Nuclear mattersNuclear bomb forensics will identify who planted it

Suppose that a terrorist group manages to get hold of nuclear material, make a bomb, and set it off in a city center somewhere. Will the fragments that are left leave clues to who did it, or where the fissile material came from? Tests on residues from the world’s first nuclear explosion suggest they might.

If someone launches a nuclear missile attack, tracking systems should quickly pinpoint where it was fired from. If the attackers did not have access to a missile, however, and delivered the bomb by truck or ship, its origins would be harder to trace.

David Shiga writes that in 2009, researchers from the Institute for Transuranium Elements in Karlsruhe, Germany, showed that when smuggled nuclear material is intercepted, its source can be deduced from details of its composition. Gleaning forensic information from an exploded nuclear bomb, though, is a different matter.

To investigate what information might be available, physicist Albert Fahey at the U.S. National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, and colleagues examined the elements, and the ratios of various isotopes, in debris from the first ever nuclear test, carried out on 16 July 1945 at the Trinity site in the New Mexico desert.

The explosion melted sand, which solidified to form a greenish glass which has been dubbed “trinitite.” Fahey’s team analyzed a 3-centimetre piece of this material.

Their preliminary results are proving promising. “You can sort of begin to figure out what elements were in the device and potentially be able to trace them to their source,” Fahey says.

One basic fact his team hoped to show could be gleaned from the trinitite was whether the weapon was powered by plutonium or uranium. As some countries produce bombs made of one or the other but not both, this could help narrow the list of possible sources. The Trinity bomb was made of plutonium, and Fahey’s team found this element in quantities of up to 400 parts per billion in the trinitite.

Shiga writes that a potentially more interesting question is whether a fragment could be traced to a specific plant, based on the ratio of the plutonium isotopes it contains. Knowing this might help identify its origin, and prevent more material passing into terrorist hands.

When the fissile isotope plutonium-239 is made in nuclear reactors it is inevitably contaminated with plutonium-240. The exact ratio of the two isotopes differs from plant to plant.

In the trinitite sample, the researchers found the ratio to be 1.6 percent, which is similar to what the Trinity bomb is though to have contained, says Fahey.

The researchers also used the trinitite to see what they could discover about the bomb’s tamper — the spherical metal shell that keeps the core from flying apart too quickly and prevents the nuclear reaction from fizzling out.

The tamper can be made of non-fissile uranium or other metals. These elements vary in their isotopic composition depending on where they were mined, so they could offer another clue to the origins of the weapon. The Trinity tamper was made of non-radioactive uranium isotopes.

In the parts of the rock where plutonium was found in high concentration, the researchers did find traces of uranium isotopes, as they had hoped.

Their measurements were not precise enough to pinpoint the material to a particular mine, but they say a more detailed analysis might yield this information.

Metals are traded internationally, so knowing only where material in a bomb was mined would not necessarily tell you where a tamper was built or who constructed it, warns Tom Bielefeld of Harvard University, who studies nuclear security.

Bielefeld recognizes, though, that these kinds of clues could prove important when combined with traditional intelligence. “You need to really gather all the information you can get,” he says. “It’s really a big puzzle and every clue may be valuable.” The work is also “an important contribution to a body of literature that’s very sparse”, he adds.

—Read more in A. J. Fahey et al., “Postdetonation nuclear debris for attribution,” Proceedings of the National Academy of Science (8 November 2010) (DOI: 10.1073/pnas.1010631107) (sub. req.)