Fluorine:

Fluorine (F^-) is the other parameter monitored and be controlled in a few reactor facilities.  High levels of F- are potentially hazardous for two reasons. First, F^- promotes corrosion of zirconium through a stress corrosion mechanism at the inner surface of the clad (fluorine could be introduced to this region since of the existence of small defects or "pinholes" which cannot be fully prevented in the fuel clad).  Second, F^- is a main contributor to radiation levels of the reactor coolant. Whereas some radioactive ¹⁸F is generate through a proton, neutron reaction with ¹⁸O, the just important possible sources are impure makeup water or contamination of chemical agents, such as NH₃ and Li that are added to the reactor coolant system.

¹⁸O(p, n) ¹⁸F (half-life F 112 minutes)

Removal of F^- is accomplished in the similar manner as the removal of Cl^- that is, through feed and bleeds or raised flow through the ion exchanger system. Removal through ion exchange, therefore, is not as effectual as for Cl^- since of the lower affinity of F^- for exchange.  Since in the case of Cl^- contamination, if feed and bleed is the method selected for cleanup, the purity of the makeup water must be established and other chemistry parameters monitored nearly to preclude aggravation of the existing conditions.