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Nuclear Energy – Learning Outcomes
 Describe the principles underlying fission and fusion.
 Interpret nuclear reactions.
 Discuss nuclear weapons.
 Describe the structure and operation of a nuclear
reactor.
 Discuss the environmental impact of fission reactors.
 Discuss the development of fusion reactors.
 Discuss fusion in the Sun.
 Discuss mass-energy conservation in nuclear reactions.
 HL: Solve problems about mass-energy conservation.
Nuclear Fission
 Nuclear fission is the splitting
up of a large nucleus into two
smaller nuclei, releasing
energy and neutrons.
 e.g. uranium-235 will fission if it
is given an extra neutron
becoming uranium-236).
 It produces a krypton-92, a
barium-141, three neutrons,
and lots of energy.
235
92𝑈
+ 10𝑛 →
141
56𝐵𝑎
92
+ 36
𝐾𝑟 + 3 10𝑛 + 𝑒𝑛𝑒𝑟𝑔𝑦
by fastfission – public domain
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Chain Reaction
 Since fission reactions
produce neutrons, they can
cause further reactions.
 Not all neutrons will cause
fission.
 If at least one neutron from
each reaction causes another
reaction, the process is called
a chain reaction.
 The minimum amount of
material needed to cause a
chain reaction is called the
critical mass.
by fastfission – public domain
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Atomic Bomb
 If each reaction causes more
than one reaction on
average, it is called
supercritical.
 Fission weapons use
supercritical reactions to
release large amounts of
energy to devastating effect.
 The ignition mechanism brings
two subcritical masses
together quickly with chemical
explosives, starting the
uncontrolled chain reaction.
by Charles Levy – public domain
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Fission Reactor
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Fission Reactors
 Uranium found in ore is mostly uranium-238, which is not
fissionable. A little is uranium-235, which is fissionable.
 Enriched uranium is processed uranium where the
amount of U-235 is increased to almost critical levels.
 U-235 releases fast neutrons, but requires slow neutrons
to start another reaction. U-238 captures fast neutrons.
 Graphite moderators slow neutrons down so the U-235
captures them instead.
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Fission Reactors
 To create a sustainable reaction, there must be a critical
mass.
 This is dangerous however.
 Uranium fuel is separated into multiple subcritical rods,
while the total amount is critical.
 Cadmium control rods absorb neutrons and can be
raised or lowered to control the rate of chain reaction
between them.
 The energy resulting from the reaction is used to boil
water and the steam runs a turbine.
Environmental Impact
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Unnecessary facts
Advantages
No CO2 emissions
Disadvantages
Radioactive waste products
No greenhouse gases
Accidents can be
catastrophic
High energy output
Mostly safe
 Around 450 nuclear reactors in operation with more on
the way.
 13 countries use nuclear power to generate more than
¼ of their energy.
 France uses nuclear power for over ¾ of its energy.
 Nuclear power supplies over 10% of the world’s energy.
Nuclear Fusion
 Nuclear fusion is the
combining of two nuclei to
form a larger one with the
release of energy.
 e.g. deuterium and tritium
fuse to form a helium
nucleus and a neutron.
 21𝐻 + 31𝐻 → 42𝐻𝑒 + 10𝑛 + 𝑒𝑛𝑒𝑟𝑔𝑦
by Wykis – public domain
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Nuclear Fusion
 Nuclei are positively charged, so the coulombic force
between them is repulsive and only gets stronger as they
get closer.
 To overcome this, the nuclei need huge energies to get
close enough to each other to fuse.
by Panoptik – CC-BY-SA-3.0
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Fusion Reactors
 They don’t exist yet.
 Temperatures > 108 K are required.
 This takes a huge amount of energy.
 To date, reactors are unable to get more energy out
than they put in.
 Some projects use magnetism to try to compact
samples so they fuse easier.
 Others use lasers to implode samples.
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Fusion vs. Fission
 Fusion produces less radioactive waste than fission.
 Fusion cannot cause a runaway reaction (because it’s
so difficult to get any reaction at all). Fission definitely
can.
 Deuterium can be easily extracted from seawater.
Tritium can be manufactured from lithium. Most
fissionable materials are difficult to get or process.
 Fusion is way cooler.
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Fusion in the Sun
 Stars use a number of different reactions to produce
energy depending on their type.
 Our Sun primarily gets its energy from fusing hydrogen.
 Elements up to iron are produced during this fusion.
 Heavier elements are not produced in our Sun, but in
other stars that died violently (supernova), using the
extreme energy to produce heavier elements.
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Mass-Energy Equivalence
 Mass is a form of energy.
 We can calculate the energy of mass using Einstein’s
famous formula: 𝐸 = 𝑚𝑐 2 .
 The result is that when things lose energy (e.g. burning
petrol), they get lighter; and when things gain energy
(e.g. accelerating something), they get heavier.
 In nuclear reactions, energy needs to be input if the
mass of the products is higher than the mass of the
reactants. If the products are lighter than the reactants,
energy is given out.
 Hence, fission only gives out energy for heavy elements
and fusion only gives out energy for light elements.
𝑐 = 3 × 108 𝑚 𝑠 −1
Mass-Energy Equivalence
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 e.g. Calculate the kinetic energy gained when a 1000
kg car accelerates from rest to 15 m/s. How much mass
does it gain?
Higher Level
 e.g. The Sun gets 4 000 000 000 kg lighter every second
due to emission of light. What is the power of the Sun?
 e.g. How much energy is given out from the fusion of
deuterium and tritium?
 𝑚𝐷𝑒𝑢𝑡𝑒𝑟𝑖𝑢𝑚 = 2.014102 𝑎. 𝑚. 𝑢.
 𝑚 𝑇𝑟𝑖𝑡𝑖𝑢𝑚 = 3.016049 𝑎. 𝑚. 𝑢.
 𝑚𝐻𝑒𝑙𝑖𝑢𝑚 = 4.002602 𝑎. 𝑚. 𝑢.
 𝑚𝑛𝑒𝑢𝑡𝑟𝑜𝑛 = 1.008665 𝑎. 𝑚. 𝑢.
 1 𝑎. 𝑚. 𝑢. = 1.660539040 × 10−27 𝑘𝑔