By Keith Eberhardt, Structural Preservation Systems
Built by Baltimore Gas & Electric (BG&E) in the 1970s on 2,300 acres of prime waterfront property, the Calvert Cliffs Nuclear Power Plant (CCNPP) became the first plant in the U.S. to earn a 20-year extension of its operating licenses from the Nuclear Regulatory Commission (NRC). While this March 2000 milestone is significant, the reactors, built in 1975 and 1977, were beginning to show their age.
CCNPP has two nuclear reactors, with each generating plant producing approximately 850 MWe net or 900 MWe gross. The company replaced the steam generators in Units 1 and 2, and when deterioration of steel on the intake floor, circular water pump bowels (CWB) and salt water pits (SWP) became severe, CCNPP sought a repair solution that would help prevent corrosion and last for a period of approximately 20 years to reduce plant maintenance costs.
Demolition Begins
In 2005, an independent engineering firm performed initial inspections of the CWB, SWP and the main floor. A percentage of each area was sounded to determine the amount of deterioration. The survey area was then extrapolated to determine the estimated amount of distressed concrete for the entire project.
Several factors contributed to the deterioration of the concrete. First, there was constant exposure to saltwater, which had inundated the system for three decades. Second, several pieces of large electrical equipment grounded to the slab were sending stray currents through the concrete. (These stray currents were small enough to be overcome by the intentional polarization that occurs through the impressed current the repair contractor imposed.) Third was the age of the structure itself.
The best repair procedure for the main floor was to remove the existing concrete at an average of 6 inches to eliminate all delaminations as well as the concrete that had a high calcium content. The scope of work on the $3.25 million project encompassed the intake floor, CWB and SWP. Structural Preservations Systems was hired to execute the repairs.
The team selected an impressed current cathodic protection (ICCP) system to help prevent corrosion of the new and existing reinforcing steel in the floor. With impressed current cathodic protection systems, a small direct current is passed from a permanent anode to the reinforcing steel. An external power supply is connected between the anode and the steel with the appropriate polarity and voltage to prevent the reinforcing steel from giving up electrons. This system repels chloride ions away from the reinforcing steel toward the installed anode and provides flexibility since the current or output can be easily adjusted.
The project began with the demolition process. Hydro-demolition was used to remove the majority of the concrete and all hand-chipping was performed under heavily wetted conditions to minimize any dust. All debris was vacuumed out of the structure with state-of-the-art equipment. The water was then pumped from the debris and neutralized to a pH level below 9.0 before being pumped back through the plant water treatment facility for final adjusting.
For the 8,800-square-foot intake floor, an average of 6.5 inches of concrete was removed, a cathodic protection ribbon was installed to prevent continued steel deterioration and a new concrete floor was poured to replace the old floor. For the CWBs and SWPs, concrete was removed to 1 inch under the reinforcing steel, a cathodic protection ribbon was installed and the concrete was replaced. In areas where concrete was removed, the ribbon was installed at 12 inches on center and 1 inch above the reinforcing steel. Then the patches could be formed and repaired with new concrete.
In areas where the concrete was sound, the ribbon still had to be embedded into the concrete. To allow for proper ribbon placement, 1-inch-by-2-inch slots were cut into the concrete. These slots then had grout placed over them. Approximately 3.5 miles of saw-cutting was required to install the ribbon in the concrete surfaces.
Placing the overlay on the intake structure floor presented other challenges. The ICCP system is 1 inch above the reinforcing steel and cannot touch the steel, which would result in a short of the system, rendering it useless. To overcome this challenge, the entire floor was decked with 2x10s. As placement and finishing operations progressed, boards were pulled up and removed from the work area. Additionally, the location of the ready-mix truck access in relation to the placement point exceeded 400 feet. This required a concrete mix design that utilized ice, retarder and plasticizer to ensure all placed concrete met strict NRC codes. This also provided the workers good workability and placement durations.
The work schedule was also a major challenge as it was based on hours not days, and all work was performed around trip-sensitive equipment that is designed to “trip” or shut down the reactors in the event of an emergency. Crews had to ensure that there was forward-thinking with each scheduled activity since all other groups and plant resources worked within the same schedule. Any delay to the repair contractor’s activities could impact these groups’ planned projects and maintenance activities.
QV Hold Points
Because the work was done in a nuclear power plant, there were several quality verification (QV) hold points, which require a third-party inspection and approval of a task prior to moving forward to the next task. QV hold points were used after demolition was completed to inspect the condition of the existing steel. During this hold point, steel that had lost cross-sectional area was inspected to verify if it had to be replaced. A hold point was also required to inspect the replaced steel. Location of bar-locks, verification that all fasteners were fully engaged in the reinforcing steel, as well as identification of all deteriorated steel was determined prior to placing the cathodic protection anode. Correct edge distances for all base plates also required a hold point. The edge distances were engineered to ensure that the anchor bolts and base plates met all seismic activity as designed for each system.
All grout and bar-locks used for the project required testing at an independent laboratory to ensure that all in-place material met or exceeded physical data as shown on the technical data sheets. Grout was not placed until this data was returned for inspection and approval by the plant engineers. All of the rebar was checked at every node (intersecting point) for continuity with the designed acceptance criteria of 0.9 mV or below. However, the repair contractor typically provided remediate action for anything greater than 0.02 mV potential.
Before, during and after the concrete pours, the anode system ground to reference cell ground, reference cell to reference cell ground, and anode to system ground were tested for static potential via a multi-meter (mV Static) and for resistance (Ohms AC) via a soil resistance meter. The compressive strengths of the cast-in-place concrete were tested in accordance with American Concrete Institute (ACI) guidelines. All grout material used in the slots and bar-locks was batch-tested at an independent testing laboratory to ensure that the grout and bar-locks met all physical properties as listed on the data sheets.
After the concrete pours, the cathodic protection system was also verified. A small current was impressed between the anode and rebar via a lantern battery to determine whether the system was polarizing, by monitoring the reaction of the reference cell to rebar leads. In all cases, the anode to rebar systems showed the ability to polarize, which proved the system was free of shorts.
Safety and Efficiency
Seventy-five percent of the zones and sub-zones achieved full protection within 30 minutes of activating the permanent impressed current. Typically, this level is reached in about two weeks. The fact that this system achieved this level so quickly demonstrates a high level of consistency with regard to anode and cathode placement during installation.
All work had to be performed while the plant was fully operational. Foreign particles such as dust and debris had to be contained so they would not get into the motors, bearings, windings and other machines. In addition, the crew needed to be extremely careful while they were working in the vicinity of operating plant equipment.
To achieve the required amount of protection, a number of systems were used. A protection net was installed to a height of 8 feet to prevent water and debris from entering equipment and panels. In addition, water-tight plastic walls were draped around the entire perimeter of each phase to prevent water and slurry from acting as foreign materials to panels, drains and sumps. At all motor casing openings, frames were built to install filtering media to trap dust so it did not get into the motor windings.
Crews often worked in very small spaces around some of the plant equipment. The hydro-demolition contractor designed and built a custom concrete removal machine to expedite the work. The machine was a three-wheeled hydraulically powered unit that could fit into areas as narrow as 28 inches. The unit was controlled by an umbilical cable connected to the main control panel so the operators could maintain a safe distance from the machine and plant equipment. The hydraulic pumps were operated on a 208v, three-phase power pack.
The hydraulic oil was food-grade and biodegradable, an important feature because of the close proximity to the Chesapeake Bay.
And, as with any project, safety was extremely important. Increased controls were incorporated into the crews’ daily activities. The team used a combination of nuclear power plant briefs and internal Job Safety Analysis (JSA) forms, as well as the concepts of “Questioning Attitude” and “STAR” (Stop, Think, Act, Review). Crews were consistently briefed with job condition updates.
The first phase of the repair project, which represented approximately 15 percent of the overall scope, took approximately five months. The remaining 85 percent of the work, which was split into five phases, was completed in eight months, which shows an increase in project efficiency. One of the reasons for this continual improvement was the concept of facilitating a “lessons learned” meeting. Key to the successful project was a partnering attitude with all team members and the owner.
Author: Keith Eberhardt, Structural Preservation Systems branch manager, has 26 years of restoration experience. Eberhardt has a B.S. in civil engineering from the Illinois Institute of Technology as well as an M.S. coursework completed, focused on bond strengths of overlays and surface preparation. A member of the American Concrete Institute and the International Concrete Repair Institute, he holds several patents and is the author of numerous trade articles.
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