q983001
(a)
What is a fission reaction?
(b)
How can an uncontrolled nuclear chain reaction occur?
(c)
What is a controlled nuclear fission reaction?
Try HSC 2012: Question 34
Check your answer by reviewing the solution guidelines on the next page.
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Solution Guidelines
(The solution is more detailed than required in a H.S.C. answer. The
solution is designed to give your greater depth of knowledge in
preparation of your H.S.C. Physics examination)
(a)
In nuclear fission, a heavy nucleus such as uranium splits into two intermediate-sized
nuclei after being struck by a neutron. In the fission process other neutrons are
released.
Typical fission reactions are:
235
U92 + 1n0 
236
U92* 
144
Ba56 + 89Kr36 + 31n0
235
U92 + 1n0 
236
U92* 
140
Xe54 + 89Sr38 + 21n0
(b)
When a nucleus splits due to a collision with a single neutron, other neutrons are
released, so that a chain reaction is possible. To maintain the chain reaction a
minimum mass of fuel (the critical mass) is required. Also, to enhance the chances of a
neutron imitating a fission reaction, a moderator is used to slow down the released
neutrons.
235U
92
139Ba
56
fission fragment
1n
139Ba
0
56
94Kr
36
94Kr
90Rb
37
94Kr
36
36
139Ba
lost neutron
initiating
neutron
144Cs
55
90Sr
90Sr
56
38
38
143Xe
54
144Cs
55
143Xe
54
90Rb
1st
generation
neutron
2nd
generation
neutrons
3rd generation
neutrons
37
4th generation
neutrons
Chain reaction.
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Because each fission event releases two or more neutrons, while only one neutron is
required to initiate the fission process, an avalanche of fissions can occur in a sample
of fissionable material giving an uncontrolled chain reaction where an immense
amount of energy is liberated in a very short period of time.
An uncontrolled chain reaction can cause an explosion of exceptional magnitude.
(c)
A chain reaction can be controlled, when one neutron per fission causes another fission
reaction. In a controlled chain reaction, the energy output per second is constant. This occurs
in a nuclear reactor. Movable control rods absorb neutrons to maintain a nuclear at a critical
level to maintain a self-sustaining chain reaction and in the event of an accident can be
dropped to be into reactor vessel to shut-down the fission process.
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