Body
Frank Pabian and Sig Hecker have published an important piece in the Bulletin of Atomic Scientists that is likely to inform debate about the DPRK’s nuclear intentions. But their claims bear close scrutiny, because they are more cautious than the headlines the study has generated.
The core of the piece rotates around some clever forensic work the two have done on the precise location of the 2006 and 2009 tests and their likely yields. Such estimates are based on the arrival times of seismic waves at monitoring stations outside of North Korea. These monitoring stations are networked through the Comprehensive Test Ban Treaty Organization's International Monitoring System; 22 registered the first test but an additional 18 stations registered the larger 2009 event including seven seismic stations in Japan, one in China, and one in South Korea.
The problem with these efforts is that they can locate the two events relative to one another but cannot easily provide the absolute location of the epicenter. Pinpointing the exact location of the test requires additional geospatial—in effect Google mapping—work that connects the predicted epicenter with plausible features of the test-site terrain, including likely tunnels in which the tests were conducted. Moreover, the depth at which the devices were placed is also consequential; Pabian and Hecker drew on intelligence from Pakistani tests and North Korean broadcasts showing the tunnel structure to make their inferences.
The bottom line: initial intelligence estimates that the first test was less than a kiloton and the second also quite low, at less than 2 kilotons, need to be revised upwards, as other researchers have already done. But Pabian and Hecker believe that they can reduce some of the uncertainty in the yields with their techniques, and they put the first test at a kiloton but the second at between 4 and 6.9.
However, it is not clear how you get from these yields to prognostication about future tests. Does a higher yield make the North Koreans more or less confident about the need to test again?
The headline in the piece is not the yields: it's in the authors’ authoritative statements about tunnel activity and the likelihood that they would either go HEU or do a combined HEU/plutonium test. These speculations are worth quoting directly because they are likely to be misused to infer that a test is imminent.
On the tunneling:
“North Korea appears to have an underground tunnel ready for testing. Commercial satellite imagery shows a recently excavated "south portal" for a tunnel in Punggye-ri, situated very close to the tunnels for the first two tests. It apparently has been under construction since 2009 and significant activity was reported at the tunnel site after the failed April space launch. Technically, North Korea could be ready to test within two weeks.”
On possible HEU or combined tests:
“For the long term, the plutonium route is at a dead end for North Korea; Pyongyang decided to shut down its five megawatt reactor in 2007, and it has no other facilities for producing plutonium. At the same time, North Korea has strong technical reasons to do a third plutonium test, in spite of its meager plutonium inventory, which we estimate to be 24 to 42 kilograms, to better calibrate its computer models and understand implosion devices. It is therefore conceivable that North Korea may conduct two tests simultaneously, using a double fishhook at the end of the tunnel, with one bomb fueled by HEU and a second by plutonium. Pakistani scientists made a similar decision to conduct multiple tests simultaneously to maximize technical results. If Pyongyang has more HEU than we surmise, it may do more than one HEU test with different designs. Two detonations will yield much more technical information than one, and they will be no more damaging politically than if North Korea conducted a single test.”
The media has had a field day with this, of course; for a sample, see The Guardian, The Chosun Ilbo and Bloomberg. But let’s be clear: Pabian and Hecker are not saying that the North Koreans will test, and in fact they seem to be cautiously optimistic that they won’t. But the Bulletin piece is a reminder that the possibility of a third test is ever-present; we suspect that the North Koreans are perfectly happy to see the resultant hand-wringing.
