Not to worry, it was "probablya dud." Ha ha.

A plutonium device should produce a yield in the range of the 20 kilotons, like the one we dropped on Nagasaki. No one has ever dudded their first test of a simple fission device. North Korean nuclear scientists are now officially the worst ever.

It was only a kiloton, these guys say. Maybe half - 500 tons of TNT. So that's not much, is it? Just a little pop. How much damage can 500 tons of TNT do? Probably no more than, say, four Foley IMs.