Effect of divalent oxides on the structure of glasses for HLW immobilisation
The mixed alkali borosilicate glass system used for high-level waste immobilisation, has been studied with different divalent additive oxides such as CaO, Srf), BaO, MnO, ZnO and PbO added to the BNFL base glass (MW). A number of techniques have been employed in order to study the effect of these oxide additives. lIB MAS NMR was performed to measure the fraction of tetrahedrally coordinated boron atoms N4 and 29Si MAS NMR to measure the Qn speciation. Durability tests were carried out and compared with electrical conduction behaviour. Densities, glass transition temperatures and thermal expansion coefficients are also reported. The change in N4 with concentration has been determined for different additive oxides. The addition of BaO, CaO and srO to the base (MW) BNFL vitrification glass caused N4 to increase with alkaline earth content, and maximum values were attained at compositions depending on the M2+ ion type. In contrast, the addition of divalent oxides such as ZnO and PbO to MW reduced the N4 values, with the rate of decrease being greater for ZnO than PbO. The 29Si NMR studies have shown the presence of significant concentrations of non-bridging oxygens which can influence the aqueous corrosion behaviour of the glass. The durability tests revealed that the alkaline earth oxides BaO, srO and CaO decrease the durability. In contrast, oxides such as ZnO and PbO improve the corrosion resistance. The presence of a surface Mn7Si012 crystallisation phase was identified on corroded MnO containing glasses. The reduction in electrical conductivity with oxide addition shows a similar trend to the corrosion behaviour for chemically resistant glasses. In contrast, the opposite trend is found for less durable glasses. UV visible transmission of MnO doped glass shows a decrease in transmittance at - 380 run. The absorption band centred at - 49Onm, moves towards longer wavelength, caused by generation ofNBOs on the [Si04] tetrahedral unit.