Because the potential for elevated temperatures exists during most conditions of facility operation, we will summarize the results from the resin in an ion exchanger overheating. Module 4 addresses resin in great detail, and the actual resin breakdown will be included there. Basically the resin of an ion exchanger is an inert polystyrene structure with ion exchange sites "loosely" attached. The basic structure is stable up to fairly high temperatures (approximately 300F), but the active exchange sites are not. There are two types of exchange sites: anion and cation. The anion resin begins to decompose slowly at about 140F, and the decomposition becomes rapid above 180F. The cation resin is stable up to about 250F. Because these temperatures are well below normal reactor coolant temperatures, the temperature of the coolant must be lowered before it passes through the ion exchange resin.

The decomposition of resin produces an alcohol form of the resin, which has no exchange capability, and trimethylamine (TMA), N(CH₃)₃. TMA is a weak base, similar to ammonia, that reacts with water as follows.

N(CH₃)₃ + H₂0 u NH(CH₃) ₃ + OH

If large amounts of TMA are released to the coolant, the pH may increase noticeably. For example, 1 ppm of TMA in reactor coolant that uses lithium form resin will cause a noticeable increase in pH. TMA may also interfere with the analysis for chloride ions (which is routinely performed on reactor coolant) by giving a false indication of high chloride concentration. Another significant property of TMA is its intense odor of dead fish. Although the presence of such an odor from reactor coolant is not definitive for TMA, it may give an indication of resin overheating.

The other product of resin breakdown, [R CH₂N(CH₃)₂], is an amine with exchange capabilities considerably less than the original form of the resin. Thus, both reactions lead to partial (or complete) loss of exchange capability. If the temperature is sufficiently high, or if a lower temperature (but greater than 180F) is sustained for a long enough period, the resin will be unfit for use. If the temperature becomes very high (greater than about 450F), the polymeric base structure of the resin will decompose, forming a complex mixture of organic tars and oils.

The preceding discussion concerned the decomposition of resins in their original forms. It should be noted that if overheating occurs after the resin has been in operation for some time, part of the resin will be in a different form due to the exchange process. As a result, some of the previously removed impurities will be released to the coolant if decomposition occurs.

A number of changes are probable if overheating of resin occurs. Reactor coolant Cl^- levels would probably increase as a result of thermal breakdown and subsequent release. Ion exchanger effectiveness would be greatly reduced for similar reasons. Radioactivity levels of the reactor coolant would increase because of the release of impurities collected and later released from the resin. pH would likely decrease because of the release of H+ ions from the resin complex and may cause acidic conditions in the reactor coolant if the temperature is sufficient (>250F). Because certain types of resin decompose at lower temperatures, pH may increase as a result of the release of TMA and be accompanied by a dead fish odor. Because of the consequences of overheated resin, stringent temperature limitations are necessary. If overheating occurs, the ion exchanger should be taken out of service immediately and the cause rectified. The resin must be replaced prior to placing the ion exchanger back in service after overheating.

Summary

The important information in this chapter is summarized below.

Chemistry Parameters Summary

Table 1 provides a summary of the parameters, why they are controlled, and the methods of control.

The following abnormal chemistry conditions are discussed:

Injection of air will cause the hydrogen inventory to become depleted, which in turn can lead to a pH swing towards the acidic end. This reduced pH leads to a loosened corrosion film hence a crud burst. The crud burst causes higher radiation levels and an increase in conductivity.

Fuel element failure allows large amounts of fission products a possible release path to the reactor coolant. This could lead to a lowered ion exchange efficiency.

Resin overheating results in the decomposition of the resin. The products of the decomposition can seep into the reactor coolant and affect the chemistry in several ways. A few of the common products and their resulting influence on the chemistry is discussed.