Boric Acid – Chemical Shim

By chemical shim, we mean that boric acid is dissolved in the coolant/moderator. Boric acid (molecular formula: H3BO3) is a white powder that is soluble in water. In pressurized water reactors, chemical shim (boric acid) is used to compensate for an excess of reactivity of reactor core along the fuel burnup (long-term reactivity control). At the beginning of the specific fuel cycle concentration of boric acid is highest (see picture). At the end of this cycle concentration of boric acid is almost zero, and a reactor must be refueled.

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.

Boron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Boron letdown curve (chemical shim) and boron 10 depletion
Boron letdown curve (chemical shim) and boron 10 depletion during a 12-month fuel cycle. At the beginning of the specific fuel cycle concentration of boric acid is highest. At the end of this cycle, the concentration of boric acid is almost zero. A reactor must be refueled because no positive reactivity can be inserted to compensate for the negative reactivity of fuel burnup (increase in reactor slagging).

See previous:

Applications of Boron in Nuclear Power

See above:

Boron 10

See next:

Boron 10 Depletion