The chemistry of uranium in borosilicate glasses

The chemistry of uranium has been investigated in two borosilicate glasses containing titanium that are prototypes of those to be used in the immobilisation of nuclear waste. The U(VI)-U(V)-U(IV) redox equilibria are established in these glasses, but the stability region of U(V) is somewhat less than that in glasses that do not contain titanium. U(IV) is the only uranium redox state found to coexist with Ti(III) in the glasses as the stability regions of U(VI) and Ti(III) do not overlap, while Ti(III) totally reduced U(V) to U(IV). The product of this reduction by mutual interaction is identified as a Ti(IV)-U(IV) entity within the glass microstructure. The presence of titanium also appears to enhance the solubility of U(IV) in the borosilicate melts through the stabilisation of U(IV) within the melt structure. (author)