radioactivity

In unit 4 we will learn about energy from the nucleus and its applications.

*

What do you know?

How do we get energy from the nucleus?

What do we mean by energy?

What do we mean by nucleus?

What do we use it for?

What do we know?

*

Ionising Radiations are used in many medical applications including X-rays and sterilising hospital equipment. They are also used in many non medical applications and it is important in many fields of work to understand radiation dose and safety. Nuclear reactors are used in the production of around 11% of the world’s energy production, and to power some military ships and submarines.

Key words: atom, protons, neutrons, electrons, radiationenergy, absorption, alpha, beta, gamma, ionisation

By the end of this lesson you will be able to:

Describe a simple model of the atom which includes protons,

neutrons and electrons.State that radiation energy may be absorbed in the

medium through which is passes.State the range through air and absorption of alpha,

beta and gamma radiation.Explain what is meant by an alpha particle, beta particle

and gamma radiation.Explain the term ionisation.State that alpha particles produce much greater ionisation

density than beta particles or gamma rays.*

Useful Radiation

Radiation has many uses in medical Physics

– different types of radiation are used

for different things.

Baggage scanning

Smoke DetectorsSmoke alarms contain a weak source made of Americium-241.

Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows.

If smoke enters the alarm, this absorbs the alpha particles, the current reduces, and the alarm sounds. Am-241 has a half-life of 460 years.

Radioactive Dating

Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues.When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a half-life of 5700 years). The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.

Leaking Pipes

Radioactivity is used in industry to detect leaks in pipes.

To have a good understanding of radioactivity we need to know a bit about the

structure of the atom

What do atoms look like?

They are very small!

Atoms are the smallest possible particles of the elements which make up everything around us

Structure of the atom

nucleus

proton

neutron electrons

The relative masses and charges of these particles are given below

PARTICLE CHARGE MASS

Proton +1 1

Neutron 0 1

Electron -1 1/ 2000

Relative size of the atom and the nucleus.

The ratio of the diameters is 10 000 : 1 !

If the diameter of a particular atom was 10 metres, its nucleus would be 1 millimetre across!!

The atoms of a particular element are identical:

All carbon atoms have 6 protons in the nucleus and 6 orbiting electrons.

*

Atoms usually have the same number of

protons and electrons so an atom has no

overall charge.Six protons – charge? +6

Six electrons – charge? -6

Overall charge? 0

Ionisation

We will learn

about typesof radiationwhich cause

ionisation.

Ionisation

Ionisation means adding orremoving an electron from anatom to produce a chargedparticle.

What happens to the chargeon an atom when an electron is

added or removed?

Atoms contain protons, which are positive as well as electrons, which

are negative

Normally atoms have equal numbers of protons and electrons and are

therefore neutral

Atoms usually have the same number

of protons and electrons so an atom

has no overall charge.Six protons – charge? +6


Overall charge? 0

If you add an electron…

Six protons – charge? +6


Overall charge?

Add one more electron – charge? -1

-1

If you remove an electron…

Six protons – charge? +6


Overall charge?

Take away an electron – charge? -5

+1

Ionisation means the addition

or removal of an electron from

a neutral atom to produce a

charged particle.Virtual Int 2 Physics -> Radioactivity -> Ionising Radiations -> Model of the Atom *

The picture below shows an ALPHA PARTICLE, consisting of 2 protons and

2 neutrons

Imagine that an ALPHA PARTICLE passes through a neutral atom – this will be

shown in slow motion!

electron

An electron has been knocked out of the atom.

This atom is now positively charged – it is a POSITIVE ION.

There are three types of ionising radiation.

Alpha radiation (Beta radiation (β)Gamma radiation (γ)

Virtual Physics Int 2 – Radioactivity -> Ionising Radiations -> Alpha, Beta, Gamma

http://www.darvill.clara.net/nucrad/detect.htm



An alpha particle is made up of twoprotons and two neutrons. It is the

sameas a helium nucleus.

It is positively charged.It is largest of all the three types

ofradiation.

Alpha radiation (α)

He42

The alpha particle that is emitted has a lot of energy and can damage human cells.

A big atom releases an alpha particle to make itself more stable.

*

An alpha particle is given the symbol

4

2

Alpha radiation (α)

He42

*

What are alpha particles?

An alpha particle is made up of two protons and two neutrons. It is the largest of the three ionising radiations. It has a lot of energy.

Summary

*

a fast moving highenergy electron

released from thenucleus –

it is very very small

Virtual Physics Int 2 – Radioactivity -> Ionising Radiations -> Alpha, Beta, Gamma

Beta radiation (β)

*

http://uk.wrs.yahoo.com/S=2114717014/K=cooling+towers/v=2/SID=e/l=IVI;_ylt=AhszNjGE7KTuBKv1Tw3BzOFWBQx.;_ylu=X3oDMTA4NDgyNWN0BHNlYwNwcm9m/*-http://carbon.cfr.washington.edu/esc110/2003Winter/projects/009/coolingtowers.jpg



A beta particle is given the symbol

Beta radiation (β)

β

βor

01−

This is what happens inside the nucleus.

*

Summary

What are beta particles?

A beta particle is a fast moving, high energy electron. The electron is released from the nucleus when a neutron changes into a proton plus electron.It is very very small.

*

A wave of energy.High frequency electromagnetic wave

(sotravels at the speed of light) No significant mass. No charge.Has the greatest amount of kinetic

energy.

Gamma Radiation (γ)

γ*

Gamma ray

It is the most energetic of all three radiations.

It is therefore the most penetrating – the most difficult to stop.

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What are gamma rays?

Gamma rays are high energy electromagnetic waves. They travel at the speed of light.

*

Radiation & Ionisation

These three radiations (α, β, γ) are called

ionising radiations because theycause ionisation of living cells.

Radiations can kill or change living cells.

This is what makes them dangerous.

Ionisation Density

We can think about how much damage

a type of radiation will cause in terms

of ionisation density.

Alpha particles are heavy and slow moving.

They cause a lot of ionisation.Beta particles are light and cause less

ionisation.Gamma rays have no mass. They cause

little ionisation.*

ALPHA PARTICLES are relatively large and cause a lot of ionisation

+

+

++

+ + +

+

++

++-

-- - - -

--

-- -

-

BETA PARTICLES are smaller, so they cause less ionisation

+ + +

+- - --

GAMMA RAYS cause least ionisation of all

+

-

Ionisation Density & Range of Particles

Each time a particle causes ionisation it

loses energy. The energy is absorbed bythe medium through which it passes.

Alpha particles cause a lot of ionisation,

therefore lose a lot of energy. This means

they have a short range in air.

Ionisation Density & Range of Particles

Beta particles cause less ionisation,therefore lose less energy. This meansthey have a longer range in air than alpha

particles.Gamma particles have the lowestionisation density. This means they have

the longest range in air.

Identifying Radiations

We can tell which radiation is

which by testing to see whathappens when they reachdifferent materials.

Virtual Int 2 – Radioactivity -> Ionising Radiations -> Absorption of Ionising Radiations

http://www.ccotrust.nhs.uk/about/sitemap/access_map.htm



What material is sufficient to absorb alpha particles?

Paper

What material is sufficient to absorb beta particles?

A few millimetres of aluminium

What material is sufficient to absorb gamma rays?

Several cm of lead

How much ionisation do alpha particles cause?

The greatest amount. Alpha particles are most dangerous when inside the body (but least dangerous outside – they can be stopped with paper!)

How much ionisation do beta particles cause?

Medium. Less than alpha, more than gamma.

How much ionisation do gamma particles cause?

The least. Gamma particles are most dangerous when outside the body because they can easily travel into the body. But they’re least dangerous when inside because they can escape.

Can you…?

Describe a simple model of the atom which includesprotons, neutrons and electrons.State that radiation energy may be absorbed in themedium through which is passes.State the range through air and absorption of alpha,

beta and gamma radiation.Explain what is meant by an alpha particle, beta particle

and gamma radiation.Explain the term ionisation.State that alpha particles produce much greaterionisation density that beta particles or gamma rays.

Quick Recap

Type of radiation

Symbol

What is this radiation?

Charge and absorption

Range in air

αA few m

Uncharged. Absorbed by

lead.

Key words: atom, protons, neutrons, electrons, radiation energy,

absorption, alpha, beta, gamma, ionisation


Describe how one of the effects of radiation is used in

a detector of radiation.

State that radiation can kill living cells or change

the nature of living cells.

Describe one medical use of radiation based on the

fact that radiation can destroy cells.

Describe one use of radiation based on the fact that

radiation is easy to detect.

Detecting Radiation

To protect those who workwith radiation it is importantto be able to detectradiation. The detection ofradiation is also vital in its use in

many applications.

Geiger Muller Tube

The Geiger counter is commonly used

to detect radiation (demo).

The Geiger counter consists of aGeiger Muller tube attached to acounter.

Geiger Muller Tube

The tube is filled with argon gas.

Where else is argon gas used?

Geiger Muller Tube

Around 400 V is applied to the thin wire.

Geiger Muller Tube

The thin window alllows radiation to enter.

Radiation causes ionisation of the gas – what do we mean by this?

Geiger Muller Tube

Ions produce electrical pulses which are counted and displayed.

Geiger Muller TubeWe can either display total counts and use a timer to determine counts per second, or use a rate meter, which displays counts per second.

Geiger Muller TubeRadiation

Ionisation in tube (lots of electrons)

Discharges central wire

Counted as a pulse

*

How the Geiger Muller tube works


Photographic Fogging

We know that photographic film can be

fogged or blackened by radiation.

Where is this commonly used in medicine?


This principle is used in film badges

worn by radiation workers.

The darker the film the more radiation

the person has received.


Why are there different materials in the film badge?


Different radiations pass through or are absorbed by different materials.

*

Radiation and the Human Body

When the source of radiation is outside thebody, alpha radiation may not be able to harmthe vital internal organs as it is easily stoppedby the air, layers of clothing or the skin.

If swallowed an alpha radiation source isextremely dangerous. It causes large amountsof ionisation (remember it has a high ionisationdensity) – it changes or kills a lot of living cells.

It can’t escape from the body.

Alexander Litvinenko

Poisoned using extremely rare radioactive substance Polonium-210 – which is 250000 more toxic than hydrogen cyanide. Swallowing a dose less than 1/10th the size of a Smartie is lethal for a grown adult male.


Beta radiation will penetrate the first 1cm or

skin and tissue though, and will damage thattissue. A small amount can penetrate the body.

If the beta source is inside the body, then it

will cause damage internally, for example toorgans.


Gamma radiation will penetrate the skin andtissue, and will deposit its energy as it travels

further into the body. It is more dangerousthan alpha or beta radiation in this case.

Gamma radiation inside the body will alsodamage tissue however it can “escape” and bedetected from outside the body, and this makes

it very useful.

Making Use of Radioactivity

Gamma radiation’s ability to travel through skin

and tissue is used in medical and non medical

applications of radioactivity.

The gamma camera

Radioactive Tracers

A radioactive tracer is a gamma emitting

substance (a radiopharmaceutical) which

can be injected into the body to allow

internal organs and functions to beinvestigated without surgery.

Radioactive Tracers

Technetium-99 and Iodine-123 arecommonly used because they emit onlygamma, which can be detected outside the body, and cause little ionisation.

However, different substances arechosen for different organs.

Radioactive Tracers

A gamma camera is used to detectradiation from outside the body.

This scan is produced after a few hours of the patient being injected with an

isotope that emits gamma radiation. A detector is moved

around the body and a computer produces an image. Dark areas

show high concentrations of

radiation coming from those parts. This

indicates increased blood flow to these

parts.

If a radioisotope that emits alpha radiation is used, no particles can be detected outside the body – why not?

Alpha radiation will be stopped within a few centimetres. Internal organs will be seriously damaged.

Isotopes that emit gamma radiation must be used – why?

Since gamma rays will pass through the body (and out) while doing the least damage.

Radioactive Tracers in Industry

Leaks in underground pipes can be detectedusing radioactive tracers and a Geiger Counter.

A rise in count rate detected would indicatemore radiation escaping the pipe and therefore

a leak or crack.

Oil companies also use radioactive tracers inshared pipelines to identify their own oil.

Radiation Therapy

Radiotherapy is commonly used as part of

treatment for cancer. It might be usedinstead of surgery, or after surgery, or

chemotherapy, to destroy any remainingcancer cells.

Treating Cancer (Radiotherapy)

Ionising radiation kills living cells. Cancers

are simply growths of cells which are outof control and have formed tumours.

By directing radiation at the tumour, theliving cells are damaged or killed, and thisshrinks the tumour. Unfortunately healthy cells

are also damaged or killed by the radiation.


It is importantto ensure thathealthy tissuedoes not receive

too muchradiation whilethe tumourreceives enoughto damage it.


Video clips. http://www.ccotrust.nhs.uk/about/sitemap/access_map.htm

The machine rotates around the patient.The tumour can be hit by radiation all of the time while minimising the damage tohealthy tissue. Each section of healthytissue receives only a small dose.

http://science.howstuffworks.com/nuclear-power2.htm









Why are alpha and beta sources unsuitable

for radiotherapy treatments?

Alpha and beta are absorbed byair/skin/bone so would not reach thediseased tissue within the body. Instead

high energy X-rays are used.*

Radiation & Sterilisation

The ability of radiation to kill living cells

makes it very useful for sterilisingequipment e.g. plastic syringes in hospital.Previously expensive metal or glasssyringes had to be used and sterilised usingheat or chemicals.

Using heat to kill germs and bacteria would melt

the plastic syringes.

Paper Thickness Measurement in Industry

Virtual Int 2 Physics -> Radioactivity -> Ionising Radiations -> Uses of Ionising Radiations

A beta source and detector is used. If thepaper is too thin then the reading on thedetector will increase. If it is too thick, the

reading will decrease.

Why is an alpha source no use for thisapplication?

Key words: activity, radioactive source, decays, decays per second,

becquerels, absorbed dose, grays, radiation weighting factor,equivalent dose, background radiation level


State that the activity of a radioactive source is the number of

decays per second and is measured in becquerels (Bq), where

one becquerel is one decay per second. Carry out calculations involving the relationship between

activity,number of decays and time.

State that the absorbed dose is the energy absorbed perunit mass of the absorbing material. State that the gray (Gy) is the unit of absorbed dose

and

that one gray is one joule per kilogram.

By the end of this lesson you willbe able to:State that a radiation weighting factor isgiven to each kind of radiation as a measure of

its biological effect. State that the equivalent dose is the product

of absorbed dose and radiation weightingfactor and is measured in sieverts (Sv). Carry out calculations involving the relationship

between equivalent dose, absorbed doseand radiation weighting factors.

By the end of this lesson you will be able

to:State that the risk of biological harm from

an exposure to radiation depends on: a) the

absorbed dose b) the kind of radiation,

e.g. α, β, γ, slow neutronc) the body organs or tissue exposed. Describe factors affecting the backgroundradiation level.

How much exposure is safe?

It should be stressed that no minimumamount of exposure to radiation iscompletely safe.In Physics we aim to understand how to

measure radiation and to estimate therisk of exposure. In many cases thebenefit of exposure significantlyoutweighs the risks.

Radioactive Decay

Radiation is caused by the unstable nucleii

of radioactive atoms splitting up.

This is called radioactive decay.

Virtual Int 2 Physics -> Radioactivity -> Dosimetry -> Activity

Activity

We talk about the activity of a

source.

What do we mean by this?The activity of a radioactive source is a

measure of the number of decays persecond.

Units of Activity

The becquerel is used to measure

the activity of a source.

1 becquerel (Bq) is one decay per second.

Activity

t

NA=

Activity (Bq)

Number of nuclei decaying

Time (s)

The becquerel

In practice, particularly in medical

treatment, the Bq is too small. Larger

units such as kBq and MBq are commonly

used.

Dosimetry: Absorbed Dose

When radiation reaches the body or

tissue it is absorbed.

This is called the absorbed dose (D).

Dosimetry: Absorbed Dose

m

ED =

Absorbed dose – units?

Energy (J)

Mass (kg)

Dosimetry

ABSORBED DOSE (D) is the energy absorbed PER UNIT MASS of absorbing tissue.

m

ED =

1 Gy = 1 J/kg

Units are GRAYS (Gy)

Dosimetry

Radiation Treatment

Absorbed dose (Gy)

Chest X-ray 0.00015

CT Scan 0.05

Gamma rays which would just produce reddening of skin

3.0

Dose which if given to whole body in a short period would prove fatal in half the cases

5.0

Typical dose to a tumour over a six week period

60.0

Biological Harm from Radiation

Radiation can damage living cells through heat

or damage to molecule structure such as DNA.

The risk of biological harm from an exposure to radiation depends on

• the absorbed dose• the type of radiation (e.g. alpha, or other nuclear particles such as neutrons)

• the body organs or type of tissue

EQUIVALENT DOSE (H) is a quantity which takes into

account the TYPE OF RADIATION.

RDWH =

Unit of equivalent dose is sieverts (Sv)

WR is the WEIGHTING FACTOR of the particular radiation

Typical Equivalent Dose

Investigation Equivalent dose (mSv)

Chest X-ray 0.1

Spine X-ray 2.0

Stomach X-ray 4.0

CT Scan 1 to 3.5

Bone Scan 2.0

Annual exposure of aircraft crew

2.0

Renogram 2.0

Astronaut in space for one month

15.0

How much is a sievert (Sv)?

If 100 people received a dose of 1 Sv, 4 would die as a result. This is the type of dose you’d receive after a nuclear accident.

We normally work in millisieverts (mSv = Sv )

or microsieverts (μSv = Sv)

1000

1

610−x

1 mSv = One thousandth of a sievert = 0.001 Sv

1 μSv = 0.000001 Sv

Example

A 50kg person is exposed to radiation of

energy 0.25J. The weighting factor forthe radiation is 20.

(a) Calculate the absorbed dose for this

radiation(b) What is the equivalent dose?

Example

(a) Calculate the absorbed dose for this radiation

Gym

ED 005.0

50

25.0===

Example

(b) What is the equivalent dose?

100mSvor 1.020005.0 SvxDWH R ===

1 mSv is about 100 times theradiation you experience when

youtravel by aircraft on

holiday. If you are part of the

aircrew, youwill experience larger

amounts due tothe amount of travel. There

areregulations about total

flying timeswhich take into account

exposure toradiation.

In the UK people receive an average of 2 mSv each year from background sources

(cosmic rays, radon gas etc).

Members of the public – an additional 5 mSv each

year

Legal limits have been set on the additional dose

equivalent which people can receive:

Workers exposed to radioactivity - an

additional 50 mSv each year

Background Radiation

Life on Earth has evolved to cope with this. Your cells have self-repairing mechanisms which

allow them to survive relatively unscathed.

The amount of background radiation varies considerably

around Britain, as shown on the map. You can see that it is

particularly high in Cornwall, because of the types of rock

there.

Background RadiationBackground radiation is present all around us

from natural and artificial sources.

Sources which contribute to backgroundradiation are:

radon from rocks and soilChernobyl and fall out from weaponstestingmedical uses of radiationgamma rays from building materialscosmic radiation from outer spaceindustrial usenuclear industry

Chernobyl (April 1986)Failure in safetyprocedures meantnuclear reactionbecame out ofcontrol30 people diedimmediately, aFurther 19 within four months.135000 wereevacuated from their

homes in a 20 mileradius.

Long term consequences

Thyroid cancer increased ten fold with

biggest increases in children under 15.

Difficult to assess – and muchcontroversy.

Key words: activity, radioactive source, half life, shielding, safety precautions


State that the activity of a radioactive source decreases with time.

State the meaning of the term ‘half-life’.

Describe the principles of a method for measuring the half-life of aradioactive source.

Carry out calculations to find the half-life of a radioactive isotope from

appropriate data

Describe the safety procedures necessary when handling radioactivesubstances.

State that the dose equivalent is reduced by shielding, by limiting the time

of exposure or by increasing the distance from a source.

Identify the radioactive hazard sign and state where it should bedisplayed.

Half-Life

Each radioactive substance has a different

half-life.

The half life is the time taken forhalf the radioactive nuclei todisintegrate OR the time taken forthe activity of a source to fall by one

half.

Radioactive Decay and Half Life

The activity of a radioactive sourcedecreases with time.

Virtual Int 2 Physics – Radioactivity – Half Life

http://library.thinkquest.org/26285/english/animation.html




The graph of activity(measured in counts persecond) against time has a

distinct shape.

Virtual Int 2 Physics – Radioactivity – Half Life

http://www.npp.hu/mukodes/anim/Uuu13-e.htm



Radioactive Decay & Half Life

Sketch a graph of activity against time

Time (s)

Act

ivity

(B

q))

Finding the half life of a source

We can find the half life of a radioactive

source but we must remember to correct

for background radiation.

Background RadiationIf we are measuring the activity of a source we must always take off the background radiation

For example:

We measure background radiation at 2 counts each second.

We then introduce a source and find that there are 47 counts each second.

What is the radiation due to the source?

Source radiation = total radiation – background radiation

Source radiation = 47 – 2 = 45 counts each second.

Time (s)

Counts per second

Corrected count rate

0

10

20

30

40

Continue to 250 seconds

Draw out this table

Measuring Background Radiation

Counts in 60 seconds =

Counts per second =

Use the data to plot a graph of corrected

count rate against time. Remember to

label axes and include units. Calculate 2 or

3 half life values from the graph and find

the average half life.

Tasks

What makes a good graph?

Measuring the Half-Life of a radioactive source

Read the time taken for the activity to half. You can choose any starting point.

Time (s)

Act

ivity

(B

q))

T1T2

The half life is found by calculating T2-T1.

Measuring the Half-Life of a radioactive source

Time (s)

Act

ivity

(B

q))

T1T2

Construct a table like this:

1st activity

2nd activity

T1

(s)

T2

(s)

Half-life(s)

80 40

70 35

60 30

Average half-life = ……………. s

Half Life Calculations

Below is a graph of corrected count rate

plotted against time

10 Time (min)

Corrected Count Rate (counts/sec)

In this case the half-life is 10 minutes.

1200

600

Time elapsed (mins)

Count rate (counts/sec)

Fraction of initial

count rate

0 1200 1

10 600 ½

20 300 ¼

30 150 1/8

40 75 1/16

50 37.5 1/32

4


A freshly prepared radioactive substancehas an initial activity of 60kBq. What will its

activity be after 1 hour if the half life is 15

minutes?

1 hour = 4 x 15 minutes

So the substance has been through ? half lives

After 1 half life the activity falls by half

From 60 kBq to ? kBq.

After 2 half lives, the activity halves again


30


15

After 3 half lives the activity halves again


After 4 half lives, the activity halves again

From 7.5 kBq to ? kBq.

7.5


3.75

Half Life CalculationsA radioactive sample has an initial activity of 800 Bq.

What is the substance’s half-life if the activity takes

24 years to decrease to 100 Bq?

Initial activity = 800 BqAfter 1 half life = 400 BqAfter 2 half lives = 200 BqAfter 3 half lives = 100 Bq

so in 24 years the substance has gone through 3 half

lives.

3 half lives in 24 years1 half life in 24/3 = 8 years.

Radiation Safety

Protection when using radiation

There are three methods by whichradiation exposure can be reduced:

1. Shielding a source with an appropriate thickness of absorber

e.g. a radiographer wears a lead lined aprone.g. radioactive sources are stored in lead containers.



2. Limiting the time of exposure

e.g. sources should be moved and used as quickly as possible



3. Distance from source

The further you are from the source the less radiation you will receive. In fact, if you double the distance you will receive only a quarter of the radiation.

Radiation Safety

What safety precautions should be taken when working with radioactive sources?

Radiation SafetyUse forceps or a lifting tool to

remove a source– never bare hands.Keep radiation window away from the

body.Never bring a source close to your

eyes.After any experiment with

radioactivity, washhands thoroughly.

Radiation SafetyThe symbol for radiation sources

being storedmust be displayed where radiation is

being usedor stored. It is an international

symbol whichcan be seen in hospitals, schools,

colleges and inindustry.

The Biological Effects of Radiation

The amount of damage caused depends

on:

1. the absorbed dose2. the kind of radiation 3. the body organs or tissue exposed

to the radiation.

The biological risk causedby radiation is represented

by the equivalent dosemeasured in

sieverts (Sv).

Questions

1. What is meant by ionisation?

2. (a) Why is ionising radiation dangerous. (b) When is ionising radiation produced? (c) Which is the most ionising of the three types of radiation? (d) Why is alpha radiation not dangerous if the source is outside the body? (e) Why is alpha radiation the most dangerous if the source is inside the body?

3. (a) Why is it possible to use photographic film to detect ionising radiation? (b) Explain how a film badge works. (c) How can fluorescent materials be used to detect ionising radiation? 4. A radioactive source gives out one type of radiation. A Geiger-Muller tube and counter are used in an experiment to determine the radiation present. The detector is placed directly above the source and the count rate measured with different substances between the detector and the source. (a) What correction must be made to the count rate before it can be used to determine the type of radiation present ? (b) The corrected count rate does not fall significantly when a sheet of paper is placed between the source and detector, however, it falls to the background level when a sheet of aluminium is used. Identify the radiation and explain the reason for your choice.

Key words: nuclear reactors, chain reaction, fission, fuel

rods, moderator, control rods, containment vessel, coolant,

nuclear waste.

By the end of this lesson you will be able to:State the advantages and disadvantages of using nuclearpower for the generation of electricity.

Describe in simple terms the process of fission.

Explain in simple terms a chain reaction.

Describe the principles of the operation of a nuclearreactor in terms of fuel rods, moderator, control rods,coolant and containment vessel.

Describe the problems associated with the disposal andstorage of radioactive waste.

Is nuclear power renewable or non-renewable?

Strictly non-renewable because theuranium fuel is a finite resource.At the current rate of use the existingreserves will last a long time. The ‘spent’ fuel can be re-processedand used again.

Nuclear Power

A lot of energy is produced per kilogramof uranium.

- 1 kilogram of coal produces 30 million

Joules 30 x 106 J or 30 MJ - A kilogram of uranium produces 5

million million Joules 5 x 1012 J of

energy.

Nuclear Power – What are the advantages?

Nuclear power plants generate relativelylittle carbon dioxide so contribute little toglobal warming.

Technology is readily available and wellestablished. It is reliable.

Large amount of electricity can begenerated by one plant.

Produces small amount of waste.

Nuclear Power – What are the advantages?

In the UK, about 50% of energy is created from nuclear sources. In France it is about 70%.

Nuclear Power – do we rely on it?

Nuclear power stations produceradioactive waste – which can be harmfulto us and the environment.

The waste must be stored safely formany years – sealed and buried.

Nuclear Power – What are the disadvantages?

Chernobyl demonstrated the risks of thistype of technology.

Nuclear power is reliable, but a lot of money has tobe spent on safety - if it does go wrong, a nuclearaccident can be a major disaster. People are increasingly concerned about this - in the 1990snuclear power was the fastest growing source ofpower in much of the world. In 2005 itwas the second slowest-growing.

Nuclear Power – What are the disadvantages?

There are 3 main types of power station:

THERMAL POWER STATION

NUCLEAR POWER STATION

HYDROELECTRIC POWER STATION

Each type has the same basic plan

ThermalHydro-electric

Nuclear

Turbinekinetic energy

Generatorkinetic to electrical

energy

Coal is burned

chemical energy to heat

Water behind dam

potentialenergy to kinetic

Nuclear reactionnuclear

energy to heat

1.Coal stockpile

2.Pulveriser which breaks the coal down – why is the coal broken up before use?

3. Boiler – coal is burnt to produce heat energy, the heat boils the water to produce steam. The steam is used to turn the turbines

4. Turbines – these have hundreds of blades. The steam from the

boiler hits the blades and turns the turbine. The turbine has a

shaft attached to it. As the turbine turns so does the shaft. The shaft from the turbine is connected to the generator.

5. Generator

5. Generator – the generator is made up of large electromagnet and coils of wire. The electromagnet is attached to the shaft from the turbine and turns inside the wire coils. As the electromagnet turns an electrical current is produced in the coil of wire.

6. Transformer

6. Transformer – the transformer increases the voltage of the electricity from 20 000 V to 275 000 V. This allows the electricity to be transported efficiently through the electrical transmission system.

7. Cooling Tower – after the steam has turned the turbine it is piped to the condenser. Cold water is pumped from the cooling towers where it is used to cool the steam. After circulating round the condenser the cooling water which is now about 10 ºC warmer, flows back to the cooling tower. The water is cooled by air and then falls back down to the bottom of the cooling tower to be recycled through the condenser (8) again. Some of the heat from the water is released into the air in the form of water vapour which you can see coming out of the top of the tower.

7. Cooling Tower – after the steam has turned the turbine it is piped to the condenser. Cold water is pumped from the cooling towers where it is used to cool the steam. After circulating round the condenser the cooling water which is now about 10 ºC warmer, flows back to the cooling tower. The water is cooled by air and then falls back down to the bottom of the cooling tower to be recycled through the condenser (8) again. Some of the heat from the water is released into the air in the form of water vapour which you can see coming out of the top of the tower.

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1. Coal stockpile2. Pulveriser3. Furnace & Boiler 4. Turbines5. Generator6. Transformer & National

Grid7. Cooling Tower8. Condenser

Stages In A Coal-Fired Power Station

A Conventional (Fossil Fuel) Power Station

Energy Changes

Energy Efficiency

What do we mean by the efficiency of

a machine?

How can we write this as an equation?

Energy Efficiency

100 x input energy total

out energy useful efficiency =

Why is the useful energy out always less than the total energy input?

Units?

Efficiency

100 x in power

out power efficiency =

It can be useful to consider the energy each second rather than total energy.

What would the equation be for efficiency using energy each second?

100 xinput energy total

out energy useful efficiency % =

Efficiency (as a percentage)

The efficiency of a power station (or any machine) tells us how much of the input energy is converted to useful output energy. Energy that is LOST has been converted to less useful forms such as heat.

Efficiency

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fuel of kgeach in storedenergy

requiredenergy total fuel of kilograms ofNumber =

Fuel Consumption

To determine the amount of fuel required:

Note that power is energy each second so for a given power output we can find the fuel needed each second.

Like fossil fuels, uranium is mined. A lengthy (and expensive) process is required to extract the uranium from the ore.

Nuclear Power

Inside the Nuclear Power Station


Inside the Nuclear Power Station

In place of the boiler found in a conventional power station,there is a reactor.

Heat energy produced during nuclear fission is carried bycarbon dioxide gas to a heat exchanger where it heatswater, turning it into steam.

The steam drives a generator to produce electrical energy.The steam is cooled (turned back into water) and pumpedround for reuse.

Inside the Reactor

To obtain energy from uranium-235 nuclei, they arebombarded with neutrons. (What is a neutron?)

The neutron is absorbed by the uranium-235nucleus making is unstable – it splits into two pieces

releasing a large amount of (heat) energy and twofurther neutrons. This process is called fission.


Chain Reaction

The two neutrons released then strike twofurther uranium nuclei. This time four new

neutrons are produced which cause furtherfissions, producing more neutrons and so on.

This continuous reaction of fissions is called

a chain reaction.

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Nuclear Fission

The total mass at the end is less than the mass at the start. The lost mass has changed into energy - E = mc2 m = the loss in mass and c = speed of light

A Chain Reaction

A Chain Reaction

The 2 neutrons released during the nuclear fission cango on to bombard further uranium nucleii which causesfurther nuclear fission releasing even more neutronswhich can in turn go on to produce more fission

An uncontrolled chain reaction is used in a nuclearbomb

In a nuclear power station the rate of reaction is controlled using boron control rods which can belowered into the reactor and absorb the neutrons thatinduce the fission process.

The fuel rods

These rods contain natural uranium whichis enriched so that fission can occur.

The amount of uranium in a fuel rod is wellbelow critical mass so that an explosion cannot

naturally occur.

Fuel rods have to be replaced every few years.

The Graphite Moderator

When the neutrons are emitted after fission theyare moving very fast.

They will not be able to be “captured” by othernuclei so fission will not occur.

If they are slowed down there is a greater chancethat fission will occur.

This is done using a graphite moderator – collisionswith graphite atoms slow the neutrons down.

Keeping the Chain Reaction

under control

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The Control Rods

The amount of electrical power required will vary with peak demand during the day and lower demand at night. Rods of boron absorb additional neutrons and control the number available for fission. They can be raised and lowered as necessary, and provide an important safety feature. In the event of an accident, all rods are lowered to absorb neutrons and stop the chain reaction.

CoolantThe heat produced during the reaction must be removed from the reactor.

This is done using the coolant – normally carbon dioxide.

The carbon dioxide is continually heat, then passes the heat to water via the heat exchanger.

The water turns to steam, which drives the turbine.

Calculating amount of fuel required for power outputTo determine the amount of fuel required

to produce a given power output

Number of kg of fuel =

total energy required

energy stored in eachkg of fuel

Energy Changes in a Nuclear Power Station

Note that in conventional fossil fuel power stations AND in nuclear power stations the energy source is used to raise steam to drive turbines to drive the electricity generator.

Containment Vessel

The key parts of the nuclear reactorwhich form the core, are contained in a

containment vessel. This is designed so

that no radiation can escape – it is several

metres thick and has a concrete top.

Disposal of Nuclear Waste

There are different categories of nuclear

waste.

High level – mainly spent nuclear fuel.After several years of use fuel rods are

taken out and sent for reprocessing –removal of useful parts which can bemade into new rods.


High level – unfortunately what remainsafter reprocessing is highly radioactive.Storage is initially in water for around ayear before the waste can be handled.However, it has a very long half life,remains extremely dangerous and there isas yet no ideal solution for long term safe

storage.


Low level – this is, for example, wastegenerated by hospitals etc. It is still dangerous

and must still be stored. It used to be dumped

at sea but this is now banned.

With either type of waste the problems are

- storage methods- storage sites – including transportation

radioactivity

Technology

protons charge

electrons charge

electron charge

overall charge

symbol alpha radiation

proton charge particle

useful radiation radiation

airandabsorption of