Fusion Reactions

Many different nuclear fusion reactions occur in the sun and other stars, but only a few such reactions are of practical value for potential energy production on earth. These all involve forms (isotopes) of the element hydrogen. Three isotopes of hydrogen are known; they are hydrogen (H), deuterium (D), and tritium (T). The nuclei of all three isotopes contain one proton, which characterizes them as forms of the element hydrogen; in addition, the deuterium nucleus has one neutron and the tritium nucleus has two neutrons. In each case, the neutral atom has one electron outside the nucleus to balance the charge of the single proton.

The three isotopes of hydrogen.

To produce net power, fusion reactions must take place at high temperatures. The power production process which can occur at the lowest temperature and, hence, the most readily attainable fusion process on earth is the combination of a deuterium nucleus with one of tritium. The products are energetic helium-4 (He4) the common isotope of helium (which is also called an alpha particle), and a more highly energetic free neutron (n). The helium nucleus carries one-fifth of the total energy released and the neutron carries the remaining four fifths. Since nuclei carry positive charges, they normally repel one another. The higher the temperature, the faster the atoms or nuclei move. When they collide at these high speeds, they overcome the force of repulsion of the positive charges, and the nuclei fuse. In such collisions energy is released. The difficulty in producing fusion energy has been to develop a device which can heat the deuterium-tritium fuel to a sufficiently high temperature and then confine it for a long enough time so that more energy is released through fusion reactions than is used for heating.

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Updated: 12 November 2004 Send questions or comments to: Anthony R. DeMeo at ademeo@pppl.gov