U Canyon—a huge, windowless concrete monolith that housed secret Cold War-era plutonium and uranium processing work at the U.S. Energy Dept.'s Hanford site, the former nuclear-weapons production facility in eastern Washington—has sat empty and inert for more than four decades. Now, however, the cavernous structure will become a beehive of activity as a technology test site, featuring a first-time DOE process in which 20,000 cu yd of specially formulated, cement-like grout is pumped beneath the edifice to stabilize its radioactivity before final demolition.
The facility's name derives from its proportions: 810 ft long, 66 ft wide and 77 ft tall. Built in 1944, it was used at first to train workers extracting plutonium from irradiated fuel rods, but the plant also housed uranium reprocessing in the 1950s. U Canyon is one of five processing plants at Hanford and one of the largest in DOE's U.S. nuclear complex. Federal stimulus funds expedited the $5-million grout project, but full demolition is not budgeted until 2015, says DOE. The project is part of an agreement reached in 2005 between state and federal agencies. DOE says the pact originally targeted 2024 for demolition.
The grout will fill 40 below-grade cells, including ducting, piping and vessels in which radioactive material processing occurred. The material will “fix in place” residual contamination before U Canyon's above-grade walls are collapsed, says Brad Herzog, lead project engineer for CH2M Hill Plateau Remediation Group, the project contractor.
Work began 18 months ago, when crews inserted tons of old equipment into the structure like a “giant three-dimensional jigsaw puzzle,” Herzog says. CH2M Hill and subcontractors AGEC, Sandy, Utah, and Central Pre-Mix Concrete Co., Spokane, Wash., activated grouting in March from a new on-site batch plant.
With about 25% of the cells filled and 12% of the overall grouting work completed, Herzog expects work to finish by Sept. 6, about three weeks ahead of a Sept. 30 deadline. “The challenge is that the 40 cells, ventilation tunnel and drain header are all interconnected,” says Al Farabee, DOE project director. “So when you pour in any one cell, you have to look at a significant number of piping diagrams to only pour in the cells you want to fill.”
Crews have placed dams to keep grout from flowing into unwanted locations. Grouting started in Cell 10, the deepest spot at about 47 ft underground, and will continue until everything below ground and at-grade fills with grout. The remaining structure, about five times the size of the filled portion, will be demolished.
Herzog cites three performance criteria for the grout mixture. First, it needs to reach 1,500 psi at 28 days; while not considered structural strength—the structure itself will remain stable and hold the grout—that strength is enough to seal contamination. Second, a minimum flow of 80 ft is needed to ensure the grout reaches tight quarters. Third, the grout cannot gain more than 13°F of heat per 100 lb, which protects both the grout and the structure from thermal cracking.
The mix design combines standard grout and cement using a sandless mix, which is self-leveling, to “cap things off,” says Herzog. To place the grout, crews so far have bored 132 of 137 injection ports throughout the structure, with bores ranging in thickness from 3 ft to more than 17 ft. A total of 11 external ports acts as a utility sleeve, Herzog says. Once inside the building, 6-ft-thick cover blocks cap each of the cells, requiring more injection ports.
Grouting a ventilation tunnel in the building's corner has proven to be the most challenging job. “It is the most remote and most difficult place to access,” Herzog says. There, nine injection ports were drilled through 17.5 ft of concrete, and crews used the entire 80-foot flow capability to fill the cavity. The team grouts in tight quarters and under stringent procedures similar to those required at a nuclear powerplant. “We have to sequence our pours so we don't upset any equipment,” Herzog says.
Weather is also a factor at the high-desert site. Crews have started using hot-weather concrete placement techniques, such as inserting ice (instead of water) into the mix to keep the grout cool. “This is definitely not like pouring grout in any other place in the world,” Herzog says. A mix of clean sand, clay and silt will eventually cover the demolished structure, acting like a giant sponge, says DOE.