Containment
Vessel Plate
Evaluation
By John F. Vincent, CTLGroup and
Trent A. Henline, Davis-Besse Nuclear
Power Station.
John Vincent
John Vincent is a principal engineer
with CTLGroup in Skokie, IL. He is in
charge of complex
projects involving
analysis of strength
and serviceability
deficiencies in a
variety of structures,
and design of
innovative repairs to
enhance structural
performance
and long-term
durability. John is a
licensed structural
engineer in the
State of Illinois
and a professional
engineer in several
states.
Evaluation Using a Special Process
Introduction
As part of the license renewal pro-
cess, FirstEnergy committed to the NRC
that it would verify that the containment
vessel had not degraded in interior re-
gions of the containment due to refueling
canal leakage at its Davis-Besse plant.
Confirmation of the material condition
of the interior of the containment vessel
plate would require excavation of nearly
five feet of reinforced concrete to expose
the plate at a critical low elevation where
water could accumulate for examination
and ultrasonic thickness testing. Further
complicating
the
interior effort was
that the area to be
investigated was in
a hallway adjacent
to the area under
the reactor vessel,
which made it a high
radiological
dose
rate region and a
congested location.
Due to the poten-
tial for irreversible
damage to occur to
the containment ves-
sel plate during the
concrete excavation
phase, and the need
to perform work efficiently in a high dose
area, the process to expose the contain-
ment vessel plate at the interior would be
treated as a Special Process per 10CFR50
Appendix B, " Quality Assurance Crite-
ria for Nuclear Power Plants and Fuel Re-
processing Plants" Criterion IX. There-
fore, procedures were developed off site
to efficiently access the containment ves-
sel plate at the interior of the containment
without damage to the plate.
Containment Concrete
Removal
Concrete removal methods needed to
be studied and/or developed to accomplish
exposure of the containment vessel plate
for visual examination and ultrasonic
thickness testing. Requirements of the
concrete removal methods included the
following:
1. Concrete over the containment vessel
plate is approximately 4 ft-6 in. thick
where concrete removal was to occur
and reinforced with a layer of No. 11
reinforcing bars spaced at 6 in. on
center in each direction at the bottom
and a layer of No. 11 reinforcing bars
spaced at 12 in. on center in each
direction at the top. For each layer
of reinforcement, a maximum of one
bar in each direction could be cut
during concrete removal.
2. The plan dimensions of the concrete
excavation could not exceed 12 in. X
12 in.
3. The location of the work was to
occur within and at the end of a
hallway that was bulk headed with
lead shielding.
4. The concrete removal method
would need to include evaluation
of the possibility of encountering
undocumented embedded items over
the depth of the excavation.
5. Damage to the containment vessel
plate caused by any task would not
be tolerated.
6. Concrete removal would need to
be performed with a dry method to
avoid contaminating liquids that may
be encountered as the removal work
proceeded so that unaltered samples
of the liquid could be collected for
laboratory analysis.
In order to address project
requirements,
full-size
laboratory
mockups of a portion of the containment
were constructed and appropriate team
members were retained to research
equipment, methods, and procedures,
and to develop the Special Process as
preparation for the outage work.
Preliminary Trials
Initially, a simple concrete block on
steel plate was constructed to research
nondestructive test (NDT) methods
and their accuracy in evaluating for
the presence of embedded items. The
block was constructed with a bottom
layer of reinforcement and 12-in.-
diameter cylindrical blockouts of various
lengths placed within the formwork.
The cylindrical holes also presented
the smallest and, therefore, the most
challenging plan areas to perform the
NDT required to evaluate the presence
of embedments. Trials with three
types of NDT revealed that ground
48
NuclearPlantJournal.com Nuclear Plant Journal, May-June 2016
