Technetium is a potentially hazardous radionuclide in the waste vitrification process as its oxides are volatile. Electrochemical measurements, i.e. potentiometry and voltammetry are applied to investigate the redox reactions of Tc in the glass melt. Re is studied for comparison. Under normal oxidizing conditions Tc is in the -T-state, and substancial losses of Tc occur as a result of compound volatility. Reducing conditions lead to the formation of TcO2, which precipitates from the melt. Stronger reducing conditions lead to the formation of metallic Tc

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Oxygen diffuses in vitreous false(normalvfalse)‐SiO2 as network O atoms and O2 molecules. The latter process is also responsible for the growth of SiO2 films on silicon during thermal oxidation. Since the distance between solubility sites of O2 molecules is ∼2.6 nm, the molecules either diffuse along randomly oriented structural channels resembling the ordering along the c‐axis in tridymite crystal, or they cover this distance by several random jumps of ∼0.26 nm (the O‐O distance). In the latter case, the incorporation of an O2 molecule at a jump site affects the structure of normalv‐SiO2 so that no further solution of O2 can occur within a volume element of ∼10−20 cm3 around it. This process is associated with structural ordering as indicated by the unusually large negative entropy term. Analysis of reported results on the growth of SiO2 films on silicon in 18O2 ambient shows that the diffusivity of O2 is about three times larger than in fused normalv‐SiO2 . From the observed concentration profile of 18O atoms we imply that the diffusivity of network oxygens is about three orders of magnitude less than in fused normalv‐SiO2 ; this may be due to the very low H2O contamination level and/or increased structural ordering in the SiO2 film. This effect is another aspect of the influence of oxygen diffusion upon the structure of normalv‐SiO2 . Under these conditions the O2 and network oxygen diffusion processes are not related to each other in the manner they are in fused normalv‐SiO2 . It is suggested that increased structural ordering in SiO2 films on silicon involves preferential alignment of structural channels more or less perpendicular to the Si surface. It is also suggested that the structural channels consist of rings haying 7–8 SiO4/2 tetrahedra; these channels cannot be considered as pores which are gross imperfections.

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