Abstract
In-service evaluation of a flaw that is detected in a Zr-2.5Nb alloy pressure tube in a CANada Deuterium Uranium (CANDU) nuclear reactor includes an analysis to demonstrate that delayed hydride cracking will not initiate from the flaw. The flaw can be assessed either as planar or volumetric with a finite root radius. Two adjacent flaws caused by fretting due to flow-induced vibration were detected by in-service ultrasonic examination in a Zr-2.5Nb alloy pressure tube in a CANDU reactor. The flaws were replicated to obtain details of flaw geometry. The two flaws are volumetric in nature, have irregular shapes, and were determined to intersect. Although both flaws are volumetric, for pragmatic reasons one flaw was treated as planar. The other flaw was treated as blunt. The flaws were evaluated for delayed hydride cracking initiation in accordance with the Canadian Standards Association Standard N285.8. A three-dimensional finite element stress analysis was performed to quantify the interaction between the two flaws, as well as the peak stress at the root of the blunt flaw. Both flaws met the acceptance criteria for prevention of delayed hydride cracking initiation, and the pressure tube was returned to service. This paper provides a summary of the delayed hydride cracking initiation evaluation of these two flaws, and highlights the methods that need to be applied for flaws with irregular shapes and that interact.
