Curve of the Binding Energy/nucleon
fission
fusion
1MeV/nucleon
10.1 Nuclear energy
Nuclear Fission
Nuclear Fusion
Energy can be released by nuclear reactions of
Fusion and Fission
Nuclear Power
Natural radioactivity
Many elements found in nature are unstable and decay
emitting radioactivity.
These include Uranium, 238U , Radon
and Potassium 40K. Carbon 14C,
224Ra
The half lives of natural radio-isotopes are long.
Not useful as sources for power.
Low Power output.
Induced Nuclear reactions
Can result in short half lives- fast reactions-high energy density
Combining nuclei (Fusion)
• Nuclear power requires induced nuclear
fission.
• Nuclear fission can be induced by
neutrons in a chain reaction.
• Nuclear fusion can be induced by
collisions at high temperature.
Fission of Uranium
Bombard uranium with
neutrons
D + 21D ⎯⎯
→ 31T + 11H
2
1
+ Energy
Fast
neutrons
Neutron reactions (Fission)
n+
1
0
235
92
U ⎯⎯
→ 141
56 Ba +
92
36
Kr + 3 01n + Energy
Slow
neutrons
n
Neutron
Source
(radon+Be)
parafilm
n
Enrico Fermi (1936)
Found induced radioactivity
uranium “Transuranium elements”
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Fission of Uranium
Liquid Drop model-
Strassman and Hahn (1939)
Explained fission due to the instability
of the higher larger nucleus.
Irradiated Uranium with neutrons
Detected Barium
Conclude Uranium nuclei splits into smaller fragments
n+
1
0
235
92
T1/2 ~10-12 s
n+
1
0
U ⎯⎯
→ 141
56 Ba + other _ products
Nuclear Chain reaction
235
92
U ⎯⎯
→
236
92
U ⎯⎯
→
Lise Meitner
Ba + products
∆E (average) =208 MeV
Two major isotopes of Uranium
235U
(0.7%) Fissionable upon neutron capture
238U
(99.3%) Non-Fissionable upon neutron capture
Chain Reaction
binding of 1 neutron releases ~3 neutrons
Each neutron can initiate another reaction
Enriched 235U
Critical Condition for Chain reaction
235U
(Fission)
Natural Uranium is a mixture of 235U (0.7%) and 238U(99.3%)
n
n
n
235U
n
(Escape)
238U (Capture)
Most Uranium nuclear reactors use uranium enriched in
(2-3%)
235U.
Nuclear weapons used highly enriched 235U. (~90%)
Nuclear reactor
Reproduction constant
K = no. of neutrons that produce a new fission event
K=1 ( self –sustained reaction)
Enrichment done by mass separation.
Gaseous diffusion
Centrifuge process.
Laser separation
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Centrifuge separation of
isotopes
Nuclear reactor
fast neutrons must
be slowed down to
react efficiently.
Moderator- slows
neutrons to thermal
velocities.
Control rods- neutron
absorbers to control
the level of neutrons
Critical condition. – When each neutron released initiates
a new reaction.
centrifugal
separation
gaseous UF6
Nuclear reactor
Plutonium
Plutonium is a fissionable material created in
a nuclear reactor.
238
92
239Pu
−
U + 01n ⎯⎯
→ 239
94 Pu + 2e
can be made into nuclear bombs.
Pu can be chemically separated from U in spent fuel
rods from nuclear reactors.
Nuclear Fusion
Question
How many gallons of gasoline (1.3x108 J of energy/gallon)
would be equivalent to a kg of Uranium? ∆E=208MeV
Fusion of small nuclei releases energy
No. of fissionable nuclei =N
N = ( fraction235U)(
⎡
⎤
1kg
m
22
) = (0.0072)⎢
⎥ = 1.8 x10
− 27
mass / atom
⎣ 238u(1.66 x10 kg / u) ⎦
Energy released = E = N(∆E)
[
E = N( ∆E) = 1.8 x10 ( 208 x10 eV ) 1.6 x10
22
6
−19
]
D + 21D ⎯⎯
→ 32 He + 01n
∆E= 3.27 MeV
D + 21D ⎯⎯
→ 31T + 11H
∆E= 4.03 MeV
D + 31T ⎯⎯
→ 42 He + 01n
∆E=17.6 MeV
2
1
2
1
J / eV = 6 x10 J
E
gallons of gasoline =V= E / gallon
11
2
1
E
6 x1011 J
=
= 4.6x10 3 gallons
E / gallon 1.3 x10 8 J / gallon
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Nuclear Fusion
High energy required to bring charged nuclei close together
+Z
v
+Z
+Z +Z
Plasma Fusion
Magnetic Confinement
Plasma is a gas of ionized atoms
Heated to high temperature
Confined by magnetic forces
~10-15 M
Requirements for fusion
High Temperatures (T~ 108 K)
High density (n) for long time (τ)
Lawson Criterion
nτ > 1014 s/cm3
Laser fusionInertial Confinement
T~108 K
Princeton Tokomak
long times
low density
Prospects
• Nuclear energy by fission is currently a source of
much of the electrical power (~15% USA).
• The problems with nuclear energy
– Radioactive waste disposal
– Atomic bomb threats
– Easily mined U is limited (~100 yr supply)
Deuterium pellet
Lawrence Livermore Lab
is heated by a short
Nova Laser
laser pulse
Short times
High density
• Nuclear fusion reactions promise an unlimited
source of energy.
– Controlled fusion reactions are not yet possible.
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