While a leaked U.S. intelligence report suggests North Korea now can build warheads small and light enough to fit inside its intercontinental ballistic missiles, nuclear weapons experts doubt that Pyongyang can yet produce an operational ICBM with a reliable warhead capable of hitting the U.S. mainland.
Reports about the intelligence community's consensus on North Korea's weapons capability came this week as Pyongyang and Washington exchanged war threats.
The July 28 analysis from the Pentagon's Defense Intelligence Agency (DIA), disclosed Tuesday by The Washington Post, concludes that Pyongyang has "produced nuclear weapons for ballistic missile delivery, to include delivery by ICBM-class missiles." On Thursday, NBC News quoted unnamed U.S. officials as saying the CIA and the Office of the Director of National Intelligence, as well as other intelligence agencies, agreed with the assessment.
Key questions unanswered
Miniaturization technology was one of the major hurdles in Kim Jong Un's nuclear and missile programs. If the DIA assessment holds true, the regime is now closer to achieving its ambition: the capability to strike the continental U.S. with a nuclear-tipped ICBM.
Determined as North Korea is to become a full-fledged nuclear power, experts say several important questions about its capabilities remain.
David Albright, a former U.N. nuclear inspector, told VOA's Korean service the DIA assessment appeared to have ignored "uncertainties and caveats" about the reliability of the miniaturized warhead, once it is loaded atop an ICBM, and little is known about the chances that the payload will reach its target.
Pyongyang already can miniaturize nuclear warheads and mount them on its medium-range Rodong missiles, which have been test-launched repeatedly since the early 1990s. Albright said it might be possible, but less likely, that they could do the same with an ICBM.
Difficulties only begin at launch
Albright, president of the Institute for Science and International Security, explained that a nuclear warhead must survive the entire ICBM mission — the rigors of blastoff, possibly from a mobile launch vehicle, the flight into space and then a blazing re-entry into the atmosphere — before it can detonate above its target.
Failures can occur, Albright noted, because of the much more exacting requirements of the Hwasong-14 ICBM missile system, which has been tested only twice, and just within the past month — not enough to establish its reliability.
"Countries spend a lot of time working this problem to try to build up what they call the reliability of the warhead in a delivery system, and it just takes time," Albright said. "I think I would be skeptical that North Korea can do it right now."
Michael Elleman, senior fellow for missile defense at the International Institute for Strategic Studies, had similar views. But in terms of volume, he said, if a warhead can fit inside the payload bay of Pyongyang's Scud-type short-range missile, which has a relatively narrow diameter, any of the regime's other missiles, including the Hwasong-14, certainly could accommodate it.
Lighter warheads travel farther
It is not yet clear by how much the North Koreans can lighten their missiles' payloads, which would extend their range.
"It is still a question mark as to whether they can threaten deep into the United States," Elleman said.
If a warhead weighs about 500 kilograms, it could reach as far as the Western portions of the continental U.S., according to Elleman.
Further undercutting confidence in the North's technical capabilities is a lack of clarity about the Kim regime's mastery of atmospheric re-entry technology for the warhead, a crucial requirement for operational ICBMs.
For long-range flights, Elleman said, "the re-entry velocity, when it comes back into the Earth's atmosphere, is much higher, and so the protection mechanisms for the re-entry vehicle [must be] more rigorous, to survive the much greater amount of heat and the vibrations as it slows down, passing through ... thicker and thicker [air] as you get closer to the surface."
Tests show 'substantial progress'
Siegfried Hecker, a former director of the Los Alamos National Laboratory, is now a professor at Stanford University's Center for International Security and Cooperation, and he has visited North Korea seven times and toured its nuclear facilities.
Hecker said in an interview with The Bulletin of the Atomic Scientists that making miniaturized warheads robust enough to survive the extreme conditions of ICBM flight "is very demanding and takes time, particularly because warheads contain materials such as plutonium, highly enriched uranium, high explosives and the like."
"These are not," he added, "your ordinary industrial materials."
However, Hecker added in a separate interview this week, Pyongyang's latest two missile tests, of their ICBMs, "demonstrate substantial progress, and most likely mean they will be able to master the technology in the next year or two."
The Bulletin of the Atomic Scientists, founded in 1945 by scientists who worked on the U.S. Manhattan Project and built the first atomic bomb, has for 70 years published the "Doomsday Clock," intended as a measure of how close the world is to a thermonuclear war — or to midnight, on the clock, because that could lead to a worldwide cataclysm.
The Doomsday Clock stood at three minutes to midnight in 2016. The scientists involved advanced it in late January this year, and it is now just two minutes and 30 seconds short of midnight.