The explosion may have been relatively small, but it was nuclear, according to U.S. intellegence:
Air samples gathered last week contain radioactive materials that confirm that
conducted an underground nuclear explosion, National Intelligence Director John Negroponte's office said Monday. North Korea
In a short statement posted on its Web site, Negroponte's office also confirmed that the size of the explosion was less than 1 kiloton, a comparatively small nuclear explosion. Each kiloton is equal to the force produced by 1,000 tons of TNT.