Suppose the unthinkable happens — terrorists explode a nuclear bomb in a major U.S. city.

The blast wave, extreme heat, nuclear radiation, and then radioactive fallout from even a simple device would have disastrous effects on the population in and around ground zero, potentially dwarfing the results of the terrorist acts of September 11, 2001. For example, an explosion the size of the Hiroshima blast (approximately 15- to 20-kiloton yield) could kill more than 100,000 people, injure a similar number, and lead to the evacuation of hundreds of square miles of radioactively contaminated land surrounding the blast site.

The economic, political, and social consequences of such an attack would be devastating to the nation and the world.

What is the chance that a terrorist would explode a nuclear weapon? Former president George W. Bush and Congress have recognized this as “a very serious threat.” President Obama has warned that nuclear terrorism is “the single biggest threat to U.S. security” in “the short term, medium term, and long term.”

Extended Retaliation

What can be done to prevent it? Threatening retaliation against terrorists may have little effect. Many terrorists seek martyrdom by death, and their leaders lack return addresses.

But nuclear weapons are very hard to come by. Terrorists would need plutonium, which is made in nuclear reactors, or enriched uranium, which is made in, for example, huge centrifuge facilities. Both materials take years to produce. In addition, terrorists would need specialized knowledge to design and assemble a weapon, knowledge known only to nuclear scientists—for example, the Pakistani scientist A.Q. Khan, who for decades illegally transferred equipment and technology to rogue nations such as North Korea, Iran, and Libya. In other words, terrorists would need partners in crime: a nuclear state; a subnational military or scientific organization; and very talented, experienced individuals. Unlike the terrorists, these “friends” might indeed be deterred.

President Obama has warned that nuclear terrorism is “the single biggest threat to U.S. security.”

Nuclear attribution—identifying the suppliers of nuclear fuel and the design of a device—requires two ingredients: first, technical nuclear forensics (TNF) to analyze the explosion debris and figure out the exact fuel and the exact device design and second, intelligence and law-enforcement information to reduce the potential suspects. The attribution capability, coupled with the threat of extending U.S. retaliation beyond the terrorists to any accomplices, would likely give rogue nations such as Iran or North Korea pause, keeping them from supplying nuclear material to a terrorist organization. It would also be an incentive for Russia, Pakistan, and other nuclear powers to invest in better safeguards to prevent the loss, theft, or diversion of their nuclear materials, weapons, and technical capabilities.

Bottom line: To deter a nuclear crime, the United States needs nuclear sleuths and a nuclear forensics crime lab with the ability to analyze the explosion debris. In the event of an attack, these sleuths would need to be ready, instantly, to get on the case—to unravel the physical evidence pointing back to the perpetrators. Los Alamos National Laboratory is the lab for the job. “It takes a nuclear weapons lab to find a nuclear weapons lab,” says Laboratory Director Charles McMillan.

Today, LANL is one of the leading nuclear crime labs for the federal interagency program called National Technical Nuclear Forensics (NTNF). Program members represent the Energy, Defense, Justice, State, and Homeland Security departments, as well as other government agencies.

To deter a nuclear crime, the United States needs nuclear sleuths and a nuclear forensics crime lab to unravel the physical evidence pointing back to the perpetrators.

Los Alamos began developing nuclear forensics in 1945, when Manhattan Project pioneers analyzed debris from the first nuclear explosion (the Trinity Test near Alamogordo, New Mexico). Today nuclear forensics is a mature science, based on the analysis of debris from over a thousand U.S. nuclear tests; extensive research and design in all aspects of nuclear weaponry; modeling of nuclear performance with some of the fastest supercomputers in the world; and use of unique radiological and nuclear facilities such as Technical Area 48, the Chemistry and Metallurgy Research building, and the Plutonium Facility.

Los Alamos now applies all this experience and capability in a new way—its nuclear detectives think the unthinkable to help deter nuclear terrorism.

Nuclear Crime Scene Investigation

A terrorist nuclear explosion anywhere on the surface of the globe would announce itself instantaneously. It would send out an intense flash of light detectable by the global array of satellite-borne instruments, many developed at Los Alamos, that look for violations of nuclear test ban treaties. The explosion would also produce an enormous blast wave, setting off earth tremors that, in minutes, would reach the treaty-monitoring seismic sensors dotting the globe. Together, these so-called “prompt signals” would be the first definitive evidence that the explosion was nuclear and would give an indication of its magnitude.

George Brooks, TNF program manager at Los Alamos and technical team lead for the field collection operations, describes what would happen next: “Almost immediately after the boom, the U.S. government would try and collect airborne radioactive debris from the mushroom cloud and downwind plume. Within 30 minutes Los Alamos would receive a request for help through the National Command Authority [the President and the Secretary of Defense] and the FBI. We would immediately begin to spin up the NTNF Ground Collections Task Force, composed of LANL staff and other program participants, and develop a collections plan to gather the physical evidence needed for nuclear forensic analysis—samples of the highly radioactive explosion debris.”

Today nuclear forensics is a mature science, based on the analysis of debris from over a thousand U.S. nuclear tests.

Ideally, the collections team, equipped with protective gear against the radiation, could be on the scene in less than 24 hours. Select staff from the Department of Defense would collect the samples. Then on-scene Los Alamos experts would place the radioactive samples inside portable gloveboxes made of Plexiglas and begin to manipulate and analyze them. The scientists would stand outside, and only their forearms and hands, covered by lead-lined gloves, would reach into these transparent but sealed containers to handle the radioactive samples. The Plexiglas would block the most dangerous radiation, allowing these samples to be safely examined. “After initial analysis at the site, we would reduce the samples to a size suitable for shipment back to our TNF radiochemistry team at Los Alamos, who would then conduct a more exacting analysis,” explains Brooks. “Los Alamos plays end to end in nuclear crime forensics. We have people trained to be involved from the onset of collections to the final analysis that determines what the weapon looked like. No other lab has that complete set of capabilities.”

Collecting the Evidence

Just as each firearm leaves unique marks on a fired bullet or cartridge case, every nuclear weapon leaves unequivocal nuclear signatures in its explosion debris. But nuclear signatures bear no resemblance to a gun’s visible marks. Literally everything in and around a nuclear explosion vaporizes; nothing recognizable of the original bomb is left. All the signs of how the bomb was made and how it worked—all the on-scene evidence that could lead to attribution—are in the vaporized debris that cools and condenses into dust and clumps of glass-like material that rain down as radioactive fallout across the landscape.

Los Alamos plays end to end in nuclear crime forensics, from the onset of collections to the final analysis that determines what the weapon looked like.

Hugh Selby, one of the newer-generation Ph.D. chemist/ nuclear sleuths at Los Alamos explains, “At a crime scene, the investigators gather fingerprints, blood samples, hairs, and such and bring them to a crime lab, where technicians check for DNA, identify blood type, and so on. That composite dataset becomes one nugget of information that says, ‘It looks like Joe Bob Dillinger shot Sue Ann Ellis with a .38 special.’ Analogously, the radiochemistry team gets different samples of explosion debris and through detailed analysis puts together the unique ‘nuclear fingerprints’ that tell how big the explosion was, the kind of bomb it was, and the materials it was made of.” Discovering that information—the nature of the weapon—is the Laboratory’s signature capability.

Once the nuclear detectives know all this about the weapon, they have important clues about where the materials could have come from and who could have built the bomb.

Why are Los Alamos nuclear detectives so confident that the debris from a nuclear explosion always contains unequivocal, detailed evidence about the nature of the bomb? They know the effects of “neutron exposure.” During the fraction of a second of detonation, the fission and fusion reactions in the fuel produce an exponentially increasing number of neutrons that strike all the materials both inside and outside the bomb, transmuting (changing) some of the materials’ nuclei and thereby creating new elements and radioactive isotopes. Those newly made radioactive isotopes are the bomb’s post-detonation nuclear fingerprints; the unique nuclei in the debris reveal the nature of the weapon.

Just as each firearm leaves unique marks on a fired bullet or cartridge case, every nuclear weapon leaves unequivocal nuclear signatures in its explosion debris.

“Exposing materials to neutrons causes nuclear changes the same way that exposing film to light causes chemical changes in the photographic emulsion,” explains Selby. “And the more neutrons in the explosion, the more nuclear changes in the material. LANL’s job is to analyze that debris for nuclear changes and thereby recreate a picture of the neutron exposure, which leads us to the original bomb materials, bomb design, and explosive performance.” This information, the result of LANL’s very specialized forensic expertise, can be used to point to or rule out certain suspects.

“At Los Alamos, we do forensics. We don’t say exactly who did it,” explains Carol Burns, division leader of LANL’s Chemistry Division, “but we say exactly what happened. We provide the scientific and technical information that either supports or refutes a particular case being made by our nation’s attribution community.”

Inside the Crime Lab

In the aftermath of a nuclear attack, the radiochemical sleuths would have some tough decisions to make about how to get the most out of the debris analysis. The mood would be tense, the time short, and the stakes high. Decision makers in Washington would need answers, and the public would be demanding a response. Neutron exposure would have transmuted the nuclear fuel as well as the other materials in the bomb into an incredibly large suite of newly made radioactive isotopes (radioisotopes), possibly hundreds of them, each with potentially useful information about the nature of the bomb. The most highly radioactive of them would have very short half-lives (hours or days), so the right measurements would need to be done quickly to get the correct diagnostic information before these key isotopes disappeared.

Discovering the nature of the weapon is the Laboratory’s signature capability.

What measurements would need to be made and in what order? “Those are very high-pressure decisions,” says Selby. “You have only one pass at getting it right, and once you’ve done the measurement on a sample, that sample is gone.”

“Like a DNA sample, radioactive debris from a nuclear explosion is incredibly complicated. It takes a team of world-class chemists working around the clock to separate all the chemical elements in the radioactive debris and parse their isotopic content for nuclear clues,” comments Ann Schake, leader of the radiochemistry team for Los Alamos and the person ultimately responsible for generating all the data.

Nuclear Fingerprints

The radiochemical measurements would determine the relative amount of each radioisotope in the samples, and those relative amounts, or ratios, would lead to or be the unique nuclear fingerprints for the bomb’s efficiency, total energy release, nuclear fuel, casing, geometry, and more.

Efficiency, the simplest nuclear fingerprint to find, is the ratio of burned to unburned fuel. That ratio reveals what fraction of the nuclear fuel actually fissioned and released energy— that is, underwent “nuclear burn.” In a car engine, close to 100 percent of the gasoline burns, releasing heat and turning the molecules of gasoline into water vapor and carbon dioxide, gases that disappear down the exhaust pipe. In, say, a plutonium fission bomb, only a small percentage of the plutonium-239 (Pu-239, the fuel) undergoes fission, and the waste products of that fission do not disappear but become part of the radioactive fallout. These fission products remain present in the radioactive debris samples taken from the site of a nuclear terrorist attack.

At Los Alamos we don’t say exactly who did it, but we say exactly what happened.

Thus, to get the efficiency fingerprint, the radiochemical sleuths would measure the amount of fission products (the burned fuel) and the amount of remaining Pu-239 (the unburned fuel) in the debris samples and calculate the ratio of the two. Manhattan Project pioneers used the identical method to determine the efficiency of the Trinity Test.

The efficiency fingerprint would give a starting point for determining all the other nuclear fingerprints. However, it would take much more detective work to determine the others— such as the fingerprint of the plutonium fuel. The sleuths know that a device’s plutonium fuel would not have been 100 percent Pu-239. Depending upon where, when, and how the plutonium was produced, it would have an identifiable set of unique ratios of different plutonium isotopes (Pu-239, -238, and -240) and other very heavy elements such as uranium. Thus, the fuel’s unique fingerprint would be a strong clue as to its source.

The fingerprint of the nuclear fuel would indicate who might have been capable of producing that material.

“The fuels produced by different reactors, reprocessing facilities, or enrichment facilities are isotopically unique,” explains Selby. “So the fingerprint of the nuclear fuel would indicate who might have been capable of producing that material.”

Unbaking the Cake

But there is a big hitch. When a bomb explodes, all those isotopes are irradiated with neutrons and transmuted, so the original isotopic ratios become new ones. In other words, the explosion itself would alter the original nuclear fingerprint of the fuel, producing a new, post-detonation fingerprint! How would it be possible to infer the make-up of the original fuel? The scientists at Los Alamos are so well versed in the physics of nuclear explosions that they can reverse engineer the post-detonation products to determine the original isotopic ratios. McMillan calls this “unbaking the cake.”

So the crucial first step to identifying the fuel would be to find the post-detonation fingerprint, that is, to measure the isotopic ratios of each of the heavy elements in the explosion debris and get them exactly right, as the guilt or innocence of an entire nation would, in part, depend on it. The radiochemists would separate all the plutonium into one flask, all the uranium into another, and so on. Then they would individually measure each isotope of each element, so the Pu-239 would be differentiated from the Pu-238 and so forth, after which their ratios would be determined.

Clearly, the isotopic ratios for each element would come out wrong unless virtually all the atoms of each element in the debris samples were separated from the rest, with no contamination by other elements. Performing such exacting separations is like picking out a few grains of sand from among the trillions spread out over a mile-long beach 100 feet wide and 3 feet deep. This ability to separate all the atoms of each element—and then count them to determine isotopic ratios—is a unique strength of the Los Alamos forensics lab. “We are the nation’s premier laboratory for this kind of work,” says Selby. “And without this capability, none of the subsequent isotopic measurements would be accurate enough to be of any use as a fingerprint of a bomb.”

After uncovering the isotopic ratios in the debris, the radiochemistry detectives would input that information to the Laboratory’s radiochemistry computer codes, which would run the neutron exposure reactions backward, reverse engineer the transmutations, and come up with the original isotopic ratios of the fuel.

Scientists at Los Alamos can reverse engineer the post-detonation products to determine the isotopic ratios of the original fuel.

Reverse engineering the fuel fingerprint based on the isotopic contents of the debris is possible only because U.S. nuclear scientists have already made precise estimates of how all the various neutron exposures can transmute all the imagined nuclear fuels and combinations of fuels. Los Alamos is the leading center of excellence for the measurements, theory, and evaluation that go into constructing these high-precision estimates.


We'll Know "Whodunit"

The next step would be to send the radiochemistry team’s fingerprinting results for the bomb materials’ performance and isotopic profiles to the members of the Los Alamos nuclear weapons modeling team. These nuclear sleuths conduct accurate simulations of nuclear detonations and combine the radiochemical results with the prompt signal data. Again using the Laboratory’s massive computing power and codes for designing nuclear weapons, they would plug in the combined data and get their answer: the weapon’s design.

Stephanie Frankle, Los Alamos project leader for TNF modeling, explains, “Using our wealth of information collected from nuclear weapons tests [collected before the halt of nuclear weapons testing] and our long history of modeling nuclear weapons with powerful computers, we are able to reverse engineer the weapon design using the data and discover what the weapon was and who would be capable of building it.”

Finally, the technical evaluations from both the radiochemistry team and the modeling team would be presented to the attributions community, whose members come from the departments of State, Justice, and Homeland Security and from other government agencies. The community’s job is to put the technical assessments from the nation’s crime labs together with intelligence and law enforcement information gathered from other sources to come up with the “smoking gun”—the conclusive evidence of those responsible. The conclusion would then go to the White House for a determination of the appropriate response.

Selby adds, “Los Alamos has spent its entire 70-year history devoted to solving forensics problems, first for the nuclear test program and now for the stockpile stewardship and technical nuclear forensics programs—determining from explosion debris how a given bomb design performed. We have excellent tools, the best in the world, to investigate a nuclear attack. In fact, we would probably know what happened better than the criminals themselves.”

We have excellent tools, the best in the world to investigate a nuclear attack. In fact, we would probably know what happened better than the criminals themselves.

He continues, “Los Alamos and the entire NTNF community are developing all the tools the United States would need to find out who did it if such an attack occurred, and when the perpetrators were named, we at Los Alamos would tell those villains what they did and how they did it. Terrorists and their accomplices’ knowing they can’t get away with it—that’s the deterrent. That knowledge should make anyone thinking about committing a nuclear crime against the United States, or against its allies, shake in their boots.”






A New Manhattan Project

According to Brooks, “The NTNF effort is like a new Manhattan Project; we’re gathering the best minds in the country, but this time it’s not to build the first nuclear weapon—it’s to deter the first nuclear terrorist attack and to make sure that, if the unthinkable happens, the perpetrators will be caught.”

The NTNF effort is like a new Manhattan Project, but this time it’s not to build the first nuclear weapon, it’s to deter the first nuclear attack and to make sure that, if the unthinkable happens, the perpetrators will be caught.

Brooks continues, “Finding out ‘whodunit’ presents the nation with fantastically difficult problems. Deterring nuclear terrorists and preventing a national catastrophe—the ‘single biggest threat to national security’—requires the best and brightest nuclear detectives.” At Los Alamos, the best and brightest are on the case 24/7. Terrorists beware.






LANL Gets it Right!

In the 50 years of U.S. nuclear testing, testing debris came in the door regularly, and radiochemists and the nuclear weapons modelers quickly donned their forensics hats and went to work. Today the United States does no nuclear testing, so how do today’s nuclear detectives stay prepared?

The answer is—exercises, very intense exercises in which teams from national labs compete under severe time constraints. Each team is given data or actual debris based on real or theoretical events, and they have a certain amount of time to come up with answers. “You have to make decisions and do calculations and measurements as quickly and accurately as possible. Failure is definitely an option. People are human and they make mistakes, and everyone has to deal with all that in real time,” says Selby.

Despite the complexities, the Los Alamos nuclear crime lab has been very successful in all these exercises, whether local or national. They always have answers for the materials and design of the device—and their answers are invariably right. These exercises demonstrate that the decision makers can rely on a very strong TNF capability should it be needed.

–Necia Gran Cooper

For more information, visit “Tracking the Isotopes” and “CSI Karlsruhe.”

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