Investigation of physical and chemical properties for upgraded SAP (SiO2[sbnd]Al2O3[sbnd]P2O5) waste form to immobilize radioactive waste salt

Abstract

Metal chloride waste generated from pyroprocessing is considered as one of the more problematic wastes in radioactive waste immobilization for the reprocessing of used nuclear fuel. This waste cannot be directly vitrified using the conventional approach using a borosilicate matrix because of the high chloride content in the waste. Thus, an indirect immobilization method using a SAP (SiO2Al2O3P2O5) composite via dechlorination of the salt waste was evaluated as an alternative with a binder glass to aid in the consolidation process. Additionally, U-SAP was developed by adding consolidation promoters such as B and Al to the SAP and then, the glass binder was not needed. In the present contribution, basic properties of U-SAP dechlorination products and U-SAP waste forms were characterized by physical and chemical analysis methods. Structure of U-SAP waste form was investigated with X-ray diffraction, transmission, scanning transmission, and scanning electron microscopies, energy dispersive spectroscopy, secondary ion mass spectrometry, and atom-probe tomography. The results showed that uniform and nano-sized droplets were composed of crystalline Li3PO4 or a mixture of Li3PO4 and amorphous phosphorous-based phases. The droplets were encapsulated by durable silicate glass. Interfacial region composed of SiOAl(B) and Al(B)OP components connected immiscible silicate and phosphorous phases. The Li3PO4, which is not chemically durable, was protected by silicate glass from leaching and very small amount of Li was contained in silicate glass. The good leaching properties of the U-SAP waste form are attributed to this structure.