FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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probe in its operating environment is the combined result of this effect plus
the temperature related resistivity changes in the pressure tube and in the
stainless steel wear cap.
The temperature variations in each fuel channel are expected to be minimal due
to the flow of coolant that exists during shutdown. As a result the
temperature sensitivity of the probe should not cause finy major problems
during inspections and may be covered by the stated accuracy of the results.
2.5 Probe Variations
The data presented thus far were obtained with one specific probe.
Experiments comparing subsequent probes to the original showed a constant
difference in signal outputs due to identical gap variations. The wall
thickness and temperature sensitivities of the alternate probe were also
verified to differ from the reference probe by the same factor. This allowed
the assignment of a compensation factor to each probe generally ranging in
value from 0.7 to 1.1, so that for an identical set of circumstances the
change in output of the alternate probe could be referred to a standard.
3. GAP ESTIMATION
The procedure for estimating gap assumes that lift off, resistivity,
temperature or any other factors do not influence the eddy current signal
obtained in channel. Before the inspection data can be used, these signal
values must be related to those recorded in the data base, and so it is
necessary to select a reference point in each channel where gap, wall
thickness and the eddy current signal value are known. This generalized
requirement does not necessarily involve the point at which the instrument is
balanced, thus reducing the function of balancing to one of locating the
output trace somewhere on the strip chart record. The wall thickness and eddy
current values may be measured at the reference point but the gap must be
inferred from the structure of the fuel channel. Two good reference points
are located near the ends of the pressure tube, as close as possible to the
rolled joints. Here the pressure tube is centered firmly in the calandria
tube and the gap may be assumed to be 8 mm at all rotational positions.
Once a reference point has been chosen in the channel, the corresponding point
in the data base may be located using the wall thickness and gap information.
From this point, the changes in eddy current signal and wall thickness data,
as measured when the probes travel from the reference point to the measurement
point in-channel, can be followed individually to arrive at the mesurement
point in the data base. It is quite likely that the measurement point in the
data base will end up between two lines of constant gap in which case simple
linear interpolation is used to determine the estimated gap value. The number
of lines of constant gap is large enough that this approximation to the true
non-linear nature of the gap response will not introduce any significant
error. This procedure may easily be implemented on a computer using only the
stored slopes and intercepts of the lines in the data base.