Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGREFLECTORSIn a reactor of finite size, the leakage ofneutrons from the core becomes somewhat ofa problem. To minimize the leakage, a reflectoris used to assist in keeping the neutronsin the reactor. The use of a reflectorreduces both the required size of the reactorand the radiation hazards of escaping neutrons.The characteristics required for a reflectorare essentially the same as those required fora moderator.Since ordinary water of high purity is suitablefor moderators, coolants, and reflectors,the inference is that it could serve all threefunctions in the same reactor. This is indeedthe case in many nuclear reactors.SHIELDINGThe shielding of a nuclear reactor servesthe dual purpose of (1) reducing the radiationso that it will not interfere with the necessaryinstrumentation, and (2) protecting operatingpersonnel from radiation.The type of shielding material used isdependent upon the purpose of the particularreactor and upon the nature of the radioactiveparticles being attenuated or absorbed.The shielding against alpha particles isa relatively simple matter. Since an alphaparticle has a positive electrical charge of 2,a few centimeters of air is all that is requiredfor attenuation. Any light material such asaluminum or plastics makes a suitable shieldfor beta particles.Neutrons and gamma rays have considerablepenetrating power; therefore, shielding againstthem is more difficult. Since neutrons are bestattenuated by elastic collisions, any hydrogenousmaterial such as polyethylene or water is suitableas a neutron shield. Sometimes polyethylenewith boron is used for neutron shields, asboron has a high neutron capture cross section.Gamma rays are best attenuated by a densematerial such as lead.NUCLEAR REACTORSThe purpose of any power reactor is toprovide thermal energy which can be convertedto useful work. Several types of experimentaland operational reactors have beendesigned. They include the Pressurized WaterReactor (PWR), the Sodium Cooled Reactor,the Experimental Boiling Water Reactor, theExperimental Breeder Reactor, and the ExperimentalGas Cooled Reactor.The first full-scale nuclear-powered centralstation in the United States was the PressurizedWater Reactor (PWR) at Shippingport, Pennsylvania.The Shippingport PWR is a thermal,heterogeneous reactor fueled with enricheduranium-235 "seed assemblies" arrangedin a square in the center of the core, surroundedby "blanket assemblies" of uranium- 238 fuelelements. Figure 24-7 shows a cross-sectionalview of the PWR reactor and core. This typeof reactor can be called a converter , since theuranium-238 is converted into the fissionablefuel of plutonium-239.A schematic diagram of PWR and its associatedsteam plant with power output andflow ratings is shown in figure 24-8. Thereactor plant consists of a single reactorwith four main coolant loops; the plant iscapable of maintaining full power on threeloops. Each coolant loop contains a steamgenerator, a pump, and associated piping.High purity water at a pressure of 2000psia serves as both moderator and coolantfor the plant. At full power the inlet watertemperature to the reactor is 508° F and theoutlet temperature is 542° F.The coolant enters the bottom of the reactorvessel (fig. 24-9) where 90 percent of the waterPLACE FOREXTRA 8LANKETASSEMBLYCONTROL ROD