Identification and quantification of organ species in power station feedwater

Abstract

It is possible for organic material to enter the steam/water cycle of a power station in number of ways. Make-up water, chemical additives and ion exchange resin leachates are usual sources of organics entering the system during normal power operation. Other sources include cleaning agents, lubricants, bacteria and condenser inleakage that can enter the steam/water by contamination during outages or damaged plant during operation. Although the steam/water cycle is a closed loop, water and steam is lost by imperfection in design and construction, material degradation and defective components. Normal operation includes interventions that occasionally require or result in a loss of steam and / or water from the secondary system. From several strategic positions on the secondary cycle on both the water and steam lines, continuous sample flows via the sampling system to the online analysers and grab sampling points. All of operational losses of steam and water require the secondary system to be made-up. Make-up water is the source of natural organic matter (NOM) that enters the system during normal operation. Water treatment that includes chlorination for disinfection causes the formation of disinfection by-products (DBP) and of specific interest are organochlorides such as chloroform as a product f ram reaction of chlorine with NOM and microorganisms. Organochloride breaks down under the plant thermohydraulic conditions resulting in the .increase of the chloride ion concentration. Chloride in contact with metal surfaces has a detrimental effect on the structural integrity for power plants. lnconel, a metal alloy consisting of nickel, chromium and iron is used for the manufacture of steam generator (SG) tubes of PWRs worldwide because of its physical properties. SG tubing is by far the most vital component of this type of nuclear power plant and lnconel is susceptible to chloride induces pitting corrosion. Chloride also causes stress corrosion cracking (SCC) in austenitic stainless steels that constitute the balance of the plant systems. For these reasons it is imperative that plant chemists must continue to strive to keep the chloride concentration as low as achievable. Over the years it has been observed that high secondary make-up at Koeberg Nuclear Power Station (KNPS) results in corresponding increased chloride levels in the steam generators even though the make-up water tanks chloride analysis is low. The elevated chloride concentration following high system make-up has been attributed to trihalomethanes (THM) being present in the make-up water. Chemicals are added to the secondary circuit with the intent to establish the most favourable chemical condition that will mitigate plant degradation by limiting corrosion mechanisms. It is usual to have additives that control pH and dissolved oxygen in the secondary system of a pressurised water reactor (PWR). Historically inorganic additives such as trisodium phosphate and later ammonia have been used to control pH. Recent developments have established that organic amines have superior properties and are often the preferred additive. Plants that use amines add to the organic loading and this has consequences that are important to consider. In power plant chemistry ion exchange (IX) resins are used to remove impurities from the process water. Ion exchange is extremely effective for retaining inorganic ionic species but organic compounds are not readily retained. These IX resins are organic polymers and under certain physical conditions break down resulting in the leaching of organic material into the process water increasing the organic load. For this work, water samples of the secondary cycle from PWR, KNPS, have been analysed to gain a better understanding of the overall effect of organic material on the system chemistry. During normal plant operation sources include NOM, chemical additives and ion exchange resin leachates. This work focuses on the analysis and profiling of organic material entering the system during normal operation with more specific focus on organochlorides because of its potential detrimental effect on the plant construction materials. In the attempt to identify and quantify organic species at KNPS methods for identification and quantification of organic species at KNPS were developed and implemented. This was achieved by purchasing Total Organic Carbon (TOC) analyser and also specific analytical column were purchased and installed in the Ion Chromatography (IC) System to enable it to have capabilities to quantify organic anions such as formate, acetate, oxalate and glycolate in single run along with the inorganic anions. The TOC and IC work was performed at KNPS while some of the samples were sent to various laboratories for further organic characterisation and also for trihalomethanes (THM) analysis. Koeberg Nuclear Power Station surveillance procedures were updated to reflect a surveillance requirement for TOC in the water production plant systems. This research project has also initiated various other projects at KNPS. One of the projects was the anion resin routine clean-up process where the organic material, that fouls the anion resins are removed by soaking the resin in a solution containing 5 % sodium chloride and 2 % sodium hydroxide and this process is also known as a "brine washed". Procedures were compiled and they are currently being utilised by KNPS chemistry staff. Another project that was initiated was to replace the anion resins in the water production plant. This project has also initiated a strategy and recommendation on how KNPS should deal with poor water quality and these recommendations were communicated to Engineering Group so that appropriate modification may be initiated. TOG analysis was performed at various strategic points around the water production plant and secondary systems to develop an organic profile which had not previously been performed at KNPS.