Quantum Physics

980 Chapter 30 Nuclear Energy and Elementary Particlestests on all containers used to transport nuclear materials. Container manufacturersmust demonstrate that their containers will not rupture, even in high-speedcollisions.The safety issues associated with nuclear power reactors are complex and oftenemotional. All sources of energy have associated risks. Coal, for example, exposesworkers to health hazards (including radioactive radon) and produces atmosphericpollution (including greenhouse gases). In each case, the risks must beweighed against the benefits and the availability of the energy source.This photograph of the Sun, taken onDecember 19, 1973, during the thirdand final manned Skylab mission,shows one of the most spectacularsolar flares ever recorded, spanningmore than 588 000 km (365 000 mi)across the solar surface. Several activeregions can be seen on the easternside of the disk. The photograph wastaken in the light of ionized heliumby the extreme ultraviolet spectroheliographinstrument of the U.S.Naval Research Laboratory.NASA30.3 NUCLEAR FUSIONFigure 29.4 shows that the binding energy of light nuclei (those having a massnumber lower than 20) is much smaller than the binding energy of heavier nuclei.This suggests a process that is the reverse of fission. When two light nuclei combineto form a heavier nucleus, the process is called nuclear fusion. Because the massof the final nucleus is less than the masses of the original nuclei, there is a loss ofmass, accompanied by a release of energy. Although fusion power plants have notyet been developed, a worldwide effort is under way to harness the energy fromfusion reactions in the laboratory.Fusion in the SunAll stars generate their energy through fusion processes. About 90% of stars,including the Sun, fuse hydrogen, whereas some older stars fuse helium or otherheavier elements. Stars are born in regions of space containing vast clouds of dustand gas. Recent mathematical models of these clouds indicate that star formationis triggered by shock waves passing through a cloud. These shock waves are similarto sonic booms and are produced by events such as the explosion of a nearby star,called a supernova. The shock wave compresses certain regions of the cloud, causingthem to collapse under their own gravity. As the gas falls inward toward thecenter, the atoms gain speed, which causes the temperature of the gas to rise. Twoconditions must be met before fusion reactions in the star can sustain its energyneeds: (1) The temperature must be high enough (about 10 7 K for hydrogen) toallow the kinetic energy of the positively charged hydrogen nuclei to overcometheir mutual Coulomb repulsion as they collide, and (2) the density of nuclei mustbe high enough to ensure a high rate of collision.It’s interesting that temperatures inside stars like the Sun are not sufficient toallow colliding protons to overcome Coulomb repulsion. In a certain percentageof collisions, the nuclei pass through the barrier anyway, an example of quantumtunneling. So a quantum effect is key in making sunshine.When fusion reactions occur at the core of a star, the energy that is liberatedeventually becomes sufficient to prevent further collapse of the star under its owngravity. The star then continues to live out the remainder of its life under a balancebetween the inward force of gravity pulling it toward collapse and the outwardforce due to thermal effects and radiation pressure.The proton–proton cycle is a series of three nuclear reactions that are believedto be the stages in the liberation of energy in the Sun and other stars rich inhydrogen. An overall view of the proton–proton cycle is that four protonscombine to form an alpha particle and two positrons, with the release of 25 MeVof energy in the process.The specific steps in the proton–proton cycle are11 H 1 1 H : 2 1 D e and11 H 2 1 D : 3 2 He [30.3]where D stands for deuterium, the isotope of hydrogen having one proton and one2neutron in the nucleus. (It can also be written as .) The second reaction is1 H