Fluorine (F^-) is another parameter monitored and controlled in some reactor facilities. High levels of F^- are potentially hazardous for two reasons. First, F^- promotes corrosion of zirconium by a stress corrosion mechanism at the inner surface of the clad (fluorine can be introduced to this region because of the existence of small defects or "pinholes" that cannot be completely avoided in the fuel clad). Second, F^- is a major contributor to radiation levels of the reactor coolant. Although some radioactive is produced by a proton, neutron reaction with , the only significant possible sources are impure makeup water or contamination of chemical agents, such as NH₃ and Li, which are added to the reactor coolant system.

Removal of F is accomplished in the same manner as the removal of Cl, that is, by feed and bleed or increased flow through the ion exchanger system. Removal by ion exchange, however, is not as effective as for Cl^- because of the lower affinity of F for exchange. As in the case of Cl^- contamination, if feed and bleed is the method selected for cleanup, the purity of the makeup water must be verified and other chemistry parameters monitored closely to preclude aggravation of the existing conditions.

Radioactivity

Radioactivity of the reactor coolant system is monitored and controlled because it is a good indicator of several conditions that may occur in the system. These include increased corrosion, crud bursts, fuel element defect or failure, and improper operation of the ion exchanger (where installed). Radioactivity levels in the reactor coolant system are normally kept at nominal values during normal conditions by the ion exchanger. In facilities that do not use ion exchangers, other methods that vary from facility to facility are used to control radioactivity. These include processes such as chemical cleanup followed by feed and bleed with subsequent disposal procedures and reactor shutdown in severe cases. Reactor coolant gaseous activity is also monitored, and it provides an indication of cladding failures and the introduction of air during reactor operation. Gaseous activity levels are normally controlled by degassification. Operating limits are established in most reactor facilities to operate the reactor safely, to limit radioactive releases to the environment, and to prevent excessive personnel exposure during operation and shutdown.