Under cover of night, a blacked-out fishing boat slips into Baltimore, Maryland’s Inner Harbor. A U.S. Coast Guard cutter moves to apprehend the intruder. But before officers can board, both boats and much of Baltimore disappear in an intense flash: A nuclear bomb hidden on the boat has detonated. As first responders rush to victims, nuclear forensics specialists scrutinize data on radiation and acoustic and seismic waves from sensors placed around the city in a breakneck effort to decipher the bomb’s design and perhaps determine who was behind the blast.
At a time when a bomb smuggled by terrorists is as big a concern as one from a foreign power, delivered by missile or airplane, an attack at a port is “definitely a more likely scenario,” says Thomas Cartledge, a nuclear engineer with the U.S. Defense Threat Reduction Agency (DTRA) in Fort Belvoir, Virginia. But forensic experts, who rely largely on nuclear test data collected years ago in Western deserts, lack a clear picture of how energy from a detonation would propagate in the highly saturated geology of many U.S. port cities. To remedy that, DTRA last October quietly staged Humming Terrapin: a 2-week test series at the Aberdeen Proving Ground in Maryland that detonated nearly 2 metric tons of conventional explosives to simulate nuclear blast effects in shallow water.
Since the 9/11 attacks, the U.S. government has mounted a major effort to prevent a nuclear bomb from being smuggled into a port. It has outfitted points of entry with radiation detectors, and it is working with foreign ports toward a goal of having all U.S.-bound cargo scanned for nuclear materials before departure. But it’s well nigh impossible to track the myriad small craft flitting in and out of the 361 U.S. ports and 153,000 kilometers of open shoreline. “There are a zillion fishing boats that leave U.S. ports and nobody inspects them when they come home,” says Matthew Bunn, a specialist on nuclear terrorism at Harvard University’s Belfer Center for Science and International Affairs. “If there is highly enriched uranium metal that’s shielded and below the water line, it’s going to be really tough to detect at long range.”
In case the unthinkable happens, a sensor array called Discreet Oculus that is being installed in major U.S. cities would capture key forensic information. The array, which DTRA is still developing, would record radiation and seismic waves emanating from the blast. “Discreet Oculus is up and running in several U.S. cities now,” Cartledge says. A sister system—a portable array that runs on battery or solar power called Minikin Echo—will be deployed at major events such as the Olympics or the Super Bowl. Data from Cold War–era nuclear testing and simulations are being used to calibrate the sensors.
Yet past U.S. testing is a poor proxy for detonations at a port, says Tamara VanHoose, a U.S. Army major and nuclear engineer at DTRA. A closer analog is a little-known campaign in 1963–64 in which the U.S. Air Force conducted a series of detonations of as much as 10 tons of chemical explosives at the bottom of Lake Superior. The tests offered a wealth of data on how seismic waves traverse the land-water interface, but they “were not instrumented to meet our needs,” VanHoose says.
Humming Terrapin aims to fill that gap. VanHoose and colleagues set up Discreet Oculus and two Minikin Echo arrays at Aberdeen, adding hydrophones, which are not currently included in either array. Another set of sensors probed how seismic signals ripple through East Coast rock layers. “These are wet-type geologies versus the granite geologies that we see out at the typical desert sites where we’ve done historic testing,” VanHoose says.
The team set out to test several scenarios. “We were looking at how a weapon might be delivered,” Cartledge says. A detonation above the water line—say in a container on the deck of a cargo ship—would produce a mostly acoustic signal, he says, whereas a detonation in a ship’s hull, below the surface, would be mostly seismic. “Really challenging,” he says, is the seismo-acoustic coupling “right at the surface”—a scenario one might expect for a detonation aboard a smaller boat.
Finally came the big bangs. Working with U.S. Navy hydrosound experts, the DTRA-led team detonated eight 175-kilogram TNT explosions at Aberdeen’s Briar Point Test Pond, as well as one 455-kilogram TNT explosion at a nearby underwater explosives facility. The team sheltered in a bunker about 450 meters away and watched the explosions on closed-circuit TV.
Less than a second after a detonation, the seismic waves arrived. The bunker “really rocks,” Cartledge says. “Wow, you don’t think it would shake us much as it does. That’s the fun part of the job.” A moment later came the airborne shock wave: “a very intense bang,” recalls Mark Leidig, a seismologist at Weston Geophysical Corp., a consulting firm in Lexington, Massachusetts, that designed the tests.
Now comes the hard work of sifting the data and “building our models to account for the coupling effects of the water we observed,” VanHoose says.
DTRA will stage its next test series back on dry land at the White Sands Missile Range in New Mexico, where an unshielded “fast-burst” nuclear reactor is normally used to test how military hardware might withstand a nuke’s high-energy neutron barrage. In June the DTRA team will verify that the speed-of-light sensors it is developing—detectors for gamma rays, radio waves, and light—can capture and model the fast burst, or the exponential rise of the nuclear reaction going critical. Such data provide “valuable forensic insight into weapon characteristics,” Cartledge says. Revealing a weapon’s design would speed the government’s response to a once-unimaginable act of terrorism, wherever it took place.