Comments on the present practice of fatigue analysis of nuclear power plant components

Abstract

In the evaluation of present operating experience with respect to the safety assessment of nuclear power plant (NPP) components, the following conclusions can be drawn. (1) Thermally induced loads such as local temperature stratification (large temperature differences in pipe cross-sections under laminar flow conditions) and fluctuations have been underestimated in the former thermal-hydraulic analysis. (2) Environmental influences are not covered adequately in the design codes. (3) The freedom of the stress analyst in grouping the specified cyclic loadings, as well as the degree of conservatism in calculating the stresses, results in a large variation of the calculated usage factor; the estimated life usage fractions for the different local regions are conservative but not necessarily representative. The goal of the fatigue analyses for NPPs should be (a) to identify the local areas where significant fatigue loadings are expected to occur, as well as (b) to show that the component is protected against fatigue failure. At present the emphasis in performing and reviewing fatigue analyses is on the licensing steps before the NPP is put into operation. The designer must evaluate the fatigue loading pattern at an early stage in order to optimize the design. With respect to the lifetime assessment of the plant, it is the opinion of the authors that only a simplified procedure should be required for the operating license. To assess the residual fatigue life at any time during operation, it makes much more sense to follow the plant operational history as it develops. To follow this approach, it is necessary to manage and process the plant operating data in such a way that it can be used for a detailed fatigue evaluation. Computerized systems which can meet these requirements are commercially available and used for viable parts in conventional power plants. The advantages of this procedure are (i) a more representative number to account for the actual fatigue load and life fraction used; (ii) a better selection of local areas for recurring in-service inspection; (iii) a more reliable data base for the application of sophisticated fatigue analyses to assess damage mechanisms including environmental and load-history effects. As a result, an increase in plant performance and safety is expected.