The science of measuring North Korea’s destructive nuclear power from afar

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• Judy Woodruff:

  The Trump administration confirmed last night that Vice President Pence was scheduled to meet secretly with top North Korean officials during his recent trip to South Korea. North Korean officials pulled out of the meeting at the last minute.

  It would have come after Pence sat near Kim Jong-un's sister during the Olympics' opening ceremony.

  The Trump White House considers North Korea's nuclear and missile programs the main threat to American national security.

  Last week, Miles O'Brien examined the North's ability to make the material required for nuclear weapons.

  Tonight, Miles looks at what we know of the devices themselves and how analysts gauge North Korea's building progress.

  It's for our weekly series on the Leading Edge of science.

• Miles O’Brien:

  North Korea has tested nuclear weapons six times since 2006 at this mountainous site in Punggye-ri. No one from the outside has witnessed such a test, but the experts believe this propaganda movie produced by the regime offers an accurate, albeit embellished, version of what happens inside.

  But how much is known about the design and size of this secret arsenal?

• Jeffrey Lewis:

  This is actually a 3-D model of North Korea's nuclear test site.

• Miles O’Brien:

  Jeffrey Lewis is director of the East Asian Nonproliferation Program at the Middlebury Institute of International Studies at Monterey.

• Jeffrey Lewis:

  So these black spheres are actually the estimated size of the cavities from the North Korea's nuclear explosions.

• Miles O’Brien:

  He and his team used terrain data gathered by satellites to build a 3-D computer model of the Punggye-ri test site.

• Jeffrey Lewis:

  We estimated where the nuclear explosions had occurred. We were able to use satellite photographs to see where the tunnels went into the mountains.

  And when we finished that, one of the things we realized is that that site could accommodate a much larger nuclear explosion than we had seen in the past.

• Miles O’Brien:

  It led him and his team to predict the North Koreans would soon test a bomb with a few hundred-kiloton yield, much larger than their previous tests.

  The prediction came at the beginning of 2017. In September, seismometers detected an explosion about that big. Even when testing occurs underground, some gases can leak out. The United Nations, the U.S., South Korea, Japan, and China have surrounded the North with sensors that can detect radioactive isotopes, as well as so-called noble gases before they decay.

  The ratio of noble gases xenon 131 and 133 can offer a clue about the design of a bomb. But the North Korean site is doesn't seem to leak very much.

• David Albright:

  The South Koreans detected a little bit of xenon-133, but not that much, and they didn't detect any other isotopes.

• Miles O’Brien:

  Physicist and former arms control inspector David Albright is president of the Institute for Science and International Security.

• David Albright:

  Not what you would expect from a large-yield weapon, so they're clearly — they're doing something, and it is an ideal test site location. The explosion is sealing the material inside.

• Miles O’Brien:

  But the North Koreans do share some tidbits about what their weapons might look like, or so it seems.

• Siegfried Hecker:

  They have actually shown us what are presumed to be photographs of their nuclear devices, and they have shown that over the last few years' time frame.

  This was two weeks after the first nuclear test.

• Miles O’Brien:

  Nuclear physicist Sig Hecker is with the Center for International Security and Cooperation at Stanford University. He ran the Los Alamos National Laboratory from 1986 through 1997 and has visited North Korea seven times.

• Siegfried Hecker:

  This was 2007 in one of their uranium processing laboratories.

• Miles O’Brien:

  He believes the first weapon they tested was akin to Fat Man, the bomb that the U.S. dropped on Nagasaki. In it, high explosives implode a plutonium core, causing an instant nuclear chain reaction, or explosion.

  North Korea's first test in 2006 was an apparent failure, but, in 2009, test number two was a success. Since then, the yields have grown steadily larger. And that leads experts to believe the North Koreans have developed a so-called boosted fission bomb.

• Siegfried Hecker:

  The boosted bomb is one where you take highly enriched uranium or plutonium, and you actually put the fusion fuel inside to sort of help light more of the plutonium or highly enriched uranium.

• Miles O’Brien:

  In 1957, Great Britain tested a one-stage boosted thermonuclear bomb. Its yield, 720 kilotons, or about 50 Hiroshima bombs.

• David Albright:

  The trouble with them is, as you increase the yield, it gets bigger and bigger in diameter, and it uses an incredible amount of weapon-grade uranium. And so all the countries are motivated to go to the two-stage weapon.

• Jeffrey Lewis:

  This is a 3-D model of North Korea's thermonuclear weapon. They released a series of pictures, and we were able to assess its size.

• Miles O’Brien:

  A two-stage, or hydrogen, bomb allows for a much larger yield in a smaller, lighter package. It begins with a plutonium or highly enriched uranium explosion, stage one. That creates enough energy to squeeze hydrogen and its isotopes, causing fusion, stage two.

  The U.S. tested more than 100 hydrogen bombs in the Pacific. The largest was 15 megatons, or 15,000 kilotons, more than 1,000 Hiroshima bombs.

  After the last North Korean test, the regime released a picture of a what looks like a two-stage device.

• Siegfried Hecker:

  It looks as if this last test, the yield, the explosive yield of that is large enough that it was most likely a two-stage bomb.

• Miles O’Brien:

  But are these weapons small, light and robust enough to be efficiently delivered on a missile? The experts believe the North Koreans have designed their bombs with all of this in mind from the outset.

• Siegfried Hecker:

  The first bomb was mostly, I think they had to prove to themselves, essentially a proof of principle that they can actually make an efficient bomb. And, from that point on, they were determined towards making that bomb deliverable.

• David Albright:

  So, they have to spend time, I think, working on getting the yield up, and not increasing the size too much, and I think that, they have done.

• Miles O’Brien:

  So it is likely the North Koreans have bombs small enough to be delivered on short-range missiles. But weapons that are mounted on medium-range or intercontinental ballistic missiles are a much bigger challenge.

• Siegfried Hecker:

  They have to be smaller. They have to be lighter. They have to be even more robust. They haven't demonstrated that they can do that.

• Miles O’Brien:

  And what about the missiles themselves? What can they deliver and where? The rocket science in our next report.

  I'm Miles O'Brien for the PBS NewsHour.