Cycle-Wise Process-Zone Model for Evaluation of DHC Initiation Under Flaw-Tip Hydride Ratcheting Conditions With Creep

A key structural integrity concern with in-service flaws in Zr-2.5Nb pressure tubes in CANDU reactors is their susceptibility to delayed hydride cracking (DHC) initiation, particularly for debris fretting flaws under flaw-tip hydride ratcheting conditions. Hydride ratcheting conditions refer to situations when flaw-tip hydrides do not completely dissolve at normal operating temperature, and accumulation of flaw-tip hydrides occurs with each reactor heat-up/cool-down cycle. A cycle-wise process-zone model was developed as an extension to the process-zone model in CSA N285.8. The model is able to predict whether DHC initiation occurs or not during a specific reactor heat-up and cool-down cycle. The development of the cycle-wise process-zone model was driven by the need to include flaw-tip stress relaxation due to creep in evaluation of DHC initiation. The model explicitly accounts for the decrease in process-zone stress due to flaw-tip hydride accumulation with reactor cool-down cycles and due to creep induced flaw-tip stress relaxation during reactor normal operation, which enables proper inclusion of the beneficial effect of creep induced stress relaxation on DHC initiation. This paper presents the cycle-wise process-zone model for evaluation of DHC initiation under flaw-tip hydride ratcheting conditions with creep, focusing on validation of the model for prediction of DHC initiation against laboratory tests on un-irradiated pressure tube specimens under an applied load and thermal cycles with a long hold at the peak temperature that simulates the creep effect under the normal operating conditions. The validation of the cycle-wise process-zone model supports taking credit for creep induced flaw-tip stress relaxation in flaw evaluation for pressure tubes in CANDU reactors.