There are many differences between nuclear fission and fusion.
Nuclear Fission is :
-the splitting of a nucleus into smaller particles, releasing a high amount of energy
-Does not occur naturally
-produces many highly reactive particles
-energy released is million times greater that in chemical reactions but lower than the energy released by nuclear fusion
Nuclear fusion is:
-the combination of 2 smaller atoms to create a large atom releasing energy
-Occurs in stars such as the sun
-produces few radioactive particles, but requires fusion trigger
-energy released is 3 to 4 times greater than the energy released by fission