This Critical Review reviews the origin and chemical and rheological complexity of radioactive waste at the U.S. Department of Energy Hanford Site. The waste, stored in underground tanks, was generated via three distinct processes over decades of plutonium extraction operations. Although close records were kept of original waste disposition, tank-to-tank transfers and conditions that impede equilibrium complicate our understanding of the chemistry, phase composition, and rheology of the waste. Tank waste slurries comprise particles and aggregates from nano to micro scales, with varying densities, morphologies, heterogeneous compositions, and complicated responses to flow regimes and process conditions. Further, remnant or changing radiation fields may affect the stability and rheology of the waste. These conditions pose challenges for transport through conduits or pipes to treatment plants for vitrification. Additionally, recalcitrant boehmite degrades glass quality and the high aluminum content must be reduced prior to vitrification for the manufacture of waste glass of acceptable durability. However, caustic leaching indicates that boehmite dissolves much more slowly than predicted given surface normalized rates. Existing empirical models based on ex situ experiments and observations generally only describe material balances and have not effectively predicted process performance. Recent advances in the areas of in situ microscopy, aberration-corrected transmission electron microscopy, theoretical modeling across scales, and experimental methods for probing the physics and chemistry at mineral-fluid and mineral-mineral interfaces are being implemented to build robustly predictive physics-based models.

Design Authority/ Date Cognzant Manager 21. DOE APPROVAL (if required) Ctrl No.

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Liquid Hanford nuclear wastes are highly concentrated caustic aqueous electrolyte solutions. They contain elevated dissolved aluminum concentrations compared to simple NaOH(aq) solutions in equilibrium with gibbsite [γ-Al­(OH)3]. The reason for this elevated solubility has been debated for years, with slow gibbsite precipitation kinetics or various unverified thermodynamic factors being historically offered as explanations. The present study determines whether there is a kinetic or thermodynamic explanation. Here, dissolved aluminum in real tank waste was equilibrated with excess gibbsite, approached from both above and below saturation at 40 °C. In both cases, the samples equilibrated to an aluminum concentration up to four times higher than in pure NaOH(aq) solutions of the same hydroxide concentration. However, in one case, when all of the gibbsite dissolved during heating, no measurable precipitation was observed from the supersaturation direction when gibbsite seed was unavailable for nucleation. These results indicate that there is a real (and, as of yet, unknown) thermodynamic effect that accounts for the elevated solubility of aluminum exhibited by the waste. There is also a kinetic effect superimposed under some conditions.