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NuclearPlantJournal.com Nuclear Plant Journal, May-June 2015
a missile shield placed across the cavity
on the other end. The missile shield
was outfitted with scaffolding handrails
for fall protection as well as a jib crane
with an electric hoist. Additionally,
two electric hoists were installed on the
refuel crane existing hoist rail. This setup
allowed four, of the six, clevis inserts to
be worked from the refuel machine with
the remaining two clevis inserts capable
of being worked from the missile shield.
AREVA
engineers
designed
innovative tool staging platforms that
were mounted on the refuel bridge rails
and overhung the cavity so that wet tools
could be handled in a controlled manner.
Tools were lifted using the electric
hoists to and from the X-Y-Z positioning
modules and the staging platforms. This
allowed for the personnel work platforms
to remain dry and reduced the potential
for personnel contaminations.
Individual tool end effectors were
delivered from the tool staging platforms
to the X-Y-Z positioning modules using
the appropriate hoist. The various tool
end effectors included:
EDM Tool - performed the
machining operations for removal of
the locking bar, machining counterbore
detent features and provided any features
on the bolt shank that may have been
needed for removing a damaged bolt or
shank remnant. EDM Filtration Skid –
provided filtration of the EDM swarf.
The skid remained submerged during
operation and featured a lid design which
allowed for remote opening and closing
to facilitate filter change-outs.
Bolt Removal Tool - used to de-
torque the existing bolts. This heavy duty
impact driver was capable of exerting
tremendous force to remove the original
hardware.
Bolt Installation Tool - used for
turning the new bolts into the threaded
hole in the clevis inserts and performed
crimping of the lock cups.
EDM Threading Tool –This tool
had capability to machine threads into a
larger hole in the event that the existing
threads had been damaged beyond repair.
Brushing Tool – this tool delivered a
rotating nylon brush that was driven into
the threaded hole to clean any debris.
All tooling end effectors were operated
remotely via the operator station. The
operator station featured a touch screen
human machine interface (HMI) control
panel which could call up the required
routines for each tool as well as control
tool positioning. The controller also
managed all pneumatic and hydraulic
functions.
A segregated storage canister was
used to store each bolt removed from the
clevis insert such that it could be positively
identified and retrieved independently
of other removed bolts. This approach
allowed bolt remnant traceability to aid
in any future destructive examination
testing needed to determine the cause of
the bolt degradation.
The following general steps were
followed for a standard bolt replacement:
Video alignment of the EDM tool to
the bolt location
EDM the locking bar
Removal of the locking bar remnant
EDM a feature on remaining cap
screw to engage removal tooling, if
required
Remove remaining cap screw
Chase the existing threaded hole
Video alignment to the threaded hole
EDM counter-bore for crimp cup
locking feature
Install replacement bolt with precise
torque
Crimp locking cup
Productivity/Efficiency
Productivity was achieved through
the use of automated underwater repair
tooling operated by a crew of trained
workers. In order to familiarize the
repair team with the complex tooling
choreography and first-of-a-kind control
system, the repair process was tested
repeatedly on a partial, full-scale mockup
of the reactor vessel at full depth. A
full scale mockup was prepared which
included all six clevises with inserts
and simulated BMI nozzles. Critical
to the success of the project was full
involvement of the key stakeholders in
pre-deployment mockup training. The
training included repair craft personnel,
ALARA engineers, and AEP radiation
protection leads. AEP containment
coordinators, construction oversight
and project engineers. The extra focus
allowed all of the repair details and
potential issues to be identified in a
controlled environment.
The project team evaluated a
multitude of scenarios to determine the
optimum operation cycle times, number
of work platforms, and number of tool
station support frame configurations for
performing the work onsite. The team
concluded that two work platforms and a
six-station tool support frame option was
best, considering both schedule efficiency
planning and risk management.
Transferability
The EDM tools for bolt removal
and threading used for this repair can be
used for any repair on a plant requiring a
threaded attachment point. Although this
is a first-of-a-kind failure at DC Cook,
other plants may experience similar
issues and may need to execute a similar
repair. Many plants are implementing
more rigorous reactor vessel internals
inspections through license renewal
commitments, such as MRP-227-A,
Materials
Reliability
Program:
Pressurized Water Reactor Internals
Inspection and Evaluation Guidelines
,
which may lead to discovery of more
conditions. Lessons learned taken from
this project for FOAK evolutions and
bolt replacement will add to the value of
future bolt and reactor vessel internals
replacements.
Contact: David Dolby, American
Electric Power, telephone: (269) 697-
5112, email:
LRSS Clevis...
