Nuclear analysts from the US and Russia settled on a new method to account for atomic weapons that may shrink estimates on the size of North Korea’s arsenal and could be used to aid future disarmament.

Accounting for nuclear material stockpiled by countries is at the heart of the global arms-control system and plays a central role in verifying disarmament agreements. Publication of the new model in a forthcoming edition of Janes Intelligence Review comes as diplomats from the two countries with the biggest nuclear stockpiles convene in Vienna to discuss an extension of a treaty to limit the number of deployed weapons.

“You cannot agree to get rid of something unless you know how many there are,” said Robert Kelley, a former nuclear-weapons engineer at the Department of Energy, who helped create the new accounting method with Vitaly Fedchenko, a Russian nuclear physicist who works at the Stockholm International Peace Research Institute.

While figuring out the number of warheads in the arsenals of nuclear states has long been the focus of intelligence and military-planning officials, it’s also increasingly become an important number for researchers and diplomats promoting new approaches to eliminating atomic weapons.

“Accounting for weapons-fissile material is one important piece of the puzzle,” said Alicia Sanders-Zakre, who coordinates research around the Treaty on the Prohibition of Nuclear Weapons. That document is gaining traction, requiring ratification from only 10 more countries for it to come into force.

“This sets up an international framework for the elimination of nuclear weapons and technical research on weapons material accounting helps fill out that framework,” said Sanders-Zakre from the Geneva-based headquarters of the International Campaign to Abolish Nuclear Weapons, which won the 2017 Nobel Peace Prize.

The new model challenges public nuclear stockpile figures that “generally estimate the size and proliferant stockpiles by using simple models, largely ignoring thermonuclear stages and competing demands for nuclear materials and tritium,” wrote Kelley and Fedchenko in their paper, which was reviewed by U.K. defense and security officials to ensure classified information wasn’t divulged.

Using North Korea as a case study, the researchers deconstructed the plutonium and highly-enriched uranium requirements for a two-stage thermonuclear weapon, which differ dramatically from simple single-stage devices modeled in most current studies. Countries with thermonuclear devices — the likes of which Kim Jong Un is now widely suspected to possess — have greater challenges when it comes to managing the material demands of plutonium, uranium and tritium in their weapons, according to the authors.

The new model may prompt security experts to reassess their figures.

“Janes concludes that North Korea’s nuclear arsenal is most likely to be in the range of 10-20 weapons if Pyongyang committed its highly-enriched uranium to thermonuclear weapons production,” the article said, which will appear in the IHS Markit Ltd. publication this week. That figure is at least two-thirds lower than the 60 nuclear-warhead estimate formulated by the Defense Intelligence Agency and published July 14 by the Congressional Research Service.