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 of the Chapter 15 on FSAR (Final Safety Analysis Report). The key steps of the entire process are basically two: a) the selection of PIE (Postulated Initiating Events) and, b) the analysis by best estimate models supported by uncertainty evaluation. Otherwise, key elements of the approach are: 1) availability of qualified computational tools including suitable uncertainty method; 2) demonstration of quality; 3) acceptability and endorsement by the licensing authority. The effort of issuing Chapter 15 is terminated at the time of issuing of the present paper and the safety margins available for the operation of the concerned NPP (Nuclear Power Plant) have been quantified.
The application of the BEPU approach is mandatory in licensing of NPP (Nuclear Power Plants) when “best-estimate” (BE) codes and realistic input condition values are adopted. Namely, the predictions by BE codes are affected by errors that must be quantified. Advantages for the industry from the application of BE approaches (BE codes & realistic input) versus “conservative” approaches (conservative codes, or even BE codes with conservative input) shall also be characterized. In some situations, conservative approaches bring the results of accident analysis outside the acceptability boundaries, while BEPU results of the same transients are acceptable. The experience gained in producing the Chapter 15 on Accident Analyses of the FSAR (Final Safety Analysis Report) of the Atucha-2 PHWR (Pressurized Heavy Water Reactor), fully based on the BEPU approach, is summarized. The following key aspects are at the basis of the BEPU application in the Atucha-2 licensing process: • The accident classification: the standard distinction between Anticipated Operational Occurrences, Design Basis Accident and Beyond Design Basis Accident is considered. However, different assumptions are proposed for the analysis of selected transients belonging to individual classes. • Conservatism is embedded into the reactor design including protection system and related set-points; furthermore, conservatism can be added in the analyses through the proper choice of boundary and initial conditions or by (typically) preventing redundant trains of emergency core cooling systems (ECCS) from contributing in recovering the plant safety functions. • A best estimate analysis of phenomena expected in the concerned class of transients implies the coupled application of system thermal-hydraulics, computational fluid dynamics and three-dimensional neutron kinetics computational tools other than structural mechanics codes. The demonstration of suitable qualification for the computational tools constitutes a challenge for the present approach.
The Best-Estimate Plus Uncertainty (BEPU) Challenge in the Licensing of Current Generation of Reactors
D'AURIA, FRANCESCO SAVERIO;
2012-01-01
Abstract
The application of the BEPU approach is mandatory in licensing of NPP (Nuclear Power Plants) when “best-estimate” (BE) codes and realistic input condition values are adopted. Namely, the predictions by BE codes are affected by errors that must be quantified. Advantages for the industry from the application of BE approaches (BE codes & realistic input) versus “conservative” approaches (conservative codes, or even BE codes with conservative input) shall also be characterized. In some situations, conservative approaches bring the results of accident analysis outside the acceptability boundaries, while BEPU results of the same transients are acceptable. The experience gained in producing the Chapter 15 on Accident Analyses of the FSAR (Final Safety Analysis Report) of the Atucha-2 PHWR (Pressurized Heavy Water Reactor), fully based on the BEPU approach, is summarized. The following key aspects are at the basis of the BEPU application in the Atucha-2 licensing process: • The accident classification: the standard distinction between Anticipated Operational Occurrences, Design Basis Accident and Beyond Design Basis Accident is considered. However, different assumptions are proposed for the analysis of selected transients belonging to individual classes. • Conservatism is embedded into the reactor design including protection system and related set-points; furthermore, conservatism can be added in the analyses through the proper choice of boundary and initial conditions or by (typically) preventing redundant trains of emergency core cooling systems (ECCS) from contributing in recovering the plant safety functions. • A best estimate analysis of phenomena expected in the concerned class of transients implies the coupled application of system thermal-hydraulics, computational fluid dynamics and three-dimensional neutron kinetics computational tools other than structural mechanics codes. The demonstration of suitable qualification for the computational tools constitutes a challenge for the present approach.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
