Conversion ratio: (in a nuclear reactor) the ratio of new fissile nuclei to fissioned nuclei. In a normal reactor this as around 0.6, at high neutron energy in a fast reactor it may exceed 1.0, due to more neutrons per fission, making it a net breeder. (Above about 0.1 MeV, ie fast neutron spectrum, the number of neutrons released per fission increases from around 2.5, and at 10 MeV it reaches about 4.)