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NuclearPlantJournal.com Nuclear Plant Journal, May-June 2016
Containment
Vessel...
Trent Henline
Trent Henline is the Manager, Site
Projects at FirstEnergy Nuclear
Operating
Company’s Davis-
Besse Nuclear
Power Station
in Oak Harbor,
OH. His team of
project managers
is responsible
for planning,
scheduling, and
executing all
projects that are
implemented on
site. Trent has
a Mechanical
Engineering degree
from the University
of Toledo and
holds a Project
Management Professional (PMP
®
)
certification.
requires experience by the operator.
2. The desired excavation is best
formed by drilling four 6-in.-
diameter (nominal) sub-cores in a
2 sub-core X 2 sub-core grid. The
sub-cores would overlap slightly
leaving interstitial “fins” of concrete.
Interstitial fins can be removed
through combinations of drilling and
impacting the concrete.
3. Practically, reinforcing bars could
only be effectively cut with a wet
coring process. Wet coring should be
used for the first few inches in order
to sever the top mat reinforcing bars.
This was considered acceptable near
the top of the excavation since the
likelihood of encountering liquids
at upper elevations was low. For
the lower mat of reinforcement,
it was decided to avoid cutting
any reinforcing by formulating a
procedure to remove concrete in the
clear space between bars.
4. A maintenance schedule or replacement
schedule for the vacuum attachment
during outage work would be needed.
Concrete Removal
near the Bottom of the
Excavation
Various
concrete
demolition
techniques were evaluated to assess their
applicability in performing the controlled
concrete removal required during the
final stage of exposing the containment
vessel plate. Factors that were considered
in the evaluation included the relative
remoteness of personnel from the location
of removal (i.e., working from the top of
an approximate 5-ft.-deep excavation),
the confined conditions within the
excavation, and the necessity of exposing
the containment vessel plate without
damaging the plate. The overall approach
to the evaluation involved applying various
demolition techniques directly to a pressure-
vessel quality steel plate to evaluate the
potential damage if the containment vessel
plate were accidentally exposed to those
techniques. Favorable techniques were then
tested on a laboratory specimen to assess
the practicality of removing concrete at the
bottom of the excavation.
Different regions of the plate were
subjected to direct application of the
various techniques.
A total of 23
methods were tested.
Observations during
trials indicated needle
scaling and abrasive
blasting
imparted
minimal
potential
damage to steel and
provided control over
the rate of concrete
removal. Therefore,
these methods were
found to be favorable.
Needle scaling
involves use of a
needle gun, which is
a tool that has a set
of very fine chisels
(termed needles) that are forced against
the surface to be tooled. Needle scalers are
typically used to remove corrosion product,
mill scale, and paint from steel. Abrasive
blasting involves the use of compressed
air to project hard media against the tooled
surface.
Needle scaling was attempted at the
base of a hole in the test block. Concrete
was removed at a relatively slow, steady
rate. Resulting debris accumulated at
the bottom of the hole and was readily
removed with a shop vacuum.
Abrasive blasting of the concrete at the
base of one of the holes in the test block
was attempted. Utilizing the fine and coarse
media at different locations, several inches
of concrete could be removed within a
few minutes. However, a large volume of
abrasive media, small chunks of concrete,
and large amounts of dust were emitted into
the air which could not be tolerated within
the containment.
Due to better control of dust and debris,
needle scaling was chosen for concrete
demolition at the final stages of excavation.
Procedure Development,
Staff Qualifications, and
Training
With the necessary methods and tools
established, the laboratory specimens
were then used to develop the concrete
removal procedure. Multiple trials of each
step of what would become the procedure
were performed to identify potential
shortcomings, make appropriate refinements
to each step, and have the appropriate
personnel gain experience in the necessary
techniques. The refined steps were then
incorporated into a step-by-step procedure.
Primary and backup personnel assigned to
the project were qualified by performing the
procedure on the laboratory specimens.
Outage Work
Utilizing the procedure, the interior of
the containment vessel plate was exposed by
procedure-trained staff. Concrete removal
work occurred without incident. Ultrasonic
thickness testing of the plate was performed
and the measured thickness was found to be
acceptable. The preparation work performed
and the efficiencies in the work achieved
by procedure development on full size
laboratory specimens resulted in no damage
to the containment vessel plate and relatively
low radiation exposures to field personnel.
The procedure that was developed can be
used for similar situations at other plants.
Contact: John Vincent, CTLGroup,
5400 Old Orchard Road, Skokie, Illinois
60077; telephone: (847) 972-3242, email:
Trent Henline,
Davis-Besse Nuclear Power Station, 5501
North State Route 2, Oak Harbor, Ohio
43449; telephone: (419) 321-7824, email:
