Glossary

Preceding page blank porous barrier more rapidly than the Glossary heavier isotopes. Diversion—The removal of material from Breeder—A nuclear reactor that produces some point in the commercial nuclear fuel more fissile nuclei than it consumes. The cycle to use in nuclear weapons. fissile nuclei are produced by the capture of neutrons in fertile material. (See definitions Enrichment—The process of increasing the below.) The resource constraint for breeder concentration of one isotope of a given ele- reactors is thus fertile material, which is far ment. more abundant in nature than fissile Fast Neutron—A fast-moving, neutral material. These reactors have not yet subatomic particle. Neutrons are emitted reached commercialization. Fast breeders when a nucleus, such as uranium-235, fis- do not contain a moderator (see definition sions. below) to slow neutrons down; i.e., fast neutrons are used. Thermal breeders do Fertile Isotope—An isotope not itself fissile contain a moderator; i.e., slow neutrons are but that is converted into a fissile isotope, used. either directly or after a short decay process following absorption of a neutron. Exam- Centrifuge—A 238 239 rotating vessel that can be ple: U can capture a neutron to give U . 239 used for enrichment of uranium. The U 239 then decays to Np which in turn heavier isotopes of the UF gas tend to con- 6 decays to fissile Pu239. centrate at the walls of the rotating centrifuge. Fissile Isotope—An isotope that will split, or fission, into two (or more) lighter elements Chain Reaction—A series of nuclear fissions, plus extra neutrons when it is struck by a each one stimulated. . by a neutron emitted in neutron, a Previous fission. A chain reaction occurs when at least one of the two or more Fission—The splitting of a nucleus usually neutrons released in a fission initiates into two or more lighter elements. The total another fission. mass of the resulting particles is less than Critical Mass—The minimum amount of that of the original atom, the difference fissile material required to sustain a chain being converted into energy. reaction. The exact mass varies with many Fresh Fuel—Nuclear fuel ready for insertion factors such as the particular fissile isotope present, its concentration and chemical into a power reactor. form and the geometrical arrangement of Fuel Cycle—The set of chemical and physical the material. operations needed to prepare nuclear Dedicated Facility—A facility built in- material for use in reactors and to dispose digenously (possibly clandestinely) in of or recycle the material after its removal order to produce fissile material for nuclear from the reactor. Existing fuel cycles begin weapons. It might be a plutonium produc- with uranium as the natural resource and tion reactor, a uranium enrichment plant or create plutonium as a byproduct, Some a reprocessing plant. future fuel cycles may rely on thorium and produce the fissile isotope uranium-233. Denaturing—A technique to render fissile nuclear material unsuitable for explosive Fuel Fabrication Plant—A facility where the weapons by mixing in other isotopes of the nuclear material (e.g., enriched or natural same element. uranium) is fabricated into fuel elements to be inserted into a reactor. Diffusion—A technique for enrichment of uranium based on the fact that the lighter Gun-Type Nuclear Weapon—A device in isotopes of a gas will diffuse through a which gun propellants are used to move

Preceding page blank 261 two or more subcritical masses of fissile Nuclear Fission Weapons—Devices that material together to produce an explosion. derive their explosive force from the energy released when a large number of nuclei fis- Implosion-Type Nuclear Weapon—A device sion in a very short period of time. in which high explosives surrounding a subcritical configuration of fissile material Plutonium-239 (Pu239)-A fissile isotope cre- ated as a result of capture of a neutron by compress it into a condition of 238 supercriticality to produce an explosion. U . It is excellent material for nuclear weapons. Isotopes-Atoms of the same chemical ele- ment whose nuclei contain different num- Plutonium-240 (Pu240)—A fissile isotope bers of neutrons and hence have different whose presence complicates the construc- masses, even though chemically identical. tion of nuclear explosives because of its Isotopes are specified by their atomic mass high rate of spontaneous fission. It is pro- number, that is, the total number of protons duced in reactors when a Pu239 atom ab- plus neutrons, and a symbol denoting the sorbs a neutron instead of fissioning. chemical element, e.g., U 235 for Protons—Positively charged particles which, uranium-235. together with neutrons, comprise the nucleus of an atom. Mixed-Oxide Fuel—Nuclear reactor fuel composed of plutonium and uranium in ox- Reactor—A facility that contains a controlled ide form. The plutonium replaces some of nuclear fission chain reaction. It may be the fissile uranium, thus reducing the need used to generate electrical power, to con- for uranium ore and enrichment. This is the duct research, or exclusively to produce form of the fuel that would be used in plutonium for nuclear explosives. plutonium recycle. Reactor-Grade Plutonium—Plutonium that Moderator—A component (usually water, contains more than 7 percent of the isotope heavy water, or graphite) of some nuclear plutonium-240. It is created in most power reactors that slows neutrons, thereby in- reactors under normal operating condi- creasing their chances of being absorbed by tions, although the liquid metal fast breeder a fissile nucleus. reactor does produce weapons-grade plutonium in one portion of the reactor. Multinational Fuel-Cycle Facilities (MFCF)- A concept for joint national ownership and Recycle—The reuse of unburned uranium management of certain steps of the nuclear and plutonium in fresh fuel after separation fuel cycle-especially those steps that are from fission products in spent fuel at a particularly vulnerable to national diver- reprocessing plant. sion. Multinational reprocessing plants and Reprocessing-Chemical treatment of spent spent-fuel storage facilities are currently reactor fuel to separate the plutonium and under study. uranium from the fission products and (under present plans) from each other. Nth Country—A nation judged to have high potential of becoming a nuclear-weapons Safeguards-Sets of regulations, procedures, state—because of its technical and economic and equipment designed to prevent and ability and its political motivations. detect the diversion of nuclear materials from authorized channels. Neutron—Neutral particles which, together with protons, comprise the nucleus of an Special Nuclear Material (SNM)-Plutonium, atom. or uranium enriched in U235 or U233. Non-State Adversary—Any individual or Spent Fuel—Fuel elements that have been group that wishes to use destructive force to removed from the reactor because they con- further its own goals. tain too little fissile material and too high a

262 concentration of radioactive fission prod- Uranium-233 (U233)-A fissile isotope bred by ucts. They are both physically and radioac- fertile thorium-232. It is similar in weapons tively hot. quality to plutonium-239.

235 Strategic Special Nuclear Material (SSNM)- Uranium-235 (U )-The only naturally OC- Plutonium, U233, or uranium enriched to 20 curring fissile isotope. Natural uranium has percent or more in U235. 0.7 percent of U235; light water reactors use about 3 percent and weapons materials nor- Spiking—A technique to deter theft of nuclear mally consist of 90 percent of this isotope. fuel by the addition of radioactive sub- stances. Uranium-238 (U238)—A fertile isotope from which Pu239 can be bred. It comprises 99.3 Thermal neutrons—Low energy, or slow percent of natural uranium. moving neutrons. Weapons-Grade Plutonium—Plutonium that Thorium-232 (Th232)-A fetile, naturally oc- contains less than 7 percent of curring isotope from which the fissile plutonium-240, an isotope that complicates isotope uranium-233 can be bred. the design of nuclear weapons.

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