A fatigue assessment device, Fatiguemètre, has been developed by EDF especially for sensitive zones of components of French PWR NPP. The main objective of this device is the evaluation of a realistic usage factor (damage criterion) by using operating data. Two requirements are to use as less specific transducers as possible and simplified thermomechanical methods for the calculation of stress variation. Some monitoring devices had nevertheless been necessary to get correlations between in service data, for instance flow rate and thermal stratification distribution. This instrumentation, called a stratimeter, consists of a ring of thermocouples installed around a section of a pipe.

The present studies were carried out on stabilized thermal stratification states and also on transients of different rates. These input data are always representative of the operating and design transient occurring on the FWL next to the connection with the steam generator. The first use of the Fatiguemètre device revealed the need of optimization on some thermomechanical methods. Investigating steady-state (stabilized) or slow enough transients of temperature stratifications, it was found that it is possible to calculate sufficiently accurate stresses without the use of any stress intensification factor, when applying temperatures from outer wall measurement as mean wall temperatures. The new discretization of Duhamel formula gives a satisfying evaluation of the local stratification stress value.

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