In addition to fission, a second possible method for obtaining energyA system's capacity to do work. from nuclear reactions lies in the fusing together of two light nuclei to form a heavier nucleus. As we see when discussing Figure 1 from Mass-Energy Relationships, such a process results in nucleons which are more firmly bonded to each other and hence lower in potential energy. This is particularly true if is formed, because this nucleus is very stable. Such a reaction occurs between the nuclei of the two heavy isotopes of hydrogen, deuteriumThe isotope of hydrogen having one neutron in its nucleus. and tritiumThe isotope of hydrogen that has two neutrons in its nucleus.:
For this reaction Δm = – 0.018 88 g mol–1 so that ΔHm = – 1700 GJ mol–1. Although very large quantities of energy are released by a reaction like Eq. (1) such a reaction is very difficult to achieve in practice. This is because of the very high activation energyThe energy barrier over which a reaction must progress in order for reactants to form products; the minimum energy that reactants must have if they are to be converted to products., about 30 GJ mol–1, which must be overcome to bring the nuclei close enough to fuse together. This barrier is created by coulombic repulsion between the positively charged nuclei. The only place where scientists have succeeded in producing fusion reactions on a large scale is in a hydrogen bomb. Here the necessary activation energy is achieved by exploding a fission bomb to heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. the reactants to a temperatureA physical property that indicates whether one object can transfer thermal energy to another object. of about 108 K. Attempts to carry out fusion in a more controlled way have met with only limited success. At the very high temperatures required, all molecules dissociate and most atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. ionize. A new state of matterAnything that occupies space and has mass; contrasted with energy. called a plasma is formed. It is neither solid, liquid, or gas and behaves much like the universal solventThe substance to which a solute is added to make a solution. of the alchemists by converting any solid material which it contacts into vaporThe gaseous state of a substance that typically exists as a liquid or solid; a gas at a temperature near or below the boiling point of the corresponding liquid..
Two techniques for producing a controlled fusion reaction are currently being explored. The first is to restrict the plasma by means of a strong magnetic field rather than the walls of a container. This has met with some success but has not yet been able to contain a plasma long enough for usable energy to be obtained. The second technique involves the sudden compression and heating of pellets of deuterium and tritium by means of a sharply focused laser beam. Again, only a limited success has been obtained.
Though these attempts at a controlled fusion reaction have so far been only partially successful, they are nevertheless worth pursuing. Because of the much readier availability of lighter isotopes necessary for fusion as opposed to the much rarer heavier isotopes required for fission, controlled nuclear fusion would offer the human race an essentially limitless supply of energy. There would still be some environmental difficulties with the production of isotopes such as tritium, but these would be nowhere near the seriousness of the problem caused by the production of the witches brew of radioactiveDescribes a substance that gives off radiation‐alpha particles, beta particles, or gamma rays‐by the disintegration of its nucleus. isotopes in a fission reactor. It must be confessed, though, that at the present rate of progress, the prospect of limitless clean energy from fusion seems unlikely in the next decade or two.