Neutrons capture

Neutrons are captured by the nuclei they encounter in the reactors. The probability of the capture varies greatly with the energy of neutrons. The most common and simplest type of capture is the radiative capture, where the capture cause only excitation in the nucleus, which gets rid of its excitation by emitting a gamma ray. The nucleus with a extra neutron can be radioactive : neutrons are able to activate matter. For a handful of very heavy nuclei (uranium, plutonium), the capture leads to fragmentation of the nucleus. That is fission.