A SPLIT-COIL EDDY-CURRENT TECHNIQUE FOR THE MEASUREMENT OF ANNULAR GAPS

Eddy current testing techniques have been used for the measurement of the gap between the pressure tube and calandria tube in coolant channe l assemblies of a pressurised heavy water reactor. Standard rotating ' absolute' probes are generally used for the measurement of this gap, b ut with an absolute probe the eddy current signal is strongly dependen t on the wall thickness of the pressure tube and is not linear for all gap values, particularly with gaps larger than 4 mm. A modified diffe rential split-coil probe has been designed in the authors' laboratory which shows only a very small effect due to wall thickness variation o f the pressure tube. Experimental results show an accuracy of +/- 0.6 mm, without measurement of the wall thickness.