Page 39 - Nuclear Plant Journal Digital Edition: January - February 2013
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Nuclear Plant Journal, January-February 2013
39
Since the location of thermals in a reactor
core depends on the placement of old
and new fuel bundles, it was necessary
to develop software algorithms that could
compensate for every condition; from
virtually no thermals to situations where
the entire image was distorted.
Complicating the development of
thermal mitigation algorithms was the
issue of lighting. Since old fuel bundles
are virtually black compared to the shiny
new fuel bundles, the lighting would need
to be significantly increased to get the
fine details of the fuel assembly. This was
especially true of the old fuel bundles, as
their dark oxide layer build up made them
virtually black to the camera under normal
lighting conditions. This resulted in the
development of a very high intensity LED
light array. However, since the light would
then produce extreme reflections on the
shiny new fuel bundles, the algorithms
were improved to operate in such a high
contrast lighting environment.
Thefinalmajor engineering challenge
of the project rested in designing
equipment capable of withstanding not
only the underwater condition and the
heat present in the water, but also enduring
a relatively long life in a high radiation
environment. The trade-off engineers
faced was determining the distance to
place the camera from the top nozzles.
The closer to the fuel the camera gets, the
higher the radiation levels get but the less
thermal distortion is present. The opposite
is true as well; place the camera farther
from the fuel and the radiation levels are
lower, but the thermal activity is higher.
Because the thermal mitigation
algorithms proved to be so effective, the
camera head was then able to remain at
a relatively far distance above the fuel
assembles (~5 feet). This meant that the
camera head would be exposed to smaller
levels of radiation, prolonging the life of
the system by orders of magnitude. In
addition, since the camera head did not
have to be designed for extremely high
levels of radiation, its production cost
was dramatically reduced.
Remarkably, during testing the
prototype unit was placed within a foot
of the spent fuel assemblies in the reactor
core in a very high radiation field and still
produced clear images without damage to
the equipment. The device created such
superior images even under those extreme
radiation fields that the camera head was
used to read some serial numbers during
one of the deployments after lowering it
close enough to the fuel assemblies.
Both the user interface and system
output have been designed to provide
simple operation and easily interpreted
results to the refuelling crew and reactor
engineering. A live image overlaid with
the computer vision results is displayed,
along with a color-coded map of the core
to easily identify areas of concern.
Before performing a mapping,
actual plant design data is input into the
system. This includes fuel assembly and
baffle wall shapes, along with ideal gap
specifications and the planned reload
pattern. While the system is currently
configured for Westinghouse 4 loop
PWRs, it has the potential to be modified
for use at any PWR. Currently Exelon is
in the process of updating the system to
be used at TMI. The necessary test data
was acquired at TMI during their outage
in November of 2011.
The system has been put to the test
during both the 2010 and 2011 outages
at Exelon’s Byron and Braidwood
Nuclear Stations. The NM200E was
able to accurately measure the location
of the fuel assemblies and deliver that
information to both the refuelling crew
and the outage/engineering personnel,
as well as permanently store the data
for later review. The system’s ability to
overcome the known issues of thermals,
the differential between old and new
bundles was outstanding. Since the correct
placement of fuel assemblies is a constant
concern for the industry, this extremely
accurate system is a welcome addition
to the current tools for determining fuel
assembly placement.
Safety Response
The most significant safety feature
of this tool is that it ensures the fuel
assemblies are placed properly in the
reactor core. If the fuel assemblies are
not properly positioned in the core, fuel
damage may occur. This has recently
happened at one US facility as well as
multiple times by EDF in France in
the last three years. While the existing
methods used at Exelon since 1994 have
been effective in ensuring the gaps and
misalignments in the fuel are acceptable,
they rely on visual inspections only and
do not determine the actual placement
of the fuel in comparison to design
locations. Due to the recent issues in
the industry, several utilities are now
requiring that actual fuel gap checks
are performed. While this process is
much more accurate and thorough than
simple visual checks, it still relies on
an individual manually obtaining the
data. The NM200E system performs
these measurements automatically and
much more efficiently. This tool helps
to prevent a much more significant issue
from occurring, such as fuel or reactor
component damage. Recovery from
such damage is complicated, risky, dose
intensive and very time consuming. The
dose savings from using this tool on
an ongoing basis is not significant, but
can still save approximately 50 mrem
per outage. The savings from a major
recovery could be several rem.
Transferability
The existing tool is configured for
Byron and Braidwood cores but the
software is currently being reconfigured
for use at TMI. It can be configured
for any PWR core shape and size,
and eventually will support any PWR
fuel type. A variation of this tool (3D
underwater laser scanner) has also
been used to gather as built dimensions
for performing modifications in BWR
reactors. This will help to reduce outage
extensions by gathering more accurate
as-built dimensions.
Contact: David Kelly, Exelon
Nuclear, 4300Winfield Road, Warrenville,
Illinois 60555; telephone: (630) 657-
