The potential for elevated temperatures exists during most conditions of facility operation, we will examine in detail the processes that occur if the resin in an ion exchanger is overheated. Although the inert polystyrene basic structure of resin is stable up to fairly high temperatures (approximately 300F), the active exchange sites are not. 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 anion resin (hydroxyl form) decomposes by either of two mechanisms with approximately equal probability.

or

Reaction (4-24) 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.

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 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 methyl alcohol (CH₃0H) produced by Reaction (4-25) is not expected to have a harmful effect on the reactor coolant system. The other product of this reaction [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 (greater than 180F) is sustained for a long enough period, the resin will be unfit for use.

Cation exchange resin begins to undergo thermal decomposition at temperatures above about 250F by the following reaction.

This reaction destroys all exchange capacity of the cation resin and also produces an acid. The Reactions (4-24) through (4-26) are the initial reactions when resin is overheated. 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. For example, the chloride form of the anion resin will form CH₃C1 by the reaction corresponding to Reaction (4-25).

The CH₃C1 (chloromethane) will be released to the coolant and will decompose in the radiation field of the reactor core, producing Cl^- ions. Similarly, the sodium form of the cation resin will release Na' ions by the following reactions.

A number of changes are probable if overheating of resin occurs. Reactor coolant Cllevels 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 cation resin complex and may cause acidic conditions of the reactor coolant if the temperature were sufficient (>250F). Because the anion resin decomposes 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.