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.”
Nearly two years of negotiations went into the landmark deal, which would limit Iran’s production of uranium and plutonium in exchange for the end of international oil and financial sanctions. It was finalized in July and is set for a congressional vote this month. Last week,President Obama secured commitments for enough votes to put the agreement in place over fierce Republican opposition.
But in the dauntingly complex analyses that preceded that political alignment, questions and criticism revolved almost exclusively around uranium — how much of it Iran would be allowed to enrich and stockpile, and how compliance would be verified.
Atomic experts call the uranium focus potentially misleading, because it is the lesser path to the bomb.
In secret, three decades ago, Iran began exploring the plutonium path and was perhaps only months from inaugurating a plant for its production when, last year, as negotiations gained momentum, it abruptly agreed to a fundamental redesign that would end the facility’s potential for making substantial amounts of bomb fuel.
Tehran’s vow was a major turnaround, say nuclear experts, who express frustration that political jousting and technical naïveté have largely obscured what they call one of the accord’s main triumphs.
“It’s a real success,” said Frank N. von Hippel, a physicist who advised the Clinton administration and now teaches at Princeton. “I was surprised that they were willing to give it up.”
Richard L. Garwin, a principal designer of the world’s first hydrogen bomb and a longtime adviser to Washington on nuclear weapons and arms control, called the redesign “a great achievement.” He and other scientists signed a letter to President Obama last month praising the Iran deal as innovative and stringent.
Bomb veterans say the central importance of Tehran’s plutonium concession becomes strikingly clear in the light of history.
After the Manhattan Project began, in 1942, plutonium became a superstar and uranium a sideshow. Purifying uranium into bomb fuel turned out to be extraordinarily difficult, whereas plutonium was an atomic byproduct, easing its manufacture. Moreover, it took far less plutonium to produce a blast of equal size. “It’s got twice the punch,” said Ray E. Kidder, a retired arms designer at the Livermore weapons lab in California. “All things being equal, it makes for a more powerful weapon.”
The plutonium was made in reactors. Tiny particles known as neutrons would zip through fuel rods, splitting atoms of uranium in two. That released energy and more neutrons in multiplying chain reactions.
In a kind of modern alchemy, some of the uranium atoms would also absorb neutrons and turn into plutonium. The Manhattan engineers refined that process so plutonium became the main product. The work was far more dangerous than purifying uranium, in part because the fresh plutonium had to be scavenged from highly radioactive fuel rods. But the results were spectacular.
On July 16, 1945, the world’s first atom bomb lit up the New Mexico desert. Its plutonium core was 3.6 inches wide. In his diary, President Harry S. Truman called the blast “startling — to put it mildly.” The shock wave, he said, knocked down men nearly six miles away.
The nature of a detonating atom bomb is that it rapidly tears itself apart, stopping the chain reactions long before all the atoms are split in energetic bursts.
In New Mexico that day, the bomb’s core started with 6.2 kilograms of plutonium. About a fifth of those atoms split in two, producing waves of smaller atoms as well as a gargantuan flash of pure energy. The plutonium behind that flash is estimated at one gram – the weight of a dollar bill.
The secret, and that of all nuclear arms, lies in the colossal divide between matter and energy that Einstein laid out decades earlier in his famous E = mc², where energy equals mass times the speed of light squared, a staggeringly large number.
On Aug. 9, 1945, when the United States dropped a plutonium bombon Nagasaki, a gram of matter again flashed into energy. Some 75,000 people died. More plutonium bombs were in preparation as Japan surrendered.
The Soviet Union, Britain and France used plutonium to power their first atom bombs. The metal liberates more energy than uranium in part because its atoms emit more neutrons when split, speeding chain reactions and increasing the weapon’s explosive yield. The high multiplication factor also means that plutonium warheads can be smaller and lighter, so missiles can fire them over longer distances.