The movie strongly implies that the bomb tested at the Trinity site was an uranium bomb. It was in fact a plutonium bomb similar to that used at Nagasaki. The only uranium bomb produced by the Manhattan Project was the bomb used at Hiroshima.
In part one they describe the transmutation of Uranium into other elements. The last mutation is into lead, and they symbolize it with a supposed lead ingot. But if it were an actual ingot of lead, it would be far to heavy to pick up with the ease shown.