Abstract

In case of flaw detection during In-Service inspection of nuclear components, ASME Code Section XI provides Acceptance Standards to determine whether the flaw can be considered as harmless or should be further evaluated.

For ferritic steel materials, the size of allowable planar flaws is given in Table IWB-3510-1. To facilitate the assessment, the allowable flaw size only depends on three parameters : the component thickness, the flaw aspect ratio and the proximity of the flaw to the surface. However, a graphical analysis of the impact of those parameters highlights some inconsistencies. The allowable flaw depth evolution is discontinuous between the ranges of thickness. Besides, the allowable flaw depth increases indefinitely with component thickness, which is not consistent with finite fracture toughness of materials. Finally, while the aspect ratio dependence of the allowable flaw depth has been clearly described in EPRI NP-1406-SR (1980), no clear technical basis has been found to explain the component thickness dependence. Consequently, the need to revise the allowable planar flaws of ASME Code Section XI Acceptance Standards using a robust technical basis is brought to light.

This paper details the inconsistencies related to the present allowable planar flaws table and proposes improvement points to revise the allowable planar flaw Table IWB-3510-1.

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