One of the goals of ASME Section XI is to ensure that systems and components remain in safe operation throughout the service life, which can include plant license extensions and renewals. This goal is maintained through requirements on periodic inspections and operating plant criteria as contained in Section XI IWB-2500 and IWB-3700, respectively. Operating plant fatigue concerns can be caused from operating conditions or specific transients not considered in the original design transients. ASME Section XI IWB-3740, Operating Plant Fatigue Assessments, provides guidance on analytical evaluation procedures that can be used when the calculated fatigue usage exceeds the fatigue usage limit defined in the original Construction Code. One of the options provided in Section XI Appendix L is through the use of a flaw tolerance analysis. The flaw tolerance evaluation involves postulation of a flaw and predicting its future growth, and thereby establishing the period of service for which it would remain acceptable to the structural integrity requirements of Section XI. The flaw tolerance approach has the advantage of not requiring knowledge of the cyclic service history, tracking future cycles, or installing systems to monitor transients and cycles. Furthermore, the flaw tolerance can also justify an inservice inspection period of 10 years, which would match a plant’s typical Section XI in-service inspection interval. The goal of this paper is to demonstrate a flaw tolerance evaluation based on ASME Section XI Appendix L for several auxiliary piping systems for a typical PWR (Pressurized Water Reactor) nuclear power plant. The flaw tolerance evaluation considers the applicable piping geometry, materials, loadings, crack growth mechanism, such as fatigue crack growth, and the inspection detection capabilities. The purpose of the Section XI Appendix L evaluation is to demonstrate that a reactor coolant piping system continues to maintain its structural integrity and ensures safe operation of the plant.
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.
Reviewing online monitoring data led engineers at a U. S. domestic power plant to discover some periodic transient activity occurring in the unisolable sections of the plant’s safety injection (SI) lines. This activity typically occurred when the plant approached hot standby conditions after an outage. After performing an investigation, it was concluded that the activity was caused by periodic SI check valve testing. Effective review and disposition of the monitoring results supported engineering feedback to operations personnel with recommendations for performing check valve testing to mitigate further thermal shocks on the downstream components. Engineering was also able to classify the observed transient activity as one of the transients in the SI line licensing basis, to justify continued operation without significant engineering analysis, using the WESTEMS™ Design Envelopes approach to transient cycle counting. This paper describes the approaches used to identify the unexpected transient activity, evaluate and classify the activity using an efficient automated approach, and to develop recommendations for mitigating actions for plant operation.
Thermal stratification is a common phenomenon in the surge lines of Pressurized Water Reactors (PWR). The stratification temperature difference (ΔT) and cyclic action severities are most prevalent during the heatup and cooldown operations of a PWR, when the system ΔT between the pressurizer and the Reactor Coolant System (RCS) hot leg is the greatest and system inventory fluctuations are highest. This paper describes the computer simulation of thermal stratification loading in a surge line nozzle connected to the RCS hot leg to correlate to unusual behavior of plant sensor data in the hot leg and the subsequent development of a monitoring model to account for thermal stratification effects in the transient and fatigue evaluation performed in the online monitoring system. What makes this particular investigation unique is the geometry of the nozzle of interest. In many PWRs, the surge line and the surge line hot leg nozzle are horizontal at the hot leg connection. This particular nozzle is oriented at an upward angle before the attached surge line piping bends into a horizontal configuration. This orientation required a more detailed treatment of the stratification effects than has been typically developed for horizontal nozzles, with respect to both the orientation and the potentially detrimental effects of increased cyclic behavior indicated by nearby temperature sensors. This investigation combined Computational Fluid Dynamics (CFD) modeling of the system to correlate the plant data with a detailed stress model that will enable the fatigue usage factor calculation in the plant’s online transient and fatigue monitoring system.
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