Hi there, I have came across a question in past exam papers that basically asks... What do the terms Conversion and breeding mean? I put conversion is the change from a fissionable element to a fissile element such as U238 to Pu239. When more of these fissile products are created than used (conversion gain > 1) breeding is achieved. Then the question goes on to ask "Explain why conversion is usual in thermal reactors but breeding is normally restricted to fast reactors". My best guess for breeding being more likely in fast reactors is that fast neutrons have a KE greater than 100Mev. The activation energy for U238 1.749Mev, therefore any energy between 100Mev and 1.749Mev will not be of the right energy to cause fission, therefore the neutron combines with the nucleus and the 2 beta decays can occur to breed fissile material. The proportion of energies lower than 1.749Mev will be lower than the proportion higher. Therefore conversion rate > 1 thus breeding occurs. As for the thermal reactor having conversions but no breeding, I have no idea and would appreciate some help please. Thank you.