Concrete Pumps and Flowable Fill Close a Chapter in U.S. Nuclear History
A South Carolina facility that was a part of the United State’s early involvement with nuclear energy is being relegated to the history books. The Savannah River Site (SRS) in Aiken, SC was conceived in the shroud of Cold War secrecy to produce nuclear weapons-grade plutonium and tritium in the early 1950s. Today, the original 198,000 acre site has been substantially returned to nature with the exception of roughly 10-percent of the property which is home to a nuclear research and development arm of the Department of Energy. Commercial grade uranium used for energy generation is produced on-site but the remains of some of the U.S. governments early nuclear reactors still exist . A two-part contract begun last June is covering up the contaminated remains of two 60-year old decommissioned reactors and an adjoining building that was used for disassembly of fuel rods and purification of reactor cooling water. Two concrete pumps are placing more than 190,000 cubic yards of site-mixed flowable fill to finalize the decommissioning by entombing the radioactive equipment and encasing the buildings in concrete.
Baker Concrete Construction, an ENR Top Specialty Contractor headquartered in Monroe, OH with twelve regional offices throughout the U.S. is undertaking the project. “We were well prepared for the project since we already had our own extensive quality program to comply with ASME NQA-1“ states Chuck Fiala, Baker’s project manager. The American Society of Mechanical Engineers Nuclear Quality Assurance program applies standards to activities that could affect the quality of nuclear material, structures, systems and components at facilities including constructing, modifying and decommissioning. Baker’s program achieved industry credibility by passing program audits by both the U.S. Department of Energy and National Nuclear Security Administration.
Baker has partnered with longtime concrete pumping allies Ramcrete, Inc. from Hamilton, OH on the project. Ramcrete, founded in 1976, has a versatile fleet of concrete pumps, which include 61-, 47-, 42-, 39 -and 32-meter boom pumps along with a 31-meter EZ Boom and line pumps to meet the exact requirements of each contractor. Through an arrangement with Baker, Ramcrete is supplying a Schwing 61-meter boom pump with operator for the duration of the project, which has included three separate contracts: P and R Reactors Grout Fill and Stack Demolition, Roof Modifications and Sealing of Exterior Openings and Disassembly Buildings Concrete Cover. Ramcrete is also supplying a Schwing 47-meter boom pump for the placement of flowable fill and concrete.
The Savannah River Nuclear Solutions (SRNS), LLC is a partnership that operates the nuclear facility. In preparation for the decommissioning of the two reactors which have been shut down for many decades, SRNS dismantled and demolished some of the above-ground structures. The remaining underground portions of the P & R Reactors, located six miles apart, were the portions to be filled with a cementious material referred to on-site as “lean fill,” “high-flow grout mix” or “flowable fill.” In the project specification documents it is referred to as “Controlled Low Strength Material (CLSM).”
Three of these mixes were specially designed for different site applications. One CLSM specified as Congested Dry Area Placements included 150 lbs of Portland Cement, 500 lbs. of flyash, 2,318 lbs of sand and 63 gallons of water resulting in a water-to-cement and flyash ratio of .59. The second CLSM was specified for Uncongested Dry Area Placements and contained 150 lbs. of Portland Cement, 500 lbs. of flyash, 1850 lbs of sand, 800 lbs of pea gravel (#8 stone) and 50 gallons of water resulting in a .64 water to cement and flyash ratio. A third mix design was specified for Underwater Placements and included 150 lbs of Portland Cement, 500 lbs of flyash, 1870 lbs of sand, 870 lbs of pea gravel and 41.5 gallons of water for a .54 water-to-cement and flyash ratio. The compressive strength of the various mixtures was approximately 100 psi for the dry area placements and 380 psi for the underwater placements. Batching of these mixtures occurred on-site at an SRNS contracted plant with Champion Concrete owned truck mixers feeding the pumps.
A 250-foot x 300-foot area defined each of the abandoned reactors which included many underground rooms, corridors, stairwells and the reactor vessels. A concrete slab covered the entire below grade portions, so the only access was from the top down. With the pour area extending 40-feet below grade, an intricate system of slickline with diversion valves was designed to fill all of the cavities with flowable fill. At each reactor site, Baker crews core drilled more than one hundred 6- to 8-inch access holes through as much as 4-feet of concrete. More than 3,000-feet of 7-gauge, single-wall heat-treated 5-inch pipe was used in the grouting of each reactor.
Four different locations were chosen for pump placement at each site. Pipeline was attached to a 90-degree elbow attached to a reducer connected directly to the pump’s Rock Valve outlet. “We were working off of blueprints from 1952 in order to route the slickline “explains Keith Jones, pump operations manager on-site. Crews worked 12-hours a day, five days a week for six months to fill every void in the structures. “We performed normal maintenance on the pumps,” Jones said “The lean fill is not that abrasive but you are pumping at a fast rate.” Crews averaged 150 to 200 yds/hr and completed the 95,000 yards for each reactor in six months. “Pump reliability was very important because in many cases it was a one-shot deal,” explained Jones.
From the four pump locations at each site, pipeline branched out and diversion valves routed the mix to the desired pour location. Cleanout of the above-grade lines at the end of the day meant running a ball through the pipe and washing out into the pour area. Contamination in the building meant that below-grade pipe sections were often abandoned. Laborers on the job received special training on clean-out procedures and pressurized pipeline. Tremie pours were used when ground water had filled the long-abandoned buildings. An eight-foot cap was placed over each reactor vessel and was accomplished in two-foot lifts of 1,500 yards each. The concrete for the cap was a 3,000 psi mixture with a waterproofing agent added. “That was a tough mix that took a lot of pressure to push,” explains Jones. The pours required 400-feet of slickline and 50-feet of five-inch rubber end hose.
The third contract required roof modifications and sealing of exterior openings in a separate building. Seven roof pours were made with the S 61 SX and S 47 SX. Both pumps utilize 4-section, Overhead Roll and Fold Booms. The 61-meter pump features 196’10” of vertical reach and the S 47 SX reaches to 151’5 inches. Both pumps use Schwing exclusive curved Super X outriggers that allow the pumps to set up as close to the pour area as possible which was important because of set-back restrictions on the building’s site. The cap mix was used for all of the roof pours which totaled 1,300-yards at each reactor site.
Exterior openings were sealed with concrete which was formed with plywood and pumped through the booms. Interior forms were left in place and a 45-degree angled “birds mouth” chute was used for charging the forms with concrete. For future testing of radioactivity, one stainless steel door was placed over the final opening.
All employees on the site underwent Radiological Worker II (RAD) training to prepare for work in radiological conditions. Thermoluminescent dosimeters (TLDs) were worn to measure the radiation exposure and were monitored regularly. Baker hired an independent radiological controls firm to monitor the workers and special protective suits were used by workers in contaminated areas. “We set our workers limits for exposure at one-half of the Savannah River site limit and we never got anywhere near our self-imposed limit,” explained Fiala.
Owner: Department of Energy
General Contractor: Savannah River Nuclear Solutions, Aiken, SC
Concrete Contractor: Baker Concrete Construction, Aiken, SC office
Pumping Contractor: Ramcrete, Inc. Hamilton, OH
Pumping Equipment: Schwing S 61 SX truck-mounted concrete pump with placing boom and Schwing S 47 SX truck-mounted concrete pump with placing boom.