Dispersant trial at ANO-2: Results from a short-term trial prior to SG replacement

Corrosion products that make their way to the secondary side of pressurized water reactor (PWR) steam generators (SGs) via the feedwater can deposit on the SG tubes. These deposits can form an occluded region which inhibits heat transfer, leads to thermal hydraulic instabilities through blockage of tube supports and creates regions where corrosive species can concentrate along tubes and tube to tube support plate crevices. The performance of the SG is compromised not only by formation of an insulating scale, but by the removal of tubes from service due to corrosion. A promising new method for significantly reducing corrosion product deposition on the secondary side of recirculating steam generators is the use of online dispersant addition to help prevent the corrosion products from adhering to the steam generator surfaces. By inhibiting the deposition of the corrosion products, they are more effectively removed from the steam generator via blowdown. After completion of a significant and comprehensive qualification program, a short-term dispersant trial was performed at Arkansas Nuclear One Unit 2 (ANO-2) in Winter/Spring 2000, lasting approximately 3 months. A high purity, high molecular weight polyacrylic acid (PAA) dispersant produced by Betz-Dearborn was injected at low concentrations (0.5 μg x kg-1 to 12 μg x kg-1) into the final feedwater. The blowdown iron removal efficiency was observed to increase by an order to magnitude and more with use of PAA. Normal chemistry parameters, such as blowdown cation conductivity and TOC/TIC, were unaffected by PAA application. The results and conclusions from the trial are presented and discussed. (orig.)