14.20. During normahoperation, most nuclear plants run continuously at full rated power (base loaded). Should there be unexpected sudden transients or disturbances affecting the load, various procedures are pro­vided to accommodate them. Our purpose here is not to consider such

cases but to examine the inherent ability of a system to “follow” changes in load should the particular generating unit not be in base-loaded service.

14.21. An example of the natural load-following capability of a PWR is initiated by an opening of the steam throttle leading to the turbine in response to a sensor detecting a slight slowing of the generator as the load is increased. As the pressure is reduced on the secondary side of the steam generator, there is some flashing of water leading to additional heat re­moval from the primary system. As the cooler water returns to the core, the increase in reactivity resulting from the negative temperature coefficient causes an increase in reactor power. For a load reduction, the process is reversed. In practice, some “fine tuning” by the control system is usually

carried out involving control rod movement or boron concentration ad­justment. However, the picture is not quite so simple. The partial insertion of rods during part-load operation can lead to an undesirable upper core axial power shift on return to full power since the upper region xenon concentration had previously been reduced. The excess upper half power is known as axial offset. However, axial problems can be avoided and load following accomplished without boron concentration adjustment by bal­ancing the operation of two banks of control rods, with each bank con­taining some weak absorbers or “gray” rods. This is known as MSHIM (mechanical shim) strategy.

14.22. By contrast, in a BWR, an opening of the steam throttle reduces the core pressure, causing more boiling, which, in turn, reduces the reac­tivity as a result of the void coefficient. Therefore, BWR load following, generally in the range of 75 to 100 percent power, is accomplished by the control system, which can reverse this tendency by increasing the recir­culating flow rate and moving the rods an appropriate amount. Although the recirculating pumps operate at constant speed, valving is provided for flow rate adjustment.