Nuclear power is produced when uranium or plutonium fuels inside a nuclear reactor core undergo fission, generating high-pressure steam that turns huge electricity-generating turbines. Controversial though it is, nuclear energy remains an important resource for developed nations. Today, more than 440 nuclear plants operate worldwide, including 104 in the U.S. In addition, 56 countries operate a total of 250 research reactors, with another 180 reactors powering ships and submarines.
Medical applications of nuclear energy -- a branch of medicine called nuclear medicine -- include magnetic resonance imaging (MRI), radioisotope imaging and radiation therapy. These techniques involve the use of radioactive isotopes to help physicians diagnose or treat certain diseases and medical conditions, including cancers and heart disease. Medical isotopes are usually called radiotracers or radiopharmaceuticals. A major benefit of nuclear medicine for the patient is that the procedures are noninvasive and usually painless.
Radioactive decay obeys a "rate law" that permits determination of the age of materials containing certain radioactive elements (isotopes). Every element decays according to its unique "half-life," the time required for decay of half of a given amount of an element. The age of a material can be computed if the ratio of the parent element(s) and their stable decay product(s) are known. Radiometric dating has been used to accurately determine the age of ancient civilizations, fossils and even Earth itself (4.6 billion years). Examples are carbon 14 dating, potassium-argon dating and uranium 235 dating.
The energy released in nuclear reactions is typically a million times greater than in chemical reactions. This hints at the enormous temperatures and pressures generated when a sufficient mass of nuclides is forced to undergo fusion or fission in a tiny fraction of a second. The uncontrolled release of nuclear energy in this way is the basic principle of nuclear weapons design. The largest recorded nuclear test explosion to date released energy equivalent to about 60 million tons of TNT (trinitrotoluene). As many as 25,000 nuclear weapons continue to menace the human race.
Nuclear energy enables stars to shine continuously, outputting huge amounts of energy, for millions or even billions of years. Almost all the light, heat and radiation received on Earth is generated in the core of our sun, where 6 million tons of hydrogen are fused into helium every second. So it is true to say that life on Earth would be impossible without nuclear energy.