Page 38 - Nuclear Plant Journal Digital Edition: January - February 2013
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Core
Verification
System
By David Kelly, Exelon Nuclear.
David Kelly
David Kelly has 36 plus years of
experience as a Nuclear Professional
specializing in Reactor maintenance
and refueling activities with more
than 100 refueling
outages of
experience. He is
NRC SROL license
holder for 8 years
at Byron Nuclear
Station. He is
responsible for
initial fuel receipt
and core loading
at both Byron
units through the
implementation of
Dry Cask Storage
campaigns at
both Byron and
Braidwood units.
Nuclear Energy Institute’s Top Industry
Practice (TIP) Awards highlight the
nuclear industry’s most innovative
techniques and ideas.
This was a 2012 NEI Process Award
Winner.
The team members who participated
included: David Kelly, Exelon Outage
Services Sr. Manager; Nathan Neal,
Exelon Site Reactor Services Manager;
Martin Wolfe, Exelon Site Reactor
Services Manager; John Bramblet,
Newton Labs President and CEO; Barry
O’Brien, Newton Labs Director of
Software.
.
Summary
Following completion of core reloads
at pressurized water reactors (PWR), an
inspection is performed to verify that all
the fuel is in proper alignment, ensuring
there are no problems encountered when
installing the Upper Internals or Reactor
Plenum. Previously this inspection has
been performed using binoculars or
underwater cameras. While this binocular
inspection or camera examination is most
often accurate enough to avoid incident,
misalignment has resulted in stuck or
damaged fuel assemblies and prolonged
outages of up to 3 months. Within the
265 PWRs worldwide, 10 documented
events involving damaged or stuck fuel
assemblies have occurred, including four
in the last two years.
After
two
years of work, we
have developed a
revolutionarysystem
that will change and
improve the way
nuclear operators
ensure that the fuel
assemblies will not
be damaged when
with the upper
internals/plenum
are lowered after
a refueling. This
system, known as
the NM200E Core
Verification System, uses a camera
head deployed from the refueling mast
coupled with proprietary computer
vision algorithms to accurately measure
the positions of all fuel assemblies
and compare them with ideal positions
(supplied by the reactor operators). The
result is detailed, accurate measurements
of all S-Hole locations, allowing reactor
personnel to determine if there will be
any issues with the installation of the
upper internals.
Locations of S-holes, not gaps, are
the critical measurements when it comes
to installation of the upper internals. The
NM200Emarks a significant improvement
over traditional gap measurement based
mapping techniques. Its software enables
the precise global mapping of fuel
assembly S-hole positions, including any
degree of misalignment or top nozzle
rotation. The legacy video method merely
gauges the relative nozzle gap variations,
limiting the operators to inferring the
actual positions of the S-holes.
There were three significant
challenges to overcome when developing
this technology. The first is the difficulty
in producing precise measurements
underwater. What is routine practice in air
becomes a major engineering challenge
underwater. Second, and especially true
for PWRs, are the thermals coming off
the hot core. Viewing these thermals with
a camera creates blurred and distorted
images that look similar to a mirage in
the desert, again making measurement
difficult. Finally, the high radiation dose
field of the nuclear core can burn out most
camera systems. Designing a camera
system that can operate over sustained
periods of time in high radiation would
be critical.
Exelon Nuclear and its technology
partner, Newton Labs, set out to overcome
these challenges. The resultant NM200E
Core Verification System uses advanced
computer vision algorithms to accurately
measure S-Hole position to +/- 0.050”, an
unheard of underwater tolerance prior to
this development.
To test the system, a PWR fuel core
quadrant was accurately mocked up at
Newton Labs and used extensively in
development. After the prototype system
was completed, the team moved to the
Byron Fuel Pool to test the device in a
realistic environment using spent fuel
assemblies. This effort was enlightening
and provided data on two environmental
characteristics that had yet to be
incorporated into the prototype system.
First, the thermal distortion that the spent
fuel bundles produced did blur the images
significantly, and second, the dark oxide
build-up on the top nozzles made s-hole
detection difficult, highlighting the need
for increased lighting.
The next major engineering
challenge in the project was overcoming
the image distortion caused by the heat
energy produced by the fuel bundles,
known as thermals. To account for this
thermal distortion, the team developed
improved software that successfully
mitigates this distortion and allows for
accurate measurements in its presence.
38
Nuclear Plant Journal, January-February 2013
