Computational Modeling of Mixed Potentials in Nuclear Waste Tanks

Abstract The Hanford Site maintains large volumes of radioactive and chemically hazardous wastes arising from weapons production, currently stored underground in 130 single-shell tanks and 27 double-shell tanks. The waste in the tanks is typically composed of a supernatant liquid over saturated settled solids, and significant differences can exist in the chemical compositions and pHs of the supernatant and interstitial liquids of the settled solids, leading to dissimilarities in polarization response between regions of the carbon steel tank. A computational finite-element modeling study was performed on mixed potentials in a selected nuclear waste tank at the Hanford Site, identified as Tank 241-AN-107. The study evaluated the electrochemical coupling effects between the supernatant liquid and the solids layer and calculated the potential and current distributions along the carbon-steel tank bottom and wall. Results indicate that a high-pH supernatant liquid layer can cathodically protect a low-pH interstitial liquid at the tank bottom through a mixed potential effect. The model was validated experimentally using a split-cell method. This paper discusses the modeling study, its laboratory validation, and the application of the results to the management of waste to ensure that corrosion risks remain low in nuclear waste storage tanks.