In certain cases also fine power changes can be controlled by the chemical shim. If it is desired to increase power, the boric acid concentration must be diluted, removing 10B from the reactor core and decreasing its poisoning effect. Compared with burnable absorbers (long-term reactivity control) or with control rods (rapid reactivity control), the boric acid avoids the unevenness of neutron-flux density in the reactor core because it is dissolved homogeneously in the coolant in the entire reactor core. On the other hand, high concentrations of boric acid may lead to a positive moderator temperature coefficient, which is undesirable. In this case, more burnable absorbers must be used.
Moreover this method is slow in controlling reactivity. Normally, it takes several minutes to change the concentration (dilute or borate) of the boric acid in the primary loop. For rapid changes of reactivity control rods must be used.