Nuclear Physics
4.5.1 – 4.8.4
The Nucleus
4.5.1 – 4.5.3
The Plum-Pudding Model
1. Electrons suspended in a background of otherwise,
uniform positive charge.
2. The positive charge is not concentrated enough to
facilitate repulsion of alpha particles.
3. Alpha particles pass through unaffected.
This resembled a plum-pudding, so it
was called the ‘Plum –pudding’ model.
This was wrong!
Rutherford’s Model
2. The source of alpha particles was
contained within an evacuated
chamber (i.e. a vacuum).
1. A source of alpha particles was
placed incident to a thin gold film
placed perpendicular to their motion.
3. The alpha particles
were fired at the gold foil
5. The alpha particles were
detected by the flashes of light
(scintillations) the produced
on interaction with the screen.
4. Behind the gold foil,
a zinc-sulfide screen
was positioned.
Rutherford’s Scattering Apparatus
You must be able to label this diagram
2. Alpha Particles
3. Gold Foil
Rutherford observed that:
1. Most
alpha
particles
were
undeflected.
2. Some
were
scattered
by
appreciable angles.
3. About
1
in
8000
were
backscattered through a very large
angle.
5. Microscope
1. Alpha Source
6.Vacuum
4. Zinc-sulfide Screen
Rutherford’s Model: conclusions
If the ‘Plum Pudding’ model of the atom was correct, the alpha particles should pass straight
through and only be slightly deflected.
Conclusions
1.
The majority of the alpha particles passed straight through the metal foil because they did not come
close enough to any repulsive charge at all
2.
All the positive charge and most of the mass of an atom formed an exceptionally small, dense core or
nucleus.
3.
The negative charge consisted of a “cloud of electrons” surrounding the positive nucleus.
4.
Only when a positive alpha particle approached sufficiently close to the nucleus, was it repelled strongly
enough to be “back scattered” through a large angle.
5.
The small size of the nucleus explains why only a small number of alpha particles that were repelled in this
way.
6.
Most of the atom is empty space.
Nuclear Radius
Determining the size of the nucleus within an atom was a challenging undertaking. This
is due to the fact that the nucleus does not possess a sharp, defined edge but instead can
be described as fuzzy.
The first meaningful attempts to ascertain the size of the nucleus came through
Rutherford’s experiments involving alpha particles. Physicists were able to measure the
distance of closet approach which allowed an approximation of the upper limit of the
size of the nucleus to be determined.
Early measurements of the nucleus size:
~10-15 m
Nuclear Radius
If we were to assume that the volume of a
nucleon (i.e. a proton or a neutron) in any
nucleus is about the same, then it is possible to
assume that the volume of the nucleus would
be directly proportional to the total number
of nucleons contained within it.
Remember that the total number of nucleons is
the mass number (A).
The equation for the nuclear radius is given as:
Where:
r
r0
A
= radius of a given nucleus (m)
= the constant of proportionality (m)
= mass number
Nuclear Density
Nuclear Density
By subbing in the equation for the nuclear radius, the mass can be rewritten as:
The mass of a nucleus can also be written in terms of the mass number and mass of a
nucleon:
Nuclear Density
This allows the mass equation to be rewritten as:
Nuclear Density
This can be rearranged to give an equation for the density of nuclear matter:
Representation
of
Nuclear
rnucleus
rnucleus
Graphical
Density
A
You should be familiar with both graphs for Nuclear Density
A1/3
Nuclear Decay
4.6.1 – 4.6.8
The Nucleus
The diameter of an atom is around 10-10m. Within every atom is a central, positively
charged nucleus with a diameter of approximately 10-15m. Therefore, an atom is
typically 100,000 times larger than its nucleus. Over 99.9% of the mass of an atom is
held within its nucleus. Atomic nuclei are unaffected by chemical reactions.
Atomic nuclei contain protons and neutrons
which are collectively known as nucleons.
Orbiting the nucleus are the electrons.
-
The Nucleus
Nucleons are held together by one of the fundamental forces of nature called the strong
interaction. This nuclear force acts over very short distances and is much stronger than
the electric force of repulsion that exists between protons within the nucleus.
Isotopes
Isotopes are nuclei with the same number of protons but differing numbers of
neutrons.
An isotope is described using two numbers and the chemical symbol of the element.
A
Z
A = the Mass number, the total number of nucleons in the
nucleus.
X
Z = the Atomic number, the total number of protons in the
nucleus.
X = the chemical symbol of the element.
Isotopes
Hydrogen
Deuterium
Tritium
Deuterium and Tritium are isotopes of Hydrogen which are used in Nuclear Fusion.
They have the same number of protons, but different numbers of neutrons.
Radioactivity
Some elements possess unstable isotopes whose nuclei disintegrate randomly and
spontaneously. This effect is known as radioactivity.
Electromagnetic Wave
Particle
Atoms which emit electromagnetic radiation or a particle by the disintegration
of their nucleus are called radioactive.
Radioactivity
In 1896, the French scientist Henri Becquerel
discovered that certain rocks containing uranium
give out strange radiation that could penetrate
paper and fog photographic film.
He called this phenomenon - Radioactivity
His students, Pierre and Marie Curie, later
identified three separate types of radiation
naming them alpha (a), beta (b) and gamma (g)
radiation after the first three letters of the Greek
alphabet
Radioactivity
The unit of measurement for activity is the Becquerel (Bq)
1 Bq = one disintegration per second
There are three types of radiation. You must understand their properties and the general
form of the decay equations for each.
1. Alpha Radiation
2. Beta Radiation
3. Gamma Radiation
-
Alpha Radiation
Alpha radiation is made up of a
stream of alpha particles
emitted from large nuclei
Alpha particles are positively
charged
and
will
be
deflected in a magnetic field.
Alpha particles have poor powers of penetration and
can only travel through about 4cm of air. They are
easily stopped by a sheet of paper
Given that alpha particles move
relatively slowly (at about 6%
of the speed of light) and have
a high momentum, they will
interact
with
matter
producing intense ionisation
Alpha Radiation
Decaying parent nucleus
Daughter nucleus remains
Alpha particle emitted
Note that both the number of nucleons (mass number) and the charge (atomic
number) is conserved.
Beta Radiation
Beta radiation is emitted from
nuclei where the number of
neutrons is much larger than the
number of protons.
Beta particles are emitted by
nuclei that contain too many
neutrons to be stable. One of the
neutrons will decay to create
one proton and one electron.
The proton will remain in the
nucleus, whilst the electron is
emitted as a beta particle.
Beta particles are negatively charged which
means they will be deflected in a magnetic field.
This deflection will be greater than that of an
alpha particle because beta particles have a
smaller mass to charge ratio.
Beta particles move much faster than alpha
particles and therefore will interact less with
matter. This means they have a greater
penetration power.
Beta Radiation
Decaying parent nucleus
Daughter nucleus remains
Alpha particle emitted
Note that the total number of nucleons (mass number) does not change, but the atomic
number (Z) of the daughter nucleus (Z+1) is greater than that of the parent (Z) by 1.
Gamma Radiation
Gamma radiation does not
consist of particles, but short
wavelength,
high
energy
electromagnetic waves known
as gamma rays.
The wavelength of gamma rays is
characteristic of the nucleus that emits
it. The wavelengths are typically in the
region of 10-10 to 10-12.
Like alpha and beta radiation, gamma
radiation comes from a disintegrating
unstable nucleus.
As there are no particles, gamma
radiation has no mass. As there are no
charged particles, a magnetic field has
no effect on gamma radiation.
A thick block of lead or concrete is
used to greatly reduce the effects of
gamma radiation but cannot stop it
completely. Gamma radiation has the
weakest ionising power.
Gamma Radiation
Decaying parent nucleus
Daughter nucleus remains
Alpha particle emitted
Note that both the total number of nucleons (mass number) and the atomic numbers do
not change.
Ionisation
Ionisation is the process by which electrically neutral atoms or molecules are converted
to electrically charged atoms or molecules, known as ions. This occurs when an alpha
particle, beta particle or gamma ray causes an electron to be ejected from the atom or
molecule.
An ion-pair is the positively charged particle (positive ion) and the negatively charged
particle (negative ion) simultaneously produced by an alpha particle, beta particle or
gamma ray interacting with the molecule.
Alexander Litvinenko
Number of unstable nuclei
The Law of Radioactivity
Radioactive nuclei will disintegrate spontaneously and
randomly. This means that we can neither tell which particular
nuclei in a given sample are going to decay, nor can be we tell
when they are going to decay.
2000
1500
However, if we have sufficiently large numbers of
nuclei in our sample, the random decay of
individual nuclei averages out in such a way as to
be governed by empirical laws.
1000
500
2
4
6
8
Time (s)
The Law of Radioactivity
Number of unstable nuclei
The rate of disintegration cannot be speeded up or slowed down by any
known means (through temperature, pressure, particle size or chemical
reactions).
2000
Therefore, the number of unstable nuclei (and hence the activity) will
decrease exponentially and is governed by the following equation:
1500
1000
500
2
4
6
8
Time (s)
Number of unstable nuclei
The Law of Radioactivity
2000
1500
1000
500
2
4
6
8
Time (s)
Number of unstable nuclei
The decay constant
2000
One of the isotopes of Protactinium has a decay
constant of 1.01x10-2 s-1. A mass of 1mg of this
isotope will contain 2.57x1018 unstable nuclei.
Therefore, at this instant the number of nuclei that
are decaying per second is given by:
1500
1000
1.01x10-2 x 2.57x1018 = 2.60x1016 nuclei
500
2
4
6
8
Time (s)
Number of unstable nuclei
The decay constant
2000
As time passes there are few unstable nuclei so the number that decay each
second gradually decreases. Therefore, the activity also decreases
exponentially with time since it is directly proportional to the number of
unstable nuclei present. Therefore, we can express the activity of a sample
as:
1500
1000
500
2
4
6
8
Time (s)
Half-life t1/2
The half-life of a radioactive nuclide, t1/2 is the time taken for half of the radioactive
nuclei present to disintegrate.
We know that:
After one half life has passed, only half the unstable nuclei remain. Therefore:
Half-life t1/2
Dividing both sides by N0 gives:
Taking natural logs of both sides gives:
Half-life t1/2
Half-life t1/2
Half life can also be defined in terms of the activity: the half life of a radioactive
material is the time taken for the activity of that material to fall to half of its
original value.
Measuring half-life
There are two common methods of measuring the half-life of a
radioactive substance:
1. Using an ionisation chamber and a source of radon gas (Rn220)
2. Using a Geiger-Muller tube, a rate meter and a source of
protactinium (Pa-234).
One emits alpha particles, and one emits beta particles!
Measuring half-life: ionisation chamber
The ionisation chamber consists of an aluminium can, with a metal rod (negative
electrode) mounted centrally within the chamber and insulation surrounding the entire
container. A direct current amplifier is connected to the central negative electrode.
As the number of radon gas atoms in the chamber gets smaller and smaller, they emit
fewer and fewer alpha particles. As a result, the number of ions gets smaller leading to
a reduced ionisation current.
The ionisation current is directly proportional to the number of radon atoms
remaining and hence to the activity of the gas within the chamber.
Measuring half-life: ionisation chamber
Current I / mA
Ln I
A measure of the half-life can be extracted through two graphical means.
Intercept = lnIo
Half life = 0.693/-gradient
Half life
Time t / s
Time t / s
Measuring half-life: ionisation chamber
The equation which governs the half-life of a sample based on the ionisation current is
given as:
Where:
Io = the current at time t = 0
λ = the decay constant
Measuring half-life: ionisation chamber
Measuring half-life: Geiger-Muller tube
The apparatus used for this experiment is the Geiger-Muller and a counter to measure
the activity of a sample of protactinium-234. When alpha, beta or gamma radiation
enters the Geiger-Muller tube, it causes some of the argon gas inside to ionise and give
an electrical discharge. This discharge is detected and counted by the counter. If the
counter is connected to its internal speaker, you can hear the click when radiation enters
the tube.
However, in the absence of all known sources of radioactivity, the Geiger-Muller tube
and counter still detects radiation. This is known as background radiation. This
radiation comes from a variety of sources:
1. The Sun
2. Cosmic rays from space
3. Hospital nuclear physics departments
4. Nuclear power stations
5. Granite rocks
The detector is a metal tube filled with gas. The tube has a thin wire down the
middle and a voltage between the wire and the casing.
Good at detecting alpha and beta, not as good at detecting gamma.
radiation
Argon
Argon gas
gas
The Argon
contains a little
bromine to act as a
quenching agent
and prevent
continuous
discharge.
mica window
collision & ionisation
When the radioactivity enters the tube, it
ionises the gas in the tube. This produces a
pulse of current which is amplified and
passed to a counter.
counter
124
125
Measuring half-life: Geiger-Muller tube
The equation which governs the half-life of a sample based on the activity is given as:
Where:
Ao = the activity at time t = 0
λ = the decay constant
Measuring half-life: ionisation chamber
Ln A
Measuring half-life: Geiger-Muller tube
Intercept = lnAo
Half life = 0.693/-gradient
Time t / s
Nuclear Energy
4.7.1 – 4.7.4
Mass-energy equivalence
In 1905, Einstein’s published paper titled “The
Special Theory of Relativity” dealt with the
speed of light for observers moving with a
constant velocity relative to each other. From
this work, Einstein made two postulates,
assumptions which hold true today.
Einstein’s Postulates
1. The laws of physics take the same form in all inertial frames of reference.
2. Light is always propagated in empty space with a definite velocity c that is
independent of the state of motion of the emitting body
Mass-energy equivalence
Mass-energy equivalence
The electron volt and unified atomic mass unit
The values of 1 joule and 1 kilogram are much too large to be useful when dealing with
atomic and nuclear processes. A much more appropriate unit for energy is the electron
volt (eV).
The electron volt is defined as “the kinetic energy possessed by an electron
accelerated from rest through a voltage of one volt.”
Remember these following quantities:
1 eV = 1.6 x10-19 J
1 MeV = 1 million eV = 1.6 x10-13 J
The electron volt and unified atomic mass unit
Nuclear binding energy
Nuclear binding energy
This reduction in mass arises due to the act of combining of the nucleons to form the
nucleus. When the nucleons are combined to form a nucleus a tiny portion of their mass
is converted to energy. This energy is called the binding energy of the nucleus.
The binding energy is defined as “the amount of energy that has to be supplied to
separate the nucleons completely (i.e. to an infinite distance apart).
Nuclear binding energy
Therefore, the binding energy of a nucleus is given by:
Binding energies can be given in joules (J) but are usually quoted in millions of
electron volts (MeV).
To determine the binding energy for a known element, follow these steps:
1.
2.
3.
4.
5.
Work out the mass of the constituent nucleons.
Work out the mass defect (difference in mass between nucleons and nucleus)
Convert the mass defect into kg
Use mass energy equivalence equation to determine binding energy (in J)
Convert to MeV (if required!)
Nuclear binding energy
Binding energy per nucleon, MeV
Nuclear binding energy
Nucleon number, A
Nuclear Fission
In nuclear fission, a massive nucleus is divided and breaks up into two less massive
nuclei. The average binding energy of these fission fragments is higher than that of the
original heavy nucleus. Because of this increase in the total binding energy, some of the
mass of the heavy nucleus is converted to kinetic energy of the fission fragments.
This is illustrated in the graph of the binding energy per nucleon against nucleon
number shown previously.
Nuclear Fusion
Nuclear Fusion is the joining of lighter nuclei to produce a heavier, more stable nucleus.
The fusion process results in the release of energy since the average binding energy of
these fusion products is higher than that of the lighter nuclei which join together. Because
of this increase in the total binding energy, some of the mass of the lighter nuclei is
converted to kinetic energy of the fusion product. This means that the mass of the
heavier nucleus is less than the total masses of the two light nuclei that fuse together.
Nuclear Fission and
Nuclear Fusion
4.8.1 – 4.7.4
Nuclear Fission
Previously the principles of nuclear fission were discussed, but the precise details on
which physicists being about the conditions in which controlled nuclear fission can
occur will be explored. Traditionally, the equation for nuclear fission is given as
follows:
However, this is merely one of several reactions that can take places inside a nuclear
reactor. Another commonly used equation is:
Remember that for both equations, the nucleon number (mass number) and the atomic
number must be conserved on both sides of the equation!
Nuclear Fission
Nuclear Fission
Regardless of which reaction occurs, the fission of uranium will always possess the
following features:
1. It always releases huge amounts of energy! Approximately 80% of the energy is
carried away as kinetic energy of the two major fission fragments. Burning atom of
carbon (as coal) would release about 5eV, whereas the fission of one uranium nucleus
releases more than 200MeV.
2. The fission fragments are often radioactive and their subsequent decay accounts
for approximately a further 10% of the total energy released.
3. Extremely penetrating and highly dangerous gamma rays are always produced
along with the fission fragments. The gamma rays, along with the kinetic energy of
the sub-atomic particles produced account for the remaining 10% of energy released.
Chain reactions
When a uranium atom undergoes fission,
further neutrons are produced, on average
around 2.5 per fission. There are three
possible fates for the fission neutrons
produced:
1. They might escape from the uranium fuel where they were formed without causing a
further fission.
2. They might be absorbed by a neighbouring nucleus, again without causing a further
fission.
3. They may cause further fission in a neighbouring nucleus.
Chain Reactions
If enough neutrons go on to cause
further fission, then the reaction
will sustain itself and fission will
continue to take places. This is
called a chain reaction.
There are two kinds of chain reactions; (a) uncontrolled and (b) controlled.
An uncontrolled chain reaction is what takes place inside an atomic bomb, where
enormous amounts of energy is released in a very short space of time. A controlled
chain reaction is what takes place inside a nuclear reactor, where energy is released to
generate electricity.
Fission Nuclear Reactors
There are four types of fission nuclear reactors:
1. The Magnox type
2. The Advanced Gas-cooled Reactor (AGR)
3. The Pressurised Water Reactor (PWR)
4. The Fast Reactor
Nuclear power accounts for approximately 11% of the world’s electricity, and is seen as
meeting the worlds electricity needs, whilst reducing greenhouse gas emissions.
However, it does have its disadvantages.
Design of a nuclear reactor
Nuclear Fuel and Moderators
Natural uranium is approximately 99.3% uranium283 and 0.7% uranium-235. Uranium-238 is
fissile, which means it can undergo nuclear
fission, but only with fast neutrons. Uranium-235
is also fissile, but only undergoes nuclear fission
with slow moving neutrons. Therefore, neutrons
emitted by the fission of uranium-235 are too slow
to cause fission of uranium-238 but must also be
slowed further to cause further fission of uranium235. To achieve this, a material known as a
moderator is used. This comes in the form of
graphite, water (H2O) and heavy water (D2O).
Another method of improving the fission process is called enrichment. This means the
addition of further uranium-235 into natural uranium in order to increase the amount of
fissile uranium-235 from 0.7% to 3%. This increases the likelihood of a fission reaction
taking place.
Critical Size
The bigger the size of
the
uranium
fuel
assembly within a
nuclear reactor, the
more likely that a
fission neutron will go
on to produce another
fission
event
and
produce
a
chain
reaction.
The critical size is defined as “the fuel assembly which is capable of sustaining a chain
reaction within it”
The critical size of uranium-235 is about the size of a small football, and typical fuel
assembly in a nuclear reactor is about 5% above this critical size.
Control Rods, Coolant and Shielding
To control the rate of reaction in a nuclear
reactor, boron-coated steel rods, called control
rods are used to capture excessive neutrons
The heat energy produced by a fission reaction is
removed by passing a coolant through the
reactor. This coolant passes its energy to water
by flowing through a heat exchanger, which
produces steam that then drives the turbines.
This then turns the electricity-producing
generators.
All reactors are surrounded by a thick concrete
shield to prevent potentially dangerous radiation,
in particular very penetrating gamma ways and
neutrons from reaching workers and the wider
community.
Nuclear Fusion
Almost all the energy we receive on Earth comes from the Sun as a result of nuclear
fusion. All the elements that make up the material world, including living organisms on
Earth were formed by fusion in stars. Stars like our Sun consist mainly of hydrogen and
helium. The fusion of hydrogen to form helium is the basis of nuclear fusion within
stars.
Remember that hydrogen nuclei can
only really fuse together when their
temperature is about 15,000,000
degrees Celsius.
Temperature for Nuclear Fusion
In nuclear fusion, two light particles are brought sufficiently close together so that they
fuse to form a more massive particle. However, due to their positive charge, as they get
closer together, they should repel each other. The closer the get to each other, the
stronger the repulsive force.
If the protons are projected
towards each
other the
repulsion
between
their
positive charges causes their
kinetic energy to decrease
and their potential energy to
increase.
The energy needed to bring a pair of protons close enough to result in fusion is about
110keV.
Temperature for Nuclear Fusion
What temperature is needed to give the protons an average kinetic energy of about
110keV?
Plasma confinement
A plasma is simply a material that has been heated to a temperature such that the
electrons have broken free of the atoms.
Even if we were able to create a plasma, containing it for long enough for fusion to take
place is the biggest challenge.
There are three main types of plasma confinements.
1. Gravitational confinement
2. Inertial confinement
3. Magnetic confinement
Gravitational confinement
Stars are made of plasma, which is confined
under gravitational forces. Stars exist under a
state of hydrostatic equilibrium, where their
spherical shape is obtained from a delicate
balance between the internal pressure (created
from photon interaction with particles and
radiation) and the effects of gravity.
The inward pull of the stars huge mass balances
the pressure created by the plasma. On Earth this
would not be possible as this would require an
enormous mass of material to provide
gravitational forces strong enough to balance the
forces tending to dissipate the plasma.
Inertial confinement
Inertial confinement is a process that involves
directing intense ion or laser beams at a solid
fuel pellet
The beams provide the energy to heat the
material to the required temperature. The idea is
to produce fusion for long enough to extract the
energy before the plasma escapes.
Magnetic confinement
This approach takes advantage of the fact that
particles within a plasma are charged. If
particles move through a magnetic field, they
will experience a force.
To produce the required magnetic field, an electric current flows through a coil
would round a shape known as a toroid.
The magnetic field produced is circular within the highly evacuated toroidal chamber.
• Charged particles moving parallel (or anti-parallel) to the magnetic field will not
experience a force and will move in a straight line at a constant speed.
• Particles moving perpendicular to the magnetic field will experience a force that
makes them move in a circular path around the field line.
Magnetic confinement
Magnetic confinement
Magnetic confinement
The main challenges facing these methods of confinement:
1. How do we keep the plasma well contained and at a
sufficiently high temperature for long enough to obtain
adequate number of fusion reactions.
2. How do we ensure that the ensure that the energy produced by
fusion exceeds that supplied by the operators to maintain the
reactor at its enormously high temperature.
Deuterium-tritium (D-T) reaction
Deuterium-tritium (D-T) reaction
Advantages of Nuclear Fusion
1. Supplies of fuel are readily available and
virtually inexhaustible. Sea water contains 1
atom of deuterium for every 5000 hydrogen
atoms. Tritium is created from Lithium.
2. There is non of the toxic and highly
radioactive waste associated with fission.
Waster products are irradiated with neutrons
and less damaging that fission products.
3. There is a greater yield of
energy per kilogram of fuel
consumed from a hydrogen
fusion reactor that from a
fission reactor.
4. Fission reactors are considered fail
safe because fuel is continually fed
into them. If the feed stops, the
reaction stops. In a fission reactor, all
the fuel is in place before starting.
Disadvantages of Nuclear Fusion
1. Requires a large initial energy input
before a useful output is obtained.
2. The process is still unproven as a means of
providing electricity on a commercial
scale.
3. Commercial power plants would be extremely expensive to build. The
highest cost is likely to be the need for large superconducting magnets to
provide the magnetic confinement of the plasma.
International Thermonuclear Experimental
Reactor (ITER)
1. At the centre of the complex is a tokamak
vacuum vessel, which contains the plasma
where fusion takes place.
2. The walls of the vacuum vessel are lined
with beryllium blankets approximately
0.5m thick with a mass of almost 5000kg.
These blankets provide shielding from the
high energy neutrons produced by the
fusion reactions.
3. As the neutrons are slowed, their kinetic
energy is transformed into heat energy and
collected by the water coolant. This is used
to produce electrical power.
4. Cooling water is circulated through the
vessels steel walls to removed heat
generated.
5. The heat removed is used to produce steam to drive the turbines to produce electricity.
International Thermonuclear Experimental
Reactor (ITER)
The reactor uses the deuterium-tritium (D-T) reaction to create the high temperature fusion
plasma. The international thermonuclear experiment reactor uses three methods to heat the plasma
to the very high temperatures needed for fusion:
1. The plasma is a good conductor of electricity. The changing magnetic fields that are
used to contain the plasma also produce a very large current by electromagnetic
induction. The current passes through the plasma causing the electrons to gain ions
to gain kinetic energy and collide. These collision result in resistance which
results in a heating effect (similar to resistance in a metal).
2. A beam of deuterium ions is accelerated by an electric field to a high velocity.
However, before the ions enter the plasma they pass through another electric field
and as a result gain an electron to make them electrically neutral. These high
velocity particles will collide with the ions and electrons, exchanging energy and
causing further heating.
International Thermonuclear Experimental
Reactor (ITER)
The reactor uses the deuterium-tritium (D-T) reaction to create the high temperature fusion
plasma. The international thermonuclear experiment reactor uses three methods to heat the plasma
to the very high temperatures needed for fusion:
3. The final step is to direct high energy microwaves into the plasma. These waves
transfer their energy to the particles in the plasma. The ensure the energy is
transferred as efficiently as possible, three specific frequencies of microwave are
used, each one being match to a specific type of ion or electron within the plasma.
The expectation is that this fusion reactor will produce approximately 500MW of
power, about the same amount obtained from a coal-fired power plant. However, this
reactor is first and foremost, a large-scale physics project designed to develop a reliable
fusion reactor rather than to produce electricity on a commercial basis.
Nuclear
energy,
environment
society
and
the
Opponents of nuclear energy argue that the environment, health and security risks make it
unsuitable as a replacement for fossil fuels. Some of the issues with nuclear power are:
1. Nuclear energy (from fission) produces large quantities of toxic, radioactive waster
and that must be stored safely and securely between 10,000 years and 240,000
years. Finding safe storage facilities for this waste poses problems for many
countries.
2. Nuclear power creates employment opportunities, but living close to nuclear power
plants and radioactive waster storage sites have many concerns. The Chernobyl
disaster in 1986 was the result of a flawed reactor design being operated by
inadequately trained personnel. The subsequent disaster caused huge economic,
health and environmental damage to the area surrounding the power plant.
Nuclear
energy,
environment
society
and
the
Opponents of nuclear energy argue that the environment, health and security risks make it
unsuitable as a replacement for fossil fuels. Some of the issues with nuclear power are:
3. Nuclear power is a secure source of energy provided the countries that provide the
uranium ore have stable governments and societies. However, some sources of
uranium ore are in countries that might be considered to be less stable*. Theses
sources of uranium will become more important in the future as supplies from
stable sources become depleted.
4. Some of the technology used for nuclear power can also be used to produce nuclear
weapons. A “fast breeder” reactor uses a moderator that does not slow down fission
neutrons. Instead these neutrons are absorbed by uranium-238 to create plutonium239, which can be process to create fuel for nuclear weapons. The Nuclear NonProliferation Treaty (1970) was devised as a means of controlling the spread of
such weapons.
*stable is a subjective term. What constitutes stable, and who determines what
countries are stable?