This paper presents the results of an engineering review that examined the accuracy and effectiveness of specific enhancements to an automated data validation algorithm used as part of an on-line fatigue monitoring system. One of the major challenges in developing an effective fatigue monitoring system that processes plant computer data is the quality of the original data stream. In these systems, the goal is to minimize, and eventually eliminate, spurious conservative fatigue monitoring results caused by bad input data passed through the plant computer. Reaching this goal frees engineering users from having to correct the faulty data in the original input stream and reprocess it. The WESTEMS™ stress and fatigue monitoring program at the Beznau Nuclear Power Plant Units 1 and 2 was used to develop and test the enhancements to the data validation algorithm designed to reach this goal. The engineering review included both quantitative and qualitative assessments of data-related errors and warnings, as well as a detailed comparison of the automated data validation and correction algorithm against a parallel user-assisted (manual) process. Effectiveness of the method was quantified by comparing the overall fatigue accumulation rates resulting from the manual data checking process with those produced by the fully automated approach. The positive results of the review demonstrate the effectiveness of the enhanced automated approach presented.
In ASME Code Section III NB-3222.4 fatigue evaluations, selecting stress states to determine the stress cycles according to Section NB-3216.2, Varying Principal Stress Direction, can become a challenging and complex task if the transient stress conditions are the result of multiple independent time varying stressors. This paper will describe an automated method that identifies the relative minimum and maximum stress states in a component’s transient stress time history and fulfills the criteria of NB-3216.2 and NB-3222.4. Utilization of the method described ensures that all meaningful stress states are identified in each transient’s stress time history. The method is very effective in identifying the maximum total stress range that can occur between any real or postulated transient stress time histories. In addition, the method ensures that the maximum primary plus secondary stress range is also identified, even if it is out of phase with the total stress maxima and minima. The method includes a process to determine if a primary plus secondary stress relative minimum or maximum should be considered in addition to those stress states identified in the total stress time history. The method is suitable for use in design analysis applications as well as in on-line stress and fatigue monitoring.
This paper deals with a unique and severe stratification phenomena in the pressurizer surge line piping system. The methodology used in the calculation of total stress state includes pressure stresses from internal pressure, global thermal stresses from the constraining of the system by supports, local thermal stresses from hot/cold stratified condition, and transient thermal stresses in the pipe and at structural discontinuities. Special discussion on the interpolation to obtain moment loadings from one state of thermal condition to the next with nonlinear gapped support system is presented. Finally, the thermal cycling associated with operating transients in the surge line system is also addressed.
A number of technical challenges have been identified in applying current methods to evaluate the effects of reactor water environment in fatigue evaluations of reactor components in license renewal applications. One challenge in using more conservative approaches is the large environmental penalty factor that may be calculated for stainless steels. The use of more sophisticated methods leads to more complex challenges, like the determination and application of transient strain rates required by the current proposed equations. For example, the definition of transients for component evaluations must consider competing effects of strain rate and stress range to assure a conservative design. As discussions extend to the possibility of addressing environmental effects in fatigue design requirements of the ASME Code, it is important to consider the implications of the challenges met in application of the current methods. This paper describes a plant-specific application of environmental fatigue penalty factors, the aspects of the challenges encountered in the application, and the ramifications of the various considerations on incorporation of fatigue environmental evaluations in future design requirements.
Fatigue usage factor evaluations including the effects of reactor water environment have been performed in numerous nuclear plant license renewal efforts. A large number of these evaluations have used the environmental fatigue penalty factor, Fen, approach prescribed in various regulatory documents. The Fen equations require input of strain rate, but the prescribing documents do not provide methodology or criteria for the quantification of the strain rate to be input. As a result, numerous approaches have been offered and studied. This paper presents an approach used by Westinghouse to include strain rate in an automated calculation of Fen based on the modified rate approach (MRA) to integrated strain rate applications. The starting point of the approach is ASME Code Section III NB-3200 fatigue analysis. With environmental fatigue evaluations in new plant designs now emerging in ASME Code criteria, strain rate considerations remain part of the discussion. The intent of this paper is to provide further insight into this process.
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