The Physics behind Nuclear Fusion
The binding energy curve shows that energy can be released if two light nuclei combine to form a single larger
nucleus. This process is called nuclear fusion. The process is hindered by the electrical repulsion that acts to
prevent the two particles from getting close enough to each other to be within range and "fusing."
To generate useful amounts of power, nuclear fusion must occur in bulk matter. That is, many atoms need to fuse
in order create a significant amount of energy. The best hope for bringing this about is to raise the temperature of
the material so that the particles have enough energy--due to their thermal motions alone--to penetrate the
electrical repulsion barrier. This process is known as thermonuclear fusion. Calculations show that these
temperatures need to be close to the sun's temperature of 1.5 X 10 7K.
Thermonuclear Fusion in the Sun and other Stars
The sun radiates energy at the rate of 3.9 X 1026 W (watts) and has been doing so for several billion years. The sun
burns hydrogen in a "nuclear furnace." The fusion reaction in the sun is a multistep process in which hydrogen is
burned into helium, hydrogen being the "fuel" and helium the "ashes." The figure below shows the cycle.
Fusion cycle of the Sun
The cycle starts with the thermal collision of two protons ( 1H + 1H) to form a deuteron (2H), with the simultaneous
creation of a positron (e+) and a neutrino (v). The positron very quickly encounters a free electron (e -) in the sun
and both particles annihilate, their mass energy appearing as two gamma-ray photons. Once the deuteron has
been produced, it quickly collides with another proton and forms a 3He nucleus and a gamma ray. Two such 3He
nuclei may eventually (within ten thousand years) find each other, as the bottom row shows.
Overall, this amounts to the combination of four protons and two electrons to form an alpha particle (4He), two
neutrinos, and six gamma rays. Thus, the overall equation is
.
The energy release in this reaction is
where 1.007825u is the mass of a hydrogen atom and 4.002603u is the mass of a helium atom; neutrinos and
gamma-ray photons have no mass and thus do not enter into the calculation of disintegration energy.
The burning of hydrogen in the sun's core is alchemy on a grand scale in the sense that one element is turned into
another. The medieval alchemists, however, were more interested in changing lead into gold than in changing
hydrogen into helium! Hydrogen burning has been going on in the sun for about 5 billion years and calculations
show that there is enough hydrogen left to keep the sun going for about the same length of time into the future.
If the core temperature increases to about 108K then energy can be produced by burning helium to make carbon.
As a star evolves and becomes still hotter, other elements can be formed by other fusion reactions. However,
elements more massive than those with atomic number equal to 56 (iron) cannot be manufactured by further fusion
processes as atomic number equal to 56 makes the peak of the binding energy curve. If nuclides were to fuse after
that, then energy would be consumed as opposed to produced.
Fusion here on Earth
The first thermonuclear fusion reactions to take place on Earth occurred at Eniwetok Atoll on October 31, 1952,
when the United States exploded a fusion device, generating an energy release equivalent to 10 million tons of
TNT. The high temperatures needed to initiate the reaction were triggered by a fission bomb.
A sustained and controllable source of fusion power, a fusion reactor, is considerably harder to achieve. The goal,
however, is being pursued vigorously in many countries around the world because many look to the fusion reactor
as the power source of the future, at least as far as the generation of electricity is concerned. The scheme for
fusion on the sun is not suitable for an Earth-bound fusion reactor because the scheme is hopelessly slow. The
reaction succeeds in the sun only because of the enormous density of protons in the center of the sun.
The three requirements for a successful thermonuclear reactor are:
o
o
o
A High Particle Density The density of interacting particles must be great enough to ensure that
the collision rate is high enough.
A High Plasma Temperature The plasma must be hot. Otherwise the colliding particles will not be
energetic enough to penetrate the electrical barrier that tends to keep them apart.
A Long Confinement Time A major problem is containing the hot plasma long enough to ensure
that its density and temperature remain sufficiently high for enough of the fuel to be fused. It is
clear that no solid container can withstand the high temperatures that are necessary, so clever
confining techniques are called for.
Possible Implementation on Earth
The Tokamak
Tokamak is a type of thermonuclear fusion device first developed in the USSR. Large tokamaks have been built
and operated in several countries, and several major new machines are in the design stage.
In a tokamak, the charged particles that make up the hot plasma are confined by a magnetic field in the shape of a
doughnut. The magnetic forces acting on the moving charges of the plasma keep the hot plasma from touching the
walls of the chamber. The current that generates the field is induced in the plasma itself, and it serves also to heat
the plasma.
However, the abilty for self-sustaining the thermonuclear reaction still hasn't been achieved. In spite of the rapid
progress being made at present, many formidable engineering problems remain, and a practical thermonuclear
power plant does not seem possible before the early decades of the next century.
Laser Fusion
A second technique for confining the plasma is called inertial confinement. It involves compressing a fuel pellet by
"zapping" it from all sides by laser beams (or particle beams), thus compressing it and increasing its temperature
and particle density so that thermonuclear fusion can occur. By comparison with devices such as the tokamak,
inertial confinement invovles working with much higher particle densities for much shorter times.
Laser fusion is being investigated in many laboratories in the United States and elsewhere. At the Lawrence
Livermore Laboratory, the laser pulses are designed to deliver, in total, some 200 kJ of energy to each fuel pellet in
less than a nanosecond. This is a delivered power of about 2 X 10 14 W during the pulse, which is roughly 100 times
the total sustained electric power generating capacity of the world! The feasibility of laser fusion as the basis of a
thermonuclear power reactor has not been demonstrated as of yet, but research is continuing at a vigorous pace.