System Contaminants and Water Quality Guidelines
During nuclear power operation, sodium will concentrate in crevices and points of evaporation, resulting in high sodium alkalinity, which contributes to stress corrosion cracking in reactors, steam generator tubes, and turbine components.
Silica is also of concern in the reactor. It easily complexes with metals that form zeolite compounds and accumulate on heat transfer surfaces, undermining efficient heat transfer. Many nuclear plants have adopted a reactive silica limit for the makeup water of ≤ 10 ppb. This concentration limit provides protection since the boiler water silica specification is ≤ 1,000 ppb. Boroflex® (BASF Corporation) coated fuel racks used to store spent fuel in the spent fuel pool are a significant source of silica and contribute to primary coolant treatment issues.
Total chloride and sulfate ion concentrations introduced into the systems (if not from system leaks or makeup) typically come from the ion exchange resins themselves. These impurities are present as a combination of inorganic chlorides and sulfates and organically bound chloride (OBCl) and sulfate (OBS). These compounds degrade thermally or radiolytically in the reactor, producing chloride and sulfate ions, potentially leading to intergranular corrosion (IGC) and stress corrosion cracking. The typical allowable level of sulfate in the coolant is 1.50 ppb and the permissible level of chloride is < 0.5 ppb.
Water Quality Guidelines
Although the nuclear industry does not regulate the type or quality of ion exchangers used in nuclear plant operations, it does establish guidelines on makeup water, primary chemical volume control and shutdown chemistry. It also regulates secondary chemistry coolant that impacts condensate polishing and steam generator blowdown quality. These guidelines strongly drive the selection and specification of resins by nuclear operations to maintain consistent quality and cleanliness within circuits. Individual nuclear power companies set specifications for resins purchased for the different plants. However, these are generally based on resin manufacturers’ specifications. Recently, the Electric Power Research Institute (EPRI) has implemented new guidelines on the organic sulfate and chloride impurities extracted from resin. These new specifications are designed more for the boiling water reactor (BWR) condensate resins but cover all nuclear resins.
Make-up water chemistry parameters have been referenced by the Institute of Nuclear Power Operations (INPO) Chemistry Guidelines, the World Association of Nuclear Operators (WANO) and EPRI Chemistry Guidelines. All emphasize maintaining extremely low contaminant concentrations in the makeup water and minimizing corrosion products and trace impurities from coolant water and condensate streams.