Put simply, a nuclear reactor splits atoms and releases the energy that holds their parts together.
If it's been a while since you took high school physics, we'll remind you how nuclear fission works: Atoms are like tiny solar systems, with the nucleus where the sun would be, and electrons orbiting around it. The nucleus is made up of particles called protons and neutrons, which are bound together by something called strong force. Perhaps it was named "strong force" because it's almost too powerful for us to imagine -- many, many billions of times stronger than gravity, in fact [source: Bryson]. Despite the strength of strong force, it's possible to split a nucleus -- by shooting neutrons at it. When that's done, a whole lot of energy is released. When atoms split, their particles smash into nearby atoms, splitting those as well in a chain reaction. (Think a multi-car crash on the freeway.)
Uranium, an element with really big atoms, is perfect for atom splitting because its strong force, though powerful, is relatively weak compared to other elements. Nuclear reactors use a particular isotope called uranium-235 [source: Union of Concerned Scientists]. Uranium-235 is rare in nature; the ore from uranium mines only contains about 0.7 percent uranium-235. That's why reactors use enriched uranium, which is created by separating out and concentrating the uranium-235 through a gas diffusion process [source: NRC].
This process is what gives an atomic bomb, like the ones that were dropped on Hiroshima and Nagasaki, Japan, during World War II, such terrible power. But in a nuclear reactor, the chain reaction is controlled by inserting control rods made of a material like cadmium, hafnium or boron, which absorb some of the neutrons [source: World Nuclear Association]. That still allows the fission process to give off enough energy to heat water to a temperature of about 520 degrees Fahrenheit (271 degrees Celsius) and turn it into steam, which is used to turn turbines and generate electricity [source: Union of Concerned Scientists]. Basically, a nuke plant works like a coal-powered electrical plant, except that the energy to boil water comes from splitting atoms instead of burning carbon [source: Nuclear Regulatory Commission].
In the next section, we'll talk about the different types of reactors and how their key parts work.