Воробева Нуцлеар Реацтор Тыпес (Леарн то реад бы реадинг) 2010

eg. U-235, U-238. Some isotopes are unstable and decay (qv) to form isotopes of other elements.

L

Light water: Ordinary water (H20) as distinct from heavy water. Light water reactor (LWR): A common nuclear reactor cooled and

usually moderated by ordinary water.

Low-enriched uranium: Uranium enriched to less than 20 % U-235. (That in power reactors is usually 3.5 — 5.0 % U-235.)

Low-level wastes: Mildly radioactive material usually disposed of by incineration and burial.

M

Megawatt (MW): A unit of power, = 106 watts. MWe refers to electric output from a generator, MWt to thermal output from a reactor or heat source (eg the gross heat output of a reactor itself, typically three times the MWe figure).

Metal fuels: Natural uranium metal as used in a gas-cooled reactor. Micro: one millionth of a unit (eg microsievert is 10-6 Sv).

Milling: Process by which minerals are extracted from ore, usually at the mine site.

Mixed oxide fuel (MOX): Reactor fuel which consists of both uranium and plutonium oxides, usually about 5 % Pu, which is the main fissile component.

Moderator: A material such as light or heavy water or graphite used in a reactor to slow down fast neutrons by collision with lighter nuclei so as to expedite further fission.

N

Natural uranium: Uranium with an isotopic composition as found in nature, containing 99.3 % U-238, 0.7 % U-235 and a trace of U-234. Can be used as fuel in heavy water-moderated reactors.

Neutron: An uncharged elementary particle found in the nucleus of every atom except hydrogen. Solitary mobile neutrons travelling at various speeds originate from fission reactions. Slow (thermal) neutrons can in turn readily cause fission in nuclei of "fissile" isotopes, e.g. U-235, Pu-239, U-233; and fast neutrons can cause fission in nuclei of "fertile"

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isotopes such as U-238, Pu-239. Sometimes atomic nuclei simply capture neutrons.

Nuclear reactor: A device in which a nuclear fission chain reaction occurs under controlled conditions so that the heat yield can be harnessed or the neutron beams utilised. All commercial reactors are thermal reactors, using a moderator to slow down the neutrons.

Nuclide: elemental matter made up of atoms with identical nuclei, therefore with the same atomic number and the same mass number (equal to the sum of the number of protons and neutrons).

O

Oxide fuels: Enriched or natural uranium in the form of the oxide UO2, used in many types of reactor.

P

Plutonium: A transuranic element, formed in a nuclear reactor by neutron capture. It has several isotopes, some of which are fissile and some of which undergo spontaneous fission, releasing neutrons. Weap- ons-grade plutonium is produced in special reactors to give > 90 % Pu-239, reactor-grade plutonium contains about 30 % non-fissile isotopes. About one third of the energy in a light water reactor comes from the fission of Pu-239, and this is the main isotope of value recovered from reprocessing used fuel.

Pressurised water reactor (PWR): The most common type of light water reactor (LWR), it uses water at very high pressure in a primary circuit and steam is formed in a secondary circuit.

R

Radiation: The emission and propagation of energy by means of electromagnetic waves or particles. (cf ionising radiation)

Radioactivity: The spontaneous decay of an unstable atomic nucleus, giving rise to the emission of radiation.

Radionuclide: A radioactive isotope of an element.

Radiotoxicity: The adverse health effect of a radionuclide due to its radioactivity.

Reactor pressure vessel: The main steel vessel containing the reactor fuel, moderator and coolant under pressure.

Repository: A permanent disposal place for radioactive wastes.

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Reprocessing: Chemical treatment of used reactor fuel to separate uranium and plutonium and possibly transuranic elements from the small quantity of fission product wastes, leaving a much reduced quantity of high-level waste (which today includes the transuarnic elements). (cf Waste, HLW).

S

Separative Work Unit (SWU): This is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, i.e. the extent of increase in the concentration of the U-235 isotope relative to the remainder. The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms.

E.g., to produce one kilogram of uranium enriched to 3.5 % U-235 requires 4.3 SWU if the plant is operated at a tails assay 0.30 % or 4.8 SWU if the tails assay is 0.25 % (thereby requiring only 7.0 kg instead of 7.8 kg of natural U feed).

About 100 000— 120 000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Enrichment costs are related to electrical energy used. The gaseous diffusion process consumes some 2400 kWh per SWU, while gas centrifuge plants require only about

60kWh/SWU.

Sievert (Sv): Unit indicating the biological damage caused by radia-

tion. One Joule of beta or gamma radiation absorbed per kilogram of tissue has 1 Sv of biological effect; 1 J/kg of alpha radiation has 20 Sv effect and 1 J/kg of neutrons has 10 Sv effect.

Spent fuel: Used fuel assemblies removed from a reactor after several years use and treated as waste.

Stable: Incapable of spontaneous radioactive decay.

T

Tailings: Ground rock remaining after particular ore minerals (e.g. uranium oxides) are extracted.

Tails: Depleted uranium (See enriched uranium), with about 0.3 % U-235.

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Thermal reactor: A reactor in which the fission chain reaction is sustained primarily by slow neutrons, and hence requiring a moderator (as distinct from Fast Neutron Reactor).

Transmutation: Changing atoms of one element into those of another by neutron bombardment, causing neutron capture and/or fission. In an ordinary reactor neutron capture is the main event, in a fast reactor fission is more common and therefore it is best for dealing with actinides. Fission product transmutation is by neutron capture.

Transuranic element: A very heavy element formed artificially by neutron capture and possibly subsequent beta decay(s). Has a higher atomic number than uranium (92). All are radioactive. Neptunium, plutonium, americium and curium are the best-known.

U

Uranium (U): A mildly radioactive element with two isotopes which are fissile (U-235 and U-233) and two which are fertile (U-238 and U-234). Uranium is the basic fuel of nuclear energy.

Uranium hexafluoride (UF6): A compound of uranium which is a gas above 56В°C and is thus a suitable form in which to enrich the uranium.

Uranium oxide concentrate (U3O8): The mixture of uranium oxides produced after milling uranium ore from a mine, sometimes loosely called yellowcake. It is khaki in color and is usually represented by the empirical formula U3O8. Uranium is sold in this form.

V

Vitrification: The incorporation of high-level wastes into borosilicate glass, to make up about 14 % of it by mass. It is designed to immobilise radionuclides in an insoluble matrix ready for disposal.

W

Waste:

High-level waste (HLW) is highly radioactive material arising from nuclear fission. It can be what is left over from reprocessing used fuel, though some countries regard spent fuel itself as HLW. It requires very careful handling, storage and disposal.

Low-level waste (LLW) is mildly radioactive material usually disposed of by incineration and burial.

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Y

Yellowcake: Ammonium diuranate, the penultimate uranium compound in U3O8 production, but the form in which mine product was sold until about 1970. See also Uranium oxide concentrate.

Z

Zircaloy: Zirconium alloy used as a tube to contain uranium oxide fuel pellets in a reactor fuel assembly.

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