Reliability-based partial safety factors for dual performance level design of prestressed inner containment shells in Indian nuclear power plants

Partial safety factors (PSFs) used in reliability-based design are intended to account for uncertainties in load, material and mathematical modeling while ensuring that the target reliability is satisfied for the relevant class of structural components in the given load combination and limit state. This paper describes the methodology in detail for developing a set of optimal reliability-based PSFs for the design of prestressed concrete inner containment shells in Indian NPPs under Main Steam Line Break (MSLB)/Loss of Coolant Accident (LOCA) conditions at two performance levels in flexure: cracking and collapse. The methodology follows current design practices in the country, accounts for uncertainties in loads and material properties and dependence among capacities and demands, develops the limit states from first principles, explicitly lays down the target reliabilities and criteria for PSF optimization. The optimization of the PSFs is based on reliability indices for each representative group of components obtained from importance sampling and a local linear response surface fit. A detailed numerical example on a typical 220 MWe Indian PHWR demonstrating the methodology is provided.