Development of glasses for the vitrification of high level liquid waste (HLLW) in a joule heated ceramic melter

A vitrification process was developed at Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgungstechnik (INE), for solidifying in borosilicate glasses High Level Waste (HLW) solutions from the nuclear fuel cycle. To optimise melter operation the glass melt should have a flat viscosity curve and a relatively high specific electrical resistance of ≤6.5 ω•cm at 1150°C. Further requirements are: no liquid-liquid immiscibility and no crystallization of the glass, waste loading ≥15 wt% and, in view of repository storage of the HLW glass, a chemical durability comparable to that of other HLW glasses. The main emphasis of experimental work was put on finding out how the viscosity, the slope of the viscosity curve, the specific electric resistance and the chemical durability depend on the chemical composition of the glasses. Especially, the effect of the mixed alkalis Li and Na on the glass properties was studied. It was found that by increasing from 0 to 1 the Li2O/(Na2O + Li2O) molar ratio of the glass FRIT WAW, the viscosity of the melt decreases roughly linearly and the slope of the viscosity curve decreases as well. The specific electric resistance passes through a maximum and the Soxhlet leach rate through a minimum at an alkali ratio of about 0.5. As a final result, a range of optimum glass compositions was determined which meet the required properties.