Nuclear Fuels and Fission
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How do we get energy from atoms?
Atoms contain huge amounts of energy in their nuclei.
There are two ways in which this energy can be released.
One way is to split atomic nuclei in
a process called nuclear fission.
Another way is to join nuclei
together in a process called
nuclear fusion.
nucleus
electrons
The energy that holds particles together in a nucleus is much
greater than the energy that holds electrons to a nucleus.
This is the why the energy released during nuclear reactions
(involving nuclei splitting apart or joining together) is much
greater than that for chemical reactions (involving electrons).
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Einstein and E = mc2
In 1905, Albert Einstein made the connection between
energy and matter.
Einstein predicted that a
small amount of matter could
release a huge amount of
energy in a nuclear reaction.
He expressed this in what is
probably the most famous
equation in physics: E = mc2.
 E is energy produced
 m is mass lost
 c is the speed of light in a vacuum
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Proof of Einstein’s theory
In 1933, work by Irène and
Frédéric Joliot-Curie proved
Einstein’s prediction.
They produced a
photograph that showed
the creation of two
particles (mass) when a
particle of light (carrying
energy) was destroyed.
This was the first proof of the conversion of energy into mass.
However, the strongest evidence of Einstein’s theory came
with discoveries about nuclear fission and fusion reactions,
in which huge amounts of energy are released from atoms.
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What is nuclear fission?
Nuclear fission occurs when a stable isotope is struck by a
neutron. The isotope absorbs the neutron, becomes unstable
and then splits apart, releasing large amounts of energy.
Unlike natural radioactive decay,
fission is not a natural event.
Isotopes that undergo fission include
uranium-235 and plutonium-239.
Most nuclear reactors use uranium-235.
The fission of 1 kilogram of uranium-235
releases more energy than burning
2 million kilograms of coal!
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How is uranium used in nuclear reactors?
There are two major isotopes of uranium – 238 and 235.
Uranium-238 is more common, but it does not undergo
nuclear fission.
Only 0.7% of naturally-occurring
uranium is uranium-235, which
does undergo nuclear fission.
Before it can be used as the
fuel in nuclear power stations,
uranium needs to be enriched
until it has 3% uranium-235.
The enriched fuel is made
into rods which are used in
the reactor.
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What happens in nuclear fission?
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What are the products of fission?
When fission of uranium-235 occurs, it splits into two smaller
nuclei, known as daughter nuclei.
Many possible daughter nuclei may be formed in a
fission process. One example is shown below.
fission
+


+
+
uranium + neutron  uranium  strontium + xenon + neutrons
235
236
90
144
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Where does the energy come from?
Barium and krypton are often the daughter nuclei formed by
the fission of uranium-235. The decay equation for this is:
235
U
92
+
1
n
0

90
Kr
36
143
+
Ba
56
+
1
30n
In this decay equation, the number of protons and the mass
numbers on both sides of the equation balance.
However, the particles after decay have slightly less mass
than the particles before decay.
The mass that has been lost has turned into energy.
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How does nuclear power work?
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Nuclear fission – true or false?
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What happens in a chain reaction?
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What is a chain reaction?
Nuclear fission results in a chain reaction
because each time a nucleus splits it
releases more neutrons, which can
go on and cause more fission
reactions to occur,
and so on.
+
This is why a chain reaction
releases a lot of energy so rapidly.
If a chain reaction is uncontrolled, heat
builds up very quickly. A chain reaction must
be controlled to maintain a steady output of heat.
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What are the stages of a chain reaction?
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How are neutrons controlled?
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Controlling a reactor
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Why must chain reactions be controlled?
Chain reactions can generate a lot of heat and can be
extremely dangerous if they are not properly controlled.
This is what happened in 1983 in the
world’s worst nuclear power accident
at Chernobyl, Ukraine.
Most of the control rods had been
removed from a reactor during a test.
The chain reactions were uncontrolled
and generated too much heat.
The reactor overheated and caused a
steam explosion, which blew the
building apart and released a lot of
radiation into the environment.
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How do nuclear weapons work?
Nuclear bombs use uncontrolled chain reactions.
For such a chain reaction to occur, there must be a certain
amount of uranium atoms. This is called the critical mass.
A nuclear weapon works
by forcing together two
masses of uranium-235
to create a critical mass.
This results in uncontrolled
chain reactions releasing
huge amounts of energy.
This four tonne uranium bomb is similar to one used during
the Second World War in Hiroshima, Japan. It has the
same power as 20,000 tonnes of high explosive.
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Chain reactions – key words
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Fukushima Daiichi nuclear disaster
A massive earthquake on
11th March 2011 caused the six
reactors at the Fukushima Dai-ichi
nuclear power plant in Japan to
shut down automatically.
Though fission had stopped, the
reactors continued to produce heat.
The tsunami that followed the
earthquake disabled the pumps driving the cooling
systems, causing the reactors to overheat.
Reactors 1, 2 and 3 subsequently experienced ‘meltdown’.
Why was heat still produced after fission had stopped?
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What happens to the daughter nuclei?
Some of the daughter nuclei produced during nuclear fission
are stable isotopes, but many are unstable and radioactive,
e.g. strontium-90.
Unstable daughter nuclei in the
fuel rods decay into other
radioactive isotopes. The decay
process continues until a stable
isotope is formed.
This process is what caused
the meltdown at the
Fukushima nuclear plant, as
it produces large amounts of
heat and radiation.
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Strontium-90
half-life = 28 years
Yttrium-90
half-life = 64 hours
Zirconium-90 (stable)
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Making materials radioactive
Materials that are placed inside a nuclear reactor can
become radioactive themselves.
This is because their atoms absorb some of the neutrons
released during fission, creating new isotopes.
Some of these isotopes are unstable
and decay, giving out radiation.
All parts of a nuclear reactor must be
carefully disposed of as nuclear waste
when the reactor is decommissioned.
Some substances, like medical tracers,
are deliberately put inside nuclear
reactors to make them radioactive.
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Nuclear waste
Waste from spent nuclear fuel rods contains highly toxic
substances and may remain radioactive for thousands
of years. It can harm both people and the environment.
Plutonium-239 can be recovered
and used as a nuclear fuel or to
make nuclear weapons. Many of the
other isotopes in the remaining
waste have no practical purpose.
Strict regulations are followed when
handling and storing nuclear waste.
Long-term storage of nuclear waste
is a major problem. Why is it so
difficult to find suitable sites?
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Categorizing nuclear waste
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Using nuclear fission
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Glossary
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Anagrams
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Multiple-choice quiz
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