A nuclear reactor is a facility that produces electricity by turning turbines with steam that is produced by boiling water with radioactive material. The radioactive material is usually uranium-235, consisting of small ceramic pellets encased in a metal rod. The chemical reaction that produces radiation occurs when a neutron strikes an atom of uranium, turning it into U-236 and causing it to split into two different atoms and expel neutrons. The ongoing splitting of atoms is called fission.
Despite common fears, a nuclear reactor cannot explode in the same manner as a nuclear bomb because the chain reaction is controlled with rods that absorb neutrons. A nuclear bomb is the result of an uncontrolled chain reaction. Also, the fissile material in a reactor is different from a thermonuclear bomb. Nuclear reactors operate under stringent safety guidelines.
Two types of fission reactors exist: boiling water reactors and pressurized water reactors. Both have the same function, but BWRs make use of steam from the reactor core to turn generator turbines directly. PWRs include two separate closed water systems. The primary one is pressurized to prevent boiling, and the heat transfers to the other one, which boils and powers the turbine.
One goal of the nuclear science community is to devise a fusion reactor, or a reactor that produces its radiation through fusion, a more powerful process seen in stars. However, nuclear fusion requires
enormous amounts of heat that make it cost-prohibitive to attempt.