A probabilistic approach to update lower bound threshold stress intensity factor (KIH) for delayed hydride cracking

Zirconium alloys are susceptible to a stable cracking process called delayed hydride cracking (DHC), which can compromise the integrity of pressurized components in the nuclear reactor. Therefore, the fitness for service assessment of pressure retaining components requires demonstrating that there is sufficient margin against the rupture. The CSA-Standard N285.8 specifies a lower-bound value of the threshold stress intensity factor (K-IH) that would provide adequate protection against fracture that can potentially originate from existing cracks. Since fracture toughness is a variable material property, a probabilistic approach is necessary to establish the lower bound of K-IH. The paper presents a probabilistic definition of the lower bound K-IH, and its Bayesian interpretation to address both aleatory and epistemic uncertainties. The paper proposes a new method to update the lower bound K-IH as new surveillance data become available from the testing of components removed from operating reactors. A concept of a control chart is introduced to support the disposition of new surveillance data. The main advantage of the proposed approach is that it provides a practical, risk-informed basis for fracture toughness assessment of pressurized components. (C) 2010 Elsevier B.V. All rights reserved.