Breeder reactor
A reactor that breeds more fuel than it burns.
Bkleinf2 · CC BY-SA 3.0
A breeder reactor is a type of nuclear reactor that produces more fissile material than it consumes. It does this by maintaining a high neutron economy, meaning enough extra neutrons are available to be absorbed by fertile materials—such as uranium-238 or thorium-232—loaded into the reactor alongside fissile fuel. These fertile materials then transmute into new fissile material that can itself undergo fission. Unlike conventional reactors that rely on the rare uranium-235, breeders can use these more abundant isotopes.
Breeder reactors were initially seen as attractive because they used uranium fuel far more efficiently than light-water reactors. However, interest waned after the 1960s when new uranium reserves were discovered and enrichment methods became cheaper. Several countries, including Russia, India, Japan, the United States, France, and China, have developed and operated breeder reactors. As of April 2026, only Russia operates a commercial fast breeder reactor.
Many types of breeder reactor are possible. In principle, almost any reactor design can be modified to become a breeder by increasing its neutron economy enough to achieve a conversion rate above 1.0. For example, the light-water reactor, a thermal design, evolved into the reduced-moderation water reactor (RMWR) concept, which uses supercritical light water to boost neutron economy. Other envisioned designs include molten-salt cooled, gas cooled, and liquid-metal cooled variants. These can be fueled with uranium, plutonium, minor actinides, or thorium, and may aim to create more fuel, achieve steady-state operation, or burn nuclear waste.
Breeder reactors are broadly divided into two categories based on their neutron spectrum. Fast breeder reactors (FBRs) use unmoderated, fast neutrons to breed plutonium (and possibly higher transuranics) from uranium-238, and can also breed uranium-233 from thorium. Thermal breeder reactors use moderated, slow neutrons to breed uranium-233 from thorium; commercially, a thermal breeder is considered feasible only with thorium fuel, as it avoids building up heavier transuranics.
As of 2026, all large-scale FBR power stations are sodium-cooled liquid metal fast breeder reactors (LMFBRs). These come in two designs: loop type, where primary coolant circulates through heat exchangers outside the reactor tank but inside the biological shield; and pool type, where heat exchangers and pumps are immersed in the reactor tank. As of 2017, only two commercial breeder reactors were operating: Russia’s BN-600 (560 MWe) and BN-800 (880 MWe), both sodium-cooled. Liquid metals other than sodium have been used in FBRs, including mercury, lead, lead-bismuth eutectic, molten tin, and NaK (a sodium-potassium alloy). Mercury and NaK are liquids at room temperature, convenient for experiments but less attractive for full-scale plants due to toxicity and cost. Lead-cooled and lead-bismuth-cooled reactors saw extensive Soviet use, with lead in the BREST reactor and lead-bismuth in Alfa-class submarine reactors and the SVBR-100.
Three of the proposed Generation IV reactor types are FBRs: the gas-cooled fast reactor (cooled by helium), the sodium-cooled fast reactor (based on existing LMFBR and integral fast reactor designs), and the lead-cooled fast reactor (based on Soviet naval propulsion units). FBRs typically use a mixed oxide fuel core of up to 20% plutonium dioxide and at least 80% uranium dioxide, or metal alloys of uranium, plutonium, and zirconium. Enriched uranium can also be used alone. Many designs surround the core with a blanket of tubes containing non-fissile uranium-238, which captures fast neutrons to become plutonium-239, later reprocessed as fuel. Other designs rely on fuel geometry to achieve sufficient fast neutron capture. In a fast spectrum, the fission cross-section for plutonium-239 or uranium-235 is much smaller than in a thermal spectrum, as is the ratio of their fission cross-section to uranium-238’s absorption cross-section. This increases the concentration of fissile material needed to sustain a chain reaction, as well as the breeding ratio.
- type
- Nuclear reactor
- key_feature
- Generates more fissile material than it consumes
- first_developed
- Before 1960s
Lore & Background
Breeder reactors are nuclear reactors designed to generate more fissile material than they consume, achieved through high neutron economy. They can utilize fertile materials such as uranium-238 and thorium-232, which are more abundant than the rare uranium-235 used in conventional reactors. These fertile isotopes are loaded into the reactor alongside fissile fuel; extra neutrons produced during fission are absorbed by the fertile material, transmuting it into new fissile fuel. The defining characteristic of a breeder is a conversion rate greater than 1.0. Many reactor types can be adapted for breeding, including light-water, molten-salt, gas-cooled, and liquid-metal cooled designs. Fast breeder reactors (FBRs) use unmoderated, fast neutrons to breed plutonium from uranium-238 and can also breed uranium-233 from thorium. Thermal breeder reactors use moderated, slow neutrons and are considered commercially feasible only with thorium fuel to avoid heavy transuranic buildup. As of 2026, the only commercially operating breeder reactors are the Russian BN-600 and BN-800, both sodium-cooled liquid metal fast breeder reactors. These use liquid metal as primary coolant to transfer heat to steam for electricity generation. Other coolants used historically include mercury, lead, lead-bismuth eutectic, molten tin, and NaK. FBRs typically use a mixed oxide fuel core of plutonium dioxide and uranium dioxide, or metal alloys of uranium, plutonium, and zirconium. Many designs include a blanket of non-fissile uranium-238 around the core to capture fast neutrons and convert to fissile material.
Reader's Guide
The integral fast reactor (IFR) design specifically addresses waste disposal by using on-site pyroprocessing to recycle uranium and all transuranics, leaving only short-half-life fission products. Breeder reactors are central to several Generation IV reactor concepts, including gas-cooled, sodium-cooled, and lead-cooled fast reactors. Despite their potential, challenges remain in fuel cladding materials, coolant interactions, and economic viability, with current materials like austenitic stainless steel having known limits, while oxide dispersion-strengthened alloy steel is viewed as a long-term solution.
Did You Know?
- The integral fast reactor design includes an on-site electrowinning fuel-reprocessing unit that recycles uranium and all transuranics.
- Fast breeder reactors use unmoderated neutrons, while thermal breeder reactors use moderated neutrons.
The Thermal Advantage of Liquid Metal
The choice of liquid metal as a primary coolant is driven by a cluster of engineering advantages that water simply cannot match. Because metals conduct heat exceptionally well, a liquid metal loop can pull enormous thermal energy away from the core, enabling very high power density. That compactness is precisely what makes these designs appealing for naval vessels and submarines, where every kilogram and cubic centimeter counts. Water-based reactors, by contrast, must be cranked up to very high pressures just to keep the coolant from boiling, and that pressure introduces ongoing safety and maintenance headaches that a liquid metal system sidesteps entirely. The elevated operating temperatures of the metal also feed directly into the thermodynamic efficiency of the power-conversion cycle, improving both electrical output and fuel economy. On the mechanical side, the electrical conductivity of molten metal means the coolant can be circulated with electromagnetic pumps rather than conventional impeller machines. Finally, the very high boiling points of these metals keep vapor pressure low enough that the primary loop can run at or near ambient pressure, and some designs submerge the entire core and heat exchangers in a large pool of coolant, virtually eliminating the risk of losing inner-loop cooling.
A Century of Coolant Experiments
The search for the right liquid metal has been anything but straightforward. Mercury seemed like the obvious first pick because it is liquid at room temperature, and it powered Clementine, the very first liquid metal cooled nuclear reactor. Yet mercury's high toxicity, its relatively high vapor pressure even at room temperature, its low boiling point that produces noxious fumes when heated, its modest thermal conductivity, and a high neutron capture cross-section collectively pushed it out of favor. Lead offers excellent neutron reflection and gamma shielding, but its high melting point makes refueling and servicing difficult, and alloying it with bismuth to lower the freezing point introduces severe corrosion of structural metals.
The Breeder Connection and Fast-Neutron Physics
Liquid metal cooled reactors were the first reactor type adapted specifically for breeder power generation, and that connection remains their defining identity. Most fast-neutron reactors to date have been liquid metal cooled, earning the label liquid metal cooled fast reactors, or LMFRs. When such a reactor is surrounded by a breeding blanket that converts fertile material into additional fissile fuel, it becomes a liquid metal fast breeder reactor, or LMFBR. The physics behind this is elegant: sodium, for instance, is a poor neutron moderator, so it naturally lets the core operate with a fast neutron spectrum, which is exactly the spectrum a breeder needs. Liquid metal coolants have also been applied to thermal-neutron designs, such as the Sodium Graphite Reactor, where graphite serves as the moderator and allows the use of natural uranium instead of enriched fuel. Those SGRs enjoyed high-temperature operation and a strong prompt negative fuel temperature coefficient that made them comparatively easy to control, but they were experimented with in the 1950s beginning with the Sodium Reactor Experiment and soon lost ground to competing light-water designs and persistent sodium-handling problems.
Hazards, Demonstrations, and the Road Ahead
No coolant is without its perils, and liquid metals carry a distinctive set of risks. Inspecting or repairing a reactor buried in opaque molten metal is inherently difficult, and alkali-metal coolants like sodium present a genuine fire hazard. Neutron activation turns sodium into an intensely radioactive medium during operation, though the short half-life means the radioactivity does not create a long-term disposal burden. Today, the Russian BN series and the Chinese CFR series operate commercially, and two Generation IV sodium-cooled LMFR proposals, one oxide-fueled and one a metal-fueled integral fast reactor, point toward the next chapter. The Natrium system, built by TerraPower at Kemmerer, Wyoming, using a GE Vernova Hitachi PRISM reactor, represents the most visible near-term deployment.
Gallery
Frequently Asked Questions
What is a breeder reactor?
A breeder reactor is a nuclear reactor engineered to produce more fissile fuel than it actually burns during operation. Instead of simply depleting its initial fuel load, it converts surrounding fertile material into additional usable fuel through neutron absorption.
What is a breeder reactor's key ability?
Its defining trait is high neutron economy, which spares extra neutrons to transmute non-fissile isotopes into new fissile ones. This lets the reactor sustain itself far longer than conventional designs that rely solely on their starting fuel inventory.
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