Stockpile S&TR July/August 2015 Stockpile Stewardship In the two decades since it was established, at 20 Years the nation’s Stockpile Stewardship and Management Program has served as a highly effective tool for maintaining confidence in the U.S. nuclear deterrent.

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OLLOWING the collapse of the Soviet weapons laboratories (Livermore, Los FUnion and the end of the Cold War, the Alamos, and Sandia national laboratories), United States took a series of bold steps to DOE officials, and congressional leaders. strengthen nuclear nonproliferation. These The new approach relied on advanced steps included halting underground nuclear scientific understanding through a testing, ceasing the development of new combination of theoretical advances, nuclear weapons, reducing the size of the nonnuclear (including subcritical) existing nuclear stockpile, and beginning to experiments, simulations, close nonessential elements of the nuclear and enhanced stockpile surveillance—not weapons production complex. In September on additional nuclear testing—to predict, 1996, President Clinton formally announced identify, and correct stockpile problems. his decision to seek a Comprehensive A key architect of the fledgling Nuclear-Test-Ban Treaty (CTBT) and program, DOE’s Assistant Secretary for directed the Department of Energy (DOE) Defense Programs Vic Reis, addressed to take the required actions for sustaining Livermore employees in October 1995 confidence in the stockpile without nuclear to discuss the ambitious effort. Reis told testing. Thus, the Stockpile Stewardship employees the “awesome responsibility” and Management Program was formally for decisions regarding the safety and established to maintain the safety, security, reliability of the nation’s nuclear weapons and reliability of the U.S. nuclear deterrent stockpile “has been put right back where it without full-scale testing. belongs—with the labs.” Details of the program were arrived at The new program posed significant following months of close consultation challenges for the DOE weapons between the directors of the three DOE laboratories. Weapons typically were

(left) Kevin Vandersall, a researcher in Livermore’s High Explosives Applications Facility (HEAF), inspects equipment and diagnostics prior to an energetic materials experiment. HEAF is home to some of the best- equipped high-explosives research and testing laboratories in the world. (Photograph by George A. Kitrinos.) (right) The Laboratory conducted many highly complex underground nuclear tests at the (now the Nevada National Security Site) prior to the commencement of the Stockpile Stewardship and Management Program.

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designed for a 20-year service life and (NNSA) national security laboratories— an x-ray computed tomography system to most often were replaced before then to provide the U.S. president an annual detect aging defects on critical components with newer designs. For more than four assessment of the safety and reliability in nuclear weapons. Developed by decades, nuclear testing had provided of the stockpile for more than two Livermore scientists and located at the means to assure confidence in the decades with no identified need for an NNSA’s Pantex Plant in Texas, this high- stockpile. Bruce Goodwin, associate- underground nuclear test. Verdon says, resolution imaging system assembles two- director-at-large for Global Security “This work is a remarkable achievement dimensional (2D) digital radiographs into a Research, described the challenge as given the major shift in methods and three-dimensional (3D) tomographic image analogous to maintaining a 40-year-old the many uncertainties at the start of that scientists analyze to discover any car. “New parts cannot be obtained, and the program.” He notes, however, that the anomalies. Since its installation in 2009, in some cases you can’t make replacement program faces continuing challenges as the rate and quality of surveillance efforts parts because they require materials that weapons age. have increased dramatically. Researchers are environmentally prohibited or are no are continuing to develop more efficient longer available,” he says. “You have to Enhanced Surveillance scintillators for converting x rays to light ensure that the car runs, but you’re not A central focus of the Stockpile and to increase the speed of tomographic allowed to start it.” Nevertheless, “it has Stewardship and Management Program image reconstruction. to work with 100 percent reliability.” was to enhance routine surveillance of As a complement to existing The weapons laboratories now had weapons with a combination of predictive x-ray diagnostic tools such as to understand in far greater detail how aging models and nondestructive testing CoLOSSIS, Livermore researchers materials’ aging processes might affect to forecast and detect age-related material are building a prototype high-energy weapons performance years into the changes at an early stage. For example, (10-megaelectronvolt) neutron imaging future and to use this information for plastics break down, metals corrode, and facility. The machine, scheduled for anticipating when serious issues might coatings deteriorate in response to long- completion in 2019, is designed to detect initially arise. To succeed in the radically term exposure to radiation, fluctuating cubic-millimeter-scale voids, cracks, and reworked assignment, weapons scientists temperatures, and other environmental other defects in heavily shielded or low-Z and engineers required more advanced conditions. Advanced computational materials such as plastics and polymers. nonnuclear experimental capabilities. models show how materials outgas; It will be capable of producing 2D Toward this end, the National Ignition polymers slowly change their chemistry; radiographic and 3D tomographic images. Facility (NIF) was built at Lawrence gas bubbles form, grow, and diffuse; Livermore, and some of the Laboratory’s explosives recrystallize and interact with Aging Gracefully other facilities, such as the Contained binders; materials interact with atmospheric Since the end of nuclear testing, Firing Facility (CFF) at Site 300 and gases, such as water vapor, hydrogen, scientists have been particularly concerned and the High Explosives Applications and oxygen; and how long-term radiation about how plutonium ages because Facility (HEAF), were significantly exposure changes materials. (See the article unexpected changes in the element’s upgraded. A series of progressively beginning on p. 22.) chemical structure (or phase) could more powerful was also Physical chemist Bill McLean, compromise weapons performance. In acquired. These computational machines enhanced surveillance campaign manager, particular, scientists worried about the integrated existing nuclear test data and says, “With a shrinking stockpile, damage accumulated over decades as increasing amounts of scientific data on disassembling and discarding assets to find plutonium-239 decays and self-irradiates. stockpile materials’ performance and potential problems is not always practical.” In response, Livermore scientists launched aging characteristics. They also served as Stockpile stewardship has enabled the experiments at the Laboratory’s Superblock, a surrogate test platform using far more development of new nondestructive a city-block-long assemblage of actinide accurate predictive models. surveillance technologies to identify research facilities. (Complementary efforts Charlie Verdon, principal associate potential safety or reliability problems such were conducted at Los Alamos National director for the Weapons and Complex as corrosion, cracks, and compositional Laboratory.) The ongoing experiments Integration (WCI) Principal Directorate, changes without the need for destructive involve samples of plutonium-239 taken notes that stockpile stewardship has dismantlement. from old weapons and from plutonium-239 enabled the three directors of the DOE One important surveillance tool, called artificially aged to 200 equivalent years at weapons laboratories—now DOE CoLOSSIS (Confined Large Optical present. The tests showed no significant National Nuclear Security Administration Scintillator Screen and Imaging System), is changes in pertinent physical properties

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Samples of naturally and artificially aged plutonium, each 2 or 3 centimeters long, are measured in dilatometry experiments to check for changes in the samples’ volumes over time.

Sample holder Plutonium sample

such as density and strength. As a result, scientists determined the minimum lifetime for plutonium in existing weapons is at least 85 years. (See S&TR, December 2012, pp. 11–14.) Chemist Pat Allen, the enhanced surveillance deputy program leader who also leads plutonium aging studies, notes samples of plutonium-239 to obtain data use projectiles moving at 8 kilometers that Livermore researchers have gained on its dynamic behavior. These tests are per second to impact plutonium targets a tremendous amount of knowledge about called subcritical experiments because the fabricated at the Superblock. the aging mechanisms of both and configuration and quantities of explosives “We’re just beginning to understand plutonium as well as their static properties. and special nuclear materials such as dynamic behavior,” says Allen. The However, “We have to better understand plutonium cannot create a self-sustaining new studies are part of what he terms the behavior of plutonium under dynamic nuclear chain reaction, or criticality. “Stockpile Stewardship 2.0,” an effort conditions, which is a real challenge,” Subcritical experiments are thus consistent dedicated to understanding the dynamic says Allen. He points to a recent series of with the U.S. nuclear testing moratorium behavior of both nuclear and nonnuclear plutonium experiments at NIF conducted and CTBT. materials, including those produced under a regime of extreme pressures and Underground U1a experiments through new manufacturing techniques temperatures that are beginning to yield complement those conducted at the Joint such as additive manufacturing. NIF significant new data. Actinide Shock Physics Experimental experiments are expected to play a large Dynamic plutonium experiments also Research (JASPER) Facility. JASPER role in this effort. continue at two facilities located at the captures data on the properties of Nevada National Security Site. At the plutonium at high shock pressures and Long-Lasting High Explosives underground U1a complex, chemical high temperatures close to those experienced One of stockpile stewardship’s most explosives (HEs) are detonated next to in nuclear weapons. JASPER experiments important advances has been increased

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understanding of HEs. Composed of HE powder and inert plastic binder, HEs are used in the main charge, booster, and detonator of a ’s firing system. Jon Maienschein, director of Livermore’s Energetic Materials Center, says, “We have to understand how high explosives change over time and how these changes might affect future stockpile functionality.” He notes, however, that Livermore formulations have proven themselves extremely stable. “We expect our HEs to be long-lasting because we designed them that way,” he says, adding that surveillance activities are validating this expectation. High-fidelity simulations mimic the extremely rapid physical and chemical detonation processes of energetic materials. They reflect accurate physical models that are used to predict likely changes in weapons’ HEs during the next one to two decades. HE computational models also allow scientists to assess data from HE and hydrodynamic tests and precisely reproduce these experiments. “From a scientific perspective stockpile stewardship has been beneficial for developing a much deeper scientific understanding of all materials,” says Maienschein. “We’ve progressed from observing high explosives being blown up to carefully studying their chemistry, morphology, crystallinity, and aging characteristics.” HEAF is home to some of the best- equipped HE research and testing laboratories in the world. Researchers at the facility conduct all aspects of explosives research, from synthesizing new materials to characterizing their safety and detonation properties. In 2008, NNSA named Lawrence Livermore its High Explosives R&D Center of Excellence. Working in HEAF laboratories, impervious to heat, shock, and impact, HEAF research complements Livermore chemists have developed including from small arms fire. A new experiments at CFF involving up to “insensitive” HEs that are much less likely Livermore HE formulation, LX-21, is 60 kilograms of HE. The world’s largest to accidentally detonate than the already based on the Laboratory-developed HE indoor firing facility, CFF capabilities safe conventional HEs used in most molecule LLM-105, and has proven notably include high-resolution imaging and weapons. Insensitive HEs are remarkably insensitive in advanced testing. velocity measurements of detonating

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Researchers Kevin Vandersall and Joe Tringe prepare to conduct an explosives test in one of HEAF’s seven indoor firing tanks. (Photograph by George A. Kitrinos.)

used by Livermore and Los Alamos for conducting hydrotests.

Extending Weapons’ Lives A vital element of stockpile stewardship, life-extension programs (LEPs) address weapons issues discovered through routine surveillance and annual stockpile assessments, such as aging effects that could lead to future performance degradation. Depending on the nature of these changes, parts may need to be replaced or refurbished to meet safety, security, and reliability requirements. In this way, LEPs can extend the weapons’ lifetimes for an additional 20 to 30 years without the need to conduct underground nuclear tests. NNSA national security laboratories design the parts required for LEPs and certify the life-extended weapons when they enter the stockpile. Currently, Livermore has been assigned an LEP for the W78/88-1, a system to be interoperable between Air Force and Navy missile systems. However, the effort is currently paused to accelerate the W80 LEP in support of the U.S. Air Force Long-Range Standoff missile. Engineer Hank O’Brien, W78/88-1 LEP program manager, recounts that at the start of stockpile stewardship, scientists and engineers did not envision LEPs. “The focus of stockpile stewards was on building new experimental and computational resources and increasing our scientific knowledge,” says O’Brien. “A few years later it became apparent we needed to perform work on the W87, and nonnuclear weapons assemblies. These Alamos National Laboratory. It uses two so we started thinking of life-extension hydrodynamic tests create temperatures large x-ray machines instead of one to programs.” The W87 LEP took about and pressures so great that solids behave record interior images of materials and 10 years to complete. The W78/88-1 LEP like liquids. CFF’s sister facility, the fashion them into ultrafast motion pictures will take about 20 years from start to finish. Dual- Axis Radiographic Hydrodynamic of the detonation. CFF and DARHT offer Weapons manufacturing processes are Test (DARHT) Facility, is located at Los complementary capabilities and are jointly currently five decades old, and sustaining

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these legacy processes for producing LEP those in stars and detonating nuclear components is an increasing challenge. weapons. Producing these extreme Many of the original materials and high-energy-density environments is methods are no longer used because of critical to validating theoretical models environmental and health considerations. and simulation codes that improve According to O’Brien, Livermore stockpile understanding of weapons physics. stewards search for ways to manufacture NIF experiments have provided replacement components that are less invaluable data about materials properties expensive, more environmentally friendly, at regimes otherwise inaccessible in the and simpler to certify. One promising absence of nuclear testing. Key material new technique is additive manufacturing, properties data include equation of state in which materials such as a polymer or and material strength. “NIF has been metal are added layer by layer to produce phenomenally useful,” says physicist objects with complex shapes and desirable Desmond Pilkington, program director material properties. (See S&TR, January/ for Weapon Physics and Design. During February 2015, pp. 4–11.) “With additive the past two years scientists have made manufacturing, we can design our own significant progress toward realizing material,” says O’Brien. “We can pick the ignition and energy gain on NIF, which is density of a plastic cushion or dictate the relevant to understanding thermonuclear stiffness of a metal.” processes in weapons. Pilkington says, “Achieving ignition will move our Experiments at the National Ignition Facility help Re-Creating the Sun’s Temperatures research to the next energy phase, but it scientists understand thermonuclear processes In the post-test era, scientists must is not the only goal. NIF will continue in weapons. A fully assembled ignition target rely on alternative methods for measuring to open up many more doors to scientific incorporates a capsule assembly, hohlraum, and the dynamic properties of weapons discovery for stockpile stewardship.” a surrounding thermal–mechanical package with materials. The 192-beam NIF, the most Equipped with approximately 70 optical, silicon cooling “arms.” energetic laser in the world, is stockpile x-ray, and nuclear diagnostics, the giant stewardship’s flagship facility for high- laser also helps address questions regarding energy-density physics experiments. design options being considered in LEPs. for fully validating LEP concepts prior NIF is capable of reaching temperatures Changes to weapons systems through to implementation. of 100 million degrees and pressures LEPs can have unintended consequences. 100 billion times that of the Earth’s Together with computational models, Surrogate Testing with Computers atmosphere—conditions similar to NIF experiments provide valuable data When stockpile stewardship was first formulated, scientists knew that unprecedented computational capabilities were needed to integrate the vast amount Life-extension programs of scientific knowledge of nuclear necessitate the adoption weapons processes and materials and of new manufacturing the accumulated experimental data from processes such as additive hundreds of nuclear tests. The Advanced manufacturing. Livermore Simulation and Computing (ASC) Program researchers produced (formerly known as the Accelerated a silicone cushion with Strategic Computing Initiative, or ASCI) programmable mechanical was launched to address this need. energy absorption properties Dedicated to improving computational through a three-dimensional power by at least a millionfold, ASC printing process using a quickly became one of the most significant silicone-based ink. accomplishments of stockpile stewardship. The program has also contributed to making high-performance computing an

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essential element of scientific research larger calculations.” Most full-system he cautions that the Laboratory must and spurred the U.S. high-performance simulations are still in 2D. Graduating to maintain its expertise and guard against computing industry. 3D simulations will require more powerful being overly confident as weapons ASC simulations offer a computational supercomputers. Livermore scientists continue to age and stockpile stewards surrogate for nuclear testing by accurately are preparing for the next-generation are called upon to enhance weapons modeling the extraordinary complexity of supercomputer, , which will provide safety and security and package weapons nuclear weapons systems. Major advances four to six times ’s sustained on new platforms. “Experiments play in hardware and software have made performance, with a peak performance an essential role in providing the data to possible a clearer understanding of the speed of 120–150 petaflops. develop, improve, and ultimately validate issues involved in stockpile stewardship. A key accomplishment of Livermore’s understanding of our models,” says Full 3D, high-fidelity simulations ASC-supported research has been the Dunning. “Equally important, experiments allow physicists to observe phenomena refining quantification of margins and provide opportunities for our workforce to nanosecond by nanosecond, with a level of uncertainties (QMU), an approach similar to develop the judgment required to make the spatial resolution and a degree of physics engineering safety factors. This methodology compromises inherent in taking a design realism previously unobtainable. brings together data from simulations, from a computer model to something NNSA laboratories continue to house experiments, and theory to establish that can be engineered, built, assembled, some of the world’s fastest, most powerful confidence factors for the key potential and diagnosed. We really do and will supercomputers, including Livermore’s failure modes in weapons systems. For continue to benefit from these capability 20-petaflop (quadrillion floating-point example, QMU helps weapons scientists investments.” operations per second) Sequoia machine. rank the design options identified for LEPs. Stockpile stewardship is a continuous “The calculations we run on Sequoia are process that requires the right tools for incredibly involved,” says Pilkington. Taking Better Care of Weapons the job. “Nothing is wrong with the “Often, we find the physics is more Mike Dunning, principal deputy for stockpile now, but we have a much smaller complex than we first assumed. To WCI, asserts that stockpile stewardship production complex than we used to if we understand this complexity, particularly has been so successful because of the need to make changes,” says Verdon. “I’m the interaction between all aspects of a significant investments made over the pleased that the nation is making important nuclear weapon, we need a high-resolution, past two decades in facilities, computing investments in production facilities.” He full-system model, which requires much power, and technical expertise. However, notes that a healthy complex is a crucial

Researchers used the Laboratory’s BlueGene/L supercomputer to create (left) a high-resolution simulation of turbulent thermonuclear burning in a Type 1a supernova. (bottom) The Livermore Computing Complex houses most of the Laboratory’s supercomputers. (Photograph by George A. Kitrinos.)

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munitions.” A prototype was designed Finding a Balance and virtually tested using advanced computational techniques. Computers One of the most notable accomplishments of stockpile stewardship was a theoretical then guided researchers to select the most advance, namely, solving a mystery that had confounded some of the smartest physicists stressing experiments to validate the design. for five decades. Livermore physicist Omar Hurricane won the Department of Energy’s prestigious E.O. Lawrence Award for leading a team that solved the mystery of missing In the 20 years since it was energy produced during nuclear tests. established, the Stockpile Stewardship “The energy balance problem was first recognized in the 1960s when Livermore and Management Program has developed the first two-dimensional radiation hydrodynamic simulation tools. Over many developed highly effective capabilities decades when those tools were applied to conducted nuclear tests, it appeared that the tests for maintaining confidence in the U.S. violated a basic principle of physics known to every college freshman physics student: nuclear deterrent. In 2012, NNSA conservation of energy,” says Hurricane. Many scientists had their own hypotheses about commemorated the 20th anniversary of the the missing energy, and all sorts of arguments raged, but the issue was never resolved. “It last U.S. nuclear explosives test and the was something weapons designers needed to be aware of but didn’t have time to address success of the Stockpile Stewardship and because they were fighting the Cold War,” he says. “Back then, solving energy balance was Management Program, noting that the less important because weapons designers could always conduct an underground nuclear United States has no plans to conduct such test and see to what degree the principle was violated. The test data would then be folded into their codes. The entire Cold War was fought with that gap in understanding. The tests tests in the future. usually worked, but the issue still nagged at people because of the potential problems if Stockpile stewardship is not without you got the energy balance wrong.” risk as weapons continue to age and When underground testing ended in 1992, solving the energy balance mystery became national security requirements change. important because weapons designers could no longer rely on new test data to validate their However, it has proven itself as the best codes. Starting in the early 2000s, Hurricane began leading a decade-long scientific effort approach to ensuring a safe, secure, and to find the missing energy. “We got to the bottom of why we had a discrepancy between the effective nuclear stockpile as long as results from our models and test results,” he says. “Most of the components of the solution nuclear weapons exist. “I believe we are had been hypothesized previously by designers. However, a novel part crossed many taking better care of these weapons today disciplinary boundaries, which is why it evaded solution for so long.” than when we were conducting nuclear Hurricane says that without the team’s solution, scientists would be less certain tests,” says Goodwin. “We have developed whether changes made to the stockpile would affect the energy balance. “We are more confident executing life-extension programs and using new processes and materials. a greater understanding of the way nuclear Stockpile stewardship would be much different today without this solution.” weapons work because we could not test them.” —Arnie Heller component of sustaining the nation’s facilities. Physicists and engineers still nuclear deterrent and provides a hedge have to work together and make judgments Key Words: Advanced Simulation and against technological surprise. with incomplete information, just like they Computing (ASC), Confined Large Optical Pilkington observes that the enduring did during nuclear testing.” He adds that Scintillator Screen and Imaging System stockpile “is a testament to the people who Livermore people working on LEPs also (CoLOSSIS), Comprehensive Nuclear-Test- Ban Treaty (CTBT), Contained Firing Facility designed the weapons.” One of the major gain valuable real-world experience by (CFF), Dual-Axis Radiographic Hydrodynamic concerns of stockpile stewardship architects collaborating with NNSA production plants Test (DARHT) Facility, Energetic Materials was preserving the core intellectual and on LEP components. Center, energy balance, High Explosives technical competencies of the weapons Verdon also notes that the stockpile Applications Facility (HEAF), Joint Actinide laboratories. As Pilkington notes, stewardship “methodology” has resulted Shock Physics Experimental Research (JASPER) Facility, life-extension program “People, not models, make decisions and in greater ability and confidence in (LEP), LLM-105, LX-21, National Ignition certify LEPs.” computational design and testing. These Facility (NIF), National Nuclear Security Retaining the skills, knowledge, and advances have led to tighter design cycles Administration (NNSA), Nevada National abilities of stockpile stewards is paramount. for new conventional weapons systems such Security Site, plutonium-239, Sequoia, Sierra, “Weapons designers used to receive critical as the low-collateral-damage bomb called Stockpile Stewardship and Management Program, U1A complex, W78/88-1. experience by performing underground BLU-129/B. (See S&TR, March 2013, tests,” says Verdon. “Now, they are pp. 4–9.) He says, “We developed BLU- obtaining similar experience working on 129B on a much shorter timescale than For further information contact Charlie Verdon experiments at NIF, CFF, and other NNSA had ever been accomplished with previous (925) 423-4449 ([email protected]).

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