Online Trace Analysis of Anions in the Primary Water-Steam Circuits of Nuclear Power Plants

Water-steam circuits that are used in both fossil and nuclear power plants are inherently prone to corrosion, as the metal components of these circuits are constantly in contact with water. It is therefore imperative for both fossil and nuclear power plants to constantly monitor and measure possible sources of corrosion that occur in these circuits.

In addition to the two other water reactors typically found in nuclear power plants, pressurized water reactors (PWRs) are generally designed with a third “primary” water circuit. In this primary circuit, the primary coolant water is pumped under high pressure to absorb heat generated by nuclear fission, which further transfers heat to the secondary circuit. The PWR ensures that radioactive materials remain contained within the system and do not disperse to the secondary circuit, as this misdirection of these materials could be released into the environment and cause adverse effects.

Analysis of Corrosion of Metals by Anions

The corrosion of metals by anions typically occur when exposed to high temperature and pressures. The increase susceptibility of these materials to corrosion therefore warrants their constant monitoring by a sensitive analytical technique, as even trace amounts of these metals can have devastating consequences to the system. The analytical challenge in the primary circuit is that the detection of anions in the μg/L range is typically alongside gram quantities of boric acid and lithium hydroxide applied within the system.

In addition to the standard anions such as fluoride, chloride, nitrate, and sulfate, other important organic degradation products such as glycolate, formate, and acetate, can also be present and therefore indicative of the presence of defective ion exchangers that are used to condition the boiler feed water. Phosphates are often added to systems in an effort to prevent corrosion, as this element is capable of forming corrosion-resistant protective films on metal surfaces and cracks, while also phosphatizing defects in their presence. As phosphate plays an important role in maintaining adequate conditions of these systems, it is therefore necessary to also monitor the levels of phosphate concentrations.

Diagram of a 3-water circuit nuclear power plant. Water sample from the primary circuit of a pressurized water reactor containing 2 g/L H₃BO₃ and 3.3 mg/L LiOH spiked with 2 μg/L anions; preconcentration volume: 2000 μL. Image credit: Metrohm.

Metrohm’s Process IC and Process IC TWO

The precise and reliable analysis of the aforementioned chemicals requires and automated method, such as the Process Ion Chromatograph that is combined with both the Inline Preconcentration and Inline Matrix Elimination offered by Metrohm Process Analytics. With one injection, the Process IC is capable of measuring numerous different ionic compounds in an aqueous media from ng/L to % concentrations. The analysis system of the Process IC is continuously fed with samples directly through the bypass. The Process IC provides an automatic calibration that ensures excellent detection limits, a high reproducibility and excellent recovery rates.

The Process IC also provides an alarm if the system indicates that intervention concentration limits are reached, or a pre-set warning is detected in an effort to save costs by preventing irreparable damage that occurs following corrosion of the materials within these nuclear power plant systems.

The Process IC TWO is capable of monitoring the concentrations of both anions and cations with a single ion chromatograph and two detector blocks, thereby providing a comprehensive overview of the water circuit chemistry. The potential to connect one Process Ion Chromatograph to up to 20 sample streams allows for multiple areas within the water-steam circuits in a single power plant to be monitored by a single instrument. Equipped with a built-in eluent production module and optional PURELAB^® flex 5/6 from ELGA^® for pressureless ultrapure water, the Process IC can be configured to run trace anion analyses autonomously for up to several weeks at a time.

This information has been sourced, reviewed and adapted from materials provided by Metrohm.

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