IPFM International Panel on Fissile Materials

Additional Protocol. The voluntary agreement between a state and the International Atomic Energy Agency to accept more stringent safeguards than those originally required to verify compliance with the Nonproliferation Treaty or other safeguards agreements. Devised in the 1990s following the discovery of Iraq’s clandestine uranium-enrichment programs, it broadens the information on nuclear activities a state declares to the IAEA and provides additional rights for IAEA inspectors to verify this declaration, including taking swipe samples to check for possible undeclared nuclear activities in a country.

Americium-241. A fissile isotope with a half-life of 433 years produced from decay of plutonium-241. There is no public information that americium has ever been used to build a nuclear weapon but it is considered an “alternative nuclear material” by the IAEA.

Breeder reactor. A nuclear reactor that produces more fissile material than it consumes in “fertile” material (U-238 or thorium). Most R&D has been focused on fast-neutron reactors cooled with liquid sodium. Despite many attempts, breeder reactors have not been successfully commercialized.

Burn-up. A measure of the fission energy generated by a mass of fuel in a reactor usually given at the time of discharge from the reactor, measured in units of thermal megawatt-days per kilogram or thousand thermal megawatt-days per metric ton.

Cascade. The arrangement of isotope separation elements (for example, centrifuges) in a uranium enrichment facility. The cascade is organized as a series of “stages” in each of which separation elements operate in parallel. The stages are connected in series so that material from one stage is passed to another for further enrichment or depletion of the uranium in the isotope U-235. The final output streams when the feed is natural uranium are enriched and depleted uranium.

Centrifuge. A rapidly rotating cylinder used for the enrichment of uranium in which the heavier isotope (uranium-238) in uranium hexafluoride gas is forced to higher concentrations near the cylinder’s walls, while the lighter isotope (uranium-235) concentrates towards the center of the cylinder.

Chain reaction. A continuing process of nuclear fissioning in which the neutrons that are released from one fission trigger other nuclear fissions. In a nuclear weapon, an extremely rapid, multiplying chain reaction causes an explosive release of energy. In a reactor operating at constant power, the chain reaction is controlled so that each fission causes on average exactly one fission.

Critical mass. The minimum amount of a fissile material required to sustain a chain reaction. The exact mass of material needed to sustain a chain reaction varies according to its geometry, the mixture of fissile isotopes and other elements it contains, its density (e.g. whether it is in metal or oxide form), and the neutron-reflecting properties and thickness of the surrounding materials.

Depleted uranium. Uranium having a smaller percentage of uranium-235 than the 0.7 percent found in natural uranium. It is a by-product of the uranium enrichment process.

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).

Fertile material. Nuclear isotopes that are transmuted by neutron absorption and radioactive decay into fissile materials. One such element is uranium-238, which, after it absorbs a neutron, decays in two steps into plutonium-239.

Fissile material. Material that can sustain an explosive fission chain reaction, notably plutonium of almost any isotopic composition and highly-enriched uranium.

Fission. The process by which a fissionable nucleus splits either after absorbing a neutron or, in some cases, spontaneously. During the process of nuclear fission, typically two or three high-speed neutrons also are emitted, along with gamma rays.

Fissionable material. A heavy isotope with an atomic nucleus that can be caused to undergo fission when struck by a neutron. Uranium-238 is a fissionable isotope, in that it can be fissioned by high-energy neutrons, although, unlike uranium-235, it cannot sustain a fission chain reaction.

Fizzle yield. The reduced explosive energy that is released by a nuclear weapon when the chain reaction is initiated at the first moment when the explosive assembly becomes critical. This is termed pre-initiation. In an implosion weapon using reactor-grade plutonium, a fizzle yield could be the equivalent to the explosion of one kiloton of TNT.

Gaseous diffusion. A method of isotope separation based on the fact that gas molecules carrying isotopes with different masses diffuse through a porous barrier (or membrane) at different rates. The method is used to separate uranium hexafluoride molecules containing uranium-235 from molecules containing uranium-238. It requires significant amounts of electric power to pump the gas through the membranes.

Half-life. The time required for one-half of the nuclei in a quantity of a specific radioactive isotope to decay.

Heavy metal. The uranium, thorium, and transuranic elements in reactor fuel, usually measured in metric tons.

Heavy-water reactor. A reactor that uses heavy water as a neutron "moderator," i.e. to slow the neutrons between fissions. Most of the hydrogen in heavy water is deuterium, whose nucleus, unlike that of ordinary hydrogen, contains a neutron as well as a proton. Only about one in ten thousand hydrogen atoms in nature is deuterium. Heavy water is made by concentrating water molecules containing deuterium. Heavy water reactors typically use natural uranium as fuel. It is impossible to sustain a chain reaction in natural uranium in a reactor moderated by ordinary water because the "light" hydrogen in the water absorbs too many neutrons.

High-level waste. The radioactive waste containing fission products and non-plutonium "transuranic" elements (i.e. neptunium, americium and curium) resulting from the reprocessing of spent fuel.

Highly enriched uranium (HEU). Uranium in which the percentage of uranium-235 nuclei has been increased from the natural level of 0.7 percent to 20 percent or more. A large fraction of HEU is 90-percent enriched or higher because it was originally produced for weapons use.

International Atomic Energy Agency (IAEA). A separately funded organization, established in 1957 under the United Nations, that is responsible for promoting the peaceful use of nuclear technology and implementing "safeguards" agreements with non-weapon states under which it checks that fissile material is not diverted from peaceful uses and (for states that are members of the Nonproliferation Treaty) that no fissile material is made in undeclared facilities.

Isotope. A form of any element that is designated by the sum of the number of protons and neutrons that its nucleus contains (e.g. uranium-235 has 92 protons and 143 neutrons). Because all isotopes of an element have the same number of protons in the nucleus and therefore the same number of electrons, they have virtually the same chemical properties. But, because they have different numbers of neutrons in the nucleus, they have different atomic weights and nuclear properties. Uranium-235, for example, can sustain a fission chain reaction while uranium-238, whose nucleus contains three more neutrons, cannot.

Kiloton TNT (kt). A unit used to measure the energy of a nuclear explosion, roughly the energy released by the explosion of one thousand tons of TNT, by definition, equal to 10¹² calories (4.184x10¹² joules). The fission of 1 kilogram of fissile material releases about 18 kilotons of TNT equivalent.

Light water. Ordinary water (H₂O) as distinguished from heavy water (D₂O) that contains deuterium, a heavier isotope of hydrogen.

Light-water reactor. A reactor that uses ordinary water to cool the reactor and to "moderate" the speeds of neutrons between fissions and usually uses low-enriched uranium as fuel.

Low-enriched uranium (LEU). Uranium in which the percentage of uranium-235 nuclei has been increased from the natural level of 0.7 percent to less than 20 percent. The fuel of light-water reactors is usually enriched to 4-5 percent. Fuel rods containing low-enriched uranium can sustain a chain reaction when immersed in ordinary water.

Megawatt (MW). One million watts. Used to measure the rate of energy output of a nuclear power plant: 1 million watts of electricity (megawatts-electric, or MWe). Also used to measure the rate at which heat is released in research or plutonium-production reactors: 1 million watts of thermal energy (megawatts-thermal, or MWt). A typical light water power reactor today has a peak electricity generation capacity of approximately 1000 megawatts-electric – that is, 10⁹ watts. Such a reactor would generate about 3000 megawatts-thermal.

Megawatt-day (MW-day). A unit of energy. The cumulative amount of heat that would be released in a day at a rate of one megawatt. The fission of one gram of uranium or plutonium releases approximately one megawatt-day of thermal energy.

Metric ton (sometimes tonne). One thousand kilograms. A metric weight equivalent to about 1.1 short tons. A short ton equals 2000 pounds. Mixed-oxide fuels (MOX). Nuclear reactor fuel composed of a mixture of plutonium and natural or depleted uranium in oxide form, commonly referred to as MOX fuel. The plutonium replaces the uranium-235 in low-enriched uranium as the primary fissioning material in the fuel. MOX is used in Europe -- and planned in India and Japan -- to recycle plutonium recovered from spent fuel through reprocessing. The United States and Russia hope to dispose of some of their excess weapon plutonium in MOX fuel.

Natural uranium. Uranium as found in nature, containing 0.7 percent of uranium-235, 99.3 percent of uranium-238, and trace quantities of uranium-234 formed by the decay of U-238.

Neptunium-237. A 2-million-year half-life fissile isotope, produced in nuclear reactors by two successive neutron captures on uranium-235. There is no public information that neptunium-237 has actually ever been used in a nuclear weapon but its properties make it as suitable as U-235 and the IAEA considers it an "alternative nuclear material."

Neutron. An uncharged elementary particle with a mass slightly greater than that of a proton. Neutrons are found in the nuclei of every atom heavier than hydrogen. Neutrons provide the links in a fission chain reaction. Nuclear fuel. Basic chain-reaction material, usually including both fissile and fertile materials. Commonly used nuclear fuels are natural uranium and low-enriched uranium. Highly enriched uranium and mixed-oxide fuel (see above) are also used to fuel some reactors.

Nuclear fuel cycle. The 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 the mining of uranium ore and produce fissile plutonium as a by-product by absorption of neutrons in uranium-238 while the fuel is in the reactor. Some proposed fuel cycles would use natural thorium as a fertile material to produce the fissile isotope uranium-233, which would then be recycled in reactor fuel. An "open" fuel cycle stores the spent fuel indefinitely. A "closed" fuel cycle reprocesses it and recycles the fissile and fertile material once or more and stores the fission products and other radioactive isotopes.

Nuclear reactor. An arrangement of nuclear and other materials designed to sustain a controlled nuclear chain reaction that releases heat, which can be used to generate electricity, or mechanical power to propel a ship. Since reactors can also produce fissile material (for example, plutonium) in the irradiated fuel, they may be used as a source of fissile material for weapons. Nuclear reactors fall into three general categories: power reactors, production reactors (for producing fissile materials such as plutonium and U233, and also radioactive isotopes used in medicine) and research reactors.

Nuclear Supplier Group (NSG). A group of nuclear technology and material supplier countries organized in 1977 which have agreed to guidelines for nuclear exports, currently including a "trigger list" of items that suppliers agree to export to non-nuclear weapon states only when the receiving state has brought into force an agreement with the IAEA that allows the Agency to safeguard all nuclear activities within the state.

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

Nuclear-Weapon-Free Zone. A region of non-nuclear weapon states that have reaffirmed collectively through a treaty their decision not to manufacture, acquire, test, or possess nuclear weapons and their requirement that nuclear weapon states not store nuclear weapons there or threaten to use nuclear weapons against the signatories.

Plutonium-239. A fissile isotope produced when uranium-238 captures an extra neutron. The plutonium that has been used in the core of nuclear weapons typically contains more than 90 percent Pu-239. It has a half-life of about 24,000 years.

Plutonium-240. An isotope produced in reactors when a plutonium-239 atom absorbs a neutron instead of fissioning. Its concentration is limited in weapons plutonium because of its high rate of spontaneous fission. It has a half-life of 6600 years.

Plutonium-241. A fissile isotope produced in reactors by neutron absorption by plutonium-240. Pu-241 has a half-life of only 14 years and decays into americium-241.

Power reactor. A reactor designed to produce heat to generate electricity, as distinguished from reactors used primarily for research or for producing plutonium or other isotopes.

Production reactor. A reactor designed primarily for the large-scale production of plutonium for weapons and/or tritium.

Radioactivity. The spontaneous disintegration of an unstable atomic nucleus, resulting in the emission of electrons (beta decay), helium nuclei (alpha decay), and/or gamma rays (high-energy X-rays).

Reactor-Grade Plutonium. The United States defines reactor-grade plutonium as containing more than 18 percent plutonium-240 -- much more than in weapon-grade plutonium. Reactor-grade plutonium can be used, however, to make a nuclear explosive.

Recycle. The reuse of the uranium and/or plutonium in spent fuel after separation from fission products by a reprocessing plant.

Reprocessing. The chemical treatment of spent reactor fuel to separate plutonium and uranium from fission products. Because of the intense radioactivity of the fission products, this has to be done remotely behind heavy shielding. Research reactor. A reactor designed primarily to supply neutron irradiation for experimental purposes. It may also be used for training, the testing of materials, and the production of radioisotopes.

Safeguards. Measures aimed at detecting in timely fashion the diversion of significant quantities of fissile material from monitored peaceful nuclear activities. For non-nuclear weapon states that are parties to the Nonproliferation Treaty, the safeguards are implemented by the IAEA. See Significant Quantity.

Separative Work Unit (SWU). A measure of the work done by a machine or plant that separates uranium into streams with higher and lower fractions of U-235. Sometimes referred to as a kilogram-SWU to distinguish it from a ton-SWU (1000 SWUs).

Significant quantity (SQ). The IAEA defined amount of fissile material required to manufacture a first-generation nuclear explosive device: Plutonium containing less than 80% Pu-238 -- 8 kg, Uranium-233 -- 8 kg, and Uranium-235 (in HEU) -- 25 kg.

Spent fuel. Fuel elements that have been removed from the reactor because the fissionable material they contain has been depleted to a level near where it can no longer sustain a chain reaction. The high concentration of radioactive fission products in spent power-reactor fuel creates a gamma-radiation field around it that makes light-water reactor fuel "self protecting" for about one hundred years. At a distance of a meter, the gamma field would be lethal in minutes a few years after discharge and in hours a century after discharge.

Strategic Offensive Reduction Treaty (SORT). An agreement between the United States and Russia that entered into force in June 2003 to reduce the number of their operationally deployed strategic nuclear warheads to 1700-2200 warheads each by the end of 2012.

Thermonuclear explosive. A type of nuclear weapon that produces much of its energy through nuclear fusion reactions of heavy hydrogen isotopes (also known as a hydrogen bomb). These fusion reactions only proceed at temperatures around one hundred million degrees that are created by a fission explosive "trigger." Thermonuclear weapons can have yields much larger than simple fission weapons.

Thorium-232. The naturally-occurring isotope of thorium that is "fertile" in that neutron absorption in it produces the fissile isotope Uranium-233.

Transuranic. Any element whose atomic number is higher than that of uranium. All transuranics are produced artificially and are radioactive. The most commonly produced transuranic isotopes, in order of increasing weight, are neptunium, plutonium, americium and curium.

Tritium. The heaviest hydrogen isotope, containing one proton and two neutrons in the nucleus, produced most effectively by bombarding lithium-6 with neutrons. In a fission weapon, the fusion of tritium with deuterium to make helium produces an extra neutron that can be used to cause additional fissions. Tritium-deuterium gas is used in modern fission weapons to produce extra neutrons in this way to "boost" the weapon’s explosive power.

Uranium. A radioactive element with the atomic number 92. The two principal natural uranium isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is not.

Uranium dioxide (UO₂). The chemical form of uranium used in heavy-water and light-water power reactor fuel. Produced as a powder, uranium dioxide is pressed and then sintered into ceramic fuel pellets.

Uranium hexaflouride (UF₆). A volatile compound of uranium and fluorine. UF₆ is a solid at atmospheric pressure and room temperature, but can be transformed into gas by heating. UF₆ gas is the feedstock in gas-centrifuge and gaseous-diffusion uranium enrichment processes.

Uranium oxide (U₃O₈). The most common oxide of uranium found in typical ores. Uranium oxide is extracted from the ore during the milling process. The ore may contain only 0.1 percent uranium oxide. Yellowcake, the product of the milling process, contains about 80 percent uranium oxide.

Uranium-233. A fissile isotope produced by neutron absorption in fertile thorium-232. Like HEU and plutonium, it is theoretically an excellent material for nuclear weapons. It has been used in at least one nuclear test but not in deployed nuclear weapons -- perhaps because a small amount of U-232 is produced with it. A decay product of the U-232 produces gamma radiation at levels higher than the levels produced by weapon-grade plutonium. U-233 is also a potentially attractive reactor fuel for heavy and light-water moderated reactors because it releases more neutrons than U-235 per neutron absorbed.

Uranium-235. The only naturally occurring fissile isotope. Natural uranium contains 0.7 percent uranium-235; light-water reactors use fuel containing 4-5 percent; and weapon-grade highly enriched uranium normally contains at least 90 percent of this isotope.

Uranium-238. A fertile material. Natural uranium contains approximately 99.3 percent uranium-238.

Weapon-grade. Fissile material with the isotopic makeup typically used in fission explosives, that is, uranium enriched to over 90 percent uranium-235 or plutonium that is more than 90 percent plutonium-239. The HEU used in the Hiroshima weapon was enriched to about 80 percent. Uranium enriched to greater than 20 percent and plutonium containing less than 80 percent Pu-238 are considered weapon-usable, however.

Yellowcake. A uranium concentrate produced during the process of extracting uranium from ore ("milling") that contains about 80 percent uranium oxide (U₃O₈). In preparation for uranium enrichment, the yellowcake is converted to uranium hexafluoride gas (UF₆). In the preparation of natural uranium heavy-water power reactor fuel, yellowcake is processed into purified uranium oxide.

Yield. The total energy released in a nuclear explosion -- usually measured by the number of kilotons of TNT whose explosion would release the same amount of energy.