Chapter 24. -NUCLEAR POWER PLANTS 



are nearly equal in value. As the power in- 

 creases, the average temperature remains 

 constant but the inlet and outlet temperatures 

 diverge. Since the colder leg of the primary 

 coolant is the longer, the net effect in the 

 pressurizer is a decrease in level to make 

 up for the increase in density of the water in 

 the primary loop. The reverse holds true with 

 a decreasing power level. Electrical heaters 

 and a spray valve with a supply of water from 

 the cold leg of the primary coolant assist in 

 maintaining a steam blanket in the upper part 

 of the pressurizer and also assist in main- 

 taining saturation conditions of 2000 psia and 

 636° F. 



PRINCIPLES OF REACTOR CONTROL 



Reactor control principles' which are of 

 particular interest to this discussion include 

 the negative temperature coefficient , the de - 

 layed neutron action , and the poisoning of fuel. 



The term "negative temperature coefficient" 

 is used to express the relationship between 

 temperature and reactivity— as the temperature 

 decreases, the reactivity increases. The nega- 

 tive temperature coefficient is a design re- 

 quirement and is achieved by the proper ratio 

 of elements in the reactor, the geometry of the 

 reactor, and the physical size of the reactor. 

 The negative temperature coefficient makes it 

 possible to keep a power reactor critical with 

 minimum movement of the control rods. 



The concept of negative temperature coef- 

 ficient may be most easily understood by use 

 of an example. Assume that, in the PWR plant 

 shown in figure 24-8, the reactor is critical 

 and the machinery is operating at a given 

 power level. Now, if the valve is opened to 

 increase the turbine speed, the rate of steam 

 flow, and the power level of the reactor, the 

 measurable effect with installed instrumenta- 

 tion is a decrease in the temperature of the 

 primary coolant leaving the steam generator. 

 The decrease in temperature is small but 

 significant in that it results in an increase in 

 density of the coolant. As the density of the 



coolant increases, so does the magnitude of the 

 neutron scattering cross section. The higher 

 value of the scattering cross section allows the 

 coolant, in its capacity as moderator, to therma- 

 lize neutrons at faster rate, supplying more 



SPRAY NOZZLE 



^ DOME 



STANDPIPE 



HEATER 

 SECTION 



John F. Hogerton, The Atomic EnergyPeskbook (New 

 York: Reinhold Publishing Corp., 1963), p. 463. 



147.161X 

 Figure 24-11.— Cutaway view of PWR 

 pressurizing tank. 



625 



