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Option C: Energy
C.3 : Nuclear Fusion and Fission
What is a nuclear reaction?
• A nuclear reaction is
any reaction that
involves the nucleus.
• These reactions change
the identity of an atom,
as opposed to chemical
reactions which only
involve valence
electrons.
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The nucleus
• The nucleus is made up
of protons and neutrons.
• We know what the
protons do – they
provide an electrostatic
attraction to the
electrons close… but
what about the
neutrons?
The Neutrons
• The major function of
the neutrons is to hold
the nucleus together.
• The neutrons provide a
strong nuclear force of
attraction within the
nucleus, counteracting
the repulsion between
the positively charged
protons.
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How is the nucleus held together?
• In the 1930’s it was first
observed that the mass
of an atoms nucleus is
less than the sum of the
masses of the protons +
neutrons…?
• Some of the mass of the
nucleus is converted
into energy to hold the
nucleus together.
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Mass Defect
• The difference in mass
of the nucleus and it’s
parts is referred to as
the mass defect, and
the energy (e=mc2) it
provided is called the
nuclear binding energy.
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Nuclear vs. Chemical Reactions
• This nuclear binding energy is released during
nuclear reactions (fission & fusion), and is
~1,000,000X greater than the chemical bond energy
released during chemical reactions.
What makes an isotope radioactive?
• Elements are
radioactive when their
nucleus is unstable.
• The stabilizing force of
the neutrons is effective
for smaller elements,
though all elements
above lead are
radioactive.
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Band of Stability
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All Radiation is not the same
• Radiation, the particles
and/or energy given off
by nuclei undergoing
radioactive decay, comes
in a number of forms.
• The forms we need to
know are:
– Alpha emission
– Beta emission
– Gamma emission
Alpha Emission (too many protons)
Emits an alpha particle
• Made up of 2 protons and 2 neutrons
• Effect on parent:
– Mass lost, resulting in a new element
Danger Level
Extremely
ionizing but least
penetrating
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Beta Emission (too many neutrons)
• A neutron is converted into a proton and an
electron. The electron is emitted as a β
particle
• Result:
– parent changes slightly in mass (new element
produced)
Danger Level
Less ionizing but
more penetrating
than alpha
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Gamma Rays
• A form of EMR given off
when nucleons rearrange
themselves inside a
nucleus
• Don’t result in a change in
the identity of the atom
• Gamma rays typically
come from the nucleus,
while x-rays come from
the electrons.
Danger Level
Least ionizing but
much penetrating
than alpha
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How long will radioactive
isotopes decay?
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It depends
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Types of Nuclear Reactions
Fission & Fusion
Nuclear Fission
• Involves the splitting of a heavy nucleus into smaller
elements, releasing the nuclear binding energy.
• This can be done for isotopes larger than Fe-56,
though is primarily done with U-235 & Pu-239.
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Artificially Induced Fission
• First discovered by
German scientists in
1938 when they
discovered barium after
bombarding uranium
with neutrons.
A chain reaction
• Assuming a critical mass of fissionable material is
present, a chain reaction can occur as each fission
releases additional neutrons.
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Controlling the Reaction
• The fission reaction has to
occur at an acceptable
speed in power plants, or
else the energy would be
lost.
• Moderators are used to
slow down the neutrons,
while control rods (often
graphite) can absorb the
neutrons
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Fuel Rods
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Control Rods
Uranium Enrichment
• U-235 is the most fissionable isotope, but…
• Naturally occurring uranium is <1% U-235.
• Uranium enrichment increases the percentage U-235,
increasing the likelihood of fission.
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Breeder Reactors
• As the supply of U-235 is limited, “breeder reactors”
are used to convert the more common U-238 into
fissionable Pu-239.
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The problem with breeders…
• Plutonium is highly toxic and these reactors are more
susceptible to accidents.
• Plutonium can be concentrated from reactor grade to
weapons grade very easily.
Issues with nuclear energy
• Nuclear waste is radioactive for a very long time.
– High Level Waste (ex. Control rod)
• high activity; long half-life
– Low-level Waste (ex. Fuel containers, clothing)
• low activity; short half-life
• And of course there is always the potential for a
major accident.
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Three Mile Island (1979)
Chernobyl
(1986)
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Fukushima (2011)
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Fukushima (2011)
Disposal of high-level waste
• Requires a geologically-stable area with impervious
rock and away from water supplies.
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Uses of Nuclear Power
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Nuclear Fusion
• Occurs when light
nuclei are brought
together to form
heavier elements.
• This typically involves
the fusion of two
hydrogens to form
helium.
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The up-side of fusion
• It releases 3-4X the energy of a fission reaction.
• It produces almost no radioactive waste.
• The supply of deuterium (H-2) is cheap and almost
unlimited (from the ocean)
The down-side of fusion
• It requires way more
energy than fission to
begin (reactants must
be in a plasma state).
• It is much harder to
control.
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Cold Fusion
• Many attempts have
been made to design a
controllable fusion
reactor (pg. 533) and
will certainly continue,
as success would mean
electricity would cost us
pennies per day.
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Fusion reactions power stars!
• Fusion reactions generate the energy released from
stars, which explains why absorption data shows us
that the Sun is made up of primarily hydrogen and
helium.
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The Electromagnetic Spectrum
• Every object above 0 K emits EMR of different
wavelengths.
• The Sun is hot enough to emit all wavelengths of
visible light (white)
• Atmospheric gases absorb some wavelengths of this
light, which allows us to identify them.
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The End
No HW except reading C.4 (pages 542-550)
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BONUS COVERAGE!!!
Natural Sources
Terrestrial
Cosmic
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Terrestrial
• Consists of radioactive
elements found here on
Earth
• By far the major
contributor to
terrestrial radiation is
Radon-222, which is a
naturally occurring
decay product of
Uranium-238, which is
found in the soil
Radon Gas
• A colorless, odorless gas • The EPA estimates that
whose concentration
it causes 21,000 deaths
can build up in poorly
from lung cancer each
ventilated homes. Not
year.
uncommon in our area.
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Stanley Watras
• The build-up of radon
gas in homes was made
famous when an
engineer repeatedly set
off radiation detectors
at a new nuclear power
plant – that didn’t yet
have radioactive
material.
Testing in his home revealed that his families cancer risk would
be the same as if they smoked 135 packs of cigarettes daily.
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Radon Abatement Systems
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Natural Sources
Cosmic
Cosmic Radiation
• Consists of a variety of
high energy photons
primarily from the sun
and supernovas
• Though mostly blocked
by the earth’s
atmosphere and
magnetic field, it still
increases our cancer risk
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Poses a serious stumbling block to any thoughts of space travel
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