Abstract

As part of the effort for a nuclear plant to undergo license renewal, the effect of reactor water environment on fatigue life must be addressed for limiting component locations. One method to incorporate the effects of reactor water environment into the fatigue evaluations of metal components is to apply an environmental fatigue penalty factor (Fen) to the partial usage factor obtained from the design fatigue curve for each stress cycle. Fatigue evaluations have historically been performed by assuming that temperature transient loads occur at conservatively high rates to maximize the stress response and corresponding fatigue usage values. However, with consideration of reactor water environmental effects on fatigue, transients with slower rates generally produce higher Fen values that could potentially result in higher environmental fatigue usage values than transients with identical temperature changes but faster rates. A generic parametric study was performed in MRP-218 to characterize limiting transient ramp rates with respect to environmental fatigue usage for a range of piping geometry and material configurations. This paper describes the application of the parametric study results to optimize thermal hydraulic and stress response modeling assumptions with respect to transient rates and downstream effects on environmental fatigue results in both design and monitoring fatigue evaluations.

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