The Best Estimate Plus Uncertainty (BEPU) approach in licensing of current nuclear reactors

Within the licensing process of the Atucha-II PHWR (pressurized heavy water reactor) the BEPU (Best Estimate Plus Uncertainty) approach has been selected for issuing the Chapter 15 of FSAR (Final Safety Analysis Report) dealing with accident analysis. The key steps of the entire process at the bases of the BEPU approach are basically two: (a) the selection of PIE (postulated initiating events) and (b) the analysis of each PIE by best estimate models supported by uncertainty evaluation. Otherwise, key issues which should be addressed to pursue the BEPU approach are: (1) availability of qualified computational tools including suitable uncertainty method; (2) demonstration of quality of any calculation result; (3) achieving the acceptability and endorsement by the licensing authority. The key features of the BEPU approach are discussed in the paper. The approach constitutes a valuable and, under some circumstances, an unavoidable tool to demonstrate the safety of the concerned NPP.