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There are two ways to release nuclear energy: fission, breaking apart a large nucleus into two smaller parts, and fusion, combining two small nuclei to make a larger one. It will be convenient to use the common names for some of the light nuclei: the hydrogen isotope, 21H, is called the deuteron; 31H is called the triton. These are the names of the nuclei. If the nuclei have an electron around them, so that we have a hydrogen atom, the atoms are called deuterium and tritium, respectively. Deuterium is often called heavy hydrogen; and water in which one of the hydrogen atoms is deuterium is called heavy water. |
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| Fusion Reactions | The simplest fusion process one might imagine would be combining a proton and a neutron to form a deuteron:
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This is not practical as an energy source because free neutrons are not found in nature. One might also consider combining two protons:
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But as you can see from your
nuclear table, helium-2 does not exist. (That is to say, two protons do not hold together. A neutron and a proton do hold together, but two protons don't. Why? Also, bearing in mind the fact that there is an attractive nuclear force, think about the following question: Why is it that 3He holds together, but 2He does not?) Most fusion reactions are not really "complete" fusion, where two nuclei combine to one. Rather, you have a reaction in which you end up with some larger combinations than you had at the beginning. Two examples:
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These are called deuterium-deuterium (or D-D) reactions. When two deuterons come close together they may just bounce off each other, or they may undergo a change such as (3) or (4). There is a certain probability for each of these processes to occur. (3) and (4) can be viewed as transfer reactions. (What is transferred?)
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| Release of Energy | Notice that in order for these reactions to occur, the two deuterons must move fast toward each other to overcome their electrical repulsion (as in all, or most, nuclear reactions). There is a certain amount of energy in the two deuterons. But after the collision, as the two products move apart from each other, they have more energy than there was before. This is basically because of binding energy: There is less mass on the right side of these reactions than on the left side, and the mass difference has been converted to energy. |
| The Plasma The D-T Reaction | If we want energy production from a human-sized sample of hydrogen, we must raise the hydrogen to very high temperature (10 to 100 million degrees). Under those conditions, first of all, the electrons will be stripped away from the nuclei. We will have a system with no atoms, just electrically charged nuclei and free electrons. This is called a plasma. Nuclei and electrons will be moving very rapidly in random directions. If the plasma is hot enough, nuclei overcome the electrical barrier, undergo reactions (3) and (4), and produce more energy, increasing the plasma's temperature further. To create energy by fusion we have to first put in energy, and then hope that we will gain more than was put in at the start. (It's like using a match to start a wood fire. The burning wood produces much more heat energy than the match, but the match is needed to start the wood burning.) It turns out that if you start with 21H and 31H you don't need to go quite as high in temperature to initiate fusion. Therefore much of the interest in fusion centers on the deuterium-tritium (D-T) reaction:
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| Controlled Fusion | Since the invention of the hydrogen bomb, scientists have sought to carry out fusion reactions in a controlled way. The object is to gradually heat a system of hydrogen fuel until it reaches the point of fusion. Then the fusion reaction proceeds itself, heating the hydrogen further. This heat would then be used as in an ordinary power plant, to generate electricity. (Use of fusion as an energy source seems now to be pretty far in the future, so not much has been done to design a practical electric power plant.) The problem is that the very hot plasma cannot be kept in a container, because any solid container -- steel, for example -- would be vaporized at temperatures of millions of degrees. If the plasma is not contained, then as it is heated it will just dissipate, like cigarette smoke into a room. There are two approaches to containing the plasma: |
| (1) Magnetic Confinement | The trajectories of fast-moving electrically charged particles are bent in a magnetic field. Plasma physicists have devised very complex arrangements of magnets, so that the electrons and ions are kept within a finite volume; when a particle approaches the edge of the volume, it feels a magnetic force that turns it back into the volume. |
| (2) Inertial Confinement | In inertial confinement, a small pellet of solid hydrogen fuel is hit on all sides by many laser beams. This compresses the pellet, and heats it to a fusion temperature. The pellet is quickly vaporized and begins to dissipate, but it may stay together long enough so that fusion can generate additional heat. |
| Break-even | The object in each of these techniques is not to keep the plasma confined forever, but to keep it confined long enough so that fusion can generate more heat energy than was used to trigger the fusion. When the system can generate as much energy as is needed to start it, that will be the "break-even" point. Both methods of confinement have produced fusion; neither has quite reached break-even, but there has been steady progress over the past 20 years or so. Estimates are that fusion might be practical in 40 or 50 years. |
| Fuel | Although deuterons make up only .015% of all the hydrogen nuclei on earth, there is so much hydrogen (in the H2O of the oceans) that if we could produce energy by the D-D reactions, there would be enough hydrogen fuel to last thousands of years. The D-T reaction is easier, and much research has been done on it. Hydrogen bombs all use the D-T reaction. |
| Making tritium | The problem with the D-T process is that tritium is not found naturally on earth. It has to be manufactured in a nuclear laboratory. One method is to use the lithium isotope, 63Li, and the following reaction:
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| Here the neutron would have to be produced in some prior nuclear reaction. Lithium-6, however, is rare. So if we had to rely on 6Li to obtain tritium fuel, the D-T reaction would not be a very long-term solution to the energy problem. | |
| Helium from the moon | An alternative to Eq. (5) for generating energy via fusion is the following, |
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| starting with 3He, rather than 3H. The disadvantages of this reaction are (1) it takes considerably more starting energy to ignite fusion this way, and (2) 3He is a very small fraction of naturally found helium, as the Nuclear Table shows. Hence not much attention has been paid to this approach. However, one iconoclastic scientist (Harrison Schmitt, a former astronaut) has argued that we can mine 3He on the moon, where its concentration in the soil is considerably higher than on earth -- and has worked out some details about fusion energy along these lines. | |
| The outlook | Although fusion energy will not be available in the near future, it seems worthwhile to continue research into it, (1) because fuel is effectively unlimited, and (2) because there is almost no problem of radioactive waste. There are no fission fragments and no trans-uranic elements produced. The only radioactivity problem is that reaction (4) produces the triton, 3H. This nucleus is beta radioactive with a half-life of 12 years. One would have to guard against the release of this isotope to the environment, but only for a period of a few half-lives, less than a century. In contrast, nuclear fission produces plutonium which has a half-life of 24,000 years. |
| New research | Although the U.S. scaled down support for fusion research over the past few years, a recent (late 2003) report by the Secretary of Energy suggests that this trend may be changing. In an analysis of major science projects to be considered for support through the next two decades, the project given highest priority is American participation in an ongoing international research program aimed at developing a thermonuclear (fusion) reactor. |
KEY CONCEPTS
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