Probabilistic assessment of thermal fatigue in nuclear components

A probabilistic framework is set up to assess the fatigue life of components of nuclear power plants. It intends to incorporate all kinds of uncertainties, such as those appearing in the specimen fatigue strength (number-of-cycles-to-failure of specimens), design margin factors (taking into account the size, surface finish and environmental effects), mechanical model (precisely, the uncertainty on the model input parameters) and the thermal loading. This paper presents the global methodology and details the statistical treatment of the fatigue specimen test data. A first analytical example shows that the reliability of a structure submitted to a periodic stress cycle S changes significantly with respect to the value of S, although the codified (deterministic) design criterion is equally fulfilled. A more comprehensive example involving a mechanical model of a pipe submitted to a deterministic inner temperature loading is finally analysed. The use of the first-order reliability method (FORM) allows to compute the probability of failure as a function of the foreseen lifetime and to rank the input random variables according to their importance in response sensitivity.