When a neutron collides with a larger atom, it excites it and splits it into two smaller atoms known as fission products. Additional neutrons are released, potentially triggering a chain reaction. When an atom splits, a tremendous amount of energy is released.Uranium and plutonium are the most commonly used fission fuels in nuclear power reactors because they are easy to initiate and control. In these reactors, the fission energy released heats water into steam. Steam is used to spin a turbine in order to generate carbon-free electricity.

When two atoms collide, they fuse to form a heavier atom, such as when two hydrogen atoms fuse to form one helium atom. This is the same process that powers the sun and generates enormous amounts of energy—many times more than fission. It also does not produce fission products that are highly radioactive. Scientists are studying fusion reactions, but they are difficult to sustain for long periods of time due to the enormous amount of pressure and temperature required to join the nuclei together.