The reported size of the North Korean “nuclear” explosion suggests it either failed or was just high explosive.
South Korea’s geological institute estimated the force of the explosion to be equivalent to 550 tons of TNT, far smaller than the two nuclear bombs the U.S. dropped on Japan in World War II.
France’s atomic energy commission similarly estimated the blast measured at around 1 kiloton or less — equivalent to the explosive force of 1,000 tons of TNT.
This post analyzes the three explanations for such a low yield:
1. It was a mini-nuke
2. The test fizzled
3. It was a fake using high explosive.
First some background.
The weapon would have used either a gun-type mechanism to detonate enriched Uranium, or an implosion mechanism to detonate Plutonium 239 – there’s a good overview here.
North Korea is believed to have enough Plutonium for 10 bombs (about 110 pounds), and may have enriched Uranium using centrifuge designs supplied by the “rogue” Pakistani A Q Khan.
However gun-type Uranium weapons don’t need testing – the Hiroshima bomb wasn’t. So any test would be of a Plutonium bomb. These need testing because they use the complex implosion mechanism:
The core of fissile material that is formed into a super-critical mass by chemical high explosives (HE) or propellants. When the high explosive is detonated, an inwardly directed implosion wave is produced.
This wave compresses the sphere of fissionable material. The decrease in surface to volume ratio of this compressed mass plus its increased density is then such as to make the mass supercritical…
A neutron generator emits a burst of neutrons to initiate the chain reaction at the proper moment —- near the point of maximum compression in an implosion design…
The Nagasaki bomb yielded 21 kilotons from 13.5 pounds of Plutonium. You can make much smaller implosion bombs by using more explosives, less Plutonium and neutron generating boosters. Ted Freeman, the king of fission weapons, built a tiny 1 kiloton bomb and suggests designs as small as 0.1 kilotons (pages 49-56 of this book).
But mini-nukes are built on a huge raft of technology and experience, and there’s no way the impoverished North Koreans could have reached that level at their first attempt.
So this was not a mini-nuke.
A fizzle occurs if you don’t get the implosion mechanism to work properly. In that case detonation is partial – the 18th US test in 1956 was the first that failed, and it just blew the top off the supporting tower.
A 0.55 kiloton fizzle is possible, but should be detectable because a fizzled underground test (WSJ, $): …
…is more likely to vent xenon gasses and isotopes, such as krypton and cesium, into the air than a large device, which can melt rock and seal the site.
If the explosion was a fizzle, the radiation detectors now being deployed off North Korea should sniff it out.
This is highly probable – a military dictatorship like North Korea would have no problem in covertly collecting, burying, and detonating 550 tons of high explosive.
Whether fizzle or fake, Armageddon is not as close as we feared.