The reason for controlling pH in the reactor coolant system is to minimize and control corrosion. As discussed in Module 1, the presence of excess H+ ions in solution results in an acidic condition. In reactor facilities (except those containing aluminum components), acidic conditions are detrimental to the materials of construction in a number of ways. An acidic condition in the primary coolant results in processes that are potentially harmful to the system as follows. First, a low pH promotes rapid corrosion by deteriorating or "stripping off" the protective corrosion film, and second, corrosion products such as ferrous oxide (Fe₃0_a), which is predominant in the corrosion film, are highly soluble in an acidic solution. Figure 2 shows how the corrosion rate increases as the pH decreases. Thus for facilities not using aluminum components, a neutral or highly basic pH is less corrosive.

Figure 2 Corrosion Rate vs. pH for Iron

In nuclear facilities that do not use chemical shim to control reactivity, pH is normally maintained at a relatively high value, such as a pH of about 10. In these facilities the upper limit for pH is set based on caustic stress corrosion considerations because caustic stress corrosion becomes more probable as higher pH values are approached.

In facilities that use chemical shim reactivity control (chemical shim involves the addition of boron in the form of boric acid) the pH is maintained at a much lower value. A low pH is necessary because of the large amounts of boric acid added to the reactor coolant. Accordingly, pH in these facilities is maintained as high as possible consistent with the reactivity requirements of the nuclear facility, with pH range from 5 to 7 being common.

In facilities using aluminum components, pH is maintained on the acidic side of the scale because of the corrosion characteristics of aluminum discussed in Module 2. In these facilities pH may be controlled by the addition of a dilute nitric acid (HN0₃) solution to the reactor coolant system in conjunction with an ion exchange system of some type.

Regardless of the pH range maintained, most facilities use an ion exchange process (described in Module 4) to help control pH. For the high pH facilities, the most common means of control is the use of a lithium or an ammonium form cation and a hydroxyl form anion. When lithium is used, it must be ⁷Li because other lithium isotopes produce tritium, which represents a significant biological hazard to personnel. In facilities that employ high pH chemistry control and do not use chemical shim reactivity control, it is sometimes necessary to add a strong base solution such as ammonium or lithium hydroxide. When chemical additions are used for pH control, facility design and operating procedures are utilized to preclude over concentration at any point in the system, which may lead to caustic stress corrosion conditions. Many reactions that take place in the reactor coolant system can affect pH; therefore chemistry control must be considered carefully to preclude upsetting the pH balance provided by the ion exchanger.