To understand the risks associated with deep geologic storage of nuclear waste over geologic time frames, thermodynamic constants describing the equilibrium behavior of solubility-controlling radionuclide phases are needed, particularly for the elevated temperature conditions expected in repository environments. Specifically, this work seeks to quantify the solubility of technetium dioxide (TcIVO2) at elevated temperatures (up to 80 °C) and to define the variable temperature TcIV hydrolysis constants over the pH range 1–11.5. Additional solubility experiments conducted from oversaturation of TcO2 at variable ionic strengths will further provide a means to quantify ion interaction coefficients appropriate for activity corrections using the specific ion interaction theory (SIT) or the Pitzer model. Preliminary TcO2 solubility data collected under anoxic conditions are in good agreement with TcIV solubility simulations generated using the geochemical code PHREEQC v.3.7 and the thermodynamic constants currently available in the LLNL.v8.r6+ database. This work will increase the accuracy of predictive models of Tc solubility and transport in elevated temperature and ionic strength environments, providing a means to better assess the specific risks associated with the storage of technetium in deep geologic repositories.

Technetium dioxide solubility at elevated temperatures

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