In addition to fission , a second possible method for obtaining energy 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 \({}_{\text{2}}^{\text{4}}\text{He}\) is formed, because this nucleus is very stable. Such a reaction occurs between the nuclei of the two heavy isotopes of hydrogen, deuterium and tritium: \[{}_{\text{1}}^{\text{2}}\text{D + }{}_{\text{1}}^{\text{3}}\text{T }\to \text{ }{}_{\text{2}}^{\text{4}}\text{He + }{}_{\text{0}}^{\text{1}}n \label{1} \] For this reaction, Δ m = – 0.018 88 g mol –1 so that Δ H m = – 1700 GJ mol –1 . Although very large quantities of energy are released by a reaction like Equation \(\ref{1}\), such a reaction is very difficult to achieve in practice. This is because of the very high activation energy, 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 heat the reactants to a temperature of about 10 8 K. Attempts to carry out fusion in a more controlled way have met only limited success. At the very high temperatures required, all molecules dissociate and most atoms ionize. A new state of matter called a plasma is formed. It is neither solid, liquid, nor gas. Plasma behaves much like the universal solvent of the alchemists by converting any solid material that it contacts into vapor. 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 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 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 radioactive 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.