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NuclearPlantJournal.com Nuclear Plant Journal, May-June 2014
reactor vessel mockup as well as in the real
reactor vessel at the Ulchine nuclear power
plant in Korea. Prior to the underwater
experiments, we performed the experiment
in air to confirm our laser guidance
control method. Positioning accuracy
is examined, including the rotation and
inclination accuracy of the underwater
mobile robot, scan path accuracy of the
manipulator, angle measurement accuracy
of the pan-tilt unit of the laser pointer, and
the overall positioning accuracy of this
system. The above results of the functional
tests indicate that the prototype inspection
system satisfied most of the acceptance
criteria.
In order to determine whether the weld
has defects or not, we also have conducted
the ultrasonic testing. After emitting an
ultrasonic wave to the suspected welds,
we monitored its reflected signal. Usually
a reactor pressure vessel is manufactured
by welding several parts together. The
welds to be inspected in the vessel are
largely classified as nozzle welds and
circumferential welds. Circumferential
welds include the welds of the flange to
upper shell, the upper shell to the middle
shell, the middle shell to the lower shell,
and the lower shell to the bottom head,
while the nozzle welds include the welds
of the nozzle to the middle shell, and the
nozzle to the nozzle pipe so called the safe
end. Besides, a flange ligament should be
inspected.
When inspecting each weld, we
have to use various incident angles of the
ultrasonic wave for a more accurate and
stricter inspection. For example, in the
case of a reactor shell welds inspection,
we used incident angles of 0, 45, 60, 50/70
degrees, respectively. In addition, for each
incident angle, we have to scan the welds
in four directions: upward, downward,
and in a clockwise and counter clockwise
direction by using ultrasonic probes with
a specified incident angle. Thus we have
to inspect the welds seventy seven times in
total.
A Korean standard reactor vessel has
six nozzles, thus the number of nozzle
inspections becomes sixty, and the number
of circumferential weld inspections
becomes sixteen. The probe assembly
should be cleverly designed to contact
the probe plate to the weld surfaces with
a suitable compliance by tilting adjusters
and springs.
Summary
In order to improve the reactor ves-
sel inspection system, we have developed
an innovative robotic inspection system
based on underwater mobile robot. We
completed the laser induced control of the
mobile robot, and the method is thought to
be applicable to other industries. Ultrason-
ic testing for Ulchine 6 reactor vessel was
successfully completed at Doosan Heavy
Industries and Construction Co. When our
system is used for a reactor vessel inspec-
tion instead of the conventional machines,
a lot of benefits are expected to be the re-
sult, such as a critical path process reduc-
tion, handling safety improvement, exami-
nation reliability and positioning accuracy,
and so on.
Contact: Jae-Hee Kim, Korea Atomic
Energy Research Institute, 989-111
Daedeok-daero, Yuseong-gu, Daejeon,
305-353, Korea; telephone: 82-42-868-
2085, fax: +82-42-862-7269, email:
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