| The CANDU (CANadian Deuterium Uranium) reactor uses deuterium as a moderator and uranium as a fuel. Uranium fuel elements consist of pellets stacked within a sealed Zircaloy-2 sheath. In the rare occurrence of a defective fuel sheath, the primary coolant can enter into the element and flash to steam in the fuel-to-sheath gap, where the steam can subsequently react with the fuel matrix. This reaction can cause the fuel to oxidize, which can affect the element's thermal performance since the oxidization of the fuel may result in a reduced thermal conductivity and a lower incipient melting temperature. Moreover, when the element is breached, there is also a release of fission products and fuel debris from the element into the primary heat transport system. Consequently, it is important to discharge the defective bundle as soon as operationally possible in order to minimize such effects and occupational exposure of station personnel. A post-irradiation inspection is required to: 1) Establish the frequency of fuel failure events and 2) evaluate the severity of such events.;Visual techniques are normally used for the post in-reactor inspection of discharged CANDURTM fuel bundles to help identify fuel failures (and the possible root cause of the failure). These inspections look for obvious evidence of damage to Zircaloy-2 fuel sheathing. In this work, a more sensitive method based on ultrasonic testing (UT) is studied for the detection of water in compromised fuel elements. Experiments were therefore performed at the Stern Laboratories on both intact and defective fuel elements to assess the capability of the UT method. These tests utilized fuel elements containing unirradiated uranium dioxide pellets, and simulated elements which used copper pellets for instrument calibration purposes. Underwater angled beam inspections demonstrated that the introduction of water into the fuel elements acts as a couplant for sound waves, allowing for a clear demarcation of the fuel pellets within the element in observed scans. Optimum results were obtained for angled beams in the range of 30°-35° (shear wave mode) and 50°-55° (lamb wave mode), with respect to the normal incidence for sound waves passing from water into the Zircaloy-2 sheath. This study demonstrated that the inspection of the outer-ring (i.e., higher-power) elements in a fuel bundle is possible. As such, this work can be taken as a first step towards the development of a post-irradiation UT inspection technique for a complete irradiated fuel bundle. General design concepts for implementation of such a system are also presented. |
