Corrosion and biological problems in closed water systems are a serious concern in the nuclear industry. Loss of heat exchanger efficiency, costly maintenance repairs and cleanings, and expensive equipment and piping replacements are recurring problems.
Corrosion products and biomass activity are the two main operating problems in nuclear water systems and may cause the following operation problems:
Corrosion particulate matter can become radioactive and result in undesirable environmental concerns.
Reduced flow rates and increased pump head pressures caused by deposit buildup on the surface of the heat exchanger tubes or piping
Loss of heat exchanger efficiency as a result of deposit buildup on the metal surface
Formation of deposits that provide an environmental for anaerobic bacteria corrosion attack
Deposit buildup that plugs tubes and/or piping and can affect the normal operation of valves, orifices, sensing equipment, and strainers
Retards the formation of a strong protective film on the metal surface by the corrosion inhibitor
Corrosion products are the reaction product of the corrosive attack on carbon steel. If this reaction is allowed to continue unabated, then in time the carbon steel heat exchanger, auxiliary equipment and/or piping will no longer operate efficiently or will have to be replaced.
Biomass accumulation may cause the following operational problems:
Interference with the function of a corrosion inhibitor forming a strong protective film on the metal surface
Reduction in heat exchanger efficiency
Plugging or reduction of flow rates in piping and/or heat exchangers
Provides an environment for anaerobic bacteria type corrosion
Promotes the formation of obnoxious odors
Some nuclear plants experience a scaling phenomenon in their condensers which reduces the condenser efficiency and can cause condenser material degradation. These plants have a scaling mechanism that is directly attributed to their cooling water calcium hardness and alkalinity.
Following are some case histories for the nuclear power industry.
Case History 1: Nuclear - Utility - Braidwood Nuclear Station (Figure PO-5) is a two-unit, 2350 MW plant located south of Chicago. Units 1 and 2 began commercial operation in 1988. Braidwood's source of water is the Kankakee River. The makeup demineralizer is designed for 120 gpm capacity. River water pretreatment consists of polymer feed and lime softening clarification followed by sand and activated carbon filtration. The water is further treated and demineralized in a multimembrane trailer which includes UF, EDR and RO. The RO permeate is then fed to an ion-exchange demineralizer system consisting of cation and anion-resin beds followed by a mixed-bed polisher.
Case History 2: Nuclear Power - LGS (PECO Energy's Limerick) in 1994, initiated the first stage of it quest for a more cost-effective, superior quality makeup water with the installation of a triple membrane trailer (TMT) system.
The new process design continued the use of LGS' existing clarification, anthracite, and carbon filtration equipment as pretreatment to the new membrane system. The primary membrane filtration process is a 50,000 dalton cutoff, spiral-wound ultrafiltration membrane. The UF follows the existing equipment as the final pretreatment process, providing a clean, manageable water for the high performance demineralization membrane systems to follow.
The secondary filtration and primary demineralization stage of the process is a reverse osmosis system using thin-film composite membranes. This RO acts as the bulk demineralizer in the process design. An electrodeionization membrane system follows the RO and operates as the primary polisher. The substantially purified water is then placed in contact with ozone for TOC/trihalomethane (THM) reduction and then ultraviolet for ozone destruction. Final polishing is accomplished by two stages of off-site regenerated, nuclear grade mixed bed (MD) ion-exchange polishers and a resin trap to prevent any resin from entering the system.