Abstract
When in-Service inspection detects flaw in nuclear components, Acceptance Standards of ASME Code Section XI provide allowable flaw sizes to assess the flaw severity and to identify the need for further evaluation. For ferritic vessel welds, the size of allowable planar flaws is given in Table IWB-3510-1.
Considering some inconsistencies highlighted in this table, a revision using a robust technical basis is conducted. For that purpose, a methodology is developed following ASME philosophy and including some basic criteria to ensure component safety by preventing plastic collapse and brittle failure. As far as the prevention from plastic collapse is concerned, a uniform limit load reduction is considered whatever the flaw aspect ratios. For the prevention of brittle failure, a reference surface flaw configuration (flaw size, aspect ratio and component thickness) is defined to derive a reference stress intensity factor that should not be exceeded. Then, all the flaw configurations corresponding to this reference stress intensity factor are identified to derive a set of allowable flaw size curves. At last, the most penalizing flaw sizes between both failure modes are retained to define the new allowable flaws.
This methodology is applied to surface flaws with various aspect ratios and component thicknesses. It is also coherently applied to subsurface flaws considering the proximity of the flaw to the surface as an additional parameter. Finally, a revision of the allowable planar flaw Table IWB-3510-1 of ASME Code Section XI Acceptance Standards is proposed.
