Sulfate Dissolution and Retention in High Level Nuclear Waste Containment Glass Systems

Abstract

This is a two-part study of sulfate dissolution and sulfate retention in nuclear waste containment glasses. Sulfates present in nuclear wastes limit the amount of nuclear waste which can be incorporated (waste-loading) into a glass waste form due to the limited solubility of sulfates. Another factor that limits the waste-loading is the glass composition. In the case of volatile components (e.g., sulfur), the time and temperature of melting and volume of the melt processed will influence the waste-loading in terms of the retention of the volatile elements. The baseline borosilicate glass system and the simulated nuclear waste composition used in this study are based on developments at the Savanna River National Laboratory (SRNL).

The dissolution part of this study was designed to increase the sulfate solubility in a proven nuclear waste containment glass system after the addition of specific elements into that system. Sulfate was introduced into these glasses in the form of sodium sulfate, which was then normalized into the glass network to target sulfur levels of 0.80 and 1.00 weight % (wt. %). In order to increase sulfate solubility, additions of barium, lead, and vanadium were introduced in small quantities (1.00-5.00 wt. %) into the predetermined glass matrix. These additives have shown to help increase sulfate solubility by creating a less-connected glass network.

The retention part of this study was to determine the volatilization of sulfur from the glass melt as a function of time of melting and volume of melt. Additives consistent with the dissolution study in 1.0 wt. % and a target sulfur level of 0.80 wt. % were introduced into a glass frit provided by SRNL.

The sulfate solubility and volatilization of these glasses were then determined for the entire study using inductively coupled plasma optical emission spectroscopy (ICP-OES) along with other thermal and spectroscopic methods. Both the dissolution and retention studies confirmed that additions of BaO to the raw batch components show an increased sulfur concentration when compared to samples where V2O5 was added; and, the sulfur concentration continues to increase with increasing BaO addition and does not display a limit within the compositional space of this study. Glasses where V2O5 was added show a higher sulfate concentration at lower V2O5 additions, and samples where PbO was added show similar sulfate concentrations to those where V2O5 was added when targeting 0.80 wt. % sulfur but show lower sulfur concentrations when targeting 1.00 wt. % sulfur. In addition, Fourier transform infrared (FT-IR) and Raman spectroscopic techniques were used to determine whether the sulfate was integrated into the glass structure. Within the limits of detection of these techniques, no evidence of sulfur was found.