Reactor Water Chemistry
In addition to the direct contribution to corrosion, oxygen reacts with nitrogen to lower the pH
of the reactor water, which also results in an increased rate of corrosion. Oxygen and nitrogen
react to form nitric acid by the following reaction.
In all the reactions presented, it can be seen that oxygen concentrations promote corrosion. It
follows then that if corrosion is to be minimized, oxygen concentrations must be maintained as
low as possible. In most nuclear facility reactor coolant systems, the limit for dissolved oxygen
concentrations is expressed in ppb (parts per billion). Concentration may be monitored on a
continuous basis by using an in-line analyzing system or periodically by withdrawing a sample
volume and analyzing that sample. Monitoring oxygen levels is done not only to ensure that
no oxygen is available for corrosion, but also to indicate the introduction of air into the system.
Because the presence of dissolved oxygen contributes to most mechanisms of corrosion, the
concentration of oxygen is controlled and reduced by the addition of scavenging agents in most
facilities. Hydrogen gas (H ) and hydrazine (N H ) are the scavenging agents normally used to
eliminate dissolved oxygen from the reactor coolant system. These substances scavenge oxygen
by the following reactions.
Because hydrazine decomposes rapidly at temperatures above about 200 F (forming NH , H ,
and N ), hydrogen gas is used as the scavenging agent during hot operation and hydrazine is
used when the reactor coolant system is cooled below 200 F.
The decomposition reactions of hydrazine pose additional problems in chemistry control. Even
if sufficient hydrazine were added to overcome the loss due to decomposition, instability of
coolant pH would probably occur by the following reactions.