Nuclear power is generated by reactions that take place inside the nuclei of certain atoms. One of two types of fuel powers today's nuclear plants: uranium or plutonium. Nuclear power generation relies on a process called fission, which begins when a fast-moving free neutron "bullet" strikes the nucleus of a uranium or plutonium atom. Because a neutron possesses no charge, it can approach a uranium or plutonium nucleus, which, like all atomic nuclei, carries a positive charge. A neutron also carries sufficient mass to move other particles with which they come in contact.
Heavy elements like uranium and plutonium contain large numbers of protons and neutrons bound tightly together in the nuclei of their atoms. Charged with extra internal energy from the neutron "bullet," the atom's nucleus becomes elongated and begins to split into two positively charged sides. Repulsive forces between the two positive ends overcome the attractive forces holding the nucleus together, and the nucleus separates into two smaller nuclei, called fission fragments. The energy that bound the individual nuclei is carried away by the moving fragments, which generate heat through their collisions with other particles, and by the two or three free neutrons that are produced. Energy is also released in the form of radiation: gamma rays and beta particles.
If sufficient fissile uranium or plutonium atoms are present, the neutrons released in fission become "bullets" in their own right, striking surrounding nuclei and causing them to break apart and produce more neutron bullets and more energy as well. This self-sustaining and accelerating cause-and-effect phenomenon is called a chain reaction. Electric power plants can use the heat generated by a carefully controlled chain reaction to drive steam turbines, just as in oil- or natural-gas-fired plants. In the detonation of an atomic bomb, the reaction is controlled for only as long as it takes for most of the material to undergo fission: a few millionths of a second.
Not all uranium and plutonium atoms can be split in this way. The most common type of uranium, for example, called U-238 because its atomic weight is equal to 238, is not fissile. Another type, U-235, is. This isotope, chemically identical to U-238 but with three fewer neutrons, is unstable -- some forces try to keep its nucleus together, and some try to disrupt it -- and will readily undergo fission. U-235 exists in minute quantities, however; only seven out of a thousand atoms of raw uranium are U-235. For nuclear fission to happen on a large, useful scale, then, this fuel must be separated from the U-238 and prepared in both sufficient quantity and concentration.