Reactor Water Chemistry
In facilities using aluminum components, pH is maintained on the acidic side of the scale
because of the corrosion characteristics of aluminum discussed in Module 2. In these facilities
pH may be controlled by the addition of a dilute nitric acid (HNO ) solution to the reactor
coolant system in conjunction with an ion exchange system of some type.
Regardless of the pH range maintained, most facilities use an ion exchange process (described
in Module 4) to help control pH. For the high pH facilities, the most common means of control
is the use of a lithium or an ammonium form cation and a hydroxyl form anion. When lithium
is used, it must be Li because other lithium isotopes produce tritium, which represents a
significant biological hazard to personnel. In facilities that employ high pH chemistry control
and do not use chemical shim reactivity control, it is sometimes necessary to add a strong base
solution such as ammonium or lithium hydroxide. When chemical additions are used for pH
control, facility design and operating procedures are utilized to preclude overconcentration at
any point in the system, which may lead to caustic stress corrosion conditions. Many reactions
that take place in the reactor coolant system can affect pH; therefore chemistry control must be
considered carefully to preclude upsetting the pH balance provided by the ion exchanger.
Control of the dissolved oxygen content in the reactor facility system is of paramount
importance because of its contribution to increased corrosion. The base reactions of concern
regarding high concentrations of dissolved oxygen are the following.
They are dependent on both the concentration of oxygen and temperature. Reaction (3-19) is
predominant at high temperatures (>400 F) in the presence of lower oxygen concentrations.
This corrosion film, ferrous oxide, is also known as magnetite and is a black, generally
tightly-adherent film that provides a protective function to surfaces within the facility.
Reaction (3-20) occurs at temperatures below about 400 F in the presence of higher oxygen
concentrations. Ferric oxide (Fe O ) is more commonly known as rust and is generally a
reddish color. This corrosion product adheres loosely to surfaces and is therefore easily
removed and transported throughout the system for subsequent deposition and possible
irradiation. In either of the reactions, the corrosion rate is accelerated by increased
concentrations of dissolved O and can be aggravated further by the presence of other
substances that may be present in the system.