Reactor Water Chemistry
The key point of this discussion is the realization that a theoretical, or calculated, relationship
does exist, and measurements that vary appreciably from the theoretical values should be
investigated and corrective action taken.
Excessively high conductivity levels are an indication of the presence of undesired ions. This
condition warrants further investigation to locate the source of the impurity because, in addition
to other chemistry problems, it contributes to general corrosion by increasing the reaction rates
of the electrochemical cells. The purity of the makeup water, and any pH control agents added,
should be verified to determine the cause. pH should also be checked because of the
relationship of these parameters. Other chemistry parameters should also be checked, such as
Cl and F . After the cause of high conductivity has been determined, appropriate steps should
be taken to return conductivity to its normal value. One method that is often used is a feed and
bleed procedure whereby water is added to and drained from the facility at the same time. If
this method is used, verification of makeup water purity must be ensured to prevent
compounding the problem.
Low conductivity is also an indicator of a potential problem because, in high purity basic
systems, the only possible cause of low conductivity is a low pH. For example, in a system
using high pH ammonium hydroxide control, the introduction of air into the facility could result
in the formation of nitric acid (HNO ) with a reduction in pH by the following reaction.
Conductivity decreases even more than would be expected because of the formation of
NH NO . NO is not as conductive as OH , so the NH NO results in a lower conductivity than
NH OH. This neutralization of NH OH is shown by the following reaction.
The water formed is only slightly ionized, so the solution conductivity is lowered even further.
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.