Approaches and methods to evaluate the uncertainty in system thermal-hydraulics

The evaluation of uncertainty constitutes the .necessary supplement of Best E stimate (BE) calculations performed to understand accident scenarios in water cooled nuclear reactors. The needs come from the imperfection of computational tools on the one side and from the interest in using such tool to get more precise evaluation of safety margins. In the present paper the approaches to uncertainty are outlined and the CIAU (Code with capability of Intemal Assessment of Uncertainty) method proposed by the University of Pisa is described including ideas at the basis and results from applications. An activity in progress at the Intemational Atomic Energy Agency (IAEA) is considered. Two approaches are distinguished that are characterized as “propagation of code input uncertainty” and “propagation of code output errors”. For both methods, the thermalhydraulic code is at the centre of the process of uncertainty evaluation: in the former case the code itself is adopted to compute the error bands and to propagate the input errors, in the latter case the errors in code application to relevant measurements are used to derive the error bands. The CIAU method exploits the idea of the “status approach” for identifying the thermalhydraulic conditions of an accident in any Nuclear Power Plant (NPP). Errors in predicting such status are derived from the comparison between predicted and measured quantities and, in the stage of the application of the method, are used to compute the uncertainty.