There is a transformation of gravitational energy into kinetic energy. Nuclear energy is the energy released when nuclear reactions take place. In certain nuclear reactions, the sum of the masses of the reactants is more than the sum of the masses of the products. For more details, please consult some physics textbook at college level. The former is a spontaneous process, which however requires to build up to a chain reaction before large quantities may be extracted.
Fusion requires extreme conditions of temperature and sufficient density.
All present day nuclear reactors are fission reactors. These neutrons can then go on to produce more fission reactions. So there is a chain reaction. This is because some of the mass of the original large nucleus is converted into energy. In future it may be possible to build reactors to produce energy from fusion.