Determination of Trace Fluoride, Chloride, and Sulfate in Lithium-Containing Borated Waters

Importance of Topic

In pressurized water reactors, boric acid and lithium hydroxide are added to the coolant to control neutron reactivity and maintain pH. Trace levels of fluoride, chloride and sulfate in the borated coolant can accelerate corrosion of fuel cladding and system components. Reliable monitoring of these anions at low microgram-per-liter concentrations is therefore essential to support plant safety, reliability and regulatory compliance.

Objectives and Study Overview

The primary goal was to develop a simple and robust ion chromatography method for the determination of trace fluoride, chloride and sulfate in lithium-containing borated waters. By adapting the Dionex IonPac AS14 column and a manually prepared borate eluent with a two-step isocratic program, the method aims to achieve baseline separation of the borate matrix and the target anions without requiring complex matrix elimination.

Methodology

Simulated reactor coolant samples were prepared containing 1 000–2 500 mg/L boron and 1.8–5.0 mg/L lithium. Standards and samples were injected directly (1.0 mL) onto a 2 mm Dionex IonPac AS14 column protected by an AG14 guard. A two-step gradient was applied using Eluent A (100 mM boric acid/75 mM NaOH) and Eluent B (DI water). The program held 10 mM boric acid/7.5 mM NaOH for 4 min, stepped to 65 mM boric acid/48.75 mM NaOH for 4.01–8.50 min, then returned to initial conditions. Suppressed conductivity detection with an ASRS 300 suppressor was used at 55 mA.

Used Instrumentation

• Thermo Scientific Dionex ICS-5000 system (SP pump, DC compartment)
• Dionex IonPac AG14 guard (2 × 50 mm) and AS14 analytical column (2 × 250 mm)
• Dionex ASRS 300 anion self-regenerating suppressor
• Dionex AS-AP autosampler with 5 mL syringe and 1 mL sample loop
• Chromeleon 7.1 chromatography data system software

Main Results and Discussion

The large borate matrix peak eluted early (~1.7 min) and returned to baseline before fluoride elution, eliminating overlap. Baseline separation of fluoride, chloride and sulfate was achieved within a 15 min run time at 0.5 mL/min. Calibration was linear (r2>0.999) over 0.1–10 µg/L. Method detection limits in DI water were 0.021 µg/L (F–), 0.094 µg/L (Cl–) and 0.11 µg/L (SO4^2–); in a 2 500 mg/L boron matrix: 0.048, 0.19 and 0.31 µg/L, respectively. Recoveries for spiked simulated samples (1 000–2 500 mg/L B) ranged from 92 to 109 %. Retention time RSD was ≤0.6 % and peak area RSD ≤5 % over seven injections.

Benefits and Practical Applications

• Direct injection of large volumes simplifies sample handling and enhances sensitivity.
• Early elution and resolution of the borate peak avoid complex matrix removal steps.
• High precision, low detection limits and broad linear range support QA/QC of reactor coolant.
• Reduced manual eluent preparation through a single borate/NaOH solution.

Future Trends and Potential Applications

• Integration of automated eluent generation systems to further reduce manual preparation.
• Coupling ion chromatography with mass spectrometry for enhanced specificity.
• Extension of the method to monitor additional anions and radionuclide signature species.
• Implementation in online or at-line process monitoring for real-time coolant quality control.

Conclusion

The presented ion chromatography method enables accurate, precise and robust determination of trace fluoride, chloride and sulfate in lithium-containing borated waters. By resolving the borate matrix and employing a straightforward two-step eluent program with large-volume injection, the approach meets low detection limits and stringent recovery and precision criteria, making it well suited for nuclear power plant water chemistry monitoring.

References

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4. Dionex Application Note 56: Trace Anions and Organic Acids in High-Purity, Ammoniated and Borated Waters. Thermo Scientific; 1988.
5. Dionex Application Note 114: Trace Anions in High-Purity Waters Using Direct Injection and Two-Step Isocratic IC. Thermo Scientific; 1997.
6. Dionex Application Note 185: Trace Organic Acids and Inorganic Anions in Boric Acid-Treated Power Plant Waters. Thermo Scientific; 2008.
7. Dionex Application Update 175: Organic Acids and Inorganic Anions in Lithium-Containing Boric Acid-Treated Waters. Thermo Scientific; 2010.
8. Dionex Application Note 166: Eluent Generation for Trace Anion Analysis of Borated Waters. Thermo Scientific; 2004.