13.12. Operational reactivity control in a PWR is provided primarily by boric acid dissolved in the coolant water supplemented somewhat by control rods. In a procedure known as chemical shim (§5.187), the boric acid concentration is varied to control reactivity changes during the op­erating cycle, such as those resulting from fuel depletion and fission product buildup. However, selected fuel assemblies contain burnable absorber rods to limit power peaking (§10.31). Therefore, the soluble boron concentra­tion adjustment is made in accordance with the effects on reactivity con­tributed by these burnable absorber rods as shown in Fig. 13.3. Although

this figure is for a first core in which solid burnable poison is needed to substitute for the fission products accumulated in subsequent cycles, the general effects on soluble boron concentration are typical of boron burn­able absorber rod use [2]. Also shown in Fig. 13.3 is the residual poison reactivity penalty associated with this practice. Gadolinia rods behave somewhat differently. The control rods are used primarily for startup, safety shutdown, and to follow load changes. Some rods are partial length or partial strength to aid power distribution “shaping.”

13.13. The control rods are stainless steel tubes encapsulating an ab­sorber material such as hafnium, boron carbide, or a silver-indium-cadmium alloy. These rods are arranged in clusters which move within guide tubes (thimbles) which replace some fuel rods in the rod lattice of the fuel as­semblies. Perforations over a portion of the thimble length allow the escape of water as the rods are inserted. However, the lower end of the tubes is closed to decelerate the rods at the end of their drop. The clusters are operated in groups to accomplish their various functions. In assembly po­sitions not reserved for control rod insertion, the empty thimbles may be used for burnable absorber rods. However, other fuel designs provide for more flexibility (§10.32).