Online Trace Analysis of Cations in the Primary Circuit of Nuclear Power Plants

In power plants, the leading factor to costly and critical downtimes is typically a result of corrosion present within the systems. In addition to the two other water reactors that are often found in thermal power plants, such as boiling water reactors (BWRs) or coal-fired and geothermal power plants, pressurized water reactors (PWR) 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.

Elements in PWR

In the PWR, light water is used as a primary coolant, however a number of other chemicals are added to the reactor for their specified properties. For example, boron in the form of boric acid (H3BO3) is added to the coolant to control reactivity, as this compound readily absorbs neutrons, whereas the addition of monoisotopic lithium hydroxide (LiOH) is added to ensure that pH values within the reactor are greater than 7 to prevent corrosion. The presence of lithium is therefore a critical parameter to monitor in PWRs.

Nickel is also an important alloying metal that increases the corrosion resistance of steel, however, when present in its dissolved form, this element can promote corrosion. It is therefore imperative that regular checks on the concentration of Ni2+ are performed. In addition, metals released by any type of corrosion can undergo nuclear reactions and thus increase radiation within the power plant. To limit the corrosion of materials, as well as prevent the formation of radioactive products, water in power plants is often treated with depleted zinc oxide. The concentration of zinc in the PWRs must also be monitored to ensure that any potential radioactivity does not spread.

Diagram of a 3-water circuit nuclear power plant. Simulated 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 nickel, zinc, calcium, and magnesium; preconcentration volume: 1000 μL.

Diagram of a 3-water circuit nuclear power plant. Simulated 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 nickel, zinc, calcium, and magnesium; preconcentration volume: 1000 μL.

Analysis of Elements Present in PWR

The use of ion chromatography can determine the concentration of to a precision as low as the sub-μg/L range in a single analysis. It is important to note that to ensure reliable trace analysis of these elements through IC, the method must be automated as much as possible.

To address this challenge, Metrohm Process Analytics offers the Process Ion Chromatograph featuring the combined Inline Preconcentration and Inline Matrix Elimination. With one injection, the Process IC is capable of measuring numerous ionic compounds present within a single aqueous media from ng/L to % concentrations.

The analysis system is continuously fed with a direct supply of sample solutions through a bypass. Automatic calibration guarantees excellent detection limits, a high reproducibility and excellent recovery rates of samples by use of the Process IC. Additionally, the Process IC will provide an alarm to the user upon identifying either a pre-set warning or whether the intervention concentration limits are reached. This automated system further helps to prevent unnecessary costs of the irreparable damage that can occur due to corrosion in these systems.

Conclusion

The Process IC TWO is capable of monitoring the concentration of both anions and cations with a single ion chromatograph and two detector blocks to provide the user with a comprehensive overview of the water circuit chemistry. The possibility to connect one Process Ion Chromatograph to up to 20 sample streams could allow for the automated monitoring of multiple areas within the different water-steam circuits in a power plant to be possible by the use of a single instrument.

With a built-in eluent production module and the optional addition of the PURELAB® flex 5/6 from ELGA® for pressureless ultrapure water, allows the Pressure IC TWO to be a flexible instrument that can be configured to autonomously run multiple trace anion analyses for total duration of up to several weeks.

The Process IC is available with either one or two measurement channels, along with integrated liquid handling modules and several automated sample preparation options.

The Process IC is available with either one or two measurement channels, along with integrated liquid handling modules and several automated sample preparation options.

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

For more information on this source, please visit Metrohm.

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