As with many heat exchangers, nuclear steam generators are subject to the accumulation of corrosion product deposits on heated tube surfaces and other internal components on the secondary (shell) side. These deposits can have adverse effects on thermal efficiency, material integrity, and in some cases plant operability. Accordingly, plant operators routinely devote substantial resources to limiting the generation and accumulation of deposits, including periodic removal of deposits through various mechanical and chemical approaches. Since the 1980s, chemical cleaning processes with solvents designed to remove all (or nearly all) of the iron- and copper-based deposit material from the steam generator (SG) secondary side have been employed in dozens of different units worldwide. Although these full-bundle “hard” chemical cleaning processes have generally been quite successful in removing large fractions of the deposit mass (typically >95%), they are complex, costly evolutions that require additional plant downtime. Additionally, a few units have unexpectedly experienced declines in SG heat-transfer efficiency and plant production following a successful cleaning application due to the removal of thermally beneficial scale layers. During the past 15 years, alternative processes designed to periodically remove only a fraction of the secondary SG deposit inventory have been developed and qualified by the authors. One of these is an advanced scale conditioning agent (ASCA) treatment, a dilute or “soft” chemical cleaning method that focuses on penetrating through consolidated tube scale layers. By removing a fraction of the ironbased matrix material throughout the scale layer thickness—but without fully removing the layer— ASCA applications increase the average scale porosity and thereby improve the associated scale thermal properties, raising SG thermal performance levels. ASCA processes have three principal benefits: 1) immediate, reliable improvements in SG heat-transfer efficiency as demonstrated by analysis of more than a dozen field applications, 2) reduction in the long-term rate of deposit accumulation within the SG (lowering long-term risks of component corrosion and heat-transfer fouling), and 3) substantially reduced application complexity and cost, including no added plant downtime. A second type of process developed by the authors, consolidated deposit extraction (CODE) removal, focuses on the dissolution of non-iron binding species such as oxides of aluminum and silicon from the deposits that form at the lower SG tubesheet elevation in vertically oriented SGs. These deposits typically are very hard and non-porous, making them at least partially resistant to removal through conventional chemical and mechanical removal methods. Laboratory test results and one successful plant application indicate that the use of CODE in conjunction with other cleaning methods can increase the cleaning yield for these hardened, consolidated deposits.
Key words: conference papers, 2017 conference papers, nuclear steam generator, tube fouling, deposits, chemical cleaning, heat transfer, tube corrosion