Principles of naval engineering - Historic Naval Ships Association

Chapter 24. -NUCLEAR POWER PLANTSREACTOR FUELSThe form and composition of a reactorfuel may vary both in design and in the fissionableisotope used. Many commercial powerreactors use a solid fuel element fabricatedin plate form, with the fissionable material beingenriched uranium in combination with aluminum,zirconium, or stainless steel. Fuel elementsmay be arranged in thin sandwich layers, asshown in figure 24-6. This construction providesa relatively large heat transfer area betweenthe fuel elements and the reactor coolant.The outer cladding on the fuel elementsconfines the fission fragments within the fuelelements and serves as a heat transfer surface.Cladding materials should be resistant to corrosion,should be able to withstand high temperatures,and should have a small cross sectionfor neutron capture. Three common claddingmaterials are aluminum, zirconium, and stainlesssteel. The fuel elements may be assembledin groups, some of which may contain controlrods. Several groups of fuel elements placedwithin a reactor vessel make up the reactorcore. It is not necessary that all fuel groupswithin the reactor contain control rods.CONTROL RODSControl rods serve a dual purpose in areactor. They keep the neutron density (neutronflux) constant within a critical reactor and theyprovide a means of shutting down the reactor.The material for a control rod must havea high capture cross section for neutrons andPLATE OFCLADDINGMATERIALvi '. '?;,,-• v'.,iv >'--",: '-'.v-vjv •-'-- 'i-l-v:j-?^ -j•ri ''7~V-^\/ilt'-:-7y^ir&:^?Al^jiiLLCii_j_i.lCl.l,;ilficiZ^lj^:lJi._li:^II^l:^FUELCLADDINGCOOLANT"CHANNEL147.156Figure 24-6.—PWR fuel element.a low fission cross section. Three materialssuitable for control rod fabrication are cadmium,boron, and hafnium. Hafnium is particularlysuitable for control rods because it has arelatively high capture cross section and becauseseveral daughter products after neutroncapture are stable isotopes which also havegood capture cross sections.The control rods are withdrawn from thereactor core until criticality is obtained; thereaftervery little movement is required. It isimportant to note at this point that aftercriticality is reached, movement of controlrods does not control the power output of thereactor; it controls only the temperature ofthe reactor.Control rod drive mechanisms are so designedthat, should an emergency shutdown ofthe reactor be required, the control rods maybe inserted in the core very rapidly. A shutdownof this type is called a scram.MODERATORSA moderator is the material used to thermalizethe neutrons in a reactor. As previouslystated, neutrons are thermalized byelastic collisions; therefore, a good moderatormust have a high scattering cross section anda low absorption cross section to reduce thespeed of a neutron in a small number of collisions.Nuclei whose mass is close to that ofa neutron are the most effective in slowingthe neutron; therefore, atoms of low atomicweight generally make the best moderators.Materials which have been used as moderatorsinclude light and heavy water, graphite, andberyllium.Ordinary light water makes a good moderatorsince the cost is low; however it must befree from impurities which may capture theneutrons or add to the radiological hazards.REACTOR COOLANTSThe primary purpose of a reactor coolantis to absorb heat from the reactor. The coolantmay be either a gas or a liquid; it must possessgood heat transfer properties, have goodthermal properties, be noncorrosive to thesystem, be nonhazardous if exposed to radiation,and be of low cost. Coolants which have beenused in operational and experimental reactorsinclude light and heavy water, liquid sodium, andcarbon dioxide.621