The process in which a nucleus of an atom breaks up, emitting subatomic particles and/or energy is called radioactive decay. This process always involve a nuclear fission reaction and is a spontaneous process. Fission reaction is one type of nuclear reaction. This type occurs when a nuclei is being divided. Fission reaction takes place in any of the heavy nuclei after capture of a neutron. The process of radioactive decay would always discharge a particle such as alpha, gamma, beta, positron or neutron. It is a process where there is a loss of the elementary particles from the unstable nucleus of an atom.