Ending the Production of Fissile Material for Nuclear Weapons: Key Terms

Key Terms

Actinide—A heavy, radioactive element with an atomic number greater than 89 (actinium) and less than 103 (lawrencium). The actinide series includes uranium (atomic number 92) neptunium (93), plutonium (94), and americium (95).

Americium—A fissionable, artificial element that can be used to produce nuclear explosives. The principal isotope, americium 241, is created as a result of the decay of plutonium 241. Other important isotopes are americium 242m and 243.

Atomic number—The number of protons carried by the nucleus of a chemical element.

Burnup—The percentage of heavy metal atoms fissioned, or the thermal energy produced per mass of fuel (usually measured in megawatt days per tonne, MWd/t).

Chemical processing—Chemical treatment of materials to separate specific usable constituents.

Cladding—The material which encases the nuclear fuel, reducing the risk of radioactive materials leaking from the fuel.

Closed-down facility—An installation or location where operations have been stopped and the nuclear material removed but which has not been decommissioned.

Critical mass—The minimum mass required to sustain a chain reaction. The exact mass varies with many factors, such as the particular isotope present, its concentration and chemical form, the geometric arrangement of the material, and its density.

Declared facility—A facility that has been declared to the International Atomic Energy Agency (or other inspection authority) and is made available for inspection in accordance with relevant safeguards obligations. In a non-nuclear weapon state party to the NPT, this includes all operating nuclear facilities. In a nuclear weapon state it includes only facilities designated by the state.

Decommissioned facility - An installation or location at which residual structures and equipment essential for its use have been removed or rendered inoperable so that it is not used to store and can no longer be used to handle, process or utilize nuclear material.

De facto nuclear weapon state—A non-nuclear weapon state that is not a party to the NPT and possesses unsafeguarded nuclear facilities. Typically refers to India, Israel, or Pakistan.

Depleted uranium—Uranium containing less than 0.71 percent uranium 235. Produced as a by-product of the uranium enrichment process.
Deuterium—A heavy isotope of hydrogen containing one proton and one neutron in the nucleus. Deuterium (D) occurs naturally in water at about 150 parts per million.

Disposition—The disposal of plutonium or enriched uranium, especially stocks arising from dismantled nuclear weapons.

Diversion—The deliberate removal of fissionable material in civil fuel cycles for other uses.

Downblending—The mixing of a particular enrichment of uranium, usually HEU, with a lower enrichment of uranium to decrease the concentration of uranium 235 in the final product.

Downstream facilities—Those facilities that store, process, or handle fissile material after it has been produced.

Enrichment—The process of increasing the concentration of one isotope of a given element (in the case of uranium, increasing the concentration of uranium 235)

Feedstock or feed—Material introduced into a facility at the start of the process, such as uranium hexafluoride in an enrichment plant.

Fertile material—Material composed of atoms which readily absorb neutrons to produce fissile material. One such material is uranium 238, which becomes plutonium 239 after it absorbs a neutron. Fertile material alone cannot sustain a chain reaction.

Fissionable material—Material whose nuclei can be induced to fission by a neutron.

Fissile material—Material composed of atoms that fission when irradiated by slow or “thermal” neutrons. The most common are uranium 235 and plutonium 239. The term is often used to describe plutonium and HEU, e.g., a cutoff in the production of fissile materials. Uranium 233 is also fissile.

Fuel-grade plutonium—Plutonium containing from 7 to 18 percent plutonium 240.

Gamma radiation—High-energy electromagnetic radiation emitted from nuclei as a result of nuclear reactions and decay.

Half-life—The time taken for a quantity of an isotope to halve through radioactive decay.

Heavy water—A highly enriched form of water containing more than 99.5 percent D2O. It is used as a moderator and also as a coolant, especially in natural uranium fueled reactors. Water with deuterium concentration greater than 1 part in 5,000 can be subject to safeguards.

Highly enriched uranium (HEU)—Uranium in which the percentage of uranium 235 is raised (“enriched”) from its natural level of 0.71 percent to greater than 20 percent, usually 90 percent. All HEU can be used to make nuclear explosives, although a very large quantity is required for HEU enriched to 20 percent.

Hot cell—Shielded room or interconnected rooms with remote handling equipment where highly radioactive materials can be safely examined and processed. Hot cells are typically used to handle, process, and inspect remotely spent reactor fuel or targets.

Immobilization—Process of isolating materials, either directly or indirectly, in a matrix of vitrified high-level nuclear waste. Often refers to a particular plutonium disposition technique.

Isotope—Atoms having the same number of protons but a different number of neutrons. Two isotopes of the same atom are chemically similar to each other, and therefore difficult to separate, but may have different nuclear properties. Isotopes are designated by their atomic mass numbers (total number of protons and neutrons). Uranium 235 and uranium 238 are isotopes.

Low-enriched uranium (LEU)—Uranium containing over 0.71 and less than 20 percent uranium 235. Most modern light water power reactors use 3 - 6 percent LEU. LEU is insufficiently enriched in uranium 235 to be used for nuclear explosives.

Mass number—The number of protons and neutrons in the atomic nucleus. Elements may occur in forms displaying a range of mass numbers - i.e., plutonium 238, 239, 240, 241, 242.

Milling process—Installation for refining uranium ore to produce yellowcake (U3O8).

Mixed oxide (MOX) fuel—Nuclear fuel composed of a mixture of uranium and plutonium oxide.

Model Protocol—Legal instrument containing important aspects of the International Atomic Energy Agency’s strengthened safeguards. Needs to be ratified by the individual state before these measures can be implemented. See also strengthened safeguards.

Natural uranium—Uranium containing 0.71 percent uranium 235.

Naval propulsion reactor—See propulsion reactor.

Non-nuclear weapon state—Any state that did not manufacture and explode a nuclear weapon or other nuclear explosive device prior to January 1, 1967. Non-nuclear weapon states that are party to the NPT are obligated to sign comprehensive safeguards agreements with the International Atomic Energy Agency to ensure that declared, civil nuclear materials are not being diverted to military purposes, and (under recently strengthened safeguards) to verify the absence of undeclared fissile material production facilities.
Neptunium 237—A fissionable, artificial isotope that can be used to produce nuclear explosives. Neptunium is created by irradiating uranium 235 or uranium 238 in a nuclear reactor or through the decay of americium 241.

Nuclear fuel—Basic chain-reacting material, including both fissile and fertile materials. Commonly used nuclear fuels are natural uranium and low-enriched uranium; highly enriched uranium and plutonium are used in some reactors.

Nuclear fuel cycle—The set of chemical and physical operations needed to prepare nuclear material for use in reactors and to dispose of or recycle the material after its removal from the reactor. Existing fuel cycles begin with uranium as the natural resource and create plutonium as a by-product. Some future fuel cycles may rely on thorium and produce fissionable isotope uranium 233.

Nuclear fuel element—A rod, tube, plate, or other mechanical shape or form into which nuclear fuel is fabricated for use in a reactor.

Nuclear fuel fabrication plant—A facility where the nuclear material (e.g. enriched or natural uranium) is fabricated into fuel elements to be inserted into a reactor.

Nuclear power plant—Any device that converts nuclear energy into useful power. In a nuclear electric power plant, heat produced by a reactor is used to produce steam to drive a turbine that in turn drives an electricity generator.

Nuclear waste—The radioactive by-products formed by fission and other nuclear processes in a reactor. Most nuclear waste is initially contained in spent fuel. If this material is reprocessed, new categories of waste result.

Nuclear weapon state—Any state that manufactured and detonated a nuclear weapon or other explosive device prior to January 1, 1967. Refers to China, France, Russia (as the successor to the Soviet Union), the United Kingdom and the United States.

Plutonium 239—A fissile, artificial isotope created when uranium 238 captures a neutron through irradiation. Plutonium 239 is one of the principal materials used for nuclear weapons, the other being uranium 235.

Plutonium 240—An isotope produced in nuclear reactors when uranium 239, instead of decaying to plutonium 239 or fissioning, absorbs a second additional neutron. Its presence complicates the construction of nuclear explosives because of its high neutron emission and heat output.

Power reactor—A nuclear reactor designed to produce electricity, as distinguished from other reactors used primarily for research or for the production of radiation or fissionable materials.
Production reactor—A nuclear reactor designed principally for the large-scale production of weapon-grade plutonium.

“Programme 93+2”—See strengthened safeguards.

Propulsion reactor—Nuclear reactor configured for the propulsion of naval ships and submarines.

Purex process—One particular method of reprocessing spent fuel. See also reprocessing.

Radioactivity—The property of certain nuclides of spontaneously emitting particles or gamma radiation or of emitting “x” radiation following orbital electron capture or of undergoing spontaneous fission.

Reactor-grade plutonium—Plutonium containing over 18 percent plutonium 240.

Reprocessing—Chemical treatment of irradiated fuel to separate one or more elements (in most cases, plutonium and uranium) from unwanted radioactive byproducts and (under present plans) each other. A reprocessing plant may also include a waste treatment facility and liquid and solid waste storage.

Research reactor—A nuclear reactor primarily designed to produce neutrons for research purposes. Such reactors are also used for training, materials testing, and radioisotope production.

Safeguards—Technical and inspection measures for verifying that nuclear materials are not being diverted from civil to other uses. See also strengthened safeguards.

Separative work—A measure of the effort required in an enrichment facility to separate uranium of a given uranium 235 content into two fractions, one with a higher percentage of uranium 235 and one with a lower percentage. The unit of measure is the kilogram separative work unit (SWU).

Significant quantity—The approximate amount of nuclear material (not just fissile material) which the International Atomic Energy Agency considers sufficient for a state to manufacture its first nuclear explosive taking into account process losses. Eight kilograms of plutonium and 25 kilograms of uranium 235 in HEU are considered significant.

Spent fuel—Fuel elements that have been removed from the reactor after use because they contain too little fissile and fertile material and too high a concentration of unwanted radioactive by-products to sustain reactor operation. Spent fuel is both thermally and radioactively hot.

Strengthened safeguards—Set of measures adopted by the International Atomic Energy Agency (IAEA) to better enable IAEA inspectors to verify that states have not initiated clandestine fissile material programs. Developed through “Programme 93+2.” Certain aspects of strengthened safeguards are formalized in the Model Protocol.

Tails—The waste stream of an enrichment facility. The percentage of the uranium 235 in the tails is called the “tails assay.”

Tonne—Metric ton, equivalent to 1,000 kilograms or 2,200 pounds.

Undeclared facility—A nuclear facility that has not been declared to the International Atomic Energy Agency (or other inspection authority). In the non-nuclear weapon states, undeclared facilities may constitute a violation of relevant safeguards agreements.

Uranium 233—A fissile, artificial isotope that can be used to produce nuclear explosives. Created when thorium 232 captures a neutron through irradiation.

Uranium 235—The only natural occurring fissile isotope. Natural uranium contains only 0.71 percent of uranium 235.

Uranium 238—The principal isotope (99.3 percent) of natural uranium, the other being uranium 235.

Uranium hexafluoride—A volatile compound of uranium and fluorine. Uranium hexafluoride is a solid at atmosphere pressure and room temperature, but can be transformed into gas by heating. Uranium hexafluoride gas (alone, or in combination with hydrogen or helium) is the feedstock in most uranium enrichment processes.

Uranium oxide—The most common oxide of uranium found in typical ores. Uranium oxide is extracted from the ore during the milling process. The ore typically contains only 0.1 percent uranium oxide; yellowcake, the product of the milling process, contains about 80 percent uranium oxide.

Weapon-grade plutonium—Plutonium containing less than 7 percent plutonium 240.

Weapon-grade uranium—Uranium enriched to more than 90 percent uranium 235.

Weapon-grade uranium equivalent (WGU-eq)—The amount of weapon-grade uranium (93 percent) that is equivalent to an HEU stock of another enrichment. Often refers to the amount of weapon-grade uranium that could have been produced from the total separative work output, assuming a typical tails of 0.3 percent. This procedure is used when the actual enrichment level of the HEU is unknown, although most of the HEU is believed to be weapon-grade uranium.

Weapon-usable material—- Usually separated plutonium or HEU. Because of the use of this term in UN Security Resolution 687, the IAEA Action Team formally defined it as uranium enriched to 20 percent or more in uranium isotopes 233, 235 or both; plutonium containing less than 80 percent plutonium 238; any of the foregoing in the form of metal, alloy, chemical compound or concentrate; and any other goods containing one or more of the foregoing, other than irradiated fuel.

Yellowcake—A concentrate produced during the milling process that contains about 80 percent uranium oxide. In preparation for uranium enrichment, the yellowcake is converted to uranium hexafluoride gas. In the preparation of natural uranium reactor fuel, yellowcake is processed into purified uranium dioxide.

Zirconium—A grayish-white lustrous metal which is commonly used in an alloy form (i.e., zircalloy) to encase fuel rods in nuclear reactors.