The wire services are carrying the following quote:
“It amounts to detonating 550 tons of trinitrotoluene, or TNT, at once,”
said Park Chang-soo, a spokesman of the Korea Institute of Geoscience
and Mineral Resources.
Hmmmm.
1) The report from Mr. Park could be a misquotation.
2) Mr. Park could have mis-estimated the yield. For example, it is possible,
with some clever work, to "decouple" an explosion so as to reduce its seismic
signature.
3) A 0.55 kt result might represent a triumph, namely the world's first
mini-nuke. See below.
4) Or it might represent a fizzle, perhaps 20x less than what they were
expecting.
5) Or ... maybe a pile of TNT dolled up to look like a nuke???
We're talking about a chain reaction here. Critical mass is critical
mass. The yield you naturally get from critical mass is on the order
of 10 kt. The Hiroshima weapon yielded 15 kt. If you design for less
than that, you are likely to get nothing at all; a decaying exponential
is qualitatively different from a growing exponential in the chain
reaction!