Fission and Fusion
Graphic: www.lab-initio.com
Energy and Mass
Nuclear changes occur with small but measurable
losses of mass. The lost mass is called the mass
defect, and is converted to energy according to
Einstein’s equation:
DE = Dmc2
Dm = mass defect
DE = change in energy
c = speed of light
Because c2 is so large, even small amounts of
mass are converted to enormous amount of
energy.
Nuclear Fission and Fusion
Fission: Splitting a heavy nucleus into two
nuclei with smaller mass numbers.
1
0
n
235
92
U
142
56
Ba  Kr  3 n
91
36
1
0
Fusion: Combining two light nuclei to form
a heavier, more stable nucleus.
3
2
He  H  He  e
1
1
4
2
0
1
Fission
A uranium-235 atom absorbs a neutron,
and fissions into two new atoms (fission
fragments), releasing three new
neutrons and some binding energy.
One of those neutrons is absorbed by
an atom of uranium-238, and does not
continue the reaction. Another neutron
is simply lost and does not collide with
anything, also not continuing the
reaction. However one neutron does
collide with an atom of uranium-235,
which then fissions and releases two
neutrons and some binding energy.
Both of those neutrons collide with
uranium-235 atoms, each of which
fission and release between one and
three neutrons, and so on.
Source: Wikimedia Commons
Fission Processes
A self-sustaining fission process is called
a chain reaction.
Neutrons
Causing
Event
Fission
subcritical
<1
critical
=1
supercritical
>1
Result
reaction stops
Sustained reaction
violent explosion
By increasing the mass of the sphere
to a critical mass, the reaction can
become self-sustaining
Critical Mass
A sphere of fissile material is too small
to allow the chain reaction to become
self-sustaining as neutrons generated by
fissions can too easily escape.
Surrounding the original sphere with a
neutron reflector (such as tungsten carbide)
increases the efficiency of the reactions
and also allows the reaction to become
self-sustaining.
Graphic: Wikimedia Commons
Source: Wikipedia
Fission Bomb Design
Little Boy
Fat Man
A Fission Reactor
Fusion
The deuterium-tritium
fusion reaction
2
1
H  H  He  n
3
1
4
2
1
0
Graphics: Wikimedia user Borb
The proton-proton
chain dominates in
stars the size of the
Sun or smaller.
Stellar Fusion
The CNO cycle dominates in
stars heavier than the Sun.