YAKIMA — The U.S. Department of Energy has notified officials in Washington state that it may be unable to meet the legal deadline for operating a multibillion-dollar waste treatment plant at the Hanford nuclear reservation.

If the Energy Department fails to have the plant up and running by 2011, it would mark the fourth time the federal government has missed a deadline to complete its largest construction project. The deadline already has been pushed back three times.

The plant is being designed to treat highly radioactive waste left from decades of plutonium production for the nation's nuclear weapons arsenal. The Energy Department halted construction on major portions of the plant last month amid skyrocketing costs stemming from seismic issues and construction problems.

Federal officials have repeatedly refused to release a new cost estimate for the plant - currently tagged at more than $5.8 billion. Congress has estimated the latest problems could push the cost as high as $10 billion and delay the start by four years.

The Energy Department notified state officials yesterday that a new cost estimate and schedule for completing construction on the plant will not be ready before June 2006, the state Department of Ecology said in a statement today.

"We continue to be frustrated by this update, but at the same time agree that USDOE and the contractors should do the job right and not make promises they cannot keep," the statement said.

An Energy Department spokesman did not immediately return a telephone call seeking comment.

The Energy Department also notified the state that it may not meet two deadlines for cleaning up sludge from two leak-prone pools of water near the Columbia River. The K East and K West basins were built at Hanford to store spent nuclear fuel, but cleaning them up has proven more difficult than envisioned.

The federal government was to have sludge removed from the K East basin by July 31, 2006, and all sludge from the K West basin in containers by June 30, 2006. The Energy Department warned it may miss both deadlines.

The waste treatment plant has long been considered the cornerstone of cleanup at Hanford, which was created in the 1940s as part of the top-secret Manhattan Project. Today, it is the nation's most contaminated nuclear site.

The greatest risk is posed by 53 million gallons of decades-old radioactive waste brewing in 177 underground tanks. Retrieval of the waste is a priority because some of the tanks are known to have leaked, threatening the aquifer and the Columbia River less than 10 miles away.

The plant will use a process called vitrification to turn the waste into glasslike logs for permanent disposal in a nuclear waste repository. Once completed, it will stand 12 stories tall and be the size of four football fields.

The operating deadline already had been pushed back three times from the original deadline of 1999. Critics argue the current slowdown could have been avoided if the federal government had conducted a more thorough seismic review.

Three years ago, the Defense Nuclear Facilities Safety Board raised concerns that the agency's seismic review was inadequate, and a scientific report in 2004 found that the force of the ground movements at the plant site during a severe earthquake would be 38 percent greater than previously estimated.

Under the Tri-Party Agreement, the legal pact signed by the state Department of Ecology, Environmental Protection Agency and the Energy Department, which manages cleanup at Hanford, the plant was to have been fully operating by 2011.

The plant is being designed as it is being built — a method that has proven costly. Design of the plant is about 75 percent complete, while construction is only about 35 percent complete.

The price tag already has grown from $4.3 billion to the current $5.8 billion. If the cost jumps to $10 billion as Congress estimated, that would push it closer to the $15.2 billion estimate former contractor BNFL Inc. proposed in 2000. The Energy Department fired the company shortly thereafter.