PSI - Issue 60

Available online at www.sciencedirect.com Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2024) 000 – 000

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ScienceDirect

Procedia Structural Integrity 60 (2024) 44–59

Third International Conference on Structural Integrity 2023 (ICONS 2023) An assessment of delayed hydride cracking initiation in PHWR pressure tubes using in-house ZIPTAS code

D. Sen a* , I.A. Khan a , R. Rastogi a , J. Chattopadhyay a a Reactor Safety Division, Bhabha Atomic Research Center, Mumbai, Maharastra, India

© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the ICONS 2023 Organizers Abstract The earlier fitness-for-service (FFS) evaluation of a service-induced volumetric flaw in the pressure tube of a Pressurized Heavy Water Reactor (PHWR) was based on the threshold peak stress that can lead to initiation of Delayed Hydride Cracking (DHC) as a function of number of reactor Heat-up/cool-down cycles. This design curve was a conservative lower bound to the limited test data available at that time. It, however, did not account for the effect of flaw geometry explicitly. To overcome this restriction, an improved flaw evaluation procedure that accounts for the stress relaxation due to hydride formation and decohesion process by a non-linear process zone lying at the tip of a blunt flaw was proposed by Scarth and Smith (2001). This model has been adopted by the Canadian Standards Association in their FFS code of practice. The process zone model is, however, tedious as it involves a set of non-linear equations that need to be solved simultaneously. In the present work, the process zone model is implemented in an in-house computational code (ZIPTAS) and a set of parametric studies are performed to assess the influence of flaw geometry, flaw size and service loads on the maximum nominal stress that will not lead to initiation of DHC from a volumetric flaw in a pressure tube. The developed ZIPTAS code is validated with several published results and is expected to be useful to both plant operators and regulators for quick and robust assessment of flaw size that can be safely permitted for continued operation of PHWRs. © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the ICONS 2023 Organizers Keywords: Delayed hydride cracking (DHC), volumetric flaw, stress relaxation, fitness-for-service (FFS). 1. Introduction One of the important failure mechanism in the pressure tubes used in Pressurized Heavy Water Reactors (PHWRs) is “Delayed Hydride Cracking” (DHC). During service , hydrogen/deuterium is picked up by the pressure tubes (PT) from the heavy water which serves as coolant in the reactor. The hydrogen has a tendency to migrate to the region of high stress concentration. Once the hydrogen concentration at such locations exceeds the terminal

2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the ICONS 2023 Organizers

2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the ICONS 2023 Organizers 10.1016/j.prostr.2024.05.030