Toshinori Okura
Kogakuin University, Japan
Title: Immobilization of simulated high level nuclear waste with magnesium-zinc-phosphate glasses
Biography
Biography: Toshinori Okura
Abstract
The disposal of radioactive waste generated by the nuclear fuel cycle is among the most pressing and potentially costly environmental problems. The high level nuclear wastes (HLW) are immobilized in a stable solid state and completely isolated from the biosphere. Nuclear waste glasses are typically borosilicate glasses, and these glass compositions can experience phase separation at elevated concentrations of P2O5. For some waste streams, this can require considerable dilution and a substantial increase in the volume of the waste glass produced. Magnesium and Zinc phosphate glasses are classified as ‘anomalous phosphate glasses’, which exhibit anomalies in the relationship between physical properties, such as density and refractive index, and MO/P2O5 (M/P) (M=Mg, Zn) molar ratio around the metaphosphate composition (M/P=1). Most of the phosphate glasses form high polyphosphate consisting of chains of phosphate ions, while the structures of M-P glasses and Z-P glasses are of 2 types, one includes 4 membered rings of PO4 tetrahedra at M/P<1 (type T) and the other contains dimers of PO4 tetrahedra at M/P>1 (type P). In this study, M-P, Z-P, M-Z-P glasses are chosen as the base glass. Simulated HLW was incorporated into the base glass to study its effects on the leaching behavior of M-P, Z-P, M-Z-P glasses for nuclear waste immobilization. The gross leach rates and the leach rates of each constituent element of the sample in water at 90°C were determined from the total weight loss of the specimen and chemical analysis of leachate solution.