The Challenge:
Cut the largest-ever access hole in an active radioactive waste storage tank to allow for installation of a robotic system to remove the radioactive and chemical waste.
The Players:
AK Services
Washington River Protection Solutions
The Process:
In order to remove 247,000 gallons of radioactive and chemical waste from a storage tank at the U.S. Department of Energy's (DOE) Hanford Site in Washington, AK Services was asked to cut a 55-inch-diameter hole in the top of the underground tank.
The waste, which was stored in the tank during the Manhattan Project and the Cold War is being vitrified for safer storage so the DOE needed an access hole to install the robotic system that will remove the waste. The tank is one of more than 100 similar underground tanks that the DOE is cleaning out as part of its environmental cleanup effort at the Hanford Site, which was once home to nine nuclear reactors that produced plutonium for the Manahattan Project and the U.S. nuclear arsenal. Hanford is considered the most contaminated nuclear site in the U.S.
AK Services had to battle freezing temperatures on the job so they installed air dryers and used air line antifreeze to keep the system from freezing up. As the temperature dropped, the UHP lines were blown out with air. Since the project dealt with radioactive waste, the contractor used a remote-controlled ultra-high pressure water jet from a 300-foot distance. "The biggest challenge was running the equipment from 300 feet away," said Carl Franson, AK Services vice president of operations. "The tank top is in a radiation area and entry to the area is regulated to the extent that anything that enters the tank farm must be scanned and cleared prior to being released out of the fenced-in restricted area. The pumps and control systems had to be outside of the restricted area to ensure they did not become contaminated with radiation and then becoming the property of the DOE."
AK Services worked with Washington River Protection Solutions to carefully plan and ensure worker safety and protection of the environment. AK Services also spent a year developing and testing the water jet cutting system and abrasives for the project.
"The motion device had to be specially developed to secure itself inside a 75-inch steel riser and cut a 55-inch hole," Franson said. "We also needed to be able to level the motion device to make sure it did not drag the abrasivejet focusing tubes on the concrete surface."
AK Services engineered a three-piece stainless steel motion system with two circles separated by guide bushings with a ring gear in the middle. The ring gear features four redundant Jet Edge High Flow Abrasivejet cutting heads in case of failure and is driven by a set of reduction gears and a drive motor to achieve the proper cutting rate. Pneumatically actuated rams are placed on the sides and bottom to level and secure the system.
The specially engineered abrasive water jet cutting machine made the cut through 15 inches of concrete and steel rebar in just 22 hours. The contractor was able to cut 8 inches per hour using an abrasivejet of garnet grit mixed with three gallons per minute of water pressurized to 48,000 psi with a Jet Edge waterjet intensifier pump.
"We were able to do the entire cut with the one lead cutting head," Franson noted. "We used a Jet Edge High Flow Abrasivejet cutting head with a Roctec 100 focusing tube rather than the standard tungsten carbide and we ordered diamond orifices from Jet Edge to ensure the longest life possible."
After the concrete plug was lifted from the tank, it was immediately wrapped in a plastic sleeve to prevent spread of contamination and then was placed in an isolated area where it is being staged for disposal at the Hanford Site. A large riser was placed in the hole to support the robotic retrieval system.
See a video of AK Services and the water jet cutting project on YouTube.
