Electrochemistry of PUREX: R is for reduction and ion transfer

ABSTRACT

The PUREX (Plutonium, Uranium, Reduction, EXtraction) process is the foundation of all industrial activities that involve the recycling of plutonium and uranium from used nuclear fuel. With over 70 years of research, engineering, and operations, it can be argued that little opportunity is left for further discovery and new fundamental understandings regarding PUREX, in general, and tri-n-butyl phosphate (TBP), in particular. Through use of an advanced electroanalytical approach, molecular-level insights regarding the back-extraction of Pu(IV) by reductive separation are reported. Studies of model PUREX systems, with lanthanide ions extracted into 20% TBP in n-dodecane, are used to demonstrate the approach. These studies reveal knowledge pertinent to mass-transfer aspects of the biphasic reductive stripping reaction central to the separation of U and Pu in PUREX. Two lanthanide ions – Ce(IV) and Yb(III) – were extracted from their concentrated solutions in aqueous 3 M HNO₃ to expressly provide electrically conducting third phases to facilitate electrochemical data acquisition. To simplify the diabolically-complicated reductive separation aspects of PUREX, the controlled polarization of electrode surfaces was used to reduce Ce(IV) and Yb(III) in place of chemical reductants. Electroanalyses of the biphasic systems with Ce(IV) and Yb(III) using three-phase electrode differential pulse voltammetry demonstrate the effects of mass transfer across water–oil interfaces on the electrode potentials for the reduction to Ce(III) and Yb(II). These reductions trigger nitrate transfer out of the oil phase for Ce and proton transfer into the organic phase for Yb. This is in correlation with the transfer of the charge-neutral ion pairs Ce·3NO₃ and Yb·2NO₃ out of the oil phases. The differences in ion-transfer steps reflect significant variations in the equilibrium speciation of multinuclear Ce(IV) and mononuclear Yb(III) solvates of TBP in the oil phases and the loading of the organic phase.

KEYWORDS:

• PUREX
• voltammetry
• third phase
• reduction
• ion-transfer

Acknowledgments

E. R. B. and J. C. S. acknowledge support provided by the U.S. Department of Energy, Office of Science Graduate Student Research (SCGSR) Program and the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program at Colorado School of Mines under Award Number DE-SC0020189. M. R. A. acknowledges the support of the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under contract no. DE-AC02-06CH11357.

Supplementary material

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