Answer is : Chain reaction
Nuclear fusion: two or more atomic nuclei collide to form one or more different atomic nuclei and subatomic particles.
Alpha decay (α-decay) : The radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or ‘decays’ into an atom with a mass number that is 1/4th of the original, atomic no 1/2 of the original nucleous. example U-238 decays to thorium-234.
Nuclear chain reaction: occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions. (suitable for our case)
Gamma Decay: when Radioactive decay rxn takes place 3 kinds, alpha, beta and gamma rays emitting particles are formed. the fission rxn emitting gamma rays particles called gamma decay