FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The eddy current and wall thickness probes are located at the same rotational
position on the inspection head, however they could not be used simultaneously
during an axial scan due to the conflicting requirements of the two systems
(the eddy current scan must be continuous whereas the ultrasonic probe must be
stopped for each measurement). This separation of the data collection scans
caused some problems on numerous occasions as the inspection drive mechanism
would tend to slip rotationally and corkscrew without affecting the rotational
display counter. This had serious consequences for the gap system since a
small rotation in a typical pressure tube during an axial scan would result
in a sharp rotational wall thickness change being sensed as a gradual axial
change when in fact there was none. Such a situation, especially if only
experienced during the eddy current or wall thickness scan, would result in
misleading gap results.
4.4 Sample Results
The axial gap profile of a fuel channel exhibiting accelerated pressure tube
sag due to the shift of a garter spring from its design location is shown in
Figure 10. The gap system was originally conceived to discover situations
such as this where the PT/CT spacing is very small in some areas.
4.5 Sources of Errors
Errors can generally be grouped into two categories, those that affect the
accuracy of the measurements and those that affect the reliability of the
system. The latter group tend to severely distort the data and would include
such conditions as; cabling problems, misalignment of the wall thickness and
eddy current probes, operator or analyst error and excessive temperature or
resistivity changes. Many times these problems will cause the data or results
to be unusual or even unreasonable when compared to historical data and known
characteristics. Some checks that can be made to give credibility to the data
are listed here:
1. The eddy current signal trace should be smooth with no
abrupt discontinuities or excessive noise or wandering.
2. The wall thickness data is usually fairly constant in an
axial direction.
3. Some sign of both gap and wall thickness changes should
be seen in the eddy current signal trace since it is
highly unlikely that gap and wall thickness would vary in
exactly opposing manners so as to cause the eddy current
signal to remain constant.
4. The gap profile should be fairly smooth.
5. The gap near the rolled joints at both ends of an axial
scan should be 8 mm.