Most plants place considerable emphasis on controlling the operational chemistry of the secondary side so that it is consistently within acceptable levels. In the steam generator, tubing life is directly related to local chemistry conditions at the tube secondary side surfaces. In the crevices at tube supports and in the tube sheet sludge pile region, successful long life requires maintaining the crevice chemistry within ranges that minimize tubing corrosion. Subtle changes that can significantly affect tubing life may result from variations in feedwater impurity. If these impurity fluctuations are within normal bulk water specifications, their potential to increase tubing corrosion damage, particularly in secondary side SG crevices, could go unnoticed until extensive damage becomes evident.

To assess crevice conditions for corrosion damage potential with given operational chemistry parameters, plant staff need knowledge of the local chemistry in the steam generator tube bundle crevices, in addition to the bulk chemistry of the water surrounding the tubing. In the past this was a rather difficult and time intensive task that could only be done by chemistry and corrosion experts not usually found at the plants. However, recently, tools have been developed to provide a nuclear SG crevice chemistry prediction that can be used by the plant operator on-line. The effects of impurity ingress to the secondary side water, on local crevice chemistry and fouling in the steam generator are identified and where of concern, flagged.

This type of on-line monitoring and prediction system gives the plant operator an important life management tool for maintaining good steam generator health and for attaining long life, by providing early indication of any change in chemistry parameters that could result in damage to the SG tubing. Ready on-line access by the operators to current and past chemistry conditions, including chemistry predictions in the critical crevice regions of the SG, enables appropriate responses while on-line (such as diagnosis of any change in corrosion susceptibility) and planning of future shutdown maintenance actions (such as inspections to verify local condition, and the need for cleaning specific areas).