In PWRs, the reactivity control is accomplished in part by varying the concentration of boric acid (H3BO3) dissolved in the coolant. Such a chemical shim cannot be made to respond as quickly as control rods. Therefore, the chemical shim is used to control the long-term reactivity changes due to such factors as fuel burnup and Xe transients. It substantially reduces the number of control rods required in the reactor. Furthermore, because boric acid is more or less uniformly distributed through the reactor core, its concentration changes can be made without disturbing the neutron flux in the core.

Figure 1.9 shows an example of the changes of the boric acid concentration in a PWR. It is observed in the figure that the chemical shim compensates for the negative reactivity due to 135Xe and 149Sm and for the reactivity decrease with fuel burnup.

If a chemical shim is present, a decrease in coolant density due to temperature rise will also lead to a decrease in boric acid concentration. Thus it has a positive reactivity effect on moderator temperature coefficient as shown in Fig. 1.10. The requirement for a negative moderator temperature coefficient will limit the amount of boric acid concentration allowed.