Computational Modeling of Mixed Potentials in Hanford Nuclear Waste Tanks Containing Stratified Waste Layers

Radioactive and chemically hazardous wastes stored in carbon‑steel double‑shell tanks at the Hanford Site are commonly stratified, with an alkaline supernatant liquid overlying settled solids that can contain chemically distinct alkaline interstitial liquids. Such stratification can produce dissimilar electrochemical behavior and localized corrosion risk at the tank wall and bottom. In this work, mixed‑potential theory and finite‑element analysis were used to evaluate electrochemical coupling between stratified waste layers in Tank 241‑AN‑107, which contains a relatively low‑pH interstitial liquid in contact with the tank bottom. Polarization curves measured in representative simulants of the tank waste were applied as boundary conditions to compute potential and current distributions along the tank wall and bottom. The model predicts that coupling produces a nearly uniform mixed potential across wetted tank surfaces and that the high‑pH supernatant cathodically polarizes the bottom region, increasing the margin to critical pitting and cracking potentials. Split‑cell coupling/decoupling experiments provide direct validation of the predicted mixed potential shift and currents. The results have important implications for corrosion threat assessment, tank potential monitoring, waste remediation strategies, and operational decisions related to waste transfer.