Section 21.2 Nuclear Reactions and Energy
Objectives: Compare and Contrast Nuclear fission
and Fusion, Demonstrate Equations that Represent
the Changes that Occur During Radioactive Decay,
Trace the Operation and Structure of a Nuclear
Reactor
The Power of the Nucleus
Nuclear reactions involve enormous energy
changes.
Albert Einstein was the first scientist to realize
the enormous amount of potential energy
available in matter and related it in the
equation, E = mc2.
Nuclear Reactions & Energy:
EINSTEIN!
E= mc2
c = speed of light, 3.00 x 108 m/s
m = mass of the nuclear particle
Small amount of mass = large amount of energy
Nuclear Fission
Nuclear fission is the splitting of an atomic
nucleus into two or more smaller fragments,
accompanied by a large release of energy.
Nuclear Fission (cont.)
Nuclear power plants use fission to produce
electricity by striking uranium-235 with
neutrons.
– Fission of an atom of uranium-235 releases
two neutrons.
– Each of those neutrons can split an atom of
uranium, which releases more neutrons.
– The self-sustaining process is called a
chain reaction.
– In order for this reaction to be useful, and
not explosive, it must be controlled
Nuclear Fission (cont.)
Nuclear Fission
 Bombardment of the radioactive nuclide with a
neutron starts the process.
 Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.
Nuclear Fission
If there are not enough radioactive nuclides in the
path of the ejected neutrons, the chain reaction will
die out.
Nuclear Fission
Therefore, there must be a certain minimum
amount of fissionable material present for the chain
reaction to be sustained: Critical Mass.
This is what fission “looks like”
Example:
U-235 + n0 → U-236 → Kr-93 + Ba-142 + 3n0
ENERGY
235U
92
+
1
n
0
→
93Kr
36
+
140 Ba
56
+
3 1n
0
Nuclear Reactors
In nuclear reactors the heat generated by the
reaction is used to produce steam that turns a
turbine connected to a generator.
Nuclear Reactors
 The reaction is kept in
check by the use of control
rods.
 These block the paths of
some neutrons, keeping the
system from reaching a
dangerous supercritical
mass.
Nuclear Reactors
 Only 3% of the uranium in the fuel rods is U-235,
enough to sustain a chain reaction, but far less than
required for a nuclear explosion
 These fuel rods are surrounded by control rods
generally made out of graphite. They help control the
rate of the reaction.
 Water functions as a coolant/moderator and heat
transfer medium, BUT it also has the ability to capture
(slow down) neutrons, thereby slowing down fission
reactions is some reactors
 Why is there so much resistance?
Nuclear Reactors
 Fission reactions produce
more than 200
radioisotopes, having
half-lives up to millions
of years…where do we
dispose of it?
 U-235 is more fissionable
than U-238
(nonfissionable), so it is
desirable to concentrate
U-235 by converting the
naturally occurring
uranium to UF6 (a gas)
Pros and Cons
Pros:
1) Don’t produce a lot of pollution like fossil fuels
2) Relatively inexpensive
3) Efficient source
Cons:
1) Chain reactions can be difficult to control
Can be controlled w/ control rods (block n0)
2) Produce radioactive waste- hard to dispose of
safely (storage/transport)
3) Limited supply of fissionable fuel, higher cost
Nuclear Fission (cont.)
Stop!
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words or words you do not know…
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Nuclear Fusion
Nuclear fusion is the process of combining
two or more nuclei to form a larger nucleus.
The enormous amount of energy that is
generated by fusion reactions in the Sun
sustains all life on Earth.
Nuclear Fusion
 Occurs when 2 or more nuclei combine to form a
larger nucleus
 Hydrogen isotopes are common “participants”:
 Fusion produces 20 times more energy than fission,
but it requires a LARGE input of energy to make it
happen (100 million kelvins!)
Nuclear Fusion (cont.)
In one common fusion reaction, two different
isotopes of hydrogen combine to form
helium and a neutron.
What does fusion “look like”
Process of combining two or more nuclei to form a larger
nucleus (occurs in the sun and stars)
00 + 000 → 00000
Ex:
2H +
3H
→ 4 He + 1 n
1
1
2
0
Nuclear Fusion (cont.)
The isotope of hydrogen with a mass number
of 2 that is one of the reactants is called
deuterium (D).
The hydrogen isotope with a mass number
of 3 is called tritium (T).
Nuclear Fusion
Pros:
1) Fusion of H produces 20x the energy produced by the
fission of an equal amount of uranium
2) No radioactive waste
3) Fusion reactors easier to control (no chain reaction)
Cons:
1) H nuclei less tendency to react
2) Large input of energy must be provided- large temps (100
million K) and pressures-May require more energy than it
creates
3) Expensive
Nuclear Fusion
 Fusion would be a superior
method of generating
power.
 The good news is that the
products of the reaction are
not radioactive.
 The bad news is that in order
to achieve fusion, the material
must be in the plasma state at
several million kelvins.
Nuclear Fusion
 Tokamak apparati like the
one shown at the right show
promise for carrying out
these reactions.
 They use magnetic fields to
heat the material.
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 https://www.youtube.com/watch?v=FU6y1XIADdg