Nuclear Fission and Fusion
Week of March 18-22, 2013
Fission
• Fission occurs when an unstable heavy nucleus splits apart
into two lighter nuclei, forming two new elements.
• Fission can be induced by free neutrons.
• Mass is destroyed and energy produced according to E =
mc2.
• http://library.thinkquest.org/17940/texts/fission/fission.h
tml
• http://www.atomicarchive.com/Movies/index.shtml
Neutron-induced fission
• Neutron-induced fission produces a “chain
reaction.” What does that mean?
• Nuclear power plants operate by
harnessing the energy released in fission in
by controlling the chain reaction.
• Nuclear weapons depend upon the initiation
of an uncontrolled fission reaction.
Critical Mass
• The neutrons released from an atom that
has undergone fission cannot immediately
be absorbed by other nearby fissionable
nuclei until they slow down to “thermal”
levels.
• How can this concept be used to explain
why a chain reaction in nuclear fission will
not occur unless a “critical mass” of the
fissionable element is present at the same
location?
Nuclear Reactors
• Nuclear reactors produce electrical energy through fission.
• Advantages are that a large amount of energy is produced
without burning fossil fuels or creating greenhouse gases.
• A disadvantage is the production of highly radioactive
waste.
• Another simulation appears at
– http://www.howstuffworks.com/nuclear-power.htm
University of Wisconsin Nuclear
Reactor Tour
Nuclear Weapons
Nagasaki, Japan
• Nuclear weapons have
been used only twice,
although they have been
tested thousands of times.
• Weapons based on nuclear
fission involve slamming
together enough material
to produce an uncontrolled
fission chain reaction.
Little Boy was dropped on Hiroshima and
contained U-235 produced in Oak Ridge, TN.
Fission
• Fission occurs only with very heavy elements,
since fissionable nuclei are too large to be
stable.
• A charge/mass calculation is performed to
balance the nuclear equation.
• Mass is destroyed and energy produced
according to E = mc2.
1
0
n
239
94
Pu 

133
51
Sb 
Tc  6 n
100
43
1
0
Sample problem
Complete the following reaction.
U  n 

235
92
1
0
132
51
Sb  ? 5 n
1
0
Fusion
• Fusion occurs when two light nuclei come together to
form a new nucleus of a new element.
• Fusion is the most energetic of all nuclear reactions.
• Energy is produced by fusion in the sun.
• Fusion of light elements can result in non-radioactive
waste.
1
H
1
1
H
1
He
2
2
Fusion
• Fusion is the reaction that powers the sun,
but it has not been reliably sustained on
earth in a controlled reaction.
• Advantages to developing controlled fusion
would be the tremendous energy output
and the lack of radioactive waste products.
• Disadvantages are – we don’t know if we’ll
be technical able to do it on earth!