Are North Korea's nuclear weapons small enough to fit a ballistic missile?

North Korea claims to have made progress on miniaturized nuclear devices. Some experts credit that claim, but much of what North Korea can or can't do remains unknown.

Can North Korea make a nuclear warhead small enough to fit atop a ballistic missile? That’s a key question facing the US and its allies in East Asia as tensions continue to roil the Korean Peninsula.

Following its Feb. 12 nuclear test, Pyongyang boasted that it had detonated a “miniaturized and lighter nuclear device." Some US experts interpreted this to mean that North Korea is claiming to have developed a weapon that would fit on a Nodong medium-range rocket or, perhaps, even the untested, longer-range KN-08. This would threaten South Korea and perhaps Japan, but not the US mainland itself.

“The question is, do we believe them?” writes nuclear expert Jeffrey Lewis of the James Martin Center for Nonproliferation Studies in Foreign Policy magazine.

The public US government position is that it does not know how far down the road toward miniaturized nuclear devices North Korea has progressed. In 2005, the director of the Defense Intelligence Agency told a congressional hearing that North Korea had the capability to arm a missile with a plutonium-based nuclear device, but Pentagon officials later backed away from that conclusion, according to a 2013 Congressional Research Service report on technical issues related to North Korea’s nuclear weapons.

“It is possible that Pakistani scientist A.Q. Khan may have provided North Korea the same Chinese-origin nuclear weapon design he provided to Libya and Iran,” writes CRS expert Mary Beth Nikitin. “Even though that design was for a [highly enriched uranium] based device, it would still help North Korea develop a reliable warhead for ballistic missiles – small, light, and robust enough to tolerate the extreme conditions encountered through a ballistic trajectory.”

For his part, Mr. Lewis of the Center for Nonproliferation Studies is inclined to take seriously North Korea’s statements about its progress in miniaturization. He bases this view, in part, on circumstantial evidence: the smallish yields of the first two North Korean nuclear tests, in 2006 and 2009.

“North Korea’s disappointing yields in 2006 and 2009 [nuclear tests] are not the result of technical incompetence so much as outsized ambition,” Lewis writes. “The North Koreans tried to skip some steps and go directly to miniaturized devices.”

In other words, Pyongyang is not following the same process of weapons development used by superpowers such as the US and the Soviet Union. Instead,  North Korea may be attempting to leapfrog ahead to missile-deliverable devices based on the experience and knowledge gathered by smaller members of the nuclear club, particularly Pakistan.

David Albright, president of the Institute for Science and International Security, largely agrees with this assessment. He writes in a Feb. 13 report on the subject that North Korea likely has the ability to mount a plutonium-based device on a Nodong, which has a range of about 800 miles.

“Pyongyang still lacks the ability to deploy a warhead on an ICBM, although it shows progress at this effort,” Mr. Albright writes.

The duration of North Korea’s nuclear efforts is one piece of evidence Albright cites. The North Koreans have been working on such weapons for some 30 years – plenty of time for them to have learned miniaturization’s secrets.

In addition, North Korea is known to have carried out extensive testing with high explosives at a site adjacent to its Yongbyon nuclear facility. Compression of a fissile core with precise high explosives is a key to making smaller nuclear designs.

China is thought to have provided a number of nations with a simple weapons design in the 1980s. This design had a diameter of about 0.8 meters, according to Albright. Over two decades, Pakistan refined this design further, “levitating” the core and shrinking it to 0.6 meters in diameter. This design fit on Pakistan’s Ghauri missile, which is based on technology acquired from (you guessed it) North Korea.

As noted above, Pakistanis notorious A.Q. Khan likely passed the Chinese bomb design on to North Korea, completing the circle of dangerous weapon technology proliferation.

“Given that North Korea started at least 20 years ago working on a warhead for the Nodong missile, it is likely that it finished developing one able to fit on the Nodong in the early to mid-2000s,” writes Albright.

These assessments underscore the difficulty of developing policies that might push Pyongyang to give up its nuclear program. That program is already well-developed, and North Korea and its young dynastic leader Kim Jong-un likely see it as a guarantor of regime survival.

But North Korea does not have the ability to produce weapons that might reach the United States, note most experts. That would require reaching a much higher level of scientific expertise. In any case, Pyongyang knows that if it launches a nuclear attack on anyone, the military response from the US and its regional allies will be devastating and result in the end of North Korea as it is currently constituted, points out Siegfried Hecker, a Stanford University nuclear expert, who has visited North Korea a number of times.

That means there is still time to curb the dangers North Korea’s activity pose.

“Despite its recent threats, North Korea does not yet have much of a nuclear arsenal because it lacks fissile materials and has limited nuclear testing experience,” says Dr. Hecker, in a conversation posted on the website of Stanford’s Center for International Security and Cooperation. “In the long term, it’s important to keep it that way; otherwise North Korea will pose a much more serious threat.”