LNSP is pleased to announce that it will offer Stanton Foundation Fellowships in the area of technical nuclear-security studies. The fellowship is given annually for up to two years and is open to postdoctoral scholars and tenure-track faculty working at the intersection of technology and nuclear security. For more information, qualification requirements, and application instructions, please see the Stanton Fellowship page.
By Meg Murphy | MIT News
Will the recent U.S. withdrawal from a 2015 accord that put restrictions on Iran’s nuclear program make it easier for Iran to pursue the bomb in secret? Not likely, according to Scott Kemp, an associate professor of nuclear science and engineering at MIT. “The most powerful insights into Iran’s nuclear program come from traditional intelligence, not from inspections by the International Atomic Energy Agency,” says Kemp.
But covert nuclear-weapon programs, whether in Iran, North Korea, or elsewhere in the world, are a major unsolved problem, according to Kemp. He recently explained the technical challenges involved in the hunt for clandestine sites. And he floated a possible solution.
What inspectors look for
Inspectors want to search for the secret production of plutonium or highly enriched uranium, says Kemp. Manufacturing an actual explosive device can be accomplished quickly and discreetly once either of these ingredients is secured in enough quantity. “The assembly work can be done in an office building, underground facility, or even in a big kitchen. It’s nearly impossible to detect once the program reaches this point.”
The good news, relatively speaking, is that manufacturing these explosive materials can leave telltale clues. “All international efforts to prevent nuclear proliferation focus on preventing the production of plutonium and highly enriched uranium,” says Kemp. “The hope is to stop the material from ever being produced in the first place, or at least in sufficient quantities to make a nuclear bomb.”
What are the telltale clues of covert production?
“The production of either plutonium or highly enriched uranium is a major operation that requires people and time,” says Kemp. The involvement of many people means traditional intelligence has some chance of finding the program. But traditional intelligence can be unreliable, especially in closed societies like North Korea. Technical mechanisms would provide a useful overlay.
Detecting plutonium production, Kemp says, is easier than detecting enriched-uranium production for several reasons. The first clue is the heat signature. “Nearly all plutonium production occurs in nuclear reactors, and they obviously produce a lot of heat,” he says. “There are clever things a country could do to hide the heat signature, but they are not simple. Infrared satellites can search for waste heat leaving buildings, or being pumped into rivers or oceans.
A second clue comes from chemical signatures. The processing of reactor fuel to extract plutonium creates chemical effluent, which could be another promising detection pathway. “In addition to plutonium, the nuclear reactor will also produce a mix of other radionuclides — and while most are trapped in the reactor, a few leak out to the environment,” says Kemp, “especially the noble gases, such as radioactive isotopes of xenon and krypton.”
Scientists may be able to detect these isotopes — xenon-131, xenon-135, and krypton-85 —when they seep into the environment. “Governments already use detectors to look for those small signatures of the operation,” he says. “But a country could do all sorts of fancy things, like cryogenically freezing the off-gas, to eliminate the chemical signature if they wanted to. So we may or may not find signs of plutonium production this way.”
And what about uranium enrichment? “It also produces a distinct chemical signature,” says Kemp, which is caused when uranium hexafluoride (UF6) gas leaks into the atmosphere. The probability of a leak is very small, but it happens. When the gas escapes into open air, water vapor causes it to decompose into hydrofluoric acid and a specific kind of dust-like aerosol. The hydrofluoric acid is not useful in terms of detection. It is too reactive and disappears whenever it touches dirt, or a building, or a tree. “You are not going to detect it at any meaningful distance,” says Kemp. But the other byproduct, the dust-like aerosol, is another story.
A new way to track secret nuclear activity
The dust produced by uranium enrichment is an aerosol called uranyl fluoride (UO2F2), and it has a chemical form that is unique to uranium processing operations, says. Kemp. He is interested in working with his colleagues on the engineering faculty to develop detectors that can identify the molecule's distinctive chemical bonds. “There are many techniques for identifying molecules, but the sensitivity required in this case is exceedingly high, and the aerosol form presents a number of other challenges,” he says.
“If we could come up with extremely sensitive detectors that are cheap enough to put around a country without a lot of fancy equipment or maintenance, we would make significant inroads into the problem of detecting clandestine uranium-enrichment programs.” Imagine, he says, something like small weather stations with a solar-powered box that has a tamper-proof seal on it. It has a tiny fan that blows air over a sensor that searches the telltale U-F bond, and then sends an alert signal if the molecule is detected.
“After a localized detection, you could use weather data to project backward and estimate the most probable places this molecule came from. If you could eventually narrow it down to a few buildings or a couple city blocks, then it would be feasible for international inspectors to request access under existing legal provisions to see what is inside.” A return to the politics
The ongoing presence of the International Atomic Energy Agency, which monitors Tehran’s most sensitive factories and research labs, is provided for by the long-established Treaty on the Non-Proliferation of Nuclear Weapons, or NPT, which Iran is unlikely to withdraw from, says Kemp. That means inspection teams can continue to check known nuclear facilities as before.
However, a special provision, called the Additional Protocol, has allowed the IAEA to have wide-ranging access over the past three years, including the right to venture out to investigate tips about suspicious sites. This provision also permits the IAEA to deploy environmental sensors of the kind Kemp wants to build. It is these extra privileges that would be at risk if Iran withdraws from the 2015 accord, says Kemp. The IAEA has used these privileges to make at least 60 visits to facilities that are not part of Iran’s declared nuclear program.
“But politics ultimately drives this in the end,” he adds. “If inspectors learned something, whether from intelligence or sensors, but were refused the additional access needed to follow up on the lead, then the international community would probably presume the worst. It would therefore still be in Iran’s interest to provide follow-up access even if they did not technically have to — that is, unless they were really hiding something.”
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.”
Q&A — Nuclear security expert R. Scott Kemp
How to detect clandestine nuclear weapons programs
A “policy physicist” explores practical ways to sniff out uranium processing from afar
By Alexandra Witze 02.21.2018
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.