FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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The next step in the development program was the designing and building
of a large inside differential coil close to the inside diameter of the
pressure tube. The first probe consisted of two, two hundred turn coils
separated by 15 mm. This arrangement provided a strong signal from
vertically oriented spring assemblies. However, springs that had flopped
over and lay slanted produced almost no signal at all (see Figure 3). A
probe was quickly produced that had the coils on a 20° slant. This set
up worked fine on slanted springs providing the spring was tilted in the
same direction as the coils. To pick up springs inclined in the opposite
direction or slued somewhere in between meant rotating the probe until
the maximum signal was achieved. This was far too time consuming a
procedure when it is considered that 1720 springs per reactor had to be
located and mapped.
AECL's Chalk River Laboratories had also tackled the problem of designing
a single probe that could locate springs irrespective of orientation.
Their solution was a one 25 mm. wide exitation coil with 6 mm. receive
coils on either side. This configurate gave consistently strong signals
from all springs with the exception of those within the 150 mm. long
belled portion at each end of a calandria tube. Unfortunately, many of
the springs had migrated to the bells and these had to be located as
well. As an interim measure to solve this problem, a shielded isotope
camera holder was fabricated that allowed a camera to be directly
attached to an end fitting. It was then possible to radiograph the
adjacent bell without evacuating the area. Paper radiography with
instant development speeded the process to the point where results were
available in less than five minutes. Later, Chalk River was able to
provide a probe with segmented coils that was capable of detecting
springs in the belled ends.
The next hurdle was that of accuracy. In order to have any effect on the
garter spring, the capacitor discharge coxl had to be placed with an
accuracy of + 1 mm. and this up to eight meters down a tube.
The initial scans had been performed by a probe mounted on an automatic
stem unit. The heart of the stem unit is two foil elements wrapped on
drums that form a strong lightweight tube as they are unrolled. They
extend in much the same manner as a tape measure. A wheel encoder riding
on top of the elements provides a digital readout equivalent to the axial
distance (see Figure 4).
To perform an inspection, the stem unit is placed on an end fitting and
by means of a controller the probe is driven to the far end of the tube.
The probe is retracted and as it does so, the axial distance and the eddy
current signals are recorded on a strip chart. The finished chart is
about a meter long and is scaled in proportion to the tube length. The
accuracy of the chart is approximately + 10 mm. The encoder that
provides the axial scan distance is also subject to error due to skipping
on the element, deformation of the rubber rim of the wheel and slight
variations in the wheel diameter. With a fine tuned system, accuracies
of + 3 mm. can be achieved. However, during general use, a tolerance of