FOR centuries, militaries have tapped the extraordinary energy locked in the molecules of energetic materials: shock waves with speeds approaching 10 kilometers per second, pressures up to 500,000 times that of Earth’s atmosphere, rapidly expanding gases reaching temperatures of 4,000 kelvins, and 20 billion watts of power per square centimeter of detonation front. Since Lawrence Livermore’s inception in 1952, Laboratory researchers have been among the nation’s leaders in understanding, synthesizing, formulating, testing, and modeling the chemical high explosives (HE) that are an integral part of every nuclear weapon system. Such violent reactions were once extremely difficult to accurately predict, characterize, and control. However, scientific understanding of these physical and chemical phenomena has progressed significantly during the last few decades.
Since its founding in 1991, Livermore’s Energetic Materials Center (EMC) has been the focal point for research and development of explosives, propellants, and pyrotechnics at the Laboratory. EMC provides oversight and direction for HE efforts at Livermore, ensuring full advantage is taken of Laboratory-wide capabilities. In 2008, the National Nuclear Security Administration (NNSA) named Livermore its HE R&D Center of Excellence because of EMC and the people and research facilities that support it. The center is located at the High Explosives Applications Facility (HEAF), where the majority of high-explosives synthesis, formulation, experiment, and theory are performed at Livermore.
Explosives, such as the well-known trinitrotoluene (TNT), react in millionths of a second, and their high-power properties are used in a variety of warheads and bombs. Propellants release similar amounts of chemical energy as explosives but over a longer period (seconds). The gases they generate are thus useful in launching objects such as artillery shells or rockets. In contrast, pyrotechnics generate only heat and light and are used to weld and cut. Pyrotechnics are more commonly found in fireworks. EMC provides expertise on all three types of energetic materials to the Department of Energy, NNSA, Department of Defense, Department of Homeland Security, Transportation Security Administration, Federal Bureau of Investigation (FBI), and other law-enforcement and government organizations.
According to EMC Director Jon Maienschein, “We needed to strengthen collaborations and integrate all the disparate high-explosives activities, bringing them together in a facility where everything—synthesis, formulation, experiment, and theory—could be done.” Scientists work on theoretical models of the behavior of energetic materials; advanced simulations to better understand the fundamental physics and chemistry of energetic materials; synthesis and formulation of new energetic molecules; experimental characterization of energetic material properties and reactions; and high-speed diagnostic instruments for measuring the chemical and physical processes that occur during a detonation. (See the box below.)
Strong Growth of Simulation
Livermore computer codes mimic the extremely rapid physical and chemical detonation processes of hundreds of energetic materials. CHEETAH is the nation’s leading chemical simulation code for the performance of energetic materials, with uses ranging from nuclear weapons HE to gun propellants to improvised explosive devices (IEDs). Running on desktop computers, CHEETAH reliably runs molecular calculations based on thermodynamic properties and density parameters and converts these calculations into explosives performance measures such as detonation velocity and pressure.
The work with CHEETAH is led by computational physicist Sorin Bastea and colleagues and is based on more than a half-century of explosives experiments at Livermore. With libraries of hundreds of reactants and 6,000 products in its code, the program is used throughout the Department of Defense; version 6.0 is currently in use.
CHEETAH is linked to Livermore’s hydrodynamics code ALE3D. Such a linkage, which requires hundreds of processors working in parallel, permits realistic modeling of both chemical and physical reactions across a wide envelope of detonation conditions and is invaluable in determining an explosive’s safety characteristics. “ALE3D allows researchers to simulate the thermal environment of any situation,” says Randy Simpson, associate program leader for Livermore’s Weapons and Complex Integration Principal Directorate. “The code will track thermal expansion and chemical changes and, when an explosion finally occurs, the violence of the reaction.”
Livermore codes are routinely updated to incorporate new theoretical and experimental findings. Computational physicist Larry Fried leads quantum simulations on Livermore supercomputers for NNSA’s Advanced Simulation and Computing Program to strengthen the chemical and physical reaction assumptions on which CHEETAH is based. Involving thousands of microprocessors, the simulations have revealed fleeting unexpected details.
For example, simulations show that for less than 100 picoseconds, detonating high explosives act similar to a metal; that is, they become electrically conductive. Hydrogen ions become extremely mobile, while other elements remain firmly bonded to each other. In addition, simulations show that during detonation, high-explosive constituents possess characteristics of both a soup of molecules as well as a collection of high-energy ions resembling plasma. (Scientists had long debated which model was more accurate.) Finally, advanced hydrodynamics simulations using CHEETAH reveal that HE safety characteristics are highly dependent on the size of an explosive’s powdered grains.
Searching for Energetic Molecules
“Synthesis is both an art and a science,” says chemist Phil Pagoria. For example, a chemist can attempt many multistep approaches before finding a method that produces the first few grams of a new molecule. Selecting the easiest, fastest, safest, and most environmentally friendly path requires knowledge, experience, and often the help of computer simulations. Synthesizing the first gram of an explosive may take three to six months, followed by several more months of effort to produce an optimized production process.
New molecules must pass a battery of tests that determine sensitivity to impact, friction, heat, electrostatic discharge, and shock as well as resistance to chemical decomposition. Promising molecules that pass performance and safety tests are sent to other chemists for incorporation into a mixture of ingredients, in particular binders. Additional ingredients reflect necessary trade-offs among sensitivity, performance, ease of manufacturing, safety, cost, and environmental considerations.
Synthesis and formulation chemists have helped pioneer high explosives that are remarkably insensitive to heat, shock, and impact. The most important insensitive high explosive (IHE) used in modern nuclear warheads is 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), which is virtually invulnerable to significant energy release in plane crashes, fires, and explosions or to deliberate attack with small arms fire. TATB can be found in nuclear weapons, conventional munitions, and explosives used for mining and oil production. Livermore researchers developed a more environmentally benign and lower-cost method for producing TATB, coined the vicarious nucleophilic substitution of hydrogen method. They also developed a technique to recycle TATB crystals of superior quality from existing supplies. (See S&TR, June 2009, Dissolving Molecules to Improve Their Performance.)
One IHE effort is synthesizing new molecules for use as boosters to set off the main HE charge of a nuclear device. A promising molecule is Lawrence Livermore Molecule-105 (LLM-105), which produces about 10 percent greater power than the IHE booster material ultrafine TATB. It also has greatly improved mechanical properties and performance at extreme temperatures, with safety properties similar to TATB.
For conventional munitions, LLM-172 is under development as a possible replacement material for TNT because of the latter’s environmental drawbacks. LLM-172’s melting point of 84°C aids processing safety because it is much lower than the temperature at which the material becomes unsafe. Livermore chemists have also developed new propellant ingredients for large guns to modify the burn rate and generate performance-improving gases. One new formulation, LLM-137, is composed of about 75 percent nitrogen, which extends the life of large gun barrels.
In addition, researchers are rethinking the very nature of new energetic materials. For example, chemist Alex Gash and materials scientist Troy Barbee are leading an effort to develop environmentally safe energetic nanolaminates that would augment or replace chemical explosives in conventional weapons. A nanolaminate is a dense solid composed of thousands of nanometer-scale layers. These materials store chemical energy in a manner similar to conventional explosives and remain inert until activated. (See S&TR, November/December 2008, Atom by Atom, Layer by Layer.)
A two-stage gun at HEAF fires smaller-diameter projectiles to velocities of about 8,000 meters per second. The two-stage gun is a companion to one at the Joint Actinide Shock Physics Experimental Research Facility at the Nevada National Security Site, where actinide research is conducted. Other resources include a range of guns for testing small-caliber ammunition and a microdetonics laboratory for studies at the millimeter
HEAF laboratories are used to characterize energetic materials for their safety, sensitivity, and mechanical and thermal stability. The detonation products of these energetic materials are determined as well. Tests are thoroughly instrumented with high-speed diagnostics including x-ray radiography, x-ray computed tomography, high-speed photography, and laser velocimetry.
“We run experiments with high-fidelity diagnostics that collect a large amount of precise data,” says Maienschein. “Where we once would run dozens of experiments to obtain the information we wanted, we now need only a few, especially if we use computer simulations to design the initial experiment.”
Maienschein cites long-standing efforts to develop ever-more-accurate diagnostics. One of Livermore’s most important diagnostic instruments is the photonic Doppler velocimeter (PDV), which uses standard fiber optics and commercial digitizers. Easy to operate and extremely accurate, PDV records continually changing velocities by measuring the Doppler-shifted frequency of light reflected off a surface. (See S&TR, July/August 2004, This Instrument Keeps the Beat.)
Site 300 Operations
The casting of large amounts of high explosives at Site 300 is supervised by formulation chemist Sabrina DePiero and others, who work both at HEAF and at Site 300. Much of the casting work supports counterterrorism programs that test large explosive devices for determining their properties as IEDs and as part of improvised nuclear devices, which would presumably use HE along with some type of nuclear material.
Some “melt castable” explosives, such as Composition B (a mixture of TNT and RDX [1,3,5-trinitro-1,3,5-triazacyclohexane]), are gently heated and poured into molds. A team of chemists, mechanical engineers, and technicians oversees the explosives casting at Site 300’s Building 827 complex, which includes a control room as well as three earth-covered cells for explosives processing. “Casting Composition B in a large kettle is similar to making caramel candy,” says Site 300 Manager John E. Scott. Once melted, the explosive is poured into a mold, slow-cooled by a water bath, and later machined to the precise shape needed to accommodate instruments.
Other types of explosives are pressed into rough shapes, machined into precise shapes, and fitted with a variety of diagnostic sensors. With still other explosives, such as LX-20, binders are added to HMX (tetranitro tetraazacyclooctane), and mixing is done remotely under closed circuit cameras. The mixture can then be extruded in molds, where it cures into a solid.
Up to 45 kilograms of energetic materials are detonated at one outdoor facility, where detonation characteristics are examined for myriad applications and programs. Up to 60 kilograms of energetic materials can be detonated inside the 2,600-square-meter Contained Firing Facility (CFF), the largest indoor firing facility in the world. Energetic parts heavier than 60 kilograms are transported to the Nevada National Security Site or a Department of Defense site for testing.
CFF offers the nation’s most extensive suite of diagnostic equipment for studying the detonation of explosives. Most experiments conducted at CFF support the Laboratory’s stockpile stewardship efforts. The facility’s flash x-ray machine, the only wide-angle penetrating radiography accelerator in the NNSA complex, captures the density and symmetry of compressed metals in 65-billionths of a second. Ultrahigh-speed rotating mirror cameras capture up to 160 consecutive frames of images at 3 million frames per second, forming a movielike record of an experiment. Testing capabilities are complementary to those at the Dual-Axis Radiographic Hydrodynamic Test Facility at Los Alamos National Laboratory, according to Jack Lowry, who leads explosives firing operations at CFF.
EMC scientists with forensic training also support the Forensic Receival Facility at Site 300. The facility is operated by Livermore’s Forensic Science Center (FSC), where experts determine the composition and often the source of minute samples of evidence to counter terrorism, aid domestic law enforcement, and verify compliance with international treaties.
The Forensic Receival Facility is designed to ensure chain of custody of evidentiary materials if a terrorist on U.S. soil detonates an explosive. At the FBI’s request, explosives involved in a terrorist event would be received, sampled, and temporarily stored at the facility. A climate- and contamination-controlled, air-filtered transportainer is dedicated to processing HE-contaminated evidence delivered to the facility following such an event. Samples would eventually be transported by a specialized van to Livermore for detailed analysis at the FSC analytical laboratories and HEAF. An FBI-led exercise in 2009 demonstrated the utility of the Forensic Receival Facility, if it should be required.
Such international exposure and academic collaboration help strengthen the expertise and influence of EMC researchers, notes Maienschein. From precise explosive devices that destroy terrorists (but not close-by civilians), to the devastating explosive power of highly penetrating bombs, to remarkably insensitive formulations in nuclear weapons, energetic materials are the focus of myriad innovative research efforts at the Laboratory.
Key Words: ALE3D, CHEETAH, Composition B, Contained Firing Facility (CFF), Energetic Materials Center (EMC), flash x ray, Forensic Receival Facility, high explosive (HE), High Explosives Applications Facility (HEAF), improvised explosive device (IED), insensitive high explosive (IHE), National Explosives Engineering Sciences Security Center, photonic Doppler velocimeter (PDV), Site 300, Stockpile Stewardship Program, tetranitro tetraazacyclooctane (HMX), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), trinitrotoluene (TNT).
For further information contact Jon Maienschein (925) 423-1816 (firstname.lastname@example.org).
Lawrence Livermore National Laboratory
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