All nuclear reactors in existence today are powered by the heat generated by the fission of uranium atoms. Here is the concept in a nutshell: Fuel rods containing enriched uranium (U-235) give off tremendous amounts of heat as the uranium atoms are bombarded with neutrons, whereupon they split into smaller nuclei. This process, known as nuclear fission, is harnessed in a controlled chain reaction which generates energy as long as a sufficient supply of uranium is present in the fuel rods.
The heat produced by nuclear fission is used to boil vast quantities of water. As the water boils into steam, it turns the blades of a turbine, which is a device analogous to a water wheel or windmill hooked up to a generator. As long as the turbine spins, electricity is produced. Electricity passes through transformers, devices that convert the electron flow from DC (direct current) to AC (alternating current). AC power lines transport electricity in the form of a low current at high voltage, making it possible to construct electrical grids encompassing thousands of square miles.
Now for the rest of the story.
The fundamental breakthrough that made nuclear energy a reality was the controlled chain reaction. The first group to demonstrate the feasibility of this concept was led by the physicist Enrico Fermi at the University of Chicago during World War II. In late 1942, Fermi and his colleagues achieved a self sustaining fission chain reaction by bombarding a sample of uranium with neutrons. This caused its nuclei to break into smaller pieces, yielding heat as well as more neutrons, which in turn broke apart more uranium nuclei and so forth, generating energy in accordance with Einstein's famous equation E=mc2.
Given the circumstances of the time, it was all but inevitable that nuclear power would be used as a weapon. Less than three years after Fermi's breakthrough, WWII ended with the destruction of Hiroshima and Nagasaki by atomic bombs. Although the ensuing Cold War and nuclear arms race made governments and civilians alike wary of nuclear power, the proverbial genie was out of the bottle. By 1964, the U.S., Britain, France, Soviet Union, and China had acquired stockpiles of nuclear weapons; all except China had begun constructing nuclear power plants as well. Today, at least 30 countries have operational nuclear power plants, and only Germany is considering shutting down its facilities.
Although the fuel rods used in nuclear power plants contain a much smaller percentage of enriched uranium than nuclear war heads, the entire structure of the plant is (ideally) designed to avoid a meltdown. A meltdown occurs when the heat from the chain reaction (as well as decay heat from spontaneous fission) is not dissipated quickly enough by the plant's cooling system, allowing the chain reaction to spiral out of control. The resulting surge in energy output does not culminate in a mushroom cloud explosion but does result in the ejection of large amounts of radioactive material into the power plant's immediate environment as well as into the atmosphere.
To prevent this worst case scenario, the fuel rods themselves are housed inside a steel containment vessel, which in turn is surrounded by a concrete shell many feet thick. The plant's cooling towers are also insulated with massive amounts of concrete. Another important precaution is the addition of cadmium rods to the reactor core. Cadmium absorbs neutrons and slows the chain reaction to a manageable level. Yet another safety measure, at least in the United States, is the location of the nuclear power plants themselves. The majority are located at least 30 miles away from large cities.
Nuclear power plant disasters are rare but can be catastrophic. The first nuclear power plant accident to gain global attention happened at Three Mile Island, near Harrisburg, PA in 1979. This was followed by a far worse disaster in the Ukraine at Chernobyl in 1986, leading to over 50 deaths and hundreds of cases of cancer in the following years. In 2011, a powerful earthquake destroyed the cooling systems in at least three nuclear power plants in Japan, resulting in a meltdown at the Fukushima Daiichi plant. The scale of this disaster is still not fully known.
Although no deaths were ever directly linked to the Three Mile Island accident, no new nuclear power plants have opened in the U.S. since 1979.