Refinement of the Nepheline Discriminator: Results of a Phase I Study

摘要

The performance of a glass used for immobilization of high-level nuclear waste (HLW) is generally quantified by its resistance to chemical degradation, or durability. The durability of a HLW glass is dependent on its composition. If crystalline phases form within a glass during cooling, the composition of the residual glass network is altered, therefore affecting the durability of the glass. Crystallization of nepheline (NaAlSiO(sub 4)) has been shown to adversely impact the durability of HLW glasses since it removes glass forming species (in this case, Al and Si) from the glass network. The propensity for nepheline crystallization in a HLW glass increases with increasing concentrations of Al(sub 2)O(sub 3) and Na(sub 2)O in the glass. Nepheline crystallization is therefore of concern for processing of HLW at the Defense Waste Processing Facility (DWPF) since the sludge waste streams at the Savannah River Site (SRS) can contain high concentrations of Al(sub 2)O(sub 3) and Na(sub 2)O. Currently, a 'nepheline discriminator' is included as a process control constraint at the DWPF. The nepheline discriminator relates the concentrations of SiO(sub 2), Na(sub 2)O and Al(sub 2)O(sub 3) (as weight percentages in glass) to a critical value of 0.62. The discriminator defines a boundary line on the SiO(sub 2)-Na(sub 2)O-Al(sub 2)O(sub 3) ternary diagram above which (or toward the SiO(sub 2) corner of the ternary) nepheline is not predicted to crystallize in the glass upon quenching or slow cooling.