Advances in liquid electrodes for spent fuel reprocessing in the nuclear industry

Abstract

For several decades, nuclear power has been of interest as an energy source. Advantages include reliability and zero carbon emissions, however there are disadvantages in waste management and the risk of proliferation. Current research aims to address these by developing proliferation resistant methods to reprocesses the spent fuel and close the fuel cycle.

Whilst aqueous liquid-extraction reprocessing techniques are in use, these are confined to specific fuel types and produce significant additional low-level radioactive waste. Separation of spent fuel in molten salts has been proposed as an alternative to such aqueous methods. These systems include using liquid metals as electrodes at which selective metal deposition takes place. There are several technical and materials hurdles to overcome, these include: characterisation and optimisation of the processes taking place at the liquid electrode/ molten salt interface; and the development of the necessary on-line analysis techniques for process monitoring.

This thesis takes a step-by-step approach to: characterise the behaviour of liquid metal electrodes; explore how molten salts interact with materials to inform the development of microelectrodes for on-line monitoring, and; develop methods to prepare molten salts for rigorous study.

First the fundamental response of liquid cathodes is demonstrated through comparative studies in aqueous systems. By comparing electrochemical studies of mercury cathodes at ambient temperatures and liquid cathodes in molten salts, characteristic responses for deposition, alloying and dendritic growth are observed. In ambient conditions, the bounded Cottrell equation is applied to a liquid cathode for, what is believed to be, the first time to explain alloy formation in the upper layers of a liquid cathode.

The contact angles of lithium chloride - potassium chloride eutectic (LKE) on materials used in microelectrode fabrication have been measured. These wettability measurements confirm silicon nitride and tungsten as suitable materials in design and fabrication of microelectrodes for online process monitoring.

Ensuring the correct speciation is studied in the molten salt is of significant importance in developing a pyroprocessing system. Characterisation must be performed in a high purity, rigorously dry molten salt, and a new method for removing water from lithium chloride-potassium chloride eutectic melt (LKE) has been developed.