Addressing Iran’s Plutonium Pathway

Editor’s Note: One sticking point in U.S.-Iran discussions is the fate of Iran’s nuclear program. Although most attention focuses on stopping Iran from enriching uranium, my Center for Strategic and International Studies colleague Bailey Schiff argues that the United States must also block the plutonium pathway, ensuring that Iran does not use fuel reprocessed from the Bushehr Nuclear Power Plant to build nuclear weapons. 

Daniel Byman

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In June 2025, Israel struck and destroyed Iran’s Arak heavy-water reactor, the site every previous negotiation treated as part of Tehran’s pathway to a plutonium bomb. With Arak gone and Iran’s enrichment program set back, some analysts have turned to the country’s remaining plutonium source: the 210 metric tons of spent fuel at the Russian-staffed Bushehr Nuclear Power Plant. A recent Bulletin of the Atomic Scientists exchange and subsequent commentary in The Hill claimed Iran could produce more than 200 nuclear weapons from that fuel. Yet spent fuel reprocessing was absent from the June 2026 Islamabad Memorandum of Understanding, which centered on uranium enrichment, and the Trump administration has said little about the issue since. So, how should a future deal address Iran’s plutonium route?

Today, Tehran’s path to a plutonium weapon would be slow and detectable, limited by the difficulty of scaling up reprocessing, acquiring controlled equipment under sanctions, and rebuilding lost weapons expertise. But with enough time and will, Iran could erode these barriers, and if its relationship with Russia deepens, Moscow could ease that effort rather than obstruct it. Once negotiations resume, Washington should constrain the plutonium route by committing Iran to ship out spent fuel and accept a verifiable ban on reprocessing.

What Plutonium Weaponization Would Require Today

For two decades, Iran’s route to a plutonium weapon relied on the heavy-water reactor at Arak, spent fuel from which would have yielded weapons-grade plutonium. The 2015 Joint Comprehensive Plan of Action (JCPOA) required Iran to neutralize Arak by filling the reactor core with concrete. The United States backed out of the JCPOA in 2018 without a clear alternative in place to halt Iran’s nuclear program. Israel then destroyed the reactor in June 2025 and struck the adjacent heavy-water production plant in March 2026, leaving the reactor-grade material in Bushehr’s spent fuel as the country’s remaining plutonium source.

Spent fuel from a reactor like Bushehr yields reactor-grade plutonium, a poorer weapons material than what a heavy-water reactor like Arak would produce, but is weapons-usable in a pinch. Turning the spent fuel into a device would mean overcoming three obstacles: scaling plutonium separation from lab traces to weapons quantities, procuring the specialized equipment that sanctions bar Iran from buying, and rebuilding the weapons expertise that strikes and assassinations have eroded.

Iran demonstrated it was capable of the underlying chemistry required to separate plutonium-239 from spent fuel during reprocessing experiments at the Tehran Nuclear Research Center from 1988 to 1993, where it recovered about 100 milligrams. To put this in perspective, this is roughly five orders of magnitude below the International Atomic Energy Agency’s (IAEA’s) significant quantity of 8 kilograms for a weapon. Iran has not closed that gap in the decades since because scaling to weapons quantities requires an industrial capability that Iran lacks. Its one dedicated reprocessing site, the Molybdenum, Iodine, and Xenon Radioisotope Production (MIX) Facility, never became operational after its 2005 completion, reportedly because Iran could not procure components for hot cells, the shielded chambers needed to handle radioactive material. The leap to weapons quantities would require Iran to both separate plutonium from intensely radioactive power-reactor fuel, which is far more hazardous than the lab targets Iran worked with, and build the shielded infrastructure to handle it, neither of which it has done.

Today, procuring specialized equipment required for separating plutonium—including shielding, remote-handling tools, and corrosion-resistant materials—would be even more difficult. Export controls under the Nuclear Suppliers Group trigger list, together with UN sanctions restored by the 2025 snapback and standing U.S. measures, would force Iran to rely on the same front-company networks its Atomic Energy Organization used for Arak. It could likely obtain some of this equipment, but not quickly or without the procurement signatures that make a covert effort difficult to hide. Alternatively, Iran could improvise a crude plant in a small warehouse, using repurposed oil-industry equipment, though a U.S. Government Accountability Office review estimated it would take two years to build such a plant capable of separating plutonium. And even that assumes conditions Iran does not have, since it would be attempting all of this under sanctions, in a war-torn country facing its own economic crises. Iran must also recognize that the United States or Israel would likely bomb such a crude plant if they discovered it.

Even if Iran were able to separate plutonium-239 from the spent fuel, it would face the bigger issue of whether it retained enough indigenous knowledge from its historical weaponization work to convert the plutonium into a workable nuclear device. It would have to cast the plutonium into metal, machine it into a precise core, miniaturize a warhead to fit existing medium-range ballistic missiles, and engineer the device that detonates it, each requiring specialized skills built over years. Iran developed a foundation for some of this under its pre-2004 Amad nuclear weapons program, documented in the 100,000-page archive Israel seized in a 2018 Mossad operation. Still, to guard against leaks, Iranian leadership confined that work to a small cadre of scientists whom Israel targeted for years, from the 2020 assassination of Mohsen Fakhrizadeh through the 2025 and 2026 strikes that killed additional scientists and engineers, thinning what was already likely a small pool of personnel with expertise in later-stage weapons-related activities. Recent strikes also destroyed nuclear facilities, and with them, likely, some records of work. Whether enough people, equipment, and documentation survived to reconstitute work is unknown.

With time and will, Iran could erode these technical barriers, though not without warning signs. But first, Iran would have to divert the material from Bushehr, where, unlike at Tehran’s uranium enrichment and conversion sites at Natanz, Fordow, and Isfahan, Russia plays a role in its operation and fuel access.

Watching Russia

Bushehr is Iran’s facility, but Russia is deeply woven into it. Rosatom, Russia’s state-owned nuclear corporation, deploys about 600 Russian personnel to assist Iranians with daily operations, is building two more units at the site under a multibillion-dollar arrangement, and supplies the fuel. Today, Moscow has commercial and strategic reasons to constrain Iranian nuclear activity, at Bushehr and more broadly. But if the partnership deepens, a Russia more aligned with Tehran could stop constraining Iran, or even help it erode the technical barriers. This scenario should give Washington pause.

A 2005 nuclear fuel agreement obligated Russia to take back Bushehr’s spent fuel to prevent reprocessing, but that never happened, so roughly 210 metric tons have since accumulated in the plant’s spent fuel pool, an amount Rosatom’s Alexey Likhachev confirmed in March 2026. Much of the delay is unavoidable, as freshly discharged fuel must cool for years before shipment. Estimates of how long have varied; Russian officials told U.S. counterparts in 2005 three to five years, a later Moscow statement said around 10, and Bushehr’s own storage pool design assumed roughly eight. The plant first discharged spent fuel in 2014, so even under conservative estimates Bushehr’s earliest assemblies presumably reached the point where transport became feasible in the early to mid-2020s. The rest of the lapse likely reflects a lack of will on both sides to resolve existing technical, financial, and safety concerns around shipment, which stalled the return clause for years during negotiations. And unlike most of Rosatom’s customers, who want the spent fuel gone, Iran has reason to keep it.

Regardless of the timeline for fuel return, Moscow is not indifferent to what becomes of it. Bushehr is one project in Rosatom’s reactor-export business, which spans roughly a dozen countries and rests on a promise to other host governments that a Russian-built reactor forecloses proliferation risks by supplying the fuel and taking it back. An Iranian diversion at the facility would undercut those guarantees, Moscow’s standing in the Gulf, and its efforts to portray itself as a responsible voice on non-proliferation. Notably, Russia pressed Iran to readmit inspectors at Bushehr after the 2025 U.S. strikes, and whether Moscow acted out of a standard Rosatom legal obligation or in response to these concerns, Bushehr has become the only site open to the IAEA in the entire country. The agency verified the plant’s declared design as recently as June 2026. Should Iran show signs of noncompliance, Russia also maintains leverage in the fuel it supplies, which it exercised in 2007 when it threatened to withhold deliveries unless Iran suspended enrichment and met UN demands.

Analysts and former Russian officials have claimed for years that the Kremlin prefers Iran as a nuclear threshold state with international uncertainty about its program to a nuclear-armed Iran or a pro-Western Iran. Nevertheless, this relationship is changing, and as the partnership deepens, Moscow could come to see a nuclear-hedging or nuclear-armed Iran as beneficial. The two countries have grown closer since Russia’s 2022 invasion of Ukraine, bound by shared antagonism toward the West and mutual arms transfers. Iran supplied Moscow with Shahed drones, Russia helped Tehran develop ramjet engines for supersonic cruise missiles, the two signed a 20-year strategic partnership, and Moscow provided Iran with intelligence to target U.S. forces in March 2026. While Russian support in the 2025 and 2026 wars was more limited than some expected, the Iranian regime’s survival points toward deeper military cooperation. A Moscow more aligned with Tehran could aid its nuclear program. One path for Russia to do so is diplomatic protection in international fora, which is already underway. Just this year, Russia opposed IAEA Board of Governors resolutions finding Iran noncompliant in June and September, and in September joined China in vetoing the UN Security Council Panel of Experts extension, ending the only independent body monitoring Iran sanctions.

Alternatively, Russia could go further and actively lower the barriers to a nuclear weapon Tehran might otherwise face. Moscow has shown with North Korea a willingness to trade sensitive nuclear-relevant technology for wartime support, and while specifics on nuclear collusion between the two countries remain hazy, it is not unimaginable that Moscow would help Iran acquire sanctioned reprocessing equipment in the future.

However unlikely today, both prospects are reasons for U.S. negotiators to include the plutonium route in a future deal.

Constraining Iran’s Plutonium Pathway

When negotiations resume, the U.S. delegation’s focus will be, and should be, on Iran’s uranium enrichment route, as President Trump, members of the negotiating team, and the June Islamabad memorandum have underscored. But Tehran could also pursue the plutonium pathway. As a result, negotiators must secure a verifiable ban on reprocessing, which requires an agreement to capture an explicit prohibition while also restoring IAEA authorities that lapsed in 2021. When Iran suspended the Additional Protocol and Modified Code 3.1 that year, it cut off access to undeclared locations and environmental sampling, and the IAEA has since said it cannot verify whether Iran suspended reprocessing activities. A deal must restore both authorities so that the IAEA can resume inspections nationwide. It must also establish specific provisions on limits to reprocessing similar to the JCPOA’s Annex I Section E on spent fuel reprocessing activities, which limited hot cells and gloveboxes, barred plutonium and uranium metal production, and restricted activities at the Tehran Research Reactor, the MIX Facility, and the Jabr bin al-Hayyan Laboratory. Finally, a deal should require Iran to ship out all irradiated fuel on its territory. Since most of this fuel sits at Bushehr, ideally Russia would take it back, with later-stage talks establishing a defined schedule based on the fuel’s cooling timeline.

Leaving Iran’s plutonium pathway unaddressed offers Iran a potential route to a nuclear weapon if it can circumvent sanctions and its ties to Russia grow. A future agreement should include the necessary, and straightforward, steps needed to close off this pathway from the start.