As the PT is a primary coolant pressure boundary, the probability of such cracking must be reduced to very low levels. Hence it is very important to measure the gap between the two tubes.

As access to the PT bore is available after shutdown (and fuel unloading for the particular tube), it is possible to use the Eddy Current Test (ET) method to measure the annular gap. We used conventional single frequency eddy current test equipment of the impedance phase angle analysis type generally used for testing heat exchanger tubes. The measurement principle is based on lift-off.

The main problem is separating the signals from wall thickness variation from the signal for the gap. At low frequency where the sensitivity is good, the wall thickness variations gives signals of the same phase as that of the gap signal. At high frequency the phase angles are different but sensitivity is lower. To neutralize other variables such as resistivity changes in the material (including radiation-induced), a self comparison differential type probe was built.

The probe design was unusual in that different depths of penetration were designed into the main and balancing coils by making them in different sizes, and this approach largely cancelled thickness variation effects. A very useful aspect was that the probe could measure gaps right up to the maximum possible, i. e., 17 mm, with sufficient confidence in measurement. This permits summing of readings obtained from diametrically opposite locations (which sum must be a constant), as a means of verifying test accuracy.

In practice, we found that the wall thickness variation was not fully compensated for smaller gaps, and hence we used one more single frequency eddy current test equipment set at high frequency and recorded eddy current test signals corresponding to wall thickness variations. This permitted a computed compensation of wall thickness variation. Results have been accurate to 1 mm. Accuracy, as can be expected, is higher for smaller gaps. This paper describes the design details as well as the actual working experience at Rajasthan Atomic Power Station, Kota, India.