The chemistry of uranium in borosilicate glasses

The basic chemistry of uranium has been investigated in some simple borosilicate glasses whose only multivalent element is uranium. The redox chemistry of uranium in these glass-forming melts can be described by U(VI)-U(V)-U(IV) equilibria which are strong functions of melt temperature and imposed oxygen fugacity but are not strong functions of base composition (for those studied) and total uranium concentration. The conditions necessary to stabilise U(III) in the glasses were also determined. The assigned coordination sites of U(VI) as UO22+ species, U(V) in sixfold coordination with slight tetragonal distortion, U(IV) in eightfold coordination, and U(III) in sixfold coordination in borosilicate glasses were inferred from spectroscopy from the visible to the near infrared. The solubility of uranium in a typical borosilicate was dependent on the U(IV) concentration, the U(IV) saturation at 11500C being about 9 wt%, about twice that of a typical aluminosilicate. U(VI) was over four times more soluble than U(IV) in these glasses. The information collected in this study sets a basis for the chemistry of uranium in glasses relevant to the immobilisation of nuclear waste. (author)