FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

Each firing of the capacitor banks causes localized heating in the tube
adjacent to the coil. The rise in temperature changes the material
resistivity to the point where meaningful eddy current readings are
unobtainable. Natural cooling through conduction and convection takes up
to thirty minutes. This is an intolerable delay so cooling manifolds
were fabricated that provided a cooling air blast through several jets on
the effected area. This, in conjuction with working on several adjacent
tubes in parallel, reduced down time to an insignificant amount.
Unfortunately there are always mysterious signals that must be explained
away. Reactivity mechanisms run vertically through the reactor adjacent
to the calandria tubes. These produce a signal easily misinterpreted by
the inexperienced as they are the mirror image of a garter spring
indication. A spring adjacent to a mechanism produces signals that
cancel each other out and results in undetectable springs. For this
situation and where it was expected that two springs were nestled
together, radiographie equipment had to be always ready.
A general sequence of events established for repositioning in any tube is
as follows:
(1) Perform eddy current inspection and locate all four springs.
(2) Feed spring locations into a computer which would suggest
various options with respect to which springs to move and by how
much.
(3) Select a spring to move, place capacitor discharge coil and fire.
(4) Cool tube and eddy current inspect to determine motion.
The above sequence or slight variations on the above would be
repeated until the tube was accepted by the computer.
One of the alternate systems for moving a spring involved flexing the
pressure tube repeatedly with a hydraulic jack located in the center of a
2 1/2 meter long rigid beam. An eddy current coil was built into the
beam adjacent to the jack and it was possible to monitor spring movement
in real time. It turned out to be self defeating as the coil diameter
interfered with the flexing of the tube and only modest movement was
achieved. The probe was eventually mounted on the end of the beam which
allowed progress readings to be taken by partially withdrawing the jack
assembly.
Another system that was developed by Ontario Hydro research utilized a
large sonic vibrator that generated high intensity sound over a wide
range of frequencies. The vibrator was located near the end of the
selected pressure tube. An expanding head was fed into the prassure tube
and clamped near the spring to be moved. An aluminum tube coupled the
vibrator to the head. Various combinations of frequency and intensity
were selected until the real time eddy current system indicated spring