The search for an underground nuclear test

Inspection team members, with 150 tons of equipment, set up base in the ISP to begin their work. They were allowed 12 hours’ flying time to scan the sites by helicopter. Flight crews examined the ground visually and with a gamma spectrometer. For the first time in a CTBTO exercise, multispectral imaging was used, in the air and on the ground.

From the overflights, inspectors were able to eliminate two of the sites: the earthquake site and the suspicious construction site. The inspection then moved to the ground to investigate the quarries. Active seismic systems similar to those used in oil exploration were deployed to detect aftershocks that would result from the collapse of the cavity formed by a nuclear explosive.

The team that controlled the scenario, comprising mostly individuals from the US, the UK, Russia, and China, used buried explosives to simulate seismic events. Gordon MacLeod, a CTBTO official who was project manager for the exercise, says the team had to figure out “how to create something that will hold people’s interest, be realistic, and could stand up to all these techniques being used against it.” That included faking a cavity. “We had the ability from the control team to inject data needed to make a cavity that didn’t exist,” MacLeod says. They made assumptions about the depth of the explosion and radionuclide release rates. Numerous visible and buried clues, such as boreholes and underground cables, had been planted in advance. Containers of noble gas radioisotopes argon-37, xenon-131m, and xenon-133—considered smoking guns of a nuclear test—were hidden.

In addition to using ground-penetrating radar and magnetometers in the search for those clues, the inspectors tested three new noble gas detectors. China provided an ³⁷Ar analyzer, the only field-mobile device of its kind in the world. A highly sensitive radioxenon analyzer developed by Swedish researchers was provided by the European Union. A second Chinese-made device extracted radioxenon from air samples and measured it either on a silica pin detector or with high-purity germanium detectors. The US supplied a portable version of a xenon detection system that has been incorporated into the CTBTO’s international monitoring system.

The ISP attributed a hot spot of iodine-131 (kept in sealed sources for safety) and an elevated noble-gas concentration at one site to a chemical spill by a departing contractor. But the iodine’s decay could have explained only one of the xenon isotopes.

The exercise entailed a political component: An ISP watchdog team shadowed the inspectors and ensured strict compliance with the treaty. Legal foot-dragging by the host delayed access to the sites and thus diminished the chances of finding telltale evidence of a nuclear explosion.

“The ISP was quite firm on their rights under the treaty,” says Malich. “There were a couple points that were not clearly spelled out in the treaty and subordinate operational manual, so we had some tough discussions early on.” The ISP provided good logistical support, which may or may not be realistic depending on the motivation of the state, he says.

In the end, the inspectors detected multiple signatures consistent with an underground nuclear explosion, but not at the location that was thought to have been the epicenter of the earthquake. Malich notes that pinpointing an epicenter can sometimes be imprecise.

In real life, the inspection team’s evidence would be submitted without conclusions to the CTBTO executive council. That body would determine whether a test had occurred and decide what international sanctions should be imposed for a treaty violation. If results are inconclusive, the council also could authorize inspectors to continue their work for up to 130 days.

The problem is that although 183 states have signed the CTBT, eight other nations—China, Egypt, India, Iran, Israel, North Korea, Pakistan, and the US—must ratify the pact before it will take effect. There is little near-term likelihood that the US will do so, much less North Korea, the only nation to carry out nuclear tests in recent years. The US Senate rejected the treaty in 1999, in part because many senators were concerned that the international monitoring system hadn’t been completed.

Still, the CTBTO has assembled a worldwide network of 321 monitoring stations equipped with seismometers, hydrophones, and radionuclide and infrasound detection instruments. The network also has 16 laboratories and a center for analyzing the data. Now considered 90% complete, the network has detected all three North Korean tests. (For more on the monitoring system, see the article by Matthias Auer and Mark Prior, Physics Today, September 2014, page 39.)

“Through this exercise, we have shown the world that it is absolutely hopeless to try to hide a nuclear explosion from us. We’ve now mastered all components of the verification regime,” CTBTO executive secretary Lassina Zerbo said in a statement.

MacLeod says that the $10 million exercise involved mainly volunteers. Unlike the International Atomic Energy Agency, the CTBTO does not have a permanent force of inspectors. About 100 volunteers have been trained to conduct on-site inspections, but MacLeod estimates that 200–400 people would be needed to ensure continuous inspection readiness.

All the available technologies are essential to the inspections, says Malich. “If you have a perfectly contained underground nuclear explosion, theoretically you might not see a single radionuclide unless you drill into the cavity hundreds of meters down,” he says. “You can run radiation overflights and run ground-based radiation samples and surface samples through labs and find nothing. But you might find the aftershock from the collapse of the chimney or cavity.” A chimney is a narrow portion of the cavity that could vent radionuclides.

The last full CTBTO field exercise took place in 2008 in Kazakhstan. Three smaller tests were held in the past two years in preparation for Jordan. That frequency may not be enough to maintain the cadre of expertise that’s needed, says MacLeod. “What we are going to have to do is have these exercises on a regular basis so you can maintain the capabilities and make sure people can work as a team,” he says. “You need to have that capability as a deterrent.”