Nuclear fusion and nuclear fission are two different types of energy-releasing reactions in which energy is released from high-powered atomic bonds between the particles within the nucleus. The main difference between these two processes is that fission is the splitting of an atom into two or more smaller ones while fusion is the fusing of two or more smaller atoms into a larger one. Comparison chart Embed this chart Nuclear Fission, Nuclear Fission is the splitting of a large atom into two or more smaller ones. Fusion is the fusing of two or more lighter atoms into a larger one. Natural occurrence of the process: Fission reaction does not normally occur in nature. Fusion occurs in stars, such as the sun. Byproducts of the reaction: Fission produces many highly radioactive particles. Few radioactive particles are produced by fusion reaction, but if a fission "trigger" is used, radioactive particles will result from that. Conditions: Critical mass of the substance and high-speed neutrons are required. High density, high temperature environment is