectively. The changes in energy of spin-orbital splitting (SOS) — Δδnl± of inner nl-orbitals of the uranium atom were calculated from the data of ChSh of spin-doublet lines. For UO2+x oxides, a linear decrease in Δδnl± values with increasing degree of uranium oxidation was found. Sign inversion of Δδnl± for uranium levels was found on passing to solid solutions. No change in the SOS energy of inner thorium levels was detected. The values of Δδnl± were found to correlate with the experimental values of the effective magnetic moment of uranium in oxides. On the basis of the comparison of experimental Δδnl± values with Dirac–Hartree–Fock atomic calculations, it was concluded that the observed variations in Δδnl± values are due to the redistribution of electron and spin density between the 5f7/2- and 5f5/2-levels of the fine structure of the uranium atom without changes in atomic charge state. On the basis of the hypothesis of intraatomic relativistic U5f7/2↔U5f5/2 transition, a model of paramagnetic moment formation on the uranium atom in uranium dioxide was proposed.
Radiochimica Acta, Volume 92, Issue 2/2004, Pages 73–80, ISSN (Print) 0033-8230, DOI: 10.1524/ract.92.2.73.27465, September 2009

Thermodynamic modeling of phase equilibria in BeO-UO2 system has been performed. The assessed state diagram has been plotted in the framework of subregular solutions theory. The activities of the components have been calculated at the temperature 2873 K.