Jeffrey LewisCan North Korea Mate a "Simple" Fission Weapon to the Taepo Dong 2?

Underground facility at Hagap, once thought an HE testing site.

Was Lowell Jacoby correct about North Korea being able to mate a miniaturized nuclear warhead on a Taepo Dong 2?

Probably not without Pyongyang testing both.

The current CIA estimate assesses North Korea “has produced one or two simple fission-type nuclear weapons and has validated the designs without conducting yield-producing nuclear tests.”

“Simple” is a term of art within the intelligence community. Proliferation: Threat and Response defines a “simple” fission weapon:

There are many uncertainties associated with potential proliferant employment of nuclear weapons. We do not know how successful the proliferant will be in implementing fusing, yield enhancement, delivery system accuracy, and other technologies. For immediate purposes, it is reasonable to use a baseline case of a weapon using 1950s vintage U.S. technology—a simple fission weapon with a tens of kilotons yield that could be delivered by aircraft or tactical missiles.

The term took on special meaning within the Comprehensive Nuclear Test Ban Treaty (CTBT) debate. The argument is that North Korea – or any other new nuclear state – could not develop a nuclear weapon small enough for a long range missile without nuclear testing:

  • Then-Acting Under Secretary of State for Arms Control and International Security Affairs John Holum testified that a CTBT “would constrain the development of nuclear weapons beyond simple fission designs. … It would be a challenging task for an emerging nuclear weapon state, likely requiring nuclear explosive tests, to design nuclear weapons in the sizes, shapes and weights most dangerous to us—compact weapons deliverable in long-range airplanes and missiles …”
  • Then-Secretary of Defense William Cohen argued “potential proliferators can develop and stockpile simple fission weapons without testing. However, the CTBT can keep the nuclear threat from becoming greater than it otherwise could become. Proliferators, for example, could not develop with confidence small, high yield strategic warheads suitable for delivery by ballistic missiles.”
  • John Holdren, then a member of President Clinton’s Committee of Advisors on Science and Technology and chair of the National Academy of Sciences Committee on International Security and Arms Control explained “Certain types of simple and relatively heavy and inefficient nuclear weapons can be developed without any nuclear testing at all. But independent development of efficient and compact fission weapons, or thermonuclear weapons of any kind, could not be accomplished by countries new to nuclear weapons without nuclear testing highly likely to be detected.”

Holdren chaired the National Academies Committee on Technical Issues Related to Ratification of the Comprehensive Test Ban Treaty (2002). Holdren et al describe a “simple” weapon:

Nagasaki was destroyed by an implosion weapon containing about 6 kg of plutonium. It weighed 9,000 pounds and had an explosive yield of about 20 kilotons. Fifty-five years later, and with all the information that has since been declassified, a state with the requisite technical skills in explosives, electronics, and metallurgy could with some confidence reproduce the Nagasaki device without the full-scale test the United States conducted in New Mexico on July 16, 1945. Many non-nuclear tests would be needed to demonstrate the mastery of the technology, and there would be some uncertainty in yield. A weapon weighing 1,000–2,000 pounds might similarly be built, with somewhat less confidence; this might resemble the U.S. Mark-7 bomb of 1951 that weighed 1,800 pounds.

The task of perfecting an implosion weapon is more difficult than the path leading to a U-235 gun-type weapon, but is essential if plutonium is to be used and also provides, as noted above, a path to a weapon using less U-235 than a gun design requires. Technology transfer—authorized or unauthorized, and ranging from tips about dead-end or productive approaches, to transfer of computer codes, to precise working drawings and specifications, to actual transfer of nuclear explosive devices—could greatly ease a recipient state’s path to relatively light and compact implosion weapons and could reduce the number of nuclear tests needed to master these. A single full-yield test would validate both the legitimacy of a blueprint and success in reproducing the object, but that test might be of yield too high to be concealed. Access to plutonium for an implosion weapon, moreover, would require either indigenous production in a nuclear reactor or acquisition from outside sources. Either acquisition or clandestine reprocessing of plutonium from nuclear reactors incurs risk of detection.

The size and weight of fission bombs that could be developed confidently without nuclear testing limit the available means of delivery. Transport aircraft, ships, trucks, and trains can carry any nuclear weapon. The most common missile of 300-kilometer range, the SCUD, has a payload capacity of 1,000 kg. The extended-range SCUD used by Iraq against Israel and Saudi Arabia in the Gulf War can carry 500 kg. The 3-stage Taepo Dong-2, under development but as yet untested by North Korea, could deliver a 700-kg payload anywhere in the United States.

For the Taepo Dong 2 payload/range curve, Holdren et al cite National Intelligence Officer Robert Walpole. Walpole testified:

North Korea is more likely to weaponize the larger Taepo Dong–II as an ICBM that could deliver a several-hundred kilogram payload (sufficient for early generation nuclear weapons) to the United States.

So, could North Korea build a Mk 7-like warhead?

According to Chuck Hansen’s Swords of Armageddon, Los Alamos dramatically improved implosion techniques, permitting a six-fold weight reduction from the aptly named Fat Man to the Mk-7 –which Hansen estimates as 1,600 lbs with a 30” diameter.

Expertise in implosion techniques is clearly driving CIA concern about smaller North Korea warheads. David Sanger’s July 2003 article was prompted by the detection of an “advanced nuclear testing site in an area called Youngdoktong” in North Korea. What was special about that testing site? The revelation that “equipment has been set up to test conventional explosives that, when detonated, could compress a plutonium core and set off a compact nuclear explosion.” It stands to reason that the CIA saw the North Koreans working on implosion and inferred that they were trying to reduce the weight of their warhead designs.

So, if (and it is a big if) a couple of shacks at a test site prove that North Korea has become very proficient at implosion, then Pyongyang might be able to manufacture a warhead small enough to fit on a Taepo Dong 2—assuming the untested Taepo Dong 2 performs as advertised.

The warhead, however, would not be very reliable without nuclear testing. I doubt the North Koreans can run the table, building an ICBM and miniaturized warhead without testing either. At the very least, Pyongyang has to worry that doubts like mine will be widespread, severely undermining whatever existential deterrence Pyongyang enjoys.

Reuters is reporting the CIA believes North Korea could test as early as June. This, in my view, an excellent reason to pursue diplomatic measures that would re-impose the moratorium on missile flight testing and prevent North Korea from conducting nuclear tests.

After all, ICBMs and miniaturized warheads are skills I’d rather the North Koreans not master.


  1. AHM (History)

    Nice work on the Mk-7; I hadn’t checked through Swords to see what the US had.

    1600 lbs w/30” diameter means that a 34” design (i.e. the one A.Q. Khan sold to Libya) should weigh 1.46×1600 lbs/2.2 kg/lb = 1060 kg, which works out about right for, er, not fitting on a Taepo Dong 2.

    Time to add another footnote…

  2. Jeffrey Lewis (History)

    The Chinese-provided design for CHICOM-4 that ended up in Libya probably has a more advanced implosion design than whatever the North Koreans have slapped together.

    Glenn Seaborg described CHICOM-1 as “more sophisticated in design than our own Hiroshima U‐235 weapon, employing an advanced form of implosion trigger to detonate fission materials.”

    So, the Chinese may have been a lot better at implosion in 1966 than North Korea today. In that case, David and Corey’s description of CHICOM-4 as a 500 kg physics pacakge is consistent with the hypothesis that North Korea — which has never tested a weapon — has much heavier warhead designs.

    Presumably, the new testing facility and rumors of Pakistani assitance have the IC rethinking that position. The June 2002 NIE on North Korea reportedly claims Pakistan provided some of that expertise with implosion to North Korea.
    On CHICOM-1, see: Glenn T. Seaborg, Stemming the Tide: Arms Control in the Johnson Years (Lexington, MA, Lexington Books, 1987) p.116. Seaborg’s passage
    is based on his diary entries for 20 and 21 October 1964, republished in Journals of Glenn Seaborg,
    Volume 9 (Lawrence Berkeley Laboratory, University of California, 1979) 261.

  3. AHM (History)

    Hmm. I’ll take a look at the Seaborg reference, but I would expect the CHICOM-1 U-235 implosion design to be lighter than the gun-type Hiroshima weapon on general principles. After all, the Hiroshima design was 8900 lbs.

  4. tom (History)

    I’m sure you’ve already noticed, but NK appears to have test-fired a rocket into the strait b/w itself and Japan this very afternoon.