At the Hanford Site in Washington State, U.S., nearly 210,000 m3 of nuclear waste generated during 45 years of plutonium production is slated to be vitrified in the Hanford Tank Waste Treatment and Immobilization Plant (WTP). The current strategy is to separate the low-activity waste (LAW) fraction from the tank waste, mix it with glass-forming and -modifying additives, and charge it into electric melters at the WTP. A similar process will be followed for the high-level waste (HLW). Molten glass will be poured from the melters, which are operated at ∼1150 °C, into stainless-steel canisters or containers where it will cool and solidify, immobilizing both the HLW and LAW fractions in the form of borosilicate glass, a stable waste form suitable for long-term disposal.
Technetium-99 (99Tc) is a long-lived (half-life = 211,100 years) fission product, abundant in Hanford waste, that is highly mobile in subsurface soils, creating challenges for the safe, long-term disposal of Hanford nuclear waste. In addition to these environmental challenges, the volatility of Tc creates a significant challenge during the waste vitrification itself. The volatilized Tc compounds have to be captured from the off-gases and be either recycled into the melter or stabilized in alternative waste forms, increasing the processing costs and the amount of waste created. As a result, a significant amount of literature has been published both on the immobilization of Tc and its nonradioactive surrogate, rhenium (Re), whose volatility generally exhibits behavior analogous to that of Tc, regardless of the differences in their speciation.
The retention of Tc/Re during the melting of LAW feeds is strongly affected by their composition. Addition of reducing agents, such as sucrose, increase Tc/Re retention. Sulfate has an opposite effect: perrhenate/pertechnetate dissolves in the sparsely soluble sulfate phase that inhibits Re incorporation in the glass-forming melt and promotes its Re volatilization when segregated on the glass melt surface. However, effects of other feed components on the Tc/Re retention are still poorly understood.
Recently, we observed that the chemical form of alumina might affect Tc/Re retention – the Re retention in AP-107 LAW glass was lower than in a compositionally similar AN-105 LAW glass, the difference being that AP-107 LAW melter feed contained kyanite (Al2SiO5) whereas AN-105 LAW feed contained mostly gibbsite (Al(OH)3) with a low fraction of kyanite. To investigate this effect, we measured the Re retention in a series of AP-105 and AN-102 LAW melter feeds with different alumina sources – nominal feeds with kyanite, feeds in which 50 % of kyanite substituted with gibbsite, and feeds with kyanite fully replaced with gibbsite. All feeds were spiked with Re2O7 solution corresponding to 300 ppm Re in glass. Approximately 50 g of dry powder feed was heated in glazed porcelain crucibles at 10 K min−1 under air atmosphere from room temperature to 1150 °C. After quenching, the samples were ground and crushed to below 100 um grain size.
In both AP-105 and AN-102 feeds, the Re retention increased significantly when gibbsite replaced kyanite. On heating, Al(OH)3 produces amorphous alumina with a high specific surface area. We reason that adsorption of sulfate-perrhenate melt on amorphous alumina increased its contact area with the glass-forming phase and the high contact area then enhanced the rate of sulfate-perrhenate dissolution in the glass-forming melt, reducing the rate of sulfate-perrhenate segregation and volatilization.
Because the differences in the chemical and/or mineralogical alumina sources also affect the feed-to-glass conversion process, we used the feed expansion test to visually observe the effect of the Al-source on high-temperature foaming. We observed that Al(OH)3 promotes foaming. This is because gibbsite is an early dissolving Al-sources, which increases the glass-forming melt viscosity at a lower temperature than kyanite, a late dissolving Al-source. This keeps the glass-forming melt viscosity low up to a high temperature. A high viscosity leads to extended foaming, a low viscosity leads to a faster foam collapse.