Nuclear power has been used in a wide variety of applications for decades. Most people are at least vaguely familiar with the existence of nuclear power plants for civil utilities and aboard naval vessels, but perhaps not so familiar with the details surrounding them. Beyond that is a whole class of nuclear-based, electricity-producing devices that are sometimes called atomic batteries, which are used on spacecraft and are relatively unknown to the public at large.
There are two fundamentally different means of harnessing nuclear energy to produce electricity. The first and best known is the fission reactor. Reactors split the atoms of highly enriched uranium or plutonium fuel in a controlled reaction to release energy, including heat. The heat is used to boil water into steam, which is converted into mechanical energy by a turbine. That mechanical energy is then used to run a generator and produce electricity. Another form of nuclear power is the radioactive decay battery (sometimes called an atomic battery), which uses the radiation released by the decay of certain radioactive substances as a source of heat to generate electricity. There are a variety of designs for both fission reactors and atomic batteries, and some are especially well-suited to particular functions.
The most common use for a nuclear power plant is to generate electricity for civilian consumption. In 2009, 15 percent of the world's total electrical output was met through these utility-scale nuclear power plants, with 436 such facilities in service in 30 different countries.
Nuclear reactors are also used on sea-going vessels, where they provide both electricity and mechanical power for propulsion. The reactor design used for these vessels is almost always the pressurized water type. This type is widely used in civilian reactors designs, but is prized for use in ships due to its ability to function well in an unstable platform (like a ship pitching and heaving at sea) Instead of piping steam to a turbine, it transfers heat energy through super-heated water kept under very high pressure. Examples in use are found among aircraft carriers, submarines, missile cruisers, and icebreakers.
The major application of the atomic battery power plant is on space satellites that have a substantial, long-term demand for electricity that cannot be met through other means, such as solar panels. A good example of an atomic battery in operation is the radioisotope thermoelectric generator, which uses heat from radioactive decay to operate thermocouples and produce electricity. The Cassini space probe, for example, used a radioisotope thermoelectric power source.
The process of creating fuel for and running nuclear power plants produces a number of useful byproducts, or can be tailored to do so. Enriching uranium for fission reactors, for example, creates an isotope commonly known as depleted uranium, which the American military uses in the manufacture of both armor and ammunition due to its extremely high density. Then there are breeder reactors, which produce new fissile material (usually plutonium) at a rate greater than that at which the reactor consumes them, thereby creating a second generation of fuel. This reactor design is useful for its high fuel economy.