Chapter 21: Nuclear Power
The Binding Energy Curve
21.1 Fission and Fusion
p. 922
(figure 21.1 p. 922)
http://staff.orecity.k12.or.us/les.sitton/Nuclear/313.htm
This curve is the heart of nuclear physics.
 Iron (A= 56) has the maximum binding energy nucleon.
 Therefore, iron must have the most stable nucleus and, given time, all nuclei will form iron.
 The products of nuclear reactions have more binding energy than their reactants.
 The corresponding mass defect releases energy as kinetic energy of the product particles.
 The binding energy differences are greatest at the extremes of the curve
 Fusion of small nuclei (H  He) produces a lot of energy
 Fission of large nuclei (U  Ba + Kr) produces a lot of energy.
Nuclear Fusion
the joining of two light nuclei into a heavier nucleus, releasing energy
.
The following are fusion reactions believed to occur in the Sun
1
1
H 11H 12H  10e   v
1
1
H 12H 23He
2
1
H 12H 23He  01n
3
2
He  23He 24He  211H
Sunlight from Fusion
In stars like our Sun, the temperature and pressure are so great that the hydrgen nuclei have sufficient energy to overcome the Coulombic repulsive
forces and successfully collide. The resulting fusion reactions eventually form helium-4 nuclei and releases energy.
Hydrogen and helium nulcei can continue to undergo fusion to form larger, denser nuclei. These larger nuclei, (approaching A=56), would tend to
migrate toward the centre of the star where they could continue to undergo fusion reactions. As a result, the Sun and all stars should have an iron
core. (See http://www.scienceagogo.com/news/20031002191731data_trunc_sys.shtml for the hypothetical link between the iron core and solar
flares as well as the birth of our solar system.
Nuclear Fission
the splitting of a large nucleus into lighter nuclei, releasing energy
There are many different possible products for each reactant nuclei.
Many fission reactions begin with a neutron being absorbed by a large nucleus, which then undergoes radioactive decay
example:
Uranium-235 is the fuel used in CANDU reactors (http://www.candu.org/)
141
92
1
U  01n 236
92 U  56 Ba  36 Kr 30 n
235
92
135
90
1
U  01n 236
92 U  54 Xe 38 Sr  110 n
235
92
The production of neutrons is a key feature of fission reactions.
The neutrons that are produced can initiate subsequent fission reactions.
Chain Reaction
a self-sustaining sequence of reactions where a reactive product causes additional reactions.
The fission rate will remain constant if only one of the neutrons produced results in another fission reaction.
Critical Mass
the minimim mass of the nuclear fuel (
235
U
92
) required to maintain a nuclear chain reaction.
Energy Released in a Nuclear Reaction
Mass Defect
= the difference between the total mass of reactants and the total mass of the products.
The energy released is the energy equivalent of the mass defect.
Fusion
2
1
H 12H  23He  01n
, given the masses of deutrium, helium-3, and neutron.
Mass Defect
= Mass of Reactant Nuclei – Mass of Product Nulcei
= [2 x mass of deutrium]– [mass of helium-3 + mn]
= (4.0282 u) – (4.0247 u)
= 0.0035 u
Energy = 0.0035 u × 931.5 MeV/ u = 3.26 MeV
3.26 MeV of energy is produced by the fusion of 2 deuterium nuclei.
Fission
141
92
1
U  01n 236
92 U  56 Ba  36 Kr 3 0 n
235
92
Mass of reactants (U-235 + n)
Mass of products (Ba-144 + Kr-89 + 3n)
Mass Defect
Energy = 0.185875 u × 931.5 MeV/ u
= 236.0526 u
= 235.8667250 u
= 0.185875 u
= 173.14 MeV
173 MeV of energy is produced by the fission of 1 uranium-235 nucleus.
Practice Problems
# 1 – 3, p. 925