Reactor Water Chemistry
Figure 2 Corrosion Rate vs. pH for Iron
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 (Fe O ), 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.
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.