Fast Breeder Reactor (FBR)
A nuclear reactor that produces more fissile material than it consumes — a net generator of fuel. It does this by using fast (unmoderated) neutrons to convert fertile materials (U-238 or Th-232) into fissile ones (Pu-239 or U-233).
How It Differs from Conventional Reactors
Conventional thermal reactors (PWRs, PHWRs, BWRs) use a moderator (water, heavy water, graphite) to slow neutrons down, because slow (“thermal”) neutrons are more likely to trigger fission in U-235. But the majority of natural uranium is U-238, which absorbs thermal neutrons without fissioning — a fuel efficiency problem.
FBRs work differently:
- No moderator: Neutrons remain fast.
- Fast neutrons + U-238: Fast neutrons have a high probability of converting U-238 into Pu-239 through neutron capture and beta decay. The U-238 “blanket” surrounding the FBR core is transmuted into usable fissile plutonium.
- Breeding ratio > 1: The reactor produces more Pu-239 from the blanket than the Pu-239 it consumes as fuel. Net result: fuel stock grows over time.
This turns U-238 — normally a mostly inert waste product in enrichment — into a vast energy resource. Estimates suggest FBRs could extract 60–70× more energy from natural uranium than conventional thermal reactors.
Coolant: Liquid Sodium
Because water would slow neutrons (defeating the fast-spectrum design), FBRs use liquid sodium as a coolant. Sodium is an efficient heat transfer medium at the temperatures involved (~500°C) but reacts violently with water and burns in air, demanding exceptional engineering containment. This is a primary reason FBRs are more complex and expensive to build than conventional reactors.
India’s PFBR (Kalpakkam)
The Prototype Fast Breeder Reactor (500 MWe) at Kalpakkam, Tamil Nadu is India’s Stage 2 entry in the india-three-stage-nuclear-programme. Built by BHAVINI; technology developed by IGCAR. First criticality: 6 April 2026.
Key design features:
- Fuel: Uranium-Plutonium Mixed Oxide (MOX) from reprocessed Stage 1 spent fuel
- Blanket: Uranium-238 (converting to Pu-239); future extension to Thorium-232 (converting to U-233 for Stage 3)
- Coolant: Liquid sodium
- Closed fuel cycle: Spent fuel reprocessed and recycled
Global Context
As of 2026, Russia operates the only commercial FBRs: the BN-600 (operating since 1980) and BN-800 (since 2016), with the BN-1200 planned. India’s PFBR makes India the second country with a commercial FBR.
The World Nuclear Association’s history notes that multiple countries (US, UK, France, Russia) ran experimental FBRs from 1959, but most closed by 2009. The technology fell out of favour when uranium prices dropped in the 1980s (if uranium is cheap, you don’t need to breed more). India’s calculus is different: because it has almost no uranium but vast thorium, breeding is a strategic necessity rather than an economic choice.
Connection to Nuclear Fission Science
FBRs exploit the same nuclear-fission physics as conventional reactors but optimise for a different outcome: fuel production over immediate power generation. The breeding insight — that fertile material absorbs a neutron and becomes fissile — was understood as early as 1940 (Bretscher and Feather at Cambridge; McMillan and Abelson in the US). Plutonium’s identification as “element 94” by Glenn Seaborg in 1941 was the foundational discovery enabling the entire plutonium-breeder pathway.