Oh, and while we’re speculating...
Was the North Korean test of a nuclear weapon a failed test of an atomic bomb, or a successful test of a neutron bomb?
IMHO, it was just a leak of nuclear accident they probably had. There was no earthquake that is usually is a tell tale sign of a nuclear explosion.Everything you wanted to know on seismic verification of nuclear weapons are on these links.
https://str.llnl.gov/Mar09/walter.html
http://www.fas.org/ota/reports/8838.pdf