Principles of naval engineering - Historic Naval Ships Association

Chapter 24. -NUCLEAR POWER PLANTSare nearly equal in value. As the power increases,the average temperature remainsconstant but the inlet and outlet temperaturesdiverge. Since the colder leg of the primarycoolant is the longer, the net effect in thepressurizer is a decrease in level to makeup for the increase in density of the water inthe primary loop. The reverse holds true witha decreasing power level. Electrical heatersand a spray valve with a supply of water fromthe cold leg of the primary coolant assist inmaintaining a steam blanket in the upper partof the pressurizer and also assist in maintainingsaturation conditions of 2000 psia and636° F.coolant increases, so does the magnitude of theneutron scattering cross section. The highervalue of the scattering cross section allows thecoolant, in its capacity as moderator, to thermalizeneutrons at faster rate, supplying moreSPRAY NOZZLEPRINCIPLES OF REACTOR CONTROLReactor control principles' which are ofparticular interest to this discussion includethe negative temperature coefficient , the de -layed neutron action , and the poisoning of fuel.The term "negative temperature coefficient"is used to express the relationship betweentemperature and reactivity— as the temperaturedecreases, the reactivity increases. The negativetemperature coefficient is a design requirementand is achieved by the proper ratioof elements in the reactor, the geometry of thereactor, and the physical size of the reactor.The negative temperature coefficient makes itpossible to keep a power reactor critical withminimum movement of the control rods.The concept of negative temperature coefficientmay be most easily understood by useof an example. Assume that, in the PWR plantshown in figure 24-8, the reactor is criticaland the machinery is operating at a givenpower level. Now, if the valve is opened toincrease the turbine speed, the rate of steamflow, and the power level of the reactor, themeasurable effect with installed instrumentationis a decrease in the temperature of theprimary coolant leaving the steam generator.The decrease in temperature is small butsignificant in that it results in an increase indensity of the coolant. As the density of the^ DOMESTANDPIPEHEATERSECTIONJohn F. Hogerton, The Atomic EnergyPeskbook (NewYork: Reinhold Publishing Corp., 1963), p. 463.147.161XFigure 24-11.— Cutaway view of PWRpressurizing tank.625