Spent nuclear fuel must be treated and managed safely and securely to ensure that it poses no risk to people or the environment, now or in the future. But what’s the best way to do this? A research group under the supervision of Professor Andy Mount (based at King’s Buildings, Edinburgh) are investigating the fabrication and characterization of microelectrodes for use as sensors during the reprocessing of nuclear waste.
The electrochemical extraction of the material from nuclear waste in baths of molten salts, such as lithium chloride potassium chloride eutectic (LKE), is known as pyroprocessing. Once attached to an anode and suspended in a bath of molten salt, an electric current dissolves the waste. Uranium is then extracted by plating onto a cathode; remaining actinides are extracted by alloying with an active electrode such as aluminium or bismuth . This alloying lowers the potential at which the material is deposited and groups the elements together, therefore prevents the extraction of pure plutonium (as per requirements to prevent nuclear proliferation). Macroelectrodes can extract large amounts of material quickly; microelectrodes are beneficial for sensing and understanding the fundamental aspects of the alloying processes.
One line of research in the Mount group is the development of microelectrodes that can survive in the harsh conditions of molten salts. Microelectrodes are being fabricated with inert metals to study plating reactions as well as active metals to study alloying reactions. One metal the Mount group use for active microelectrodes is aluminium. Unfortunately, on exposure to air, aluminium forms an oxide layer which prevents its effective use. Regular methods of removing this layer do irreparable damage to microelectrodes, so Simon Reeves is attempting to remove it electrochemically. Currently this can’t be elaborated due to intellectual property constraints.
Bismuth microelectrodes are also being developed. During plating, the solid bismuth can form tree-like structures on the electrode’s surface, affecting its ability to function. Justin Elliott is developing an aqueous bath to ensure smooth plating of thin layers of bismuth. The bismuth microelectrodes can then be used in molten salt to aid in understanding alloy formation.
To improve fabrication success and efficiency, another group member, Hannah Levene is investigating factors such as how many working electrodes each batch yields, what the failure mechanisms are, and how long each microelectrode lasts.
As well as fabricating novel microelectrodes, Prof. Mount’s group are trying to refine the refining process, by answering questions like: Which potentials can the desired materials be extracted at? Which alloys are formed and how many? During alloying, specific intermetallic compounds are formed that show unique features in their voltammograms (a plot of current vs potential). These fingerprint peaks help give greater understanding of which elements are present.
The Mount group are also currently setting up a new laboratory with a continuous system of four large connected glove boxes. Here, the entire reprocessing cycle can be simulated, using non-radioactive surrogate materials. Once completed, the lab will be an open access national facility for molten salt research.
Not only do advances in nuclear fuel reprocessing help to increase safety, but to be able to recover and reuse plutonium and uranium effectively closes the fuel cycle. This means that up to 30% more energy could be gained from the original fuel material; a valuable energy security goal for any country. The lessons learnt while researching this topic can also be put to good use in other applications of microelectrodes, such as medical biosensing and water pollution monitoring.
This article was written by Samuel Stanfield and edited by Bonnie Nicholson.