Polymeric Dispersants for Control of Steam Generator Fouling

Abstract

Fouling of steam generators by corrosion products from the feedtrain leads to loss of heat-transfer efficiency, disturbances in thermalhydraulics, and potential corrosion problems owing to the development of sites for localized accumulation of aggressive chemicals. This paper summarizes studies of the use of polymeric dispersants for the control of fouling, which were conducted at the Chalk River Laboratories. High-temperature settling studies on magnetite suspensions were performed to screen available generic dispersants, and the dispersants were ranked in terms of their dispersion efficiency; polyacrylic acid (PAA) and the phosphonate - HEDP - were ranked as the most efficient. Polyacrylic acid was considered more suitable than HEDP for nuclear steam generators and more emphasis was given to the former in these studies. The dispersants had no effect on the particle deposition rates under single-phase forcedconvective flow, but did reduce the deposition rates under flow-boiling conditions. The extent to which the deposition rates were reduced increased in proportion to the dispersant concentration. Preliminary corrosion tests indicated negligible pitting or general corrosion of steam generator tube materials in the presence of PAA. Corrosion of carbon steel, although higher in a magnetite-packed crevice under heat flux than in bulk water, was lower in the presence of PAA than in its absence. Some impurities (e.g., sulphate, sodium) were observed in commercially available PAA products at small, though significant concentrations, making them unacceptable for use in nuclear plants. However, the PAA could be purified by ion exchange. Preliminary experiments, to assess the thermal stability of PAA at steam generator operating temperature, showed the polymer to break down in deaerated solutions and under argon cover to give hydrogen and carbon dioxide as the 2 major products in the gas phase and variable concentrations of acetate and formate in the aqueous phase. These results suggest that the predominant breakdown mechanism is decarboxylation, rather than depolymerization. More detailed studies on thermal degradation of PAA are in progress. The implications of the results obtained so far with respect to a field trial of dispersant at a nuclear power plant are discussed.