On Saturday, the U.S. bombed three nuclear sites in Iran in an effort to ensure the Islamic Republic is no longer able to enrich uranium, or continue to pursue the development of nuclear weapons. President Trump proclaimed the operation a “spectacular military success,” saying that “Monumental Damage was done to all Nuclear sites in Iran … Obliteration is an accurate term!”
Trump called the attacks necessary to prevent Iran from building a nuclear weapon, crediting the U.S. mission as “taking the ‘bomb’ right out of their hands.” On June 19, the White House press secretary insisted that Iran would only need “a couple of weeks to complete the production of that weapon, which would, of course, pose an existential threat not just to Israel but to the United States and to the entire world.” Israel’s leaders agreed and Mossad, Israel’s intelligence agency, reported that Iran could achieve a nuclear weapon in 15 days.
But is Iran actually close to becoming a nuclear power?
Not everyone agrees on this point. Until recently, U.S. intelligence agencies uniformly maintained that Iran has yet to decide whether to make a nuclear bomb. Senator Chris Murphy (D-Conn.), who sits on the Senate Foreign Relations Committee, wrote on social media on Saturday night: “I was briefed on the intelligence last week. Iran posed no imminent threat of attack to the United States. Iran was not close to building a deliverable nuclear weapon.” And while the International Atomic Energy Agency (IAEA) has censured Iran for its nuclear program, the U.N. watchdog also said that it “did not have any proof of a systematic effort to move into a nuclear weapon.”
The key to understanding these contradictory assessments lies in Murphy’s statement that Iran is nowhere close to building a deliverable nuclear weapon. To understand the status of Iran’s nuclear efforts, it’s important to differentiate among three critical components of nuclear weapons: the production of weapons-grade fuel; the fabrication of a workable nuclear device; and the presence of nuclear delivery systems. We do know that Iran has made considerable progress on the first of these components. However, Iran’s capacity to produce either a nuclear device or delivery system is debatable. Here is what we know.
Fueling the bomb
The first component to building a nuclear weapon is suitable fuel, either uranium or plutonium (we will leave tritium and deuterium, fuel necessary for thermonuclear weapons, out of this discussion). Uranium is a naturally occurring element, and uranium ore can be mined. Plutonium is produced in nuclear reactors, a byproduct of the fission process that creates power.
Whether a nuclear aspirant chooses to use uranium or plutonium is largely a matter of access – not every country has access to uranium mines or nuclear power plants. Iran has one operating commercial nuclear reactor, Bushehr, which is monitored by the IAEA and fueled by uranium produced in Russia. Israel attacked and damaged Iran’s heavy water reactor in Arak, which could have produced plutonium, but was not operational: Iran had agreed to redesign the reactor after the 2015 Joint Comprehensive Plan of Action.
Iran extracts a limited amount of uranium through two mines. This uranium ore typically contains two isotopes, U-238 and U-235. U-238 is far more abundant, making up 93% of the mined uranium. But, unlike U-235, it is not “weapons grade.”
For a fission bomb to work, a supercritical mass of fissile material must undergo a rapid, uncontrolled chain reaction. That means that the mined uranium needs to be transformed into highly enriched uranium (HEU), to the point that over 90% of it is U-235. Most programs rely on centrifuges, which spin at high speeds to separate and remove U-238, leaving the fissile material behind.
Iran’s uranium enrichment program goes back decades
In 2002, an Iranian dissident group revealed the existence of Natanz, Iran’s main enrichment site. In 2009, Iran declared the presence of a second enrichment site at Fordow, informing the IAEA just days before the U.S. and U.K. planned to expose the news of a second site at a G20 summit.
Iran has always claimed that its uranium enrichment is legal under the Nuclear Non-Proliferation Treaty, because it is designed for peaceful purposes, such as medical research. Much of Iran’s behavior, however, undercuts these claims. Fordow, for example, was built clandestinely and buried under a mountain. And Iran’s enrichment of uranium to 60% U-235 goes well beyond the 20% HEU level needed for medical research.
It is in uranium enrichment that Iran has made the most progress towards a nuclear weapon. Iran has reportedly stockpiled enough uranium, enriched to 60%, to fabricate nine to ten weapons. To do so, Iran would have to enrich that uranium further, to more than 90% HEU, a process that would take two to three weeks – hence the language that Iran could have a “weapon” in a short time span.
Going critical: the importance of the nuclear device itself
But it is not enough to have weapons-grade fuel. The tricky part of a nuclear explosion is to generate that “rapid, uncontrolled chain reaction” at the right place and right time. And that means building a device designed to do just that. When the U.S. built the first atomic weapons, scientists relied on two different designs.
The first, the type of bomb the U.S. dropped on Hiroshima on Aug. 6, 1945, was a “gun-type,” which fires one subcritical mass of HEU into another, combining them into a supercritical mass that then initiates the uncontrolled chain reaction. This is the most simple, and also the most inefficient of nuclear devices – think of a bullet firing into a target. The second design uses implosion to create the supercritical mass. This nuclear weapon can use U-235 as fuel, but more commonly involves a plutonium “pit” surrounded by high explosives. These high explosives compress the pit into a supercritical mass; at the same time, a burst of neutrons starts the reaction at its peak density.
Unlike Iran’s status in enriching uranium, there is less certainty on its progress in nuclear weapons design. There is considerable evidence that Iran was working on weaponization technology, such as neutron initiators and high-explosive compression tests, through 2003, but the current status of that research remains unclear. Israeli intelligence reported that Iranian scientists had conducted successful experiments in the design of a nuclear weapon, a factor that may have helped push the Israeli government to strike Iran’s program now, rather than waiting for negotiations.
Our knowledge of Iran’s nuclear device research is probably best summed up through the IAEA director general’s assessment: “Certainly, it was not for tomorrow, maybe not a matter of years… But this is speculation.”
Most experts believe that if Iran were to build a device quickly, it would likely be the most primitive, gun-style weapon described above. How dangerous would that be? When the Enola Gay dropped the gun-style atomic bomb on Hiroshima, it produced the equivalent of 15 kilotons of high explosives, a fireball with a temperature of 1 million degrees Celsius at its epicenter. Blast waves, then the fireball, and then fire itself consumed the city. Many who were not killed instantly suffered radiation sickness. An estimated 140,000 people died from the Hiroshima bombing.
But to cause that type of damage, Iran would need a viable delivery system. And this is where the story gets even more complicated.
Delivering the bomb
To truly have nuclear weapons capability, a country has to be able to deliver a nuclear weapon to a target. “Little Boy,” the device used at Hiroshima, was anything but. It was ten feet long, and weighed almost 10,000 pounds. “Fat Man,” the bomb the U.S. dropped on Nagasaki three days later, was even larger. The standard U.S. bomber at the time, the B-29, could not accommodate the weapon, and had to undergo extensive modifications to the bomb bay, release mechanisms, and other structural components of the aircraft. Today the U.S. still relies on nuclear-capable bombers, such as the B-52 and the B-2 models. Both aircraft types can carry conventional weapons as well. The U.S. nuclear strategy combines these delivery capabilities with intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs), in what is known as the “nuclear triad.”
It is in developing a delivery system where Iran faces the most difficulty. Iran has significant missile capability, to be sure: It has the largest and most diverse missile arsenal in the Middle East, with thousands of ballistic and cruise missiles, some capable of striking as far as Israel and southeast Europe. On Monday, in fact, Iran launched a retaliatory strike against a U.S. base in Qatar. In January 2020, Iran launched ballistic missiles at U.S. troops in Iraq in retribution for the U.S. killing of General Qasem Soleimani, injuring more than 100 U.S. service personnel. Iran has also been using its missiles to attack Israeli cities.
Iran’s missiles can’t carry a nuclear bomb
But the key point here is that any nuclear device Iran develops in the short term will be far too large to put on a missile. “Miniaturization” would require would require Iran to develop thermonuclear capability, a far more complex process than the fission weapons described above.
In fact, any initial weapon Iran develops will be too large to fit on any of Iran’s military aircraft. And even if Iran managed to modify aircraft to deliver a nuclear device, a bomber would be too slow and visible to evade air defenses. When the U.S. bombed Hiroshima and Nagasaki, Japan lacked any form of air defense to stop American aircraft.
Thus, Murphy’s phrase, “deliverable nuclear weapon,” conveys the crux of the issue. Yes, it was entirely likely that Iran could have produced enough HEU for nuclear weapons within a few weeks. It is unclear where the 60% enriched uranium is now. The U.S. struck the site in Isfahan where the material was suspected to be housed, but this uranium, stored in scuba tank-sized cylinders, was easily moveable. And experts believe the material was moved prior to the strike.
In any case, there is no credible threat of a nuclear strike by Iran. How this important point shapes the assessment of Israeli and U.S. decisions to attack Iran’s nuclear program is a topic for another post.
