Another parameter that is carefully monitored and controlled in most nuclear facilities is chloride (Cl^-). The reason for maintaining the chloride ion concentration at the minimum level practicable is that several forms of corrosion are affected by the chloride ion, and the type of greatest concern is chloride stress corrosion. The mechanics of chloride stress corrosion were discussed in detail in Module 2 and will therefore not be repeated in this section.

When high levels of Cl^- are suspected, or detected, immediate steps must be taken to eliminate the source and remove Cl^- from the system because of the potential consequences. If Cl is present in the reactor coolant system, one method of removing it is to initiate a feed and bleed operation after determining that makeup water supplies are not the source of contamination. Because of the large volume of water normally contained in the reactor coolant system, cleanup by this method involves considerable amounts of pure water and a significant amount of time.

Additional problems associated with feed and bleed operations include changes in pH and a loss of H_Z from the reactor coolant system during the cleanup. Changes to either, or both, of these parameters have the potential to further aggravate the occurrence of chloride stress corrosion because: pH changes from the optimum operating limits cause increased general corrosion; and a loss of H_Z from the reactor coolant makes the dissolved oxygen that is normally present from either radiolysis or contained in the makeup water available to interact with the Cl^- ions. This would promote chloride stress corrosion (recalling that Cl^-, O₂, and tensile stress are all necessary for chloride stress corrosion to occur).

The fact that a large amount of makeup water is being introduced will result in hydrogen depletion because even deaerated water contains small amounts of oxygen. H₂ will also be lost because of the draining of coolant from the system. During conditions that require the use of feed and bleed to correct a chemistry anomaly of any type, increased attention to all parameters becomes increasingly important.

Another method that may be used to control Cl^- concentrations in the reactor coolant is to route more bypass flow through the ion exchanger system, where installed. Use of this system precludes the requirements of a large inventory of makeup water as well as the disposal problems that arise from the bleed (draining) of coolant from the system. Because the ion exchanger has minimal effect on dissolved gases, depletion of hydrogen does not contribute to the potential of the problem. Increasing flow through the ion exchanger(s) may cause changes to the pH of the system, however, and as in the case of feed and bleed, increased vigilance is necessary to ensure pH, as well as other parameters, are properly controlled and maintained.