By Tom Z. Collina
A new study in the March 2012 issue of Science & Global Security suggests that North Korea carried out a small nuclear explosive test in May 2010. If true, this would be the third nuclear test by North Korea and its first that was not announced.
CTBTO radionuclide monitoring station, Okinawa, Japan
The study argues that because there was no seismic reading to indicate a nuclear explosion at that time, the explosive yield of any such event would have been less than 50 tons (or .05 kilotons). The fact that a test this small could have been detected at all is a promising sign for the Comprehensive Test Ban Treaty (CTBT) verification system.
The paper, authored by Lars-Erik De Geer, Research Director at the Swedish Defense Research Agency, Stockholm, bases its conclusion on radionuclide data from May 2010 collected at stations in South Korea, Japan and Russia that are part of the CTBT International Monitoring System (IMS) and national nuclear monitoring networks. In addition to radionuclides (radioactive gases), the IMS detects seismic (ground shock), hydroacoustic (water), infrasound, and other signals.
De Geer argues in his paper that: "The fact that such experiments were still detected by another technology in the currently evolving CTBT verification system ... suggests that there are fewer and fewer grounds for countries to refuse ratifying the CTBT by questioning the effectiveness of its verification regime."
It is also important to note that when the CTBT does enter into force, ambiguous events such as these in states of concern would very likely lead to requests for on-site inspections to resolve concerns about clandestine nuclear testing. Although the CTBT has been signed by 182 countries and ratified by 156, and the IMS is nearly complete and operational, 8 more states including the United States and China, must ratify to trigger its formal entry into force.
This possible test highlights the point that even explosions of very low yield can be detected, given the verification system's overlapping and complementary technologies.
De Geer concludes that "the CTBT verification system sometimes is capable of detecting underground nuclear tests of significantly lower yields than ... was anticipated when the treaty was opened for signature 15 years ago."
Meaning of Data Questioned
De Geer's conclusions about the radionuclide data and further North Korean nuclear testing are not shared by other nuclear test monitoring experts.
Frank von Hippel, a physicist at Princeton University, told Geoff Brumfiel at Nature that De Geer's analysis provides convincing evidence of some kind of nuclear fission explosion.
But, Brumfiel reports, von Hippel does not agree that it involved two weapons tests or a fusion boost. "I hope that other experts will analyze it and see whether they can put forward alternative, simpler explanations," he said.
Dr. Ola Dahlman, a retired geophysicist also questioned the absence of any seismic vibrations to support the radioisotope data.
The data analyzed in the De Geer study were produced by the CTBTO's network of sensors, but the organization itself has never officially analyzed all these data.
Lassina Zerbo, director of the CTBTO's International Data Center in Vienna told Nature that although the data are processed and shared quickly after such an event, formal analyses are done only if requested by CTBTO member states.
None of the 182 signatories to the treaty ever made such a request Zerbo said, and the CTBT does not have access to the data collected by South Korea's national monitoring network.
If this indeed was a nuclear test, its raises important new questions about North Korea's nuclear weapons program that should be examined more carefully, but not about the CTBT verification system.
For more on the CTBT and nuclear test monitoring and verification, see the Project for the CTBT Web site.