Quantum Physics

30.3 Nuclear Fusion 981followed by either hydrogen–helium fusion or helium–helium fusion:or11H 3 2He :42He e 32He 3 2He : 4 2He 2( 1 1H)The energy liberated is carried primarily by gamma rays, positrons, and neutrinos,as can be seen from the reactions. The gamma rays are soon absorbed by thedense gas, thus raising its temperature. The positrons combine with electrons toproduce gamma rays, which in turn are also absorbed by the gas within a few centimeters.The neutrinos, however, almost never interact with matter; hence, theyescape from the star, carrying about 2% of the energy generated with them. Theseenergy-liberating fusion reactions are called thermonuclear fusion reactions. Thehydrogen (fusion) bomb, first exploded in 1952, is an example of an uncontrolledthermonuclear fusion reaction.Fusion ReactorsThe enormous amount of energy released in fusion reactions suggests the possibilityof harnessing this energy for useful purposes on Earth. A great deal of effort isunder way to develop a sustained and controllable thermonuclear reactor—afusion power reactor. Controlled fusion is often called the ultimate energy sourcebecause of the availability of its fuel source: water. For example, if deuterium, theisotope of hydrogen consisting of a proton and a neutron, were used as the fuel,0.06 g of it could be extracted from 1 gal of water at a cost of about four cents.Such rates would make the fuel costs of even an inefficient reactor almost insignificant.An additional advantage of fusion reactors is that comparatively few radioactiveby-products are formed. As noted in Equation 30.3, the end product of thefusion of hydrogen nuclei is safe, nonradioactive helium. Unfortunately, a thermonuclearreactor that can deliver a net power output over a reasonable timeinterval is not yet a reality, and many problems must be solved before a successfuldevice is constructed.We have seen that the Sun’s energy is based, in part, on a set of reactions in whichordinary hydrogen is converted to helium. Unfortunately, the proton–protoninteraction is not suitable for use in a fusion reactor because the event requiresvery high pressures and densities. The process works in the Sun only because ofthe extremely high density of protons in the Sun’s interior. In fact, even at thedensities and temperatures that exist at the center of the Sun, the average protontakes 14 billion years to react!The fusion reactions that appear most promising in the construction of a fusionpower reactor involve deuterium (D) and tritium (T), which are isotopes of hydrogen.These reactions areAPPLICATIONFusion Reactorsand21D 2 1D : 3 2He 1 0nQ 3.27 MeV21 D 2 1 D : 3 1 T 1 1H Q 4.03 MeV21D 3 1T : 4 2He 1 0n Q 17.59 MeV[30.4]where the Q values refer to the amount of energy released per reaction. As notedearlier, deuterium is available in almost unlimited quantities from our lakes andoceans and is very inexpensive to extract. Tritium, however, is radioactive (T 1/2 12.3 yr) and undergoes beta decay to 3 He. For this reason, tritium doesn’t occurnaturally to any great extent and must be artificially produced.One of the major problems in obtaining energy from nuclear fusion is the factthat the Coulomb repulsion force between two charged nuclei must be overcomebefore they can fuse. The fundamental challenge is to give the two nuclei enoughkinetic energy to overcome this repulsive force. This can be accomplished by