The Role of Kinetics in Advanced Nuclear Fuel Reprocessing

Jackson, Alex and Boxall, Colin and Woodhead, David and Taylor, Robin J. (2023) The Role of Kinetics in Advanced Nuclear Fuel Reprocessing. PhD thesis, Lancaster University.

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Abstract

Liquid/liquid extraction processes are designed to selectively partition actinides with especially long-lived radiotoxicity from nuclear fuel waste streams. Uranium and plutonium partitioning is possible through application of the Plutonium Uranium Reduction Extraction (PUREX) and Advanced PUREX processes using tributyl phosphate (TBP). Furthermore, the technology for the fabrication of nuclear fuel using mixed actinide salt precursors relies heavily on the availability of the Innovative Selective Actinide Extraction (i-SANEX) and the European Grouped Actinide Extraction (EUROGANEX) processes. These processes utilise the organic ligand N,N,N’,N’- tetraoctyl diglycolamide (TODGA) to extract trivalent lanthanides and actinides from nitric acid. As such, key properties and behaviours of TBP and TODGA are the object of the student’s research at Lancaster University. Using a Rotating Diffusion Cell (RDC), it is possible to interrogate the various chemical kinetic and diffusive contributions to actinide extraction by tracking the transfer of solutes over a fixed area membrane between the compartmentalised phases of nitric acid, containing dissolved metal ions, and TBP- or TODGA-loaded solvent. Furthermore, via manipulation of the rotation speed of the RDC several regimes of mass transfer may be reproduced, from low shear diffusion limiting conditions to high shear chemical kinetic control. The rate of extraction of the metal ions investigated is determined by ultraviolet-visible light absorbance spectroscopy. An analytical model coded in the Process System Enterprise (PSE) General Process Modelling System (gPROMS) has been developed to estimate unknown constants, for example to obtain values for kinetic rate constants, based on the experimental extraction rates found. Information about unknowns such as these, as well as insight into the dominant mechanisms of transfer, are fundamental in building the safety case for state of the art reprocessing and partitioning plants.