Employees at the U.S. Department of Energy's Idaho National Engineering and Environmental Laboratory (INEEL) have successfully reduced the amount of liquid radioactive waste stored over the Snake River Plain Aquifer in 11 underground storage tanks. They also successfully demonstrated remote-operated technologies for cleaning the insides of tanks.

Reducing the volume of liquids in the tanks and closing them supports the INEEL's goals of accelerating cleanup and reducing risk to the environment, public and workers. It also addresses one of the highest cleanup priorities set by the state of Idaho.

Under the INEEL's Accelerated Cleanup Plan, tank closure will be completed by 2012, four years ahead of the original schedule.

Closing tanks is a three-step process. First, the tanks must be emptied down to the heel level - the lowest level possible using existing equipment. Second, the tank interior is washed or cleaned using remote-operated cleaning equipment that removes the heel from the tank. Third, the tank is filled with grout and closed. As the grout is put into the tank, it pushes the remaining heel material to a tank outlet nozzle where the material can be removed.

INEEL project manager Keith Quigley said the washing technologies were successful. Radiation levels in the tank were reduced by more than 99 percent.

"We are very pleased with the results of the tank washing," he said. "We exceeded our cleaning estimates and will move to the next tank and use the same washing techniques. We are hopeful we will get the same type of results as with the first tank."

The INEEL has 11 underground tanks at the Idaho Nuclear Technology and Engineering Center. The tanks, each with a 300,000-gallon capacity, have been used since 1952 to store liquid radioactive waste generated from reprocessing spent nuclear fuel and other decontamination activities. The liquid waste from reprocessing was placed in the tanks. Over the years, the liquid waste volumes have been reduced by calcining, a process that turned the liquid into a granular solid, and by processing the liquid through a high-level waste evaporator, which removed some of the liquid and concentrated the waste.

During the past fiscal year, workers emptied two tanks down to the heel, bringing the number of emptied tanks to six. Workers then used two tank-cleaning technologies to wash the interior of one tank, removing the heel and flushing it into another tank.

The cleaned tanks will be filled with grout creating a solid mass and left in place permanently, in line with the closure plan approved by the state.