NUCLEAR PHYSICS: FISSION & FUSION
(see Chapter 12)
Since you've asked almost no questions regarding this
area, very little will be covered.
You already know that the atomic nucleus consists of a
cluster of protons and neutrons, held together by a
powerful force called the strong nuclear force, which is
strong enough to overcome the natural tendency of the
protons (being like positive charges) to repel away from
each other.
Nuclear Fission
Though each chemical element has a unique atomic
number Z (the number of protons in the nucleus, e.g. 1 for
hydrogen, 6 for carbon, 8 for oxygen, etc), atoms of the
same element can exist with several different numbers of
neutrons. These slightly different atoms are called
isotopes. Some isotopes are very stable and take an
extremely long time to "decay" (or never "decay"), while
other isotopes are much less stable and can "decay" quite
quickly.
By the "decay" of an atom, we mean that components of
its nucleus can "fly apart". This can happen either
spontaneously (with nothing to trigger it), or as a result of
the nucleus being struck by another particle. When it
happens spontaneously, the isotope is said to be
radioactive, and any radiation (particles or waves) given
off as result of the decay is called radioactivity.
There are many well known examples of elements with
more than one isotope. In addition to its most common
2
form, hydrogen can occur as 1 H (also called "deuterium"
or "heavy hydrogen"), with one extra neutron.
The most stable isotope of carbon is
12
6
C , but a less stable
14
6
isotope, C (known simply as carbon 14) has proven very
useful in dating ancient organic artifacts, since it has a
half-life of 5,700 years. That is, half of the carbon 14 in
any sample decays spontaneously every 5,700 years, so a
sample of old wood fibre (say) with one-quarter of the
normal expected concentration of carbon 14 would be
about 2(5,700) = 11,400 years old.
The actual decay of carbon 14 results in one of the extra
neutrons (which can be thought of a proton and an
electron "glued" together) splitting apart, with the extra
proton remaining within the nucleus while the electron is
ejected from the nucleus at high speed. The result is
referred to as beta decay with the ejected high speed
electron called a beta particle. In the end, the beta decay
of carbon 14 can be expressed as
C 147N  e  ,
14
6
that is, the carbon 14 decay produces an atom of nitrogen!
The process by which an isotope of one element undergoes
radioactive decay, resulting in an isotope of a different
element, is called fission (a word which basically means
splitting apart). Thus, the above is an example of nuclear
fission.
At the "heavy" end of the periodic table there are more
and more radioactive isotopes. This shouldn't be
surprising since larger nuclei can be expected to be less
stable. Thus, nuclear fission reactions becomes more
common for heavier elements like uranium, thorium,
radon, etc.
238
92
Let's consider the example of uranium 238, or U . This
is the most stable isotope of uranium, whose half-life of 4.5
billion years just happens to be approximately the same as
the age of our solar system. This means that about half of
the uranium 238 present when the Earth was formed has
already "disappeared" after undergoing nuclear fission.
The entire process actually includes many individual
radioactive decays:
4
U  234
Th

90
2 He (HL = 4.5 billion years)
234
234

Th

Pa

e
90
91
238
92

Pa 234
U

e
92
(HL = 24 days, total for both)
234
230
4
U

Th

92
90
2 He (HL = 250 thousand years)
234
91
4
Th 226
Ra

88
2 He (HL = 75 thousand years)
226
222
4
Ra

Rn

88
86
2 He (HL = 1,600 years)
230
90
etc, ending with a stable isotope of lead
210
84
4
Po206
Pb

82
2 He (HL = 3.8 days in total)
As seen above, nuclear fission often results in the ejection
from the original nucleus of a helium nucleus, which is
also called an alpha particle. This type of decay is thus
called alpha decay. Alpha radiation is considerably less
dangerous than beta radiation. In other fission situations,
a third form of radiation, called gamma radiation, is
released in the form of high energy photons (x-rays or
gamma rays).
Alpha radiation is the least dangerous to living tissue and
can be blocked with a sheet of ordinary cardboard. Beta
radiation would require something like a sheet of
aluminum to absorb most of the high energy electrons.
Gamma radiation, the most dangerous by far, can
penetrate deep into tissue. The thick lead vest worn when
having dental x-rays is an example of the kind of
protection required in this case. All three forms of
radiation can kill, however, if the dosage is high enough.
The so-called nuclear energy produced within a nuclear
reactor is simply the result of a nuclear "chain reaction",
where the ejected alpha particles from many nuclear
fissions (e.g. of uranium) strike other nearby uranium
nuclei and cause them to undergo further identical
fissions in turn. The rate of this chain reaction within a
nuclear reactor must be controlled by inserting other
materials (control rods) which will absorb most of the
ejected alpha particles and prevent the rate of fission from
accelerating.
If the reaction gets out of control, the result can be a
"nuclear meltdown" such as that which happened at
Chernobyl, Russia. So much heat is generated by the
runaway chain reaction, that the entire reactor literally
"melts" and sinks into the ground, releasing huge
amounts of radioactive steam and dust into the
atmosphere. As long as the chain reaction is properly
controlled, there is no significant danger associated with
living near a nuclear reactor. However, the by-products
are still radioactive, and the problem of how to safely
dispose of or store "nuclear waste" is an issue of
considerable controversy.
In fact, the Earth itself can be thought of as a gigantic
nuclear reactor! This is because the release of energy
from the long chain of decay reactions which turn
uranium 238 into lead, as outlined above, is responsible
for most of the heat which continues to keep the core of
the Earth in a hot liquid (or "molten") state.
Nuclear Fusion
If fission ultimately causes heavy elements (like uranium
and thorium) to decay into less heavy elements (like lead),
with the release of large amounts of energy, fusion is like a
"reverse" process, in which the nuclei of light elements
(like hydrogen and helium) are "smashed together" to
produce heavier elements (like carbon), again with the
release of energy. A major difference is that fusion
requires tremendous amounts of energy to occur in the
first place, whereas we have already seen that fission can
commence spontaneously.
Nuclear fusion can start naturally when matter is in the
form of a plasma, which in turn requires temperatures of
the order of 10 million degrees. This condition exists in
the cores of stars, and fusion is thus the energy source
ultimately responsible for the fact that stars "shine" and
for the outward pressure that balances inward gravity
and keeps stars in perfect equilibrium.
Theoretically, plasmas (and hence fusion reactions) can be
produced here on Earth, but the cost of creating suitable
and safe environments where this can be accomplished
has so far proven to be too high to make it feasible and
cost effective for energy generation.
In a plasma, electrons are stripped from the atoms, but
the nuclei remain intact and, at such high temperatures,
they move extremely fast. With so much kinetic energy,
they can actually "fuse" together when they collide.
To illustrate, hydrogen can fuse into helium in a stellar
core in the following steps (there are other possible
combinations of steps, as well):
H 11H 12H  e 
1
1
2
1
3
2
H 11H 23He  
He  23He 24He  211H
Likewise, helium can fuse into carbon in several ways,
including:
He  24He48Be  
Be  24He 126C   ,
4
2
8
4
with still heavier elements being created by repeated
further fusion with helium:
C  24He 168O  
12
6
20
O  24He 10
Ne  
16
8
20
10
24
12
24
Ne 24He 12
Mg  
28
Mg  24He14
Si   , etc.
The fusion of helium into heavier elements requires
temperatures about 10 times hotter than fusion of
hydrogen, or about 100 million degrees.
Eventually, when fusion begins to produce iron, energy is
absorbed instead of released.