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].
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).