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Reducing Deposits in Steam Generators By Electric Power Research Institute. A Costly Performance- Robbing Problem Corrosion products entering the secondary side of pressurized water reactors (PWRs) via feedwater can deposit on steam generator tubes and other internal surfaces. These tenacious deposits inhibit heat transfer, block tube supports, and create crevices where corrosive impurities can accumulate. If not corrected, these deposits may lead to stress corrosion cracking and tube failure. To combat the problem, nuclear utilities have tried reducing the amount of corrosion products in the feedwater or removing deposits by chemical cleaning or mechanical sludge lancing using highpressure water jets. These approaches are often effective, but can be costly and carry risks related to extended outages or incomplete cleaning. An alternative approach is to inject dispersants, which prevent corrosion products from depositing on steam generator surfaces so they can be removed via blowdown. Duke Energy performed a fullscale long-term trial of a high-purity polyacrylic acid (PAA) dispersant at its McGuire Unit 2 PWR. Findings show that dispersant application resulted in a signifi cant reduction in the rate of steam generator fouling. Collaborative R&D The McGuire trial caps more than a decade of collaborative research and development involving EPRI, utilities, vendors, and consultants. The longterm trial also corroborates results from previous proof-of-concept studies and short-term fi eld tests. In the 1990s, Commonwealth Edison (now Exelon) conducted corrosion tests to qualify a high-purity version of Source: Electric Power Research Institute's Success Story number 1018533, January 2009. PAA dispersant and transferred details of the PAA program to industry through EPRI. EPRI subsequently collaborated with Entergy on a short-term fi eld trial at Arkansas Nuclear One Unit 2 (ANO-2). The three-month trial took place during the fi rst half of 2000 just prior to steam generator replacement, demonstrating that PAA can increase the blowdown iron removal effi ciency by an order of magnitude. Details of the trial are documented in two EPRI reports, Dispersants for Tube Fouling Control, Volume 1: Qualifi cations for a Short-Term Trial at ANO-2 (1001422) and Dispersants for Tube Fouling Control, Volume 2: Short-Term Trial at ANO-2 (1003144). Following the ANO-2 trial, researchers completed additional qualifi cation work to technically justify a longer-term trial of 6-9 months at a plant with replacement steam generators tubed with Alloy 690. Qualifi cation efforts focused on two main areas: materials/chemistry and steam generator thermal performance. The materials/ chemistry evaluations indicated that a long-term trial would not result in any adverse conditions in balance-of-plant or steam generator materials. The thermal performance analyses indicated that all candidate steam generator designs had suffi cient thermal margin to accommodate a slight decrease in heattransfer effi ciency that could occur during a long-term trial. This work, which was documented in the EPRI report, Dispersants for Tube Fouling Control, Volume 3: Qualifi cation for a Long-Term Trial in a Replacement Steam Generator Tubed with Alloy 690 TT (1002774), led to technical concurrence with a long-term trial by Westinghouse and Babcock & Wilcox Canada. Continuing the R&D momentum, Duke Energy committed to a full-scale long-term trial at McGuire Unit 2 in 2004. Starting in 2005, plant personnel injected PAA into the feedwater piping upstream of the individual loop lines. Concentrations of PAA ranged from 0.25 ppb to 4 ppb. McGuire personnel devoted considerable time and effort to ensure that the trial would be implemented safely. Despite several challenges over the course of the trial from August 2005 to September 2006 (such as injection pump issues and plant transients unrelated to PAA), the plant staff made signifi cant contributions that were invaluable in making the trial a success. Results and Benefi ts • PAA dispersant injection at 2–4 parts per billion increased the corrosion product removal effi ciency from about 5% to 45–50%. • McGuire Unit 2 exhibited a slight benefi cial increase in thermal performance level during the trial. • Secondary chemistry parameters • were not adversely affected. Demineralizer performance was not compromised. Results of the McGuire trial are providing the basis for steam generator vendor technical concurrence with industrywide long-term dispersant use. Findings have also contributed to the development of EPRI’s recently published Dispersant Applications Sourcebook (1015020). This publication provides guidance and comprehensive information for utilities planning to use PAA dispersant. Duke personnel were an invaluable resource on the Sourcebook development committee, providing signifi cant input to the reference. Going Forward Several utilities are planning longterm dispersant applications. For example, Exelon is planning to begin PAA injection at Byron Unit 1 in January 2009, and plans on adding dispersant at Byron Unit 2 and Braidwood Units 1 and 2 starting later in 2009. (Continued on page 44) 42 www.nuclearplantjournal.com Nuclear Plant Journal, March-April 2009
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Page 21 and 22: Bechtel Nuclear: Building on the Pa
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