>>13443034>Why aren't we using it?
The short answer is unironically "muh terrorism".
The long answer is in the late 70's President Carter in the stopped development of reprocessing of spent fuel and development of fast breeder reactors due to proliferation of nuclear weapon concerns.
Fast breeder reactors require more fissile material than thermal reactors, pressurized water reactors or boiling water reactors because the fission cross section of the fissile isotopes is smaller at the higher neutron energies in the fast reactor spectrum than in the thermal energy spectrum. In the higher energy spectrum the reactor can convert more non-fissile uranium to fissile material than it consumes fissile material. So the fast reactor breeds Pu-239 and allows the 99.3% of U that is non-fissile U-238 to be converted into fissile Pu-239 for use as fuel. Using Pu-239 with natural U as fuel negates the need for enrichment of natural U, but requires reprocessing of the spent fuel to recover the breed Pu-239 from the rest of the radionuclides in the spent fuel for blending with natural U for new fuel.
As a resualt the new fuel with Pu and natural U allows for the Pu to be chemically separated from the U for use in a weapon where as standard pressure water reactors fuel contains U enriched in U-235 which cannot be chemically separated from the U-238.
In simple terms:
Pressure water reactors = Can't reuse spent fuel, can't make nukes with spent fuel.
Breeder reactors = Can reuse spent fuel, can make nukes with spent fuel.