J.W. Elling, G.D. Jarvinen, S. Yarbro, J. Gatewood, P.T. Lueangrujiwong
Molten Salt Solutions,
United States
Keywords: lithium isotope enrichment 6Li
Summary:
Nuclear fusion is a promising emission-free energy source. Most fusion energy concepts are based on fusion of the hydrogen isotopes deuterium (D) and tritium (T) to generate energy. Tritium is an unstable isotope of hydrogen with a relatively short half-life and so it needs to be generated. Tritium breeding can be achieved in the fusion reactor by bombarding the 6Li isotope with neutrons. The required 6Li content is substantially higher than in natural lithium – driving the need for large scale, commercial lithium isotope enrichment. In the near term, enriched 6Li will be required for private and public fusion energy pilot plant development. Further, to enable the economic analysis of fusion reactors, the cost of lithium material enriched in 6Li must be established. Ultimately, large quantities of lithium material enriched in 6Li will be required for commercial deployment of fusion power plants. Lithium-6 was separated at large scale during the cold war in order to generate tritium for thermonuclear weapons. Intense research at Y-12 in Oak Ridge, Tennessee resulted in the development of the COLEX (column exchange) process in which an amalgam of lithium and mercury was used to exchange lithium with an aqueous solution of lithium hydroxide. The COLEX process was scaled up in two large facilities to meet the demand for lithium-6 production, which unfortunately resulted in severe mercury contamination of the environment. The Y-12 COLEX production was decommissioned in 1965 and there are no remaining large-scale lithium isotope enrichment capabilities. Many laboratories have developed alternative methods for enriching lithium isotopes. Of these, a solvent extraction method developed at Berkley is considered a promising candidate for scalability. Solvent extraction (also called liquid-liquid extraction) exploits the difference in the partition coefficients between the isotopes in an organic solvent phase and an aqueous phase. The Berkley method uses a costly crown ether extractant that increases the solubility of lithium in the chloroform organic solvent while providing a tight cage around the lithium ion that energetically favors the lithium 6.. With an enrichment coefficient of 1.03, the Berkley method will require more than 3000 stages of extraction to enrich 6Li from the 7.4% natural abundance to 30%. Implementing 3000 stages of separation with commercial solvent exchange contactors (mixer-settlers) is a costly proposition and the volumes of solvents required to fill a cascade of this size would be very costly in reagents and the volume of chlorinated organic solvents. Molten Salt Solutions is developing an alternative system for solvent extraction using high-speed countercurrent chromatography (HSCCC). HSCCCs use centrifugal force by rotating coiled tubing around a central axis, generating hundreds of g's in alternating directions that provides very efficient mixing and separation with countercurrent flow. The proven efficiency of mixing and separation will reduce equipment size, capital cost, and waste generation in the lithium isotope enrichment process by orders of magnitude.