Did North Korea Conduct a Secret Nuclear Test in 2010?
At the core of this problem is a reversal in how we think about detecting underground nuclear tests. The traditional thinking is that the correct way to “detect” an underground nuclear test is to spot it seismically. If radionuclides later appear, that helps “characterize” the seismic event as a nuclear explosion, rather than a conventional one. Generally, policymakers have been reluctant to rely only on radionuclide readings alone to “detect’ events for reasons that should become clear. The radionuclide community, however, is very excited about getting the same recognition as seismologists, especially now that computer simulations promise reliable methods to model the transport of radionuclides based on weather data. So, there may a bit of a disciplinary food fight here.
In May 2010, the DPRK released a series of statements that a “thermonuclear” reaction had occurred in April. In the months following the announcements, a well-respected Swedish radiochemist, Lars-Erik De Geer, correlated these statements with certain radionuclide readings collected by the Comprehensive Test Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS). The data includes xenon isotope ratio measurements at a national radionuclide monitoring site near Geojin (South Korea) and an IMS site near Takasaki (Japan) and Barium/Lanthanum measurements at CTBTO IMS sites near Usurriysk in Russia and Okinawa in Japan. (Only Lanthanum (La) was detected at Ussuyriysk.) All these measurements occurred between May 13-18, 2010.
De Geer published his findings in a 2012 article in Science and Global Security. I was skeptical of the original De Geer paper because it posited an extraordinarily artificial scenario of the observed radionuclide readings. De Geer posited two undetected nuclear tests, conducted in the same chamber approximately one month apart.
A number of radiochemists reviewed and agreed with De Geer’s initial paper. One concluded that the evidence suggested a nuclear explosion, although he argued the radionuclide evidence was best explained by a single explosion and dismissing the xenon detections at Takasaki in Japan as coincidental.
De Geer himself concluded that the initial paper was in error, publishing a second paper in the Journal of Radioanalytical and Nuclear Chemistry. While De Geer’s first paper posited two undetected tests, the second paper posits only a single test on May 11.
Now, there are two ways to respond to this revision: I took it as confirmation of my original complaint that the scenario was being fitted to the data, raising serious methodological warnings. My colleagues, quite reasonably said, “Yeah, but the new scenario is pretty clean. What’s your objection to it now?”
Then along came another group of radiochemists, Ihantola et al, who agreed that a nuclear explosion occurred, but estimated the likely time of the event to be much later than De Geer’s estimate. De Geer and Ihantola et al posit very different explosion times, each outside of the error range posited by the other. Only the confidence intervals overlap, and just for a few hours.
So, here we are. Is De Geer right? Are Ihantola et al right? Or do we just shake our heads, muttering about how the data, like Jay from Serial, seems to always tell us what we want to hear? I don’t blame Wright for concluding that some of the readings might be unrelated to a test, but once we start tossing out awkward data, our thin methodological ice starts to crack.
Moreover, false alarms are possible. Nuclear power stations, reprocessing plants and other human events can result in releases that appear to be nuclear explosions. Early operation of a radionuclide monitoring system in Germany detected xenon spikes from nearby reactors. (The false alarm has led to better methods of characterization that emphasize isotoptic ratios, but these methods still struggle to distinguish an explosion from a fresh load of fuel.) In another instance, in 2004, a radionuclide station detected 140La that was later determined to have been from a military decontamination exercise. We are so worried about false negatives—missing a nuclear test—but we seem to never worry about false positives.
I still wonder about other possible explanations. Japan brought its Monju fast breeder reactor online on May 9, 2010. The reactor experienced a number of alarms on May 9 and 10, indicating radiation leaks. Although Japanese authorities later stated that the alarms were false alarms and turned off the alarm system, this possibility should be examined far more thoroughly than it has been to date. Similarly, China brought its first fast reactor online a few weeks later. Maybe the Chinese had a false start? These hypotheses strike me as equally likely as a North Korean test. They deserve the same scrutiny.
Sadly, the radionuclide background in Asia is getting worse, not better as more reactors come online. In particular, South Korea is planning to build a medical isotope production reactor planned for Busan that will produce a lot of radionuclide “noise.” North Korea’s 2013 nuclear test would likely have been lost in the background had this facility been operating at the time.
After De Geer’s initial report, seismologists began looking for events that would confirm an explosion.
Schaff et al closely examined seismic data from an IMS station in China on the days hypothesized in the original De Geer paper—April 14-16 and May 10-11. They found no evidence of an explosion in either period. For the crucial period of May 10-11, Schaff et al found no explosion down to a threshold of Mb=1.15.