Heat transfer characteristics of porous sludge deposits and their impact on the performance of commercial steam generators

Steam generator (SG) fouling, in the form of corrosion deposits on the secondary sides of SG tubes, has been known to occur in almost all commercial US nuclear PWR (pressurized water reactor) plants. The level of fouling, as measured by the quantity of corrosion products that form, varies widely from plant to plant. In addition, the effect of SG fouling, as measured by a decrease in effective heat-transfer coefficient, has also varied substantially among commercial US plants. While some have observed large decreases in heat transfer, others have noted little change in performance despite the presence of significant quantities of secondary corrosion layers on their SG tubes. This observation has led to considerable confusion about what role secondary deposits play in causing heat-transfer degradation in SGs. As will become clear later in this report, secondary deposits can have a wide range of effects on heat transfer, from highly resistive to slightly enhancing (reflected by negative fouling). These different behaviors are the result of differences in deposit thickness, composition, and morphology. The main focus of this report is an investigation of the effects of secondary deposits on SG thermal performance. This investigation includes compilation of detailed information on the properties of tube scale at five commercial US nuclear plants and corresponding information characterizing SG thermal performance at these plants.