PNNL

Abstract

A thermodynamic model of U phase solubility and paragenesis was developed for Hanford tank residual waste that will remain after tank closure. The model was developed using a combination of waste composition data, waste leach test data, and thermodynamic modeling of the leach test data. The testing and analyses were conducted using actual Hanford tank residual waste. Positive identification of the U phases by X-ray diffraction (XRD) was generally not possible because solids in the waste were amorphous, or below the detection limit of XRD for both as-received residual waste and leached residual waste. Three leachant solutions were used in the studies, dionized water, CaCO3 saturated solution, and Ca(OH)2 saturated solution. Thermodynamic modeling verified that equilibrium between U phases in the initial residual waste samples and the leachants was attained in less than a month. The paragenetic sequence of secondary phases that occur as waste leaching progresses for two closure scenarios was identified. These results have significant implications for tank closure design.