Evaluation of Concentric Tube Model for Pressure Tube to Calandria Tube Gap Measurement

The CANDU
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nuclear reactor fuel channels consist of a pressure tube (PT) contained within a calandria tube (CT). The gap between the hot (~300° C) PT and cooler (~50° C) CT is required since the contact could lead to hydride blister formation with subsequent cracking. This gap is measured using a drive-receive eddy current technology, providing assurance that the contact is not imminent. The analytical models of probe response to gap use flat plates to approximate the PT and CT geometry. In this paper, a semi-analytical model that approximates the PT within the CT geometry as two concentric tubes with correct PT curvature, but a CT diameter, which is adjusted to change gap, is compared with a flat-plate model of probe response to changing gap against experimental measurements using standard CT and PT samples, with varying wall thickness and resistivity that simulates in-reactor variation of these parameters. The concentric and flat plate models both predict gap values with an average error of 0.1 mm between contact and 5 mm gap at 4.2 kHz and 8.0 kHz. The concentric model has an average relative gap error at the maximum gap of 2% at 4.2 kHz and 8.0 kHz, whereas that of the flat-plate model is 10% and 12%, respectively. The improved accuracy of the concentric model at larger gaps is attributed to the incorporation of tube curvature, which limits the probe's magnetic field spread when compared to the flat-plate model.