Projects / Research
Last update: January 2016
Our research centers on technical and policy analysis on nuclear arms control, nuclear nonproliferation, and emerging nuclear energy technologies. It combines technically-driven research with the policy-driven agenda of the Woodrow Wilson School’s Program on Science and Global Security (SGS). Our work is underpinned by state-of-the-art neutronics calculations and nuclear fuel-cycle simulation and analysis. There currently are three broad research areas: nuclear verification, nuclear nonproliferation, and nuclear energy.
AREA I: NUCLEAR VERIFICATION
Future arms-control initiatives and progress toward nuclear disarmament may be difficult and perhaps impossible unless viable technical verification approaches are available to provide policy makers confidence in such agreements. A major area of our technical and policy research aims to contribute to this effort by developing new verification approaches for confirming the authenticity of nuclear warheads and accounting for the fissile materials that make these weapons possible. Our work in this area includes a radical new approach to identifying whether an object declared to be a nuclear warhead or warhead component is what it is claimed to be while learning nothing about the design of the item itself.
Zero-knowledge Nuclear Warhead Verification
How can an inspector be convinced that a nuclear weapon is real without learning anything about its design? This is a long-standing challenge in arms-control verification, and a central theme of our recent research. Previously proposed inspection systems using radiation detection techniques are in principle as well-suited to accomplish this key verification task, but such systems can also reveal highly classified information and so measurements had to be processed through an “information barrier” to strip out sensitive data. This raised a new challenge of simultaneously certifying and authenticating the electronics in the information barrier, which may ultimately be impossible. To resolve this dilemma, we have proposed a fundamentally different approach to warhead verification, one that avoids the measurement of sensitive information at the outset as a way to address concerns about its potential leakage.
The core of the idea is the concept of zero-knowledge proofs, a class of interactive proof systems developed by mathematicians working in cryptography and intended to prove an assertion while yielding nothing beyond the validity of the assertion being proved. Although widely used today in the digital domain, our proposal is the first relevant real-world physical application of a zero-knowledge proof [66, 73].
How can one be convinced that a nuclear weapon is real without learning anything about its design? We have proposed a new approach following a “zero-knowledge protocol” combining neutron radiography with non-electronic preloaded detectors. If a valid item is presented, then no information will be registered in the detector bank; in contrast, an invalid item will both fail the inspection and leak information—an additional incentive for the host not to cheat .
In our work so far, among the many possible strategies to implement zero-knowledge, we have chosen to demonstrate first an approach that is based on the following two key elements:
- Differential neutron radiography measurements: We interrogate a test object, which stands in for a nuclear warhead or warhead component with 14-MeV neutrons from a deuterium-tritium (DT) neutron generator making what in effect are differential measurements of both neutron transmission and neutron emission. This method is particularly sensitive to changes in geometry and elemental composition. 14-MeV transmission radiography is less sensitive, however, to the isotopics of the material. As discussed below, we are therefore also launching new efforts to see fission signatures driven by lower energy neutrons.
- Preloadable non-electronic detectors: To overcome the difficulties in certifying and authenticating electronic equipment such as that connected to a radiation detector for signal-processing or used in information barriers, we have decided to use non-electronic detectors. We have chosen to use superheated drop detectors, which consist of small over-expanded “superheated” fluid droplets that are suspended in a liquid matrix. Neutrons interacting with the fluid in the droplets produce charged particles through recoil interactions that in turn trigger the runaway growth of detectable macroscopic bubbles with a diameter of about 0.5 mm. The number of bubbles produced in the detector scales with the neutron fluence, and their detection threshold can be tuned to a minimum neutron energy. For radiography with 14-MeV neutrons, we can set the threshold, for example, to 10 MeV and are therefore immune to scattered neutrons. Bubble detectors also are completely insensitive to gamma radiation, which is another important advantage for our application. We are working with a group at Yale University that has pioneered the process of making bubble detectors and is supplying specially formulated detectors for us.
The first major publication featuring this research was the 2014 Nature article . The core of the paper is a series of large-scale computer simulations in which we showed that small diversions of heavy metal from a representative test object can be reliably detected with this combination of technologies and concepts, if bubble counts on the order of a few thousand can be demonstrated. Among other things, we have developed the mathematical formalism to understand how, within our zero-knowledge system, both the host and inspector can use repeated measurements to achieve desired inspection targets in terms of false-positive and false-negative rates [81, 109].
We are currently conducting the first experiments at the Princeton Plasma Physics Laboratory (PPPL) to demonstrate the viability of the concept. The initial experimental results have confirmed the main findings of our simulations, and we were able to present the first measurements and results at the 2015 INMM Annual Meeting in a special session dedicated to zero-knowledge approaches, the first of its kind on this topic .
For more information, on nuclear warhead verification, see also our overview page on the topic.
There are several additional directions that we are planning to explore over the next years. These are summarized in the research outlook section at the end of this page.
Verifying Past Production of Nuclear-weapon Materials
Today, there are about 16,000 nuclear weapons in the arsenals of the nine nuclear weapon states–but global fissile material stocks are estimated to be sufficient for 150,000-200,000 weapons. The range reflects uncertainties of the order of 20-40 percent in the fissile material production and stockpiles for some weapon states. For the international technical and policy communities concerned with nuclear disarmament and nonproliferation issues, adequately accounting for fissile material stocks is a major challenge that is expected to grow in importance as nuclear arsenals are reduced and when the long sought treaty to ban new fissile material production for weapons purposes is achieved.
Our research in this area is aimed at developing the science and technology needed to reduce these uncertainties in understanding the production histories and estimating existing stockpiles of fissile materials. Our research is also aimed at developing the technical capability to independently verify national fissile material declarations. In particular, to support the work of the International Panel on Fissile Materials (IPFM), we have developed detailed computer models of the types of reactors used for plutonium production by weapon states and helped develop detailed histories of uranium enrichment programs to generate new estimates of national fissile material holdings [52, 56, 57].
In 2014, H. Feiveson, A. Glaser, Z. Mian, and F. von Hippel authored Unmaking the Bomb: A Fissile Material Approach to Nuclear Disarmament and Nonproliferation (MIT Press). The book provides a technical and policy overview of nuclear weapon-usable (fissile material) issues and ideas on how to deal with them drawing on the work we have done as the core research team of the International Panel on Fissile Materials.
Another area in which we are working to combine nuclear arms-control verification and nuclear forensics is the emerging field of “nuclear archaeology,” which seeks to develop methods to analyze the isotopics of trace impurities in structural materials or in waste materials at former fissile material production sites. The idea was originally proposed in the 1990s for one particular type of plutonium production reactor. In a series of papers, we have sought to expand the horizon of nuclear archaeology. For example, we have examined how nuclear archaeology can be used in heavy water reactors , such as the one Iran is currently building, and in enrichment plants that have been used to make the bulk of weapon-grade uranium .
One particular goal of our work is to lay the technical basis for verification of a Fissile Material Cutoff Treaty, which would end new production of these materials for weapons. The treaty has been on the agenda of the United Nations Conference on Disarmament for two decades. One key problem that our group has focused on is how to determine when a particular sample of plutonium or highly enriched uranium was produced and thus determine if it was produced before or after a possible FMCT treaty came into force. The International Panel on Fissile materials has also produced a “Draft Fissile Material Cutoff Treaty” and its verification provisions [35, 48, 50]. In late 2009, this draft treaty was introduced by the Governments of Canada, the Netherlands, and Japan as an official document of the UN Conference on Disarmament to inform its discussions (CD/1878, 15 December 2009).
AREA II: NUCLEAR PROLIFERATION
A major area of our research combines technical and policy analysis to understand and reduce the risks from today’s large global fissile material stockpiles and from the nuclear technologies now associated with civilian nuclear energy programs around the world. Our overall perspective is straightforward: the only reliable way to reduce and end the threat from fissile materials is to verifiably end production and use of these materials, and to reduce existing stockpiles as transparently and irreversibly as possible. The threat today includes not just diversion by a state to a nuclear weapons program, but also possible terrorist acquisition of the kilogram quantities of fissile material that would be sufficient to make a simple nuclear explosive device. The key technical analysis and policy arguments that support this fissile material perspective view on arms control and nonproliferation are laid out in Unmaking the Bomb published by MIT Press.
Ending the Use of Highly Enriched Uranium
Highly enriched uranium (HEU) is one of the two fissile materials that can be used for making nuclear weapons, the other being plutonium. HEU poses unique proliferation challenges because, in contrast to plutonium, it can be used in simple weapon designs and even to make crude improvised nuclear explosive devices. Starting in the 1950s weapon-grade HEU was exported to scores of countries for use as research reactor fuel. Growing concerns about nuclear proliferation and the risk of nuclear terrorism in the 1970s spurred international efforts to convert HEU-fueled research reactors to low-enriched uranium (LEU, less than 20 percent uranium-235) fuel.
For many years, our team has contributed to the technical and policy debate on ending the use of HEU in the civilian and military nuclear fuel cycle. This includes: laying out the relevance and urgency of the ongoing efforts [6, 14, 15]; assessing the potential of very high-density fuels that are currently under development to new enable reactor conversions [9, 11, 16]; and introducing the idea for a “convert-and-upgrade strategies” to maintain or improve the performance of a research reactor once it switches from HEU to LEU fuel [22, 24, 69].
The neutrons available for scientific experiments can be increased significantly with state-of-the-art neutron guides using “supermirrors” and advanced guide geometries. Shown are the simulated neutron intensities at the exit of a 35-meter long guide . Upgrading these guides or other relevant equipment, such as the cold neutron source at the entrance of the guide, easily offsets small losses due to reactor conversion to low-enriched (non-weapon usable) uranium fuel.
Uranium enrichment technologies relying on gaseous diffusion and on electromagnetic separation were developed by the United States during its World War II Manhattan Project to make HEU for weapons and later to make LEU for nuclear power reactor fuel. By the 1970s, however, new gas centrifuge uranium enrichment technology emerged and proved to be much more efficient, less energy intensive, and required facilities much smaller in size. This technology has been demonstrated and deployed by a number of countries, including some non-weapon states. This introduced a new era in nuclear proliferation. In a series of articles, we have analyzed the technical characteristics of gas centrifuges and how a state might use this technology in a crash program to make highly enriched uranium suitable for nuclear weapons—a prospect commonly described as a breakout [10, 21, 32, 33, 34].
We have contributed to the research exploring the benefits of moving from national control to multinational ownership and control of uranium enrichment facilities [39, 49]. The early work was part of a study for the International Commission on Nuclear Non-proliferation and Disarmament, established in 2008 by the governments of Australia and Japan. More recently, in an article for Science, we applied this idea to Iran, arguing that converting Iran’s national enrichment plant into a multinational one, possibly including as partners some of Iran’s neighbors and one or more of the six countries that reached a comprehensive nuclear agreement with Iran in July 2015 .
AREA III: NUCLEAR ENERGY
The third major area of our research is exploring the potential and limits of emerging nuclear energy technologies, many still in the R&D stage, which could be game changers in shaping the nuclear future. This includes next-generation nuclear fission systems that seek to overcome key problems with current nuclear reactors as well as possible nuclear fusion energy systems. A large-scale expansion of nuclear power will be needed if it is to play a significant role in reducing greenhouse-gas emissions to address the threat of climate change. The research of our group aims to inform the technical assessments and policy debates that set the requirements for these next-generation nuclear fission and fusion technologies, with a particular emphasis on understanding and establishing possible criteria for the impacts of these technologies on nonproliferation efforts.
Next-Generation Nuclear (Fission) Reactors
We have focused, in particular, on the technology and deployment choices for Small Modular Reactors (SMRs), which are reactors with electrical power outputs below 300 megawatts, significantly smaller than the 1000-1500 megawatt reactors that dominate today’s nuclear market. Proposed designs range from evolutionary designs based on existing light-water reactor technology to reactors using life-time cores (“nuclear batteries”) that can be deployed in remote locations or countries with small electric grids [75, 99]. Molten salt reactor (MSR), in which nuclear fuel materials are dissolved in a liquid carrier salt, are one important example in this category. Our analysis confirms that MSRs could indeed offer significant advantages with regard to resource efficiency compared to existing nuclear power reactors based on light-water technology. Depending on the particular design, uranium and enrichment requirements could be reduced by a factor of 3-4, even without chemical processing of the fuel [71, 93]. These design choices also have important implications for proliferation risks.
Determining the fuel consumption and composition of next-generation nuclear reactors with computer simulations is critical for assessments of their overall viability, including proliferation-risk attributes. The table summarizes the key data for a 200 MWe-MSR compared to a light-water reactor with of the same power. In this example, we find that resource requirements are significantly lower than those of typical water-cooled reactors of the same size .
Nuclear fusion has long held the potential of solving many of humanity’s long-term energy needs, but the technical challenges involved mean commercial deployment is still over the horizon, and may only be achieved in the second half of this century. This period offers an opportunity to shape the technology and, in particular, minimize its potential proliferation risks. Our research on fusion energy seeks to inform the technical and policy debate with analyses that illuminate the possible proliferation impacts of candidate fusion energy systems and aims to provide a basis for building in safeguards-by-design for this technology.
Like traditional nuclear (fission) systems, nuclear fusion that is based on the deuterium-tritium reaction involves intense neutron fluxes. This means a fusion reactor could in principle be used to make weapon-usable fissile material. In fact, some fusion energy concepts explicitly envision fusion-fission hybrids as a “bridge technology” before pure fusion energy can be commercialized. We have published a detailed analysis of the proliferation risks of (magnetic confinement) nuclear fusion technology in Nuclear Fusion  and participated in a consultancy meeting on Non-Proliferation Challenges in Connection with Magnetic Fusion Power Plants organized by the International Atomic Energy Agency in June 2013 .
Beyond the work on the proliferation risks from fusion, we have also sought to understand the potential of nuclear fusion for the broader energy system of the future using an integrated assessment model to examine the “net value” of nuclear fusion as an energy source given the remaining technical and political uncertainties . This work is connected to broader efforts of our group to improve the characterization of nuclear energy in these models.