An Alternative View to North Korea's Bomb Acquisition

Abstract

The small yields of Pyongyang's 2006 and 2009 nuclear tests have left analysts wondering how the North designed its devices. One preeminent Chinese expert has a worrisome theory.

Opinions about the sophistication of North Korea's nuclear devices have diverged since Pyongyang's first two nuclear tests in October 2006 and May 2009. Some analysts, such as Yale University geologist Jeffrey Park, have argued that the two North Korean devices were meant to yield 20 kilotons of explosive power each, a size comparable to the yields of other countries' first nuclear tests. As such, they believe that even the North's second test, which was around 4 kilotons, was a failure. 1 Other analysts, including some prominent Chinese physicists, have contended that both tests aimed for 4 kilotons, and therefore were successful, despite the first test's approximately 1-kiloton yield. 2 They suggest that the North chose to test 4-kiloton devices in order to ensure that the nuclear explosions were contained in an underground test tunnel.

Yet both of these assessments raise a number of questions and leave open the possibility that an alternate set of circumstances and motivations led the North Koreans to build the specific nuclear devices they tested in 2006 and 2009.

The first explanation runs counter to prevailing evidence. Prior to its first test, Pyongyang reportedly told Chinese officials that it was preparing to conduct a 4-kiloton nuclear test. 3 The yield of the first test was indeed significantly lower than 4 kilotons, but the yield of the second test was very close, suggesting that this was the projected yield for both tests. 4 Park's strongest argument is that no other country tested low-yield devices as part of their first series of tests. If North Korea were developing its first-generation atomic bomb independently, this argument would make a lot of sense. But we cannot rule out the possibility that the North had access to nuclear weapon technology from an existing nuclear weapon state–akin to how Britain received help from the United States and China help from the Soviet Union when they built their first bombs.

The second hypothesis, that both tests aimed for yields of 4 kilotons, has merit, but its explanation about why is lacking. The depth required to contain a 20-kiloton explosion is only about 1.7 times the depth necessary to contain a 4-kiloton explosion. In other words, if the North Koreans could build tunnels for 4-kiloton explosions, they could also have done so for 20-kiloton explosions.

The North Koreans could have chosen to build 4-kiloton devices for two other reasons. First, a 4-kiloton weapon would make their device small enough to be deliverable by ballistic missile. But this can be better accomplished by lessening the chemical explosives as opposed to lessening the device's yield. 5 Here, practice is important. Inexperienced nuclear weapon designers do not know the minimum amount of chemical explosives necessary to make a plutonium core go critical and typically use more than is necessary to ensure success. By conducting nuclear tests, they can better understand the minimum amount of chemical explosives necessary for a fissile core to reach criticality and/or develop mechanisms to increase the device's compression efficiency, allowing a device to be small enough to be mated to a single missile. 6 (I will discuss two exceptions later.)

The other possible explanation for why the North Koreans chose a 4-kiloton design is that they wanted to preserve their relatively small plutonium stockpile. If North Korean designers used 6 kilograms of plutonium in each device, roughly the size of the atomic bomb dropped on Nagasaki, Pyongyang's current plutonium stockpile would allow for between four and eight devices. 7 Reducing the amount of fissile material by between one-half and two-thirds might double or triple the size of its nuclear stockpile. (North Korea has said that it used only 2 kilograms of plutonium in the devices it tested. 8 ) However, such a reduction would contribute very little to the North's overall nuclear strategy or its political bargaining power, while significantly increasing the risk of technical failure, since it is more difficult to achieve criticality with a smaller amount of plutonium. Thus, the North Koreans are unlikely to increase their technical burden in order to save some of their fissile material.

An alternative theory. If we combine two of the main assumptions proposed above–that the North Koreans aimed for a 4-kiloton yield and that they could not complete such a design without the benefit of previous experience–a potential workable explanation emerges: Pyongyang had access to an existing nuclear weapon design. When they started formulating their first nuclear weapon, North Korean scientists likely would have chosen the easiest path to their goal. Their choice of a 4-kiloton design suggests that they believed building such a weapon would be easier than building a larger 20-kiloton device. But the only way a 4-kiloton design is easier is if the scientists had access to a working 4-kiloton model. Even then, a 4-kiloton design obviously posed challenges for the North Koreans, evidenced by the low yield of their first test. However, following that path helped speed up their weapons research. If they had started by designing a 20-kiloton device, they would have had to repeat many of the steps already taken by previous nuclear weapon states in order to explore appropriate specifications, which would have delayed their progress.

We cannot rule out the possibility that the North had access to nuclear weapon technology from an existing nuclear weapon state–akin to how Britain received help from the United States and China help from the Soviet Union when they built their first bombs.

That the North Korean government preannounced the expected yield of its first nuclear test suggests that it had confidence in the design's projected yield. The North's weapon designers did not have much experience building nuclear weapons, let alone ones that use less fissile material than first-generation rudimentary designs. If they did not have benchmarks against which to measure their work, they would neither have had confidence in the device's yield nor made their pre-announcement. Between their first and second nuclear tests, which were less than three years apart, the North's scientists must have worked to improve the device's yield. They likely stuck to a 4-kiloton design and were confident in the success of their second test, which ended up having a yield very close to what they intended. It is also important to note that the yield of the first North Korean test was significantly lower than the first nuclear tests of other countries. If the North's scientists did not have some confidence in their design, they would worry that it might be flawed and would need more time to review or tweak it. Therefore, they might not have been able to do their second test so quickly. Their confidence can be understood if they worked from an existing model.

The biggest question that emerges from this alternative hypothesis is the source of the design of the 4-kiloton device and why it was chosen. The original weapon designers could have designed a device of this size to mate it with a missile or another delivery system. But as I explain above, reducing the amount of chemical explosives in a nuclear weapon is typically sufficient to miniaturize a device in most cases. Two special exceptions to this rule exist for some experienced nuclear weapon states. The first exception would be if a nuclear weapon state wanted to increase the accuracy of its multiple independently targetable reentry vehicles (MIRVs) with the goal of attacking a small target, such as a hardened missile silo. To do so, a MIRV needs to have a small flare angle, and the nuclear device needs to be situated close to the vehicle's nose. This requires a very small hydrogen bomb primary, and reducing the amount of chemical explosives might not shrink its size enough to meet such a requirement. However, reducing the amount of fissile material in a bomb's core and subsequently lowering its yield to about 4 kilotons would help.

The second exception would be if a nuclear weapon state wanted to produce small tactical nuclear weapons. Two kinds of tactical nuclear weapons may have smaller amounts of fissile materials in their fissile cores. The first kind is tactical nuclear weapons that have very small sizes and weights so they can be delivered by artillery or other small launch systems, such as the U.S. Davy Crockett tactical nuclear recoilless gun. For the same reason as with MIRVs, the amount of fissile materials and yield of these tactical weapons may be reduced. The second kind of tactical nuclear weapon is the so-called neutron bomb, a small thermonuclear device in which neutron radiation is increased by raising the yield from fusion while radioactive contamination from fission products is reduced. One way to do this is to reduce the amount of fissile material in the primary and subsequently, its fission yield.

Both the United States and Russia (and the Soviet Union before it) have MIRVs intended for precise preemptive strikes and small tactical nuclear weapons in their arsenals. Other nuclear weapon states designed and tested neutron bombs as well. Therefore, any of them could be the origin of the North Korean design.

Improvements in nuclear forensics could help scientists trace where the North Korean weapon came from, if it was originally created by foreign weapon designers. 9 Such an investigation could help identify a fission primary with a yield of 4 kilotons in either the existing nuclear arsenals of weapon states or in the lists of tested nuclear devices. If such a model proves to exist and its origin is located, it could help clarify the North's nuclear capability and help fix a dangerous flaw in the protection of sensitive nuclear weapon technology.

An alternative story line. Based on the preceding analysis, a potential, alternative narrative emerges about the development of North Korea's nuclear devices: During the Cold War, a nuclear weapon state developed a 4-kiloton nuclear device that used less plutonium in its fissile core than other nuclear designs with the purpose of creating a weapon that could be delivered very precisely or in a tactical attack. This weapon model was illicitly transferred to North Korea sometime in the early 1990s. With the refined model of a working device in hand, the North Koreans chose to build a 4-kiloton device that used less fissile material than a 20-kiloton device, the model for most weapon states' initial implosion designs.

The design that the North Koreans would have been working from in this scenario might not have included every production detail of the original, so they would have been forced to do their own research to complete and adjust the design. Another potential complication is that the North Koreans may have been unable to reproduce all design components even if the specifications they had were explicitly described. In other words, it is unclear if the North Koreans could have produced a device of the same quality as a device produced by an experienced nuclear weapon state. This could explain the relatively low yield of the North's first test, along with the fact that the yield of a 4-kiloton device is more sensitive to the amount of chemical explosives used than a larger-yield weapon. To increase the chance that the second test would have a full yield of 4 kilotons, the North Koreans probably sacrificed other elements of the design, for example, its small size. And while the second test reached the projected yield, the North might need to conduct further tests to learn how to control the yield and reduce the amount of explosives used in successive devices.

An alternative narrative emerges: During the Cold War, a nuclear weapon state developed a 4-kiloton nuclear device with the purpose of creating a weapon that could be delivered very precisely or in a tactical attack. This weapon model was illicitly transferred to North Korea sometime in the early 1990s.

Assuming this narrative holds, we can deduce that the North Koreans are capable of producing a workable nuclear device based on an existing model and their own research, but they may not be able to mate this device to missiles for delivery. They could be stuck working through the technical dilemma between size and yield reliability. If they are able to conduct additional nuclear tests, they may gain enough experience to produce an implosion device small enough for missile delivery. Thus, it is important for the international community to dissuade the North from testing again. It is also very important to stop Pyongyang from developing its uranium program, because technically, it would be much easier to produce a deliverable uranium bomb than the plutonium bomb North Korea is working on.

The international community is sincerely trying to accomplish these goals–most especially by encouraging Pyongyang to come back to the Six-Party Talks. And it seems to recognize that the coming months are a critical juncture that must not be squandered. If the international community acts now, it can still prevent the North from acquiring further nuclear know-how and freeze Pyongyang's nuclear program at this current, rather limited stage.