A method for calculating a fatigue usage factor of a component in a nuclear reactor includes determining a flow scaling factor for each stress component of a stress cycle, applying the flow scaling factor to rapid cycling stress conditions, and performing a fatique evaluation by calculating a fatigue usage factor using the flow scaling factor. The flow scaling factor is a ratio of a temperature fluctuation ratio at a first reactor operating condition divided by a temperature fluctuation ratio at a second reactor operating condition. The temperature fluctuation ratio is defined as a ratio of the metal temperature range over the fluid temperature range.