Development of a Partial Structural Factor Approach for Direct Fracture Toughness Implementation into the ASME Boiler and Pressure Vessel Code

Direct implementation of best estimate fracture toughness models is a growing priority to enable an accurate assessment of the fracture safety of nuclear pressure boundary components for extended licensing periods. Models currently adopted for use in ASME Section XI of the Boiler and Pressure Vessel Code are based in linear elastic fracture mechanics methodologies developed 40 or more years ago at a time when drop weight tear tests and Charpy V-notch impact tests were the accepted standards used for characterizing a material's resistance to brittle fracture. The current ASME guidance uses: a conservatively estimated RTNDT reference temperature, to index the linear elastic, lower-bound K-Ic curve that describes a material's resistance to fracture, and then divides this lower-bound curve by a structural factor. This approach includes multiple implicit and explicit treatments of uncertainty. It is not transparent, it produces inconsistent levels of conservatism, and it makes it difficult to incorporate improved knowledge, obtained from either new data or new models, into a flaw evaluation. Code Case N830-R1 proposes use of a suite of best-estimate models to describe the temperature dependence and scatter of fracture toughness from lower shelf through upper shelf. CC-N830-R1 proposes that the 5th percentile lower bound of these models be used in flaw evaluations along with the current IWB 3611 and 3612 requirements that includes division of this 5th percentile curve by a structural factor. The intent in the next revision of this Code Case (Revision 2) is to move away from these current methods that inconsistently, and in some cases multiply, treat uncertainties. Uncertainty treatment that relies on combinations of lower bound models, structural factors that do not consistently bound data, and margins needlessly complicate and confuse the characterization of material toughness and make it nearly impossible to assess the true level of safety. CC N830-R2 will therefore propose the use of a partial structural factor (PSF) approach. PSFs will be assigned to each of the key terms (i.e., toughness, stress, flaw size) in the fracture mechanics equations used in assessing safety of flaws. The value of each PSF will be defined to account for the uncertainty contribution of that term in a consistent manner. Partial safety factors have been successfully adopted by other codes and standards as will be further described in this paper, along with a proposed approach for defining partial safety factors for use with the CC N830-R1 models.