核电站单次回路中阳离子的在线痕量分析

AN-PAN-1043

2021-02

核电站一次回路中阳离子的在线痕量分析在发电厂中，腐蚀是导致代价高昂的关键停机时间的主要因素。在核电站中，被称为压水堆（PWR）的独立水回路可确保放射性物质不被污染，同时还能向其他回路输送热量和能量。硼酸和氢氧化锂是专门添加到压水堆回路中的，其用量会使其他分析测量复杂化。锂可以防止腐蚀，必须与锌、镍和铵等其他阳离子一起进行监测。为了在单次分析中在线测量这些阳离子的含量（亚微克/升），2060离子色谱过程分析仪结合了在线预浓缩和在线基质消除功能。一次进样可分析多种阳离子，自动样品制备使精确可靠的测量变得更加容易。

In power plants, corrosion is the primary factor leading to costly and critical downtimes. Pressurized water reactors (PWR) are generally designed with a third water circuit in addition to the two found in other thermal power plants (e.g., boiling water reactors (BWRs) or coal-fired and geothermal power plants). In this so-called «primary circuit», the primary coolant water is pumped under high pressure to absorb heat generated by nuclear fission, further transferring this heat to the secondary circuit. The PWR ensures that radioactive materials remain contained and do not disperse to the secondary circuit and therefore potentially to the environment.

Figure 1. Diagram of a 3-water circuit nuclear power plant with stars noting areas where online process analysis can be integrated into the system.

This additional water circuit entails some specific requirements regarding chemical analysis and monitoring. In the PWR, light water is used as primary coolant. Boron (as boric acid, H₃BO₃) readily absorbs neutrons and is added to the coolant to control reactivity. Additions of monoisotopic lithium hydroxide (LiOH) to the primary circuit assures a pH value greater than 7 to prevent corrosion. Lithium is therefore a critical parameter to monitor in PWRs.

Nickel is an important alloying metal that increases the corrosion resistance of steel. However, in the dissolved form, nickel promotes corrosion, making regular checks of the Ni²⁺ concentration necessary.

In addition, metals released by corrosion undergo nuclear reactions and thus increase radiation in the power plant. To limit corrosion of the materials and to prevent the formation of radioactive products, the water is often treated with depleted zinc oxide. The zinc concentration must be monitored as well to ensure radioactivity does not spread.

Figure 2. 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.

Figure 3. The 2060 IC Process Analyzer is available with either one or two measurement channels, along with integrated liquid handling modules and several automated sample preparation options.

These cations and more can be determined down to the sub-μg/L range in a single analysis using ion chromatography. However, precise, reliable trace analysis requires the method to be automated as much as possible. Metrohm Process Analytics offers a complete solution for this task: the 2060 Ion Chromatograph (IC) Process Analyzer featuring combined Inline Preconcentration and Inline Matrix Elimination. With one injection, the 2060 IC Process Analyzer is able to measure numerous ionic compounds in aqueous media from ng/L to % concentrations. The analysis system is fed directly and continuously with samples via a bypass in the process. Automatic calibration guarantees excellent detection limits, a high reproducibility, and excellent recovery rates.

The 2060 IC Process Analyzer provides alarms if preset warning or intervention concentration limits are reached, helping to save costs by preventing irreparable damage due to corrosion. The 2060 IC Process Analyzer can monitor both anions and cations with a single ion chromatograph and two detector blocks, giving a comprehensive overview of the water circuit chemistry. The possibility to connect one analyzer to up to 20 sample streams means multiple areas within water-steam circuits in a power plant can be monitored by a single instrument. With a built-in eluent production module and optional PURELAB® flex 5/6 from ELGA® for pressureless ultrapure water, the 2060 IC Process Analyzer can be configured to run trace anion analyses autonomously for up to several weeks.

The analysis is carried out fully automatically using a combination of Inline Matrix Elimination (for the H₃BO₃) and Inline Neutralization (for the LiOH). Analyte detection is by conductivity.

Inline eluent preparation ensures consistently stable baselines
Protect valuable company assets (e.g. pipes, PWR, and turbines, which are prone to corrosion)
Safe working environment and automated sampling)
High precision analyses for a wide spectrum of analytes with multiple types of detectors