The containment structure enclosing a boiling water nuclear reactor vessel. The drywell is connected to a pressure suppression system and provides a barrier to the release of radioactive material to the environment under accident conditions.

Dual-purpose cask:

See Cask.

Fissile material:

Material that undergoes fission from thermal (slow) neutrons. Although sometimes used as a synonym for fissionable material, the term “fissile” has acquired this more restricted meaning in nuclear reactor technology. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239.


Splitting of a nucleus into at least two nuclei accompanied by the release of neutrons and a relatively large amount of energy.


Material that is capable of undergoing fission from fast neutrons. Fission products: Nuclei resulting from the fission of elements such as uranium.

Fuel assembly:

A square array of fuel rods.

Fuel pellet:

A small cylinder of uranium usually in a ceramic form (uranium dioxide, UO2), typically measuring about 0.4 to 0.65 inches (1.0 to 1.65 centimeters) tall and about 0.3 to 0.5 inch (0.8 to 1.25 centimeters) in diameter.

Fuel reprocessing:

Chemical processing of reactor fuel to separate the unused fissionable material (uranium and plutonium) from waste material,

Fuel rod:

Sometimes referred to as a fuel element or fuel pin. A long, slender tube that holds the uranium fuel pellets. Fuel rods are assembled into bundles called fuel assemblies.

Gamma ray:

Electromagnetic radiation (high-energy photons) emitted from certain radioactive isotopes when they undergo radioactive decay.

Half-life (radioactive):

Time required for half the atoms of a radioactive substance to undergo radioactive decay. Each radioactive isotope has a unique half-life. For example, cesium-137 decays with a half-life of 30.2 years, and plutonium-239 decays with a half-life of 24,065 years.

Independent Spent Fuel Storage Installation (ISFSI):

A facility for storing spent fuel in wet pools or dry casks as defined in Title 10, Part 72 of the Code of Federal Regulations.


Process of exposing material to radiation, for example, the exposure of nuclear fuel in the reactor core to neutrons.


Elements that have the same number of protons but different numbers of neutrons. For example, uranium-235 and uranium-238 are different isotopes of the element uranium.

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