There was an enormous explosion in North Korea. “There was no indication that was a nuclear event of any kind. Exactly what it was, we/’re not sure,” our Secretary of State Colin Powell told ABC/’s This Week. Powell added that there were “some activities taking place and some sites that we/’re watching carefully, but it is not conclusive that they are moving toward a test.”
How do we know whether an explosion is nuclear or not? Well, in this case, the conclusion is pretty straightforward. Reports of a large mushroom cloud suggest an atmospheric nuclear test, which would also produce fallout that would be detected by radionuclide monitoring stations. That didn/’t happen.
But how do we know North Korea isn/’t conducting underground nuclear tests? Underground nuclear tests are primarily monitored with seismic, as well as radionuclide, signals. Earthquakes have a different seismic signature (the relationship of body- and surface-wave amplitudes are different for the two types of seismic event). It is more difficult, though possible, to distinguish large chemical explosions from nuclear using a similar methodology.
Underground explosions also often release radiation, which can be detected by radionuclide monitoring stations, and satellite imagery can reveal test preparations and a blast cavity. Perhaps the best argument that suggests this event was not a test is that the North Koreans haven/’t said so. The principle rationale for testing a simple nuclear device (which is certain to work) is to make a political point.
How do we knoe North Korea may, in fact, be moving toward a nuclear test? The New York Times reports that satellite imagery detected “the movement of materials around several suspected test sites, including one near a location where intelligence agencies reported last year that conventional explosives were being tested that could compress a plutonium core and set off a nuclear blast.” At the same time, there were no “classic indicators of preparations at a test site” such as “cables…laid to measure an explosion in a deep test pit.”
An interesting discussion of using satellite monitoring to detect test preparations is found in The Chances of an Imminent Communist Chinese Nuclear Explosion, a Special National Intelligence Estimate from 1964. Classic indeed.
The possibility exists that the North Koreans are deliberately revealing some test preparations in order to place pressure on the Bush Administration in Six Party Talks. India was able to rather successfully disguise preparations for its May 1998 nuclear tests, although it had the benefit of having been caught once before, in 1996, and was aided by a disbelief that India would conduct a test.
Sources on verification:
Seismic Verification of Nuclear Testing Treaties (Office of Technology Assessment, May 1988).
John Holdren et al, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty (Washington, DC: National Academy of Sciences Press, 2002).
Steve Fetter, Toward a Comprehensive Test Ban (Cambridge, MA: Ballinger Publishing Company, 1988).
The Chances of an Imminent Communist Chinese Nuclear Explosion, Special National Intelligence Estimate, SNIE 13-4-64 (August 26, 1964).