Nuclear Disaster Glossary: Terms and Definitions

Becquerel, Cesium, Hibakusha, Pressurized water reactor, and more

by Catherine McNiff

Below are terms and definitions frequently used in the aftermath of the nuclear disaster in Japan.

A-M | N-Z
  • Acute radiation syndrome (ARS)
    Also known as radiation sickness, this serious illness is caused by high doses of radiation. The first symptoms are nausea, vomiting, and diarrhea, and often include skin damage.
  • Apocalypse
    A great disaster, usually equated with the end of the world.
  • Becquerel
    A measurement of radioactivity; its symbol is Bq.
  • Cesium
    Cesium is a naturally occurring element found combined with other elements in rocks, soil, and dust in low amounts. Nuclear explosions or the breakdown of uranium in fuel elements can produce two radioactive forms of cesium 134Cs and 137Cs. Both isotopes decay into non-radioactive elements. The half-life of 134Cs is two years, and 30 years for 137Cs.
  • Boiling water nuclear reactor (BWR)
    In the boiling water reactor (BWR), the water that passes over the reactor core to slow down the neutrons and acts as a coolant is also the steam source for the turbine, which in turn powers the generator to produce energy.
  • Chernobyl
    The Chernobyl nuclear power station was the site of the world's worst nuclear accident, in 1986. The result of a flawed reactor design that was operated with inadequately trained personnel, the accident released at least 5% of the radioactive reactor core into the atmosphere and claimed 30 lives within the first few weeks, and unconfirmed numbers over the ensuing years.
  • Cold shutdown
    The term used to define a reactor coolant system at atmospheric pressure and at a temperature below 200 degrees Fahrenheit following a reactor cooldown.
  • Containment vessels
    A gas-tight shell or other enclosure around a nuclear reactor to confine fission products that otherwise might be released into the atmosphere in the event of an accident. Such enclosures are usually dome-shaped and made of steel-reinforced concrete.
  • Cooldown
    The gradual decrease in reactor fuel rod temperature caused by the removal of heat from the reactor coolant system after the reactor has been shutdown.
  • Core
    The central portion of a nuclear reactor, which contains the fuel assemblies, moderator, neutron poisons, control rods, and support structures. The reactor core is where fission takes place.
  • Exposure
    Absorption of ionizing radiation or ingestion of a radioisotope. Acute exposure is a large exposure received over a short period of time. Chronic exposure is exposure received over a long period of time, such as during a lifetime.
  • Fission
    The splitting of an atom, which releases a considerable amount of energy (usually in the form of heat) that can be used to produce electricity. During fission, the heavy nucleus splits into roughly equal parts, producing the nuclei of at least two lighter elements. In addition to energy, this reaction usually releases gamma radiation and two or more daughter neutrons.
  • Fuel rod
    A long, slender, zirconium metal tube containing pellets of fissionable material, which provide fuel for nuclear reactors. Fuel rods are assembled into bundles called fuel assemblies, which are loaded individually into the reactor core.
  • Fukushima Daiichi
    Explosions, fire, and a failed cooling system caused by a magnitude 9.0 earthquake and a 23-foot tsunami in this Japanese nuclear power plant in March 2011 released radioactivity directly into the atmosphere and into the sea.
  • Half-life
    The time required for half the amount of a substance (as a drug, radioactive tracer, or pesticide) in or introduced into a living system or ecosystem to be eliminated or disintegrated by natural processes.
  • Hibakusha
    The surviving victims of the atomic bombings of Hiroshima and Nagasaki, meaning "explosion-affected people"
  • International Atomic Energy Agency (IAEA)
    The International Atomic Energy Agency, the center of worldwide cooperation in the nuclear field, through which member countries and multiple international partners work together to promote the safe, secure, and peaceful use of nuclear technologies. The United Nations established the IAEA in 1957 as "Atoms for Peace." The IAEA and its then director, Mohamed ElBaradei, shared the 2005 Nobel Peace Prize.
  • International Nuclear and Radiological Event Scale (INES)
    The International Nuclear and Radiological Event Scale is a tool for promptly communicating to the public in consistent terms the safety significance of reported nuclear and radiological incidents and accidents.

A-M | N-Z
  • Meltdown
    The melting of a significant portion of a nuclear-reactor core due to inadequate cooling of the fuel elements, a condition that could lead to the escape of radiation.
  • Plutoniuim
    A heavy, radioactive, man-made metallic element with atomic number 94. Its most important isotope is fissile plutonium-239, which is produced by neutron irradiation of uranium-238. It exists in only trace amounts in nature.
  • Pressurized water reactor
    In the pressurized water reactor (PWR), the water which passes over the reactor core to act as moderator and coolant does not flow to the turbine, but is contained in a pressurized primary loop. The steam which drives the turbine is produced in a separate, secondary loop.
  • Radiation
    Consisting of alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions, ionizing radiation can cause changes in living cells.
  • Radioactive iodine (RAI)
    Used to treat overactive thyroid patients (hyperthyroid), RAI kills the cells of the thyroid gland.
  • Radioactive contamination
    Undesirable radioactive material (with a potentially harmful effect) that is either airborne or deposited in (or on the surface of) structures, objects, soil, water, or living organisms (people, animals, or plants) in a concentration that may harm people, equipment, or the environment.
  • Reactor
    The heart of a nuclear power plant, in which nuclear fission may be initiated and controlled in a self-sustaining chain reaction to generate energy. Although there are many types of nuclear reactors, they all incorporate certain essential features, including the use of fissionable material as fuel, a moderator (such as water) to increase the likelihood of fission, a reflector to conserve escaping neutrons, coolant provisions for heat removal, instruments for monitoring and controlling reactor operation, and protective devices (such as control rods and shielding).
  • Spent fuel pool
    An underwater storage and cooling facility for spent (depleted) fuel assemblies that have been removed from a reactor.
  • Three Mile Island
    Located in Pennsylvania, the Three Mile Island nuclear power plant suffered a meltdown in its No. 2 reactor in 1979, due to a cooling malfunction. While some radioactive gas was released a couple of days after the accident, no injuries or adverse health effects resulted.
  • Uranium
    A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. A particular kind of uranium, U-235, is used as fuel in nuclear power plants for fission because its atoms are easily split apart.

Source: Nuclear Regulatory Commission. Web: