Quantum Physics

978 Chapter 30 Nuclear Energy and Elementary Particlesneutrons. In order for the chain reaction to continue, therefore, the neutronsmust be slowed down. This is accomplished by surrounding the fuel with a substancecalled a moderator.In order to understand how neutrons are slowed down, consider a collisionbetween a light object and a massive one. In such an event, the light objectrebounds from the collision with most of its original kinetic energy. However, ifthe collision is between objects having masses that are nearly the same, the incomingprojectile transfers a large percentage of its kinetic energy to the target. In thefirst nuclear reactor ever constructed, Fermi placed bricks of graphite (carbon)between the fuel elements. Carbon nuclei are about 12 times more massive thanneutrons, but after about 100 collisions with carbon nuclei, a neutron is slowedsufficiently to increase its likelihood of fission with 235 U. In this design the carbonis the moderator; most modern reactors use heavy water (D 2 O) as the moderator.Neutron CaptureIn the process of being slowed down, neutrons may be captured by nuclei that donot undergo fission. The most common event of this type is neutron capture by 238 U.The probability of neutron capture by 238 U is very high when the neutrons have highkinetic energies and very low when they have low kinetic energies. The slowing downof the neutrons by the moderator serves the dual purpose of making them availablefor reaction with 235 U and decreasing their chances of being captured by 238 U.APPLICATIONNuclear Reactor DesignControl of Power LevelIt is possible for a reactor to reach the critical stage (K 1) after all neutron lossesdescribed previously are minimized. However, a method of control is needed to adjustK to a value near one. If K were to rise above this value, the heat produced inthe runaway reaction would melt the reactor. To control the power level, controlrods are inserted into the reactor core. (See Fig. 30.3.) These rods are made of materialssuch as cadmium that are highly efficient in absorbing neutrons. By adjustingthe number and position of the control rods in the reactor core, the K value can bevaried and any power level within the design range of the reactor can be achieved.A diagram of a pressurized-water reactor is shown in Figure 30.4. This type ofreactor is commonly used in electric power plants in the United States. FissionSecondary loopSteam turbineand electric generatorControl rod+–Uraniumfuel rodNuclearreactorSteamCondenser (steam from turbineis condensed by cold water)HeatexchangerMolten sodiumor liquid waterunder highpressure (carriesenergy to steamgenerator)PumpFigure 30.4PrimaryloopCold waterMain components of a pressurized-water nuclear reactor.Warm water