Conductivity of reactor facility water is measured to provide an indication of dissolved ionic substances in the coolant. Conductivity measurements provide quantitative rather than qualitative information because it is possible to determine the total conductivity of the ions present, but not the specific types of ions present. Because many ions such as iron (Fe +++), chromium (Cr+++), copper (Cu++) and aluminum (Al +++) are susceptible to forming oxides and plating out as scale on heat transfer surfaces, reactor coolant conductivity is normally controlled at a level as low as practicable and consistent with pH. By monitoring conductivity levels in the reactor facility systems, the operator is able to cross check the chemistry of these systems, thereby achieving a higher confidence level in the parameters measured.

Regardless of the operating limits specified for a given reactor facility, operating relationships can be established between pH and conductivity levels of the coolant. Figure 4 shows a typical relationship of the pH and conductivity of a reactor coolant system using high pH, ammonium hydroxide chemistry control as a function of the ammonia (NH₃) concentration.

Figure 4 pH and Conductivity as a Function of NH, Concentration

Figure 5 shows the theoretical relationship of pH versus conductivity in a solution containing pure water. A second curve is added to the graph that illustrates the relationship that exists when nitric acid (HN0₃) is used as a pH control agent (such as may be utilized in facilities with aluminum components).

Figure 5 Theoretical Conductivity as a Function of pH

In both Figure 4 and Figure 5, a definite relationship exists between pH and conductivity, assuming no foreign ions are present. A similar graph could be constructed for those facilities using cation resins of a different base such as lithium or barium.

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 (HN0₃) with a reduction in pH by the following reaction.

Conductivity decreases even more than would be expected because of the formation of NH₄NO₃. N03 is not as conductive as OR, so the NH₄NO₃ results in a lower conductivity than NH₄0H. This neutralization of NH₄0H is shown by the following reaction.

The water formed is only slightly ionized, so the solution conductivity is lowered even further.