The name “fission products” applies to all members of the decay chains— including the original fission fragments. The term fragment describes the initial flying pieces that de-excite before they come to rest as fission products. The majority of fission products have half-lives that range from fractions of a second to about 30 years.
The kinetic energy of the fission fragments accounts for most of the energy released in the fission reaction. The energetic fragments slow and stop in about 10-13 s, (after travelling about 10-3 mm), heating the fuel by passing their energy to the surrounding fuel atoms.
Each fission product consists of a substantial piece of the original nucleus. They are likely to have mass numbers between 70 and 160, with mass numbers near 95 and 140 the most probable. Symmetrical fission (equal fragments) is rare. Fission products are a potential radiological hazard. They also absorb neutrons that might otherwise cause fission, limiting the long-term energy output from the fuel. They have one important useful effect; reactor control would not be possible without the delayed neutrons that result from fission product decay.
