Handling Nuclear Evidence

ACS Meeting News

Before the early 1990s, the world's primary concerns about nuclear threats centered on military arsenals and power plants. But in 1989, the Berlin Wall fell, and soon after, the Soviet Union disintegrated. In the resulting chaos, the controls that helped safeguard Eastern Europe's considerable stocks of nuclear material fell into disarray. Soon, reports began appearing about a new commodity on the black market: nuclear and radioactive material, pilfered from spottily guarded Russian weapons and industrial facilities.

Since 1992, there have been hundreds of cases involving the seizure of suspected nuclear contraband-including cesium, cobalt, plutonium, and enriched uranium-from smugglers at the borders of European countries. These smugglers presumably hoped to sell the contraband to shadowy figures intent on building weapons. Paralleling drug-smuggling practices, the quantities are usually quite small, likely introductory samples for potential buyers. Frequently, the material is junk-non-weapons-grade scraps-but sometimes plutonium and highly enriched uranium have been uncovered.

In response, the field of nuclear forensics sprang up, charging itself with developing ways to deduce the material's origin, age, and probable intended use. Nuclear forensics is now a full-fledged discipline, making use of spectroscopy, crystallography, isotopic analysis, and modeling, noted Maria Wallenius and Klaus Mayer, nuclear chemists at the Institute for Transuranium Elements in Karlsruhe, Germany.

Like ordinary crime fighters, nuclear forensic scientists use high-tech instruments and techniques to study their samples. But instead of fingerprints and DNA, they're measuring -ray emission, isotopic ratios, surface roughness, and particle size-all characteristics that can lead them to the nuclear material's source.

Nuclear forensics also involves a significant complication: Along with the radioactive evidence may be clues in the form of fingerprints and DNA, the composition of a container, or fragments of traditional explosives, and scientists are developing ways to handle both.

Two symposia at the recent American Chemical Society national meeting in Washington, D.C., focused on the subject: one on nuclear forensics (sponsored by the Division of Nuclear Chemistry) and one on sensors and instrumentation for counterterrorism (cosponsored by the Divisions of Analytical Chemistry and of Nuclear Chemistry).

According to news reports, in March 1992, officials in Augsburg, Germany, seized from two Russian salesmen 72 pellets of the type of uranium that is used in power reactors. Since then, numerous such cases have become part of nuclear forensic lore.

Perhaps most famous is the case of a 35-year-old Turkish man named Uskan Hanifi. According to officials, in May 1999, Hanifi was stopped by guards as he tried to cross into Bulgaria. They searched his car and found documents alluding to a purchase of uranium-235. He had no luggage, save for an air compressor, which they discovered was hollowed out. Inside was a storage vial for nuclear material that contained 10 g of fine uranium powder.

The sample was sent to Lawrence Livermore National Laboratory (LLNL), where forensic chemists determined that the powder consisted of more than 70% ²³⁵U, likely from a nuclear reactor. Because the quantity was so small, Hanifi served only a few months in prison. Officials say that some time after his release, he was found dead in his car. The case is still open.

Many of the techniques used to study nuclear samples involve long-standing chemical tools such as spectroscopy. For example, the radiation emitted by nuclear material can reveal its age-that is, the time since it was first processed. As radioactive elements decay, they produce radioactive daughter isotopes, which in turn produce their own daughters, branching into a radioactive family tree. From the characteristic spectra of the tree's radiation comes information about the ratios of the various elements, and thus, how much time the original element has spent decaying.

Uranium-235, used in power plants and bombs, is of particular interest to nuclear forensics experts. Clifford Rudy, a nuclear chemist at Los Alamos National Laboratory in New Mexico, tested a -ray analysis technique on a chunk of ²³⁵U whose age was already known and came up with the correct age. Mass spectrometry can give them the same information, but it requires expensive, not always available, equipment. And although spectrum analysis is cheaper, much more material is required, Rudy said. So there are trade-offs.

Just as with drug dealing, smugglers often peddle weapons-grade materials that later turn out to be nothing of the sort. Red mercury, for example, is a mythic substance that is claimed to have fantastic superconductive, medicinal, and other properties. In the nuclear smuggling game, it's touted as an amazing component that could set off a nuclear fusion bomb without the need for an initiating fission reaction, according to LLNL nuclear chemist Patrick M. Grant. He and his LLNL colleagues Kenton J. Moody and Ian D. Hutcheon analyzed red mercury and found that it is nothing more than ordinary mercury, mercuric iodide, or mercuric oxide.

With the heightened concern about nuclear smuggling and the potential for domestic nuclear incidents, the Federal Bureau of Investigation, in particular, is augmenting its expertise, said Martine C. Duff, a forensic chemist at Savannah River National Laboratory (SRNL) in Aiken, S.C., and a coorganizer of the nuclear forensics symposium. FBI agents traditionally have dealt in ink forgeries, fingerprints, paint analyses, and so forth. Now they have to try to do that with radiologically contaminated items, Duff said. That's a whole new twist on their typical venue.

The inherent incompatibility between the two areas is immediately obvious. The usual way to check for radioactivity-swabbing the sample and then exposing the swab to a Geiger counter-destroys fingerprints in the process. Unlike traditional forensic scientists, who work on the benchtop, radiochemists work in hoods, Duff noted. But any hairs and fibers in a delicate sample will blow right up into the hood exhaust. The two fields clash, she said.

The FBI doesn't have the infrastructure to deal with radiological materials, so it is now fostering two major collaborations with SRNL and LLNL. We've got the radioactive expertise; they have traditional forensic evidence expertise, Grant said. We're basically blending those two worlds together at a nuclear lab.

To help protect workers from radioactivity while they collect evidence, SRNL has developed radiological glove boxes that can be custom-built to suit the size of the samples-as big as a two-story building or as small as a bread box. The glove boxes allow investigators to study contaminated samples in a fresh environment, avoiding trace levels of contamination that might be present in a radiological hood, which could present a problem in court cases, Duff said. The boxes are outfitted with lenses for photography, through which investigators can look for fingerprints and other evidence. A motorized stage moves the samples around. All of these cut down on the amount of time the worker spends with the glove box, she said.

The boxes, which are made of polyvinyl chloride, do have limitations. Though the plastic blocks radiation and low levels of radiation, it doesn't protect against and high levels of radiation.

When a chunk of material, like a bomb fragment, contains both regular explosives and radioactive materials, how do you separate the two? Duff and her colleagues believe that solid-phase microextraction (SPME) could be one solution. SPME is a technique that uses hair-sized, silica-based fibers coated with a liquid or sorbent. Depending on the coating, they selectively take up hydrophobic compounds such as explosives residue and fire debris: gasoline, kerosene, or diesel fuel.

Most radionuclides likely to be encountered-plutonium, uranium, cesium, and strontium-are inorganic. Duff's group tested SPME fibers having several different coatings, including polyacrylate and polydimethylsiloxane, and showed that they don't have much affinity for inorganics. They take up the residue and leave radionuclides behind, she said.

Forensics also plays a role in monitoring nuclear activities at legitimate facilities. In 1996, the International Atomic Energy Agency (IAEA) in Vienna began taking swipe samples from nuclear facilities in countries around the world. Today, inspectors from IAEA visit about 900 to 1,000 nuclear facilities in more than 100 countries that have signed the Treaty on the Nonproliferation of Nuclear Weapons. Their aim, IAEA nuclear chemist Stephan Vogt said, is to establish baseline levels of nuclear signatures and check for deviations from the baselines that might hint at undeclared activities, such as enrichment of uranium for weapons production.

Two major processes produce material for atomic weapons: the enrichment of ²³⁵U from natural ore and the production of ²³⁹Pu from ²³⁸U. Scientists flag samples if their analyses show more plutonium or more highly enriched ²³⁵U than the facility has declared it will produce.

Although the IAEA's program wasn't formally established at the time, environmental swiping led to the discovery in 1991 of Iraq's clandestine nuclear program, Vogt said. Though the program's deterrent effect is difficult to put into numbers, he said, it certainly is effective.