Conference Paper

Overview of International Implementation of Environmental Fatigue

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Abstract

For many nuclear power plants worldwide the operation period will be extended from 40 to 60 years in the coming years. As the operation period increases the importance of knowledge of ageing mechanisms like fatigue increases. Knowlegde of the influence of the environment is crucial, since environmental fatigue is a relatively new development which is a modification to the existing assessment method and has to be projected to 60 years as well. This paper contains the results of a literature survey of environmentally assisted fatigue in nuclear power plants. It describes the current status and developments in the world. The main regulatory rules, guidelines and methods from the US, Germany, Japan, Finland and France are presented. At this moment different approaches for incorporating the effect of the coolant water environment exist, although the general trend is towards a more uniform approach worldwide. The most common approach is the incorporation of an environmental fatigue correction factor (Fen) in the fatigue derivation of the cumulative usage factor. The Fen formulas and the S-N fatigue curves differ but the general equations are: Display Formula Fen = N air / N water and Display Formula CUF = Σ U partial * Fen partial Alternatives like using fatigue curves including the environmental effects, using threshold criteria and calculation of an allowable Fen based on testing, are described. Research and material tests are still on-going and subject of international development. An overview of the current international state-of-the-art is presented. Copyright © 2013 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal

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... In strain-controlled fatigue conditions, the effects of temperature and LWR environments are addressed by the concept of environmental fatigue factor, Fen [15]. Fen is defined as the ratio of fatigue life in air at room temperature to that in water reactor environments: ...
... With some caution, Fen can be compared with the value calculated from an explicit formula established on strain-controlled fatigue data. Many formulas have already proposed and yield slightly different Fen values [15]. In all cases, Fen depends essentially on temperature, strain rate and dissolved oxygen level. ...
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Influence of mean stress on fatigue life of the austenitic stainless steel 316 L in air and light water environments (boiling water reactor/hydrogen water chemistry) at 288 °C was determined with a series of tests carried out in load-control mode. Fatigue life was found to increase with application of compressive and tensile mean stress in air and light water reactor environments. Secondary hardening was regarded as the main reason for this behavior. A modified Smith-Watson-Topper (SWT) model was considered to account for mean stress and was shown to predict fatigue life accurately in air and water environments. The reduction of fatigue life in water environment, determined with the SWT curves, was about 2.5. Observations of the end-of-life dislocation arrangements by transmission electron microscopy showed that the dislocation microstructure depends essentially on plastic strain amplitude, which in turn is strongly correlated to stress amplitude and mean stress. The microstructures were found consistent with those usually observed after strain-controlled experiments. At rather low plastic strain amplitudes, corduroy structure consisting of small dislocation loops was observed. Acting as significant obstacle to dislocation motion, corduroy structure affects overall dislocation mobility therefore contributing to notable secondary cyclic hardening.
... Extension of the life-time of current NPPs is an efficient means to provide low carbon energy and contributes to the climate change fight. Accordingly, different proposals are currently being discussed to further improve guidance for assessing EAF in NPPs [4][5][6][7][8]. ...
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A ratio of shoulder to gauge displacements (S2G) is calculated for three different fatigue specimens in a pressurized water environment. This ratio needs to be known beforehand to determine the applied shoulder displacements during the experiment that would result in the desired strain amplitude in the gauge section. Significant impact of both the applied constitutive law and specimen geometry on the S2G is observed. The calculation using the fully elastic constitutive law results in the highest S2G values and compares very well with the analytical values. However, this approach disregards the plastic deformation within the specimens that mostly develops in the gauge section. Using the constitutive laws derived from actual fatigue curves captures the material behaviour under cyclic loading better and results in lower S2G values compared to the ones obtained with the fully elastic constitutive law. Calculating S2G values using elastic-plastic constitu-tive law based on the monotonic uniaxial tensile test should be avoided as they are significantly lower compared to the ones computed with elastic-plastic laws derived from hysteresis loops at half-life.
... Effects of environment are specified in Code Cases N-761 and N-792 [14,15] and also described in [16] and [17]. More information is presented in Table 1. ...
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Different pressure vessel and piping design codes and standards have adopted different fatigue analysis methods. In order to make some contribution to current efforts to harmonize international design codes and standards, a review of fatigue analysis methods for a number of selected nuclear and non-nuclear design codes and standards has been carried out. The selected design codes and standards are ASME Boiler and Pressure Vessel Code Section III Subsection NB and Section VIII Division 2, EN 12952, EN 13445, EN 13480, PD 5500, RCC-M, RCC-MRx, JSME, PNAEG and R5. This paper presents the initial review results. The results of the study could be used as part of the on-going work of the Codes and Standards Task Force of the World Nuclear Association (WNA) Cooperation in Reactor Design Evaluation and Licensing (CORDEL) Working Group.
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