The purpose of this work is to establish a method for safety assessment with respect to plastic collapse, which is consistent for design assessment in accordance with Section III of the ASME Boiler and Pressure Vessel Code and assessment of in-service defects.
In the Swedish nuclear power industry, a procedure is used for assessment of in-service defects that is based on failure assessment diagrams according to the R6-method combined with a safety evaluation system corresponding to Section III and XI of the ASME Code. The Swedish procedure was originally based on R6 Revision 3 combined with the 1995 edition of the ASME Code. In recent years the procedure has been updated to correspond to current versions of R6 and ASME. These updates include a transition to the updated definition of flow stress as well as structural factors against plastic collapse that was included in the 2002 Addenda of ASME 2001 Section XI.
Following these updates, discrepancies were found between plastic collapse analyses made in accordance with Section III and assessments of in-service defects. Due to this, a project was initiated to make the procedure for assessment of in-service defects consistent with Section III analyses. An in-depth study of the flow stress definitions and structural factors in Section XI was initiated. This study found that the structural factors are based on approximations to get material independence. These approximations uses mean values based on a number of materials which was found to be quite inaccurate for many materials used in the Swedish nuclear power industry and can differ up to about 15 % compared to Section III analyses.
A method has been developed to calculate material specific structural factors that gives a near perfect match between plastic collapse according to Section III and the Swedish procedure for assessment of cracks. The method is based on the methodology for structural factors in Section XI, but excludes the assumptions made to get material independent structural factors. The method is presented and comparisons show the agreement to Section III and highlight cases of large discrepancies in Section XI.