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Nuclear fuel waste extraction: Third phase revisited
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
The problem of third phase formation plays a key role in the plutonium and uranium extraction process (PUREX). This process is responsible for the recycling of used nuclear fuel in order to save fuel usage and more importantly to reduce the amount of waste created (and afterwards disposed or stored) after the end of a nuclear fuel cycle. Understanding the role and behaviour of its components in the aqueous and organic phase, and more in depth on the process' extractant tributyl phosphate (TBP), will help give a better understanding of what causes the phase separation of the organic phase and the interactions occuring at that interface. The focus of this project is on the mean activity coefficients of aqueous uranyl nitrate (and other salts) under varying concentrations using the Statistical Associating Fluid Theory (SAFT). Also, apart from the thermodynamic aspect of the above, molecular dynamic simulations were performed on tributyl phosphate and its interactions with other third phase components to study the effects on the structure and behaviour of TBP. By studying TBP in dodecane mixtures it was found that TBP forms aggregates and filament structuring throughout the organic diluent at most TBP concentrations. Also nitric acid hinders this formation by contacting the polar group (P=O) of TBP via intermolecular forces; its action is physical (based on intermolecular interactions) as opposed to chemical. It is expected that this structuring of TBP in the organic phase has an important effect in the transport of metal nitrates from the aqueous to the organic phase in the PUREX process.