Characterisation and interaction of simulated high level, radioactive waste (HLW) with borosilicate glass

Alkali borosilicate glasses have been widely used as an immobilisation network for high level, radioactive waste for the past 50 years. Increasing the waste loading would increase the efficiency of the process, but also increase the tendency of the waste towards devitrification in final storage. Devitrification could have a negative effect on the long term properties of the glass. “Yellow phase” is often observed within devitrified glass, and consists of many of the low solubility elements of the waste stream composition (Mo, Sr, Te, Cr and S). Yellow phase may also incorporate some of the more radioactive fission products such as l37Cs and 90Sr. The increased corrosion of the Inconel 601 melters in the early stages of melting time presents a problem, as decreased melter life means increased process inefficiency. Simulated waste streams have undergone a range of heat treatments and it has been shown that elemental segregation of the calcine is inevitable. Scanning electron microscopy and transmission electron microscopy showed a characteristic segregation of the rare earths and actinide surrogates to a fluorite structured phase. Segregation of a yellow phase to the melt surface, containing loosely bonded and highly soluble (in aqueous solution) radionuclides was observed using X-ray powder diffraction. It was demonstrated that the calcination process currently in use is ineffective and the temperatures to which it currently operates are overestimated. Simulated (non-radioactive) glasses have been melted in Inconel 601 crucibles using a variety of atmospheres, a range of waste loadings and waste streams which have undergone preliminary heat treatment. Some basic mechanisms for the dissolution of the waste into the glass have been proposed. The formation of a rare-earth enriched solute layer around the waste particles was shown using SEM, occurring concurrently with nucleation of yellow phase and segregation to the melt surface. The yellow phase, once incorporated into the body of the melt via convective mixing was seen to nucleate around bubbles within the viscous matrix. Fluorite-structured phases did not dissolve readily in the glass and were found to contain many rare-earths and actinide simulants.