Geology Reference
In-Depth Information
Box 11.2 Nuclear fusion and fission
Nuclei are held together by an immensely powerful, short-
range force called the strong force. It acts between nucle-
ons only over very short distances similar to the size of
the nucleus itself (~10
−14
m). The more nucleons present
in the nucleus, the stronger is the binding force that each
one experiences. Counteracting the attractive force
exerted by the strong force, however, is the electrostatic
repulsion acting between the
Z
positively charged protons
present, which - because the protons are held in such
close proximity in the nucleus - is also an extremely
powerful force.
The relative stability of nuclei can be expressed in terms
of the mean potential energy per nucleon in the nucleus,
relative to the potential energy each nucleon would pos-
sess as an isolated particle (set by convention at zero).
Because every nucleus represents a more stable state
than the same number of separate nucleons, the mean
potential energy per nucleon is a negative quantity. Its vari-
ation with mass number
A
for the naturally occurring
nuclides is sketched in Figure 11.2.1.
The shape of the graph reflects the interplay between
the strong force and the electrostatic repulsion between
protons. Where the curve drops steeply on the left-hand
side the strong force is clearly the dominant force, but
the curve flattens out around Fe (a region of maximum
nuclear stability) and then rises gently as the proton-
proton repulsion exerts a steadily more powerful influ-
ence; here the increase in strong force obtained by
adding further nucleons to a nucleus is slightly out-
weighed by the consequent increase in electrostatic
repulsion.
Nuclei on the extreme left of the diagram, therefore, can
in principle reduce their potential energy by fusing with
other light nuclei to form heavier ones. Fusion of these
lighter nuclei thus
releases
energy (it is an exothermic
reaction) and this provides the source for the thermo-
nuclear energy output of stars and hydrogen bombs. On the
right of the diagram, on the other hand, is a region where
fusion, were it to occur, would be energy-consuming (endo-
thermic). Nuclei in this
A
range (>60) cannot be generated
by fusion (see main text). On the contrary, the heaviest
nuclei, such as thorium and uranium, are radioactive and
decay by emitting alpha-particles (Box 10.1; also Box 3.3);
this is one mechanism for shedding mass and attaining a
Mass number (
A
)
0
50
100
150
200
250
-2
Fusion
exothermic
-4
-6
A
-range of
fission
products
U
Fe
-8
Fission
exothermic
Figure 11.2.1
Sketch of how potential energy per nucleon
varies with mass number
A
for naturally occurring nuclides.
lower energy per nucleon (greater stability). The energy
released by the decay of such elements within the Earth
constitutes the largest component of terrestrial heat flow.
Certain heavy nuclides (
235
U being the only naturally
occurring example) are also fissile: on absorbing a neu-
tron they split into two lower-mass nuclei. These fission
products, comprising various nuclides in the
A
range 100-
150, have two important properties in common:
(a) They lie on a lower segment of the potential-energy
curve than the parent nuclide. Thus fission is an exo-
thermic process: it is the energy source for present
nuclear-power reactors and for the original 'atom bomb'.
(b) although several neutrons are released in the fission
process (which by colliding with other
235
U nuclei
prompt further fission), the fission products still have
higher
N
:
Z
ratios than stable nuclides in the same
A
range, which makes them radioactive (Box 10.1). The
β
-decay of fission products such as
90
Sr,
131
I and
137
Cs
(Figure 10.1.1) is the prime cause of the intense initial
radioactivity of reactor wastes.
8
8
Reactor wastes also give rise to longer-term radioactivity
which is due to
α
-emitting isotopes of actinide elements
like plutonium,
239
pu.
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