Examining the Viability of Ultrasonic Inspection of Discharged CANDU® Fuel Bundles

Abstract

Post in-reactor inspection of discharged CANDU® fuel bundles is usually performed using visual techniques, which are only practical for observation of obvious evidence of damage to Zircaloy-2 fuel sheaths. Detected occurrences of damage due to fabrication flaws or in-service debris fretting or cracking have been less than 0.1%. The application of ultrasonic testing (UT) methods may provide a more sensitive means of detecting compromised fuel sheaths within bundles through identification of water ingress. This work presents an examination of the capability for UT to differentiate between conditions of water ingress and absence of water in fuel elements containing un-irradiated uranium dioxide pellets. Results obtained from tests of underwater angled beam UT established that the presence water in the fuel rod can act as a couplant for sound waves, allowing for generation of acoustic signatures from between fuel pellets within the element. Either shear wave coupling or, more effectively, the generation of axial Lamb waves in the Zircaloy-2 fuel sheath could be used to generate acoustic signatures. The results demonstrate the viability of detection of water within compromised fuel elements. This is the first step towards development of a post-service UT inspection of a complete irradiated fuel bundle. Considerations for the demonstration of UT on irradiated fuel elements, the next step in proving the viability of UT of discharged fuel bundles, will also be presented.