Radioactivity
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Transcript of 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
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
Six electrons – charge? -6
Overall charge? 0
If you add an electron…
Six protons – charge? +6
Six electrons – charge? -6
Overall charge?
Add one more electron – charge? -1
-1
If you remove an electron…
Six protons – charge? +6
Six electrons – 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
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)
*
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.
*
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
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
By the end of this lesson you will be able to:
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?
Photographic Fogging
This principle is used in film badges
worn by radiation workers.
The darker the film the more radiation
the person has received.
Photographic Fogging
Why are there different materials in the film badge?
Photographic Fogging
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.
Radiation and the Human Body
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.
Radiation and the Human Body
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.
Treating Cancer (Radiotherapy)
It is importantto ensure thathealthy tissuedoes not receive
too muchradiation whilethe tumourreceives enoughto damage it.
Treating Cancer (Radiotherapy)
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.
Treating Cancer (Radiotherapy)
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
By the end of this lesson you will be able to:
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
By the end of this lesson you will be able to:
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
Radioactive Decay and Half Life
The graph of activity(measured in counts persecond) against time has a
distinct shape.
Virtual Int 2 Physics – Radioactivity – Half Life
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
Radioactive Decay and Half Life
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
Half Life Calculations
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
From 30 kBq to ? kBq.
30
Half Life Calculations
15
After 3 half lives the activity halves again
From 15 kBq to ? kBq.
After 4 half lives, the activity halves again
From 7.5 kBq to ? kBq.
7.5
Half Life Calculations
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.
Protection when using radiation
There are three methods by whichradiation exposure can be reduced:
2. Limiting the time of exposure
e.g. sources should be moved and used as quickly as possible
Protection when using radiation
There are three methods by whichradiation exposure can be reduced:
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.
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
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
http://science.howstuffworks.com/nuclear-power2.htm
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.
http://library.thinkquest.org/26285/english/animation.html
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.
http://www.npp.hu/mukodes/anim/Uuu13-e.htmhttp://www.npp.hu/mukodes/anim/div2a-e.htm
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
http://www.npp.hu/mukodes/anim/sta1-e.htm
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.
Disposal of Nuclear Waste
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.
Disposal of Nuclear Waste
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