A classified replica of Iran’s nuclear facilities in Tennessee is but one part of an extensive crash program within the nation’s nine atomic laboratories to block Tehran’s nuclear progress.

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WASHINGTON — When diplomats at the Iran talks in Switzerland pummeled Department of Energy scientists with difficult technical questions — like how to keep Iran’s nuclear plants open but ensure that the country was still a year away from building a bomb — the scientists at times turned to a secret replica of Iran’s nuclear facilities built deep in the forests of Tennessee.

There inside a gleaming plant at the Oak Ridge nuclear reservation were giant centrifuges — some surrendered more than a decade ago by Libya, others built since — that helped the scientists come up with what they told President Obama were the “best reasonable” estimates of Iran’s real-life ability to race for a weapon under different scenarios.

“We know a lot more about Iranian centrifuges than we would otherwise,” said a senior nuclear specialist familiar with the forested site and its covert operations.

The classified replica is but one part of an extensive crash program within the nation’s nine atomic laboratories — Oak Ridge, Los Alamos and Livermore among them — to block Iran’s nuclear progress.

As the next round of talks begins Wednesday in Vienna, the secretive effort remains a technological obsession for thousands of lab employees living the Manhattan Project in reverse. Instead of building a bomb, as their predecessors did in a race to end World War II, they are trying to stop one.

Ernest Moniz, the nuclear scientist and secretary of energy who oversees the atomic labs, said in an interview that as the Obama administration sought technical solutions at the talks, diplomats would have been stumbling in the dark “if we didn’t have this capability nurtured over many decades.”

Given the stakes in the sensitive negotiations, the labs would check and recheck one another, making sure the answers held up. The natural rivalries among the labs sometimes worked to the negotiators’ advantage: Los Alamos National Laboratory, in the mountains of New Mexico, the birthplace of the bomb, was happy to find flaws in calculations done elsewhere, and vice versa.

“It’s what our people love to do,” said Thom Mason, the director of Oak Ridge National Laboratory. “It can be very rewarding.”