Hanford 'big dig' under way

Hanford workers are preparing to make the largest hole ever in one of the nuclear reservation's underground tanks that hold high-level radioactive waste.

The hole will be used to insert a newly developed robotic arm that's much larger and tougher than technologies used to date, in an attempt to find a way to better empty waste from enclosed underground tanks.

"This is one of the most significant projects that has ever taken place in Hanford tank farms," Scott Sax, Washington River Protection Solutions tank retrieval and closure manager, told workers.

By the end of this month, the company expects to replace the current access to the tank -- a 12-inch-diameter riser, or pipe -- with a 42-inch-diameter riser large enough for the robotic arm to fit through.

The work must be done with minimal exposure to workers from the radioactive "shine" from the tank and without spreading any radioactive contamination.

Last week, workers equipped with shovels began what they're calling the "big dig," removing the 6 to 7 feet of dirt covering Tank C-107 a shovelful at a time.

The tank, which is large enough to hold 530,000 gallons of waste, holds an estimated 247,000 gallons of radioactive and chemical sludge. It's one of a group of 16 leak-prone single-shell tanks that were built between 1946 and 1953 to hold processing waste from the production of plutonium for nuclear weapons.

Workers will dig a hole 30 feet across at the surface. Work includes cutting and removing a waste line above the tank and demolishing the concrete pad around the old riser.

The dig will leave a 6-foot diameter circle at the top of the 37-foot diameter tank exposed. The soil provides radiation shielding, so as workers get closer to the tank, they'll be exposed to more radioactivity.

Exposure is being managed with steps such as limiting worker time and using matting. In a worst-case scenario, a worker standing on the exposed top of the tank would be exposed to 40 millirems per hour. DOE limits worker exposure to 3,000 millirems per year and the contractor's administrative controls limit workers to 500 millirems per year.

Washington River Protection Solutions does not anticipate that a worker will get anywhere near the administrative limit, said Kent Smith, the contractor's deputy manager for tank retrieval and closure.

A bigger worry is avoiding spread of contamination as the tank is opened and then closed again as quickly as possible, he said.

DOE, its regulators and Washington River Protection Solutions chose a cutting technology with a goal of eliminating any waste back splash as the tank is opened. Tests on a tank mock-up with a conventional saw showed it would fling contamination as it was cut, Smith said.

Instead, workers will use pulverized garnet blasted at high pressure to cut through the foot or two of rebar-reinforced concrete that forms the tank lid. The garnet particles are expected to fall into the tank, which is under a vacuum. Tests were done to make sure adding garnet to the tank would not harm equipment that will be used to treat the tank waste at the vitrification plant now under construction.

Work will be done inside a tent set over the tank, and workers will operate equipment at a distance to cut the 55-inch diameter hole in the top of the tank.

"A cut of this size has not been done on a tank already in service at Hanford," said Steve Pfaff, DOE project director.

As soon as the hole is cut and the concrete plug is wrapped in plastic and lifted out of the way, the larger riser will be lowered and attached to the tank. Workers have been practicing on a tank mock-up.

Although work to install the larger riser is expected to be completed this month, Washington River Protection Solutions then will take about six months to install piping, valve boxes and a ventilation system and to train operators for the new robotic arm.

The arm, called the Mobile Arm Retrieval System, or MARS, is expected to begin retrieving waste from the tank in July.

Pfaff said MARS has shown in tests that it can retrieve a wide variety of solid material from tanks. The arm can rotate 360 degrees and reach 40 feet total.

Within Tank C-107, it will use a water cannon to move waste toward a pump and will have small nozzles with high pressure and low volume to break up materials. Depending on the type of waste, it's projected to remove waste at rates that range from 85 gallons per hour up to nearly 1,000 gallons per hour.

The Washington State Department of Ecology is enthusiastic about the robotic arm, including its expected ability to remove more of the waste from the bottom of the tanks, said Jeff Lyon, tank waste storage project manager for the state, the regulator on the DOE project.

Each tank seems to be unique, Lyon said, as workers have found with the latest tank they've attempted to empty.

This fall, workers have been focused on Tank C-111, which had just an estimated 57,000 gallons of solids. But a high-pressure spray planned to move the waste toward a center pump bounced off what turned out to be a hard crust of waste at a 45-degree angle.

Workers tried soaking the crust in hot water to loosen it, but the high-pressure spray technology, called modified sluicing, still didn't move the waste.

Work has been halted on Tank C-111 while other options are considered and Washington River Protection Solutions will return to Tank C-104. Modified sluicing had been used successfully there until an obstruction in the tank -- some old equipment -- prevented the waste retrieval pump from being lowered farther as the level of waste remaining dropped. Workers have been modifying a small robotic arm that can fit through a 12-inch riser to pull the old equipment out of the way.