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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report E GLOSSARY Actinide: Any of a series of chemically similar radioactive elements with atomic numbers ranging from 89 (actinium) through 103 (lawrencium). This group includes uranium and plutonium. Alpha particle: Two neutrons and two protons bound as a single particle (a helium nucleus) emitted from certain radioactive isotopes when they undergo radioactive decay. Bare-fuel cask: See Cask. Beta particle: A charged particle consisting of a positron or electron emitted from certain radioactive isotopes when they undergo radioactive decay. Beyond-design-basis accidents: Technical expression describing accident sequences outside of those used as design criteria for a facility. Beyond-design-basis accidents are generally more severe but are judged to be too unlikely to be a basis for design. Boiling water reactor (BWR): A type of nuclear reactor in which the reactor’s water coolant is allowed to boil to produce steam. The steam is used to drive a turbine and electrical generator to produce electricity. Burn-up: Measure of the number of fission reactions that have occurred in a given mass of nuclear fuel, expressed as thermal energy released multiplied by the period of operation and divided by the mass of the fuel. Typical units are megawatt-days per metric ton of uranium (MWd/MTU) or gigawatt-days per metric ton of uranium (GWd/MTU). Canister-based cask: See Cask Cask: Large, typically cylindrical containers constructed of steel and/or reinforced concrete that are used to store and/or transport spent nuclear fuel. Casks designed for storage of spent nuclear fuel can be of two types: “bare-fuel” or “canister-based.” In bare-fuel casks, spent fuel is stored in a fuel basket surrounded by a heavily shielded and leak-tight container. In canister-based casks, the fuel is enclosed in a leak-tight steel cylinder, called a canister, which has a welded lid. The canister is placed in a heavily shielded cask overpack. Casks can be single-, dual-, or multiple-purpose, indicating that they can be used, respectively, for storage (also called storage-only casks), for storage and transportation, and for storage, transportation, and geologic disposal. There are no true multi-purpose casks for spent fuel currently available on the market. Cesium-137: Radioactive isotope that is one of the products of nuclear fission Chain reaction: A series of fission reactions wherein the neutrons released in one fission event stimulate the next fission event or events.
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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report Cladding: Thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents corrosion of the nuclear fuel and the release of fission products into the coolant. Zirconium alloys (also called zircaloy, see below) are common cladding materials in commercial nuclear fuel. Conduction: In the context of heat transfer, the transfer of heat within a medium through a diffusive process (i.e., molecular or atomic collisions), Containment structure: A robust, airtight shell or other enclosure around a nuclear reactor core to prevent the release of radioactive material to the environment in the event of an accident. Convection: Heat transfer by the physical movement of material within a fluid medium. Cooling time: The amount of time elapsed since spent fuel was discharged from a nuclear reactor. Core: That portion of a nuclear reactor containing the fuel elements. Criticality: Term used in reactor physics to describe the state in which the number of neutrons released by the fission process is exactly balanced by the neutrons being absorbed and escaping the reactor core. At Criticality, the nuclear fission chain reaction is self-sustaining, Decay heat: Heat produced by the decay of radioactive isotopes contained in nuclear fuel. Decay, radioactive: Disintegration of the nucleus of an unstable element by the spontaneous emission of charged particles (alpha, beta, positron) or photons of energy (gamma radiation) from the nucleus, spontaneous fission, or electron capture. Depleted uranium: Uranium enriched in the element uranium-238 relative to uranium-235 compared to that usually found in nature. Also, uranium in which the uranium-235 content has been reduced through a physical process. Design basis phenomena: Earthquakes, tornadoes, hurricanes, floods, and other events that a nuclear facility must be designed and built to withstand without loss of systems, structures, and components necessary to ensure public health and safety. Design basis threat: In the context of this study, hypothetical ground assault threat against a commercial nuclear power plant. Some generic elements of the design basis threat are described in Title 10, Section 73.1(a) of the Code of Federal Regulations (10 CFR73.1(a)). Dirty bomb: See Radiological Dispersal Device. Dry storage: Out-of-water storage of spent nuclear fuel in heavily shielded casks.
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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report Drywell: 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. Fission: Splitting of a nucleus into at least two nuclei accompanied by the release of neutrons and a relatively large amount of energy. Fissionable: 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. Irradiation: Process of exposing material to radiation, for example, the exposure of nuclear fuel in the reactor core to neutrons. Isotope: 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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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report Loss-of-pool-cootant event: A postulated accidental or malevolent event that results in a loss of the water coolant from a spent fuel pool at a rate in excess of the capability of the water makeup system to restore it. Megawatt: One million watts. MELCOR: A computer code developed by Sandia National Laboratories for use in analyzing severe reactor core accidents. The code has been adapted to model fluid flow, heat transfer, fuel cladding oxidation kinetics, and fission product release phenomena associated with spent fuel assemblies in spent fuel pools in loss-of-pool-coolant events. Metric ton: Weight unit corresponding to 1000 kg or approximately 2200 pounds. Metric tons of uranium: See MTU. Moderator: Material, such as ordinary water, heavy water, or graphite, used in a reactor to slow down high-energy neutrons. MTU (metric tons of uranium): Unit of measurement of the mass for spent nuclear fuel. also expressed in metric tons of heavy metal (MTHM). It refers to the initial mass of uranium that is contained in a fuel assembly. It does not include the mass of fuel cladding (zirconium alloy) or the oxygen in the fuel compound. Multi-purpose cask: See Cask, MWe: Megawatts of electrical energy output from a power plant MWt: Megawatts of thermal energy output from a power plant. Neutron: Uncharged subatornic particle contained in the nucleus of an atom. Neutrons are emitted from the nucleus during the fission process. Open rack: A storage rack in a spent fuel pool that has open space and lateral channels between the cells for storing spent fuel assemblies to permit water circulation. Overpack: Metal or concrete cask used for storage or transportation of a canister containing spent nuclear fuel. See Cask. Owner-controlled area: That part of the power plant site over which the plant operator exercises control. This usually corresponds to the boundary of the site. Pellet: See Fuel pellet. Penetrate: To pass into, but not completely through, a solid object. Perforate: To produce a hole that goes completely through a solid object. Plutonium-239: A fissile isotope of plutonium that contains 94 protons and 145 neutrons.
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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report Pressurized water reactor (PWR): A type of nuclear reactor in which the reactor’s water coolant is kept at high pressure to prevent it from boiling. The coolant transfers its heat to a secondary water system that boils into steam to drive the turbine and generator to produce electricity, Probabilistic risk assessment: A systematic, quantitative method to assess risk (see below) as it relates to the performance of a complex system. Protected area: A zone located within the owner-controlled area of a commercial nuclear power plant site in which access is restricted using guards, fences, and other barriers. psia: Unit of pressure, pounds per square inch absolute, that is the total pressure including the pressure of the atmosphere. Radioactivity: Spontaneous transformation of an unstable atom, often resulting in the emission of particles (alpha and beta) or gamma radiation. The process is referred to as radioactive decay. Radiological Dispersal Device (RDD): A terrorist device in which sources of radioactive material are dispersed by explosives or other means. Also referred to as a dirty bomb. Radiological sabotage: Any deliberate act directed against a nuclear power plant or spent fuel in storage or transport that could directly or indirectly endanger the public health and safety by exposure to radiation. Radionuclide: Any form of an isotope of an element that is radioactive. Re-racking: Replacement of the existing racks in a spent fuel pool with new racks that increase the number of spent fuel assemblies that can be stored. Risk: The potential for an adverse effect from an accident or terrorist attack. This potential can be estimated quantitatively if answers to the following three questions can be obtained: (1) What can go wrong? (2) How likely is it? (3) What are the consequences? Safety: In the context of spent fuel storage, measures that protect storage facilities against failure, damage, human error, or other accidents that would disperse radioactivity in the environment Safeguards: As used in the regulation of domestic nuclear facilities and materials, the use of material control and accounting programs to verify that all nuclear material is properly controlled and accounted for, and also the use of physical protection equipment and security forces to protect such material. Safeguards information: Information not otherwise classified as National Security Information or Restricted Data that specifically identifies a U.S. Nuclear Regulatory Commission licensee’s or applicant’s detailed (1) security measures for the physical
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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report protection of special nuclear material or (2) security measures for the physical protection and location of certain plant equipment vital to the safety of production or utilization facilities (10 CFR 73.2). The U.S. Nuclear Regulatory Commission has the authority to determine whether information is “safeguards information.” Security: In the context of spent fuel storage, measures to protect storage facilities against sabotage, attacks, or theft. Shaped charge: A demolition and wall penetration or perforation device that uses high explosive to create a high-velocity jet of material. Single-purpose cask: See Cask, Special nuclear material: Fissile elements such as uranium and plutonium. Spent fuel: See Spent nuclear fuel. Spent fuel pool: A water-filled pool that is used at all commercial nuclear reactors for storage of spent (used) fuel elements after their removal from a nuclear reactor Spent fuel pools are constructed of reinforced concrete and lined with stainless steel. The inside of the pool has storage racks to hold the spent fuel assemblies and may contain a gated compartment to hold a spent fuel cask while it is being loaded and sealed. Spent (or used or irradiated fuel) nuclear fuel: Fuel that has been “burned” in the core of a nuclear reactor and is no longer efficient for producing electricity. After discharge from a reactor, spent fuel is stored in water-filled pools (see Wet storage) for shielding and cooling. Storage-only cask: See Cask. Thermal power: Total heat output from the core of a nuclear reactor. Uranium-235: A fissile isotope of uranium that contains 92 protons and 143 neutrons. It is the principal nuclear fuel in nuclear power reactors. Uranium-238: An isotope of uranium that contains 92 protons and 146 neutrons. Vital area: A zone located within the protected area of a commercial nuclear power plant site that contains the reactor control room, the reactor core, support buildings, and the spent fuel pool. It is the most carefully controlled and guarded part of the plant site. Watt: Unit of power. Watt-hour: Energy unit of measure equal to one watt of power supplied for one hour Wet storage: Storage of spent nuclear fuel in spent fuel pools.
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Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report Zircaloy: Zirconium alloy used as cladding for uranium oxide fuel pellets in reactor fuel assemblies. Zirconium cladding fire: A self-sustaining, exothermic reaction caused by rapid oxidation of zirconium fuel cladding (zircaloy) at high temperatures.
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