Workers have emptied the ninth underground tank holding high-level radioactive waste at the Hanford nuclear reservation, the Department of Energy announced Thursday.
Although workers still have 140 single-shell tanks to go, this was the second tank DOE believes it has emptied to regulatory standards this year.
Before that, workers made progress on several tanks, but failed to finish any tank for five years.
DOE is confident it has broken that bad streak, however, and will have an additional two tanks emptied by the end of October.
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It is on schedule to meet a legal requirement to have all 16 single shell tanks in the group called the C Tank Farm emptied by fall 2014, said Tom Fletcher, DOE assistant manager of the tank farms.
All but one of the nine tanks DOE believes have been emptied are in the C Tank Farm.
The Washington State Department of Ecology, the regulator on the project, has not had a chance to confirm that all but 360 cubic feet of waste, the equivalent of less than 1 inch at its bottom, has been removed from the huge tank.
"We're very encouraged" that waste retrieval appears to have been completed on another tank, said Cheryl Whalen, cleanup section manager for the Department of Ecology's Nuclear Waste Program.
But it has taken about nine years to reach that point and the remaining half of the C Farm tanks must be completed in the next two years to meet the deadline, she said.
"We've got our fingers crossed," she said. "If they could get four done this year it would be absolutely fabulous."
DOE has 56 million gallons of radioactive waste stored in underground tanks from the past production of plutonium for the nation's nuclear weapons program. It is moving the waste from single shell tanks, many of which have leaked, into 28 newer double shell tanks until the waste can be treated for disposal.
The ninth tank emptied is Tank C-104, a 530,000-gallon capacity single shell tank that once was one of the fullest in C Farm, including a significant amount of plutonium and uranium.
"It's good to see C-104 in the rearview mirror," said Kent Smith, single shell tank retrieval and closure manager for DOE contractor Washington River Protection Solutions.
It was one of the tanks where two technologies were needed to remove waste after liquid waste years earlier was pumped to a double shell tank.
Work started to retrieve about 259,000 gallons of sludge and solid waste from the tank in January 2010, using a modified sluicing system lowered into the enclosed tank that sprays water on the waste and then removes it with a pump.
But work soon was halted because of concerns about chemical vapors from the tank when the waste was disturbed.
Changes were made, including raising the vent stack higher into the air above worker breathing space, and giving workers the option of wearing supplied-air respirators.
The pumping of the sludge also was plagued with mechanical problems, including hitting an old piece of equipment hidden in the sludge that kept the pump for the retrieval system from being lowered.
But by May 17, 2011, all the sludge had been removed with modified sluicing and only a hard layer at the bottom of the tank remained.
That was removed this year with a chemical soak.
A series of water soaks helped remove water-soluble salts. Then concentrated sodium hydroxide, a common industrial caustic already used in newer tanks to preserve the correct pH level, was added to the tanks.
It converted the aluminum gibbsite salts in the waste to sodium aluminate, which also can be removed with water.
Washington River Protection Solutions was able to finish the chemical soak at almost $3 million under its $5.7 million budget and eight months ahead of schedule because the lengthy process of collecting and analyzing samples was skipped.
Instead, the contractor used historical data on the chemistry of the waste in the tank and knowledge about what should be in the tank after retrieval of the sludge to make the correct decision on how to remove the hard layer at the tank's bottom.
DOE expects to be able to announce soon that a 10th tank has been emptied, Tank C-109, where a chemical soak also is under way.
The 11th tank to be emptied is expected to be Tank C-107, where a new robotic arm -- the Mobile Arm Retrieval System or MARS -- is being used for the first time.
It's the largest and most robust system used yet to retrieve waste from a single shell tank, requiring a hole to be cut in the top of the tank to insert a new riser large enough for MARS to be lowered inside.
MARS retrieved about 80 percent of the 253,000 gallons of waste in Tank C-107, before hitting a hard layer.
That waste now is being attacked with a combination of the sluicing nozzles used on the sludge and the high-pressure nozzles also attached to MARS.
Work stopped briefly when a leak, likely of the liquid waste used for sluicing, was discovered in the rotary union that links lines in and out of the tank to the robotic arm.
Work resumed because the leak was so small, but a repair may be attempted next week.
After Tank C-107 is emptied, the six tanks that would remain with waste in the C Tank Farm include three tanks that already have had initial retrieval campaigns that removed all but the hard waste at their bottoms.
The only uncertainty remaining in meeting the 2014 deadline is for one of the other three, Tank C-105, Fletcher said. It is planned to be emptied using a MARS unit equipped with a vacuum because it is believed to have leaked in the past.
The larger opening into the first tank using a MARS unit, Tank C-107, was cut with pulverized garnet blasted at high pressure.
However, new concerns have been raised this spring about whether the bits of garnet that fall into the tank could cause erosion of metal in the Hanford vitrification plant being built to treat the waste.
DOE is working through the issue, Fletcher said.