Workers at the nation's most contaminated nuclear site are approaching a key turning point in building a massive waste treatment plant there, more than two years after the federal government shut down the project over seismic concerns.

The vitrification plant at south-central Washington's Hanford nuclear reservation is among the largest industrial construction projects nationally, both in cost and sheer size. In recent years, the project has been mired in technical problems, delays and escalating costs, even as state and federal officials underscored its importance for ridding Hanford of radioactive waste.

But workers expect to have completed 50 percent of the project by early fall, and just two of a long list of technical problems remain to be resolved.

Neither means the end is in sight, but recent progress can't be overlooked, said Suzanne Dahl, tank waste treatment manager for the Washington Department of Ecology, which regulates the federal government's cleanup efforts.

"Wouldn't it be great to be on the other side of 50 percent and heading downhill?" Dahl said. "That's a really big deal. It means we're just getting closer and closer to being able to turn it on."

John Eschenberg, project manager for the U.S. Department of Energy, said the project is in as good a situation as it's ever been.

"It's been a bumpy road," he said. "But the things that made us sweat blood are key to the success."

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb. Plutonium production continued through the Cold War, leaving a mess of radioactive debris and waste to be cleaned up on the 586-square-mile site.

The government spends $2 billion each year on Hanford cleanup - one-third of its entire budget for nuclear cleanup nationally. About $690 million of that goes for design and construction of the vitrification plant, long considered the cornerstone of cleanup.

The plant is designed to convert millions of gallons of highly radioactive waste into glass logs for safe disposal underground. At least 1 million gallons of waste have leaked from storage in aging underground tanks at Hanford, contaminating the groundwater and threatening the nearby Columbia River.

The delays pushed the operating date to 2019, a step state officials were unhappy with but accepted. The cost of the project also ballooned from $4.3 billion in 2000 to $12.2 billion.