Fast reactor

A nuclear reactor that sustains its chain reaction with fast neutrons, typically cooled by liquid sodium or lead instead of water.

• Ordinary reactors use water to slow neutrons so uranium-235 captures them; fast reactors deliberately leave out this moderator.
• Fast neutrons can split or convert heavier isotopes, including the plutonium in spent fuel, extracting far more energy per tonne of uranium.
• Sodium boils at 883 °C, so the reactor runs hot at near-atmospheric pressure, with no thick pressure vessel.
• Sodium burns in air and reacts with water, past plants had leaks and poor uptime, and most designs need scarce fuel enriched to 5–20% (HALEU).