Exploitation of BEPU Approach for Licensing purposes

Best estimate codes supplemented by uncertainty evaluation (i.e. BEPU = Best Estimate Plus Uncertainty) achieved a suitable maturity and can be applied to the licensing process of water cooled nuclear power plants. The Final Safety Analysis Report (FSAR) constitutes the key element for demonstrating the safety of a nuclear power plant. Thus BEPU approach is relevant to the FSAR. The present paper deals with a proposal to apply a BEPU approach to the FSAR of Atucha- 2 PHWR (Pressurized Heavy Water Reactor) now in construction in Argentina with operation start expected in 2010. Atucha-2 is a vessel type two loop KWU-Siemens- AREVA plant with vertical core channels and hydraulically separated moderator and coolant circuits. The following key aspects are at the basis of the possible application of the BEPU approach 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