Former Los Alamos National Laboratory director Siegfried Hecker assesses North Korea’s claim to have detonated a hydrogen bomb in an underground nuclear test this week. Hecker is one of the world’s top experts on the North Korean nuclear program. He has visited North Korea seven times since 2004, and is the only Western scientist known to have ever been inside a North Korean uranium enrichment facility. He is currently a senior fellow at Stanford’s Center for International Security and Cooperation, and a research professor of Management Science and Engineering.

Do you believe that North Korea actually detonated a hydrogen bomb in its latest nuclear test?

I don’t believe it was a real hydrogen bomb, but my greatest concern is not so much whether or not they actually tested a hydrogen bomb, but rather that they tested at all. Since this test worked, they will have achieved greater sophistication in their bomb design – that is the most worrisome aspect. This is their fourth test – with each test they can learn a lot.

What makes a hydrogen bomb a more threatening weapon than a conventional atomic bomb?

A hydrogen bomb can be a hundred or a thousand-fold more powerful than a fission bomb. Certainly a blast of a megaton will be much more destructive than the Hiroshima bomb, but the more important part is the ability to deliver at long range and to do it accurately. That is what would threaten the United States and its allies most; even with the size of nuclear blasts they have already demonstrated.

White House officials say that initial data from nearby monitoring stations are not consistent with a hydrogen bomb test. How will we know for sure whether it was a hydrogen bomb or not?

The short answer is that we may never know. The telltale signs of a hydrogen bomb are very difficult to pick up in a deeply buried test. Typically hydrogen bombs have greater explosive power or yield. This test is currently believed to have resulted in a seismic tremor of 5.1 on the Richter earthquake scale. That would make it roughly equivalent to the third nuclear test in February 2013. At that time, North Korea claimed it tested a miniaturized atomic bomb – there was no mention of a hydrogen bomb. My estimate of the yield for the 2013 test is roughly 7 to 16 kilotons – which is in the range of the 13-kiloton Hiroshima blast. As far as destructiveness, a Hiroshima-scale explosion is bad enough. Detonated in Manhattan, it may kill as many as a quarter million people. The power of the 2013 and the current explosion is more consistent with fission bombs than hydrogen bombs.

Can you rule out the possibility that it was a hydrogen bomb?

I find it highly unlikely that the North tested a real hydrogen fusion bomb, but we know so little about North Korea’s nuclear weapons design and test results that we cannot completely rule it out. A modern hydrogen bomb is a two-stage device that uses a fission bomb to drive the second stage fusion device. A two-stage device is very difficult to design and construct, and is likely still beyond the reach of North Korea today. However, by comparison, China’s early nuclear weapon program progressed rapidly. It tested its first fission bomb in 1964 and less than three years later demonstrated a hydrogen bomb – and that was 50 years ago. North Korea has now been in the nuclear testing business for almost 10 years, so we can’t rule anything out for certain.

If it wasn’t a hydrogen bomb, what kind of bomb might it have been?

What may be more likely than a two-stage hydrogen bomb is that they took an intermediate step that utilizes hydrogen (actually hydrogen isotopes) fuel to boost the explosive yield of the fission bomb, a sort of turbocharging. Such a device has a fusion or “hydrogen” component, but is not a real hydrogen bomb. It allows miniaturization – that is making the bomb smaller and lighter. Moreover, it would be the first step toward eventually mastering a two-stage hydrogen bomb.

The most important aspect then is to miniaturize, whether it is a fission bomb, a boosted fission bomb, or a hydrogen bomb. The Nagasaki bomb weighed 5,000 kilograms. It was delivered in a specially equipped B-29 bomber. North Korea wants to demonstrate it has a deterrent. To do so, it needs to be able to credibly threaten the U.S. mainland or our overseas assets. For that, you have to make the bomb (more correctly, the warhead) small enough to mount on a missile. The smaller and lighter, the greater the reach. At this point, what makes their nuclear arsenal more dangerous is not so much explosive power of the bomb, but its size, weight and the ability to deliver it with missiles.

How close is North Korea to being able to credibly threaten a nuclear strike against the mainland United States?

North Korea is still a long way off from being able to strike the US mainland. It has only had one successful space launch. It needs a lot more, but it has a large effort in that direction.

Do you think North Korea conducted this test for political or technical reasons?

North Korea had very strong technical and military drivers for this test, as well as follow-on tests. The political environment is mostly what has constrained it from testing earlier and more often. However, this test demonstrates that Pyongyang is willing to weather the political storm this test will bring. It has done so for all previous tests.

What are your current estimates on the size of North Korea's stockpile of nuclear weapons and materials?

Much like in the area of sophistication of the bomb, we have little information of what North Korea actually possesses. The best we can do is to estimate how much bomb fuel, plutonium and highly enriched uranium, they may have produced and estimate how many bombs they can produce from that stockpile. My best estimate at this time is that they may have enough bomb fuel for 18 bombs with a capacity to make 6 to 7 more annually. That, combined with the increased sophistication they surely achieved with this test, paints a troublesome picture.

How should the U.S. respond?

I am concerned about we haven’t done to date. Washington has lost many opportunities we have had since North Korea began its nuclear weapon production in earnest in 2003. One thing that’s clear is that doing what we and the rest of the world have done so far – half-hearted diplomacy, ultimatums, and sanctions – have failed, so these are not the answer. I have previously argued that we should focus on three “No’s” for three “Yes’s” – that is no more bombs, no better bombs (meaning no testing) and no export – in return for addressing the North’s security concerns, its energy shortage and its economic woes. This could have worked when I first proposed it 2008 after one of my seven visits to North Korea. It will be more difficult now.