The presence of dissolved gases, suspended solids, and incorrect pH can be detrimental to the water systems associated with a reactor facility. Therefore, these conditions must be minimized or eliminated to reduce corrosion in the systems of the facility. The way these conditions are controlled and the difficulties in controlling them are discussed in this chapter.

EO 1.6EXPLAIN the three basic methods used to remove dissolved gases from water.

EO 1.7LIST five filtration mediums used to remove suspended solids from water.

EO 1.8EXPLAIN how mixed-bed ion exchangers may be used to control pH.

EO 1.9DISCUSS resin malfunctions, including the following:

a. Channeling b. Breakthrough c. Exhaustion

Removal of Dissolved Gases

Dissolved gases result from different sources depending upon which system we examine. In the following discussion, we will address makeup water, reactor coolant systems, secondary facility water systems, the sources of dissolved gases, and methods used to reduce their concentrations to acceptable levels.

Many facilities use raw water as a source for makeup water systems. Pretreatment of this water is accomplished in various ways from distillation to a series of distinct processes as shown in Figure 5. In a pretreatment system similar to that shown in Figure 5, a resin column containing a cation resin (hydrogen form) is used to remove cations. The water entering the cation exchanger contains numerous ions including sodium (Na+), bicarbonate (), and others (HC0₃^- is one of the major impurities in many raw water systems). ions result from the water softener located upstream in the pretreatment system. In addition to the HC0₃^- ions, raw water contains large amounts of magnesium (Mg++) and calcium (Ca++), as well as small amounts of other ionic impurities.

Figure 6 A Typical Pretreatment System

The reactions that occur in the water softener include the removal of both Mg++ and Ca++ ions. The water softener contains resin in which the insoluble exchange site is the S0₃^_ molecule, and the soluble ions attached to the exchange site are Na+ ions. When water containing Mg++, Ca++, and ions is passed over the resin in the softener, the ions are exchanged by the following reaction (Mg" removal is similar).

Note that electrical neutrality is maintained before and after the exchange reaction. One calcium ion with two positive charges is attached to two exchange sites that release two sodium ions with one positive charge each. The HC0₃^- ion is not affected by the reaction and passes through the resin of the softener.

To obtain pure water, it is necessary to demineralize the water completely, which is accomplished using a cation exchanger, an aerator, and an anion exchanger.

The cation exchanger contains resin in the hydrogen form. In this treatment step, essentially all cations entering the ion exchanger will be held at the exchange site, and H+ will be released as shown in the following typical reaction (the anions, specifically the HC0₃^- ions, are unaffected by the cation exchanger).

The water leaving the resin is somewhat acidic (depending on the incoming ion concentration) because it contains H+ ions and whatever anion was associated with the incoming cation. After passing through the cation resin, the HC0₃^- ions combine with the H+ ions to form carbonic acid (H₂C0₃). Carbonic acid is a weak acid that will decompose to water and C0₂ by the following reaction.

Because the carbonic acid readily dissociates, the aerator is used to remove the C0₂ from the makeup water at this point in the system. If we aerate the water by some means, such as spraying it through a tower or blowing air through the water, the C0₂ is "stripped" from the water and vented to the atmosphere. The removal of C0₂ forces Reaction (4-8) to shift to the right, which converts more H₂CO₃ to CO₂. With sufficient aeration, all bicarbonate (HCO₃^-), and therefore CO₂, can be removed.

Similar reactions occur in the anion exchanger. For example, anion resin, which has hydroxide ions at the exchange sites, will react as indicated in the following typical reaction.

In this pretreatment system, the anion resin is downstream of the cation resin, and the only cations present are hydrogen ions. When the hydroxyl ions are released from the anion exchange sites, they combine with the hydrogen ions to form water. As a result, pure water appears at the effluent (this is somewhat overstated because a very small amount of other cations and anions pass unaffected through the resin columns in actual practice).

Another method sometimes used to remove dissolved gases from water is deaeration. In this process, the water is stored in vented tanks containing electric heaters or steam coils. The water is heated to a temperature sufficient for slow boiling to occur. This boiling strips dissolved gases from the stored water, and these gases are then vented to the atmosphere. Usually, the vented gases are directed through a small condenser to limit the loss of water vapor that would escape as steam along with the gases. This method is particularly effective in removing dissolved oxygen as well as other entrained gases (C0₂, N₂,and Ar).

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 (MR), 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 200F, gaseous hydrogen is added and maintained in the primary coolant to scavenge oxygen by the following reaction.

The other scavenger is hydrazine (N2H4). Hydrazine is thermally unstable and decomposes at temperatures above 200F to form ammonia (NH₃), nitrogen (N₂), and hydrogen (H₂). Consequently, the use of hydrazine as an oxygen scavenger is limited to temperatures below 200F. 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 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 (Na₂SO₃). Scavenging of oxygen occurs by the following reaction.

As can be seen by Reaction (4-12), oxygen is consumed in the reaction resulting in the formation of sodium sulfate, Na₂SO₄ (a soft sludge). Addition of this scavenging agent is limited to drumtype 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.

Sodium sulfite reacts rapidly with oxygen and is a very efficient scavenger. However, being a solid and the source of another solid (Na₂SO₄) that is produced during the reaction, sodium sulfite has the potential of fouling heat transfer surfaces. An additional problem associated with the use of sodium sulfite is corrosion of secondary system components resulting from its decomposition products. At the temperatures present in the steam generators, sodium sulfite can decompose as follows.

Sulfur dioxide (S0₂) is a gas and is carried over to the remainder of the steam facility. With water (in the steam or in the feed/condensate system), the S0₂ reacts in the following manner.

This acidic condition is corrosive to all components in the secondary system.

Because of the problems associated with sodium sulfites, many facilities use volatile chemistry control of the secondary steam system to control dissolved gases in conjunction with air removal systems. This control utilizes hydrazine (Reaction 4-11) and morpholine (Reaction 415) to eliminate oxygen and carbon dioxide, respectively.

As can be seen by Reaction (4-11), no solids are formed; thus, the tendency of fouling heat transfer surfaces is reduced. An additional benefit results from the decomposition of hydrazine by the following reactions.

These reactions result in an alkaline pH condition that decreases corrosion in the steam facility. As can be seen in Reaction (4-15), the consumption of C0₂ takes place. Two benefits result from this reaction; 1) the inventory of dissolved gases in the steam facility is reduced, and 2) is the reaction contributes to maintaining a higher pH by eliminating carbonic acid (H₂CO₃), thus reducing corrosion.