Keywords: best estimate, BE calculations, code input uncertainty, code output errors, statistical tools, deterministic method, accuracy, safety margin, thermal-hydraulic calculations, accident scenarios, water-cooled nuclear reactors, nuclear energy, nuclear power, nuclear accidents, nuclear safety
Evaluation of uncertainties in system thermal-hydraulic calculations and key applications by the CIAU method
The evaluation of uncertainty is a necessary supplement to Best Estimate (BE) calculations that are performed to understand accident scenarios in water-cooled nuclear reactors. The need comes from the imperfection of computational tools, on the one side, and from the interest in using such a tool to get more precise evaluation of safety margins, on the other. In the present paper, the approaches to uncertainty are outlined and the Code with capability of Internal Assessment of Uncertainty (CIAU) method proposed by the University of Pisa, Italy is described. Two approaches are distinguished and identified as 'propagation of code input uncertainty' and 'propagation of code output errors'. The CIAU method exploits the idea of the 'status approach' for identifying the thermal-hydraulic conditions of an accident in any water cooled nuclear reactor. Errors in predicting such a 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.