Identifying Foreign Nuclear Explosives

National defense programs need special knowledge to disable and defeat a foreign or terrorist nuclear device. LANL has that knowledge.

After more than 60 years of development, nuclear weapons technology has become a global commodity. The technology's distribution is still somewhat limited, but it is growing, and the size and sophistication of foreign nuclear arsenals is increasing.

Since late 1992, the United States has chosen to maintain its existing nuclear weapons stockpile without supercritical testing (achievement of a sustained nuclear chain reaction), although there are no international legal prohibitions against such testing. Furthermore, it has embarked on a policy of reducing the size of its nuclear stockpile. Since the height of the Cold War, the United States has reduced its nuclear weapons stockpile by over 90 percent.

Other countries, however, have taken a different path. Three nations, Pakistan, India, and North Korea, have conducted overt nuclear testing and presumably evolved their nuclear weapon technologies as a result of lessons learned from those tests. Russia has publicly stated its intent to develop and field new nuclear weapons, and Iran is widely believed to be developing its first ones. Moreover, the danger has spread to the subnational level: some terrorist groups are actively working to obtain nuclear explosives.

A Critical Technical Discipline

The National Nuclear Security Administration (NNSA) relies upon many different technical capabilities that can be applied to understanding the threat posed by nuclear explosives. Los Alamos National Laboratory provides many unique capabilities, including the ability to interpret various foreign nuclear explosives tests and development activities and to diagnose whether full-scale nuclear yields are achieved during those activities.

But special knowledge is also needed to field viable mechanisms for pre-detonation intervention, that is, disabling and defeating the physics package (the nuclear explosive, apart from attendant systems) of a foreign or terrorist nuclear device. Intervention requires a sufficiently detailed understanding of foreign nuclear weapons design, and the Laboratory is the NNSA's prime source for a technical discipline critical to that understanding: nuclear explosives design physics.

Beyond Stockpile Stewardship

Since the United States ended nuclear testing, the design physics community (composed of staff at the Los Alamos and Lawrence Livermore national laboratories) has helped steward the U.S. nuclear weapon stockpile, with a focus on developing, assessing, and maintaining it. But as concerns about nuclear proliferation have grown, it has become increasingly apparent that the design physics community's expertise should be applied to broader national security concerns and projects, for example, to understanding non-U.S. nuclear explosive designs. For a long time, nuclear explosives design physics of foreign devices was conducted at only an individual level, perhaps best described as "hobby-like." Now, however, a more concerted and concentrated effort is necessary.

Consequently, in 2006, LANL formed the X Theoretical Design (XTD) Division's Improvised and Foreign Designs group (XTD-4) to allow the weapon design physics community to coherently and deliberately engage in applied research and mission support for design physics topics not specifically related to stockpile stewardship. Today, XTD-4 has unique assets, including 12 nuclear design physicists, who are the nation's top experts in both foreign nuclear weapons and improvised nuclear explosives, and access to 2 of the world's fastest supercomputers. XTD-4 develops and maintains the nuclear-explosives-design-physics knowledge that can be applied to improvised nuclear devices and foreign nuclear weapons. This knowledge is critical to many national security programs; indeed, they could not be successfully conducted without integrating XTD-4 expertise.

Nuclear-explosives-design physicists train for years to become established in their science, so they are a precious national resource. They understand how to integrate a broad range of physics disciplines, assisted by computational tools and validated by small- and large-scale physics experiments needed to assess the performance, safety, and detailed operation of nuclear explosive packages. This expertise enables them to recommend how, where, and when to introduce "upsets" into a nuclear explosive package in order to neutralize it. This operation can be employed against a nuclear explosive that is static (just sitting there, but perhaps with a timer running). Or it can be used against one that is dynamic (in the middle of its detonation process), a procedure that requires perfect timing.

In nuclear device disablement work, scientists establish what is called a "working point" to evaluate and disable a potential nuclear threat. In this case, the threat is hidden in the pipe labeled with the number 5.

Nuclear Forensics

Nuclear-explosives-design physicists are also best positioned and qualified to develop post-detonation forensics signatures—the physical, chemical, and isotopic characteristics that reveal the integrated effects associated with specific nuclear device designs. To do this, XTD-4 works with LANL radiochemists who are specialists in all details concerning radioactive materials and their physical analysis. Developing and documenting nuclear forensics signatures will enable investigators to analyze radioactive debris from a nuclear explosion and identify what nuclear materials were used and what industrial processes produced them. Ultimately, the signatures could enable attribution of the materials to the originating reactor or enrichment facility. Effectively assigning attribution can serve as a contingency against acts of nuclear terrorism. If nuclear states know that the United States can trace their nuclear material back to them, they should be less likely to cooperate with terrorists.

Understanding the materials used in foreign-designed systems is another challenge LANL's design physicists are tackling. This knowledge could improve the United States' post-detonation forensics science. For example, the presence or absence of materials that either moderate neutron energy or absorb neutrons can affect the neutron-induced changes found in structural materials after a detonation. So, assessing those changes, found in post-detonation debris, could lead back to the perpetrators of a nuclear terrorist act.

Understanding Foreign Design

XTD-4 also takes on the challenge of understanding the designs and technology used in foreign nuclear devices. One challenge lies in determining whether some action or defeat mechanism used by counterterrorism responders could inadvertently produce a nuclear yield. LANL's design physicists can make that determination. They have spent years studying the problem from all angles, conducting thousands of computer simulations to inform their intuition and judgment and participating in the design, prediction, execution, and analysis of relevant experiments.

In addition, LANL is working to understand the hydrodynamic (fluid-like) and nuclear properties of explosive and nonexplosive materials to better predict how they might perform in a foreign system.

During a training exercise, the Los Alamos nuclear forensics team consults with military personnel and gives them recommendations before these personnel disable and dismantle a mock nuclear device.

Developing and Testing Design Physics Capabilities

XTD-4's capabilities support several national programs, including the National Technical Nuclear Forensics (NTNF) program and the Nuclear Counter Terrorism (NCT) program. For the NTNF program, XTD-4 applies its nuclear forensic capabilities to infer nuclear explosive technology from analyzed weapon debris. XTD-4 understands numerous nuclear explosive technologies, ranging from the most basic improvised nuclear explosive to the most sophisticated nuclear weapon systems. The group stays abreast of developments made in the modern U.S. Weapons Program while also using data from historical nuclear testing of less sophisticated, intermediate-technology nuclear explosives.

While the Stockpile Stewardship Program provides the resources and impetus for applied science at the high end of the nuclear technology spectrum, the NCT program focuses on the low end: understanding improvised nuclear devices. XTD-4 also studies low-technology nuclear explosives and is designing and executing experiments to test hypotheses about how these explosives may perform—as predicted by the Advanced Simulation and Computing program's weapon design codes. Identifying the successes and failures in those predictions permits the refining and retesting of physics models to correct deficiencies in the hypotheses.

Using this experimental data, the NCT program also fosters an environment in which "what-if" design scenarios can be tested, through physics simulations, to determine if a specific combination of materials, engineering conditions, and design philosophy could be used to construct a viable nuclear explosive. The results of these theoretical simulations are used in conjunction with real-world intelligence to inform decision makers in the U.S. government.

Using ever-improving confidence in the predictions of how nuclear weapons designs will perform, XTD-4 can also apply this knowledge to forensics signatures from a wide range of nuclear explosive devices, across the entire technology spectrum, to NTNF program needs.

The results of XTD-4's research efforts are also applied directly to the training and operation of the national Joint Technical Operations Team program. This program provides the technical capability to respond to nuclear threats from foreign nuclear states and to terrorists' improvised nuclear explosives.

Applying XTD-4 Expertise to Nuclear Threats

In addition to using the scientific approach to broaden LANL's physics knowledge of nuclear explosives, the group's design physicists seek to understand nuclear threats based on information collected by the U.S. Intelligence Community.

XTD-4 staff members collaborate with intelligence analysts, some of whom also work at the Laboratory, to interpret, contextualize, and assess U.S. intelligence information. When information pertains to nuclear explosives, intelligence analysts usually request support from the XTD-4 physics designers to assess the credibility, vulnerability, safety, and potential yield of these nuclear explosives. Responding to those requests is a primary role of the XTD-4 design physicists.

By working with the Intelligence Community, XTD-4 group members learn how to better focus their capabilities to provide the best technical support to national interests. The expertise provided by XTD-4 to the intelligence analysis process also helps inform LANL's other technical program offices about evolving technical priorities, based on evolving foreign nuclear explosives technology.

Through research managed across the Laboratory, XTD-4 continually improves the United States' ability to develop the gamut of nuclear counterterrorism measures, including the interdiction and disablement of improvised and foreign nuclear devices and the use of post-detonation forensics.

–Matt Kirkland, XTD-4 Group Leader

Department of Defense (DoD) and FBI personnel assess hazards at the working point during an exercise. They rely on technical advice from NNSA team members who recommend ways to disable a potential nuclear threat.

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