Environmental Problems, Their Causes, and Sustainability 1

nn23592 UUranium-235nucleus9236Kr14156 BanUnstablenucleusnn23592 U23592 U9236Fission fragmentKr14156 BannnnFission fragment14156 Ba92369236Kr14156 BanKrnnEnergyFigure 3-14 Fission of a uranium-235 nucleus by a neutron (n).23592 Unnnn23592 U23592 U23592 Untogether at extremely high temperatures until theyfuse to form a heavier nucleus. Lots of energy is releasedwhen this happens. Temperatures of at least100 million °C are needed to force the positivelycharged nuclei (which strongly repel one another)to fuse.Nuclear fusion is much more difficult to initiatethan nuclear fission, but once started it releases farmore energy per unit of fuel than does fission. Youwould not be alive without nuclear fusion. Fusion ofhydrogen nuclei to form helium nuclei is the source ofenergy in the sun and other stars.After World War II, the principle of uncontrollednuclear fusion was used to develop extremely powerfulhydrogen, or thermonuclear, weapons. These weaponsuse the D–T fusion reaction, in which a hydrogen-2, ordeuterium (D), nucleus and a hydrogen-3 (tritium, T)nucleus are fused to form a larger, helium-4 nucleus, aneutron, and energy, as shown in Figure 3-16.Scientists have also tried to develop controlled nuclearfusion, in which the D–T reaction is used to pro-FuelD–T Fusion+Hydrogen-2 ordeuterium nucleusReaction Conditions++ProductsNeutronEnergyFigure 3-15 A nuclear chain reaction initiated by one neutrontriggering fission in a single uranium-235 nucleus. This figureillustrates only a few of the trillions of fissions caused when asingle uranium-235 nucleus is split within a critical mass ofuranium-235 nuclei. The elements krypton (Kr) and barium(Ba), shown here as fission fragments, are only two of manypossibilities.controlled so that under normal operation only one ofevery two or three neutrons released is used to splitanother nucleus. In conventional nuclear fission reactors,the splitting of uranium-235 nuclei releases heat,which produces high-pressure steam to spin turbinesand thus generate electricity.+Hydrogen-3 ortritium nucleusD–D Fusion+Hydrogen-2 ordeuterium nucleus+100 million °C+++ +Helium-4nucleus++Helium-3nucleusEnergyWhat Is Nuclear Fusion? Forcing LightNuclei to CombineExtremely high temperatures can force thenuclei of isotopes of some lightweight atoms tofuse together and release large amounts ofenergy.Nuclear fusion is a nuclear change in which two isotopesof light elements, such as hydrogen, are forcedHydrogen-2 ordeuterium nucleus+Proton1 billion °CNeutronNeutronFigure 3-16 The deuterium–tritium (D–T) and deuterium–deuterium (D–D) nuclear fusion reactions, which take place atextremely high temperatures.50 CHAPTER 3 Science, Systems, Matter, and Energy