Scientists Provide New Tools For Policing Nuclear Test Ban

Charles Carrigan says he's come up with a way to find the "smoking gun" if a nation tries to buck a global ban on nuclear tests.

By conducting experiments in a Nevada cavern, Dr. Carrigan and his colleagues have found that gases from nuclear explosions hidden deep within the earth seep to the surface. Such wisps of evidence could prove crucial in the verification of a test-ban treaty.

Being able to prove whether a country stands by its word to halt nuclear testing has long been the Achilles' heel of nuclear treaty negotiation and administration. But as diplomats in Geneva try this week to wrap up talks aimed at banning nuclear tests worldwide, scientists and engineers across the US are perfecting ways to police ban-violators.

More and more scientists are working together to provide tools for inspectors to accurately detect when nuclear weapons have been exploded - even if there is no visible evidence of a blast on the earth's surface.

From a new generation of satellite sensors to intricate databases on everything from mining practices to the earth's crust, researchers are weaving a high-tech security net to discourage the most ardent nuclear wannabe from detonating a device.

"The key is adequate verification," says Spurgeon Keeney, president and executive director of the Arms Control Association in Washington, D.C. "You want to make committing the perfect crime more complicated."

A significant step in on-site verification came during Department of Energy sponsored tests in 1993 - and was published in a recent issue of the journal Nature. Then, Carrigan and his colleagues at the Lawrence Livermore National Laboratory in California, placed canisters of helium and sulfur hexaflouride in a cavern holding 1,400 tons of high explosives. The test was buried deep enough to avoid leaving a surface crater - strong evidence of a nuclear test. The two gases were used as stand-ins for two radioactive gases that would result from a real nuclear test. They found that traces of the gases were detectable on the surface from 50 to 80 days after the explosion.

Another Lawrence Livermore team's research saw promising evidence from this same simulated explosion that on-site inspectors could use seismic techniques to distinguish the difference between a small underground nuclear blast and a large chemical explosion. This could help put to rest a longtime concern of officials that chemicals explosions would be mistaken for nuclear ones or that nuclear detonations could be masked as chemical ones.

The team noticed that the tiny aftershocks that followed the experiment's explosion looked different from those following chemical explosions or collapsing shafts in a mine.

This meant that small seismometers - one of the most basic nuclear-test detection instruments - could be used to distinguish between the two types of explosions if a treaty violator tried to claim the suspect blast was part of a mining operation. By locating the source of the aftershocks, seismometers also could help inspectors pinpoint a blast's "ground zero."

Indeed, the vast majority of mining explosions would not be confused with nuclear explosions, says Paul Richards, a seismologist at Columbia University's Lamont-Doherty Earth Observation. His research has shown that, with few exceptions, when chemical explosives are used in mines and for construction they are set off in a sequence. In addition, his studies show most chemical blasts generate seismic signals that can be detected only nearby. Compared with underground nuclear blasts, little energy from chemical explosions goes into generating seismic waves, Dr. Richards says. By one account, this analysis eliminates 99.7 percent of the signals that once would have been deemed suspicious.

Beyond information gleaned in the 1993 experiment, Comprehensive Test Ban Treaty negotiators are turning to four verification technologies to ensure that if violators blast, they can't hide.

Assuming the test-ban treaty is ratified, a network of 170 seismic stations will monitor for underground tests. Eleven undersea stations will monitor the oceans. And devices designed to detect a surface test's radioactive byproducts and its "infrasound" signature - vibrations that occur at frequencies too low for humans to hear - will monitor for above-ground tests. An International Data Center (IDC) will be established to evaluate and publish the data from the various monitoring networks. Its analyses, along with those from data centers within individual countries, would underpin a request for on-site inspections.

Some of elements of the monitoring network are nearly complete. More than 120 automated seismic stations are on-line, according to an official with the Defense Department's Center for Monitoring Research in Arlington, Va.

Since January 1995, the center has been operating a prototype of the IDC and is posting data from seismic stations and four of 11 hydrophone stations on the World Wide Web. Data from the other types of sensors will be brought on line as well. Once the treaty is ratified and takes effect, the whole IDC data processing and interpretation set up will be shipped to the IDC's permanent location - expected to be Vienna or Geneva.