The U.S. hasn't tested a nuclear weapon in decades, and since the 1990s has used simulations and experiments to verify they're working properly. NPR was granted a rare look at how they do it.

But these tunnels stayed open ... a nuclear weapons scientist from Los Alamos National Laboratory, which tested the first bomb in 1945. Still, he adds, the decision whether or not to test is ...

But these tunnels stayed open. The U.S. still needed to ensure ... a nuclear weapons scientist from Los Alamos National Laboratory, which tested the first bomb in 1945. Still, he adds, the decision ...