Assessment of Sivablasting Efficiency Based on Characteristics of Steam Generator Primary Side Tube Deposits

Abstract

Mechanical cleaning of the primary side oxide in CANDU steam generators has been carried out at several stations since 1995. The most recent applications were the Sivablast TM campaign at Darlington Unit 1 in 2004 and the CANDU Clean campaign at Bruce Unit 5 in 2005. The Darlington campaign was characterized by a mixed outcome with the oxide removal efficiency achieved being significantly lower in two of the four steam generators (SGs) cleaned. A study was undertaken focusing principally on differences between the characteristics of the primary side oxide in various CANDU SGs which possibly impact cleaning effectiveness. The oxide characteristics examined included: Overall steam generator oxide loadings and their variation with tube length; Composition and morphology; Chemical profile across the thickness of the oxide layer; and Porosity, surface roughness and thermal conductivity. It was concluded that differences in bulk oxide characteristics of SG tubes at various stations are unlikely to significantly influence the outcome of mechanical cleaning. In general, the key factor limiting the oxide removal efficiency may be the abrasion behavior of the oxide adjacent to the metal interface. Such behavior may be influenced by both the major and minor constituents of the tube base metal. Up to approximately 80-85% of the oxide loading, consisting principally of the outer magnetite layer, appears to be amenable to removal by media blasting. Removal of the remaining oxide may not be beneficial as it is not expected to significantly reduce surface roughness further nor significantly improve the heat transfer characteristics of the SG tubes. Thus, additional improvements in reactor inlet header temperature and primary heat transport system flow rate would be unlikely to occur.