North Korean leader Kim Jong Un watches the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) released in September 2017.
North Korean leader Kim Jong Un watches the launch of a Hwasong-12 missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) released in September 2017.

In January 2004, the director of North Korea’s Yongbyon Nuclear Scientific Research Center handed me a sealed glass jar with plutonium metal inside in an effort to convince me that his country had a nuclear deterrent. To make the same point last week, Pyongyang lofted a missile 2,800 miles into space and declared it had a nuclear-tipped missile that could reach all of the United States. Has the country’s nuclear program really come that far?

As global anxiety over North Korea grows and the war of words between U.S. President Donald Trump and North Korean leader Kim Jong Un escalates, it is more important than ever to be precise about what we know, and what we don’t, about Pyongyang’s nuclear weapons program and delivery systems. In 2004, nothing I saw on my visit persuaded me that Pyongyang could build a bomb and deliver it. But more recent visits, along with several kinds of open-source analysis, leave little doubt of North Korea’s impressive progress in producing bomb fuel, building powerful nuclear devices, and test-launching a wide variety of missiles—and its determined efforts to integrate all three into a nuclear-tipped missile.

Extensive experience with shorter-range missiles and 11 years of nuclear tests most likely enable North Korea to mount a nuclear warhead on missiles that can reach all of South Korea and Japan. That capability, along with massive artillery firepower trained on Seoul, should be enough to deter Washington. By my assessment, however, North Korea will need at least two more years and several more missile and nuclear tests before it can hit the U.S. mainland.


A credible nuclear deterrent requires not just fuel for a nuclear bomb, but also the ability to weaponize (that is, design and build the bomb) and to field delivery systems that can get the bomb to a target. It also requires demonstrating these capabilities—and the will to use them—to an adversary. There may be little doubt of Kim’s willingness to use a nuclear weapon if the situation required it. Assessing his exact capabilities, however, has been a greater challenge, even for the U.S. government.

Pyongyang has often aided such efforts by allowing peeks at its key assets. It has built much of its nuclear and missile complex in full view of satellites and routinely released footage of its leaders’ inspections of weapons and facilities. It has also allowed foreign, nongovernment specialists to visit those facilities. My assessment of North Korean capabilities is based on my own seven visits and ongoing analysis of all open-source information.

There are two basic types of nuclear fuel: plutonium, which is produced in reactors, and uranium, which is enriched to weapon grade in centrifuges. North Korea’s plutonium inventory can be estimated with high confidence because the design details of Yongbyon’s 5-megawatt reactor are well known, and its operation is easily monitored by commercial satellite imagery. International teams have inspected North Korea’s reactor complex during times of diplomacy, and I have visited the plutonium facilities and met Yongbyon’s very capable technical staff several times. I estimate that North Korea has 20 to 40 kilograms of plutonium, sufficient for four to eight bombs.

Estimates of highly enriched uranium are much less certain. Centrifuge facilities are virtually impossible to spot from afar. Yet in November 2010, during my last visit, North Korea allowed me to view its recently completed modern centrifuge facility. (To my knowledge, no outsider aside from those on our small Stanford University team has seen this or any other North Korean centrifuge facility.) Based on that visit, satellite imagery, and probabilistic analysis of the import and production of key materials and components, I estimate that North Korea has 250–500 kilograms of highly enriched uranium—sufficient for roughly 12 to 24 additional nuclear weapons. (This assumes the existence of one or more covert centrifuge facilities, necessary for testing technology before deploying it in the large-scale facility I saw.)

North Korea likely has the ability to produce a small number of hydrogen bombs.

North Korea also likely has the ability to produce a small number of hydrogen bombs. These require heavy forms of hydrogen—deuterium and tritium—for the fusion stage of the device, which is triggered by a plutonium or uranium fission bomb. North Korea has demonstrated the ability to produce deuterium and tritium, as well as a lithium compound, Lithium-6 deuteride, which can produce tritium in situ in the fusion stage of a hydrogen bomb’s detonation.


Since 2006, North Korea has conducted six underground nuclear tests. Seismographs around the world have picked up the tremors, allowing estimates of the likely explosive power of each bomb. Two of the most recent tests, in 2016, have had a destructive power of 10–25 kilotons, equivalent to the bombs that destroyed Hiroshima and Nagasaki. The sixth test, on September 13, 2017, was 10 times stronger, with a probably explosive power of 200–250 kilotons—suggesting the successful detonation of a two-stage hydrogen bomb. (Pyongyang’s claims that its fourth test, in January 2016, was a hydrogen bomb did not appear credible at the time.) A few hours earlier, the government had released photos of Kim with a mock-up of such a device. Though such designs are generally considered to be among any government’s most closely guarded secrets, North Korea has publicized them more than once.

This record of tests conclusively demonstrates that North Korea can build nuclear devices with the power of the fission bombs that destroyed Hiroshima and Nagasaki, as well as bombs with the destructive power of modern hydrogen bombs. Given that record, and estimates of nuclear materials inventories, I estimate that the upper range of nuclear materials inventories is sufficient for roughly 25 to 30 nuclear weapons, with an annual production rate of 6 to 7. (David Albright of the Institute for Science and International Security has come up with a similar estimate: 15 to 34 weapons and annual production rates of 3 to 5.) This assessment is lower than a leaked U.S. intelligence community estimate of 60 weapons.

A view of the newly developed intercontinental ballistic rocket Hwasong-15's test that was successfully launched is seen in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang, November 2017.


It is another question whether those weapons are small enough to fit on short- and long-range missiles. (Official photos of nuclear devices are strategically positioned in front of diagrams of re-entry vehicles, but there is no way of being sure that the photographed devices are really identical to those tested, whatever the claims from Pyongyang.) For many years, North Korea’s missile program appeared to lag far behind its nuclear advances. Although the acquisition and development of short-range missiles dates back to the mid-1980s, work on longer-range systems has started to speed up only recently. In the past two years, North Korea has test-fired more than 40 missiles, most of which were of intermediate or long range.

Today, missile tests are the most visible part of North Korea’s nuclear weapons quest. Successful launches are easily picked up by international monitors and featured in official North Korean photos and videos, many showing Kim Jong Un present and in charge. In July 2017, North Korea passed an important milestone with the test of two Hwasong-14 missiles—intercontinental ballistic missiles, or ICBMs, that have a range greater than 3,400 miles. Last week, it tested an even more powerful missile, a Hwasong-15, with an estimated range of 8,000 miles, capable of reaching the entire continental United States. Such tests have been accompanied by diversification of North Korea’s missiles, allowing it to progress toward a stated goal of launching at any time and from any place, including submarines.

Such impressive progress at producing fuel, building devices, and launching a wide variety of missiles begs the question of whether North Korea can put it all together in a single package that can deter Washington. At the time of my 2004 visit, the leadership in Pyongyang may have believed that a handful of primitive bombs was deterrent enough. By 2009, it felt the need to conduct a second nuclear test to prove it had a working bomb. More recently, it has focused on missile delivery of growing reach. And this year, as leadership in Washington changed, it focused on a more ambitious goal: demonstrating the ability to reach the entire United States with an ICBM, possibly one tipped with a hydrogen bomb.

There is little doubt that North Korea could mount a nuclear warhead on a missile that could reach South Korea or Japan.

There is little doubt that North Korea could mount a nuclear warhead on a missile that could reach South Korea or Japan. But ICBMs require smaller and lighter warheads that are nonetheless robust enough to survive the entire flight trajectory, including re-entering the atmosphere. And acquiring that capability will, by my estimate, take at least two more years of tests.


How has North Korea, one of the most isolated countries in the world, been able to make such progress? It got some outside assistance. Beginning in the 1960s, the Soviet Union helped Pyongyang pursue peaceful applications of nuclear technologies and educated its technicians and scientists. After 1991, collaboration with Russian and possibly Ukrainian missile factories continued for some time, and North Korea has also taken advantage of a leaky international export control system to acquire key materials for the production of fissile materials, particularly for gas centrifuges to enrich uranium. But for the most part, Pyongyang has built its nuclear facilities and bombs on its own. Its program is now mostly self-sufficient.

After the most recent missile test, North Korea declared that it had achieved its “goal of the completion of the rocket weaponry system development” needed to deter U.S. aggression. Domestically, this was an important milestone, because the regime had stated in 2013 that it would develop a nuclear deterrent so it could turn its focus to economic development. With this achievement, will Kim be ready to engage in diplomacy with Washington? Although he needs more time in order to be able to credibly threaten the entire continental United States, the fact that Kim can already inflict enormous damage on American allies and bases in Asia may give him sufficient assurance to start a dialogue, in an effort to reduce current tensions and head off misunderstandings that could lead to war.

Washington should be ready to reciprocate—or if necessary, to initiate the discussion. Talking would not represent a reward or concession, or a signal of U.S. acceptance of a nuclear-armed North Korea. It would instead be a first step toward reducing the risks of a nuclear catastrophe and developing a better understanding of the other side. Ultimately, that understanding may even help inform a negotiating strategy to halt, roll back, and eventually eliminate North Korea’s nuclear program.

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  • SIEGFRIED S. HECKER is a Senior Fellow at Stanford University’s Center for International Security and Cooperation and former director of the Los Alamos National Laboratory.
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