Disposability Assessment of Glass Waste Forms for Immobilization of Iodine-129 and Spent Nuclear Fuel

Abstract

Silver tellurite glasses with melting temperatures < 700 °C were prepared to immobilize the 129I that normally volatilizes during high-temperature melting. Glasses have densities of 6.31 g/cm3 and glass transition temperatures of 165 °C that provide thermal stability at the disposal site. Iodine waste loading in glasses was 11.21 wt.%. Normalized elemental releases obtained from the product consistency test were well below US regulation of 2 g/m2. Iodines are surrounded by four Ag+ ions forming [Ag4I]3+ units that are further connected to tellurite network through bonds with non-bridging oxygens. Elemental leaching was extremely low and significant changes did not occur on the surface of the glass leached in DIW. In contrast, Ag continuously leached up to 128 days and other elements, Te, Bi and I, leached up to 16 days and rate dropped thereafter in 0.1 M acetic acid. Concentration of Ag decreased and H increased from interior to surface of the glass. Cracks and Ag precipitations were observed throughout the surface. Ratio of TeO4 and BO decreased during the dissolution in low pH solution. Iron phosphate glasses with low melting temperatures of ~ 1300 °C were developed to immobilize spent nuclear fuels (SNFs). The glasses have densities of ~ 3.15 g/cm3 and glass transition temperatures of ~ 555 °C that are high enough to endure the temperatures in geological repositories. The waste loading is 33.73 wt.%. Normalized elemental releases from the product consistency test were well below the regulated limit of 2 g/m2. Most of U in glass is 4+ and U atoms are directly connected to phosphate network. Effective neutron multiplication factor keff was 0.755, which is below nuclear criticality safety regulation (keff = 0.95). Vitrified SNF occupies one-eleventh of the repository space compared to direct disposal of the same amount of UO2, excluding left-over low-level wastes.