QUALIFICATION OF A RADIOACTIVE HIGH ALUMINUM GLASS FOR PROCESSINGIN THE DEFENSE WASTE PROCESSING FACILITY AT THE SAVANNAH RIVER SITE

At the Savannah River Site (SRS) the Defense Waste Processing Facility (DWPF) has been immobilizing SRS's radioactive high level waste (HLW) sludge into a borosilicate glass for approximately eleven years. Currently the DWPF is immobilizing HLW sludge in Sludge Batch 4 (SB4). Each sludge batch is nominally two million liters of HLW and produces nominally five hundred stainless steel canisters 0.6 meters in diameter and 3 meters tall filled with the borosilicate glass. In SB4 and earlier sludge batches, the Al concentration has always been rather low, (less than 9.5 weight percent based on total dried solids). It is expected that in the future the Al concentrations will increase due to the changing composition of the HLW. Higher Al concentrations could introduce problems because of its known effect on the viscosity of glass melts and increase the possibility of the precipitation of nepheline in the final glass and decrease its durability. In 2006 Savannah River National Laboratory (SRNL) used DWPF processes to immobilize a radioactive HLW slurry containing 14 weight percent Al to ensure that this waste is viable for future DWPF processing. This paper presents results of the characterization of the high Al glass prepared in that demonstration. At SRNL, a sample of the processed high Al HLW slurry was mixed with an appropriate glass frit as performed in the DWPF to make a waste glass containing nominally 30% waste oxides. The glass was prepared by melting the frit and waste remotely at 1150 C. The glass was then characterized by: (1) determining the chemical composition of the glass including the concentrations of several actinide and U-235 fission products; (2) calculating the oxide waste loading of the glass based on the chemical composition and comparing it to that of the target; (3) determining if the glass composition met the DWPF processing constraints such as glass melt viscosity and liquidus temperature along with a waste form affecting constraint that prevents the precipitation of nepheline (NaAlSiO4) crystals in the glass melt; (4) measuring the durability of the glass using the ASTM Standard Product Consistency Test (PCT) leach test to determine if the durability of the glass based on B, Li, and Na releases met the requirements for acceptance in a US geologic repository; (5) measuring the leachability of several radionuclides using the ASTM Standard PCT leach test and comparing them to the B, Li, and Na releases; and (6) examining the glass by scanning electron microscopy and energy dispersive X-ray spectrometry to determine if any crystals had formed in the glass melt. Results indicate that the high Al glass met all the requirements for processing and product quality in the DWPF