Uncertainty Methods and Approaches in Nuclear System Safety

Two approaches can be distinguished for uncertainty evaluation that are characterized as “propagation of code input uncertainty” and “propagation of code output errors”. For both methods, the thermal-hydraulic 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 (Code with capability of Internal Assessment of Uncertainty) Method proposed by the University of Pisa is described including ideas at the basis and results from applications. The CIAU method exploits the idea of the “status approach” for identifying the thermal-hydraulic conditions of an accident in any Nuclear Power Plant (NPP). Errors in predicting the status of the NPP 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.