Seismic re-evaluation of EDF Bugey 900 PWR nuclear power plant in the frame of the 3rd periodic safety review

Abstract

As compared to the previous Periodic Safety Review (PSR), main features of the seismic facet of the Bugey NPP (France) 3rd PSR are relating to (i) a 45% increase of the Review Level Earthquake (RLE), (ii) a dramatic increase of computer code capabilities and (iii) a certain evolution of safety standards. This paper presents the method implemented to cope with this situation. Seismic input motion is determined according to an updated Basic Safety Rule published in 2001. Concerning buildings, 3D models are established, accounting for torsion effects, which were not represented by the initial design spring-mass models; analyses are carried out as per the IAEA methodology, accounting for ductile capacity through a F μ factor; finally the major impact of the seismic re-evaluation is a reinforcement of electrical building, so that it is less torsion sensitive. Regarding equipment, the concept of Safe Shutdown Equipment List (SSEL) is enlarged to all the Safety Related Equipment List (SREL). An equipment hierarchy classification is introduced, based on two complementary approaches: a system approach that identifies components associated to success paths for a safe shutdown, and a ruggedness approach that considers experience feedback about seismic behavior of components. On these bases, all the safety related equipments are classified in two categories: High Hierarchy level (HH) and Medium Hierarchy level (MH). MH equipment is mainly verified by walkdown, whereas HH equipment is by calculation, taking benefit from design margins. The optimization procedures enable to focus on the more relevant components. Bugey NPP seismic re-evaluation stands out by its realistic calculation chain and the completeness of its assessment, and eventually, the 3rd PSR is an opportunity to improve the plant safety and increase design margins.