PNNL

Abstract

Nuclear fuel recycling flowsheets that do not fully separate plutonium (Pu) from uranium (U) are of interest due to reduction in the Pu attractiveness level, and potentially simplifying the manufacturing of mixed oxide fuel. Our research in this regard has not been directed at development or testing of a particular flowsheet for industrial application, but rather at examining how well a particular U/Pu mass ratio (chosen to be 7/3 for this project) can be maintained using laboratory-scale counter-current solvent extraction equipment (2-cm centrifugal contactors). Integral to this effort is the deployment of real-time optical spectroscopic tools to monitor the U/Pu ratio in the product stream, so that adjustments to process parameters can be made to achieve and maintain the targeted U/Pu ratio. In the co-decontamination (CoDCon) flowsheet tested in this work, the co-extraction of U and Pu follows standard protocol for a tri-butyl phosphate (TBP) based flowsheet. The Pu is stripped from the solvent using U(IV). The control point for adjusting the U/Pu ratio is the flowrate of a TBP solvent stream used to extract U(IV) from the aqueous phase in the Pu stripping contactor bank. In a conventional recycling operation, the flowrate of this TBP solvent stream would be sufficiently high to quantitatively extract the U(IV) so that a pure Pu product is obtained. However, as the flowrate of this stream is lowered, the amount of U(IV) remaining in the aqueous phase (i.e., with the Pu) increases. The novel aspect of our approach is the real-time monitoring of the U/Pu ratio in the U/Pu product stream using optical spectroscopy coupled with chemometric modeling. This real-time feedback allows adjustment of the TBP solvent flowrate to achieve the target U/Pu ratio. Several runs of the flowsheet have been conducted using simulated dissolved fuel solutions. Monitoring and control of the U/Pu ratio has been demonstrated in all cases. Once steady-state is reached, very stable control of the U/Pu ratio can be achieved. Our observations are that the uncertainties in the final U/Pu product are dominated by the uncertainties in the analytical methods used to analyze the product, rather than in operation of the solvent extraction equipment.