Nuclear energy comes in two varieties.
Fission is the decay of heavy nuclei under the bombardment of "slow" neutrons.
Fusion fuels the stars converting hydrogen into helium.    
dynamic fission  
light, less massive elements fuse under stellar pressures. heavy, more massive elements fission or break up.  
fusion fission  
strong weak  
Strong force binds Weak force decays  

Einstein's Cosmos

Murray Gell-Mann

Atomic energy