A Global Fouling Factor Methodology for Analyzing Steam Generator Thermal Performance Degradation

Abstract

Over the past few years, steam generator (SG) thermal performance degradation has led to decreased plant efficiency and power output at numerous PWR nuclear power plants with recirculating-type SGs. The authors have developed and implemented methodologies for quantitatively evaluating the various sources of SG performance degradation, both internal and external to the SG pressure boundary. These methodologies include computation of the global fouling factor history, evaluation of secondary deposit thermal resistance using deposit characterization data, and consideration of pressure loss causes unrelated to the tube bundle, such as hot-leg temperature streaming and SG moisture separator performance. In order to evaluate the utility of the global fouling factor methodology, the authors performed case studies for a number of PWR SG designs. Key results from two of these studies are presented here. Uncertainty analyses were performed to determine whether the calculated fouling factor for each plant represented significant fouling or whether uncertainty in key variables (e.g., steam pressure or feedwater flow rate) could be responsible for calculated fouling. The methodology was validated using two methods: by predicting the SG pressure following chemical cleaning at San Onofi-e 2 and also by performing a sensitivity study with the industry-standard thermal-hydraulics code ATHOS to investigate the effects of spatially varying tube scale distributions. This study indicated that the average scale thickness has a greater impact on fouling than the spatial distribution, showing that the assumption of uniform resistance inherent to the global fouling factor is reasonable. In tandem with the fouling-factor analyses, a study evaluated for each plant the potential causes of pressure loss. The combined results of the global fouling factor calculations and the pressure- loss evaluations demonstrated two key points: 1) that the available thermal margin against fouling, which can vary substantially from plant to plant, has an important bearing on whether a given plant exhibits losses in electrical generating capacity, and 2) that a wide variety of causes can result in SG thermal performance degradation. These include changes in primary control temperature, tube plugging, and measurement errors, as well as secondary tube scale. The analyses of San Onofie 2 and Callaway, as well as similar analyses performed at other plants, suggest a broad categorization of tube scale effects on heat transfer. Specifically, scale thinner than 100 microns (0.004 inches) was found to have little effect on heat transfer, while scale thicker than 225 microns (0.009 inches) was found to be highly thermally resistive, consistent with the presence of a consolidated inner scale layer adjacent to the tube interface.