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.