At first glance, the metals that give atom bombs their destructive fury might seem interchangeable: Uranium and plutonium are both more valuable than gold. Both captivate would-be atomic powers. And both fueled bombs that leveled Japanese cities — uranium at Hiroshima and plutonium at Nagasaki.

But to see them as equal is to ignore a crucial difference: Of the 15,000 or so nuclear warheads on the planet, atomic experts say, more than 95 percent rely on plutonium to ignite their firestorms.

As a fuel for weapons, plutonium packs a far greater punch than uranium, and in bulk can be easier and cheaper to produce. Which is why some nuclear experts voice incomprehension at what they see as a lopsided focus on uranium in evaluations of the deal reached with Iran — under which Tehran would forsake the production of plutonium.

“It was an incredibly big breakthrough,” said Siegfried S. Hecker, a Stanford professor and former director of the Los Alamos weapons lab in New Mexico, the birthplace of the bomb. “But nobody seems to care.”