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Gloss Terms 8Pp A4 Glossary of nuclear terms Glossary of nuclear terms Atomic number (Z) Chain reaction Number of protons in the nucleus of an A process in which one nuclear atom, which also indicates the position of transformation sets up conditions for a that element in the periodic table. similar nuclear transformation in another nearby atom. Thus, when fission occurs in Background radiation uranium-235 atoms, neutrons are The ionising radiation in the environment released, which in turn may produce to which we are all exposed. It comes fission in other uranium-235 atoms. from many sources including outer space, the sun, rocks, soil, buildings, the air we Containment, reactor breathe, the food we eat, and our own The prevention of release, even under the Accelerator bodies. The average annual background conditions of a reactor accident, of A device that accelerates charged radiation dose in Australia is about two unacceptable quantities of radioactive particles or ions to very high speeds. millisieverts (see Dose, effective). material beyond a controlled area. Also, commonly, the containing system itself. Actinides Becquerel (Bq) Elements with 89 or more protons in their Unit of activity equal to one radioactive Contamination nucleus that behave chemically like disintegration per second. Replaces the Uncontained radioactive material which actinium. All are radioactive and many are older unit, the Curie (Ci): 1 Ci = 3.7x1010 has been dispersed into unwanted long lived alpha-emitters. Bq. locations. Activity (of a substance) Beta particle (β) Control rods The number of disintegrations per unit A particle emitted from the nucleus of an Rods, plates or tubes containing time taking place in a radioactive material. atom during radioactive decay. Beta cadmium, hafnium or some other strong The unit of activity is the Becquerel (Bq), particles are electrons with either negative absorber of neutrons. They are used to one disintegration per second. or positive electric charge. High energy control the rate of the nuclear reaction in a Alpha particle (α) beta particles may travel metres in air and reactor. several millimetres into the human body; Coolant A positively charged particle emitted low energy betas are unable to penetrate from the nucleus of an atom during the skin. Most beta particles may be A fluid circulated through a nuclear reactor radioactive decay. Consists of two stopped by a small thickness of a light to remove or transfer heat generated by protons and two neutrons (a helium-4 material such as aluminium or plastic. the fuel elements. Common coolants are nucleus). Although alpha particles are water, air and carbon dioxide. normally highly energetic, they travel only Burnup Core, reactor a few centimetres in air and are stopped Either the percentage of a nuclear fuel by a sheet of paper or the outer layer of that has been ‘fissioned’, sometimes That region of a nuclear reactor in which dead skin. expressed as megawatt days per tonne the fuel is located and where the fission (MWD/t), or the percentage change in chain reaction can take place. The fuel other materials. elements in the core of a reactor contain fissile material. Carbon-14 Criticality A naturally occurring radioactive isotope: half-life approximately 5,730 years. A nuclear reactor is critical when the rate of neutrons produced is equal to the rate of neutrons lost and a self-sustaining chain fission reaction can occur. Critical mass Atom The smallest mass of fissile material that will support a self-sustaining chain A particle of matter that cannot be broken reaction under specified conditions. up by chemical means. Atoms have a nucleus consisting of positively charged Cross-section protons and uncharged neutrons of about A measure of the probability of a particular the same mass. In a neutral atom the nuclear reaction occurring between a positive charges of the protons in the Cerenkov radiation projectile and a target. The probability is nucleus are balanced by the same expressed as an area that the target The emission of light by a charged particle number of negatively charged electrons presents. The unit of measurement is the passing through a transparent non- in motion around the nucleus. barn: 10-28m2. Atomic mass unit (amu) conducting liquid or solid material at a speed greater than the speed of light in Curie (Ci) One-twelfth of the mass of a carbon-12 that material. The high energy beta A measure of radioactivity. Now atom. It is approximately equal to the particles from spent nuclear fuel superseded by the Becquerel: 1 Ci = mass of a single proton or neutron. immersed in water give rise to blue 3.7x1010 Bq. Cerenkov radiation. 2 Glossary of nuclear terms Glossary of nuclear terms Dosimeter (or Dosemeter) A device used to measure the radiation dose a person receives over a period of time. Dose limits The maximum radiation dose, excluding doses from background radiation and medical exposures, that a person may Fission receive over a stated period of time. Usually, the division of a International recommended limits, Cyclotron heavy nucleus into two unequal masses adopted by Australia, are that and the emission of neutrons, gamma A machine to accelerate charged particles occupationally exposed workers should radiation, and a great deal of energy. to high energies by the application of not exceed 20mSv per year (averaged electromagnetic forces. The accelerated over five years, no single year to exceed Fission fragments particles may be used to bombard suitable 50mSv), and that members of the public The two atoms initially formed from the target materials to produce radioisotopes. should not receive more than 1mSv per fission of a heavier atom, such as Decay, radioactive year above background radiation. uranium-235 or plutonium-239. The fission fragments resulting from each fission of The spontaneous radioactive Electron uranium-235, for example, are not disintegration of an atomic nucleus The negatively charged particle that is a necessarily the same. Various pairs of resulting in the release of energy in the common constituent of all atoms. atoms can be produced. When initially form of particles (for example, alpha or Electrons surround the positively charged formed, most fission fragments are beta), gamma radiation, or a combination nucleus and determine the chemical radioactive and emit beta particles and of these. properties of the atom. gamma rays and decay into other atoms. Decommissioning Element Fission products In relation to a nuclear reactor, its A chemical substance that cannot be The collective term for the various fission shutdown, dismantling and eventual divided into simpler substances by fragments and their resulting decay products removal. chemical means; all atoms of a given formed after fission of a heavy atom. Deuterium element have the same number of protons. Flux, neutron Also called ‘heavy hydrogen’, deuterium is The number of neutrons that pass through a non-radioactive isotope of hydrogen Enrichment, isotope one square centimetre per second. having one proton and one neutron in the The elevation of the content of a specified nucleus (that is, an atomic mass of two). isotope in a sample of a particular element Fuel cycle, nuclear It occurs in nature in the proportion of one (or compound thereof). To be used as fuel The series of steps involved in supplying fuel atom to 6,500 atoms of normal hydrogen. for power reactors, uranium usually has to for nuclear reactors and managing the waste (normal hydrogen atoms contain one be enriched – the natural isotopic products. It includes the mining, refining and proton and no neutrons). abundance of uranium-235 (~0.71 per enrichment of uranium, fabrication of fuel Dose, absorbed cent) has to be increased to about 3 per elements, their use in a reactor, reprocessing cent. Material at 20 per cent or greater to recover the fissionable material remaining A measure of the amount of energy enrichment is called high enriched in the spent fuel, possible re-enrichment of deposited in a material by ionising uranium (HEU); below 20 per cent is low the fuel material, possible re-fabrication into radiation. The unit is the joule per enriched uranium (LEU). Isotope more fuel, waste processing and long-term kilogram, given the name Gray (Gy). enrichment processes include gas storage. Dose, equivalent centrifugation and gaseous diffusion. Equivalent dose is a measure of the Fertile material biological effect of radiation on a tissue or A material not itself fissionable by thermal organ and takes into account the type of neutrons that can be converted directly or radiation. The unit is the sievert (Sv), but indirectly into a fissile material by neutron doses are usually measured in capture. There are two basic fertile millisieverts (mSv) or microsieverts (μSv). materials, uranium-238 and thorium-232. Dose, effective When these fertile materials capture Effective dose is a measure of the neutrons they are converted into fissile biological effect of radiation on the whole plutonium-239 and uranium-233 respectively. body. It takes into account the equivalent Fuel rod dose and the differing radiosensitivities of Fissile material A single rod of fissionable material body tissues. The unit is the sievert (Sv), Any material capable of undergoing fission encased in cladding. Fuel rods are but doses are usually measured in by thermal (or slow) neutrons. For assembled into fuel elements. millisieverts (mSv) or microsieverts (μSv). example, uranium-235 and plutonium-239 are fissile nuclides. Australian Nuclear Science and Technology Organisation 3 Glossary of nuclear terms Fusion Mass spectrometer The formation of a heavier nucleus from A device that uses magnetic fields, two lighter ones (such as hydrogen electric fields, or both to separate and isotopes) with an attendant release of thus analyse the masses of various energy (as in a fusion reactor or the sun). isotopes in a sample. Gamma radiation (γ) Microsievert (μSv) Gamma radiation is short wavelength One millionth of a sievert.
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