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10.1 NUCLEUS OF AN ATOM

Describe thecomposition

of thenucleus of anatom in termsof protonsand neutrons

Matter is made up of very small particles calledatoms

Each atom has a very small and very dense corecalled nucleus. Most of the mass of atom iscontained in the nucleus

The electrons move in orbits around the nucleus. There are a lot of empty spaces within atom A nucleus consists of a number of protons and

neutrons.

Protons and neutrons also known as nucleons. A proton has a unit positive charge. A neutron is an uncharged particle of about the

same mass as the proton.

An atom is neutral because it contains an equalnumber of negatively charged electrons. So the netcharge is zero.

Define protonnumber (Z)

Proton number, Z, is defined as the number of protonsin a nucleusThe number of electrons = the number of protonsAn element is identified by its proton number

Define

nucleonnumber (A)

Nucleon number, A is defined as the total number of

protons and neutrons in a nucleus.

Number of neutrons, N = A - ZWhat isnuclide

A nuclide is an atom of a particular structure. Eachelement has nucleus with a specific number ofprotons.

Nuclidenotation XAZ

Proton p11

Neutron n1

0

Electron e0

1

AX A = nucleon number

Z Z = proton number

X = chemical symbol of the elementExample C

12

6

Proton number of carbon = 6, carbon nucleus has 6protons. The nucleon number of carbon is 12. So thenumber of neutrons in carbon nucleus is 12 6 = 6

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DefineIsotopes

Example:

H

1

1 hydrogen

H2

1 deuterium

H3

1 tritium

Isotopes are atoms with the same proton number butdifferent nucleon number.

Isotopes of an element contain the same number ofprotons and the same number of electrons. So

isotopes have the same chemical propertieschemical reactions involve the electrons in anatom.

However they have different physical propertiesbecause their mass is different.

Some isotopes exist naturally. Isotopes can alsobe made artificially.

10.2 RADIOACTIVE DECAY

State whatradioactivityIs

Radioactivity

randomprocess

spontaneousprocess

3 differenttypes ofradiation

Radioactivity is the spontaneous and randomdisintegration (decay) of an unstable nucleusaccompanied by the emission of energetic particlesor photons.

The nuclei of some atoms are unstable. Thenucleus of an unstable atom will decay to becomemore stable by emitting radiation in the form of a

particle or electromagnetic radiation.

Random process means there is no way to tellwhich nucleus will decay, and cannot predictwhen it is going to decay.

A spontaneous process means the process is nottriggered by any external factors such astemperature of pressure.

Alpha

Beta Gamma radiation

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Compare the 3 kinds of radioactive emissions in terms of their natureCharacteristic Alpha Beta GammaNature Helium nuclei or

2 p and 2 nHe4

2

Electrons, e0

1Electromagneticradiation.

Mass 4 1/2000 0Charge +2e -e NeutralSpeed Slow Fast Speed of lightIonizing ability High Medium LowPenetrating power Low Medium HighStopped by A few cm of air or

a piece of paperA few mm of

aluminium foilA few cm of lead

Deflected byelectric andmagnetic fields

Yes Yes No

Ionizingeffect

Visibletracks in acloudchamber foralphaparticle, betaparticle andgamma rays

Radioactive emission has an ionizing effect The 3 types of radiation are highly energetic and use

their energy to remove electrons from the airmolecules when they pass through air.

The ionization of an atom produces positive ion andnegative ion (electron)

Due to their different charges and masses, they havedifferent ionizing abilities.

Alpha Beta Gamma

Dense strongionization powerstraight tracks -

the alphaparticle has alarge mass andmomentum so itis not easilydeflected

Very fast betaparticles - thin,straight tracks.

The slower betaparticles - short,thick trackswhich curve inrandomdirection.

The gamma raysdo not produceclear or

continuoustracks due totheir lowionizing power

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Penetratingpower

The penetrating effect of alpha, beta and gammaradiation depends on their ionizing power.

Radiation which has a stronger ionizing power willhave a lower penetrating effect.

The radiation emission loses some of its energyeach time an ion pair is produced. Alpha particles lose energy very quick as they move

through a medium. After a short distance in themedium, the alpha particles would have lost almostall energy. So alpha particles have the lowestpenetrating power.

Alpha particles can be stopped by paper, betaparticles go through paper but can be stopped bythin metal. Gamma rays can go through paper and

metal sheet and can only be stopped by thick lead orconcrete.

Interactionwithelectricalfield

Alpha and beta particles are deflected in an electricfield because they are charged. The deflections arein opposite direction because they carry oppositecharges. The deflection of beta is larger than alphabecause mass of beta < mass of alpha

Gamma rays are not deflected because they do notcarry any charge.

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Interactionwithmagneticfield

Alpha particles and betaparticles are alsodeflected when they passthrough a magnetic field

while gamma rays areunaffected.

The direction of thedeflection of alphaparticles in the magneticfield can be found usingFlemings left-hand rule.

Name common detectors for radioactive emissionsGold leaf electroscope

When an electroscope is charged, the goldleaf sticks out, because the charges on thegold repel the charges on the metal stalk.

When a radioactive source comes near, theair is ionized and starts to conductelectricity.

This means the charge can leak away, theelectroscope discharges and the gold leaf

falls.

Geiger-Muller tube (GM tube)

The radioactive emission entersthe tube through the mica windowand ionizes the neon gas.

The electrons and positive ions areattracted towards the anode andcathode respectively.

When electrons are collected bythe anode, a pulse of current isproduces.

The pulses of current are countedby a scaler or ratemeter.

The scaler gives the number ofcounts over a certain period of time

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ie counts per minute / counts persecond.

Initially the GM tube is switched onwithout the presence of any

radioactive substance. The readingdisplayed by the ratemeter isknown as the background countrates.

When the GM tube is used to detectradioactive emission, thebackground count rate issubtracted from the count rateobtained.

Notes:Background radiation givesreading to the GM tube eventhough there is no radioactive

source.Background radiation isalways present due to naturalradioactivity in the ground,bricks or buildings andcosmic radiation.

Cloud Chamber

It shows the path traveled by theionizing radiation in air.

The radioactive produces ions inthe air that is saturated withalcohol vapour.

The alcohol vapour condenses onthe ions to make the tracks of theradiation visible. Alpha particlesare best for this because it

ionization power is high.

Spark counter

The spark counter consists of awire gauze and a thin wire below it.

A high voltage is applied betweenthe gauze and the wire. Thevoltage is adjusted until it is justbelow the value required to

produce sparks. When a radioactive source isbrought near the wire gauze, theradiation ionizes the air below it.The motion of the ions to the gauzeand the wire causes sparks to beproduced.

The spark can be seen andheard.

Spark counters aresuitable for alpha particles.Beta particles and gammarays produce too few ionsto produce sparks.

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Photographic Badge

Is worn by worker in nuclear powerstations and in radiationlaboratories.

The badge contains a photographicfilm in a light-proof packet

The parts of the film whichhad received radiation willbe darkened. The degreeof darkening indicates the

amount of radiation theperson had been exposedto.

Explainwhat isradioactivedecay?

Radioactive decay is the process by which unstableatomic nuclei emit subatomic particles or radiation.

When a radioactive nucleus decays, its nucleusbreaks up, emits an alpha particle or beta particle andenergy, and forms a new atom of a different element.

A parent nuclide X changes into a daughter nuclide Y.Alpha decay Beta decay Gamma decayA A - 4 4

X Y + HeZ Z - 2 2

A A 0X Y + e

Z Z + 1 -1

A A

X Y + Z Z

The daughter nuclide has2 protons less an 2

neutrons less than theparent nuclide.The proton number Zdecreases by 2 and itsnucleon number Adecreases by 4

A neutron changes toproton and electron

during beta decay.Proton stays within thenucleus and electron isemitted as beta particle.The nucleus loses aneutron but gains aproton.Proton number increaseby 1 , nucleon numberremains unchanged

Gamma emission doesnot change the structure

of the nucleus, it justmakes the nucleus morestable. Gamma rays areemitted at the same timetogether with either analpha or beta particle.When a nucleus ejects analpha or beta particle,there is often someexcess energy producedwhich will be released as

gamma rays

HeTh4

2

226

88

230

90 + + energy

HeThU4

2

234

90

238

92 + + energy

eNC0

1

14

7

14

6 + + energy

eBiPb 01209

83

209

82 + + energy

++ HePbPo4

2

206

82

210

84

+ CoCo 602760

27

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What isradioactivedecayseries

Sometimes the daughter nuclide of a radioactive doesis still unstable. It will eventually decay into anothernuclide which is also unstable.

This process continues as a radioactive decay seriesuntil a stable nuclide is reached. Each decay will emit either an alpha particle or a betaparticle and may be gamma rays.

Example 1

Uranium-238 ( ) emits an

alpha particle and decays into athorium atom. State the nucleonnumber and proton number of thethorium, The isotope formed.

Write an equation for this decay.

U238

92

Example 2Complete:210 0

Pb Bi + e82 -1

Example 3

Strontium-90 ( ) decays to

yttrium-90 ( ) by emitting a

radioactive particle.

Sr9038

Y90

39

(a) write an equation for thisdecay.

(b) What particle is emittedduring the decay?

Example 4232 Th decays to 208 Pb ,

90 82

by emitting alpha particles andbeta particles. Determine the

number of alpha particles andbeta particles being emitte?

Example 5

The uranium isotope decays

into a stable lead isotope throughsuccessive emissions of 8 alphaparticles and 6 electrons. What isthe proton number and thenucleon number of the leadisotope?

U238

92

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Explainhalf-life

The half-life T1/2 of a radioactive substance is the time for half ofthe radioactive nuclei to decay

Determinethe half-

life from adecaycurve

256 128T1/2

T1/2= radioactivedecay

T1/2 = 3 hours

Example 1The radioactive atoms in a

substance decay to become stableatoms. It was found that after 288 s,6.25% of the atoms have notdecayed. What is the half-life of thesubstance?

Example 2The half-life of iodine-131 is 8 days.

radioactive sample contains 64 g ofiodine-131. Determine the mass ofiodine that has decayed and has notdecayed after 24 days.

Example 3A sample of lead-211 of mass 96 ghas a half-life of 36.1 minutes.(a) What fraction of the sample hasnot decayed after 108.3 minutes?(b) What is the mass of the decayedproducts after this period of time?

Example 4The figure shows the decay curve fora radioactive sample.

(a) What is the half-life of thesample?

(b) State the value of T.

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10.3 THE USES OF RADIOISOTOPE

Define

radioisotopes.

Radioisotopes are unstable isotopes which decayand give out radioactive emissions.

Radioisotopes are isotopes of an element that areradioactive.

Radioisotopes are naturally occurring or artificiallyproduced.

Artificial radioisotopes can be produced whencertain nuclides are bombarded by high energyparticles.

Describe applications of radioisotopesIn industries(a) Smoke detectors

(b) Contain a weak radioactive source such as americium-241. Alpha particles emitted from the source and ionized air molecules.

The ionized air molecules conduct electricity and a small currentflows in the smoke detector.

When smoke enters the detector, the soot particles in the smokeabsorb some of the alpha particles.

This causes a decrease in the current that flows in the smokedetector and trigger the alarm.

Americium-241 has a long half-life, 460 years so that the substancewill last longer.

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(b)Thickness control

A radioactive sendsradiation through the sheetmaterial as it comes off the

production line. Betaradiation are used for thinsheets. A radiationdetector on the other sideof the sheet measures theintensity of the radiationpassing through the sheet.

The amount of radiationreceived by the detectordepends on the thickness

of the rubber sheet.

If the sheet is too thin, the readingof the detector increases.

A signal is sent from the rollercontrol to the rollers so that the

pressure on the sheets can bereduced.

(c) detecting leaks inunderground water pipes.

A radioactive substancewhich emits beta particlesis added to a fluid in apipeline to measure theflow rate in the pipeline

and to find leaks. The radiation produced by

the radioactive substancecan be detected with a GMtube counter placed aboveground.

A larger increase in the count ratewill indicate that there is leak inthat area.

Medicine(a) Radioactive tracers

Nuclear medicine is abranch of medicine thatuses radiation to provideinformation about thefunction of the specificorgans of a patient or totreat disease.

A radioisotope is taken in by apatient through the digestivesystem, by inhalation or throughthe blood vessels by injection.

The radiation emitted enablesorgans such as thyroid, bones,heart, liver to be easily imaged byimaging equipment. Disorders canthen be detected.

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Characteristics ofradioisotope:(a) half-life is medium,

long enough to

examine metabolicprocesses in the bodyand short enough tominimize the radiationdose to the patient.

(b) emits low energy gamma rayswhich can pass through thebody without much ionization ofthe molecules.

(b)Sterilizing

medical instruments aresterilized after packing bya brief exposure to gamma

rays. Gamma rays can be used

to kill bacteria, mould andinsects in food.

(c)cancer treatment

Gamma rays can kill living cells.They are used to kill cancer cellswithout having to resort to surgery.

This is called radiotherapy. It is important to administer the

correct dosage. Excessiveradiation will damage too manyhealthy cells. Insufficient radiationwill not stop the cancer fromspreading.

Application of radioisotope in

the field of agriculture By measuring the

radioactivity of the stemand leaves, scientists canfind out how muchfertilizer has beenabsorbed by the plant.

Radioisotopes are used tokill pests and parasitesand to control the ripeningof fruits.

Application of radioisotope in

archaeology Carbon-14 is a radioisotope with a

half-life of 5730 years and decaysby emitting beta particles.

Living animals and plants have aknown proportion of carbon-14 intheir tissues which remainsconstant.

When living things die, the amountof carbon-14 in their bodydecreases at a known rate.

The amount of carbon-14 left in adecayed plant or animal can beused to tell its age.

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10.4 NUCLEAR ENERGY

Define

atomicmass unita.m.u

The atomic mass unit it the unit of mass for atomsand subatomic particles such as the proton, neutronan electron

1 atomic mass unit or 1 u is12

1of the mass of the

carbon-12 atom.

The mass of one carbon-12 atom is 1.99265 x 10-26 kg 1 u =

261099265.112

1

kg

1 u = 1.66 x 10-27 kg

Describenuclearfission

Nuclear fission is the splitting of a heavy nucleus intotwo lighter nuclei

Fission occurs when the nucleus of an atom isbombarded with a neutron.

The energy of the neutron causes the target nucleusto split into two (or more) nuclei that are lighter thanthe parent nucleus, releasing a large amount of energyduring the process.

Give

example ofnuclearfission

A slow neutron hitting a uranium-235 nucleus, causingit to split producing strontium-90 , xenon-143 andthree neutrons + energy.

energynSrXenU ++++ 109038143541023592 3 another reaction:

energynKrBanU ++++1

0

92

36

141

56

1

0

235

92 3

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Differencebetweennuclearfission and

radioactivedecay

The nucleus is divided into two large fragments ofroughly equal mass

There is a significant mass defect Other neutrons are emitted in the process

Describechainreactions

Exampleof a chainreaction

In the fission reactions, one neutron starts the fissionprocess, but three neutrons are produced.

If one of these neutrons bombards another uranium-235 nucleus then more fissions will occur, releasingmore neutrons. A chain reaction is produced.

A chain reaction is a self-sustaining reaction in whichthe products of a reaction can initiate another similarreaction.

As uranium atoms continue to split, a significantamount of energy is released during each reaction.The heat released is harnessed and used to generateelectrical energy

A controlled chain reaction is used in nuclear powerstations

An uncontrolled chain reaction is used in nuclearbombs.

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Criticalmass

Graphiteas amoderator

In order for a chain reaction to occur, the sample ofuranium must have a certain minimum mass known ascritical mass

The neutrons produced in a fission reaction are veryfast neutrons. Slower neutrons are more easilycaptured by the uranium nuclei. Graphite can act asmoderators to slow down the chain reaction to occurat a smaller critical mass

Describenuclearfusion

Nuclear fusion is the combining of two lighter nucleito form a heavier nucleus, releasing a vast amount ofenergy during the process.

Give

examplesof nuclearfusion

Nuclear fusion is believed to be the process by whichenergy is released by the Sun. When two hydrogen-2nuclei moving at high speeds collide, they can jointogether to produce heavier nucleus. A large amountof energy is released.

Another example:2 3 4 1

H + H He + n + energy1 1 2 0

energyHHeHeH +++1

1

4

2

3

2

2

1

A hydrogen bomb uses the principle of nuclear fusion for

its design.

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Relate therelease ofenergy in anuclear

reactionwith achange ofmassaccordingto theequationE = mc2

Both nuclear fission and fusion release a large amountof energy. The source of this energy is from the lossof mass in nuclear reactions.

The sum of the masses of the particles before thereaction is more than the sum of the masses of theparticles after the reaction.

Mass defect issum of the masses sum of the massesbefore reaction after reaction

Mass and energy can be exchanged one for the other. A loss of mass in a nuclear reaction means that the

mass had changed to energy.

The relationship between the mass and the energy:E = mc2

Where E = energy released, in joules, Jm = loss of mass or mass defect, in kgc = speed of light = 3.0 x 108 m/s

Example 1Below is an equation for the decay ofradium-226.

energyHeRnRa 42222

86

226

88 ++

The masses of each atom:Ra-226 = 226.02536 a.m.uRn-222 = 222.01753 a.m.uHe-4 = 4.00260 a.m.u1 a.m.u = 1.66 x 10-27 kg1 eV = 1.66 x 10-19 J

(a) Find the mass defect in

(i) a.m.u (ii) kg(b) Calculate the amount of energy

released in(i) J (ii) eV

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Example 2Below is an equation involving thefission of Uranium-235 by a fastmoving neutron.

energy31

0

92

36

141

56

1

0

235

92 ++++ nKrBanUCalculate the amount of energyreleased by an atom of Uranium 235.U-235 = 235.04 a.m.uKr-92 = 91.93 a.m.uBa-141 = 140.91 a.m.uNeutron = 1.01 a.m.u1 a.m.u = 1.66 x 10-27 kg

c = 3.0 x 108

m s-1

Example 3A possible fusion reaction isrepresented by the followingequation:

HHHH1

1

3

1

2

1

2

1 ++

Calculate the energy released in

joules.H-2 = 2.014102 uH-3 = 3.016049 uH-1 = 1.007825 u1 a.m.u = 1.66 x 10-27 kgc = 3.0 x 108 ms-1

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Describethegenerationof

electricityfromnuclearfission

A nuclearreactor

Uraniumfuel rods :

Graphite

moderator:

Boron orcadmiumcontrol rod

Concreteshield:

Coolant:

Heatexchanger:

Electricity can be generated from the energy releasedby fission reactions. A nuclear power station consistsof a nuclear reactor and a generator.

It produces tremendous amount of energy throughnuclear fission.

The nuclei are split by neutrons in a controlled chainreaction, releasing a large amount of energy. The energyreleased heats up the cold gas that passes through thereactor core.

Acts as a moderator to slow down the fast neutrons

produced by the fission. Slower neutrons are morereadily captured by the uranium nuclei.

The boron control rods absorb neutrons. It can controlthe rate of fission reaction. When rods are lowered intothe reactor core to absorb some of the neutrons, the rateof the fission reaction reduced.

Prevents leakage of radiation from the reactor core.

Take away the heat from the nuclear reactor.Substances with high specific heat capacity such aswater and carbon dioxide are used.

Heat energy from the very hot gas is used to boil thewater into steam

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Turbines:

Coil and

magnet:

Process:

The turbines are rotated by the flow of steam under highpressure

The coils are rotated by the turbines. Electricity is

generated by electromagnetic induction.

Justify theuse ofnuclearfission inthegenerationofelectricity

Advantages of using nuclear fission:

Nuclear power costs about the same as coal, so it isnot expensive

It does not produce smoke or carbon dioxide. It doesnot contribute to the greenhouse effect. It produces

less waste than fossil fuels. It produces huge amounts of energy from small

amounts of fuel.

Nuclear power stations need less fuel than stationswhich use fossil fuels

Vast reserves of nuclear fuel in the world. Safety procedures in the administration of nuclear

reactors are very advanced and safe

Produces useful radioisotopes as by-products thatcan be used in industry, medicine, agriculture andresearch.

Disadvantages of using nuclear fission

The initial cost to design and build a nuclear powerstation is very high.

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There is always a risk of accidents. If a chain reactiongoes out of control, explosion or leakage or largeamounts of radioactive substance may happen.

Used fuel rods are very hot and highly radioactivewith very long half-lives.

Expensive procedures are required to cool down therods and store them.

The hot water discharged from the nuclear powerstations can cause thermal pollution

People who work in the nuclear power station andthose living nearby may be exposed to excessiveradiation.

10.5 THE IMPORTANCE OF PROPER MANAGEMENT OFRADIOACTIVE SUBSTANCES

Sources ofbackgroundradiation

Our bodies are always exposed to the naturalbackground radiation.

The level of background radiation is very low and isnot harmful to humans.

Describe thenegativeeffects ofradioactivesubstances

When radioactive emissions strikes living cells, itcan cause ionization to the molecules of the cells.This may cause the cells to be killed, resulting intissue damage.

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Somaticeffect

Geneticeffect

At low doses of radiation, the damaged tissues canrepair itself rapidly.

High doses of radiation can cause burn effectsknown as radiation burns.

The ionization effect of radiation can also causegenetic damage to the molecules of the cells. Thismay lead to the formation of cancerous cells andtumour development.

If the radioactive substances gets inside the body,the most harmful effects come from the alphaparticles because they have the highest ionizationpower.

If the radioactive source is outside the body, thegreatest danger is from gamma sources becausegamma rays have the highest penetrating power.The alpha particles would not penetrate clothing andis highly unlikely to reach living cells in the body.

Somatic effect appears in the person exposed toradiation. The seriousness of the damage dependson the dose of radiation received.Fatigue, nausea, hair loss, radiation burns, cataractsBlood disorder (leukemia), organ failure, death

Genetic effect affects the reproductive cells and canlead to defective offspring in the future generationsof the exposed person.Birth defects, congenital defects, premature death,chromosome abnormalities, cancer in later life.

Describesafetyprecautionsneeded inthe handlingofradioactivesubstances

Radioactive substances must always be handled withthe correct procedures to prevent harmful effects topeople and the environment.

Read and follow the advice and instructions markedon radioactive sources, equipment and workmanuals.

Gloves must be worn any time an unsealed source isbeing used or whenever contamination is likely tooccur.

Laboratory coats, long pants, and closed-toefootwear should be worn.

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Eating, drinking, applying cosmetic or storing foodis prohibited.

All work surfaces and storage areas should becovered with absorbent material to contain

radioactive material contamination. When using radioactive liquids, plastic or metal

trays (stainless steel washes easily) should beutilized to contain potential spills.

Radioactive material, especially liquids, should bekept in unbreakable containers whenever possible.If glass is used, a secondary container is necessary

Before eating or drinking, wash hands and forearmsthoroughly.

Stronger radioactive sources should be handledwith robotic control systems behind steel, concrete,lead or thick glass panels.

Radiation badges containing photographic filmshould be worn to monitor exposure to radiation.The film is regularly developed. The darkness of thefilm shows the level of exposure to radiation.

Describe themanagement

ofradioactivewaste

Low levelwaste:

In a nuclear reactor, a very large amount of energy isproduced from a very small amount of fuel. However,

much of the waste is radioactive and therefore must becarefully managed as hazardous waste.Radioactive waste consists of a variety materialsrequiring different methods of management to protectpeople and the environment.

Comprises 90% of the volume but only 1% of theradioactivity of all radioactive waste.

Generated from hospitals, laboratories and industry,as well as the nuclear fuel cycle.

Comprises paper, rags, tools, clothing, filters Contains small amounts of mostly short-lived

radioactivity

Buried in shallow landfill sites. Often compacted or incinerated (in a closed

container) before disposal to reduce its volume.

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Intermediatelevel waste:

Makes up 7% of the volume and has 4% of theradioactivity of all radioactive waste.

Contains higher amounts of radioactivity and mayrequire special shielding

Comprises resins, chemical sludge, reactorcomponents and contaminated material from reactordecommissioning

Solidified in concrete or bitumen for disposal Short-lived waste (mainly from reactors) is buried. Long-lived waste (from reprocessing nuclear fuel) is

disposed deep underground

Consists of only 3% of the volume of all radioactivewaste but it holds 95% of the radioactivity.

Mainly used fueled rods or liquid waste from fuelprocessing

Contains highly-radioactive fission products andsome heavy elements with long-lived radioactivity.

Generates large amount of heat and requirescooling, as well as special shielding during handlingand transport.

Vitrified by incorporating it into borosilicate glasswhich is sealed inside stainless steel containers foreventual disposal deep underground.

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