Nuclear Reactions
Fission and Fusion
CS 4.4
State that in fission a nucleus of large mass splits into 2
nuclei of smaller mass numbers, usually with the release
of neutrons.
CS 4.5
State that fission may be spontaneous or induced by
neutron bombardment.
CS 4.6
State that in fusion, 2 nuclei combine to form a nucleus
of larger mass number.
CS 4.7
Explain, using E = mc2, how the products of fission and
fusion acquire large amounts of kinetic energy.
CS 4.8
Carry out calculations using E = mc2 for fission and
fusion reactions.
Fission
When atoms are bombarded with neutrons, their nuclei
splits into 2 parts which are roughly equal in size.
Nuclear fission in the process whereby a nucleus, with
a high mass number, splits into 2 nuclei which have
roughly equal smaller mass numbers.
During nuclear fission, neutrons are released.
Nuclear Fission
There are 2 types of fission that exist:
1. Spontaneous Fission
2. Induced Fission
Spontaneous Fission
Some radioisotopes contain nuclei which are highly
unstable and decay spontaneously by splitting into 2
smaller nuclei.
Such spontaneous decays are accompanied by the
release of neutrons.
Induced Fission
Nuclear fission can be induced by bombarding atoms
with neutrons.
The nuclei of the atoms then split into 2 equal parts.
Induced fission decays are also accompanied by the
release of neutrons.
The Fission Process
A neutron travels at high speed towards a uranium-235
nucleus.
1
0
n
235
92 U
The Fission Process
A neutron travels at high speed towards a uranium-235
nucleus.
1
0
n
235
92 U
The Fission Process
A neutron travels at high speed towards a uranium-235
nucleus.
1
0
n
235
92 U
The Fission Process
The neutron strikes the nucleus which then captures the
neutron.
1
0
n
235
92 U
The Fission Process
The nucleus changes from being uranium-235 to
uranium-236 as it has captured a neutron.
236
92 U
The Fission Process
The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
The Fission Process
The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
The Fission Process
The uranium-236 nucleus formed is very unstable.
It transforms into an elongated shape for a short time.
The Fission Process
It then splits into 2 fission fragments and releases
neutrons.
1
0
n
1
0
n
1
0
n
141
56 Ba
92
36 Kr
The Fission Process
It then splits into 2 fission fragments and releases
neutrons.
1
0
n
1
0
n
1
0
n
141
56 Ba
92
36 Kr
The Fission Process
It then splits into 2 fission fragments and releases
neutrons.
1
0
n
1
0
n
1
0
n
141
56 Ba
92
36 Kr
The Fission Process
It then splits into 2 fission fragments and releases
1
neutrons.
0n
141
56 Ba
1
0
92
36 Kr
1
n
Nuclear Fission Examples
235
1
141
92
1
235
1
138
96
1
U
n
+
92
0
U
n
+
92
0
Ba
Kr
n
3
+
+
56
36
0
Cs
Rb
n
2
+
+
55
37
0
Energy from Fission
Both the fission fragments and neutrons travel at high
speed.
The kinetic energy of the products of fission are far
greater than that of the bombarding neutron and target
atom.
EK before fission << EK after fission
Energy is being released as a result of the fission reaction.
Energy from Fission
235
1
U
n
+
92
0
138
96
Cs
Rb
n
2
+
+
55
37
0
Element
Atomic Mass (kg)
235 U
92
3.9014 x 10-25
138 Cs
55
2.2895 x 10-25
96 Rb
37
1.5925 x 10-25
1
0n
1
1.6750 x 10-27
Energy from Fission
Calculate the total mass before and after fission takes place.
The total mass before fission (LHS of the equation):
3.9014 x 10-25 + 1.6750 x 10-27 = 3.91815 x 10-25 kg
The total mass after fission (RHS of the equation):
2.2895 x 10-25 + 1.5925 x 10-25 + (2 x 1.6750 x 10-27) = 3.9155 x 10-25 kg
Energy from Fission
The total mass before fission = 3.91815 x 10-25 kg
The total mass after fission = 3.91550 x 10-25 kg
total mass before fission > total mass after fission
Energy from Fission
mass difference, m = total mass before fission – total mass after fission
m = 3.91815 x 10-25 – 3.91550 x 10-25
m = 2.65 x 10-28 kg
This reduction in mass results in the release of energy.
Energy Released
The energy released can be calculated using the equation:
E = mc2
Where:
E
m c2
E = energy released (J)
m = mass difference (kg)
c = speed of light in a vacuum (3 x 108 ms-1)
Energy from Fission
Calculate the energy released from the following fission
reaction:
235
1
U
n
+
92
0
m = 2.65 x 10-28 kg
c = 3 x 108 ms-1
E=E
138
96
1
Cs
Rb
n
2
+
+
55
37
0
E = mc2
E = 2.65 x 10-28 x (3 x 108)2
E = 2.385 x 10-11 J
Energy from Fission
The energy released from this fission reaction does not
seem a lot.
This is because it is produced from the fission of a
single nucleus.
Large amounts of energy are released when a large
number of nuclei undergo fission reactions.
Energy from Fission
Each uranium-235 atom has a mass of 3.9014 x 10-25 kg.
The total number of atoms in 1 kg of uranium-235 can
be found as follows:
No. of atoms in 1 kg of uranium-235 = 1/3.9014 x 10-25
No. of atoms in 1 kg of uranium-235 = 2.56 x 1024 atoms
Energy from Fission
If one uranium-235 atom undergoes a fission reaction
and releases 2.385 x 10-11 J of energy, then the amount
of energy released by 1 kg of uranium-235 can be
calculated as follows:
total energy = energy per fission x number of atoms
total energy = 2.385 x 10-11 x 2.56 x 1024
total energy = 6.1056 x 1013 J
Nuclear Fusion
In nuclear fusion, two nuclei with low mass numbers
combine to produce a single nucleus with a higher mass
number.
2
3
H
H
+
1
1
4
1
Energy
He
n
+
+
2
0
The Fusion Process
2
1H
3
1H
The Fusion Process
2
1H
3
1H
The Fusion Process
2
1H
3
1H
The Fusion Process
2
1H
3
1H
The Fusion Process
The Fusion Process
The Fusion Process
The Fusion Process
The Fusion Process
1
0
4
2 He
n
The Fusion Process
1
0
4
2 He
n
The Fusion Process
1
0
4
2 He
n
The Fusion Process
1
0
4
2 He
n
Energy from Fusion
2
3
H
H
+
1
1
Element
4
1
Energy
He
n
+
+
2
0
Atomic Mass (kg)
2
3.345 x 10-27
3
1H
5.008 x 10-27
4 He
2
6.647 x 10-27
1H
1
0n
1.6750 x 10-27
Energy from Fusion
Calculate the following:
• The mass difference.
• The energy released per fusion.
Energy from Fusion
2
3
H
H
+
1
1
4
1
Energy
He
n
+
+
2
0
The total mass before fusion (LHS of the equation):
3.345 x 10-27 + 5.008 x 10-27 = 8.353 x 10-27 kg
The total mass after fission (RHS of the equation):
6.647 x 10-27 + 1.675 x 10-27 = 8.322 x 10-27 kg
Energy from Fusion
m = total mass before fission – total mass after fission
m = 8.353 x 10-27 – 8.322 x 10-27
m = 3.1 x 10-29 kg
Energy from Fusion
2
3
H
H
+
1
1
m = 3.1 x 10-29 kg
c = 3 x 108 ms-1
E=E
4
1
Energy
He
n
+
+
2
0
E = mc2
E = 3.1 x 10-29 x (3 x 108)2
E = 2.79 x 10-12 J
The energy released per fusion is 2.79 x 10-12 J.