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 fuelled bombs that levelled Japanese cities - uranium at Hiroshima and plutonium at Nagasaki.
But to see them as equals is to ignore a crucial difference: Of the 15,000 or so nuclear warheads on the planet, atomic experts say more than 95 per cent 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 Teheran would forsake the production of plutonium.
"It was an incredibly big breakthrough," said Dr 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.
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.
THE PROMISE OF PLUTONIUM
The nuclear reactor complex near Arak, Iran, is ringed with anti-aircraft guns and missiles. Last year, the complex was nearly ready to begin converting uranium fuel into weapons-grade plutonium. But as part of the nuclear deal between Iran and the West, Teheran agreed to redesign the reactor and not to build other plutonium reactors for at least 15 years.
Teheran's vow was a major turnaround, say nuclear experts, who express frustration that political jousting and technical naivete have largely obscured what they call one of the accord's main triumphs.
"It's a real success," said Dr 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."
Dr Richard L. Garwin, a principal designer of the world's first hydrogen bomb and a long-time adviser to Washington on nuclear weapons and arms control, called the redesign "a great achievement". He and other scientists signed a letter to President Barack Obama last month praising the Iran deal as innovative and stringent.
Bomb veterans say the central importance of Teheran'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 Mr 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, then President Harry S. Truman called the blast "startling - to put it mildly". The shock wave, he said, knocked down men nearly 10km 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 6.2kg 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 bomb on 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.
Experts say India, North Korea, Israel and Pakistan have used reactors to make plutonium for nuclear arms.
Advanced states use plutonium mainly for hydrogen weapons, which dominate their arsenals. A small mass of the silvery metal, typically no bigger than a baseball, acts as a superhot match to light the thermonuclear fuel. The resulting warhead is up to a thousand times more powerful than an atomic bomb.
IRAN ABANDONS PLUTONIUM
Teheran's bid for plutonium was revealed publicly in late 2002, at the start of Iran's stand-off with the West. Attention focused on a sprawling, half-built reactor complex, named Arak after a nearby city. The isolated site was ringed by miles of barbed wire.
Teheran claimed that Arak would make radioisotopes for such humanitarian purposes as treating cancer. But as work on the complex progressed, nearby valleys and mountaintops came to bristle with scores of anti-aircraft weapons.
"It's pretty well defended for something that's supposedly peaceful," said Mr Forbes McKenzie, managing director of McKenzie Intelligence, a private firm in London that examined satellite images of the remote site.
Experts say Arak's anti-aircraft guns are primed for Israeli jets, which have twice hit emerging plutonium threats. In 1981, Israel bombed an unfinished reactor in Iraq and, in 2007, smashed another in Syria.
The most palpable roots of Iran's plutonium reversal go back to 2012, when arms control experts began discussing Arak's redesign. Early last year, as part of the interim diplomatic accord, Iran agreed to stop making improvements at its three nuclear fuel plants, including the unfinished reactor.
"Progress," Secretary of State John Kerry told reporters, "is frozen in place." The reactor was said to have been months from commissioning.
By the middle of last year, Iranian officials surprised Western experts by agreeing to redo the Arak reactor. They pledged a fundamental redesign so the finished plant would focus exclusively on medical isotopes rather than also producing what Western experts estimated as up to two bombs worth of weapons-grade plutonium each year. But Teheran has never admitted that it had sought plutonium for weapons.
A year ago, at the New York City residence of Iran's Ambassador to the United Nations, Iranian reactor designers laid out for American experts a summary of the detailed plan.
"It was a remarkably good redesign," recalled Dr R. Scott Kemp, a nuclear expert at Massachusetts Institute of Technology who formerly worked at the State Department.
In selling the Iran deal, the White House has stressed the plutonium step. Teheran does the opposite, telling home audiences that it will still purify uranium.
In April, when diplomats announced the preliminary accord, Mr Obama put Arak atop his list of selling points. "First, Iran will not be able to pursue a bomb using plutonium," he told reporters in the Rose Garden. "The core of its reactor at Arak will be dismantled and replaced."
The final accord, announced in Vienna on July 14, detailed the curbs on uranium before those on plutonium - perhaps as a gesture to Teheran.
Mr Obama, in his Aug 5 speech at American University, flipped the order. Iran, he said in his first technical point, "cannot acquire the plutonium needed for a bomb".
Critics fault the deal as leaving Iran free to speed ahead after the accord's main provisions expire. Its ban on plutonium reactors is to remain in place for at least 15 years. The Obama administration says the deal is better than the alternatives, including war.
Why did Iran's reversal on Arak fall off the public radar so quickly?
Nuclear experts list a number of possible factors.
They note that the military threat from an unfinished plutonium complex can be viewed as abstract compared with Iran's success at purifying uranium in its two underground plants. Worst-case estimates say Iran could enrich enough uranium for a bomb in as little as two or three months.
The plutonium deal, experts add, displayed no loose ends. It basically ended Iran's long-time bid, leaving few openings for opponents and doubters of the accord.
"There's nothing left to discuss," Dr von Hippel said.
In contrast, the overall deal lets Iran keep thousands of centrifuges spinning to purify uranium. Diplomats see that as a defensible concession to Teheran. But in the US, it has fuelled opposition to the agreement, with some critics questioning whether inspectors will be able to verify the uranium curbs.
Why did Iran give up plutonium? Dr Hecker said Teheran had probably decided to abandon its push for an arsenal. But he argued that the nation's hard bargaining to save much of its uranium complex suggested that it still wanted to hedge its bets.
"I think, at this point, Iran really doesn't want to develop nuclear weapons," he said in an interview. "But they've kept the option."
Dr. von Hippel agreed. He said it appeared that Iran had been aggressively pursuing two pathways to bomb fuel and decided that one was enough.
"They don't want an arsenal," he said. "They want the US to know that they could still go for a bomb."
Experts say the redesign and rebuilding of the Arak reactor could take up to a decade.
And if the accord falls apart? Experts say the Arak reactor could soon become a plutonium factory. Iranian troops, they add, appear to have long exercised the nearby guns and missiles, preparing for war. NEW YORK TIMES
Critics fault the deal as leaving Iran free to speed ahead after the accord's main provisions expire. Its ban on plutonium reactors is to remain in place for at least 15 years. The Obama administration says the deal is better than the alternatives, including war.
