Fusion Reactions
Equation (1.2) is an example of a charged particle reaction, since both nuclei on the left have atomic numbers greater than zero. Such reactions are difficult to bring about, for after the orbiting electrons are stripped from the nuclei, the positive charges on the nuclei strongly repel one another. Thus to bring about a reaction such as Eq. (1.2), the nuclei must collide at high speed in order to overpower the coulomb repulsion and make contact. The most common methods for achieving such reactions on earth consist of using particle accelerations to impart a great deal of kinetic energy to one of the particles and then slam it into a target made of the second material. An alternative is to mix the two species and bring them to a very high temperature, where they become a plasma. Since the average kinetic energy of a nucleus is proportional to its absolute temperature, if high enough temperatures are reached the electrical repulsion of the nuclei is overpowered by the kinetic energy, and a thermonuclear reaction results.
Two reactions based on fusing isotopes of hydrogen have been widely considered as a basis for energy production, deuterium– deuterium and deuterium–tritium:
The difficulty is that these are charged particle reactions. Thus for the
nuclei to interact the particlesmust be brought together with very high
kinetic energies in order to overcome the coulomb repulsion of the
positively charged nuclei. As a practical matter, this cannot be accomplished
using a particle accelerator, for the accelerator would use much
more energy than would be produced by the reaction. Rather, means
must be found to achieve temperatures comparable to those found in
the interior of the sun. For then the particles’ heightened kinetic energy
would overcome the coulomb barrier and thermonuclear reactions
would result. While thermonuclear reactions are commonplace in the
interior of stars, on earth the necessary temperatures have been
obtained to date only in thermonuclear explosions and not in the controlled
manner that would be needed for sustained power production.
Long-term efforts continue to achieve controlled temperatures
high enough to obtain power from fusion reactions. Investigators
place most emphasis on the D-T reaction because it becomes feasible
at lower temperatures than the D-D reaction. The D-T reaction,
however, has the disadvantage that most of the energy release
appears as the kinetic energy of 14-MeV neutrons, which damage
whatever material they impact and cause it to become radioactive.
We will not consider fusion energy further here. Rather, we will
proceed to fission reactions, in which energy is released by splitting a
heavy nucleus into two lighter ones that have greater binding energies
per nucleon. Neutrons may initiate fission. Thus there is no
requirement for high temperatures, since there is no electrical repulsion
between the neutron and the nucleus. Figuratively speaking, the
neutron may slide into the nucleus without coulomb resistance.
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Two reactions based on fusing isotopes of hydrogen have been widely considered as a basis for energy production, deuterium– deuterium and deuterium–tritium:
The difficulty is that these are charged particle reactions. Thus for the
nuclei to interact the particlesmust be brought together with very high
kinetic energies in order to overcome the coulomb repulsion of the
positively charged nuclei. As a practical matter, this cannot be accomplished
using a particle accelerator, for the accelerator would use much
more energy than would be produced by the reaction. Rather, means
must be found to achieve temperatures comparable to those found in
the interior of the sun. For then the particles’ heightened kinetic energy
would overcome the coulomb barrier and thermonuclear reactions
would result. While thermonuclear reactions are commonplace in the
interior of stars, on earth the necessary temperatures have been
obtained to date only in thermonuclear explosions and not in the controlled
manner that would be needed for sustained power production.
Long-term efforts continue to achieve controlled temperatures
high enough to obtain power from fusion reactions. Investigators
place most emphasis on the D-T reaction because it becomes feasible
at lower temperatures than the D-D reaction. The D-T reaction,
however, has the disadvantage that most of the energy release
appears as the kinetic energy of 14-MeV neutrons, which damage
whatever material they impact and cause it to become radioactive.
We will not consider fusion energy further here. Rather, we will
proceed to fission reactions, in which energy is released by splitting a
heavy nucleus into two lighter ones that have greater binding energies
per nucleon. Neutrons may initiate fission. Thus there is no
requirement for high temperatures, since there is no electrical repulsion
between the neutron and the nucleus. Figuratively speaking, the
neutron may slide into the nucleus without coulomb resistance.
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