Computational reactor safety analysis is trending to replace conservative evaluation model calculations with best-estimate analysis complemented by uncertainty evaluation of the code results. In such cases, the evaluation of the margin to acceptance criteria (e.g., the maximum fuel rod clad temperature) is based on the upper limit of the calculated uncertainty range. Uncertainty analysis is compulsory if relevant conclusions are to be obtained from best-estimate thermal-hydraulic code calculations in order to avoid presenting single values of unknown accuracy for comparison with regulatory acceptance limits.

This paper, after a thorough introduction of conservative and best-estimate methods and characterization of the main sources of uncertainties affecting best-estimate system codes, applies a best-estimate-plus-uncertainty (BEPU) method to three cases having as reference different nuclear power plants and different types of transients. Finally, the results from the BEPU approach is compared with a conservative approach and a combined approach.