Radiotracer Studies on Solute Concentration within Crevices

Abstract

Localized corrosion of steam generators is the result of the accumulation of corrosive chemicals on heat-transfer surfaces in areas of restricted coolant flow. The narrow clearances between steam generator tubes and support plates at their points of intersection may, thus, have the potential for solute accumulation and consequent corrosion damage. The present study looks at solute concentration within crevices typical of those at the broached support plates in CANDU steam generators. Solute concentration in the crevice was monitored using the radiotracer 24Na and a NaI(T1) gamma detector. The effects of heat flux, crevice dimensions, crevice fouling and bulk flow rate on solute concentration were determined, as was the rate of release from the crevice in the presence and absence of heat flux. The concentration process was found to have inherent variability. This variability was probably induced by the way steam blankets formed in the crevices and by the open structure of the junctions of the tubes with the broached support plates, which provides alternative paths for heat transfer from the crevice area to the bulk water. The observations are explained in terms of a heat-flu driven concentration process; the release from the crevices by diffusion is probably overwhelmed by the heat-flux-driven flow of water into the crevices in a direction opposing any mass flux from there.