In 1943, the US government needed a reliable centre for processing the Manhattan Project's nuclear material. Officials chose the 568-acre Hanford site in the deserts of Washington State to house nine nuclear reactors and 143 single-walled, underground waste tanks. Fast-forward 68 years and the US government is still working at Hanford — not as a research facility, but as one of the country's most polluted Superfund sites.
The plutonium production process generated billions of gallons of liquid waste and millions of tons of solid waste. The single-walled design of the holding tanks—built from a carbon steel liner surrounded by concrete walls, sunk seven to 10 feet underground, and designed to store chemical waste, the byproducts of plutonium production—are cracked and are leaking. As many as 67 of these tanks are at least suspected (and some have been confirmed) of leaching nearly a million gallons of waste into soil the soil. Thankfully, almost all of the liquid waste has already been pumped out and disposed of, leaving just the hard gunk behind.
To compound the issue, much of the material in the tanks ranges in consistency from peanut butter to soft concrete — not exactly stuff you can just suck out with a Wet/Dry Vac. And, given that it is highly, lethally radioactive, sending a cleaning crew down into the tanks to scrape it out by hand would be suicide — even if they are wearing tungsten jackets. And that's assuming you could even squeeze a worker through the small access pipe on the surface — these tanks were never designed to be emptied.
So, if you can't send a person in, why not a robot? The Mobile Arm Retrieval System (MARS) is designed to do just that — retrieve the otherwise-inaccessible waste from Hanford's single-shell tanks for transport to one of 28 new, double-walled tanks.
The MARS is an extendable mechanical arm with a maximum reach of about 12m. It's attached to a carriage which is lowered into the tank via a long mast. If necessary, the access port to the tank can be carefully widened to allow the arm entry. The MARS has both an elbow and articulated wrist joint with 360 degrees of movement, which allows it to access virtually every square inch of the tank's interior. At the end of the arm sits a high-pressure water cannon for scouring hard build-up known as "heels" as well as a low pressure nozzle for breaking up and sweeping other contaminants — salt cake and sludge — towards an extraction pump in the centre of the tank. The consistency and mix of these contaminants varies by the tank, which has until now hampered cleanup efforts.
The previous method used for cleaning these tanks is known as sluicing. Essentially, high-pressure water jets are lowered into the tanks. However, since these systems can unleash as much as 1135 litres a minute at 300psi, no one was sure if the old tanks could handle those stresses. In addition, that also meant 1135 litres of water was being irradiated every minute and would require disposal of as well — there simply wasn't enough room in the new tanks for it. The MARS system only outputs 100 gallon a minute at just 100psi and recycles the sluicing water for reuse. That's at normal pressure, mind you. For stubborn sludge, the MARS can fire 75 litres a minute at up to 5000 psi. Granted, that's not commercial water jet pressures, but it is enough to un-stick the 10 per cent of sludge that conventional sluicing leaves behind.
The MARS System will also help the government realise significant cost savings for the remainder of the project. The first tank cleaned after the superfund designation, tank C-106 cost $US100 million to clean. According to Kent Smith, the single-shell retrieval and closure manager for DOE contractor Washington River Protection Solutions, the next two tanks MARS cleans should be done for about $US5 million apiece. [Washington River Protection Solutions - Popular Mechanics, OSTI, Colombia Energy, Tri-City Herald]
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