Nuclear energy now provides about 10% of the world’s electricity from about 440 power reactors. Growing global energy demands and climate change abatement goals are driving more investment, particularly in the Asia Pacific region.
Most of these nuclear power plants (NPP) are classified as pressurized water reactors (PWRs) or boiling water reactors (BWRs). A PWR has a primary cooling circuit where water flows as a liquid through the core of the reactor under high pressure and a secondary circuit that generates steam to drive the turbine and produce electricity. Boric acid, a neutron absorber, is added to the primary water to control core reactivity. A BWR is like a PWR but uses only a primary circuit to boil water at a lower pressure to directly drive the turbine to produce electricity.
In pure water, the BWR environment is oxidizing due to the radiolytic generation of species such as oxygen and hydrogen peroxide, increasing the susceptibility of metals to undergo intergranular stress corrosion cracking (IGSCC). Injecting hydrogen into the feedwater and adding noble metal suspensions promotes recombination of hydrogen and oxidants on the metal surfaces, thus lowering IGSCC. As such, control of transition metals (e.g., zinc, iron, copper, and other impurities) in BWR final feedwater is essential.
Two ion chromatography (IC) methods have been used to determine transition metals in power plant waters. Cation-exchange chromatography with non-suppressed conductivity detection is a simple and direct technique for quantifying low-µg/L concentrations of selected transition metals. However, achieving low- or sub-µg/L detection limits for these transition metals requires the concentration of significant sample volumes, which may cause column overload due to the high concentration of lithium in PWR samples. The determination of transition metals also can be achieved by the formation of anionic complexes with a chromatographic eluent that contains pyridine-2,6-dicarboxylic acid (PDCA), separation of the complexed transition metals, post column displacement of PDCA with 4-(2-pyridylazo) resorcinol (PAR), and absorbance detection at 530 nm. This method provides broad selectivity for a variety of transition metals and yields sub-µg/L sensitivity.
This study describes the separation of iron (III), copper (II), nickel (II) and zinc (II) complexes with PDCA followed by post-column PAR reaction and absorbance detection at 530 nm. Surrogate samples include spiked deionized (DI) water (BWR surrogate) and spiked borated water containing lithium hydroxide. Detection limits <0.1 ug/L were achieved after concentration of 4.7 mL of sample. The linearity, detection limits, precision, and accuracy of the method for determining the targeted transition metals at sub-ug/L concentration in surrogate BWR are described.
Method performance for BWR surrogate samples
DI water was the surrogate matrix used to simulate the liquid circulating in a BWR. Figure 1 compares the chromatograms from the matrix blank to a spiked matrix with 1 µg/L of each of the target transition metals. All four transition metals are well resolved from each other and from the early eluting system peaks. Method performance was evaluated by spiking two concentrations of the four target transition metals in the DI water surrogate matrix (Table 1). Recoveries at 0.2 µg/L Fe, Cu, Zn, and 0.4 µg/L Ni ranged from 84 to 101%, whereas recoveries at 0.8 µg/L Fe, Cu, Zn, and 1.0 µg/L Ni ranged from 95 to 103%. These results demonstrated the ability of this method to accurately determine sub-μg/L concentrations of the target transition metals in BWR samples.
An IC method with 530 nm absorbance detection was evaluated for the determination of iron (III), copper (II), nickel (II) and zinc (II) at sub- and low-µg/L concentrations in boiling water and the primary cooling circuit of pressurized water nuclear power plant reactors. The Dionex™ IonPac™ CS5A column with a PDCA eluent allowed the separation of the four target transition metals in less than 10 minutes, even in the presence of high concentrations of boric acid and lithium that constitute the PWR primary coolant surrogate matrix.
To learn more about the study, read this application note.
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