Methodology to evaluate uncertainty in thermalhydraulic code calculations

The present paper deals with the description of a few aspects of the UMEA (Uncertainty Methodology based on Extrapolation of Accuracy) methodology suitable to evaluate the uncertainty in the prediction, by thermalhydraulic system codes, of the transient scenarios in nuclear reactors. The methodology is based upon the extrapolation of the accuracy resulting from the comparison between code results and relevant experimental data obtained in small scale facilities. Emphasis is given in this paper to the statistical treatment of data obtained from experiments and code calculation. Some results related to the full application of the UMEA to a small break Loss of Coolant Accident in a PWR, including core uncovering, are reported. (Author)