pH Control:

In reactor facilities other than those holding aluminum components or using chemical shim, reactor coolant is managing in an alkaline condition to control corrosion in the system. In practice, if the desired alkaline condition is established, and no other action is taken, the pH gradually decreases during operation of the facility.  This is because of factors such as dilution from makeup water additions to compensate for coolant losses caused through sampling, leakage, volume changes on facility cooldown, and reaction of hydroxyl ions (OH^-) along with metals and corrosion products.  Therefore, hydroxyl ions must be added to the coolant to maintain a basic pH.  The ion exchange procedure provides a convenient means of helping to control the pH of reactor coolant through adding OH^- ions from the exchange reactions of the resin.

A hydroxyl form anion resin, or a cation resin in a few form other than the hydrogen form, may be used for this reason as in a mixed-bed arrangement, same to that shown in Figure, or in separate cation and anion units whereas flow is individually controlled.  Commonly, a portion of the reactor coolant is diverted from its general path, passes by the ion exchange resin, and after that is returned to the major coolant path. In that way, element of the coolant is constantly purified, and in the procedure hydroxyl ions are released to the coolant.  These hydroxyl ions tend to rise the pH (or limit the decrease), through offsetting the effects mentioned within the preceding paragraph.

Assume that the cation resin is in the ammonium form.  As reactor coolant passes by the ion exchange system, ionic impurities that will be removed and NH₄⁺ and OH^- will be added.  Therefore, the resin serves both to purify the coolant and to help manage the pH of the coolant through releasing NH₄OH.

The main sources of ionic impurities in reactor coolant are impurities in makeup water and corrosion products.  The amount of these impurities in reactor coolant is generally extremely small; therefore, the base added to the coolant through the ion exchange resin is commonly not enough to wholly compensate for the losses described above.  For this purpose, the pH of reactor coolant is measured regularly, and additional base is added as required. The frequency of addition varies considerably along with the category of base used.