A device for suddenly producing an explosive neutron chain reaction in a fissile material such as uranium-235 (²³⁵U) or plutonium-239 (²³⁹Pu). An atomic bomb derives its destructive potency from the rapid release of nuclear energy brought about by the fission of heavy atomic nuclei. In a wider sense, any explosive device that derives its energy from nuclear reactions can be considered to be an atomic bomb; this broader definition encompasses not only the foregoing fission weapon, but also a fusion weapon (which gets its energy largely from fusion reactions of heavy hydrogen isotopes) and a fission–fusion weapon (which derives its energy from both fission and fusion). Because an atomic bomb derives its energy from nuclear reactions, it is properly called a nuclear explosive or nuclear weapon (Fig. 1). See also: Hydrogen bomb; Nuclear explosion; Nuclear fission; Nuclear fusion; Nuclear reaction; Plutonium; Uranium

The number of counts recorded by a radiation detector from background radiation. The term background radiation refers to the natural ionizing radiation on the Earth. Ionizing radiation refers to all radiations, waves, and particles that are energetic enough to remove electrons from stable atoms; they are stronger than infrared radiation, radio waves, or visible light, which cannot separate electrons from stable atoms. Radiation strong enough to cause ionization of atoms is measured in electrical units which range from 32 electronvolts up to millions of electronvolts. See also: Electromagnetic radiation; Ionization potential

The amount of fissile material (uranium-233, uranium-235, or plutonium-239) that supports a self-sustaining nuclear chain reaction. It ranges from as little as 1 lb (0.45 kg) for plutonium-239 dissolved in light water to 2 tons (1800 kg) for uranium-235 fueling a 1-GWe (1 gigawatt of electric power) nuclear power reactor. See also: Chain reaction (physics)

The removal of radioactive contamination which is deposited on surfaces or spread throughout a work area. Personnel decontamination is included. Radioactive contamination is a potential health hazard and, in addition, may interfere with the normal operation of plants, particularly when radiation detection instruments are used for control purposes. Thus, the purpose of decontamination is the detection and removal of radioactive contaminants from unwanted locations to locations where they do not create a health hazard or interfere with plant operation. The objective of a good decontamination operation is to remove the radioactive contamination permanently from the human environment, with minimum radiation exposure.

A neutron emitted spontaneously from a nucleus as a consequence of excitation remaining from a preceding radioactive decay event. Analogously, delayed emission of protons and alpha particles is observed, but the known delayed-neutron emitters are more numerous, and some of them have practical implications. In particular, they are important in the control of nuclear chain reactors. See also: Chain reaction (physics); Nuclear reactor

A pump that operates on the principle that a force is exerted on a current-carrying conductor in a magnetic field. The high electrical conductivity of the liquid metals pumped (liquid metals are used as the heat-transfer media in some nuclear reactors and magnetohydrodynamic systems) allows a pumping force to be developed within the metals when they are confined in a duct or channel and subjected to a magnetic field and to an electric current. These pumps are designed principally for use in liquid-metal-cooled reactor plants where liquid lithium, sodium, potassium, or sodium-potassium alloys are pumped. Other metallic and nonmetallic liquids of sufficiently high electrical conductivity, such as mercury or molten aluminum, lead, and bismuth, may also be pumped in nonnuclear applications. The absence of moving parts within the pumped liquid eliminates the need for seals and bearings that are found in conventional mechanical pumps, thus minimizing leaks, maintenance, and repairs, and improving reliability. In liquid-metal-cooled nuclear reactor plants, electromagnetic pumps with a capacity of up to several thousand gallons per minute have operated without maintenance for decades. See also: Magnetohydrodynamics; Nuclear power; Nuclear reactor

Radioactivity that originates from natural and anthropogenic sources, including radioactive materials in food, housing, and air, radioactive materials used in medicine, nuclear weapons tests in the open atmosphere, and radioactive materials used in industry and power generation. Natural radioactivity, which is by far the largest component to which humans are typically exposed, is of both terrestrial and extraterrestrial (cosmic) origin. About 340 nuclides are known in nature, of which 70 are radioactive and are found mainly among the heavy elements. The three nuclides responsible for most of the terrestrial component are potassium-40, uranium-238, and thorium-232 (Fig. 1). See also: Chemical element; Radiation; Radiation injury to plants and animals; Radioactivity

A branch of the environmental and occupational safety health sciences and professions concerned with the protection of people and the environment from possibly harmful effects of radiation, while providing for the utilization of radiation for the benefit of society. Health physicists are interdisciplinary radiation protection and safety specialists whose expertise draws from environmental science, mathematics, medicine, radiological health, radiation biology, chemistry, and physics. The subject requires understanding of the generation, measurement, and characteristics of radiation; environmental transport of radionuclides; dosimetry; effects of radiation in biological systems; and the regulations and recommendations governing the use of radiation. The field includes non-ionizing as well as ionizing sources of radiation (see illustration). See also: Radiation

A form of water in which the hydrogen-1 atoms with mass 1 (¹H) ordinarily present in water are replaced by hydrogen-2, known as deuterium (symbol D or ²H), the heavy stable isotope of hydrogen with mass 2. The molecular formula of heavy water is D₂O (or ²H₂O), in distinction to H₂O for ordinary, "light" water. Other kinds of heavy water besides D₂O occur (see figure). In semi-heavy water (HDO), one of the hydrogen atoms is hydrogen-1 (also known as protium) and the other hydrogen atom is deuterium. Heavy-oxygen water is enriched in heavy isotopes of oxygen (oxygen-17 or oxygen-18) rather than the most common oxygen isotope, oxygen-16 or ¹⁶O, which has a natural abundance exceeding 99.7%. Rounding up, the natural abundance of deuterium is roughly about 0.2%, meaning that deuterium composes about 1 out of every 5000 hydrogen atoms in seawater. See also: Deuterium; Hydrogen; Isotope; Molecule; Oxygen; Seawater; Water

A device in which an uncontrolled, self-sustaining, thermonuclear fusion reaction is carried out in heavy hydrogen (deuterium or tritium) to produce an explosion. Initial conceptualization of the hydrogen bomb occurred alongside development of the first fission-based atomic bombs in the United States' Manhattan Project during World War II. Seven years after dropping atomic bombs on Japan in 1945, the United States tested the first hydrogen bomb at Eniwetok Atoll in the Pacific Ocean on October 31, 1952 (Fig. 1). See also: Atomic bomb; Nuclear explosion