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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 -235 atoms, 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. 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 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 -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 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 . 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 superseded by the Becquerel: 1 Ci = mass of a single proton or . 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 -239. The fission fragments resulting from each fission of The spontaneous radioactive Electron uranium-235, for example, are not disintegration of an 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. 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 (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 -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. electromagnetic radiation of the same physical nature as light, X-rays, radio Millisievert (mSv) waves etc. However, gamma radiation is One thousandth of a sievert. highly penetrating and, depending on its Moderator energy, may require a considerable thickness of lead or concrete to absorb it. A material used in a reactor to slow down Since gamma radiation causes ionisation it high speed neutrons, thus increasing the constitutes a biological hazard. High likelihood of further fission. Examples of energy gamma rays are commonly used Hot cell good moderators include normal water, heavy water, beryllium and graphite. to sterilise medical products. A heavily shielded enclosure for handling Graphite, nuclear grade highly radioactive materials. It may be A form of carbon that can be used as a used for their handling or processing by and reflector in some remote means or for their storage. reactors. Ion Gray (Gy) An atom that has lost or gained one or A measure of absorbed dose. Replaces more orbiting electrons, thus becoming the rad. 1 Gy = 100 rad. electrically charged. Ionising radiation Half-life, radioactive (t1/2) For a single radioactive decay process, the Radiation capable of causing ionisation of the matter through which it passes. time required for the activity to decrease Monitoring, radiation to half its value by that process. Half-lives Ionising radiation may damage living vary, according to the isotope, from less tissue. The collection and assessment of radiological information to determine the than a millionth of a second to more than Ionisation a billion years. adequacy of radiation protection. Radiation Any process by which an atom or can be monitored usuing equipment such Half-life, biological molecule gains or loses electrons. as Geiger counters and scintillation The time required for the amount of a Irradiation counters. particular substance in a biological system to be reduced to one half of its value by Exposure to any kind of radiation. Megawatt (MW) biological processes when the rate of Isotopes Unit of power equal to one million watts. removal is approximately exponential. MWth denotes thermal heat output. Atoms that have the same atomic number Half-life, effective (λ) but different mass numbers. Different Neutron The time required for the amount of a isotopes of the same element have the An uncharged subatomic particle with a particular substance in a biological system same chemical properties, but different mass slightly greater than that of the to be reduced to one half of its value by physical properties. proton and found in the nucleus of every the combined effect of biological Light water reactor (LWR) atom except ordinary hydrogen. Neutrons processes and radioactive decay. are the links in a chain reaction in a Reactors that are moderated and cooled nuclear reactor. Heavy water (D2O) by normal water. They account for most Water containing significantly more than of the worlds installed analysis the natural proportion (one in 6,500) of generating capacity. Included in this group A method of analysis based on the heavy hydrogen (deuterium atoms to are pressurised water reactors (PWRs) identification and measurement of normal hydrogen atoms). Heavy water is and boiling water reactors (BWRs). characteristic radiation from radionuclides used as a moderator in some reactors formed by irradiating a sample of material Mass defect (also mass deficiency) because it effectively slows down with neutrons. neutrons to become thermalised. The amount by which the mass of an atomic nucleus is less than the sum of the masses of its constituent particles.

4 Glossary of nuclear terms Glossary of nuclear terms

Nucleus Rad The positively charged core of an atom. It Measure of absorbed dose. Now is about 1/10,000 the diameter of the superseded by the Gray (Gy). atom but contains nearly all the atom’s Radiation, nuclear mass. All nuclei contain protons and neutrons, except the nucleus of normal Radiation originating from the nucleus of hydrogen (atomic mass of one), which an atom. It includes electromagnetic consists of a single proton. waves (gamma rays) as well as streams of fast-moving charged particles Nuclide (electrons, protons, mesons, etc) and A nucleus of a species of atom neutrons of all velocities. Neutron scattering characterised by its mass number Radioactive material A technique for 'seeing' fine details of the (protons and neutrons), atomic number structure of a substance. It involves firing (protons) and the nuclear energy state. Any natural or artificial material whether in a beam of neutrons at a sample and the solid or liquid form, or in the form of a observing how they are scattered or Particle detector, bubble chamber gas or vapour, that exhibits radioactivity. diffracted. Most neutrons pass between An apparatus in which the movement and For regulatory purposes radioactive atoms. Some of the neutrons collide with collision of ionising particles is determined substances may be defined as any the nucleus of atoms and may be by the examination of trails of gas bubbles radioactive material that has an activity absorbed while others passing close to that form in the paths of the particles as level of 100 becquerels per gram or the nucleus are deflected by the intense they move through a superheated liquid. greater. electrostatic forces. Different structures Particle detector, cloud chamber and different atoms create different pathways for the neutrons. A supersaturated vapour chamber in Material that contains or is contaminated which the path of charged subatomic with radionuclides at concentrations or Neutrons, delayed particles can be detected by the formation radioactivity levels greater than clearance Neutrons resulting from fission that are of chains of droplets on ions generated by levels established by the appropriate emitted by fission products after a their passage. It is also used to infer the authority and for which no use is foreseen. significant time delay. They are important presence of neutral particles and to study in the control of a nuclear reactor. certain nuclear reactions. Neutrons, fast Plutonium Neutrons emitted from fission events. A heavy radioactive, man-made metallic They are thousands of times faster than element. Its most important isotope is slow neutrons and maintain chain fissionable plutonium-239, produced by reactions in fast reactors. neutron irradiation of uranium-238. Plutonium-239 is used as a fuel for power Neutrons, thermal or slow reactors or at high concentrations as Neutrons travelling with energy explosives for nuclear weapons. comparable to those of everyday atoms, required as links in the chain reactions in Progeny Radioactive waste, low level thermal reactors. A nuclide formed in the radioactive decay Any waste material that contains quantities of a radionuclide (called the parent). of radioactive material above the clearance Proton level (as determined in regulations) that requires minimum standards of protection A subatomic particle with a single positive for personnel when the waste is handled, electrical charge and a mass approximately transported and stored. 1,837 times that of the electron and slightly less than that of a neutron. Also, the Radioactive waste, intermediate nucleus of an ordinary or light hydrogen Any waste material that contains atom. Protons are constituents of all nuclei. quantities of radioactive material above Elements are defined by the number of clearance levels, requires shielding and protons. has a thermal power below two kilowatts per cubic metre. Nuclear reactor Quark A structure in which a fission chain Any of a group of six elementary Radioactive waste, high level reaction can be maintained and controlled. fundamental matter particles having Waste which contains large concentrations It usually contains fuel, coolant, electric charges of a magnitude one-third of both short and long lived radioactive moderator, control absorbers and safety or two-thirds that of the electron, nuclides, and is sufficiently radioactive to devices and is most often surrounded by regarded as constituents of all hadrons require both shielding and cooling. It a concrete biological shield to absorb (which include neutrons and protons). generates more than two kilowatts of neutron and gamma ray emission. Each quark type is called a flavour. Quarks heat per cubic metre. only exist inside hadrons.

Australian Nuclear Science and Technology Organisation 5 Glossary of nuclear terms

Radioactivity Sievert The ability of certain nuclides to emit A measurement of equivalent dose and particles, gamma rays or X-rays during effective dose. Replaces the rem: 1 Sv = their spontaneous decay into other nuclei. 100 rem. The final outcome of radioactive decay is Spent fuel a stable nuclide. Nuclear fuel elements in which fission Radioisotope products have built up and the fissile An isotope that is radioactive. Most material depleted to a level where a chain natural elements lighter than bismuth are reaction does not operate efficiently. Also not naturally radioactive. Three natural referred to as irradiated fuel. radioisotopes are radon-222, carbon-14 TLD Stable isotope and potassium-40. Thermo-luminescent dosimeter. A An isotope incapable of spontaneous dosimeter which uses the thermo- Radionuclide radioactive decay. luminescent properties of a material to The nucleus of a radioisotope. measure the dose of ionising radiation over a period. Tracer, radioisotope A radioisotope introduced into a system as a tracking signal, the movement of which can be followed to trace the movement of parts of that system. Transuranics

Photo: Peter Bennetts Elements with an atomic number above 92. They are produced artificially, for Synchrotron example, when uranium is bombarded Radiopharmaceutical A cyclotron in which the magnetic field with neutrons. Some are therefore A radiopharmaceutical is a molecule that strength and frequency of accelerating present in spent fuel (see also actinides). consists of a radioisotope tracer attached voltage increases with the energy of the to a pharmaceutical. After entering the particles to keep their orbital radius body, the radio-labelled pharmaceutical will constant. The isotope of hydrogen of atomic mass of accumulate in a specific organ or tumour. three (hydrogen-3). It is very rare, naturally The radiopharmaceutical will spontaneously radioactive, but can be made in a number produce specific amounts of radiation that of ways, including neutron absorption in a) can be safely used to diagnose or treat lithium, b) deuterium, or c) heavy water. diseases, injuries or infections. Uranium Radon A radioactive element with two isotopes A radioactive element and the heaviest that are fissile (uranium-235 and uranium- known gas. It is a progeny of radium in 233) and two that are fertile (uranium-238 the uranium decay series. Radon gives and uranium-234). Uranium is the basic rise to a significant part of the radiation raw material of nuclear energy. dose from natural background radiation. Uranium, depleted It emanates from the ground, bricks and Synroc Uranium having less than the naturally concrete. occurring percentage of uranium-235 A man-made rock-like ceramic material Rem (~0.71 per cent). As a by-product of which can be used to permanently trap enrichment in the fuel cycle it generally Measure of biological effect of radiation high or intermediate level radioactive (dose). Now superseded by the sievert has 0.20-0.25 per cent uranium-235, the material or highly toxic non-radioactive rest being uranium-238. (Sv): 1 Sv = 100 rems. waste for long-term storage. Uranium, enriched Reprocessing Thermal reactor Uranium in which the content of the The chemical dissolution of spent fuel to A reactor in which the fission chain separate unused uranium and plutonium fissile isotope uranium-235 has been reaction is sustained primarily by slow or increased above the ~0.71 per cent from fission products and other thermal neutrons. transuranic elements. The recovered natural content. Enriched uranium with 20- uranium and plutonium may then be Thorium 40 per cent of uranium-235 is a fuel for recycled into new fuel elements. A naturally occurring radioactive element. many research and power reactors, With the absorption of neutrons thorium- whereas higher enriched uranium with 232 is converted to the fissionable isotope over 90 per cent of uranium-235 is a fuel uranium-233. for fast breeder reactors and the explosive in nuclear weapons.

6 Glossary of nuclear terms Glossary of nuclear terms

Uranium hexafluoride (UF6) X-ray A compound of uranium that is a gas Electromagnetic with above 56°C and is thus a suitable form for wavelengths much shorter than visible processing uranium to enrich it in the light but usually longer than gamma rays. fissile isotope uranium-235. Yellowcake Vitrification The mixture of uranium oxides produced The incorporation of intermediate and after milling uranium ore from a mine. It is

high-level radioactive waste into glass for usually represented by the formula U3O8. long-term storage. If dried at low temperature it is yellow. If dried at higher temperature it is light Whole-body contamination brown. Uranium is exported from Australia monitor in this form. Also referred to as Uranium An assembly for measuring the total Ore Concentrate (UOC). gamma radiation emitted from radioactivity on clothing or skin. The monitor uses one or more radiation detectors shielded against natural background radiation.

Acronyms and abbreviations AGR Advanced Gas-cooled Reactor ICRP International Commission on Radiological Protection AINSE Australian Institute of Nuclear Science and Engineering ICRU International Commission on Radiation Units amu Atomic Mass Unit INES International Nuclear Event Scale ANA Australian Nuclear Association INIS International Nuclear Information System ANSTO Australian Nuclear Science and Technology Organisation LEU Low Enriched Uranium ARI ANSTO Radiopharmaceuticals LOCA Loss-of-coolant Accident ARPANSA Australian Radiation Protection and Nuclear Safety LWR Light Water Reactor Agency mSv Millisievert ARPS Australasian Radiation Protection Society μSv Microsievert ASNO Australian Safeguards and Non-proliferation Office MWD/t Megawatt days per tonne Bq Becquerel MW Megawatt BWR NAA Neutron Activation Analysis Ci Curie NPT Non-proliferation Treaty or Treaty on the Non-proliferation of ECCS Emergency Core Cooling System Nuclear Weapons EIS Environmental Impact Statement OECD NEA Organisation of Economic Cooperation and Development Nuclear Energy Agency Euratom European Atomic Energy Community OPAL Open Pool Australian Light water reactor FBR Fast PET Positron Emission Tomography GCR Gas-cooled Reactor PWR Pressurised Water Reactor HEU High Enriched Uranium Sv Sievert HIFAR High Flux Australian Reactor SPECT Single Photon Emission Computed Tomography HTR High Temperature Reactor TLD Thermo-Luminescent Dosimeter HWR Heavy Water Reactor UIC Uranium Information Centre IAEA International Atomic Energy Agency Z Atomic number

Australian Nuclear Science and Technology Organisation 7 The Australian Nuclear Science and Technology Organisation (ANSTO) is the home of Australia’s nuclear science expertise. This unique expertise is applied to radiopharmaceutical production and research, climate change research, water resource management, materials engineering, neutron scattering and a range of other scientific research disciplines. ANSTO is a Federal Government agency and operates Australia’s only nuclear reactor OPAL - used for research and isotope production. ANSTO applies nuclear science in a wide range of areas for the benefit of all Australians.

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Printed June 2009