Load Data Evaluation, Stress Analysis and Cycle Counting for the Qualified Thermal Fatigue Assessment of a NPP Component

Abstract

Qualified fatigue assessment based on realistic input data constitutes an essential part of an ageing management strategy for Nuclear Power Plants. In this context and as a continuation of a previous paper PVP2014-28716 the requirements of load data evaluation, stress analysis and cycle counting are detailed based on a real world example from a Brazilian Nuclear Power Plant. One essential prerequisite of any fatigue assessment approach is the availability of realistic load data. In the present analysis, selected operational plant data from the period 2003 to 2012 are used. One further prerequisite is the accurate component stress analysis based on a transient thermal-mechanical Finite Element Analyses. As an example, a highly loaded nozzle from the Chemical & Volume Control System (CVCS) is chosen to be analyzed. The influences on the fatigue assessment caused by the load-time histories, the stress analysis approaches and the cycle counting method are discussed in detail. The considered operational time period from 2003 to 2012 with respective selected plant data gives a consolidated background. It is one essential aim of the study to show the influence of the load-data input and the (design code conforming) stress analysis method on the resulting calculated cumulative usage factors (CUFs). In the present paper, the stress analysis employs the finite element method. Simplified elastic-plastic (application of ke plasticity factors) procedures are used in order to identify the margins and influences of design and actual loading histories on the resulting CUFs. The paper concludes with a comprehensive picture including quantification and discussion of the different influencing parameters on the resulting CUFs. This reveals margins in the fatigue design process and solutions of coping with the design code requirements.

Study the Cyclic Plasticity Behavior of 508 LAS under Constant, Variable and Grid-Load-Following Loading Cycles for Fatigue Evaluation of PWR Components

“…Hence, for better accuracy, it is essential to estimate the fatigue and ratcheting damage of reactor components based on the results of elastic-plastic stress analysis rather than pure elastic stress analysis alone. Recently, elasticplastic analysis and fatigue life estimation based on flaw tolerance [17][18][19][20][21][22] have increasingly become an active research area by infusing more mechanics-based prediction capability into the overall fatigue evaluation methodology. Since elastic-plastic ratcheting is a phenomenon closely related to the transient plastic deformation behavior, its accurate description requires the calculation of material hardening stressstrain states as a function of fatigue cycles or time.…”