ASME Code Section XI IWB-3500 gives the acceptance standards for in-service inspection of reactor pressure vessels, according to which the acceptable ratios of depth-to-thickness (a/t) varies along with depth-to-length ratios (a/l) for defects detected by nondestructive examination of reactor pressure vessels during the in-service stage. The acceptance standards are established based on fracture mechanics and five principal safety criteria as per EPRI NP-1406-SR, among which the safety margin between “allowable indication” in ASME Code Section XI and maximum postulated defects meeting ASME Code Section III fracture mechanics requirements is of most concern. The establishment of the acceptance standards can facilitate the development of new allowable indication standards to determine the component’s adequacy for continued service.

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