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Novel approaches in Li isotope enrichment to support the deployment of nuclear fusion reactors


   Department of Chemical Engineering & Analytical Science

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  Dr C Sharrad, Dr Kathryn George, Prof Philip Edmondson, Dr Mark Ogden  Applications accepted all year round  Funded PhD Project (UK Students Only)

About the Project

Lithium has two naturally occurring, non-radioactive isotopes – 6Li, which is 7.5% abundant and 7Li, which is 92.5% abundant. 6Li will be used in breeder blankets to generate tritium to power the proposed deuterium-tritium fusion reactors.1  

The UK Government has committed to building STEP (Spherical Tokamak for Energy Production), a prototype fusion power plant, by 2040, which will use 6Li in its breeder blankets.2 Giegerich estimates that a fusion reactor will need 112 kg of 6Li per year per GW of fusion energy.3 Approximately 52 tons of isotopically pure 6Li (based on a lithium-lead blanket enriched to 90% 6Li) is required for the operation of a 2 GW prototype fusion power plant, such as STEP, and this entire quantity of fuel is needed at the point the reactor is commissioned as current fusion reactor designs do not allow for refuelling. 

Historically, the COLEX (COLumn EXchange) process was used at the Oak Ridge Y-12 National Security Complex to separate lithium isotopes on an industrial scale in the 1950s and 1960s.4 The COLEX process is a countercurrent solvent extraction process with an aqueous lithium hydroxide phase and a liquid metallic mercury “solvent” phase which produces a lithium-mercury amalgam.5, 6 6Li preferentially dissolves in mercury over 7Li. Approximately 11 million kilograms of mercury were used at the Y-12 plant between 1950 and 1963, however, there were significant mercury discharges (both gaseous and liquid) to the local environment and clean-up is still continuing.3 As a result of the contamination, the COLEX process has been banned in the USA.  

The stockpile of 6Li generated by the Y-12 plant is used to meet the current, limited demand, along with products from China and Russia, however, this will be unable to satisfy future anticipated demand. The lack of a current industrial-scale supply chain has been described as a “major issue” and 6Li production has been assessed to be at Technology Readiness Level (TRL) 1.7  

              Numerous technologies are currently being considered to deliver the levels of 6Li necessary to support the successful deployment of fusion reactors, with no obvious stand-out option. One of these options is solvent extraction; industrial-scale solvent extraction processes have been utilised in the nuclear sector since 1954.8 Many solvent extraction plants have been operated globally, including on a commercial basis, e.g., in the UK and France,9 and therefore have well-established expertise and technology readiness for large, production-scale throughput. The application of solvent extraction methods in isotopic separations, let alone specifically Li isotope separations, is still very much in its infancy in the UK in contrast to spent nuclear fission fuel separations. It has been known for several decades that macrocyclic ethers, such as crown ethers and cryptands, can be used as extractants to enrich 6Li,10 but research focussing on understanding process requirements is needed to deliver the quantities of 6Li to support the commercial deployment of nuclear fusion reactors on currently projected timescales.

So the overall project hypothesis is:

·        A solvent extraction process can be developed using macrocyclic ether extractants to successfully enrich 6Li at the scales required to support fusion reactors that is economically viable while minimizing environmental burdens. 

The project aims and objectives are consequently as follows:

Key aim 1: Assess the key variables in solvent compositions that can maximise Li enrichment output  

·        Objective 1a: Systematic studies of macrocyclic ether extractants (e.g. 12-crown-4, benzo-15-crown-5) exploring the influence of the size-fit concept on 6Li enrichment.

·        Objective 1b: Assess diluent effects.

·        Objective 1c: Explore the influence of phase modifiers to improve enrichment performance.

A comprehensive suite of characterisation techniques will be utilised to assess both the thermodynamics and kinetics of 6Li enrichment using these solvents. Compositional analyses using, for example, ICP-MS will be conducted to determine the nature and extent of Li transfer/enrichment, while acid and water contents of the solvent phase post-contact will be determined. Physicochemical properties (e.g. density and viscosity) of the solvent phase pre- and post-contact will be ascertained and used to inform contactor studies.   

Key aim 2: Assess the key variables in aqueous phase compositions that can maximise Li enrichment (to be performed in parallel with work associated with key aim 1)  

·        Objective 2a: Assess the anion dependency (e.g. NO3-, halides, bulky anions) on 6Li enrichment.

·        Objective 2b: Determine whether aqueous phase acid content influences 6Li enrichment.

·        Objective 2c: If time allows, investigate if aqueous phase holdback reagents can improve 6Li enrichment levels.

Key aim 3: Conduct contactor studies to assess scale-up requirements for identified lead solvent extraction systems

o  Objective 3a: Assess the viability of centrifugal contactors for process scale up across single and multiple stages (accessing the UTGARD NNUF).

o  Objective 3b: Compare and contrast studies for objective 3a with pulsed column contactors.

o  Objective 3c: Estimate contactor stage numbers required to meet target 6Li enrichment levels.

o  Objective 3d: Produce a flowsheet/process model in conjunction with project sponsors that will provide the required enrichment and throughput.

This position is available to UK students only.

GREEN CDT


Funding Notes

Funding provided by the National Nuclear Laboratory and EPSRC covers the fees for home (UK) students. These funds are available for UK/EU students but EU students will need to provide funding to cover the international student fees.

References

(1) Murali, A.; Zhang, Z.; Free, M. L.; Sarswat, P. K., physica status solidi (a) 2021, 218 (19), 2100340. (2) UK Government, Towards Fusion Energy - The UK Government's Fusion Strategy. Department for Business, Energy and Industrial Strategy, Ed.; 2021. (3) Giegerich, T.; Battes, K.; Schwenzer, J. C.; Day, C., Fusion Engineering and Design 2019, 149, 111339. (4) Brooks, S. C.; Southworth, G. R., Environmental Pollution 2011, 159 (1), 219-228. (5) Ault, T.; Brozek, K.; Fan, L.; Folsom, M.; Kim, J.; Zeismer, J., Department of Nuclear Engineering University of California 2012. (6) Lewis, G. N.; Macdonald, R. T., Journal of the American Chemical Society 1936, 58 (12), 2519-2524. (7) Surrey, E., Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 2019, 377 (2141), 20170442. (8) Patterson, J. P.; Parkes, P., Recycling uranium and plutonium. The Nuclear Fuel Cycle, Wilson, P. D., Ed. Oxford Science Publications: Oxford, 1996; pp 138-160. (9) Wymer, R. G., Reprocessing of Nuclear Fuel. Chemical Separation Technologies and Related Methods of Nuclear Waste Management: Applications, Problems, and Research Needs, Choppin, G. R.; Khankhasayev, M. K., Eds. Springer Netherlands: Dordrecht, 1999; pp 29-52. (10) Nishizawa, K.; Takano, T.; Ikeda, I.; Okahara, M., Separation Science and Technology 1988, 23 (4-5), 333-345.
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