Hideout of Sodium Phosphates in Steam Generator Crevices

Abstract

Sodium phosphates were at one time commonly used as additives in the secondary coolant of nuclear steam generators to buffer the effects of impurity ingress and to minimise corrosion. However, sodium phosphates tend to sequester or to "hideout" in the crevices and sludge piles in the secondary side of steam generators during operation. During transients they may then be released or "returned" to the bulk water - often as aggressive species. This hideout-return phenomenon of sodium phosphates is well known and documented and the fact that tube degradation such as pitting or wastage can occur has led to its falling into disuse in the industry. Point Lepreau is planning to switch from phosphate dosing to AVT (all volatile treatment). It is of interest to understand the chemistry changes that are likely to occur so steps can be taken in the event that corrosive conditions result. This report discusses a two-dimensional mathematical model that describes the transport processes involved; it also discusses the proposed experiments to validate the model.