by David Chandler

In past negotiations aimed at reducing the arsenals of the world’s nuclear superpowers, chiefly the U.S. and Russia, a major sticking point has been the verification process: How do you prove that real bombs and nuclear devices — not just replicas — have been destroyed, without revealing closely held secrets about the design of those weapons?

Now, researchers at MIT have come up with a clever solution, which in effect serves as a physics-based version of the cryptographic keys used in computer encryption systems. In fact, they’ve come up with two entirely different versions of such a system, to show that there may be a variety of options to choose from if any one is found to have drawbacks. Their findings are reported in two papers, one in Nature Communications and the other in the Proceedings of the National Academy of Sciences, with MIT assistant professor of nuclear science and engineering Areg Danagoulian as senior author of both.

Because of the difficulties in proving that a nuclear warhead is real and contains actual nuclear fuel (typically highly enriched plutonium), past treaties have instead focused on the much larger and harder-to-fake delivery systems: intercontinental ballistic missiles, cruise missiles, and bombers. Arms reduction treaties such as START, which reduced the number of delivery systems on each side by 80 percent in the 1990s, resulted in the destruction of hundreds of missiles and planes, including 365 huge B-52 bombers chopped into pieces by a giant guillotine-like device in the Arizona desert.

But to avert the dangers of future proliferation — for example, if rogue nations or terrorists gained control of nuclear warheads — actually disposing of the bombs themselves and their fuel should be a goal of future treaties, Danagoulian says. So, a way of verifying such destruction could be a key to making such agreements possible. Danagoulian says his team, which included graduate student Jayson Vavrek, postdoc Brian Henderson, and recent graduate Jake Hecla ’17, have found just such a method, in two different variations.

“How do you verify what’s in a black box without looking inside? People have tried many different concepts,” Danagoulian says. But these efforts tend to suffer from the same problem: If they reveal enough information to be effective, they reveal too much to be politically acceptable.

To get around that, the new method is a physical analog of data encryption, in which data is typically manipulated using a specific set of large numbers, known as the key. The resulting data are essentially rendered into gibberish, indecipherable without the necessary key. However, it is still possible to tell whether or not two sets of data are identical, because after encryption they would still be identical, transformed into exactly the same gibberish. Someone viewing the data would have no knowledge of their content, but could still be certain that the two datasets were the same.

That’s the principle Danagoulian and his team applied, in physical form, with the warhead verification system — doing it “not through computation, but through physics,” he says. “You can hack electronics, but you can’t hack physics.”

A nuclear warhead has two essential characteristics: the mix of heavy elements and isotopes that makes up its nuclear “fuel,” and the dimensions of the hollow sphere, called a pit, in which that nuclear material is typically configured. These details are considered top-secret information within all the nations that possess such weapons. Just measuring the radiation emitted by a supposed warhead isn’t enough to prove it’s real, Danagoulian says. It could be a fake containing weapon-irrelevant materials which give off exactly the same radiation signature as a real bomb. Probes using isotope-sensitive resonant processes can be used to probe the bomb’s internal characteristics and reveal both the isotope mix and the shape, proving its reality, but that gives away all the secrets. So Danagoulian and his team introduced another piece to the puzzle: a physical “key” containing a mix of the same isotopes, but in proportions that are unknown to the inspection crew and which thus scramble the information about the weapon itself. Think of it this way: It’s as though the isotopes were represented by colors, and the key was a filter placed over the target, with areas that balance each color on the target with its exact complementary color, just like a photographic negative, so that when lined up the colors cancel out perfectly and everything just looks black. But if the target itself has a different color pattern, the mismatch would be glaringly obvious – revealing a “fake” target.

In the case of the neutron-based concept, it’s the mix of the heavy isotopes that’s matched, rather than colors, but the effect is the same. The country that produced the bomb would produce the matching “filter,” in this case called a cryptographic reciprocal or a cryptographic foil. The warhead to be verified, which can be concealed within a black box to prevent any visual inspection, is lined up with the cryptographic reciprocal or a foil. The combination undergoes a measurement using a beam of neutrons, and a detector which can register the isotope-specific resonant signatures. The resulting neutron data can be rendered as an image that appears essentially blank if the warhead is real, but shows details of the warhead if it’s not. (In the alternative version, the beam consists of photons, the “filter” is a cryptographic foil, and the output is a spectrum rather than an image, but the essential principle is the same.) These tests are based on the requirements of a Zero Knowledge Proof – where the honest prover can demonstrate compliance, without revealing anything more.

There’s a further disincentive to cheating built into the neutron-based system. Because the template is a perfect complement of the warhead itself, trying to pass off a dummy instead of the real thing would actually do the very thing that nations are trying to avoid: it would reveal some of the secret details of the warhead’s composition and configuration (just as a photographic negative lined up with a non-matching positive would reveal the outlines of the image). Danagoulian, who grew up in Armenia when it was part of Soviet Union before emigrating to the U.S. for college (he earned his bachelor’s at MIT in 1999 and his PhD at the University of Illinois at Urbana-Champaign in 2006), says he remembers vividly the Cold-War days when both the U.S.S.R and the U.S. had thousands of nuclear missiles perpetually at the ready, aimed at each others’ cities. After the fall of the Soviet Union, he says, there was a huge amount of fissile material suitable for bomb-making left in Russia and its former satellites. This material “measured in tens of tons – which could be used for making thousands, if not tens of thousands,” of nuclear bombs, he says. Those memories provided a strong motivation to find ways of using his knowledge in physics to facilitate a reduction in the amount of such material and in the number of nuclear weapons at the ready around the world, he says.

The team has verified the neutron concept through extensive simulations and now hopes to prove that it works through tests of actual fissile materials, in collaboration with a national laboratory that can provide such materials. The photon concept has been the focus of a proof of concept experiment carried out at MIT and is described in the PNAS publication.

Karl van Bibber, professor and co-chair of the Department of Nuclear Engineering at the University of California at Berkeley, says that an earlier paper from this team that outlined the concept "attracted much attention when it appeared, but as a theoretical work one could rightly reserve judgment regarding its feasibility in practice." This new paper, however, "goes far as a first scientific demonstration of the technique, particularly as the experiment was performed with the simplest and least favorable photon source available, ... simple enough for this methodology to gain currency in an actual verification program."

Thus, van Bibber says, "Danagoulian and team have passed a major bar ... The challenge up next will be tests with higher fidelity surrogates for warheads and ultimately real systems."

If a system does someday get adopted and helps bring about significant reductions in the amount of nuclear weapons in the world, Danagoulian says, “everyone will be better off. There will be less of this waiting around, waiting to be stolen, accidentally dropped or smuggled somewhere. We hope this will make a dent in the problem.”

AuthorLaboratory for Nuclear Security & Policy

Ninety-six of the United States' leading scientists—including 19 Nobel Prize laureates and 69 members of the National Academies—issued a letter to the U.S. Congress on Monday, October 30, urging preservation of the Iran nuclear agreement.

The letter, addressed to the Senate and House leaders of both parties, stated that "Congress should act to ensure that the United States remains a party to the agreement," and outlined the ways in which the agreement limited Iran's nuclear program. The letter also supported the negotiation of a subsequent agreement to address Iran's growing missile program.

Earlier this month, President Trump refused the certify Iran's compliance with the nuclear agreement, although he did not take action to terminate U.S. participation in the agreement. His action essentially shifts the fate of the agreement from the Executive Branch to Congress, which is now free to impose sanctions on Iran. If the Congress does so, its actions could unravel the agreement, terminating key restrictions on Iran's program that took years to negotiate.

LNSP's R. Scott Kemp, who participated in the design of the negotiating framework, was one of the five primary authors of the letter. The others include National Medal of Science and National Medal of Freedom recipient Richard L. Garwin; the former director of the Princeton Plasma Physics Laboratory, Robert Goldston; former U.S. Congressman and CEO of the American Association for the Advancement of Sciences, Rush Holt; and Princeton Professor Emeritus Frank von Hippel.

Ten MIT faculty members signed the agreement, including 2017 Nobel Prize laureate, Rainer Weiss; Institute Professor Jerome Friedman; and former Vice President of MIT, Claude Canizares.

The full text of the letter can be found here.

AuthorLaboratory for Nuclear Security & Policy

By Mark Wolverton
MIT Spectrum

IN SCIENCE AS IN LIFE, the seeds of good ideas can lie fallow. Michael Short ’05, SM ’10, PhD ’10 found one such seed in the form of a neglected memo from more than 70 years ago that led him to the scientific question that now drives most of his work.


Short, the Norman C. Rasmussen Career Development Professor in the Department of Nuclear Science and Engineering and an affiliate researcher of LNSP, is fascinated by the fundamental definition of material damage at the atomic level. “We don’t have a way to measure radiation damage right now,” he says. “That makes it awfully hard to quantify.” Put a piece of metal into a nuclear reactor, he says, and despite any existing tests you might run on the material afterward, “you can’t tell me how much damage is left behind.”

Such damage—invisible, but with major implications for nuclear reactor technologies and a host of other applications—comes from high-energy particles like neutrons or ions knocking atoms out of place from the ordered atomic lattice of a material, a phenomenon that scientists generally describe using the term DPA (displacements per atom). But DPA doesn’t give the complete picture, as Short explains: “It’s a measure of how many times each atom gets knocked around by ionizing radiation, but it’s not a measure of damage, because most of the atoms pop back into place like nothing ever happened. Very few of them remain as defects.” Another problem is that DPA calculations are approximations rather than precise measurements. “I always ask experts in the field why we use the DPA, and their candid answer is, well, it’s not that good, but it’s the best we’ve got.”

Short decided there had to be a better way. Unexpectedly, he found the inspiration for one in a World War II–era memo. In it, Manhattan Project physicists Eugene Wigner and Leo Szilard were discussing a phenomenon that came to be known as Wigner energy or the Wigner effect, in which certain materials exposed to ionizing radiation might actually store energy in some way.

Short first learned of the memo from his nuclear science and engineering colleague, associate professor Scott Kemp. Short also spoke with Ronaldo Szilard, a distant relative of Leo Szilard, who is a nuclear engineer at the Idaho National Laboratory. Using stored energy as a way to quantify damage had occurred to Short and his collaborators, but until hearing about Wigner and Szilard’s speculations, he’d found little encouragement to support such an idea. “We thought we were crazy until we realized this Nobel Prize–winning guy [Wigner] and this other couch-surfing theoretical physicist [Szilard] thought about it earlier.” Kemp showed Short a book noting the document’s location deep in the research library of the DuPont Corporation, where Short’s uncle, Cyril Milunsky, happened to work. “I was like, hey Uncle Cyril, go find this memo!”

Short realized that the stored energy concept could be expanded to encompass not only radiation damage in metals, but any sort of damage in materials. “Damage is defects, and it takes energy to make those defects,” he says. “Usually when you want to get rid of the defects in a material, you anneal it—you heat it up to a high temperature for a long time. If those defects go away, they should release the energy that it took to make them. That’s the crux of it, really.”

 Recent data from Short’s group shows the energy released or absorbed once irradiated stainless steel is heated. Features in heat flow for the irradiated steel (red), compared to an unirradiated piece (blue), support Short’s premise that radiation creates a “stored energy fingerprint” in materials. Two unexpected energy absorption spikes above 400 degrees Celsius helped Short and his collaborators in Kazakhstan identify another, previously undocumented effect that could assist in identifying damage: some materials become magnetic when irradiated. 

Recent data from Short’s group shows the energy released or absorbed once irradiated stainless steel is heated. Features in heat flow for the irradiated steel (red), compared to an unirradiated piece (blue), support Short’s premise that radiation creates a “stored energy fingerprint” in materials. Two unexpected energy absorption spikes above 400 degrees Celsius helped Short and his collaborators in Kazakhstan identify another, previously undocumented effect that could assist in identifying damage: some materials become magnetic when irradiated. 

Short’s insight is that the energy is released as heat in a particular pattern—what he calls a “stored energy fingerprint.” Measured with exquisite precision with a nanocalorimeter, this energy fingerprint can provide a sharp picture of the defects in the material and the specific events that created them. This ability, applied to radiation damage and nuclear technology, has startling implications both for civilian and military uses.

“Scott Kemp has called this ‘radiation forensics,’ using radiation in different ways to reconstruct the historical usage of things,” says Short. “He and I are working together on using stored energy to reconstruct historical uranium enrichment. Scott and I think we can take, say, the centrifuges in Iran, and measure the stored energy in the walls of the devices and figure out how many bombs they’ve made.” That would provide a means of technical verification for international inspectors assessing nuclear deal compliance. For nuclear reactors used to generate energy, Short envisions a “cheek swab test” to check the health of steel reactor vessels, correlating the stored energy fingerprint to embrittlement and other important material parameters, and thus providing the technical reassurance required for extending the life of existing nuclear plants.

After a year of intense simulations, the project is now moving ahead to the dedicated experimental phase, using MIT’s research reactor and other facilities. For example, with funding from the MIT International Science and Technology Initiatives, Short is working with the research group of Oleg Maksimkin at the Institute for Nuclear Physics in Almaty, Kazakhstan, a collaboration he describes as indispensable, the 14-hour flights between continents notwithstanding. Maksimkin’s group has provided some of the theoretical explanations required to interpret Short’s calorimetric findings, confirming the measurements by their own magnetic techniques.

As a nuclear scientist, Short’s main focus at present is defining a standard unit for radiation damage, but he’s actively exploring other possibilities for the stored energy idea. “Let’s say you have a piece of steel that looks fine, but isn’t on the inside, and you can’t just cut it open and look at it because that would destroy it. Can you scoop out a microgram-sized sample of material and make a stored energy measurement to figure out what’s going on?” If Short’s work is successful, the possible applications range far beyond nuclear science to practically all areas of engineering.

For Short, the project has proven not only that good ideas are planted in unexpected ways and places, but also the value of persistence. “I’ve spent pretty much my whole life until now”—including 17 consecutive years at MIT, as a student and then a researcher—“thinking of things and finding out that somebody else has already thought them through,” he says. “Finally, after four years on the MIT faculty, I have an idea that no one else had already!”

AuthorLaboratory for Nuclear Security & Policy