Download Higher Unit 1

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, radiation
energy, 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 Detectors
Smoke 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 types
of radiation
which cause
ionisation.
Ionisation
Ionisation means adding or
removing an electron from an
atom to produce a charged
particle.
What happens to the charge
on 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
Add one more electron – charge?
Overall charge?
-1
-1
If you remove an electron…
Six protons – charge? +6
Six electrons – charge? -6
Take away an electron – charge?
Overall charge?
+1
-5
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
electron
Imagine that an ALPHA PARTICLE passes
through a neutral atom – this will be shown in
slow motion!
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 (a)
Beta radiation (β)
Gamma radiation (γ)
Virtual Physics Int 2 – Radioactivity -> Ionising Radiations -> Alpha, Beta, Gamma
Alpha radiation (α)
An alpha particle is made up of two
protons and two neutrons. It is the same
as a helium nucleus. 4
2
He
It is positively charged.
It is largest of all the three types of
radiation.
A big atom releases an alpha particle to
make itself more stable.
The alpha particle that is
emitted has a lot of energy and
can damage human cells.
*
Alpha radiation (α)
An alpha particle is given the symbol
4
2
a
a
He
4
2
*
Summary
What are An alpha particle is
alpha
made up of two
particles? protons and two
neutrons.
It is the largest of
the three ionising
radiations. It has a lot
of energy.
*
Beta radiation (β)
a fast moving high
energy electron
released from the
nucleus –
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

or

0
1
This is what
happens inside
the nucleus.
*
Summary
What are A beta particle is a
beta
fast moving, high
particles? energy electron. The
electron is released
from the nucleus when
a neutron changes into
a proton plus electron.
It is very very small. *
Gamma Radiation (γ)
A wave of energy.
High frequency electromagnetic wave (so
travels at the speed of light)
No significant mass.
No charge.
Has the greatest amount of kinetic energy.

*
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 are
gamma
high energy
rays?
electromagnetic
waves. They travel
at the speed of
light.
*
Radiation & Ionisation
These three radiations (α, β, γ) are called
ionising radiations because they
cause 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 by
the 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 means
they have a longer range in air than alpha
particles.
Gamma particles have the lowest
ionisation density. This means they have
the longest range in air.
Identifying Radiations
We can tell which radiation is
which by testing to see what
happens when they reach
different 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 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 that beta particles or gamma rays.
Quick Recap
Type of
radiation
Symbol What is this Charge and
radiation?
absorption
Range in air
α
A 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 work
with radiation it is important
to be able to detect
radiation. The detection of
radiation 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 a
Geiger Muller tube attached to a
counter.
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
Radiation causes
ionisation of the gas –
what do we mean by
this?
The thin
window alllows
radiation to
enter.
Geiger Muller Tube
Ions produce
electrical pulses
which are counted
and displayed.
Geiger Muller Tube
We 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 Tube
Radiation
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 the
body, alpha radiation may not be able to harm
the vital internal organs as it is easily stopped
by the air, layers of clothing or the skin.
If swallowed an alpha radiation source is
extremely dangerous. It causes large amounts
of ionisation (remember it has a high ionisation
density) – 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 that
tissue. A small amount can penetrate the body.
If the beta source is inside the body, then it
will cause damage internally, for example to
organs.
Radiation and the Human Body
Gamma radiation will penetrate the skin and
tissue, and will deposit its energy as it travels
further into the body. It is more dangerous
than alpha or beta radiation in this case.
Gamma radiation inside the body will also
damage tissue however it can “escape” and be
detected 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 be
investigated without surgery.
Radioactive Tracers
Technetium-99 and Iodine-123 are
commonly used because they emit only
gamma, which can be detected outside
the body, and cause little ionisation.
However, different substances are
chosen for different organs.
Radioactive Tracers
A gamma camera is used to detect
radiation 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 detected
using radioactive tracers and a Geiger Counter.
A rise in count rate detected would indicate
more radiation escaping the pipe and therefore
a leak or crack.
Oil companies also use radioactive tracers in
shared pipelines to identify their own oil.
Radiation Therapy
Radiotherapy is commonly used as part of
treatment for cancer. It might be used
instead of surgery, or after surgery, or
chemotherapy, to destroy any remaining
cancer cells.
Treating Cancer
(Radiotherapy)
Ionising radiation kills living cells. Cancers
are simply growths of cells which are out
of control and have formed tumours.
By directing radiation at the tumour, the
living cells are damaged or killed, and this
shrinks the tumour. Unfortunately healthy cells
are also damaged or killed by the radiation.
Treating Cancer
(Radiotherapy)
It is important
to ensure that
healthy tissue
does not receive
too much
radiation while
the tumour
receives enough
to damage it.
Treating Cancer
(Radiotherapy)
Video clips.
http://www.ccotrust.nhs.uk/about/sitemap/ac
cess_map.htm
The machine rotates around the patient.
The tumour can be hit by radiation all of
the time while minimising the damage to
healthy tissue. Each section of healthy
tissue receives only a small dose.
Treating Cancer
(Radiotherapy)
Why are alpha and beta sources unsuitable
for radiotherapy treatments?
Alpha and beta are absorbed by
air/skin/bone so would not reach the
diseased 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 sterilising
equipment e.g. plastic syringes in hospital.
Previously expensive metal or glass
syringes had to be used and sterilised using
heat 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 the
paper is too thin then the reading on the
detector will increase. If it is too thick, the
reading will decrease.
Why is an alpha source no use for this
application?
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 per
unit 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 will
be able to:
State that a radiation weighting factor is
given 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 weighting
factor and is measured in sieverts (Sv).
Carry out calculations involving the relationship
between equivalent dose, absorbed dose
and 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 neutron
c) the body organs or tissue exposed.
Describe factors affecting the background
radiation level.
How much exposure is safe?
It should be stressed that no minimum
amount of exposure to radiation is
completely safe.
In Physics we aim to understand how to
measure radiation and to estimate the
risk of exposure. In many cases the
benefit of exposure significantly
outweighs the risks.
Radioactive Decay
Radiation is caused by the unstable nucleii
of radioactive atoms splitting up.
This is called radioactive
Virtual Int 2 Physics -> Radioactivity -> Dosimetry -> Activity
decay.
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 per
second.
Units of Activity
The becquerel is used to measure
the activity of a source.
1 becquerel (Bq) is one decay per second.
Activity
Number of nuclei decaying
N
A
t
Activity (Bq)
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
Energy (J)
E
D
m
Absorbed dose – units?
Mass (kg)
Dosimetry
ABSORBED DOSE (D) is the energy
absorbed PER UNIT MASS of
absorbing tissue.
E
D
m
Units are GRAYS (Gy)
1 Gy = 1 J/kg
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.
H  DWR
WR is the WEIGHTING FACTOR of
the particular radiation
Unit of equivalent dose is
sieverts (Sv)
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.
1
We normally work in millisieverts (mSv =
Sv )
1000
6
or microsieverts (μSv = x10 Sv)
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 for
the 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
E 0.25
D 
 0.005Gy
m
50
Example
(b) What is the equivalent dose?
H  DWR  0.005x20  0.1Sv or 100mSv
1 mSv is about 100 times the
radiation you experience when you
travel by aircraft on holiday.
If you are part of the aircrew, you
will experience larger amounts due to
the amount of travel. There are
regulations about total flying times
which take into account exposure to
radiation.
In the UK people receive an average
of 2 mSv each year from background
sources (cosmic rays, radon gas etc).
Legal limits have been set on the
additional dose equivalent which
people can receive:
Members of the public –
an additional 5 mSv each year
Workers exposed to radioactivity an additional 50 mSv each year
Background Radiation
Life on Earth has evolved to cope
with this. Your cells have selfrepairing 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 Radiation
Background radiation is present all around us
from natural and artificial sources.
Sources which contribute to background
radiation are:
radon from rocks and soil
Chernobyl and fall out from weapons
testing
medical uses of radiation
gamma rays from building materials
cosmic radiation from outer space
industrial use
nuclear industry
Chernobyl (April 1986)
Failure in safety
procedures meant
nuclear reaction
became out of
control
30 people died
immediately, a
Further 19 within
four months.
135000 were
evacuated from their
homes in a 20 mile
radius.
Long term consequences
Thyroid cancer increased ten fold with
biggest increases in children under 15.
Difficult to assess – and much
controversy.
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 a
radioactive source.
Carry out calculations to find the half-life of a radioactive isotope from
appropriate data
Describe the safety procedures necessary when handling radioactive
substances.
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 be
displayed.
Half-Life
Each radioactive substance has a different
half-life.
The half life is the time taken for
half the radioactive nuclei to
disintegrate OR the time taken for
the activity of a source to fall by one
half.
Radioactive Decay
and Half Life
The activity of a
radioactive source
decreases with time.
Virtual Int 2 Physics – Radioactivity – Half Life
Radioactive Decay
and Half Life
The graph of activity
(measured in counts per
second) against time has a
distinct shape.
Virtual Int 2 Physics – Radioactivity – Half Life
Radioactive Decay & Half Life
Activity (Bq))
Sketch a graph of activity against time
Time (s)
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.
If we are measuring the activity of a source we must
always
take off the Radiation
background radiation
Background
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.
Draw out this table
Time (s)
Counts per second Corrected count
rate
0
10
20
30
40
Continue to 250 seconds
Measuring Background Radiation
Counts in 60 seconds =
Counts per second =
Tasks
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.
What makes a good graph?
Measuring the Half-Life of a
radioactive source
Activity (Bq))
Read the time taken for the activity to half. You can
choose any starting point.
The half life is found by
calculating T2-T1.
T1
T2
Time (s)
Activity (Bq))
Measuring the Half-Life of a
radioactive source
T1
T2
Time (s)
Construct a table like this:
2nd
1st
activity
activity
80
40
70
35
60
30
T1
(s)
T2
(s)
Average half-life = ……………. s
Half-life
(s)
Radioactive Decay and Half
Life
Half Life Calculations
Below is a graph of corrected count rate
plotted against time
Corrected Count Rate
(counts/sec)
1200
In this case the half-life is 10
minutes.
600
10
Time (min)
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
Half Life Calculations
A freshly prepared radioactive substance
has 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
4
Half Life Calculations
After 1 half life the activity falls by half
From 60 kBq to ? kBq. 30
After 2 half lives, the activity halves again
From 30 kBq to ? kBq. 15
Half Life Calculations
After 3 half lives the activity halves again
From 15 kBq to ? kBq. 7.5
After 4 half lives, the activity halves again
From 7.5 kBq to ? kBq. 3.75
Half Life Calculations
A 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 Bq
After 1 half life = 400 Bq
After 2 half lives = 200 Bq
After 3 half lives = 100 Bq
so in 24 years the substance has gone through 3 half
lives.
3 half lives in 24 years
1 half life in 24/3 = 8 years.
Radiation Safety
Protection when using radiation
There are three methods by which
radiation exposure can be reduced:
1.
Shielding a source with an appropriate
thickness of absorber
e.g. a radiographer wears a lead lined apron
e.g. radioactive sources are stored in lead
containers.
Protection when using radiation
There are three methods by which
radiation 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 which
radiation 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 Safety
Use 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, wash
hands thoroughly.
Radiation Safety
The symbol for radiation sources being stored
must be displayed where radiation is being used
or stored. It is an international symbol which
can be seen in hospitals, schools, colleges and in
industry.
The Biological
Effects of Radiation
The amount of damage caused depends
on:
1.
2.
3.
the absorbed dose
the kind of radiation
the body organs or tissue exposed
to the radiation.
The biological risk caused
by radiation is represented
by the
equivalent dose
measured 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 nuclear
power 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 nuclear
reactor in terms of fuel rods, moderator, control rods,
coolant and containment vessel.
Describe the problems associated with the disposal and
storage of radioactive waste.
Nuclear Power
Is nuclear power renewable or nonrenewable?
Strictly non-renewable because the
uranium fuel is a finite resource.
At the current rate of use the existing
reserves will last a long time.
The ‘spent’ fuel can be re-processed
and used again.
Nuclear Power – What are the advantages?
A lot of energy is produced per kilogram
of 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 relatively
little carbon dioxide so contribute little to
global warming.
Technology is readily available and well
established. It is reliable.
Large amount of electricity can be
generated by one plant.
Produces small amount of waste.
Nuclear Power – do we rely on it?
In the UK, about 50% of energy is
created from nuclear sources. In France
it is about 70%.
Nuclear Power – What are the disadvantages?
Nuclear power stations produce
radioactive waste – which can be harmful
to us and the environment.
The waste must be stored safely for
many years – sealed and buried.
Nuclear Power – What are the disadvantages?
Chernobyl demonstrated the risks of this
type of technology.
Nuclear power is reliable, but a lot of money has to
be spent on safety - if it does go wrong, a nuclear
accident can be a major disaster. People are
increasingly concerned about this - in the 1990s
nuclear power was the fastest growing source of
power in much of the world. In 2005 it
was the second slowest-growing.
There are 3 main types
of power station:
THERMAL POWER STATION
NUCLEAR POWER STATION
HYDROELECTRIC POWER
STATION
Each type has the same basic plan
Thermal
Coal is burned
chemical energy to
heat
Hydro-electric
Water behind
dam
potential
energy to kinetic
Turbine
kinetic energy
Generator
kinetic to
electrical energy
Nuclear
Nuclear
reaction
nuclear
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.
Stages In A Coal-Fired Power Station
1.
2.
3.
4.
5.
6.
7.
8.
Coal stockpile
Pulveriser
Furnace & Boiler
Turbines
Generator
Transformer & National Grid
Cooling Tower
Condenser
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
useful energy out
efficiency 
x 100
total energy input
Units?
Why is the useful energy out always less than
the total energy input?
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?
power out
efficiency 
x 100
power in
Efficiency (as a percentage)
useful energy out
% efficiency 
x 100
total energy input
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 Consumption
To determine the amount of fuel required:
total energy required
Number of kilograms of fuel 
energy stored in each kg of fuel
Note that power is energy each second so for a
given power output we can find the fuel needed each
second.
Nuclear Power
Like fossil fuels, uranium is mined. A
lengthy (and expensive) process is
required to extract the uranium from
the ore.
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 by
carbon dioxide gas to a heat exchanger where it heats
water, turning it into steam.
The steam drives a generator to produce electrical energy.
The steam is cooled (turned back into water) and pumped
round for reuse.
Inside the Reactor
To obtain energy from uranium-235 nuclei, they are
bombarded with neutrons. (What is a neutron?)
The neutron is absorbed by the uranium-235
nucleus making is unstable – it splits into two pieces
releasing a large amount of (heat) energy and two
further neutrons. This process is called fission.
http://library.thinkquest.org/26285/english/animation.html
Chain Reaction
The two neutrons released then strike two
further uranium nuclei. This time four new
neutrons are produced which cause further
fissions, producing more neutrons and so on.
This continuous reaction of fissions is called
a chain reaction.
http://www.npp.hu/mukodes/anim/Uuu13-e.htm
http://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 can
go on to bombard further uranium nucleii which causes
further nuclear fission releasing even more neutrons
which can in turn go on to produce more fission
An uncontrolled chain reaction is used in a nuclear
bomb
In a nuclear power station the rate of reaction is
controlled using boron control rods which can be
lowered into the reactor and absorb the neutrons that
induce the fission process.
The fuel rods
These rods contain natural uranium which
is enriched so that fission can occur.
The amount of uranium in a fuel rod is well
below 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 they
are moving very fast.
They will not be able to be “captured” by other
nuclei so fission will not occur.
If they are slowed down there is a greater chance
that fission will occur.
This is done using a graphite moderator – collisions
with graphite atoms slow the neutrons down.
Keeping the Chain Reaction
under control
http://www.npp.hu/mukodes/anim/sta1e.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.
Coolant
The 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 output
To determine the amount of fuel required
to produce a given power output
Number of kg of fuel =
total energy required
energy stored in each
kg 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 reactor
which 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 be
made into new rods.
Disposal of Nuclear Waste
High level – unfortunately what remains
after reprocessing is highly radioactive.
Storage is initially in water for around a
year before the waste can be handled.
However, it has a very long half life,
remains extremely dangerous and there is
as yet no ideal solution for long term safe
storage.
Disposal of Nuclear Waste
Low level – this is, for example, waste
generated 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