glossary of nuclear terms - ansto · glossary of nuclear terms ... although alpha particles are...

Glossary ofnuclear terms

Glossary of nuclear terms

Accelerator

A device that accelerates chargedparticles or ions to very high speeds.

Actinides

Elements with 89 or more protons in theirnucleus that behave chemically likeactinium. All are radioactive and many arelong lived alpha-emitters.

Activity (of a substance)The number of disintegrations per unittime taking place in a radioactive material.The unit of activity is the Becquerel (Bq),one disintegration per second.

Alpha particle (α)A positively charged particle emitted from the nucleus of an atom duringradioactive decay. Consists of two protons and two neutrons (a helium-4nucleus). Although alpha particles arenormally highly energetic, they travel onlya few centimetres in air and are stoppedby a sheet of paper or the outer layer ofdead skin.

Atom

A particle of matter that cannot be brokenup by chemical means. Atoms have anucleus consisting of positively chargedprotons and uncharged neutrons of aboutthe same mass. In a neutral atom thepositive charges of the protons in thenucleus are balanced by the same number of negatively charged electrons in motion around the nucleus.

Atomic mass unit (amu)

One-twelfth of the mass of a carbon-12atom. It is approximately equal to themass of a single proton or neutron.

Atomic number (Z)

Number of protons in the nucleus of anatom, which also indicates the position ofthat element in the periodic table.

Background radiation

The ionising radiation in the environmentto which we are all exposed. It comesfrom many sources including outer space,the sun, rocks, soil, buildings, the air webreathe, the food we eat, and our ownbodies. The average annual backgroundradiation dose in Australia is about twomillisieverts (see Dose, effective).

Becquerel (Bq)Unit of activity equal to one radioactivedisintegration per second. Replaces theolder unit, the Curie (Ci): 1 Ci = 3.7x1010

Bq.

Beta particle (β)A particle emitted from the nucleus of anatom during radioactive decay. Betaparticles are electrons with either negativeor positive electric charge. High energybeta particles may travel metres in air andseveral millimetres into the human body;low energy betas are unable to penetratethe skin. Most beta particles may bestopped by a small thickness of a lightmaterial such as aluminium or plastic.

Burnup

Either the percentage of a nuclear fuelthat has been ‘fissioned’, sometimesexpressed as megawatt days per tonne(MWD/t), or the percentage change inother materials.

Carbon-14

A naturally occurring radioactive isotope:half-life approximately 5,730 years.

Cerenkov radiation

The emission of light by a charged particlepassing through a transparent non-conducting liquid or solid material at aspeed greater than the speed of light inthat material. The high energy betaparticles from spent nuclear fuelimmersed in water give rise to blueCerenkov radiation.

Chain reaction

A process in which one nucleartransformation sets up conditions for asimilar nuclear transformation in anothernearby atom. Thus, when fission occurs inuranium-235 atoms, neutrons arereleased, which in turn may producefission in other uranium-235 atoms.

Containment, reactor

The prevention of release, even under theconditions of a reactor accident, ofunacceptable quantities of radioactivematerial beyond a controlled area. Also,commonly, the containing system itself.

Contamination

Uncontained radioactive material whichhas been dispersed into unwantedlocations.

Control rods

Rods, plates or tubes containingcadmium, hafnium or some other strongabsorber of neutrons. They are used tocontrol the rate of the nuclear reaction in areactor.

Coolant

A fluid circulated through a nuclear reactorto remove or transfer heat generated bythe fuel elements. Common coolants arewater, air and carbon dioxide.

Core, reactor

That region of a nuclear reactor in whichthe fuel is located and where the fissionchain reaction can take place. The fuelelements in the core of a reactor containfissile material.

Criticality

A nuclear reactor is critical when the rateof neutrons produced is equal to the rateof neutrons lost and a self-sustainingchain fission reaction can occur.

Critical mass

The smallest mass of fissile material thatwill support a self-sustaining chainreaction under specified conditions.

Cross-section

A measure of the probability of a particularnuclear reaction occurring between aprojectile and a target. The probability isexpressed as an area that the targetpresents. The unit of measurement is thebarn: 10-28m2.

Curie (Ci)A measure of radioactivity. Nowsuperseded by the Becquerel: 1 Ci =3.7x1010 Bq.

2 Glossary of nuclear terms


Cyclotron

A machine to accelerate charged particlesto high energies by the application ofelectromagnetic forces. The acceleratedparticles may be used to bombard suitabletarget materials to produce radioisotopes.

Decay, radioactive

The spontaneous radioactivedisintegration of an atomic nucleusresulting in the release of energy in theform of particles (for example, alpha orbeta), gamma radiation, or a combinationof these.

Decommissioning

In relation to a nuclear reactor, itsshutdown, dismantling and eventualremoval.

Deuterium

Also called ‘heavy hydrogen’, deuterium isa non-radioactive isotope of hydrogenhaving one proton and one neutron in thenucleus (that is, an atomic mass of two).It occurs in nature in the proportion of oneatom to 6,500 atoms of normal hydrogen.(normal hydrogen atoms contain oneproton and no neutrons).

Dose, absorbed

A measure of the amount of energydeposited in a material by ionisingradiation. The unit is the joule perkilogram, given the name Gray (Gy).

Dose, equivalent

Equivalent dose is a measure of thebiological effect of radiation on a tissue ororgan and takes into account the type ofradiation. The unit is the sievert (Sv), butdoses are usually measured inmillisieverts (mSv) or microsieverts (μSv).

Dose, effective

Effective dose is a measure of thebiological effect of radiation on the wholebody. It takes into account the equivalentdose and the differing radiosensitivities ofbody tissues. The unit is the sievert (Sv),but doses are usually measured inmillisieverts (mSv) or microsieverts (μSv).

Dosimeter (or Dosemeter)

A device used to measure the radiationdose a person receives over a period oftime.

Dose limits

The maximum radiation dose, excludingdoses from background radiation andmedical exposures, that a person mayreceive over a stated period of time.International recommended limits,adopted by Australia, are thatoccupationally exposed workers shouldnot exceed 20mSv per year (averagedover five years, no single year to exceed50mSv), and that members of the publicshould not receive more than 1mSv peryear above background radiation.

Electron

The negatively charged particle that is acommon constituent of all atoms.Electrons surround the positively chargednucleus and determine the chemicalproperties of the atom.

Element

A chemical substance that cannot bedivided into simpler substances bychemical means; all atoms of a givenelement have the same number ofprotons.

Enrichment, isotope

The elevation of the content of a specifiedisotope in a sample of a particular element(or compound thereof). To be used as fuelfor power reactors, uranium usually has tobe enriched – the natural isotopicabundance of uranium-235 (~0.71 percent) has to be increased to about 3 percent. Material at 20 per cent or greaterenrichment is called high enricheduranium (HEU); below 20 per cent is lowenriched uranium (LEU). Isotopeenrichment processes include gascentrifugation and gaseous diffusion.

Fertile material

A material not itself fissionable by thermalneutrons that can be converted directly orindirectly into a fissile material by neutroncapture. There are two basic fertilematerials, uranium-238 and thorium-232.When these fertile materials captureneutrons they are converted into fissileplutonium-239 and uranium-233respectively.

Fissile material

Any material capable of undergoing fissionby thermal (or slow) neutrons. Forexample, uranium-235 and plutonium-239are fissile nuclides.

Fission

Usually, the division of a heavy nucleus into two unequal massesand the emission of neutrons, gammaradiation, and a great deal of energy.

Fission fragments

The two atoms initially formed from thefission of a heavier atom, such asuranium-235 or plutonium-239. The fissionfragments resulting from each fission ofuranium-235, for example, are notnecessarily the same. Various pairs ofatoms can be produced. When initiallyformed, most fission fragments areradioactive and emit beta particles andgamma rays and decay into other atoms.

Fission products

The collective term for the various fissionfragments and their resulting decay productsformed after fission of a heavy atom.

Flux, neutron

The number of neutrons that pass throughone square centimetre per second.

Fuel cycle, nuclear

The series of steps involved in supplying fuelfor nuclear reactors and managing the wasteproducts. It includes the mining, refining andenrichment of uranium, fabrication of fuelelements, their use in a reactor, reprocessingto recover the fissionable material remainingin the spent fuel, possible re-enrichment ofthe fuel material, possible re-fabrication intomore fuel, waste processing and long-termstorage.

Fuel rod

A single rod of fissionable materialencased in cladding. Fuel rods areassembled into fuel elements.

3Australian Nuclear Science and Technology Organisation


Fusion

The formation of a heavier nucleus fromtwo lighter ones (such as hydrogenisotopes) with an attendant release ofenergy (as in a fusion reactor or the sun).

Gamma radiation (γ)Gamma radiation is short wavelengthelectromagnetic radiation of the samephysical nature as light, X-rays, radiowaves etc. However, gamma radiation ishighly penetrating and, depending on itsenergy, may require a considerablethickness of lead or concrete to absorb it.Since gamma radiation causes ionisation itconstitutes a biological hazard. Highenergy gamma rays are commonly usedto sterilise medical products.

Graphite, nuclear grade

A form of carbon that can be used as aneutron moderator and reflector in somereactors.

Gray (Gy)A measure of absorbed dose. Replacesthe rad. 1 Gy = 100 rad.

Half-life, radioactive (t1/2)For a single radioactive decay process, thetime required for the activity to decreaseto half its value by that process. Half-livesvary, according to the isotope, from lessthan a millionth of a second to more thana billion years.

Half-life, biological

The time required for the amount of aparticular substance in a biological systemto be reduced to one half of its value bybiological processes when the rate ofremoval is approximately exponential.

Half-life, effective (λ)The time required for the amount of aparticular substance in a biological systemto be reduced to one half of its value bythe combined effect of biologicalprocesses and radioactive decay.

Heavy water (D2O)Water containing significantly more thanthe natural proportion (one in 6,500) ofheavy hydrogen (deuterium atoms tonormal hydrogen atoms). Heavy water isused as a moderator in some reactorsbecause it effectively slows downneutrons to become thermalised.

Hot cell

A heavily shielded enclosure for handlinghighly radioactive materials. It may beused for their handling or processing byremote means or for their storage.

Ion

An atom that has lost or gained one ormore orbiting electrons, thus becomingelectrically charged.

Ionising radiation

Radiation capable of causing ionisation ofthe matter through which it passes.Ionising radiation may damage livingtissue.

Ionisation

Any process by which an atom ormolecule gains or loses electrons.

Irradiation

Exposure to any kind of radiation.

Isotopes

Atoms that have the same atomic numberbut different mass numbers. Differentisotopes of the same element have thesame chemical properties, but differentphysical properties.

Light water reactor (LWR)

Reactors that are moderated and cooledby normal water. They account for mostof the worlds installed nuclear powergenerating capacity. Included in this groupare pressurised water reactors (PWRs)and boiling water reactors (BWRs).

Mass defect (also mass deficiency)

The amount by which the mass of anatomic nucleus is less than the sum of themasses of its constituent particles.

Mass spectrometer

A device that uses magnetic fields,electric fields, or both to separate andthus analyse the masses of variousisotopes in a sample.

Microsievert (μSv)One millionth of a sievert.

Millisievert (mSv)One thousandth of a sievert.

Moderator

A material used in a reactor to slow downhigh speed neutrons, thus increasing thelikelihood of further fission. Examples ofgood moderators include normal water,heavy water, beryllium and graphite.

Monitoring, radiation

The collection and assessment ofradiological information to determine theadequacy of radiation protection. Radiationcan be monitored usuing equipment suchas Geiger counters and scintillationcounters.

Megawatt (MW)Unit of power equal to one million watts.MWth denotes thermal heat output.

Neutron

An uncharged subatomic particle with amass slightly greater than that of theproton and found in the nucleus of everyatom except ordinary hydrogen. Neutronsare the links in a chain reaction in anuclear reactor.

Neutron activation analysis

A method of analysis based on theidentification and measurement ofcharacteristic radiation from radionuclidesformed by irradiating a sample of materialwith neutrons.


Neutron scattering

A technique for 'seeing' fine details of thestructure of a substance. It involves firinga beam of neutrons at a sample andobserving how they are scattered ordiffracted. Most neutrons pass betweenatoms. Some of the neutrons collide withthe nucleus of atoms and may beabsorbed while others passing close tothe nucleus are deflected by the intenseelectrostatic forces. Different structuresand different atoms create differentpathways for the neutrons.

Neutrons, delayed

Neutrons resulting from fission that areemitted by fission products after asignificant time delay. They are importantin the control of a nuclear reactor.

Neutrons, fast

Neutrons emitted from fission events.They are thousands of times faster thanslow neutrons and maintain chainreactions in fast reactors.

Neutrons, thermal or slow

Neutrons travelling with energycomparable to those of everyday atoms,required as links in the chain reactions inthermal reactors.

Nuclear reactor

A structure in which a fission chainreaction can be maintained and controlled.It usually contains fuel, coolant,moderator, control absorbers and safetydevices and is most often surrounded bya concrete biological shield to absorbneutron and gamma ray emission.

Nucleus

The positively charged core of an atom. Itis about 1/10,000 the diameter of theatom but contains nearly all the atom’smass. All nuclei contain protons andneutrons, except the nucleus of normalhydrogen (atomic mass of one), whichconsists of a single proton.

Nuclide

A nucleus of a species of atomcharacterised by its mass number(protons and neutrons), atomic number(protons) and the nuclear energy state.

Particle detector, bubble chamber

An apparatus in which the movement andcollision of ionising particles is determinedby the examination of trails of gas bubblesthat form in the paths of the particles asthey move through a superheated liquid.

Particle detector, cloud chamber

A supersaturated vapour chamber inwhich the path of charged subatomicparticles can be detected by the formationof chains of droplets on ions generated bytheir passage. It is also used to infer thepresence of neutral particles and to studycertain nuclear reactions.

Plutonium

A heavy radioactive, man-made metallicelement. Its most important isotope isfissionable plutonium-239, produced byneutron irradiation of uranium-238.Plutonium-239 is used as a fuel for powerreactors or at high concentrations asexplosives for nuclear weapons.

Progeny

A nuclide formed in the radioactive decayof a radionuclide (called the parent).

Proton

A subatomic particle with a single positiveelectrical charge and a mass approximately1,837 times that of the electron and slightlyless than that of a neutron. Also, thenucleus of an ordinary or light hydrogenatom. Protons are constituents of all nuclei.Elements are defined by the number ofprotons.

Quark

Any of a group of six elementaryfundamental matter particles havingelectric charges of a magnitude one-thirdor two-thirds that of the electron,regarded as constituents of all hadrons(which include neutrons and protons).Each quark type is called a flavour. Quarksonly exist inside hadrons.

Rad

Measure of absorbed dose. Nowsuperseded by the Gray (Gy).

Radiation, nuclear

Radiation originating from the nucleus ofan atom. It includes electromagneticwaves (gamma rays) as well as streamsof fast-moving charged particles(electrons, protons, mesons, etc) andneutrons of all velocities.

Radioactive material

Any natural or artificial material whether inthe solid or liquid form, or in the form of agas or vapour, that exhibits radioactivity.For regulatory purposes radioactivesubstances may be defined as anyradioactive material that has an activitylevel of 100 becquerels per gram orgreater.

Radioactive waste

Material that contains or is contaminatedwith radionuclides at concentrations orradioactivity levels greater than clearancelevels established by the appropriateauthority and for which no use is foreseen.

Radioactive waste, low level

Any waste material that contains quantitiesof radioactive material above the clearancelevel (as determined in regulations) thatrequires minimum standards of protectionfor personnel when the waste is handled,transported and stored.

Radioactive waste, intermediate

Any waste material that containsquantities of radioactive material aboveclearance levels, requires shielding andhas a thermal power below two kilowattsper cubic metre.

Radioactive waste, high level

Waste which contains large concentrationsof both short and long lived radioactivenuclides, and is sufficiently radioactive torequire both shielding and cooling. Itgenerates more than two kilowatts ofheat per cubic metre.



Radioactivity

The ability of certain nuclides to emitparticles, gamma rays or X-rays duringtheir spontaneous decay into other nuclei.The final outcome of radioactive decay isa stable nuclide.

Radioisotope

An isotope that is radioactive. Mostnatural elements lighter than bismuth arenot naturally radioactive. Three naturalradioisotopes are radon-222, carbon-14and potassium-40.

Radionuclide

The nucleus of a radioisotope.

Radiopharmaceutical

A radiopharmaceutical is a molecule thatconsists of a radioisotope tracer attachedto a pharmaceutical. After entering thebody, the radio-labelled pharmaceutical willaccumulate in a specific organ or tumour.The radiopharmaceutical will spontaneouslyproduce specific amounts of radiation thatcan be safely used to diagnose or treatdiseases, injuries or infections.

Radon

A radioactive element and the heaviestknown gas. It is a progeny of radium inthe uranium decay series. Radon givesrise to a significant part of the radiationdose from natural background radiation.It emanates from the ground, bricks andconcrete.

Rem

Measure of biological effect of radiation(dose). Now superseded by the sievert(Sv): 1 Sv = 100 rems.

Reprocessing

The chemical dissolution of spent fuel toseparate unused uranium and plutoniumfrom fission products and othertransuranic elements. The recovereduranium and plutonium may then berecycled into new fuel elements.

Sievert

A measurement of equivalent dose andeffective dose. Replaces the rem: 1 Sv =100 rem.

Spent fuel

Nuclear fuel elements in which fissionproducts have built up and the fissilematerial depleted to a level where a chainreaction does not operate efficiently. Alsoreferred to as irradiated fuel.

Stable isotope

An isotope incapable of spontaneousradioactive decay.

Synchrotron

A cyclotron in which the magnetic fieldstrength and frequency of acceleratingvoltage increases with the energy of theparticles to keep their orbital radiusconstant.

Synroc

A man-made rock-like ceramic materialwhich can be used to permanently traphigh or intermediate level radioactivematerial or highly toxic non-radioactivewaste for long-term storage.

Thermal reactor

A reactor in which the fission chainreaction is sustained primarily by slow orthermal neutrons.

Thorium

A naturally occurring radioactive element.With the absorption of neutrons thorium-232 is converted to the fissionable isotopeuranium-233.

TLD

Thermo-luminescent dosimeter. Adosimeter which uses the thermo-luminescent properties of a material tomeasure the dose of ionising radiationover a period.

Tracer, radioisotope

A radioisotope introduced into a systemas a tracking signal, the movement ofwhich can be followed to trace themovement of parts of that system.

Transuranics

Elements with an atomic number above92. They are produced artificially, forexample, when uranium is bombardedwith neutrons. Some are thereforepresent in spent fuel (see also actinides).

Tritium

The isotope of hydrogen of atomic mass ofthree (hydrogen-3). It is very rare, naturallyradioactive, but can be made in a numberof ways, including neutron absorption in a)lithium, b) deuterium, or c) heavy water.

Uranium

A radioactive element with two isotopesthat are fissile (uranium-235 and uranium-233) and two that are fertile (uranium-238and uranium-234). Uranium is the basicraw material of nuclear energy.

Uranium, depleted

Uranium having less than the naturallyoccurring percentage of uranium-235(~0.71 per cent). As a by-product ofenrichment in the fuel cycle it generallyhas 0.20-0.25 per cent uranium-235, therest being uranium-238.

Uranium, enriched

Uranium in which the content of thefissile isotope uranium-235 has beenincreased above the ~0.71 per centnatural content. Enriched uranium with 20-40 per cent of uranium-235 is a fuel formany research and power reactors,whereas higher enriched uranium withover 90 per cent of uranium-235 is a fuelfor fast breeder reactors and the explosivein nuclear weapons.


Photo: Peter Bennetts


Acronyms and abbreviations

AGR Advanced Gas-cooled Reactor

AINSE Australian Institute of Nuclear Science and Engineering

amu Atomic Mass Unit

ANA Australian Nuclear Association

ANSTO Australian Nuclear Science and Technology Organisation

ARI ANSTO Radiopharmaceuticals

ARPANSA Australian Radiation Protection and Nuclear SafetyAgency

ARPS Australasian Radiation Protection Society

ASNO Australian Safeguards and Non-proliferation Office

Bq Becquerel

BWR Boiling Water Reactor

Ci Curie

ECCS Emergency Core Cooling System

EIS Environmental Impact Statement

Euratom European Atomic Energy Community

FBR Fast Breeder Reactor

GCR Gas-cooled Reactor

HEU High Enriched Uranium

HIFAR High Flux Australian Reactor

HTR High Temperature Reactor

HWR Heavy Water Reactor

IAEA International Atomic Energy Agency

ICRP International Commission on Radiological Protection

ICRU International Commission on Radiation Units

INES International Nuclear Event Scale

INIS International Nuclear Information System

LEU Low Enriched Uranium

LOCA Loss-of-coolant Accident

LWR Light Water Reactor

mSv Millisievert

μSv Microsievert

MWD/t Megawatt days per tonne

MW Megawatt

NAA Neutron Activation Analysis

NPT Non-proliferation Treaty or Treaty on the Non-proliferation ofNuclear Weapons

OECD NEA Organisation of Economic Cooperation andDevelopment Nuclear Energy Agency

OPAL Open Pool Australian Light water reactor

PET Positron Emission Tomography

PWR Pressurised Water Reactor

Sv Sievert

SPECT Single Photon Emission Computed Tomography

TLD Thermo-Luminescent Dosimeter

UIC Uranium Information Centre

Z Atomic number

Uranium hexafluoride (UF6)A compound of uranium that is a gasabove 56°C and is thus a suitable form forprocessing uranium to enrich it in thefissile isotope uranium-235.

Vitrification

The incorporation of intermediate andhigh-level radioactive waste into glass forlong-term storage.

Whole-body contaminationmonitor

An assembly for measuring the totalgamma radiation emitted fromradioactivity on clothing or skin. Themonitor uses one or more radiationdetectors shielded against naturalbackground radiation.

X-ray

Electromagnetic radiations withwavelengths much shorter than visiblelight but usually longer than gamma rays.

Yellowcake

The mixture of uranium oxides producedafter milling uranium ore from a mine. It isusually represented by the formula U3O8.If dried at low temperature it is yellow. Ifdried at higher temperature it is lightbrown. Uranium is exported from Australiain this form. Also referred to as UraniumOre Concentrate (UOC).



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Printed June 2009

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