Thermal performance degradation and heat-transfer fouling

Abstract This chapter describes the physical phenomena, analysis techniques, and industry experience related to the thermal performance of recirculating steam generators in nuclear power plants. Changes in steam generator thermal performance may be the result of conventional tube-surface fouling, in which a thermally resistive scale layer reduces the heat-transfer coefficient, as well as various other causes such as fouling of other components inside the steam generator, instrument measurement errors, and tube plugging, among others. Detailed evaluations of approximately 40 pressurized water reactor (PWR) units performed by the authors have demonstrated that: (1) the global fouling factor (a measure of the change in the total thermal resistance to heat transfer for the steam generator) is a good metric for quantifying changes and trends in the steam generator heat-transfer efficiency and (2) collective industry experience with PWR steam generators indicates that secondary tube scale layers generally cause a thermal resistance change of between –100 and + 200 μh ft 2 °F/Btu (–0.018 and + 0.035 m 2 K/kW), with values most commonly observed between –50 and + 50 μh ft 2 °F/Btu (–0.009 and + 0.009 m 2 K/kW). The contributions of various other causes of thermal performance changes in many cases are collectively comparable to or larger than the effects of tube scale layers. In PWR steam generators, the effects of primary-side tube deposits on steam generator thermal performance are quite small due to the use of corrosion-resistant materials throughout the primary circuit, while primary-side fouling can be a much more severe problem in the steam generators in CANDU units, as these plants employ carbon steel in the primary coolant loop.