Redox-dependent solubility of technetium in low activity waste glass

Abstract

The solubility of technetium was measured in a Hanford low activity waste glass simulant. The simulant glass was melted, quenched and pulverized to make a stock of powdered glass. A series of glass samples were prepared using the powdered glass and varying amounts of solid potassium pertechnetate. Samples were melted at 1000°C in sealed fused quartz ampoules. After cooling, the bulk glass and the salt phase above the glass (when present) were sampled for physical and chemical characterization. Technetium was found in the bulk glass up to 2000 ppm (using the glass as prepared) and 3000 ppm (using slightly reducing conditions). The chemical form of technetium obtained by x-ray absorption near edge spectroscopy can be mainly assigned to isolated Tc(IV), with a minority of Tc(VII) in some glasses and TcO 2 in two glasses. The concentration and speciation of technetium depends on glass redox and amount of technetium added. Solid crystals of pertechnetate salts were found in the salt cake layer that formed at the top of some glasses during the melt. BackgroundRadioactive waste from decades of plutonium production is currently stored in large underground tanks at the Hanford Site. This waste will be mixed with glass formers and then vitrified. 1 The Hanford tank wastes vary widely in composition, but are typically largely sodium nitrate, nitrite, and carbonate with a small amount of hydroxide.2; 3 Aluminum, iron and zirconium comprise 20% or more of the waste in some cases. Chromium and manganese may be present up to several percent. not recover all of the plutonium from the irradiated uranium, and a small amount of plutonium, uranium and americium are also found in the tanks. Many of the waste components do not incorporate well into silicate glasses. A number of ionic compounds such as sulfates have low solubility in silicate glasses and may form a separate salt phase.The current plan is to chemically separate the waste into a small volume of high-level waste, intensely radioactive from 90 Sr and 137 Cs, and a large volume of low-activity waste (LAW). 1 These two fractions will be vitrified separately. Technetium will report to the low-activity waste and will be vitrified in that fraction. Tc in LAW GlassPage 2 of 21Technetium incorporates poorly into silicate glass in traditional glass melting. Technetium readily evaporates during melting of glass feeds (waste + additives) and out of the molten glass, leading to low retention in a final glass product. [4][5][6][7] To effectively manage technetium retention in the Hanford LAW glass, it is critical to understand if the solubility of technetium is a controlling factor. The speciation of technetium in glass has been previously studied, and the technetium species present in waste glass have been previously reported; [8][9][10][11] however, the solubilities of these species are unknown.The solubility of technetium and many other waste constituents depends partly on their chemical forms.A particular constituent may be better incorporated into the glass if it is adde...

Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

“…Previous studies [25,26] employed a vacuum-sealed fused silica ampoule setup to measure the solubility of Re and 99 Tc in a representative LAW glass. The measured solubility was 3000 ppm mass for Re (used as surrogate for Tc) [25] and 2000-2800 ppm mass for 99 Tc [26] (both on metal basis) at 1000°C.…”

“…The measured solubility was 3000 ppm mass for Re (used as surrogate for Tc) [25] and 2000-2800 ppm mass for 99 Tc [26] (both on metal basis) at 1000°C. Given the projected concentration of 99 Tc in LAW glass at Hanford is~3 ppm mass on average [26]; the solubility of 99 Tc is not a factor in its retention in LAW glass. In addition, the structural role of Re and Tc in glass network was discussed in Goel et al 2013 [27] and Gassman et al 2014 [28].…”

Synthesis and Characterization of 5- and 6- Coordinated Alkali Pertechnetates

“…Lower oxidation states of Tc, such as Tc(IV), may also be present during vitrification [5]. However, due to the overall oxidizing conditions of the waste feed, almost all the lower valent Tc species are expected to convert to Tc(VII) during vitrification.…”

“…The decomposition temperatures are exceeded by the glass vitrification temperature. Therefore, when combined with the previous mechanisms (equations [1][2][3][4][5], these two mechanisms could be part of the overall process for the thermal decomposition of pertechnetates. Since the Hanford LAW glass melts are expected to be strongly alkaline, the pertechnetates could be converted by reaction with Na 2 O into TcO 5 3-and TcO 6 5-, which then could thermally decompose to Tc 2 O 7 , TcO 6 6+ , and TcO 5 4+ .…”

Rhenium volatilization in waste glasses

“…Unfortunately, a substantial fraction of Tc escapes with the off-gas during vitrification [15][16][17][18]. Since the Tc solubility in a typical LAW glass is $0.2 mass% [19] and the Re solubility is $0.3 mass% [20], the solubility should not be a limiting factor for Tc/Re retention during melting. However, the Catholic University of America reported up to $80% Re and Tc losses from small-scale melters vitrifying simulated LAW feeds [16][17][18].…”