Which best describes nuclear fission

Nuclear fission is either a nuclear reaction or radio active decay process in which nucleus (the center) of an atom splits into smaller parts called nuclei. This is an extremely exothermic reaction (i.e a reaction which produces heat) resulting into release of massive amount of energy in the form of heat and sometimes light. The reaction produces much more energy as compared to a similar mass of a conventional fuel, such as Petrol/Kerosene/Petroleum Gas etc. This makes Nuclear fission an extremely dense and at times very destructive source of energy. Some common elements capable of Nuclear fission are Uranium, Plutonium etc. Though in modern days Nuclear Fission are finding application in being a source of energy (such as a Nuclear power plant), but they are also used in destructive format as Nuclear Bombs and it's one of the top most imminent threats to the existence of humanity in future (in the event of a Nuclear war).