Page 31 - Nuclear Plant Journal - March April 2013
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Nuclear Plant Journal, March-April 2013
31
like the all important cooling pipes that
circulate water through the reactor core.
In our recent laboratory study, we
tested these two monitoring methods on a
fatigued stainless steel pipe. The acoustic
emission monitoring detected signals
caused by the formation of a crack before
we visually confirmed that tiny fissure.
After we knew the crack was there,
we monitored it with the guided-wave
technique. When the waves encountered
a crack, they bounced back to the sensor;
by monitoring those received signals
we were able to follow the growth of
the defect from a starter notch that was
2.45 mm deep and 47.7 mm long to a
fissure that measured 68 mm long. This
may not seem like dramatic growth, but
such a crack would be a serious cause for
concern at an operating nuclear power
plant.
Guided-wave technology, which is
rapidlymaturing, is now regularly used for
pipe testing in the oil and gas industry. In
thenuclearindustry,regulatorsareworking
to standardize the monitoring procedures.
To use the technology inside an active
plant, however, operators must overcome
challenges like high temperatures—it can
hit 200 °C inside a light-water reactor’s
primary piping. That’s far too hot for
the most common type of transducers,
which use piezoelectric materials to
convert electricity into ultrasonic waves
in the transmitters (and vice versa in the
receivers). To get around this problem,
some researchers are testing more
rugged piezoelectric materials. Others
are experimenting with different ways
to generate the waves—for example,
a laser pulse that heats and expands a
pipe’s surface to create waves that ripple
outward.
Two other ultrasonic techniques
show potential for long-term deployment.
A kind of phased array, which is
commonly used as a diagnostic tool in
medicine, uses a grid of elements to
generate many small ultrasonic pulses.
By using electronics to control the timing
and interaction of the individual pulses,
operators can create a single wave front
and control the direction of the wave.
Phased-array technology is now routinely
used in periodic inspections of nuclear
power plants, but the technology has
the potential for continuous monitoring,
where a single transducer is fixed in place
and electronic beam steering is used to
scan critical structures. This technique
can check for degradation in coarse-grain
materials like cast stainless steels and can
also look for flaws in welded areas.
Finally, an approach drawn from
seismology could be useful to monitor
the formidable concrete structures in a
nuclear power station. In this “diffuse
field” technique, an ultrasonic pulse is
introduced into a coarse-grained material
such as rock, concrete, or cast stainless
steel. As the ultrasonic wave propagates
through the substance, the grains
interfere with the initial pulse of energy
and send echoes back to the transducer.
The resulting signal, showing all the
interactions from within the textured
material, provides a distinct signature
for that material. This signature changes
if the substance’s elastic properties
vary or if a crack or other degradation
is introduced. So far, diffuse ultrasonic
tools are being used only for research in
the nuclear industry, but their potential
for inspections and long-term monitoring
has been clearly demonstrated.
Copyright 2012 IEEE.
