Chlorides

Figure 5 Theoretical Conductivity as a Function of pH

Fluorine

Chemistry Volume 2 of 2
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2N₂ 5 O₂ 2H₂O 4 HNO₃ NH₄OH HNO₃ NH₄NO₃ H₂O Reactor Water Chemistry DOE-HDBK-1015/2-93 CHEMISTRY PARAMETERS Rev. 0 CH-03 Page 21 The key point of this discussion is the realization that a theoretical, or calculated, relationship does exist, and measurements that vary appreciably from the theoretical values should be investigated and corrective action taken. Excessively high conductivity levels are an indication of the presence of undesired ions. This condition warrants further investigation to locate the source of the impurity because, in addition to other chemistry problems, it contributes to general corrosion by increasing the reaction rates of the electrochemical cells. The purity of the makeup water, and any pH control agents added, should be verified to determine the cause. pH should also be checked because of the relationship of these parameters. Other chemistry parameters should also be checked, such as Cl and F . After the cause of high conductivity has been determined, appropriate steps should - - be taken to return conductivity to its normal value. One method that is often used is a feed and bleed procedure whereby water is added to and drained from the facility at the same time. If this method is used, verification of makeup water purity must be ensured to prevent compounding the problem. Low conductivity is also an indicator of a potential problem because, in high purity basic systems, the only possible cause of low conductivity is a low pH. For example, in a system using high pH ammonium hydroxide control, the introduction of air into the facility could result in the formation of nitric acid (HNO ) with a reduction in pH by the following reaction. 3 (3-16) Conductivity decreases even more than would be expected because of the formation of NH NO . NO is not as conductive as OH , so the NH NO results in a lower conductivity than 4 3 3 4 3 - - NH OH. This neutralization of NH OH is shown by the following reaction. 4 4 (3-23) The water formed is only slightly ionized, so the solution conductivity is lowered even further. Chlorides Another parameter that is carefully monitored and controlled in most nuclear facilities is chloride (Cl ). The reason for maintaining the chloride ion concentration at the minimum level - practicable is that several forms of corrosion are affected by the chloride ion, and the type of greatest concern is chloride stress corrosion. The mechanics of chloride stress corrosion were discussed in detail in Module 2 and will therefore not be repeated in this section.

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