Abstract

The current primary load design provisions of Section III, Division 5 of the ASME Boiler and Pressure Vessel Code, covering high temperature nuclear components, represent an allowable stress methodology using elastic analysis and stress classification procedures to approximate stress redistribution caused by creep and plasticity. This process is difficult to implement and automate in modern finite element frameworks. This paper describes an alternate primary load design approach that uses elastic perfectly-plastic analysis in conjunction with the reference stress concept to eliminate stress classification while retaining a link to the existing Section III, Division 5 allowable stresses. This global, structural allowable stress check is supplemented with a local check to guard against the initiation of creep damage at local stress discontinuities like headers, nozzles, and other stress concentrations. This check is based on a simple elastic-creep analysis with creep damage calculated with the time-fraction approach, using the current ASME minimum-stress-to-rupture values already provided in the current Code. Both the global and local checks are easily implemented in modern finite element analysis software and greatly simplify Section III, Division 5 primary load design when compared to the current design-by-elastic-analysis method. Several examples demonstrate the utility of the new approach and its potential to reduce over-conservatism.

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