Development of a novel electrochemical pyroprocessing methodology for spent nuclear fuels

Stevenson, Anthony John (2017) Development of a novel electrochemical pyroprocessing methodology for spent nuclear fuels. PhD thesis, University of Nottingham.

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Nuclear power remains the most dense and reliable primary form of energy supply worldwide. Electricity generation via fission is also inherently carbon free, with environmental footprints rivalling modern renewable options. However issues arise from the production of highly irradiated spent fuels, with current management options limited to geological repository storage or, more desirably, closing of the fuel cycle by partitioning to recover fissionable species.

Extensive research has been pursued over the past half century in an effort to address an accumulating oxide spent fuel inventory and to circumvent shortcomings of raw uranium supplies. Pyrochemical reprocessing or ‘pyroprocessing’ using molten salt electrolysis for the recovery of desirable spent fuel components is an increasingly sought after solution to the above issues. First developed in the late 1990’s, the FFC Cambridge Process has shown to be a cornerstone in the electrochemical reduction of metal oxides and potentially offers a new iteration of pyroprocessing by introducing a direct conversion of spent oxide fuels to metals. These metals are easier to reprocess and specifically recovered elements can be used directly in advanced civilian nuclear reactors and metallic fuel cycles.

This thesis considers the role that an FFC based process could play in establishing a more sustainable, efficient and safer method for the select recovery of key metals from mixed oxide composites. Using an array of surrogate materials, the appropriation of an effective procedure was investigated in both CaCl2 and LiCl-CaCl2 Eutectic (LCE) molten salts at 810oC and 600oC respectively. At all stages of the process, feeds and electrolysis products have been examined by an assortment of ex-situ analytical tools, primarily SEM, EDS and XRD.

A process engineering approach was taken to designing suitable reactors and cells with the aim of improving operational characteristics, greater electrochemical reduction efficiency and high yields of pure products.

Preparation of electrolyte and feed oxide electrodes (surrogate or spent fuel) was investigated, including unique electrochemical treatment for the two molten salts and precedent for the creation of the oxide electrode via cold pressing or slip casting to kinetically aid optimal reduction. A series of investigations considering the thermodynamic performance of CaCl2 from a standpoint of electronic conduction were carried out, and considerable improvements found via the implementation of a simple cathodic sheath.

Selective partitioning was shown possible by the intended mechanism of partial direct reduction and anodic dissolution in the 2NiO-CeO2 binary. Partitioning of Zr from ZrO2-CeO2, and Ti from TiO2-CeO2 was also achieved, however in both cases it was via the gradual chemical dissolution of partially reduced Ce(III) into the molten salt or phase separation between liquid Ce and solid Zr. Extensive CV experiments were performed to enhance understanding of redox chemistry for each species investigated. CeOCl was found to be the only semi-stable phase of Ce present at potentials between -1.0 V vs. Ag/AgCl and its final reduction potential at approximately -1.95 V in CaCl2 at 810oC.

Active CV experiments using PuO2 and a MOX fuel sample containing 5% PuO2 were initiated, revealing remarkably similar electrochemical behaviour of PuO2 and the CeO2 surrogate. Both PuO2 and the bulk UO2 content MOX could be reduced in CaCl2 and in the lower temperature LCE whilst avoiding any decomposition of the electrolyte. Consequently a route for the direct electrochemical reduction of spent oxides fuels was shown plausible and offers a promising alternative to current pyroprocessing technology, with beneficial implications to the wider materials processing field.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Chen, George
Hu, Di
Keywords: Spent Nuclear Fuels, Molten Salts, Reprocessing, Electrochemistry
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 39300
Depositing User: Stevenson, Anthony
Date Deposited: 15 Mar 2017 04:40
Last Modified: 14 Oct 2017 12:12

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