Nuclear energy comes from mass-to-energy conversions that occur in the splitting of atoms larger than Iron or joining atoms smaller than Iron.

The small amount of mass that is lost in either of these events follows Einstein's famous formula E = MC², where M is the small amount of mass and C is the speed of light, 3.0x10⁸ m/s. In the 1930s and '40s, humans discovered this energy and recognized its potential as a weapon.

Technology developed in the Manhattan Project successfully used this energy in a chain reaction to create nuclear bombs.

Soon after World War II ended, the newfound energy source found a home in the propulsion of the nuclear navy, providing submarines with engines that could run for over a year without refueling.

This technology was quickly transferred to the public sector, where commercial power plants were developed and deployed.

Fission and Fusion

• Fission is the splitting (or fissure) of a large atom such as Uranium or Plutonium into two smaller atoms, called fission products, several neutrons, and very much energy. This nuclear reaction was the first to be discovered. All commercial nuclear power plants use this reaction to generate electricity.
• Fusion is the combining of two small atoms such as Hydrogen or Helium to produce heavier atoms and energy. These reactions can release more energy than fission without producing radioactive byproducts. Fusion reactions occur in the sun, using Hydrogen as fuel and producing Helium as waste. This reaction has not been commercially developed and is a serious research interest worldwide, due to its promise of limitless, pollution-free, and non-proliferation features.

Click here to see animations of fission and fusion reactions.

The above article focuses on nuclear fission. In order to harness fusion, many daunting engineering and physics problems must be solved. The timeline for solving these problems is undefined, so we as a society must turn to other solutions to solve the energy problems.

Source: http://www.whatisnuclear.com/articles/nucenergy.html