Conservation of energy tells us that energy cannot be created or destroyed. Clearly, energy is required for radioactivity to occur. Where is this energy coming from?
The energy needed for radioactive processes comes from the the conversion of rotational kinetic energy into linear kinetic energy. This energy is contained within the movements of the protons and neutrons within the nucleus. The radioactive decay happens when the nucleus of an atom becomes unstable because of the repelling forces between protons within nucleus. If the number of protons is larger than 84 or if the difference between the number of protons and number of neutrons is extremely larger, then the nucleus becomes unstable.
There exists three types of nuclear radiation such that: alpha emission, beta emission and gamma emission. The alpha radiation does emit alpha particles which are positively charged and relatively large. The alpha particles can penetrate the matter if it is sustained. The beta particles can penetrate the matter much better than alpha particles and usually they can be stopped by aluminum shields. The gamma radiation penetrates the best the matter because it has no charge and it can be stopped by lead shields.
Yes energy can neither be created nor destroyed but energy can be transformed from one form to other.
Nuclear force which bind nucleons ( protons , neutrons) are very strong forces compared to other electrostatic forces.
Thus already a strong energy existed inside nucleus. Some part of this energy is released when an unstable nuclide
( which has unstable nuclear configuration)undergoes decay process. There are many theories behind explanation of
nuclear force and its strength.