YAKIMA, Wash. (AP) - Workers at the nation's most contaminated nuclear site have resumed work to retrieve radioactive and hazardous waste from underground tanks, nearly a year after a spill and technological glitches prompted a work stoppage.

The restart of waste retrieval Tuesday marked a significant development for contractor CH2M Hill Hanford Group, charged with emptying and closing the tanks, as it prepares to close out its cleanup contract at south-central Washington's Hanford nuclear reservation.

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb. Today, it is the nation's most contaminated nuclear site, with cleanup costs expected to top $60 billion.

Central to the cleanup is the removal of some 53 million gallons of radioactive and hazardous waste - enough to cover 123 football fields, including end zones, a food deep - left over from three decades of plutonium production for the nation's nuclear weapons arsenal. The waste is stewing in 177 underground tanks, some of which have leaked into the aquifer and threaten the neighboring Columbia River.

Work was halted last July 27, when between 85 and 114 gallons of waste spilled onto the ground from a tank known as S-102. Workers were pumping waste from the tank when a pump blocked, and the spill occurred when they tried to run the pump in reverse to unblock it.

The U.S. Department of Energy, which manages the cleanup, shut down operations at another tank, C-109, while it evaluated retrieval operations. Those operations restarted in mid-August, but regulators determined no more waste could be retrieved under the technology being used at the time.

Shortly after that, all tank waste retrieval was shut down indefinitely while the contractor and the Energy Department took corrective actions ordered because of the spill.

In May, workers lowered an 800-pound mini-bulldozer into C-109 in an attempt to break up the waste. The equipment is designed to be operated remotely from aboveground.

However, one of the rubber-like tracks came off the equipment, forcing another stoppage.

Mark Brown, director of tank farm operations for the Energy Department's Office of River Protection, said the bulldozer already had moved a large amount of solid waste toward a retrieval pump, which allowed pumping to restart again Tuesday.

Some 9,500 gallons of sludge and other solids remain in the tank. Enough of that waste has been broken up and moved to continue pumping while workers determine if the bulldozer can still be used - minus one track - without damaging the bottom of the tank, he said.

"The good news is that the equipment worked very well. We want to use that some more, even with just one track on it. We just have to do some engineering evaluations," he said. "Obviously, damaging a tank is not something we want to do."

Late last month, the Energy Department announced that Washington River Protection Solutions LLC had won the new contract to retrieve waste and close the tanks. The company will start taking over the job July 1 and officially assumes control on Oct. 1.

CH2M Hill, based in Englewood, Colo., has held the contract for 11 years. The company has paid $683,300 in penalties and settlement agreements as a result of the spill. The contractor also had $500,000 withheld from its pay by the Energy Department.

Cleanup from the spill at tank S-102 is expected to continue through the summer.