Compilation/Validation of Thermodynamic Data for Plutonium for Nuclear Waste Disposal: 1. Preliminary Results

by M. D. Siegel, Sandia Natl Lab, Albuquerque, United States,
S. L. Phillips, Sandia Natl Lab, Albuquerque, United States,
H. Nitsche, Sandia Natl Lab, Albuquerque, United States,

Abstract: A provisional thermodynamic database for plutonium has been compiled from a review of available data and other critical compilations. The database was compared to tabulations of Gibbs free energy values and calculated apparent equilibrium constants in other databases, and with the results of experiments conducted over a range of ionic strengths (0 - 5 molal). Both the database and a proposed method to calculate apparent equilibrium constants were then tested by comparing measured concentrations and redox distributions of plutonium in a reference water to the results of equilibrium calculations carried out with the HYDRAQL code. The experimental data indicate that the solutions have not yet reached steady state and that precipitates contain mixtures of crystalline and amorphous phases. The calculations suggest that the metastable solubility-limiting phase in the reference solutions is Pu(OH)4(am) and that the redox distribution of plutonium is not controlled by the O2/H2O redox couple. A preliminary sensitivity analysis suggests that the hydroxyl complexes of Pu(IV) are more stable and the carbonate complexes of Pu(VI) are weaker than suggested by our provisional database. These results illustrate a sequence of activities that we propose will lead to a defensible thermodynamic database for plutonium. The process is an iterative one and contains compilation, verification and validation steps. This paper describes partial completion of one cycle of this process as applied to a reference water for the Waste Isolation Pilot Plant.

Subject Headings: Databases | Thermodynamics | Equilibrium | Sensitivity analysis | Radioactive wastes | Standards and codes | Steady states | Mixtures

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