Chapter 24. -NUCLEAR POWER PLANTS 



REACTOR FUELS 



The form and composition of a reactor 

 fuel may vary both in design and in the fis- 

 sionable isotope used. Many commercial power 

 reactors use a solid fuel element fabricated 

 in plate form, with the fissionable material being 

 enriched uranium in combination with aluminum, 

 zirconium, or stainless steel. Fuel elements 

 may be arranged in thin sandwich layers, as 

 shown in figure 24-6. This construction provides 

 a relatively large heat transfer area between 

 the fuel elements and the reactor coolant. 



The outer cladding on the fuel elements 

 confines the fission fragments within the fuel 

 elements and serves as a heat transfer surface. 

 Cladding materials should be resistant to cor- 

 rosion, should be able to withstand high temper- 

 atures, and should have a small cross section 

 for neutron capture. Three common cladding 

 materials are aluminum, zirconium, and stain- 

 less steel. The fuel elements may be assembled 

 in groups, some of which may contain control 

 rods. Several groups of fuel elements placed 

 within a reactor vessel make up the reactor 

 core. It is not necessary that all fuel groups 

 within the reactor contain control rods. 



CONTROL RODS 



Control rods serve a dual purpose in a 

 reactor. They keep the neutron density (neutron 

 flux) constant within a critical reactor and they 

 provide a means of shutting down the reactor. 



The material for a control rod must have 

 a high capture cross section for neutrons and 



PLATE OF 

 CLADDING 

 MATERIAL 



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CLADDING 



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COOLANT 

 "CHANNEL 



■FUEL 



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Figure 24-6.— PWR fuel element. 



a low fission cross section. Three materials 

 suitable for control rod fabrication are cadmium, 

 boron, and hafnium. Hafnium is particularly 

 suitable for control rods because it has a 

 relatively high capture cross section and be- 

 cause several daughter products after neutron 

 capture are stable isotopes which also have 

 good capture cross sections. 



The control rods are withdrawn from the 

 reactor core until criticality is obtained; there- 

 after very little movement is required. It is 

 important to note at this point that after 

 criticality is reached, movement of control 

 rods does not control the power output of the 

 reactor; it controls only the temperature of 

 the reactor. 



Control rod drive mechanisms are so de- 

 signed that, should an emergency shutdown of 

 the reactor be required, the control rods may 

 be inserted in the core very rapidly. A shut- 

 down of this type is called a scram. 



MODERATORS 



A moderator is the material used to ther- 

 malize the neutrons in a reactor. As pre- 

 viously stated, neutrons are thermalized by 

 elastic collisions; therefore, a good moderator 

 must have a high scattering cross section and 

 a low absorption cross section to reduce the 

 speed of a neutron in a small number of col- 

 lisions. Nuclei whose mass is close to that of 

 a neutron are the most effective in slowing 

 the neutron; therefore, atoms of low atomic 

 weight generally make the best moderators. 

 Materials which have been used as moderators 

 include light and heavy water, graphite, and 

 beryllium. 



Ordinary light water makes a good moderator 

 since the cost is low; however it must be 

 free from impurities which may capture the 

 neutrons or add to the radiological hazards. 



REACTOR COOLANTS 



The primary purpose of a reactor coolant 

 is to absorb heat from the reactor. The coolant 

 may be either a gas or a liquid; it must pos- 

 sess good heat transfer properties, have good 

 thermal properties, be noncorrosive to the 

 system, be nonhazardous if exposed to radiation, 

 and be of low cost. Coolants which have been 

 used in operational and experimental reactors 

 include light and heavy water, liquid sodium, and 

 carbon dioxide. 



621 



