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 10⁷K.

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:

• 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 10¹⁴ 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.

Information provided by: http://library.thinkquest.org