The influence of LWR coolant environment to the lifetime of materials has been discussed recent years. Nowadays the consideration of environmentally assisted fatigue is under consideration in Codes and Standards like ASME and the German KTA Rules (e.g. Standard No. 3201.2 and Standard No. 3201.4) by means of so called attention thresholds. Basic calculation procedures in terms of quantifying the influence of LWR coolant environment by the Fen correction factor were proposed by Higuchi and others and are given in NUREG/CR-6909. This paper deals with the application of the proposed assessment procedures of ANL and the application to plant conditions. Therefore conservative assessment procedures are introduced without assuming the knowledge of detailed stress and strain calculations or temperature transients. Additionally, detailed assessment procedures based on Finite-Element calculations, respecting in-service temperature measurements including thermal reference transients and complex operational loading conditions are carried out. Fatigue evaluation of a PWR primary circuit component is used in order to evaluate the influence of plant like conditions numerically. Conclusions regarding the practical application are drawn by means of comparing the ANL approach considering laboratory conditions, conservative assessment procedures for the determination of cumulative fatigue usage factors of plant components and detailed assessment procedures. Plant like loading conditions, complex component geometries, loading scenarios and reference temperature transients shall be taken into account. Practical issues like the determination of the mean temperature or the strain rate have to be considered adequately.
Environmentally Assisted Fatigue (EAF) has been focus of various research activities and has been addressed in nuclear Codes and Standards like German safety standard KTA 3201.2 [1], 3211.2 [2] or ASME CC N-792 [3] for example. Based on experimental investigation under laboratory conditions a numerical correction factor Fen was proposed in NUREG CR-6909 [4] in 2007 after precursors in the Japanese JSME code [6]. In 2012 the EPRI Technical Report “Guidelines for Addressing Environmental Effects in Fatigue Usage Calculations” [7] introduced some practical guidelines for the application of the EAF to real plant components based on the set of formulas from 2007. Since this report the set of formulas have been adapted several times (e.g. in ANL-LWRS 47-2011 [8]) while the current revision of NUREG/CR-6909 in 2014 [9] describes the current state of the art. At E.ON Kernkraft GmbH a goal-oriented and engineering based research program called NuMEA (Numerical Methods to take Environmentally Assisted Fatigue into Account) has been established, focusing on recommendations of the EPRI guideline in the context of application to real plant components and available temperature measurement data. First main focus of the R&D activity is to calculate the EPRI sample for verifying developed procedures and taking different procedures for determining the sign to be assigned to the relevant stress intensity into account. The documentation of the procedures applied within the EPRI guideline is not comprehensive enough for real-plant evaluation application. Thus, additional definitions and procedures have been developed to ensure practical application of the procedures being developed. Additionally, updated formulas being recently introduced in the context of the NUREG/CR-6909 Rev. 1 [9] have been implemented. Second topic of the activities is to develop a procedure to take hold-time effects into account numerically based on existing experimental data. Motivated by the fact that the introduction of a potentially beneficial effect of hold times is foreseen in the framework of piping design of the German KTA safety standards, the existing engineering approach (PVP2014-2819 [10]) is appended to fatigue calculation of NPP components. This paper presents the results and the highlights of the E.ON R&D project NuMEA.
Consideration of environmentally assisted fatigue (EAF) is in discussion internationally. In German KTA Rules the effect is taken into account by means of so called attention thresholds. While the laboratory phenomena themselves are being accepted widely, numerical calculation procedures are revised continuously and transition from laboratory to real plant components is not clarified yet. Since NUREG/CR-6909, formulas for calculating the Fen factors have been modified several times. For example in ANL-LWRS47-2011 a new set of formulas was published and slightly revised by ANL in 2012. Various calculation procedures like the strain-integrated method and simplified approach have been published while each approach yields to different results. Beyond this, additional topics like weld factors or plasticity correction factors have to be taken into account. Calculation procedures depending on the level of detail and in the description of loads are yielding to significant variations in the results. Respecting these topics in context of different levels of detail in computational simulations, numerical cumulative usage factor (CUF) evaluation results are likely to differ, depending on the assumptions made. On the basis of a practical example, methods and approaches will be discussed and recommendations in terms of avoiding over-conservatism and misinterpretation will be presented.
German KTA consider the intensively discussed topic of environmentally assisted fatigue (EAF) by introducing so called attention thresholds for the cumulative fatigue usage factors (CUF). Threshold values for both austenitic steels and ferritic steels are defined. If the threshold value is exceeded, further measures like NDT, in-service inspections or detailed assessment procedures have to be performed. One way to handle those measures is to apply sophisticated procedures and to show that the numerically calculated CUF is below the defined attention thresholds. The article presents some examples to reduce calculated CUFs. Fatigue analyses performed during design phase of NPP are based on specified loads consisting of conservatively determined reference transients, covering the whole lifetime including events with significantly fewer cycles than assumed during design phase as well as events with very low probability of occurrence at all. But there might be also operational loads which are not included in the load specifications. In general, there is a lot of potential to reduce computed CUFs in terms of considering realistic loading scenarios. In German NPPs various temperature measurement planes are applied at fatigue relevant positions among others to calculate actual CUFs based on real loads. Detailed finite element (FE) calculations considering these loads can reduce computed CUFs considerably. Therefore detailed evaluation of temperature measurements in combination with FE calculations is carried out, reducing calculated CUFs below the threshold values given in German KTA.
In recent years the Environmentally Assisted Fatigue (EAF) became an item, which has to be considered additionally in terms of ensuring a conservative determination of the actual component’s health status resp. the CUF. For practical application, the consideration of the so called Fen-factor leads to the reduction of the admissible cycles in fatigue calculations. Beyond that the influence of elevated temperatures has been identified as one parameter having a negative influence on the admissible cycles as well. For example the German KTA 3201.2 defines for austenitic steels separate fatigue curves for temperatures above 80°C and for temperatures below 80°C. In summary on the one hand parameters influencing component’s lifetime negatively have to be considered in terms of conservative calculations. On the other hand, there are other parameters which influence the component’s fatigue lifetime in a positive manner. As such positive effects are neglected so far, CUF allowing for EAF tend to become over conservative leading to oversized components. Therefore, positive effects should be considered as well in the framework of a comprehensive and detailed analysis making sure not to overdesign components. When taking a closer look on the operational behavior of primary circuit components, fatigue loading is mainly defined by long steady-state periods with no significant changes in the loadings and by normally short outage periods with no thermal loading. For example fatigue of a PWR surge-line is mostly caused by short in-surge and out-surge events during start-up or shut-down of the plant. Normal operation transients mostly not cause fatigue relevant events in the surge-line. Fatigue of PWR spray-lines is primarily generated by very few spray-events during a one-year period of operation. Spray events are mainly caused by significant load ramps. Subsequently the fatigue status of primary circuit components is controlled by long periods with no fatigue relevant loading at operating temperature and few additional loading patterns in between. Experimental investigations have shown that hold time effects have a positive influence on fatigue lifetime of austenitic stainless steel materials. Anyhow, no quantification of these effects has been published in recent years. Within this publication an engineering based approach will be developed to quantify the hold time effect based on literature and published data. On the basis of a practical example the influence of hold time effects will be quantified and a direct comparison to lifetime reducing effect of EAF and temperature will be drawn.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.