Chasing the bomb
What we know, and are trying to find out, about North Korea's recent nuclear test.
What more have we learned about last week's North Korean test?
Scientists have been able to confirm that it was indeed a nuclear weapon. US intelligence is reporting that the explosive force of the bomb was less than a kiloton of TNT, and used plutonium as opposed to uranium.
How do we know all that?
The first clues came from seismic data immediately after the blast. A sharp pulse of seismic waves meant it was a man-made explosion and the size suggested it was too big to be a conventional weapon but too small to be a successful nuclear test - more likely a 'fizzle' (see ' The fizzle heard around the world').
Satellites trained on the site caught additional details, such as signs of activity around the test site (including, apparently, a volleyball game going on at the dorms nearby).
But the critical information came in the days following the blast, when Japan and the United States dispatched reconnaissance aircraft to the edge of North Korean airspace. Those planes scooped up huge volumes of air looking for radioactive traces. US officials say they detected some "radioactive debris" from the blast.
What "radioactive debris" exactly?
That information, considered highly sensitive, has not been released. The most likely candidate would be Xenon-133, a radioactive isotope created in nuclear explosions. Xenon-133 is a noble gas, which means it's chemically inert and can hang around in the air for days after the blast.
But Xenon-133 alone wouldn't be enough to tell that the bomb was made of plutonium, according to Ivan Oelrich, a chemist and nuclear expert at the Federation of American Scientists, a Washington-based nuclear watchdog. To narrow it down to plutonium would require further information about the isotopes released by the blast.
Oelrich notes that during the Cold War, the word "debris" was used to mean unexploded particles from a bomb, which sometimes leak out from underground tests. If such trace particles have been collected, they would tell us a lot more about the exact makeup of the bomb.
Why does the exact composition of the bomb matter?
The fact that it is apparently a plutonium bomb makes it highly likely that it came from North Korea's long-standing, indigenous plutonium-based programme, rather than imports. If the exact composition of the bomb can be determined, then intelligence agencies might be able to learn a great deal about how this programme refines plutonium. The composition might also provide hints as to why the bomb 'fizzled'.
The Japanese and South Koreans have said the North may be preparing a second test. What might this mean?
It might suggest that the problem with the first bomb was relatively simple: perhaps a miscalculation with the conventional explosives used to set off the nuclear charge, rather than a problem with the fissile material. If the plutonium material was not made correctly, this would present a problem that would take much longer to fix.
So does North Korea have a useable bomb?
At the moment, it seems unlikely. The test probably used a bomb too heavy to put atop a missile, and the yield suggests that the North hasn't yet perfected their device. But there are worries that they could be close to building a much improved weapon.
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