Using Ion Chromatography to Determine Anion, Cation Levels in Power Plant Water

Successful operation of a nuclear power plant demands careful control and monitoring of water chemistry in primary coolant dosed with boron and lithium, steam generator tubes dosed with ammonia, closed cooling water systems dosed with nitrite and other structural parts that are subject to corrosion and crud deposition. This can affect plant operation and radiation exposure during refueling downtime.

For nuclear power plants using a PWR (pressurized water reactor), the primary coolant includes boric acid as a water-soluble neutron absorber to control the nuclear reaction. At the high temperature and pressure in the PWR, this boric acid concentration can form crud that deposits metal oxides on the fuel rods. To prevent crud buildup, lithium hydroxide is added to increase the pH to 6.9 or higher. However, trace anionic impurities at low- or sub-µg/L (ppb) concentrations from the water source (or materials such as ion-exchange polisher resins) can serve as catalysts for corrosion. A BWR (Boiling Water Reactor) has a secondary circuit that generates steam to drive the turbine and produce electricity.

Lithium borated water and ammoniated condensate are measured for low-level µg/L concentrations of fluoride, chloride, and sulfate with the addition of a continuous regenerating cation trap column (CR-CTC III) to remove interference from mg/L concentrations of either lithium or ammonia in the samples. Other closed cooling water systems are dosed with high nitrite concentrations (up to 1000 mg/L) to prevent corrosion, but low-level µg/L chloride measurement is required for early signs of seawater ingress. This is made possible with a Thermo Scientific Dionex AS14A Eluent (sodium carbonate/bicarbonate).

In pure water, the BWR environment is oxidizing due to the radiolytic generation of species such as oxygen and hydrogen peroxide, increasing the propensity of metals to undergo intergranular stress corrosion cracking (IGSCC). Injecting hydrogen into the feedwater and adding noble metal suspensions promote recombination of hydrogen and oxidants on the metal surfaces, thus lowering IGSCC. Control of zinc, iron, copper, and other impurities in BWR final feedwater is essential.

Ion Chromatography (IC) is a safe and reliable method of choice to determine µg/L (ppb) and mg/L (ppm) levels of anions and cations including transition metals in power plant water. Cation-exchange chromatography with non-suppressed conductivity detection is a simple and direct technique for quantifying low-µg/L concentrations of selected transition metals.