Conventional thermal power plants all have a fuel source to provide heat. Examples are gas, coal, or oil. For a nuclear power plant, this heat is provided by nuclear fission inside the nuclear reactor. When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) is struck by a neutron it forms two or more smaller nuclei as fission products, releasing energy and neutrons in a process called nuclear fission. The neutrons then trigger further fission. And so on. When this nuclear chain reaction is controlled, the energy released can be used to heat water, produce steam and drive a turbine that generates electricity. It should be noted that a nuclear explosive involves an uncontrolled chain reaction, and the rate of fission in a reactor is not capable of reaching sufficient levels to trigger a nuclear explosion (even if the fission reactions increased to a point of being out of control, it would melt the reactor assembly rather than form a nuclear explosion). Enriched uranium is uranium in which the percent composition of uranium-235 has been increased from that of uranium found in nature. Natural uranium is only 0.72% uranium-235, with the rest being mostly uranium-238 (99.2745%) and a tiny fraction is uranium-234 (0.0055%).

Classification by type of nuclear reaction

Nuclear fission. Most reactors, and all commercial ones, are based on nuclear fission. They generally use uranium as fuel, but research on using thorium is ongoing. This article assumes that the technology is nuclear fission unless otherwise stated. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction:

Thermal reactors use slow or thermal neutrons. Most power reactors are of this type. These are characterized by neutron moderator materials that slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalized. Thermal neutrons have a far higher probability of fissioning uranium-235, and a lower probability of capture by uranium-238 than the faster neutrons that result from fission. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor’s systems.

Fast neutron reactors use fast neutrons to sustain the fission chain reaction. They are characterized by an absence of moderating material. Initiating the chain reaction requires enriched uranium (and/or enrichment with plutonium 239), due to the lower probability of fissioning U-235, and a higher probability of capture by U-238 (as compared to a moderated, thermal neutron). In general, fast reactors will produce less waste and the waste they do produce will have a vastly shorter halflife, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see fast breeder or generation IV reactors).

Nuclear fusion. Fusion power is an experimental technology, generally with hydrogen as fuel. While not suitable for power production, Farnsworth-Hirsch fusors are used to produce neutron radiation.

Radioactive decay. Examples include radioisotope thermoelectric generators and atomic batteries, which generate heat and power by exploiting passive radioactive decay.

A device that produces energy by nuclear fission. Heat energy is used to boil water and the steam turns a turbine and produces electricity in a nuclear power plant