DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL
Principles of Water Treatment
Removal of dissolved gases from the reactor coolant system is usually accomplished by venting
a steam space or high point in the system. In pressurized water reactors (PWR), this is normally
accomplished in the pressurizer. The steam space is the high point of the system, and the boiling
and condensing action causes a constant stripping of dissolved gases to occur. The steam space
is vented either intermittently or constantly, and the gases are carried off in the process.
In addition to the mechanical means mentioned above, the use of scavengers in a PWR prevents
the presence of dissolved oxygen. Two methods are normally used in this regard. When facility
temperature is above approximately 200 F, gaseous hydrogen is added and maintained in the
primary coolant to scavenge oxygen by the following reaction.
The other scavenger is hydrazine (N H ). Hydrazine is thermally unstable and decomposes at
temperatures above 200 F to form ammonia (NH ), nitrogen (N ), and hydrogen (H ).
Consequently, the use of hydrazine as an oxygen scavenger is limited to temperatures below
200 F. Hydrazine scavenges oxygen by the following reaction.
The presence of dissolved gases in the steam facility of a PWR is as detrimental as the presence
of these gases is in the reactor coolant systems. Because steam facility systems contain metals
other than stainless steel, they are even more susceptible to certain types of corrosion in the
presence of oxygen and carbon dioxide. Removal of dissolved gases from the steam system is
accomplished in two ways: by mechanical means such as air ejectors or mechanical pumps; and
by using chemicals that scavenge oxygen.
Because boiling occurs in the steam generators, any dissolved gases entrained in the feedwater
will be stripped out during the boiling process. These gases are carried with the steam through
the turbines and auxiliary systems and ultimately end up in the condensers. The design of the
condensers is such that noncondensible gases (for example, O , CO ) are collected and routed to
the air removal system (which consists of air ejectors or mechanical pumps), where they are
subsequently discharged to the atmosphere.
Scavenging involves the use of solid additives and volatile chemicals. One commonly-used solid
chemical additive is sodium sulfite (Na SO ). Scavenging of oxygen occurs by the following
As can be seen by Reaction (4-12), oxygen is consumed in the reaction resulting in the formation
of sodium sulfate, Na SO (a soft sludge). Addition of this scavenging agent is limited to drum-
type steam generators. Once Thru Steam Generators (OTSG) do not use this method, but instead
use controls that keep all scale-forming chemicals out of the steam generators.