FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

4. FIELD EXPERIENCE
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The gap measurement subsystem was incorporated into the CIGARette inspection
equipment along with garter spring and flaw detection capabilities for use in
Pickering Units 1 and 2/2/. Unfortunately the gap measurement function was
not one that worked right away.
4.1 Wall Thickness Measurements
Wall thickness measurements were originally to be obtained using a modified
digital thickness gauge and a highly focussed radiation resistant ultrasound
probe. This method is similar to that planned for the CIGAR inspection system
with the exception that a technician would be responsible for logging the
data from the digital display instead of a computer. Unfortunately, stable
readings could not be obtained and eventually the instrument used for the flaw
detectors, a Branson KB-6000 was utilized. Thickness measurements using this
modified system required that the probe be stationary so that the time beween
the reflection peaks of the inner and outer pressure tube walls could be
measured on a CRT display. The result was then converted to a thickness value
by multiplying by 2.389 mm/p s (the velocity of sound in Zircaloy-II divided by
2). This slow procedure limited the number of points that could be measured
in each channel and introduced a random observation error that was estimated
to be about±l% or a little more than the thickness of the screen trace.
4.2 Eddy Current-Signal Quality
Although the procedures for calculating gap outlined in section 3 indicate
that data need only be collected at the reference and measurement points, it
was soon realized that the eddy current signal would have to be recorded
during a continuous scan in order to better ascertain the signal quality.
Once this procedure was adopted, it became evident that in many cases the eddy
current signal contained excessive noise and drift which was traced to
various problems with the 80 meters of signal cabling and 8 connection points
in the CIGARette system. Finally, diligent trouble shooting and the
availability of tested spare components reduced this problem significantly,
resulting in smooth and stable signal traces for many of the channel
inspections.
4.3 Types of Scans
Two types of scans were used during the inspections, an axial scan along the
bottom of the channel to establish a gap profile and a rotational scan,
usually used at points of interest in an attempt to confirm the axial scan
results. The rotational scans were evaluated assuming an infinite gap at the
12 o'clock position but were generally not as successful as *he axial scans.
The reasons for this difficulty were never firmly established, however the
quite rapid wall thickness changes around the circumference of the pressure
tube and the relatively few data points collected required exacting
performance from the system for proper results. Axial scans were much more
successful with the added benefit that the eddy current signal trace was
usually indicative of the gap profile since very few significant axial wall
thickness variations were encountered.