The PbLi eutectic alloy serves triple duty as tritium breeder, neutron multiplier, and heat-transfer fluid in liquid blanket concepts — simplifying fusion plant architecture at the cost of managing MHD effects, tritium permeation, and corrosion.
The lead-lithium eutectic (Pb-15.7 at.% Li, often written PbLi or Pb-17Li by weight percent) is a liquid alloy that melts at 235 degrees Celsius and remains liquid well above 1000 degrees Celsius. In fusion blanket design, it performs three functions simultaneously: the lithium-6 component breeds tritium via neutron capture, the lead component multiplies neutrons through (n,2n) reactions, and the flowing liquid extracts fusion heat for power conversion. This functional integration eliminates the need for separate solid breeder ceramics, beryllium multiplier pebbles, and dedicated coolant — a compelling simplification.1
Two leading PbLi blanket architectures have emerged. The helium-cooled lithium-lead (HCLL) concept, developed primarily in Europe, uses high-pressure helium as the primary coolant and structural heat remover, with PbLi circulating slowly mainly as a breeder and multiplier. The dual-coolant lithium-lead (DCLL) concept, developed in the United States, allows PbLi to reach higher temperatures (up to 700 degrees Celsius or more) within SiC/SiC flow channel inserts while keeping the reduced-activation steel structure helium-cooled at lower temperatures.2 The DCLL approach unlocks higher thermal efficiency but demands insulating inserts to manage magnetohydrodynamic (MHD) pressure drops.
Tritium bred within the PbLi must be extracted before it permeates through structural walls into the reactor building. Because tritium solubility in PbLi is extremely low (on the order of one atomic part per billion at operating temperatures), dissolved tritium partial pressures are high, driving aggressive permeation through steel. Extraction technologies under development include vacuum permeators, gas-liquid contactors, and packed-column systems. Permeation barrier coatings — typically alumina or erbium oxide deposited on steel surfaces — are considered essential but have proven difficult to maintain crack-free under thermal cycling and irradiation.4
PbLi is corrosive to ferritic-martensitic steels, particularly at temperatures above 450–500 degrees Celsius, through dissolution of iron, chromium, and tungsten. Corrosion rates increase with flow velocity due to mass-transport effects. Alumina coatings developed for permeation barriers may also mitigate corrosion, but no single coating solution has yet been qualified for the combined corrosion-permeation-irradiation environment of a DEMO blanket.1