Structural investigations on Mo, Cs and Ba ions-loaded iron phosphate glass for nuclear waste storage application

Abstract

Borosilicate glass is considered to be the standard matrix for immobilization of fission products obtained from the reprocessing of nuclear fuel. However, nuclear waste containing constituents like molybdenum oxide (MoO3) and cesium oxide (Cs2O) are less soluble in borosilicate glass. Hence iron phosphate glass (IPG) is being contemplated as an alternate vitrification matrix since it shows better solubility for these constituents. However, radioactive 137Cs undergoes β− decay to yield 137Ba. In the present work, iron phosphate glass co-loaded with Mo/Cs/Ba ions has been synthesized using the melt quench technique to study their effect on the structure of IPG. The homogeneity, structure and composition of IPG and Mo/Cs/Ba ions co-loaded IPG has been investigated using powder X-ray diffraction, scanning electron microscopy (SEM), electron diffraction spectroscopy (EDS), Mössbauer, FTIR and Raman spectroscopy. A gradual structural modification of the undoped IPG is indicated by the FTIR and Raman spectra with increase in Mo/Cs/Ba ions loading in IPG samples. Detailed thermal analysis of all the glass samples have been done to study their glass stability and glass forming ability which has been estimated in terms of the characteristic temperatures obtained from thermogravimetric analysis (TGA) and differential thermal analysis (DTA) data. The correlation between glass forming ability and the evaluated critical cooling rate of the glasses has been analyzed. Presence of Fe2+ ions has been observed in all the IPG samples as confirmed from Mössbauer spectroscopy. The structure at the atomic scale has been studied in detail using Fe/Mo K-edge X-ray absorption spectroscopy (XAS). The IPG glass co-loaded with Mo/Cs/Ba ions exhibits higher glass transition temperature, comparable glass stability parameters, superior glass forming ability compared to unloaded IPG. Also, a slight modification in the glass structure in Mo/Cs/Ba ions co-loaded IPG as indicated by FTIR, Raman, EXAFS and XANES studies, suggests the potential application of IPG as a suitable vitrification matrix for nuclear wastes having these constituents.