How quickly could Iran build its first nuclear weapon? Look at China

The first Trump administration left the 2015 Joint Comprehensive Plan of Action (JCPOA)—also known as the Iran nuclear deal—in May 2018, significantly reducing the constraints on and monitoring of Iran’s nuclear program. Since mid-2019, Iran has significantly shortened its “breakout time”—the amount of time needed to produce enough weapons-grade highly enriched uranium (HEU) to build a nuclear weapon to mere days.

How quickly could Iran make an atomic bomb once it has acquired enough weapons-grade uranium? Some nuclear experts argue it would take Iran anywhere between several months to up to a year. But China’s experience shows that Tehran could build a bomb much faster—in as little as three to five weeks.^[1]

Weaponization and fissile materials production. Aside from acquiring the necessary fissile materials, making a nuclear warhead involves several crucial steps. These include converting uranium hexafluoride gas (UF6) to metallic form and then casting and machining the metal into the core of a bomb. Other non-nuclear components would also need to be ready before a warhead could be assembled. Such non-nuclear components include a weapon design, a neutron initiator, a detonation wave focusing system, high explosives, detonators, and an arming, fusing, and firing system. The experience of other nuclear weapon states shows that these non-nuclear components can be built in parallel with the production of nuclear weapons fuel. This means that they don’t require additional time.

When China produced its first batch of weapons-grade HEU in January 1964 for its first nuclear test that fall, all the non-nuclear components needed for its two bombs were already ready by 1963.^[2] At that time, the weaponeers were just waiting for enough HEU to be produced for those first two bombs. Once the HEU was produced, Chinese scientists and engineers spent one to two weeks converting UF6 gas into metal and purifying it and another two to three weeks for metal melting and pit casting steps. After that, they fabricated the first bomb core (made of two hemispheres) in a matter of hours in the early morning of May 1, 1964.

In early August, the Chinese weapon engineers started the final assembly of the two bombs, which they completed on August 20, 1964, at the Qinghai nuclear weapon plant. It then took only about three days to finish assembling the first bomb. After initial assembly trials, they disassembled the two bombs to prepare them for shipping to the Lop Nur test site in Xinjiang. The bomb was then reassembled in about 10 hours at the Qinghai site and prepared for China’s first nuclear explosion. In total, it took China only some three to five weeks to convert the UF6, cast pieces of metal, fabricate the core, and assemble an atomic bomb.

China built its first bombs some 60 years ago, when it lacked advanced equipment. Since then, Iran has had plenty of time to design a smaller and lighter warhead, more powerful explosives, and more advanced focusing systems. Moreover, China’s weapon was made during peacetime. If China’s weaponeers had been under a tightened schedule or pressured during wartime, they would likely have made a nuclear weapon in less than three weeks.

Portions of Iran’s “nuclear archive,” which Israel seized in January 2018, clearly show Iran’s weaponization projects running in parallel with HEU projects under the “Amad Plan,” Iran’s code name for a crash nuclear weapons program that operated from 1999 to 2003. The Amad Plan aimed to produce five 10-kiloton nuclear weapons by early 2004, including four deliverable warheads for the Shahab-3 ballistic missile, and another one for testing underground.

China’s and Iran’s nuclear bomb similarities. Iran’s early weapons designs were similar to major design features of China’s first atomic bomb (coded as device 596 and exploded in 1964) and its first missile warhead (coded as warhead 548 and tested in 1966).^[3] Both atomic bombs featured an implosion-type warhead design with a levitated HEU core, a uranium deuteride neutron source, and an advanced detonation wave focusing system.

Many Western experts^[4] often assume that China designed its first atomic bomb with the same major features as the United States’s Fat Man bomb, which exploded over Nagasaki on August 9, 1945, including the neutron initiator, the detonation wave focusing system, and the (non- levitated) core design (except for the fissile material, which was plutonium in Fat Man’s case). Newly available Chinese sources, however, show that China’s initial design was very different from that of Fat Man.

Meanwhile, China focused on designing the detonation wave focusing system—a key technical challenge for the implosion-type bomb. From the beginning, Chinese weaponeers focused on developing two focusing systems: One was the same explosive lenses as used in Fat Man. Another was the detonation wave focusing system (also referred to as a “tile” focusing system), which, in Chinese, referred to a distinct roofing tile with a special space curve, made only with high-burning explosives and a thin metal tile (hence, its name).

China’s weapon designers ultimately selected the “tile” focusing system for China’s first atomic bomb. They believed the explosives lens approach was easier to achieve, but the available high- and low-speed explosives would make the explosive lens system a “bigger size, very stout, and very bulky.” On the contrary, the tile-focusing method was easier to weaponize, although it was much more difficult to shape into the complex space curve of the metal shell. Weaponeers decided to tackle the advanced method of tile focusing as the main target and used it in China’s first atomic bomb.

To make this first nuclear bomb smaller and lighter, China used a levitated core, instead of Fat Man’s solid core. The “levitated” core design separates the fissile material pit from the tamper layer (or “flying layer”) with an air gap. (Fat Man’s solid plutonium pit was surrounded by a close-fitting tamper instead.) This type of air gap efficiently compresses the fissile pit, which allows it to achieve a higher explosive yield. To design this levitated core, Chinese weaponeers conducted many detonation experiments to determine the optimal dimensions of the air gap and a way to connect the suspending fissile pit with the flying layer without disturbing the implosion process. These advanced design features allowed China’s first atomic bomb to have a physics package that weighed less than half that of Fat Man’s for a similar test yield of about 20 kilotons.

It is not clear if Iran’s bomb designs—past and present—have been based on a foreign atomic bomb design. The nuclear black-market network of physicist Abdul Qadeer Khan—considered the father of Pakistan’s atomic weapons program—reportedly transferred the design for Chinese warhead 548 to Libya.^[5] However, it is not clear if Iran ever received the warhead 548 design.

China’s insights. Given the similarities between Iran’s bomb and China’s 596/548 atomic bomb designs, the technical bottlenecks and timelines of China’s bomb development can shed light on Iran’s possible bomb development timelines.

The major hurdles China faced when making its first atomic bomb included the weapon design, neutron sources, the detonation wave focusing system, “cold” (or subcritical) tests, and uranium metal components production. All these are non-nuclear weapon components tasks that can be completed before HEU production, the most time-consuming step in the Chinese atomic bomb program. Once China produced HEU, it took about three to five weeks from having sufficient UF6 gas to an assembled uranium bomb.

Based on Chinese experience and examination of seized Iran’s nuclear archive,^[6] I assessed that at the close of the Amad Plan in 2003, Iran had already made substantial progress in almost all other aspects of nuclear weaponization—including on the weapon design, neutron initiator, detonation wave focusing system, cold testing, casting and machining, and integration of warheads and reentry vehicles. Iran has probably made more significant progress on those projects over the past 20 years. Archives show that, after 2003, other Iranian organizations continued to work on nuclear weapons programs with a smaller, more dispersed effort. Although it is unclear how much effort Iran has put into its weaponization programs since 2003, after the failure of the Iran nuclear deal in 2018, Iran likely sped up its weaponization efforts.

After HEU fuel production creates a breakout amount of fissile material, the Chinese case shows that the time to make a bomb depends only on how quickly weaponeers can convert the fuel into metal and cast it as a bomb core. It can be assessed with some certainty that Iran would take no more time than China to complete this final step—probably less than three weeks.

Before signing the Iran nuclear deal in 2015, the US government assessed that Tehran had not mastered the necessary technologies to build a nuclear weapon. In July 2024, however, the US Office of the Director of National Intelligence determined that Iran had “undertaken activities that better position it to produce a nuclear device, if it chooses to do so.” This new US assessment confirms that Iran has significantly advanced its nuclear program over the last decade—and probably accelerated it since 2018.

Addressing Iran’s rapid nuclearization. There is no public evidence that Iran has made up its mind to build a bomb. But the recent weakening of its conventional deterrence against Israel’s attacks on its territory as well as on its allies, Hamas and Hezbollah, may motivate Tehran to pursue a nuclear deterrent. Iran is becoming a de facto nuclear threshold state: Should Iran decide to go nuclear, even as it faces the risk of Israeli or US strikes on its nuclear facilities, it will be able to make its first bombs quickly and secretly.

Since 2022, Iranian officials have talked increasingly openly about its nuclear bomb prospects and technical readiness for a weapon. Most recently, Kamal Kharrazi, an adviser to Iran’s Supreme Leader Ayatollah Ali Khamenei, stated that Iran may review its nuclear doctrine. Meanwhile, some analysts report that nearly 70 percent of Iranians seem to support the idea that the country should possess nuclear weapons.

If the new Trump administration still hopes to prevent Iran from building nuclear weapons, its best bet is to resume direct bilateral talks—either privately or publicly. Military strikes on Iran’s nuclear facilities would not prevent Iran’s nuclear proliferation goals in the long term (unless Iran is occupied permanently or its regime changed) and could drive Iran even further toward the bomb—if it has not decided to build one already.

Notes

[1] This article is based on a full report available on NPEC’s website: Hui Zhang, “Lessons from China: How Soon Could Iran Get the Bomb?” Nonproliferation Policy Education Center, December 11, 2024. https://npolicy.org/lessons-from-china-how-soon-could-iran-get-the-bomb-occasional-paper-2404/.

[2] For more details about the technical history of China’s nuclear weapon development program (1955-1996), see, Hui Zhang, China’s nuclear weapon development and testing, MIT Press (Cambridge, MA), forthcoming in 2025.

[3] Warhead 548 had a test yield of 12 kilotons. Its design was mainly based on the device 596 design (test yield of 22 kilotons), but with reduced size and weight of the major components (including the HEU core)

[4] E.g., see John Lewis and Xue Litai, China Builds the Bomb, Stanford University Press, 1988.

[5] Thomas Reed and Danny Stillman, A Political History of the Bomb and Its Proliferation, 2009, p.252.

[6] Hui Zhang, “Lessons from China: How Soon Could Iran Get the Bomb?” Appendices.