Review of PWR Steam Generator Crevice Impurity Concentration Mechanisms

Abstract

The accumulation of impurities in local regions of PWR Steam Generators (SG) such as, support plate/tube intersections, sludge piles and deep tube sheet crevices has resulted in the accelerated corrosion of SG materials. Local thermal hydraulic and mass transfer processes concentrate non-volatile impurities in the bulk SG water to aggressive levels in the crevices and sludge pile. In most cases, the crevices are "fouled" with porous corrosion product deposits which originate from balance of plant systems. Transport processes in the crevice and sludge pile occur within a complex pore structure. Boiling, flow and mass transfer processes are strongly influenced by the pore structure and capillary forces. In this paper, the mechanism by which impurities accumulate in local regions of the SG's are reviewed. The fundamental transport processes occurring within packed crevices and porous deposits are described. Several experimental and theoretical studies have been performed with the objective of quantifying impurity accumulation rates and the maximum achievable concentration within the crevice. Mathematical models which have been developed for this purpose are reviewed and the implications of the model predictions to hideout and hideout return processes in SG's are discussed.