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Nuclear expert skeptical about North Korea's H-bomb claim

In a surprising announcement, North Korea claims to have conducted its first hydrogen bomb test. However, SIPRI nuclear weapons expert Robert Kelley has doubts the yield was large enough to have come from an H-bomb.

DW: What is known about North Korea's fourth nuclear test?

Robert Kelley: Based upon what is known so far, there was a major seismic event near the previous nuclear test site in the Democratic People's Republic of Korea (DPRK). The size is roughly the same as the third DPRK nuclear test.

Wiggles in the seismic signature will give experts extremely high confidence to determine whether it was an earthquake or nuclear test. From what has been revealed so far it seems that judgment has been made, and they are just double-checking all the data.

It was a nuclear test and probably not one using conventional mining explosives. It would take far too long and be highly observable from satellites to move thousands of tons of conventional explosives into a mountain.

What are the indications that hydrogen may have been involved in the explosion?

The word "hydrogen" is meaningless in this case. It is popular slang and not a professional description.

Robert Kelley SIPRI

Kelley: 'The bottom line is that no evidence of hydrogen in the bomb has yet been found and this almost certainly will never happen'

The proper terminology for what was called a "hydrogen bomb" in the 1950s is actually a "thermonuclear bomb." Thermonuclear bombs do not actually depend on hydrogen and the popular usage is a misnomer that has its terminology roots in physics.

A different evolution of bomb design is called a "boosted" bomb. A very tiny amount of a special radioactive isotope of hydrogen might have been used in North Korea's latest test. However, military and nuclear experts would not call this a "hydrogen bomb." This type of bomb gives the designer many advantages in flexibility, including a slightly higher yield among other things such as reliability.

But the bottom line is that no evidence of hydrogen in the bomb has yet been found and this almost certainly will never happen.

How can experts confirm that Pyongyang indeed tested a hydrogen bomb?

If radioactive debris leaks out of the test and is collected by airplanes or ground sampling stations there may be clues in the debris. This will require very special laboratory analysis and will take days, even weeks, if the nuclear test leaks debris in the first place.

It is important to remember that no debris leaked out of one earlier North Korean nuclear test, so this is a theoretical possibility and probably unlikely.

Some people like to think they can deduce bomb design from seismic waves. This is not true because the seismic waves do not provide that kind of detail.

Please also note that the seismic data says that the yield of this bomb is roughly the same size as the third DPRK test which was not claimed to be a hydrogen bomb. If this one is the same size, what new information do we have suggesting it is any different other than a DPRK claim?

What exactly is a hydrogen bomb and what is it capable of?

A real hydrogen bomb uses isotopes of lithium and hydrogen after an ordinary atomic bomb is detonated a few dozen centimeters away. That produces huge amounts of energy and atomic particles that cause uranium in the thermonuclear bomb to fission more.

These thermonuclear bombs can be used to make huge explosions like in the 1950s but their best use is to engineer much smaller bombs - maybe 5 to 10 times the yield of the bomb dropped on Hiroshima - which are very efficient militarily and save lots of money on nuclear materials.

Nonetheless, should this be a hydrogen bomb, how would this boost North Korea's nuclear capabilities?

I do not believe it was thermonuclear bomb (hydrogen bomb) for many reasons including the very small yield. If it is the boosted bomb described above, it says they are trying very hard to be modern, efficient, and militarily effective.

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What does this latest test reveal about the state of Pyongyang's nuclear program?

We really only know that there has been a fourth underground nuclear explosion the same size as the third one. Pyongyang has bragged it is a "hydrogen bomb" which is neither a scientific nor meaningful engineering term.

North Korea is known to exaggerate and even lie. In any case, we know the program is continuing and that they may be making some scientific advances that could lead to more explosive yield, or, more likely, more efficient bombs - i.e. making more bombs from the same amount of material as before. That is the real impact, not bigger yield.

Robert Kelley is an associated senior research fellow within the Nuclear Weapons Project, Arms Control and Non-proliferation Program of the Stockholm International Peace Research Institute (SIPRI).

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