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Breeder and converter reactors are distinguished by what nuclear reactor physicists called "conversion ratio" or CR. CR is the ratio of the fuel produced by reactor to the fuel it consumes. A converter reactor has CR less than1 while breeder has CR greater than1.
How can nuclear reactor produces fuel while it is burning fuel? The fuel for a Light Water Reactor consist of around5% U- and and the rest mostly is U-. While operating, LWRs are burning U- (fissile isotopes) to produce energy, at the same time they bombard U- (fertile isotopes) with few high energy neutrons (majority of the neutrons in LWRs are low energy or "thermal") that eventually produce U- as fuel. However, in LWRs the amount of U- produced is less than the amount of U- they consume. The CR for LWRs is around0.5-0.6.
While in the fast reactors (where the majority of neutrons are high energy or "fast") the value CR could be around1.1-1.3. Therefore they can produces more fuel that the fuel they consume. After steadily operating, they can be loaded only with fertile isotopes.
breeding being more likely in fast reactors is that fast neutrons have a KE greater thanMev. The activation energy for U.Mev, therefore any energy betweenMev and1.Mev will not be of the right energy to cause fission, therefore the neutron combines with the nucleus and the2 beta decays can occur to breed fissile material. The proportion of energies lower than1.Mev will be lower than the proportion higher. Therefore conversion rate >1 thus breeding occurs.
