Glass products for the vitrification of HLLW in a joule-heated ceramic melter and the effects of platinoids on the glass properties

A vitrification process was developed at KfK/INE for the solidification of HLW-solutions from the nuclear fuel cycle as borosilicate glasses. To optimize melter operation the glass melt should have a flat viscosity curve and a relatively high specific electrical resistance of > 6 Ω · cm at 1,150 C. Further requirements are: no liquid-liquid immiscibility and no crystallization of the glass, waste loading > 15 wt.% and with a view to final storage of the HLW glass a chemical durability comparable to other HLW glasses. Experimental work was geared to adjusting the viscosity and the electrical resistance to the required values by varying the chemical composition of the glass. By use of mixed-alkali glasses containing Li2O and Na2O, glasses can be prepared which meet the required properties. In the technical vitrification process the platinoids Ru, Rh and Pd form precipitates and accumulate on the bottom of the melter. Dense sediments are formed with a lower resistance and a higher viscosity than those of the rest of the glass melt. Glass samples with about 16 wt.% platinoids have a very low resistance of only 0.1 Ω · cm and show a non-Newtonian, pseudoplastic flow behavior