Development of bismuth thin film and bismuth pool electrodes for the monitoring and extraction of neodymium and samarium and their applicability to civil nuclear molten salt systems

Abstract

Molten salt reactors (MSRs) with closed loop reprocessing are one type of generation IV nuclear reactor being developed to provide large and reliable amounts of low (zero in operation) carbon energy to help address the worldwide climate crisis. Within a molten salt reactor, the nuclear fuel is dissolved into a molten salt, typically either a chloride (such as a eutectic mixture LiCl-KCl, LKE) or a fluoride (such as a mixed LiF-NaF-KF) based salt. As the fuel is dissolved into the molten salt, MSRs have key potential advantages over traditional nuclear reactors, including inherent safety and the potential for online refuelling and reprocessing. One suggested approach to increase the neutron efficiency and operational lifetime of an MSR is online reprocessing through the targeted removal of key lanthanide fission product neutron poisons, such as samarium (Sm) and neodymium (Nd), via selective electrochemical reduction. Pyroprocessing, a method for the recycling of spent nuclear fuel, also uses molten salt (MS) (often LKE) to dissolve the spent nuclear fuel with the selective electrochemical reduction to recover usable fuel and separate transuranic material. Online reprocessing in an MSR and pyroprocessing therefore could both involve selective separation of lanthanides such as Sm and Nd. In both cases, the development of selective quantitative real-time online monitoring methods to monitor such extraction techniques is also of interest. One approach is to use the same active alloying material in the electrochemical monitoring as the electrochemical extraction method, as its development could directly inform extraction. This thesis investigates the development of a bismuth (Bi) active electrode for online monitoring of Sm and Nd ions in LKE, and whether and how a Bi pool can be used for the electrochemical extraction of Sm and Nd ions in LKE. In this work, pre-deposited Bi active electrode and Bi pool systems have first been developed (chapter 4). The Bi active electrode gives redox peaks due to Nd(III)/(0) insertion into and removal from the pre-deposited Bi and the formation of a Bi2Nd intermetallic compound. The Nd(0)/(III) oxidation peak is shown to enable monitoring of the concentration of Nd(III) in LKE in the highly relevant concentration range of between 0.1 mM and 70 mM. This characteristic Nd(III)/(0) response was used to inform the development of the potential for Nd(III) extraction using the Bi pool electrode (chapter 5). The Bi active electrode response to Sm ions is then investigated. Dissolved Sm(III) is shown to first undergo a reduction from Sm(III)/(II), followed by Sm(II)/(0) reduction and insertion, with the formation of a Bi2Sm intermetallic compound (chapter 6). Analysis of the Sm(0)/(II) oxidation and stripping peak demonstrates that Sm concentration can be monitored in the highly relevant concentration range of between 2.3 mM and 100 mM. Unlike Nd, it was demonstrated that Sm ions were unable to be extracted from LKE as Sm(0) through reduction at a Bi pool (chapter 7).