Overview of Uncertainty Methods in Nuclear Reactor Thermal-Hydraulics

Uncertainty analysis aims at characterizing the errors associated with experiments and predictions of computer codes, in contradistinction with sensitivity analysis, which aims at determining the rate of change in the predictions of codes when one or more input parameters varies within its range of interest. The description is provided of the features of independent approaches for estimating the uncertainty associated with predictions of system thermalhydraulic codes. The 1st approach is the “standard” one and the most used at the industrial level: it is based upon the selection of input uncertain parameters, on assigning related ranges of variations and PDF and on performing a suitable number of code runs to get the combined effect of the variations on the results. In the 2nd approach the uncertainty derives from the comparison between relevant measured data and results of corresponding code calculations. The 3rd approach is based upon a sensitivity analysis procedure and uses the experimental data to characterize the ranges of variation of ‘all’ input parameters. Emphasis is given to the requirements for setting up an uncertainty method and to the demonstration of the benefits of related applications to the licensing process.