April 2011 - Founded in 1992, AK Services, Everett, Mass., provides waterjet technologies for contractors and engineers in the United States and Canada. Recently, the company cut a 55-in.-diameter access hole in an active U.S. Department of Energy radioactive waste storage tank at the Hanford Site in southeastern Washington, which is considered the most contaminated nuclear site in the United States. The tank is one of more than 100 similar underground tanks that the DOE is cleaning out as part of an environmental effort. The hole will allow the DOE to install a robotic system that will remove 247,000 gal. of radioactive and chemical waste, which was stored in the tank during the Manhattan Project and Cold War.
Because of the hazards, the equipment used in this project had to withstand freezing temperatures and be able to cut from 300 ft. away. AK Services used a specially engineered abrasive waterjet cutting machine to cut through 15 in. of concrete and steel rebar. The cut took 22 hours, with the company moving through 8 in. of concrete per hour using an abrasivejet of garnet grit mixed with 3 gal. per minute of water, which was pressurized to 48,000 psi with a Jet Edge waterjet intensifier pump.
Protecting the environment
The finished cut represents the culmination of more than a year of careful planning and preparation, both by AK Services and Hanford Site Tank Farms' prime contractor, Washington River Protection Solutions.
According to Carl Franson, AK Services' vice president of operations, "the motion device had to be specially developed to secure itself inside a 75-in. steel riser and cut a 55-in. hole. We also needed to be able to level the motion device to make sure it did not drag the abrasive jet focusing tubes on the concrete surface."
To accomplish the task, 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 were placed on the sides and bottom to level and secure the system.
"We were able to do the entire cut with the one lead cutting head," Franson said in a press release. "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."
Cutting from afar
"The biggest challenge was running the equipment from 300 ft. away," Franson noted in the press release. "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 become the property of the DOE. We also had the added challenge of winter weather, so we installed air dryers and used air line antifreeze to keep the system from freezing up. The ultra-high-pressure lines were kept running, but as the temperatures dropped, we blew them out with air."
After the concrete plug was cut, it was lifted from the tank and wrapped in a plastic sleeve to prevent spreading contamination. A large riser placed in the hole supported the robotic retrieval system. According to the DOE, the riser has a shield plug bolted to its opening and thick gasket material around its outside to seal the hole. The DOE plans to begin retrieving waste from the tank this summer. FFJ
