Environmentally Assisted Fatigue Evaluation of Cladded Low Alloy Steel Cylinder Using Unit Usage Factor Approach

Abstract Consideration of modern in-air fatigue curves and environmental effects (where applicable) in plant fatigue evaluations is important for continued operation and lifetime extension of Light Water Reactor (LWR) power stations in the UK regulatory context. Historic fatigue evaluations performed during design are expected to be updated, taking into account modern development in fatigue assessment methodology, such as changes in fatigue curves and environmental effects. This paper describes the fatigue evaluation of a generic low alloy ferritic steel cylinder that is cladded with austenitic stainless steel, subject to typical LWR plant loadings such as pressure and thermal transients. This is used as an example with the aim of describing in detail a Fatigue Assessment Methodology (FAM) developed by EDF Energy for internal use, which makes use of existing methods from various sources considered to be relevant good practice. Stresses are derived using Finite Element (FE) analysis while transient definitions and cycles are taken from the plant design transient specification. The evaluation includes assessment points at the wetted clad surface (with environmental effects), the clad-base material interface (no environmental effects) and the outside, dry surface. The evaluation, following the EDF Energy FAM, mainly uses ASME Section III, ASME Section VIII, RCC-M, relevant ASME Code Cases and Fen factor calculations from NUREG/CR-6909 Rev. 1. Of particular note is that the EDF Energy FAM recommends explicit evaluation of the clad surface. The evaluation steps are described in detail in the paper to illustrate where assumptions are made to bridge the gap where there is ambiguity in implementing the ASME Section III fatigue evaluation procedure. Where there are different options available, such as the different strain rate calculations for Fen factors and cycle counting methods (peak-to-peak vs. rainflow) as described in the EDF Energy FAM, sensitivity studies are also undertaken to evaluate the effect of using the different options. The calculations lead to a matrix of Unit Usage Factors (UUFs) for each assessment location for each assessment case. The matrix describes the UUF, as in the fatigue usage for one cycle, for every standalone transient and transient pair evaluated. The cycles of the UUFs are then expended in accordance with the available transient cycles to calculate the Cumulative Usage Factor (CUF). This is done in descending order starting from the greatest UUF, eliminating transient / transient pairs in the process as transient cycles are used up. The UUF approach is presented as a versatile approach because it can accommodate different cycle counting methods. Moreover, it allows multiple cycles within transients to be accounted for. Finally, it offers flexibility in how transient pairs are expended in the calculation of CUFs. For example, different variants of a given transient can all form part of the UUF matrix, but the logic of how transient pairs are expended can be set up to ensure that only the most conservative variant contributes to the CUF. This avoids the need for pre-selecting the variants or having to include all variants to ensure conservatism. Another example would be to include rules to avoid unrealistic pairings that result in artificial large stress ranges. Both of these examples are included in the fatigue evaluation described in this paper.