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Giving North Korea the Bomb

30 November 2010

North Korea has nuclear weapons—or does it? According to the conventional wisdom, North Korea’s October 2006 nuclear test marked the country’s entrée into the nuclear club, and the May 2009 explosion confirmed that it is improving and presumably expanding its small arsenal. Last week’s revelation about North Korea’s uranium enrichment efforts seems to bolster the image of North Korea as a real nuclear weapons state—efficiently plugging away to expand its small but growing arsenal. 

This conventional wisdom may turn out to be right, but it is important to recognize that it goes beyond what the evidence currently shows. While North Korea appears to be making progress toward developing nuclear weapons—along both the plutonium and uranium pathways—it is not clear that Pyongyang has a functioning arsenal. Pyongyang’s tests reveal that North Korea has encountered significant obstacles in its nuclear program. Furthermore, little open-source evidence shows that North Korea has surmounted the other hurdles associated with building an arsenal, such as turning a test device into an actual weapon, or integrating its weapons with reliable delivery systems.

Of course, Pyongyang should be treated with caution. Prudence is warranted by North Korea’s conventional forces, and by the destruction they could wreak upon the South Koreans, the Japanese, and on U.S. forces. Furthermore, even faulty fission bombs could cause a lot of damage in Japan or South Korea. 

But in an era in which halting proliferation is a top foreign policy goal, it is counterproductive to “give” nuclear weapons to North Korea before they have demonstrated that they have mastered the challenges associated with building them.  For North Korea and for most countries, “going nuclear” is costly, takes many years, and requires surmounting a long list of technological challenges. North Korea’s long, winding path toward acquiring nuclear weapons should be a cautionary tale for Iran and others who may be interested in going down this road. And if Pyongyang is peddling its nuclear know-how on international markets, then by exaggerating North Korea’s progress, and by obscuring their setbacks, we may be inadvertently promoting their illicit activities. 

In short, giving North Korea credit for a not-yet functional arsenal obscures important lessons the world should draw from Pyongyang’s experience: going nuclear is a risky and uncertain process. It may result in decades of failure, and decades of international isolation, before a country can produce a reliable arsenal. The revelation of North Korea’s uranium enrichment efforts may be as much a reflection of the technical troubles they’ve experienced with their plutonium devices, as it is a reflection of their continuing commitment to develop an atomic bomb.

Testing, testing…one, two…

North Korea has conducted two apparent tests of nuclear devices, in 2006 and 2009. But what did those tests actually demonstrate?

Governments, international organizations, and researchers monitor nuclear tests through two principal means. First, the explosions create shock waves that propagate through the ground, seas, and air. Seismometers—the same devices used to study earthquakes—measure the shocks and can answer many questions about the events that produced them. For example, the global network of seismometers can pinpoint the location of a blast, estimate the explosive power of the detonation, and differentiate a man-made explosion from an earthquake (although it cannot necessarily differentiate a nuclear blast from a detonation of conventional explosives).[1] 

The second means of detection, and the gold standard for differentiating a nuclear blast from one produced by conventional high explosives, is to sample the air and look for telltale radioactive signs of fission.[2] Sensors are located around the world for that purpose, as well as air-sampling aircraft, which are flown downwind of suspected tests.

The data gathered from North Korea’s first nuclear test—in 2006—reveals that it was only a partial success. The North Koreans clearly achieved fission, meaning that their device successfully split some atoms of plutonium. Seismic monitoring stations around the world registered the explosion, and air samples contained telltale radioactive material, proving that fission occurred.

But the 2006 test also raises serious questions about the competence of the North Korean nuclear program. Prior to the test, Pyongyang reportedly told the Chinese that their device would produce roughly four kilotons of energy (roughly a quarter of the Hiroshima bomb). But data gathered by seismic sensors around the world suggest the actual yield was less than one kiloton.[3] Pyongyang should never have indicated what their device was supposed to produce; in doing so they revealed that their proto-weapon had malfunctioned.

This malfunction is telling. The United States detonated its first plutonium device sixty-five years ago. Creating a plutonium fission device is no simple feat.  It typically involves, among other key steps, surrounding a lump of plutonium with high explosives and simultaneously detonating the various explosive “lenses.” The compression of the plutonium from all sides vastly increases the density of the material beyond “critical mass,” at which point a fission chain reaction can be initiated, and the weapon explodes.[4]  

The United States attempted this first in 1945, so had much more primitive technology to work with—more primitive high explosives, more primitive timing technology, and a first-generation bomb design.  Nevertheless, the so-called “Trinity Test” worked.  And the second time the United States detonated a plutonium device—the so-called Fat Man bomb—it worked as well, destroying Nagasaki in the process.[5] 

The North Korean nuclear effort has undoubtedly involved some of the best and brightest engineers and physicists in their country, and probably used a simple bomb design. Nevertheless, the device malfunctioned. No other countries have had so much trouble with their first plutonium implosion: the Soviets, Chinese, British, French, Israelis, South Africans, Indians and Pakistanis apparently all pulled off plutonium implosion in their first try—but not the North Koreans. It’s unclear which of many possible explanations might tell us what went wrong with the first North Korean nuclear test—but something clearly went wrong.

Round two

North Korea’s second nuclear test in 2009 seems to have shown the country’s progress in its nuclear program. But once again, many questions about its capabilities remain.

The Comprehensive Test Ban Treaty Organization (CTBTO) reported that strangely, none of their sensors detected radioactive evidence of North Korea’s second nuclear test—a striking admission, given the organization’s work to convince the international community that they can reliably detect cheating if the Comprehensive Test Ban Treaty comes into force. Press reports claim that U.S. Air Force aircraft, which were sampling downwind near Korea for precisely this purpose, failed to detect any debris.[6] None of this proves that North Korea faked the nuclear test—e.g., by conducting a big simultaneous detonation of conventional high explosives—but it is possible, and the lack of radioactive emissions is puzzling.

The more interesting result from 2009 is the seismic evidence. Although the North Koreans produced a bigger explosion the second time, it is likely that they failed again to produce the desired yield. Initial reports (e.g., from Russia) suggested that the North Korean blast was between 10-20 kilotons, but those estimates have been widely discredited. The best estimates of the yield of the second device, based on data from many seismic monitoring stations, put the yield of the detonation much lower. One estimate came from Paul Richards and Won-Young Kim, from Columbia University’s Lamont-Doherty Earth Observatory. These experts in seismology and nuclear test verification say the 2009 blast registered 4.5–4.7 on the Richter scale, which corresponds to an explosion between 2.2 and 4 kilotons.[7] The CTBTO, using data from 39 seismic stations, puts the blast at 4.52 on the Richter scale, a number that would put the yield at closer to 2 kilotons—the lower end of the Richards and Kim estimate.[8]

How, then, should we interpret the 2009 test? Let’s assume that North Korea achieved fission (despite the failure of any sensing stations to detect radionuclides from the blast). If Pyongyang was testing a very simple device in 2009—and the simplest designs typically produce 10-20 kilotons—then the second test was another failure. If, on the other hand, they were testing the same basic design as the first test, and intended to produce only 4 kilotons, then they still probably got another fizzle, producing 2.5 to 3 kilotons if the CTBTO data is right (and which would be consistent with the data of Richards and Kim of Columbia University).

A key question, which seems unknowable using unclassified (and perhaps also classified) sources, is what exactly went wrong in 2006, and was it corrected? Even if the second test “worked”—or “worked better”—was it because North Korean engineers discovered and fixed the key problems in the 2006 test, or because they were just luckier the second time? Would a third and fourth test function like the second or first? In the United States (and perhaps every other country), a weapons system would never be deployed with a “1 for 2” testing record. And Pyongyang’s record might actually be worse—if the second test was close to 2 kilotons, or if its goal had been to produce a higher yield than the first test.

But there’s more 

There is a big difference between testing a “nuclear device” and fielding “nuclear weapons.” Nuclear test devices are carefully assembled in a safe, controlled environment. The devices are not jostled; they are protected from dust and the elements. They are not violently rattled. And presumably every aspect of the device is inspected multiple times by teams of designers and engineers. Furthermore, nuclear test devices can be massive—the first U.S. thermonuclear device (i.e., a fusion design) was several stories tall (see photo, below), and hence totally impractical as a weapon.

The “Ivy Mike” thermonuclear (i.e., “hydrogen bomb”) device before the 1952 test (note man seated in chair, center right, for scale). (Photo:

Weapons, by contrast, have to be small—to fit on a military aircraft or on a missile—and they must be engineered to be reliable without teams of engineers constantly fiddling with them. They must survive and function after they’ve been shaken aboard an aircraft, or rattled violently by a ballistic missile launch. Assembling a nuclear device that can be detonated in a hole in the ground during carefully controlled peacetime conditions is thus very different than the challenge of building weapons that are small, deliverable, and highly robust in a real-world context.

The ability to turn nuclear devices into nuclear bombs and warheads is still not the end of the process. Even a small arsenal of nuclear weapons requires that the bombs and warheads be integrated into reliable weapon systems. Ballistic missiles are finicky; even NASA experiences frequent launch delays. Missiles that haven’t been thoroughly tested, and meticulously maintained, will have much lower probabilities of successful launch, guidance, and delivery. 

So What?

North Korea has been working on its nuclear program for many years, and has made real progress. It is clear from the recent uranium enrichment revelations that their leaders are committed to building an atomic arsenal. It’s not at all clear, however, whether Pyongyang has any deployable weapons. Rather than proclaiming North Korea as the world’s latest member of the nuclear club, policy makers and analysts should clarify the facts: North Korea has conducted two nuclear tests. The tests reveal that the program has struggled. It’s not yet clear that North Korea wields any functioning nuclear weapons.

North Korea’s efforts at uranium enrichment may reflect their fear of a U.S. air strike on their nuclear facilities (because uranium enrichment sites are easier to bury and hide than the nuclear reactors that produce plutonium). According to this view, the revelation of the centrifuge site at Yongbyon is a warning that North Korea may have other secret enrichment sites hidden elsewhere. 

A simpler interpretation, however, is that Pyongyang is still having trouble with its plutonium program. Perhaps the anomalies in the 2006 and 2009 tests were caused by problems in plutonium reprocessing. Or perhaps the test failures were caused by problems in implosion designs. Continued frustrations with their plutonium designs could have led North Korean leaders to accelerate uranium enrichment efforts.

All that one can conclude from the newly revealed enrichment facility is that despite all of the censure North Korea has received, North Korea remains committed to building nuclear weapons. But the revelations do not demonstrate that Pyongyang has an operational nuclear arsenal.

So what, one might wonder: what difference should it make for policy making toward North Korea whether the country actually has a functioning nuclear arsenal? Indeed, the mere possibility that North Korea has nuclear weapons warrants treating Pyongyang with considerable caution. Furthermore, even a weapon that fizzled and produced “only” a 1-kiloton explosion would cause terrible damage if successfully delivered to a populated location in Japan or South Korea.

Nevertheless, it’s important to highlight the questions raised by North Korea’s tests, and the hurdles the country still may face in building an arsenal. According North Korea membership in the nuclear club may help it peddle its nuclear know-how on international markets. A more skeptical discussion of North Korean capabilities, by contrast, might cause potential customers to think twice before partnering with Pyongyang. 

Secondly, by acting as if North Korea’s tests were successful, its adversaries obviate the need for further tests of plutonium devices. More testing would eat up more of North Korea’s small stock of plutonium, and might reveal continuing problems with its bomb program. Presumably, an important reason that Pyongyang conducts tests is to demonstrate its nuclear capabilities to potential adversaries. If North Korea’s adversaries prematurely give it credit for having a functioning arsenal, they will permit Pyongyang to conserve its scarce plutonium.

Finally, by giving North Korea the bomb we miss the opportunity to impart an important lesson to potential proliferators such as Egypt, Iran, Saudi Arabia, and Turkey: building these weapons is very hard. It’s hard to acquire fissile material. It’s hard to perfect the mechanics of implosion. Tests will fail, and valuable fissile material will be wasted in this process. It will all take a long time, and you’ll be left with delivery systems which themselves will have questionable ability to reach their targets. And throughout this long and uncertain process, your country will be a global pariah. That message would strengthen the U.S. non-proliferation effort, and it also has the virtue of also being true.

[1] There is some debate whether there are unique characteristics of the seismic waves from nuclear tests, but statements by leading seismologists confirm that there is no meaningful seismic difference between the signal created by a small nuclear detonation, and a simultaneous conventional detonation.

[2] For a summary, see Jonathan Medalia, “North Korea’s 2009 Nuclear Test: Containment, Monitoring, Implications,” Congressional Research Service R41160, April 2, 2010, available at

[3] See for example, “Statement by the Office of the Director of National Intelligence on the North Korean Nuclear Test,” news release 19-06, October 16, 2006.

[4] Another key (and difficult) step is to introduce neutrons at the right moment to kick-start the fission reaction.

[5] The Hiroshima bomb, dropped after the Trinity test and before “Fat Man” destroyed Nagasaki, was a uranium bomb (rather than a plutonium bomb). Detonating a uranium bomb can, in principle, be much simpler than a plutonium bomb (because it can be detonated without imploding the fissile material). In light of the recent revelations about North Korea’s uranium enrichment efforts, it is possible that North Korea is now seeking to develop some simpler “gun type” uranium bombs while it works to fix the problems with its plutonium program, or the implosion devices that trigger the plutonium weapons. However the simpler “gun type” uranium bomb designs require more fissile material than implosion designs—so in the long run it is likely that North Korea and other new proliferators will adopt implosion designs for the uranium bombs as well as their plutonium weapons.

[6] “Verification Experts Puzzled Over North Korea’s Nuclear Test,” Science vol. 324, no. 5934, June 19, 2009, p. 1499.

[7] “Latest Korean Blast Outdid 2006 Nuke Test: Seismologists, Pinpointing Location, See Telltale Images of Bomb,” Press Release May 27, 2009, Earth Institute, Columbia University; and “Seismologists monitor North Korea’s nuclear blasts,” USA Today, May 29, 2009.

[8] CTBTO analysis is summarized at: Downloaded October 15, 2010. See also, “Verification Experts Puzzled Over North Korea’s Nuclear Test,” Science vol. 324, no. 5934, p. 1499.

Credit for photo of young North Korean girl: T.M. All rights reserved, used with permission.