Reactor Water Chemistry
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 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 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 , O , 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. H 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.
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 F is produced by a proton, neutron reaction with O, the
only significant possible sources are impure makeup water or contamination of chemical agents,
such as NH and Li, which are added to the reactor coolant system.
O (p,n) F (half-life F 112 minutes)