Panel session: safety factors in the ASME code where do we go from here?

摘要

Safety factors (design margins) are used in the SAME Code to ensure that plant components can safely perform their intended function without exceeding their structural capacity. A safety factor is generally chosen such that the structural limit (design allowable) used in the design is a fraction of the actual structural capacity. In other words, the actual structural limit is divided by the safety factor to obtain the design allowable. In addition to ensuring the safe operation of the component, the safety factor also accounts for uncertainties in loading conditions, material properties, and analytical techniques used in the design of the component. The consequence of component failure may also influence the safety factor. Thus, it follows tha the higher the uncertainties and consequences of failure, the larger the safety factor should be. It is for this reason that a relatively large safety factor of 5 was used when the ASME Code was first published. However, with progress in the understanding of material behavior, loading and analytical techniques, the nominal factor of safety was reduced to 4. With the advent of nuclear power, additional stress limits materials characterization and inspection lead to further reduction of the nominal safety factor to 3. Although these safety factors appear to be reasonable, they are subjective in nature. Section VIII of the code recently justified reducing the safety factor from 4 to 3.5. Should this be the starting poin?