Enhancing the Capabilities of the CANDU Gap Probe

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Bennett, Perryn
CANDU , Eddy Current , LISS Nozzle , Gap Probe , Nuclear , Pressure Tube , Fuel Channel , Nuclear Reactor
Fuel channels in CANDU (CANadian Deuterium Uranium) nuclear reactors consist of two non-concentric tubes; an inner pressure tube (PT) and a larger diameter calandria tube (CT), within which the PT is contained. The gas annulus space between the tubes, known as the gap, ensures that hydride blisters do not form on the outer diameter of the PT, as these can lead to potential cracking. Consequently, accurate measurements are required to ensure that contact between PT and CT is not imminent. The PT-CT gap is monitored from within the PT using an electromagnetic eddy current probe called the gap probe. The gap probe has previously been investigated to extract additional information from the channel past its intended purpose, including a method to profile the inner PT surface by examining probe lift-off effects. This thesis looks at two other areas of enhancement of the gap probe’s functionality; extracting the liquid injection shutdown system (LISS) nozzle proximity to fuel channels, and extracting PT microstructural information by monitoring local PT electrical resistivity. LISS-PT proximity information was extracted from an experimental setup that could vary PT wall thickness, PT resistivity, PT-CT gap, LISS-PT proximity and gap probe lift-off. Empirical models were identified over two effective detection ranges, and out-performed remote field eddy current methods near LISS-CT contact. In addition, as PT-CT gap algorithms become more sophisticated, the ability of being able to extract local PT resistivity from gap inspection data will become available. The utility of local PT resistivity depends on research linking resistivity to characteristics of interest to CANDU reactor operators. This work identified that as PTs experience operating temperatures between 250-320 °C and their microstructure transforms, specifically decomposition of the beta-phase, the resistivity is found to decrease by up to 10%. This result has implications for some gap inspection algorithms that assume constant electrical resistivity. This change in resistivity due to heat treatment, if measured, could be leveraged to provide axial and circumferential microstructural information that could be used by probabilistic and deterministic fuel channel degradation models.
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