Other terms can be found in IEER's on-line glossary. A reactor that is designed to produce more fissile material than it consumes. A negatively-charged elementary particle.
Fissile materials can sustain nuclear chain reactions.
Most fissionable materials that are not fissile cannot sustain chain reactions. The amount of time it takes half of a given quantity of a radioactive element to decay. Isotopes of elements have the same atomic numbers, but different mass numbers. A material used in a nuclear reactor to slow down the fast neutrons emitted in the process of fission. Free neutrons decay into a proton, an electron and an anti-neutrino.