Advanced Scale Conditioning Agent (ASCA) Applications: 2014 Experience Update

摘要

ASCAs are a group of dilute chemical treatments for removing deposited corrosion products from the secondary side of PWR steam generators (SGs). Each ASCA formulation is customized to achieve plant-specific goals that can include: 1. Partial dissolution and structural modification of the tube scale present on freespan surfaces through full-bundle treatment, lowering the deposit loading and enhancing SG thermal performance levels through the creation of a scale structure marked by increased boiling efficiency, 2. Softening and partial removal of deposits present in the broached flow holes in the tube support plates, reducing the risks of level oscillations and flow-induced vibration (FIV), 3. Chemical removal of copper (Cu) and lead (Pb) from tube scale and tubesheet deposits, reducing the risk of rapid tube corrosion caused by the oxidized conditions promoted by some copper species, and 4. Dissolution of hardness species from consolidated top-of-tubesheet (TTS) collars to enhance collar removal through water-jetting and other mechanical cleaning techniques. Regardless of the cleaning objectives for a particular plant, all ASCA processes are designed to minimize corrosion, waste disposal costs, and impact on the outage schedule. To date, more than 40 ASCA applications have been carried out in four countries. This paper provides an update of the industry experience gained during these applications, including results demonstrating the ability of ASCA processes to meet the goals outlined above. Experience at multiple units, including several repeat ASCA applications, has demonstrated significant heat-transfer benefits (i.e., steam pressure increases of up to 1.7 bar or 24 psi). ASCA applications also regularly achieve significant reductions in TSP blockage (i.e., 30% absolute increases in available flow area in broached flow holes with one treatment) and have been successful in eliminating level oscillations caused by excessive broached-hole blockage. Additionally, experience with two new SG cleaning options, iASCA and CODE, is discussed. iASCA is applied using the same simplified approach as that utilized during standard ASCA applications, but with a solvent chemistry that is intermediate between the ASCA and EPRI/SGOG chemical cleaning processes, thereby facilitating greater deposit removal. CODE solutions are unique from other commercially-available cleaning chemistries for PWR SGs in that they are not designed to remove iron oxide or copper deposits, but rather to selectively remove binding species such as oxides of aluminum and silicon, which play an important role in the formation and consolidation of TTS collars and associated tubing degradation mechanisms (ODSCC, denting, etc.).