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Glossary¹¹¹

Actinides
A series of chemical elements with atomic numbers from 89 to 109.

Enriched uranium
Uranium having a higher abundance of fissile isotopes than natural uranium. Enriched uranium is considered a special fissionable material.

Enrichment plant (or isotope separation plant)
An installation for the separation of isotopes of uranium to increase the abundance of U-235. The main isotope separation processes used in enrichment plants are gas centrifuge or gaseous diffusion processes operating with uranium hexafluoride (UF6) (which is also the feed material for aerodynamic and molecular laser processes). Other isotope separation processes include electromagnetic, chemical exchange, ion exchange, and atomic vapor laser and plasma processes.

Fast reactor (fast neutron spectrum reactor)
A reactor that, unlike thermal reactors, operates mainly with fast neutrons (neutrons in the energy range above 0.1 MeV) and does not need a moderator. Fast reactors are generally designed to use plutonium fuels and can be designed to burn actinides.

Fissionable material
In general, an isotope or a mixture of isotopes capable of nuclear fission. Isotopes U-233, U-235, Pu-239, and Pu-241 are referred to as both fissionable and fissile, while U-238 and Pu-240 are fissionable but not fissile.

Fuel element (fuel assembly, fuel bundle)
A grouping of fuel rods, pins, plates, or other fuel components held together by spacer grids and other structural components to form a complete fuel unit that is maintained intact during fuel transfer and irradiation operations in a reactor.

Fuel fabrication plant
An installation for manufacturing fuel elements.

Geological repository
Underground installation for the disposal of nuclear material, such as used fuel and/or high-level and transuranic nuclear waste.

High enriched uranium (HEU)
Uranium containing 19.8 percent or more of the isotope U-235.

High-level radioactive waste (HLW)
Highly radioactive materials produced as a byproduct of the reactions that occur inside nuclear reactors. HLW takes one of two forms: used reactor fuel when it is accepted for disposal; or second cycle aqueous rafinnate or other radioactive materials remaining after used fuel is reprocessed.

Isotope
One of two or more atoms of the same element that has the same number of protons in its nucleus but different numbers of neutrons. Isotopes have the same atomic number but different mass numbers.

Lanthanides
A series of chemical elements with atomic numbers from 57 to 71.

Light water reactor (LWR)
A power reactor that is both moderated and cooled by ordinary (light) water. LWR fuel assemblies usually consist of clad fuel rods containing uranium oxide pellets of low enrichment, generally less than 5 percent U235, or MOX having low plutonium content, generally less than 5 percent. There are two types of LWR: boiling water reactors (BWRs) and pressurized water reactors (PWRs).

Low enriched uranium (LEU)
Enriched uranium containing less than 19.8 percent of the isotope U-235.

Mixed oxide (MOX)
A mixture of the oxides of uranium and plutonium used as reactor fuel for the recycling of plutonium in thermal nuclear reactors (thermal recycling) and for fast reactors.

Natural uranium
Uranium as it occurs in nature, having an atomic weight of approximately 238 and containing minute quantities of U-234, about 0.7 percent U-235, and 99.3 percent U-238. Natural uranium is usually supplied in raw form by uranium mines and concentration (ore processing) plants as uranium ore concentrate, most commonly the concentrated crude oxide U3O8, often called yellow cake.

Nuclear fuel cycle
The nuclear fuel cycle is a system of nuclear installations and activities interconnected by streams of nuclear material. The characteristics of the fuel cycle may vary widely from state to state, from a single reactor supplied from abroad with fuel to a fully developed system. Such a system may consist of uranium mines and concentration (ore processing) plants, thorium concentration plants, conversion plants, enrichment (isotope separation) plants, fuel fabrication plants, reactors, used fuel conventional reprocessing or more advanced chemical partitioning plants, and associated storage installations. The fuel cycle can be “open” by direct disposal of used nuclear fuel or “closed” in various ways: for example, by the recycling of enriched uranium and plutonium through thermal reactors (thermal recycle), by the reenrichment of the uranium recovered as a result of used fuel dissolution and partitioning, or by the burning of actinides in fast reactors.

Plutonium
A radioactive element that occurs only in trace amounts in nature, with atomic number 94 and symbol Pu.

Reactor
Any device in which a controlled, self-sustaining fission chain reaction can be maintained. Depending on their power level and purpose, reactors are subdivided into power reactors, research reactors, and critical assemblies.

Reprocessing (conventional)—PUREX
An installation for the chemical partition of nuclear material from fission products following dissolution of used fuel. The installation may also include the associated storage, head-end (cutting and dissolution) operations, conversion and analytical sections, a waste treatment facility, and liquid and solid waste storage. Conventional reprocessing involves the following steps: fuel receipt and storage; fuel decladding and dissolution; partition of uranium, plutonium, and possibly other actinides (for example, americium and neptunium) from fission products; partition of uranium from plutonium; and purification of uranium and plutonium. Once purified, uranium nitrate and plutonium nitrate may be converted, respectively, to UO₂ and PuO₂ powder at an adjoining plant. Depending on the economics, these powders may be either indefinitely stored or recycled as MOX fuel into an LWR or advanced burner or breeder reactor. More advanced chemical partitioning involves separation of the actinides and some fission products that could simultaneously provide a fuel stream that is burnable in advanced reactors and is proliferation-resistant.

Thorium
A radioactive element with atomic number 90.

Transmutation
The conversion of one nuclide into another through one or more nuclear reactions, and more specifically, the conversion of an isotope of one element into an isotope of another element through one or more nuclear reactions.

Transuranic elements
Transuranic elements are the chemical elements with atomic numbers greater than 92.

Uranium
A naturally occurring radioactive element with atomic number 92.

Used nuclear fuel
Fuel from a reactor that is no longer efficient in power production because its fission process has slowed.

ENDNOTES

111. These definitions are taken primarily from the IAEA Safeguards Glossary, 2001 edition.