Dispersant application: (1) during steam generator wet layup for removal of existing deposits, and (2) during the long-path recirculation cleanup process of the condensate/feedwater system to reduce startup corrosion product transport to the steam generators

During the last few decades, utilities have spent considerable resources minimizing corrosion product deposition within steam generators (SGs). In the past, two basic approaches have been used: Reducing the corrosion product ingress rate (e.g., by replacing secondary components containing corrosion-susceptible materials, implementing favorable chemistry changes, etc.); Removing corrosion products which have accumulated in the SGs through top-of-tubesheet (TTS) sludge lancing and other chemical and mechanical methods. Despite the success of these methods, there are limitations, including practical lower limits on the feedwater iron concentration and the high cost and effectiveness limits of cleaning techniques (particularly for crevices). A third approach is the online addition of a polymeric dispersant to promote suspension of corrosion iron, thereby reducing deposition onto SG surfaces and facilitating more efficient removal via blowdown. More than a decade of qualification work and two full-scale plant trials - at ANO-2 in 2000 and at McGuire Unit 2 from 2005 to 2007 - addressed initial technical concerns, paving the way for routine use in nuclear SGs. Online application of dispersant at the four Exelon plants with recirculating SGs is the focus of another paper at this conference. This paper is focused on the additional benefits that could be gained from similar dispersant applications during: normal SG wet layup to remove some of the existing deposit inventory; routine long-path recirculation cleanup of the PWR secondary side prior to startup. The addition of dispersant to the SGs during full wet layup periods could provide additional benefit by dispersing loose sludge powder that has accumulated, thereby facilitating its removal. Routine dispersant-assisted wet layup applications could be performed in conjunction with normal layup protocols without affecting the planned outage schedule, and could potentially reduce the frequency of more costly deposit management operations (e.g., sludge lancing, chemical cleaning, etc.). In 2009 EPRI worked with Exelon Corporation to develop and implement a plan for a trial dispersant application during the SG wet layup at Three Mile Island Unit 1 (TMI-1). Because the SGs were scheduled for replacement during the Fall 2009 outage, this trial represented a unique opportunity to evaluate the efficacy of a dispersant wet layup application with minimal risk to the SGs. This paper discusses the technical bases supporting the addition of dispersant during wet layup at TMI-1 and the results of the Fall 2009 trial application. Additional applications, under EPRI sponsorship, have either just taken place (Doel 3 in Summer 2010) or are planned (Braidwood 1 in Fall 2010). It is anticipated that addition of a dispersant during the long-path recirculation cleanup process will more readily clean up transportable corrosion products from the system and increase their retention time in solution, thereby increasing the amount of iron removed from the condensate and feedwater systems prior to initiation of flow to the SGs. Evaluations preparing for applications of this type include: Reviews of the procedure and general characteristics of the long-path recirculation cleanup process at three plants: Byron Unit 1, Millstone Unit 2 and Three Mile Island (TMI) Unit 1; A laboratory test program to assess dispersant efficacy under the conditions present during the long-path recirculation cleanup process. A number of different dispersant chemicals, including the polyacrylic acid (PAA) used in online applications, were investigated; Generic and plant-specific evaluations (for the above three units) of the compatibility of PAA with the secondary system components anticipated to be wetted during the long-path recirculation cleanup process. Based on the results of this study, inputs concerning the recommended dispersant chemical, concentration, application schedule, and cleanup criteria were generated to aid interested utilities in the development of a plant-specific application plan for an initial industry trial. An application is planned at Byron 1 in Spring 2011 under EPRI sponsorship. (author)