FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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6. The axial gap profile should be similar when computed
using either end of the pressure tube as a reference.
Note that if the gap at both ends of the pressure tube
are not equal to 8 mm, repeating the calculations using
the other end as a reference will not just shift the gap
profile by the difference. The shape of the profile will
also change due to the non-linearity in the data base.
The accuracy of the gap measurements is primarily determined by the effect of
the ±0.05 mm uncertainty in the wall thickness measurements at both the
reference and measurement points. Referring to Figure 3, the uncertainty in
the location of the reference point, which must be located along the line of
constant 8 mm gap for axial scans, can also be expressed in terms of the eddy
current signal voltage as approximately ± 0.08V. Doing so allows the
uncertainty to be transferred to the measurement point and to consider the
reference point fixed. The additional uncertainty in the measurement point
due to its associated wall thickness measurement can also be expressed in
terms of the eddy current signal, but the exact conversion factor will depend
on the size of the gap. The wall thickness measurements can be assumed to be
indépendant and normally distributed and so both of the uncertainties in the
measurement point may be combined as the square root of the sum of the
squares. The result will vary from a high of +0.11V for a gap of 8 mm to a
low of +0.083V for a gap of 0 mm. An error bound of ±(30% + 0.1 mm) on
computed gap values approximates this overall uncertainty and is the magnitude
of the error bars that have been placed around the sample results in Figure
10.
S. AKEAS ft» DEVELOPMENT
The field experience in Pickering has served to solidify the gap measurement
procedure and also to uncover various weak points in the equipment which
effect the results. Further development is being done using the CIGAR
inspection system with the emphasis on Zr-2.5%Nb pressure tubes now that the
Zircaloy-II tubes in Pickering Units 1 and 2 are to be replaced.
The use of the CIGAR inspection system addresses a number of equipment
reliability problems encountered in Pickering which made it difficult to
isolate and solve problems originating with the measurement technique. The
most important advancement is that both the eddy current and wall thickness
data are collected automatically during the same axial scan. This ensures the
proper alignment of both probes and enables a large amount of data to be
collected which can then be smoothed, reducing any unbiased error in the
individual measurements. Another improvement is the gentler and largely
automatic handling of the signal cables and their connections which has
resulted thus far in stable and reliable eddy current signals. Very promising
and repeatable results have been obtained to date but more testing is still
required. Some development work still remains to be done in the area of
pressure tube resistivity variations and in the quality of the eddy current
probe, both in terms of its temperature sensitivity and its consistency in
manufacture.