A federal audit raised concerns Monday about cleanup delays and costs at a Hanford nuclear reservation plant where workers produced more than half the plutonium for the nation's nuclear weapons arsenal.

The federal government created south-central Washington's Hanford site in the 1940s as part of the top-secret project to build the atomic bomb, and the site produced plutonium for nuclear weapons through the Cold War. Today, Hanford is the nation's most contaminated nuclear site, and cleanup is expected to last decades.

High on the cleanup list: demolishing more than 60 buildings that make up the Plutonium Finishing Plant, where plutonium was formed into buttons the size of hockey pucks to be shipped offsite for nuclear weapons.

The U.S. Department of Energy, which manages Hanford cleanup, in 2008 awarded CH2M Hill Plateau Remediation Co. a $639 million contract for the work. The contract runs through 2016 but requires that the work be completed by 2013.

The agency's Office of Inspector General reviewed the contract just through 2013. In a report Monday, it said the project cost had grown from $528 million to $718 million — a 36 percent increase — and the work was behind schedule as of Sept. 30, despite receiving $330 million in federal stimulus money to speed the work.

The work is over budget 13 percent when the contract is reviewed in its entirety through 2016, according to the Energy Department. Those increased costs can be attributed to higher labor costs and additional work that was asked of the contractor, including structural improvements to ensure worker safety.

"Cleanup of the plutonium finishing plant is a complex project, and the safety of our workers remains a priority," said Geoff Tyree, spokesman for the Energy Department's Richland Operations office. "The contractor has done the right thing by carefully addressing safety concerns."

The report also said the 2013 deadline may be missed without improvements in the next 12 months in decontaminating and demolishing contaminated "glove boxes," which workers used to safely handle radioactive material.

Kurt Workman, CH2M Hill spokesman, said the company is testing a new chemical agent to decontaminate the boxes, which should accelerate the process once it's approved. Progress also was delayed for five weeks in May when work was stopped sitewide to address concerns about beryllium, a hazardous agent.

So far, 101 of 174 glove boxes have been decontaminated.

"With the efficiency that we're implementing and some of these new things we're using, we'll be able to pick up that schedule really easily," Workman said.

Tyree said the Energy Department does not anticipate any delays to the overall cleanup schedule from the glove box work.