The lightest metal and a cornerstone of fusion fuel cycles, lithium in the reactor blanket breeds the tritium that D-T fusion consumes.
ReviewedLast reviewed: 9 Aug 2026·Category: Glossary
Lithium (Li, atomic number 3) is the lightest metal and the third element in the periodic table. In fusion energy, lithium plays an indispensable role: it is the feedstock from which tritium is bred inside the reactor blanket, closing the D–T fuel cycle that would otherwise be unsustainable.1
Tritium Breeding Reactions
Natural lithium consists of two stable isotopes — 6Li (7.6%) and 7Li (92.4%). Both participate in neutron-induced tritium production:
6Li + n (thermal) → T + 4He + 4.78 MeV
7Li + n (fast, > 2.5 MeV) → T + 4He + n′ − 2.47 MeV
The 6Li reaction is exothermic and has a large cross-section for thermal neutrons, making it the dominant breeding channel. The 7Li reaction is endothermic but produces a secondary neutron, contributing to neutron multiplication in the blanket.2
Key fact: Achieving a tritium breeding ratio (TBR) above 1.0 — meaning the reactor produces more tritium than it burns — is a non-negotiable requirement for any commercial D–T fusion plant. Blanket designs typically target TBR of 1.05–1.15 to cover losses from radioactive decay, processing inefficiency, and inventory build-up.3
Blanket Concepts
Several blanket architectures employ lithium in different chemical forms:
Liquid lithium: Pure liquid Li metal serves as both breeder and coolant. Advantages include high tritium solubility and excellent heat transfer; challenges include MHD pressure drops in strong magnetic fields and chemical reactivity with water and air.
Lithium-lead (LiPb): A eutectic alloy (~17% Li) where lead acts as a neutron multiplier. Used in the European DEMO Helium-Cooled Lithium-Lead (HCLL) design.
Solid ceramic breeders: Lithium orthosilicate (Li4SiO4) or lithium titanate (Li2TiO3) pebble beds paired with beryllium neutron multipliers. This is the basis for ITER's Test Blanket Modules.3
Resource adequacy: Estimated global lithium reserves (roughly 22 million tonnes in 2024, per the USGS) could fuel D–T fusion for thousands of years even at civilisation-scale power output — and seawater contains an additional 230 billion tonnes at roughly 0.17 ppm.4
Beyond Breeding
Lithium also serves as a plasma-facing material in some experiments. Liquid lithium walls and limiters have demonstrated reduced recycling and improved confinement in devices such as NSTX and LTX at Princeton. These results suggest lithium may play dual roles — as both a fuel-cycle material and a plasma performance enhancer — in future reactors.