Predicting Pu Concentrations in Solutions Contacting Geologic Materials
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PREDICTING Pu CONCENTRATIONS IN SOLUTIONS CONTACTING GEOLOGIC MATERIALS RICHARD G. STRICKERT AND DHANPAT RAI Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352 ABSTRACT Knowledge of Pu solid phases present in nuclear wastes is important for predicting the geochemical behavior of Pu. Thermodynamic data and experimental measurements using discrete Pu compounds, Pu-doped borosilicate glasses (simulating a high-level waste form), and Pu contaminated sediments suggest that Pu02(c) is very stable and is expected to be present in the repository. The solubility of the stable phase, such as Pu02(c), can be used to predict the maximum Pu concentration in solutions for long-term safety assessment of nuclear waste repositories. INTRODUCTION Plutonium is present in nuclear wastes that are expected to be disposed in a deep geologic repository. To predict the potential environmental consequences of Pu migration from the repository, knowledge of the Pu concentrations in leachates is necessary. The Pu concentration in solution will be greatly affected by the solubility of the waste form and the sorption/desorption reactions of the leached Pu with the geologic material. The solubility of the waste form and sorption/desorption reactions, in turn, are affected by the nature of the solid phases, radiation and groundwater pH, pe*, ionic In a strength, nature and quantity of complexing ligands and temperature. given groundwater environment, the Pu compounds present in and near a geologic repository have specific equilibrium solubilities which could control the maximum possible concentration of Pu in the groundwater available for transport to the biosphere. Although Pu sorption reactions may be occurring with the geologic material, the equilibrium concentration would be determined by the solubility of a Pu compound as long as this compound exists. Based upon thermodynamic predictions, Rai and Serne [1] computed the stability of various Pu minerals and solid phases. From their calculations, PuO2 (c,c=crystalline) was predicted to be the most stable (least soluble) solid phase contacting groundwater solutions of pH 4 or higher, for both oxidizing (pe+pH=20.6) and reducing (pe+pH=O.8) conditions. Despite such a prediction, uncertainties in Pu solution concentrations of up to 5 orders of magnitude resulted from variations in reported thermodynamic values for different Pu solution species. As a result, experimentally determined solubilities of various Pu solids that may be present in sediments and potential waste form material were needed. We, therefore, have (i) measured Pu concentrations in solutions contacting 239 PuO2(c), 2 39 Pu(OH)4(am,am=amorphous), Pu contaminated sediments, and Pu-doped borosilicate glass, and (ii) determined the effects of alpha radiation on the solubility of Pu solids. The pe value is defined as -loglo of the electron activity and is related to the Eh value at 25°C by the equation, pe=16.9 Eh.
216 Materials and Methods Details of the materials and solubility methods used in the study
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