DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL
Principles of Water Treatment
The major sources of ionic impurities in reactor coolant are impurities in makeup water and
corrosion products. The amount of these impurities in reactor coolant is normally very small;
thus, the base added to the coolant by the ion exchange resin is usually not sufficient to entirely
compensate for the losses described above. For this reason, the pH of reactor coolant is
measured regularly, and additional base is added as needed. The frequency of addition varies
considerably with the type of base used.
Resin Bed Malfunctions
The resin beds of ion exchangers are susceptible to malfunction from a number of causes.
These causes include channeling, breakthrough, exhaustion, and overheating.
Channeling is a condition in which the resin allows a direct flow of water through the
ion exchanger. Flow channels are established from the inlet to the outlet of the ion
exchanger, which allows water to flow essentially unrestricted through the resin via
these paths. If channeling occurs, the water flowing through the resin bed has
insufficient contact with the resin beads and results in a decrease in effectiveness of the
Channeling most often results from improper filling of the ion exchanger with resin. If
insufficient water is mixed with the resin when it is added, the resin column may contain
pockets, or voids. These voids may then set up flow paths for channeling to occur.
Improper design or malfunction of the water inlet connection (flow diffuser) can also
lead to channeling.
Breakthrough and Exhaustion
To gain further insight into the processes that occur in a column of mixed bed resin as
it removes an impurity, it is worthwhile to construct a series of curves such as those
shown in Figure 6. These curves illustrate the behavior of hydrogen form resin as it
removes NH ions from solution (the behavior of an anion resin is analogous). The
two rows of curves schematically represent the concentrations of H and NH on the
resin and in the effluent (exiting solution).
In the bottom row, concentration is plotted against volume of solution that has passed
through the column. That is, the plots represent the concentration of the indicated ion
in the effluent solution after a volume of solution has passed through the resin. It is
assumed that the concentration of NH in the influent is constant. NH ions are more
strongly attracted to the resin than are H ions. Thus, NH ions readily exchange for
H ions on the resin.