The ultimate nightmare for critics of the Iran nuclear deal being debated in Congress is that somehow, despite the agreement and all its built-in safeguards, Iran will still manage to design and build a nuclear weapon, escaping detection until a large city is consumed by nuclear fire.
But building a reliable operational weapon requires testing, and since the completion of the Comprehensive Nuclear Test Ban Treaty in 1996, a sophisticated global sensing network has come online to ensure that no nuclear test anywhere in the world goes unnoticed. Known as the International Monitoring System (IMS), it is the first alert system for nuclear transgressions, employing four distinct and complementary techniques to detect and pinpoint an atomic detonation anywhere on earth. Twenty-four hours a day, about 26 gigabytes of data from IMS stations in 89 countries pour into a control center in Vienna, Austria, through satellite networks and secure ground links. It cost about $1 billion to build and was funded by the nearly 200 member states of the 1996 treaty.
One of the most crucial sensing systems offers radionuclide detection. Eighty monitoring outposts and 16 laboratories in IMS are equipped to pick up and analyze atmospheric traces of noble gases and radioactive particles that provide a definite “smoking gun” sign of a clandestine nuke blast, such as those from North Korea’s nuclear tests. The IMS radionuclide network got an unexpected but vital workout in March 2011, measuring and tracking the radioactive plume released by the damaged Fukushima nuclear plant.
A well-designed underground test might not release any detectable nuclear residues into the atmosphere. That’s where the additional technologies of the network come into play. With 50 primary and 120 auxiliary stations, the IMS seismic net identifies about 130 events per day: earth tremors, mining explosions, or anything down to the equivalent of a magnitude 3.0 quake, a nearly undetectable rumble.
“The seismic stations are extremely sensitive,” says Gerhard Graham, coӧrdinator of the International Data Centre, which is operated by an international group called the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization. As the seismic net as a whole identifies those 130 events a day, each individual station within the network is monitoring just about everything: “A seismometer can measure nanometers of ground motion,” Graham says. For geologists, the network provides an ever-deepening baseline of seismic data enabling more accurate assessments of hazards in quake-prone regions.
The capabilities of the seismic net are supplemented and extended by 11 stations listening for the acoustic underwater signature of a nuclear explosion. Five of the stations are on shore, detecting sound waves that travel through water and then change to seismic waves upon hitting the coastline. The other six stations are hydrophones—underwater microphones moored about a kilometer beneath the surface in order to detect and determine the direction of acoustic waveforms. Because sound travels so efficiently under water, 11 stations can cover the entire world, says George Haralabus, head of the IMS hydroacoustics section.
Completing the IMS network are infrasound sensors capable of hearing sounds at extremely low frequencies, well below the range of human hearing. The earth is never quiet in this acoustic realm, with infrasound generated not merely by nuclear explosions but by the motions of the atmosphere and the earth’s crust, volcanoes, human activities of all kinds, and even meteors and space junk penetrating the atmosphere. Forty-eight infrasound stations are currently operational out of a planned 60, featuring arrays of microbarometers.
But the IMS network is also being used in ways its original designers never imagined. One of the most important is as an early warning system for natural disasters. Since the devastating Indian Ocean earthquake and resulting tsunami in December 2004, the IMS has linked its considerable resources to tsunami warning stations worldwide. That arrangement paid off in March 2011, with IMS data providing enough warning to Japanese authorities to move endangered residents to higher ground during the same catastrophe that inundated Fukushima. The seismic stations can even pinpoint the crash sites of large aircraft on or near land. Meanwhile, the other scientific possibilities of the IMS are only beginning to be fully appreciated, from the monitoring of collapsing ice shelves at the poles and deep ocean temperatures for evidence of climate change, to eavesdropping on whale songs for new insights into whale behavior and migration patterns.
All the sensor data that comes into the center in Vienna is continuously processed, analyzed, and passed along to all the member states of the 1996 test ban treaty. Assuming anything suspicious turns up, the next step is an on-site inspection for evidence of treaty violations.
At least, that will be the next step when the Comprehensive Nuclear Test Ban Treaty actually goes into effect. Though the world’s declared nuclear powers have observed a voluntary testing moratorium since 1991, several of the 183 signatories (including the U.S.) are dragging their heels on ratification, mostly citing concerns about verification. But the people who run the IMS say such concerns are moot now. Graham says the technologies in the system make it “all but impossible to escape detection.”
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