Chemistry control

Chemistry control of contained fluids assists in reducing degradation from inside of components. Corrosion inhibitors can be effective in minimizing internal degradation in closed loop systems. Biocides, surfactants and biodispersants can be effective in preventing fouling, attachment and growth of microorganisms for raw water piping. Where these systems are open loop systems, restrictions on chemical discharges are generally applicable and post-treatment may be necessary.

Chemistry control of the external environment of buried piping is not a practical option; however, knowledge of external conditions is critical to understanding the corrosion potential for the pipes or tanks in question. The normal mitigation method for external corrosive environments is through the application of barrier coatings.

6.2.1. Water analysis

Most nuclear power plants routinely collect water samples from process streams (e.g. service water, cooling water, fire systems and raw or treated water used for plant make-up).

Water sampling and analysis assist in determining the degradation potential of underground piping from the ID (which will be additive to OD degradation) and assessing the performance of chemical treatment programmes designed to minimize corrosion, microbiological fouling and scale/deposition. It is necessary to develop and follow standard procedures to ensure that samples are representative of the process stream, information necessary to characterize the sample is obtained, external contamination is minimized and proper preservation is utilized.

Samples typically need to be obtained from pressurized pipes or closed conduits, but there are no universal sampling procedures. Samples can be collected by taking discrete grab samples, or by obtaining samples over a

specified period of time. Particulate sampling (collected on filter pads and/or ion exchange filters) may be utilized for measuring trace level metals in high purity water applications.

Grab samples are single samples collected from an individual location at a specific time and thus represent only a ‘snapshot’. A small portion of the process stream is obtained and used to characterize the entire process.

When the process stream is known to vary over time, discrete samples may be collected over a longer interval and statistical data evaluation can be performed to determine representative process quality. Continuous monitoring may also be done for pH, conductivity, specific ions, and so on using instruments that are inserted into the flow or a side stream.

Composite samples provide a more accurate representation of heterogeneous process streams. Composite sampling may consist of collecting a series of individual samples and integrating them into a single sample or using an automated sampler that collects samples at regular intervals as defined by a programmed sequence schedule.

Composite sampling is not recommended for some constituents that can change over time such as oil and grease, pH, temperature, dissolved oxygen and volatile organics.

6.2.2. Water treatment

There are a large number of processes that circulate water in a nuclear power plant in piping. Deposition, fouling and corrosion of inner pipe diameters may adversely impact on system performance and if not properly treated and monitored may result in damage and ultimately pipe failures. A comprehensive chemical treatment programme may be necessary to mitigate these effects in systems where treatment can be applied.

Chemical treatment programmes are commonly utilized to control corrosion (deposition, scaling and fouling) and microbial growth, which can lead to MIC. The programme should be based on the quality and variability of the make-up and circulating water. Corrosion inhibitors are selected to protect materials in the system and may be comprised of cathodic inhibitors, anodic inhibitors or a combination of both. Deposition is normally associated with preventing calcium carbonate formation but may also address other compounds such as calcium phosphate, calcium sulphate or magnesium silicates. Microbial growth is often controlled through addition of oxidizing and/

or non-oxidizing biocides. Oxidizing biocides may be fed on a continual basis at a lower dose, or intermittently at a higher dose. Non-oxidizing biocides are typically batch fed.

6.2.2.1. Chemical treatment

Once-through water systems in nuclear power plants use water in a single pass and are discharged directly; a cooling tower is not used. In open recirculation systems, water is used multiple times; heat is rejected in a cooling tower, sprays or via other processes. Chemical treatment options for open systems require greater chemical usage and attention than treatment of closed loop systems.

In all systems, minimizing corrosion, deposition and scaling and biofouling typically requires chemical treatment and operational controls. Chemical additives are designed for a given application and system metallurgy.

Most open systems may require the use of four classes of chemicals that consist of corrosion inhibitors, deposit control agents or dispersants, antimicrobials and pH adjustment chemicals. Blowdown of open recirculation systems is an integral part of the process as it removes dissolved and suspended solids. Treatment of a once-through system will generally be less effective than treatment of an open recirculation system of the same basic water chemistry and will require greater amounts of chemical addition to achieve the same levels of effectiveness.

6.2.2.2. Corrosion control

Controlling corrosion requires either changing the metal so that it will not corrode in the given environment, which is often an expensive and impractical alternative, or changing the environment to inhibit the corrosion process. Modifying the environment for a water system can be accomplished by forming a protective film at the metal surface by the addition of corrosion inhibitors. Scales (e.g. calcium carbonate) can also provide a level of corrosion protection for CSs.

In most plant water systems, the most common approaches are to use corrosion inhibitors or deaerate the system. Corrosion inhibitors are designated as anodic or cathodic, depending upon whether reactions are blocked at the anodic or cathodic corrosion sites.

6.2.2.3. Scale and deposit control

Scale deposits are a result of precipitation and crystal formation where the solubility of certain species is exceeded in water or at its surface. Some examples in cooling water application may include calcium carbonate, calcium phosphate, calcium sulphate and magnesium silicate. These compounds exhibit retrograde solubility in that they are less soluble at higher temperatures and tend to precipitate on heat exchanger surfaces and downstream piping.

Factors that play a role in scale formation are water quality, pH, temperature, cycles of concentration in cooling systems and holding time. Once scale is formed it provides a site for additional crystal growth that can occur at a more rapid rate [91].

Fouling occurs when insoluble particulates suspended in water deposit on surfaces. These materials may enter the system from make-up, airborne contaminants, or raw water treatment chemicals (aluminium and iron).

Polymers can provide good scale control inhibition as well as keep suspended solids in the water.

6.2.2.4. Biological control

Chemical treatment to control biological growth is classified into oxidizing and non-oxidizing biocides [324].

An effective treatment programme and choice of biocide takes into account the system to be treated, water analysis, organisms to be controlled and environmental impact.

Increasingly, bivalves such as zebra mussels have been a reported concern for nuclear power plants.

Impacts can include clogging of pipes, screens, condensers and heat exchangers, reductions in efficiency and increased erosion and corrosion. Systems using raw water or raw water make-up (water intakes, service water, fire systems, etc.) are particularly vulnerable, especially during lay-up periods if they are not emptied and dried.

Chemical treatment is normally needed at both intakes and discharges during applicable growing seasons.

Systems have also been developed for electrolytic biological control of organisms such as barnacles, limpets, mussels and tubeworms. The tank system shown in Fig. 87 has electrodes contained in a tank through which a flow of water is passed and then fed to the pump intake. The water becomes treated with copper/aluminium ions in the tank and the discharge of this treated water gives antifouling protection to the downstream pump(s).

FIG. 87. Cuprion system for biological control using ions (courtesy of Cathodic Protection Co. Ltd.) [325].