FATIGUE MONITORING FOR DEMONSTRATING FATIGUE DESIGN BASIS COMPLIANCE

Transcription

1 FATIGUE MONITORING FOR DEMONSTRATING FATIGUE DESIGN BASIS COMPLIANCE D. Gerber, G. Stevens, T. Gilman, J. Zhang Structural Integrity Associates,San Jose,USA Structural Integrity Associates,San Jose,USA Structural Integrity Associates,San Jose,USA B&W Tech, Inc, Houston, USA / B&W Energy T&E (Shanghai), Ltd., Shanghai, China address of main author: mherrera@structint.com Abstract. Many nuclear power plant components were originally designed to the requirements of ASME Code, Section III, Class 1. These components have a specific fatigue analysis based on a set of cyclic operating conditions considered to be conservative at the time of design. As part of the licensing requirements for most U.S. plants, plant owners agreed to limit the number of selected transients by including cyclic limitations in the plant Technical Specifications. This paper explores the relationship of component design basis to actual cyclic operation. Also, the consequences and actions associated with discovering previously unanalyzed events are discussed. It is concluded that fatigue monitoring for a few fatigue sensitive components is a technically acceptable alternative to the cycle counting requirements contained in most Technical Specifications, and can be used to fulfill the related requirements associated with the plant licensing basis. 1. Introduction The accumulation of fatigue due to plant operation represents a significant aging concern for critical components in light water reactor (LWR) nuclear power plants. In the plant design process, the effects of stress and fatigue are estimated and bounded using design rules contained in Section III of the ASME Boiler and Pressure Vessel Code [1], ANSI B31.1 [2], and/or ANSI B31.7 [3]. End-of-life cumulative usage factors are determined, in accordance with these rules, using an assumed set of conservative design transients to insure that plant components do not exceed an allowable cumulative usage factor throughout their lifetime (usually forty years). To assure that design safety margins remain adequate throughout the operating life of the plant, varying degrees of comparison are required by the plant licensing bases to demonstrate that actual operating experience remains bounded by that assumed in the original plant design. Typically, during plant operation, all significant design transient operating cycles are logged and counted in accordance with plant licensing bases to assure that the design fatigue limits are not exceeded. In practice, however, many of the actual plant operating cycles are not well characterized by the design transients. Also, classification of individual plant events into one of the design transient categories is a difficult task, for which plant operators are given relatively little guidance. As a result, some operating plants have approached the limit for allowable design transients early in plant life. In other cases, classification of plant operating cycles may be done incorrectly or inconsistently, resulting in a poor estimate of 1

2 cumulative usage accumulation. Finally, there have been occurrences of fatigue failures caused by loading not considered in the original design basis (e.g., stratification). In most cases, the design transients very conservatively bound plant operation. However, in the past, there have been no practical means by which utilities could take credit for this implicit margin, which would extend the useful life of plant components. As a result, new methods of demonstrating adequate design safety margins have been pursued. These methods include cyclic analysis of actual event history and real-time computation of cumulative usage factor from actual plant operating data. Modern computers have also made these seemingly complicated tasks easy to implement. In 1985, EPRI initiated a program to develop a prototype system for monitoring cumulative usage factor in nuclear power plant components. A unique methodology was developed for accessing plant instrumentation data and converting them directly to peak stress versus time at plant locations of interest. The key to this technique was a transfer function approach, which used Green s Functions and transfer matrices to convert plant data to peak stress versus time. The methodology was developed into a specialized software system called FatiguePro, and several essential features were included to more fully address plant cycle tracking requirements. Collectively, the following features have been incorporated into the FatiguePro software: Acquisition and analysis of plant operating data (from existing plant instrumentation). Calculation of peak stress versus time and cumulative usage factor for critical plant locations using a stress-based fatigue approach. Automated cycle counting capabilities. Computation of cumulative usage factors using a cycle-based fatigue approach. Real time analysis and detailed review capabilities of all input plant data and results. Summary reporting capability for permanent documentation purposes. Flaw tolerance assessment. This paper describes how the FatiguePro software can be used to fulfill cyclic tracking and/or other margin assessment requirements specified by plant licensing bases. 2. Nuclear Industry Issues In recent years, significant industry attention has been devoted to metal fatigue and its impact on the design qualification and serviceability of operating nuclear power plant components. Fatigue failures in safety-related systems and components have been rare, and fatigue damage in pressure-retaining equipment is typically manifested as small cracks or leaks, detected long before reaching a size that could cause a major pressure boundary rupture. Thus, fatigue rarely becomes a safety issue. However, confirmation of the adequacy of the fatigue life of metal components continues to be a pressing economic issue, especially for operation beyond the originally planned operating life (i.e., license renewal period). Therefore, fatigue of metal components has been identified as a high-impact technical issue remaining to be resolved for license renewal of nuclear power plants. 2

3 The most typical form of evaluating fatigue of metal components is by detailed fatigue analysis during the component design assuming postulated loads. There are different codes and standards for performing such analyses. The ASME Code procedure for calculating cumulative usage factors, which is the most common procedure used for modern-day evaluations, is defined in Paragraph NB of Section III of the ASME Code [1]. The procedure is illustrated schematically in Figure 1. Cumulative usage factors are developed at each critical stress location for the reactor vessel and piping systems, and are documented in a Design Stress Report. Cumulative usage factors are calculated for several locations in the design process using a conservatively estimated set of design basis transients and frequencies of occurrence, and by pairing peak stresses from the individual transients to maximize stress cycles over the entire loading history. While this method of calculating cumulative usage factor may be adequate for design, a more precise means of quantifying fatigue damage is desirable for monitoring plant operation. Several documents have been written to address the adequacy of fatigue life evaluations of metal components for the nuclear industry. These documents include the Nuclear Energy Institute (NEI) (formerly NUMARC) License Renewal Industry Reports (IRs) published by EPRI [4], Nuclear Regulatory Commission (NRC) Generic Safety Issue (GSI) 78 [5], NRC GSI 166 [6], and the NRC Fatigue Action Plan [7]. Fundamentally, all of these documents discuss fatigue-related concerns and conclude that the current fatigue licensing basis for operating plants is adequate to manage the effects of fatigue for both the current and the license renewal terms, provided that fatigue-sensitive locations are assessed and managed appropriately. SECY [8] documents the NRC resolution of the fatigue issue for operating plants. The NRC staff s technical and regulatory compliance concerns with respect to fatigue for license renewal have been subsumed into GSI 166 [6], which more recently was renumbered GSI-190. Regulatory compliance fatigue concerns also have been at issue for many years for current operating plants. For example, GSI 78 [5] was intended to address staff concerns regarding some component locations exceeding the fatigue licensing basis during the current license term. 3

4 Fatigue Usage Factor Calculation: U n = N (overall transient range pairs) where: n = design basis number of occurrences for each transient range pair. N = allowable number of cycles for each transient range pair from the applicable design fatigue curve. FIG 1: Illustration of ASME Code, Section III Fatigue Usage Factor Calculation. In GSI 78, the staff recommended compliance through programs of actual plant transient monitoring or cycle counting. Other evidence of the regulatory oversight process related to fatigue in operating plants has included information notices and bulletins, such as NRC Bulletins No [9], [10], and [11], and Information Notices [12] and [13]. Typically, nuclear plants are required to track plant transients against cycle limits. These requirements are specified in the plant licensing bases and/or Technical Specifications. The intent of these requirements is to ensure that actual plant operation remains within the envelope assumed in the design basis. When properly implemented, these requirements are consistent with the recommended compliance in GSI 78 and other regulatory documents. In many cases, plants have put in place computerized transient and fatigue monitoring systems, such as FatiguePro, to fulfill plant-specific requirements for tracking cyclic duty. The intent of the FatiguePro software is to provide an industry-approved tool that can be used by plant engineers to fulfill plant fatigue life tracking requirements by using any combination of three approaches: 4

5 (1) Counting, categorizing and tracking plant transient events, and comparing the result to the allowable cycle counts assumed in the design basis. (2) Computing cumulative usage factors, either through real-time stress-based analysis or a cycle-based approach, and demonstrating that values less than the allowable are maintained for all monitored locations. (3) Using a flaw tolerance approach to demonstrate that actual or postulated flaws remain within acceptable values. All of these approaches are intended to demonstrate that structural design margins for all critical components are maintained during actual plant operation. The applicability of EPRI s fatigue monitoring methodology was demonstrated through implementation and field testing of a prototype system at San Onofre Unit 2, a pressurized water reactor (PWR) [14]. A similar installation and field test was also performed at Quad Cities Unit 2, a boiling water reactor (BWR) [15]. 3. Using FatiguePro to Meet Plant Requirements Nuclear plants are designed and analyzed in accordance with codified design rules. The design process involves analyzing each applicable component for a set of hypothetical design transients that represent the expected plant operation in terms of temperature and pressure profiles for the life of the plant. Each component is analyzed to meet all of the applicable design requirements, including limits on primary and secondary stresses and cyclic duty limits. During the license period of the plant, the utility is responsible to ensure that plant components remain within their licensing basis. The plant Final Safety Analysis Report (FSAR) usually specifies the set of design transients that define the plant licensing basis. Requirements for tracking plant operations stem from the necessity for the plant owner to demonstrate that the plant operates within the licensing basis. Excursions outside the boundaries of the licensing basis require special analysis to demonstrate that structural design margins are maintained over the operating life of the plant, thus ensuring continued safe and reliable operation. Early in the life of the most present-day operating plants, the most direct and easiest form of ensuring that operation remained within the licensing basis was considered to be simple counting and categorization of plant transient events. Such an approach was considered straightforward, and eliminated the need to re-perform costly, labor-intensive fatigue evaluations based on plant-unique operating history. Later in plant life, however, many plants experience the occurrence of certain plant events in quantities that either exceed the number assumed in the licensing basis, or accumulate at a rate that is projected to exceed the number assumed in the licensing basis prior to the end of the desired operating period. In addition, events or loads may be experienced that were not considered in the original design. In these cases, simple cycle counting is not sufficient to demonstrate acceptable design margins. In order to demonstrate acceptable design margins in these instances, the plant owner has several options. First, the plant operator can re-perform analysis of each component to a revised set of design transients that represent the observed operation of the plant in terms of number and severity of events. Alternatively, the plant owner can initiate a condition assessment program that accounts for the actual operation of the plant, and 5

6 addresses the effect of actual operation on the structural margin of the affected components. These approaches are consistent with those recommended in ASME Code, Section XI, Nonmandatory Appendix L [16], and are depicted in Figure 2. Model Component Define Loading / Collect Operating Data Inspection Data Stress Analysis Material Properties Repeat for New Operating Data Fatigue Model Crack Growth Model Σ Fatigue vs. Time Σ Crack Growth vs. Time Remaining Life Prediction 95093r0 FIG 2: Typical Remaining Life Assessment The intention of the FatiguePro software is to provide an industry-approved tool that can be used within an integrated management program to show that design safety margins are maintained. FatiguePro can be used to fulfill plant cyclic duty tracking requirements, perform component structural margin evaluations, and accommodate flaw tolerance evaluations of components with cumulative usage factor values that exceed allowable limits. To accomplish this condition assessment, FatiguePro incorporates technical capabilities that address the following three requirements: Fulfill plant licensing basis cycle counting requirements. This requirement is accomplished by consistently and accurately counting, categorizing, and tracking plant transient events for comparison to the events assumed in the licensing basis. This activity provides a direct measure that plant cycles remain within cyclic limit requirements, and an indirect measure of structural design margin. Automatically recording the occurrence of plant cycles may eliminate the need for plant operating 6

7 personnel to do so manually, and reduces the inaccuracies and unnecessary conservatism inherent in manual cycle counting. Determine actual structural margins. This determination is accomplished by computing cumulative usage factors based on actual plant operation, and demonstrating these values remain less than the design allowable for all monitored components. This method goes one step further than cycle counting alone, in that it provides a direct measure of structural design margin. In addition, assessing the structural integrity of the reactor coolant pressure boundary can be readily accomplished subsequent to an event that exceeds the operating pressure and/or temperature limits or the number of allowable cycles specified in the plant design basis. Perform flaw tolerance evaluations that demonstrate actual or postulated flaws remain within allowable limits. Components in the plant that are expected to accumulate abnormally high cumulative usage factors may be closely monitored utilizing a fatigue crack growth fracture mechanics methodology. Plant owners can adjust plant operational procedures and inservice inspection programs accordingly to ensure that design structural margins are maintained. All of these approaches are intended to demonstrate, in an accurate, reliable, and retrievable fashion, that structural design margins for all critical components are maintained during actual plant operation. FatiguePro provides the technical tools that may be used to assure that structural design margins are maintained in accordance with the approach appropriately chosen by the plant owners. 4. Conclusions The FatiguePro software supports plant operation and maintenance by providing the following capabilities: Automatically recording the occurrence of plant thermal cycles, thereby eliminating the need for plant operating personnel to do so manually, and reducing the inaccuracies and unnecessary conservatism inherent in manual cycle counting. Assessing the structural integrity of the reactor coolant pressure boundary after an event that exceeds the operating pressure and temperature limits or number of allowable cycles, as identified in the plant Technical Specifications. Closely monitoring areas of the plant that are expected to accumulate abnormal cumulative usage factors, enabling reactor operators to adjust plant operational procedures and inservice inspection programs accordingly. The intent of the FatiguePro software is to provide an industry-approved tool that can be used by plant engineers to fulfill plant cyclic duty tracking requirements by using any combination of the above approaches. Pilot plant studies have been performed using FatiguePro for lead BWR and PWR plants [17]. Each of these studies included plant-specific application of the FatiguePro software, as well as detailed evaluation of several years worth of plant data. The objective of these evaluations was to rigorously test the FatiguePro methodology and provide further confidence in the plant-specific application of the software and its ability to fulfill plant cyclic duty tracking requirements. In addition, fatigue duty extrapolation schemes were 7

8 developed for generic use. The objective of these methodologies was to estimate cumulative usage factors in instances where plant data are not available (i.e., as in the case of establishing the cumulative usage factor at a time when data are not retrievable). All of the approaches used by FatiguePro are intended to demonstrate, in an accurate, reliable, and retrievable fashion, that structural design margins for all critical components are maintained during actual plant operation. Therefore, FatiguePro provides the technical tools that may be used to assure that structural design margins are maintained. REFERENCES [1] ASME Boiler & Pressure Vessel Code, Section III, Rules for Construction of Nuclear Power Plant Components, Division I, Subsection NB, Class 1 Components. [2] USAS B31.1, Power Piping Code. [3] USAS B31.7, Nuclear Power Piping. [4] NEI License Renewal Industry Reports: a. EPRI Report No. TR , Revision 1, BWR Primary Coolant Pressure Boundary License Renewal Industry Report; July b. EPRI Report No. TR , Revision 1, PWR Primary Coolant Pressure Boundary License Renewal Industry Report; July [5] U. S. Nuclear Regulatory Commission Generic Safety Issue 78, Monitoring of Fatigue Transient Limits for Reactor Coolant System. [6] U. S. Nuclear Regulatory Commission Generic Safety Issue 166, Adequacy of Fatigue Life of Metal Components. [7] NRC Fatigue Action Plan, Revision 1 (as documented in SECY ). [8] Policy Issue SECY , Completion of the Fatigue Action Plan, September 25, [9] NRC Bulletin 79-13, Revision 2, Cracking in Feedwater System Piping, June [10] NRC Bulletin 88-08, Thermal Stresses in Piping Connected to Reactor Coolant Systems, June 22, 1988; Supplement 1, June 24, 1988; Supplement 2, August 4, 1988; Supplement 3, April 11, [11] NRC Bulletin 88-11, Pressurizer Surge Line Thermal Stratification, December 10, [12] NRC Bulletin 91-38, Thermal Stratification in Feedwater System Piping, June 13, [13] NRC Bulletin 93-20, Thermal Fatigue Cracking of Feedwater Piping to Steam Generators, March 24, [14] EPRI Report No. NP-5835, FatiguePro: An On-Line Fatigue Usage Transient Monitoring System for Nuclear Power Plants, April [15] EPRI Report No. NP-6170-M, FatiguePro On-Line Fatigue Monitoring System: Demonstration at the Quad Cities BWR, January 1989 [16] ASME Boiler & Pressure Vessel Code, Section XI, Nonmandatory Appendix L, 1995 Edition. [17] EPRI Report No. TR , Technical Basis for the FatiguePro Fatigue Monitoring Software, December