FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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WET CHANNEL MEASUREMENT OF PRESSURE TUBE TO CALANDRIA TUBE
SPACING IN CANDU REACTORS
J.tf. Sado
Ontario Hydn.0
ToA.on.to, Ontario
ABSTRACT
The pressure tube (PT) to calandria tube (CT) spacing in CANDU reactors is an
important parameter that relates to the general condition of the fuel
channels. The measurement system that was developed to measure this parameter
during the wet channel inspections of Pickering Units 1 and 2 is described in
this paper. A send-receive eddy current probe was designed which is
primarily sensitive to variations in PT/CT spacing but is also affected by
pressure tube wall thickness. A computer simulation showed that the phase
angles of the response to these variables are similar for all usable
frequencies, thus eliminating the possibility of mul tifrequency compensation.
A marriage of technologies was proposed involving the ultrasonic measurement
of wall thickness values which are then used to extract the spacing
information from the eddy current signal. The accuracy of the system is
aproximately ±(30% +.lmm) which has been sufficient to determine if and where
any of the pressure tubes have come in contact with their calandria tube.
Field experience with the new system is discussed and areas for development
are also outlined.
1. INTRODUCTION
Each fuel channel in commercial size CANDU Reactors is installed horizontally
and contains an unsupported center span approximately 6 meters long
consisting of a pressure tube surrounded by a dry gas and enclosed by
calandria tube. These tubes are firmly centered at the rolled joint positions,
that is at the ends, but spacers (garter springs) are required to ensure
proper separation in the center span. After the reactor has been in operation
for some time, the pressure tube will exhibit sagging between the garter
springs as shown in Figure 1. However, if a garter spring is out of position
then accelerated sagging may occur resulting in premature pressure tube (PT)
to calandria tube (CT) contact.
Two techniques of measuring the PT/CT spacing (gap) have been investigated
during the development of a system for the remote inspection of pressure tubes
(CIGAR) /I/. One method processes the output of an eddy current probe during a
full 360° rotational scan in order to arrive at an estimate of the PT/CT
eccentricity. The second method utilizes both an eddy current probe and an
ultrasonic probe to produce a direct measurement of the change in gap from a
reference point.