But 68 years later, Hanford remains off limits. Not because of weapons work, which has long ago ceased, but because it is the Western hemisphere's most contaminated nuclear site with 53 million gallons (200 million liters) of radioactive waste stored in aging tanks.

With billions of dollars a year invested in cleanup, there is little palpable fear of a catastrophe among residents here in the northwestern state of Washington. Some 12,000 people work at Hanford, which at 586 square miles (1,518 square kilometers) is twice the size of Singapore.

But with a crisis in Japan raising global alarm about nuclear safety, some people are calling for a new sense of urgency to cleaning up Hanford which has been hit by delays, cost overruns and charges of causing illness.

"It's a ticking time-bomb sitting there. Sooner or later, something's going to happen," said Walt Tamosaitis, a top engineer at the plant until last year.

"It would be monumental if those tanks cracked," he told AFP. "They would have no way in heck of ever stopping it."

Tamosaitis, who had 40 years experience, said he was removed after he raised concerns about the design. He said a contractor once even voiced hope that he would choke on cherries offered at a meeting -- a joke, but one he said showed management's thinking.

"Their attitude is, why should we worry?" Tamosaitis said. "It's like driving a car with the tires worn down. You didn't have a flat tire for the first 30,000 miles, but that doesn't mean you won't have one as soon as you back out of your driveway."

Officials declined to speak in detail about Tamosaitis' case because he has filed a lawsuit. But they said they put a top priority on safety and were making concrete progress on disposal of the waste.

The key part of the cleanup is turning the waste into glass -- a state-of-the-art process that involves heating toxic sludge the consistency of peanut butter to 2,100 Fahrenheit (1,150 Celsius) and placing it in eternal storage. Managers hope to complete the plant in 2016 and start operating it in 2019.

"Operations are within our reach and I think that there is some excitement here. We are seeing some momentum," said J.D. Dowell, an official at the US Energy Department. "This is a national commitment."

Managers also said they were making progress on protections to the Columbia River, which flows past Hanford to Portland, Oregon, and pointed to the recent demolition of two powerhouses that had been operated when the site was active.

Tamosaitis said he raised a number of technical questions including the design of an instrument to move sludge as well as a decision to try to manage small hydrogen explosions in the pipes instead of avoiding them altogether.

Tamosaitis said that, in a sense, Japan's ill-fated Fukushima nuclear plant was better prepared. Its design was functional, but it was hit by an unprecedented whammy of a 9.0-magnitude earthquake and tsunami.

In Hanford, "we don't even have the design down adequately," he said.

Dowell said that a comparison between Fukushima and Hanford was like "apples and oranges." The Japanese plant involved active nuclear reactors, while Hanford is a long-term clean-up.

Hanford does have a Japanese connection, however. The plutonium produced here built the bomb that the United States dropped on Nagasaki, killing 70,000 people in the world's second and last nuclear attack.

Tom Carpenter, executive director of the advocacy group Hanford Challenge, said that the waste posed a constant risk due to the possibility of terrorists or other disturbed people entering the site.

"I don't know that governments last forever. Will there be someone here in 100 or 1,000 years to assure that the materials are protected, intruders don't get in and the groundwater isn't contaminated?" he said.

He also warned of a grave stakes if a natural disaster struck. The Pacific Northwest coast is overdue for a major earthquake, although the area near Hanford is only known to experience small tremors. Hanford is also home to a functioning nuclear power station.

The safety concerns are not just theoretical. Until the 1960s, Hanford poured some of its waste into the open. The government has acknowledged that at least one million gallons (3.8 million liters) of toxic material leaked from tanks, some entering the groundwater.

In 2005, a jury awarded damages to two people who said they suffered thyroid cancer due to Hanford. Residents have also filed a class action suit seeking broader compensation.

"We had a garden all growing up, and I lived right in town, so I was right in the immediate vicinity," said Gloria Wise, 67, who was awarded $317,251 in 2005.

"I'm sure it got onto our food. Plus, we had a dairy that was delivered when I was a baby, you'd drink that milk and, I mean, I learned all these things, they didn't tell us what was going on," she said.

The lawsuits were filed against chemical giant Dupont and General Electric Co., which were major players in Hanford.

Multiple companies have also been involved in the cleanup. In 2000, the United States awarded Bechtel an 11-year, $4.3 billion-dollar contract to spearhead the cleanup after previous British designers rose cost projections.

A 2006 report to Congress found that under Bechtel, the project has also run over goals for completion and financing -- due to technical challenges but also "the contractor's performance shortcomings" and Energy Department management problems.

With the United States now seeking to cut spending, some Hanford watchers feared there would be corner-cutting.

"We're very concerned that the cleanup momentum is going to stall with the 2012 budget," said Susan Leckband, who heads the Hanford Advisory Board which brings together stakeholders.

Richard Fleming, 57, grew up near Hanford and said he worked there until he fell ill. He believed that managers' overriding concern was political -- a desire for the project to appear to be going smoothly.

"I know every square inch," he said. "I know what's out there. I know how dangerous these things are."

"We're talking about the most complicated machine probably ever built. And we don't even know if it's going to work."

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