I’d bet on it. But a recent statement by the ODNI is raising all sorts of eyebrows:
The U.S. Intelligence Community assesses that North Korea probably conducted an underground nuclear explosion in the vicinity of P’unggye on May 25, 2009. The explosion yield was approximately a few kilotons. Analysis of the event continues. [emphasis added]
The statement does not explain the use of the hedge-word “probably,” but we can make an educated guess about it.
First, it is an established practice of the National Intelligence Council to use words like “probably” or “likely” to convey degrees of certainty about analytic judgments. See, for example, the fifth page of this memorable release from December 2007. Nothing’s ever completely certain.
Second, none of the usual telltale radionuclides were detected after the test, according to this and this. Although this phenomenon is not unheard of, it does at least admit the possibility that the seismic event actually involved a heapin’ helpin’ of conventional explosives, rather than a nuclear explosive.
But merely because it’s possible doesn’t make it plausible. This scenario was discussed at a recent scientific convention on CTBT verification in Vienna and basically dismissed.
One thing is (virtually) certain. If North Korea had been trucking 2,000 tons of TNT up a mountain and packing it into a deep hole, everyone would have noticed.