Fission sometimes occurs when certain unstable atoms (eg. U-235) collide with a neutron. This collision can split the atom into two smaller atoms and several more neutrons. If the freed neutrons split other unstable atoms, a chain reaction can occur. In power reactors, the goal is to sustain this chain reaction so that a stable amount of heat is released.

However, neutrons can travel at different speeds. Neutrons from a fission reaction are usually very energetic (around 1,000,000 eV). In typical light water reactors (LWRs), these fast neutrons are either absorbed in U-238 or bounce around in a moderator (water). As the neutrons bounce around, they lose energy as the water molecules gain energy. Once they are “thermal” (around 0.025 eV), the neutrons are 1000 times more likely to split the U-235 than at “fast” energies.



All of today’s commercial reactors exploit the thermal neutron reactions. Another way to release heat is to exploit fast neutrons. That means some different advantages and disadvantages.


  • fast neutrons can “burn” long-lasting actinides resulting in better fuel utilization and less long-lasting waste
  • new fuel sources include current LWR waste, depleted uranium, and thorium
  • extra neutrons per reaction opens possibility of “breeder” reactors that create more fuel than they use


  • fast reactors are expensive compared to thermal reactors
  • higher enrichment is necessary possibly raising proliferation risk
  • sodium (liquid metal) as a coolant presents new risks and challenges
    • alternative: helium (gas) as a coolant presents its own challenges
  • reprocessing would be required

Many nations have experimented with fast reactors. The US program was canceled in 1994 but before that, the experimental EBR-II (19 MWe) produced more than 2 TWh of electricity from 1963 to 1994. France attempted a commercial project called SuperPhenix, which ran from 1985 to 1998. It was shuttered due to problems with corrosion and excessive costs. Russia, India, China, and Japan have fast reactors still in operation.

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