Even with the best secondary side water chemistry control, corrosion products from the secondary side systems will continue to be carried into the steam generator tube bundle during its lifetime. A large percentage of these corrosion products come to rest in the steam generator as deposits, mostly on the tube surfaces. Those deposits that end up on horizontal surfaces, and particularly the tubesheet, can be difficult or impossible to remove while the steam generator is operational. While considerable effort is made to maintain good bulk water chemistry in the steam generator itself, and advanced chemistry control methods are now available to reduce such fouling, these deposits, if allowed to accumulate, become hard (consolidated) and create crevice conditions at the tubesheet-to-tube interface, as well as fouling the tube-to-support gaps. Feedwater impurities diffuse to these crevices and, as a consequence of boiling in the crevice, can concentrate by factors of up to 106. Hence, a proactive ageing management programme for SGs should include secondary side cleaning (particularly tubesheet flushing or lancing), with regular application, even before the presence of significant SG deposit is detected.

Although there is considerable world experience to support this activity, plant operation and maintenance staff sometimes question the benefits of cleaning because of the considerable costs involved. To provide enhanced tools (and the science behind them) that will aid cleaning decisions, models are being developed to predict tubing corrosion damage using a variety of laboratory and field data. It is known that Alloy 800, similarly to all other SG tubing, is not immune to pitting corrosion or stress cracking, particularly under deposits (like tubesheet sludge piles) where aggressive contaminants such as chlorides and lead may have concentrated. Pitting is likely the highest concern for CANDU SG tubing. Pitting potential

modelling is being coordinated with the crevice/SG chemistry modelling to provide a qualitative guide for operators to assess the adequacy of current chemistry control and to provide input to plan tubesheet sludge lancing.