PRINCIPLES OF NAVAL ENGINEERING 



REFLECTORS 



In a reactor of finite size, the leakage of 

 neutrons from the core becomes somewhat of 

 a problem. To minimize the leakage, a re- 

 flector is used to assist in keeping the neu- 

 trons in the reactor. The use of a reflector 

 reduces both the required size of the reactor 

 and the radiation hazards of escaping neutrons. 

 The characteristics required for a reflector 

 are essentially the same as those required for 

 a moderator. 



Since ordinary water of high purity is suit- 

 able for moderators, coolants, and reflectors, 

 the inference is that it could serve all three 

 functions in the same reactor. This is indeed 

 the case in many nuclear reactors. 



SHIELDING 



The shielding of a nuclear reactor serves 

 the dual purpose of (1) reducing the radiation 

 so that it will not interfere with the necessary 

 instrumentation, and (2) protecting operating 

 personnel from radiation. 



The type of shielding material used is 

 dependent upon the purpose of the particular 

 reactor and upon the nature of the radioactive 

 particles being attenuated or absorbed. 



The shielding against alpha particles is 

 a relatively simple matter. Since an alpha 

 particle has a positive electrical charge of 2, 

 a few centimeters of air is all that is required 

 for attenuation. Any light material such as 

 aluminum or plastics makes a suitable shield 

 for beta particles. 



Neutrons and gamma rays have considerable 

 penetrating power; therefore, shielding against 

 them is more difficult. Since neutrons are best 

 attenuated by elastic collisions, any hydrogenous 

 material such as polyethylene or water is suit- 

 able as a neutron shield. Sometimes polyethylene 

 with boron is used for neutron shields, as 

 boron has a high neutron capture cross section. 

 Gamma rays are best attenuated by a dense 

 material such as lead. 



NUCLEAR REACTORS 



The purpose of any power reactor is to 

 provide thermal energy which can be con- 

 verted to useful work. Several types of experi- 

 mental and operational reactors have been 

 designed. They include the Pressurized Water 

 Reactor (PWR), the Sodium Cooled Reactor, 

 the Experimental Boiling Water Reactor, the 



Experimental Breeder Reactor, and the Ex- 

 perimental Gas Cooled Reactor. 



The first full-scale nuclear-powered central 

 station in the United States was the Pressurized 

 Water Reactor (PWR) at Shippingport, Pennsyl- 

 vania. The Shippingport PWR is a thermal, 

 heterogeneous reactor fueled with enriched 

 uranium-235 "seed assemblies" arranged 

 in a square in the center of the core, surrounded 

 by "blanket assemblies" of uranium- 238 fuel 

 elements. Figure 24-7 shows a cross-sectional 

 view of the PWR reactor and core. This type 

 of reactor can be called a converter , since the 

 uranium-238 is converted into the fissionable 

 fuel of plutonium-239. 



A schematic diagram of PWR and its as- 

 sociated steam plant with power output and 

 flow ratings is shown in figure 24-8. The 

 reactor plant consists of a single reactor 

 with four main coolant loops; the plant is 

 capable of maintaining full power on three 

 loops. Each coolant loop contains a steam 

 generator, a pump, and associated piping. 



High purity water at a pressure of 2000 

 psia serves as both moderator and coolant 

 for the plant. At full power the inlet water 

 temperature to the reactor is 508° F and the 

 outlet temperature is 542° F. 



The coolant enters the bottom of the reactor 

 vessel (fig. 24-9) where 90 percent of the water 



PLACE FOR 



EXTRA 8LANKET 



ASSEMBLY 



CONTROL ROD< 



SEED ASSEMBLY 



(FOUR SUB 

 ASSEMBLIES) 



THERMAL 

 SHIELD 



147.157X 

 Figure 24-7.— Cross-sectional view of 

 PWR reactor and core. 



622 



